3 talks
The heavy-ion program in the ATLAS experiment at the LHC originated as an extensive program to probe and characterize the hot, dense matter created in relativistic lead-lead collisions. In recent years, the program has also broadened to a detailed study of collective behavior in smaller systems. In particular, the techniques used to study larger systems are also applied to proton-proton and proton-lead collisions over a wide range of particle multiplicities, to try and understand the early-time dynamics which lead to similar flow-like features in all of the systems. Another recent development is a program studying ultra-peripheral collisions, which provide gamma-gamma and photonuclear processes over a wide range of CM energy, to probe the nuclear wavefunction. This talk presents the most recent results from the ATLAS experiment based on Run 1 and Run 2 data, including measurements of collectivity over a wide range of collision systems, potential nPDF modifications — using electroweak bosons, inclusive jets, and quarkonia — and photonuclear dijet production.
4 talks
In this talk, I will discuss the description of the early stages of a heavy ion collision in the color glass condensate framework. Concerning the issue of isotropization/hydrodynamization in the final state evolution, I will present the results of various approaches (classical statistical simulations, 2PI formalism, and kinetic theory) in order to clarify their relationships.
I will review the latest developments in the global analysis of nuclear PDFs.
Diffractive vector meson production measured at HERA provides information on the size of the proton and fluctuations of the proton shape. In particular the differential incoherent cross section allows us to constrain a model for proton shape fluctuations within the dipole model. We then incorporate the constrained fluctuating protons in simulations of p+Pb collisions within the IP-Glasma + MUSIC framework, where IP-Glasma models the initial state within a classical Yang-Mills framework and MUSIC is a 3+1 dimensional relativistic viscous hydrodynamic simulation. We demonstrate that proton shape fluctuations constrained by HERA data are also compatible with the flow harmonics of charged hadrons measured in p+Pb collisions at the LHC. We further present more detailed measurements, such as HBT radii, and identified particle spectra and flow.
In ultra-relvativistic heavy-ion collisions, the intense electromagnetic fields produced by the nuclei give rise to large fluxes of equivalent photons. Photons that are emitted coherently by the entire nuclear charge is enhanced by a factor of Z^2, resulting in interaction rates much higher than in pp collisions. These reactions are the dominant interaction mechanism when the nuclei have large transverse separation and are referred to as ultra-peripheral collisions (UPCs). They have qualitatively different features than those collisions in which the nuclei interact hadronically, which allows them to be identified experimentally. Photon-induced reactions have been studied extensively in high-energy physics experiments. Such measurements were a major part of the physics program at HERA where they were used to study a variety hardscattering and diffractive processes. Similar measurements in ion collisions provide information about the geometric structure of the nuclei and about nuclear parton distributions in a domain where non-linear evolution effects may be important. This talk will summarize recent experimental results and theoretical advancements in the subject of UPCs. Emphasis will be placed on how these developments can improve the understanding of the initial conditions and early-time dynamics of heavy-ion collisions
and how they bear directly on the physics program of a future electron-ion collider.
3 talks
3 talks
Elliptical and triangular flow, $v_n$, are approximately linear correlated with the initial eccentricities $\varepsilon_n$'s. Traditional single-harmonic measurements such as ratios of multi-particle cumulants $v_n\{m\}$ can, therefore, provide unique information about the initial state. In this talk, the skewness of the initial eccentricity fluctuations, the fluctuations of $v_2$ vs. $v_3$ on an event-by-event basis, and the magnitude of flow fluctuations are explored using relativistic viscous hydrodynamics, which naturally reproduces experimental data in large systems. The same techniques are then applied to small systems to investigate if one could differentiate between a hydrodynamic vs. partonic picture of multi-particle correlations.
At first, a brief overview of high order (beyond Navier Stokes) linear hydrodynamics will be given.
Then the focus will be shifted to fluid-gravity correspondence. Basic ideas behind the formalism
will be reviewed and the main results on high (all) order transports presented. If time permits,
I will also talk about anomaly-induced transports computed from holography.
The quark-gluon plasma (QGP) created in heavy-ion collisions possesses a
high degree of momentum-space anisotropy in the local rest frame due to
rapid longitudinal expansion. The degree of momentum-space anisotropy
is largest at early times after the initial nuclear impact, e.g. P_L/P_T
~ 0.2-0.3, and only slowly relaxes toward unity in the center of the
fireball. Additionally, large momentum-space anisotropies persist for
longer and eventually never approach unity as one moves toward the
transverse and longitudinal edges of the QGP. As a consequence,
traditional viscous hydrodynamics approaches which rely on linearization
around an isotropic background can result in particle distribution
functions which violate positivity. In order to address this and other
issues related to the high-degree of QGP momentum-space anisotropy, I
will discuss recent progress in anisotropic hydrodynamics, which is a
reorganization of traditional viscous hydrodynamics that takes into
account momentum-space anisotropies from the outset and guarantees, for
example, positivity of the one-particle distribution function.
3 talks
In the last years the LHCb experiment has started to provide novel inputs to heavy ion physics by exploiting some of its unique features. Particle production can be studied in p-p, p-Pb and Pb-Pb collisions at LHC energies for pseudorapidity between 2 and 5, providing measurements which are highly complementary to the other LHC experiments.
The excellent vertexing and particle identification capabilities allow exclusive measurements of open and hidden charm states, including baryons, distinguishing the prompt component from the charm hadrons produced in b decays.
Collisions of LHC beams on fixed targets are also studied using the internal gas target SMOG. Production measurements in this unique kinematic range provide crucial constraints on models of key interest for cosmic ray physics as well as for cold nuclear matter effects. An overview of the LHCb heavy ion program will be given, and selected results will be presented.
The PHENIX experiment has collected a wealth of data from
small systems ranging from p+p, p+Al, p+Au, d+Au, 3He+Au as well as an
energy scan for d+Au collisions. The ability to control initial
geometry and energy deposition has yielded new insights on the initial
stages of small system collisions. We present a suite of new results
and discuss the initial stage implications through comparisons to
various calculations and models.
4 talks
The study of particle production in proton-nucleus collisions provides essential information about high-density effects (like gluon saturation) in the nuclear wavefunction. The corresponding cross-sections can in principle be computed within perturbative QCD, using the framework of the Color Glass Condensate. However, a recent calculation of single-inclusive hadron production at next-to-leading order (NLO) met with an unexpected difficulty: the cross-section suddenly turns negative at transverse momenta of the order of a few GeV, in a range where perturbation theory is expected to be reliable. We argue that this problem is an artifact of some of the approximations inherent in the usual factorization scheme at high energy, ``the $k_T$-factorization'', which is local in rapidity. We present a more general factorization scheme, valid through NLO, which is non-local in rapidity but yields a manifestly positive result for the cross-section. We also clarify a recent difficulty with the use of a running coupling in the context of the NLO calculation.
The color glass condensate (CGC) description of a high energy proton or nucleus, based on stong classical color fields, provides a way to understand the initial stages of a heavy ion collision in first principles QCD. In order to make this description quantitative, the color fields have to be studied by probing them with dilute probes. This talk concentrates on the description of two such dilute-dense processes: particle production in forward rapidity proton-nucleus collisions and the total deep inelastic scattering cross sections. The CGC description of these processes is currently advancing to next-to-leading order accuracy in the QCD coupling. In both cases the NLO calculation poses similar challenges. Most importantly the leading high energy logarithm that manifests itself in a "rapidity divergence" needs to be absorbed into a BK renormalization group evolution of the target in a way that avoids oversubtracting. Another challenging aspect is to develop a running coupling scheme that can be used to consistently describe both processes, which is made difficult by the fact that to relate the two one needs to pass between momentum and coordinate space. This talk will discuss the recent progress in these very closely related calculations.
Dijet production in the gluon saturation regime provides
a unique probe for the gluonic content of hadrons.
This is because such process is, in general, sensitive to several
small-x transverse momentum dependent (TMD) gluon distributions.
I this talk I shall discuss a factorization-like approach which is suitable for production of dijet system with rather large transverse momenta.
Although it involves several not known small-x TMD gluon distributions (breaking thus ordinary factorization), they all can be calculated in the Gaussian approximation of the Color Glass Condensate.
I shall present some numerical predictions for the LHC, both for pA collisions and for ultra-peripheral AA collisions. The last process is very interesting as it probes directly TMD gluon distribution not accessible otherwise.
While most initial-state calculations for heavy-ion collisions are performed in the asymptotic high-energy limit, real-world kinematics involve large but finite energies. Sub-eikonal corrections may therefore be important for calculating the initial conditions at RHIC or the LHC, especially those corrections which are enhanced by the size of the medium. Therefore, we study the first sub-eikonal corrections to the classical Weizsacker-Williams gluon field in the McLerran-Venugopalan model of a heavy nucleus. Starting with the first sub-eikonal corrections to a single color source (quark), we see that the nature of the field changes drastically: new kinematic dependences are introduced, and the gluon field acquires new components which are not seen in the eikonal limit. In this talk, we will present preliminary results on the leading modifications to the gluon field of a heavy nucleus at large but finite energies.
6 seminars, 20 min. each
The study of the charged particle production as a function of the multiplicity in small colliding systems, as pp and p-Pb, is an attractive tool to understand the similarities and differences between small and large colliding systems. Evidence of similarities, like coherent and collective effects, well known in nucleous-nucleous (A-A) collisions, has been found experimentally in small systems.
New results on the primary charged-particle pseudo-rapidity density and transverse momentum ($p_{\rm{T}}$) distributions at central pseudo-rapidity in pp ($\sqrt{s} = 5.02$ and $\sqrt{s} = 13$ TeV) and p-Pb ($\sqrt{s} = 5.02$ and $8.16$ TeV) collisions as a function of multiplicity are going to be presented.
In order to study the hard component of particle spectra in pp collisions, a power law fit of the distributions for $p_{\rm{T}}> 4$ GeV/$c$ is performed and the evolution with multiplicity of the resulting fit parameters is discussed. To the same purpose, the ratio of multiplicity-dependent yields over minimum bias yields integrated over $4 < p_{\rm{T}} < 8$ GeV/$c$ is studied and compared to results for heavy-flavour particles. Results are presented using two multiplicity estimators, at mid pseudo-rapidity ($|\eta| < 0.8$) and forward-backward pseudo-rapidity ($- 3.7 < \eta < - 1.7$ and $2.8 < \eta < 5.1$) to study the differences caused by the multiplicity selection in different pseudo-rapidity windows.
A comparison with Monte Carlo event generators and models, like EPOS-LHC and PYTHIA 8, will be shown.
ATLAS measurements of two-particle correlations in $\Delta\phi$ and $\Delta\eta$ are presented for $pp$ collisions at 2.76, 5.02 and 13~TeV, and for $p$+Pb collisions at 5.02 and 8.16~TeV. A template fitting procedure is used to subtract the dijet contribution and to extract the genuine long-range ridge correlations. In all collision systems, the ridge correlations are shown to be present even in events with a low multiplicity of produced particles, implying that the long-range correlations are not unique to rare high-multiplicity events. The measured correlations in $pp$ collisions are shown to exhibit only a weak energy dependence and are found to be remarkably similar to those observed in $p$+Pb collisions. The measurements are extended to correlations where one of the particles comes from decays of heavy-flavor particles. Significant long-range correlations are observed for particle pairs containing the heavy-flavor muon with similar features as the inclusive hadron pairs. This presentation also includes a new, detailed study of ridge properties in collisions with hard processes, characterized by large four-momentum transfer. This may help answering the question whether the ridge arises from hard or semi-hard processes, or if it is the result of mechanisms unrelated to the initial hardness scale.
Hanbury Brown and Twiss (HBT) radii with respect to the 2nd-order event plane are measured in central p+Pb collisions at √s_NN = 5.02 TeV with the ATLAS detector at the LHC. A total integrated luminosity of 28 nb^−1 is sampled. The radii and their relative modulation are presented as a function of the magnitude of the flow vector |q_2| measured at the side of the calorimeters that the Pb beam faces with pseudorapidity η < −2.5. Modulations of the transverse HBT radii are observed with the same orientation as in ion-ion collisions, in which they are attributed to hydrodynamic evolution from an elliptic initial geometry. This modulation is consistent with a hydrodynamic evolution of a short-lived medium.
Multi-particle correlations in hadronic colliding systems at both RHIC and the LHC are under detailed investigation in the past year. Indeed, a wealth of experimental evidences in recent years has suggested the presence of collective phenomena and a possible QGP medium being formed also in high-multiplicity pp and pPb collisions. In particular, multi-particle cumulant analyses have established the collective nature of these correlations. Nevertheless, despite the fact that a common paradigm seems to emerge for all hadronic systems, the exact underlying mechanism still need to be understood. In particular, the measurement of azimuthal anisotropy coefficient ($v_n$) down to low multiplicities is still challenging experimentally. Based on recent data collected by CMS experiment, multi-particle correlation results will be presented for pp and pPb collisions. In addition, new methods introducing sub-events in the cumulant method and the first results will be presented. These results provide new insights on the origin of observed long-range correlations in small colliding systems and might be able to disentangle between different interpretations of the observed collective behavior.
Azimuthal momentum anisotropies have now been observed in
small collision systems at RHIC and the LHC. At RHIC, using the unique
ability to select different colliding species, the PHENIX experiment has
taken data in p+Al, p+Au, d+Au, and 3He+Au at 200 GeV center-of-mass
energy. This geometry engineering allows for a unique test of
explanations of azimuthal anisotropy
in terms of final state effects versus initial state momentum domain
effects. In this talk, we report on elliptic anisotropies in all systems
for unidentified hadrons as well as particle identified hadrons. We
also report on triangular anisotropies comparing d+Au and 3He+Au
results. Detailed model comparisons with all observables are discussed.
The event structure analysis allows to gain insight into the details of the physics mechanism which causes the novel collective phenomena observed in high multiplicity pp collisions at the LHC energies. In this context, transverse spherocity can be used to isolate hard and soft events in pp collisions. We present the study of the inclusive and identified charged particle production as a function of mid-rapidity charged particle multiplicity and transverse spherocity for pp collisions at $\sqrt s=$13 TeV.
The results include measurements of transverse momentum distributions of charged pions, kaons and (anti)protons as well as the mean transverse momentum and particle ratios using data recorded with the ALICE detector. For a fixed multiplicity, the evolution of radial flow-like effects as a function of transverse spherocity is studied.
Comparisons with QCD inspired event generators such as EPOS LHC and PYTHIA 6 and 8 will be shown and discussed.
6 seminars, 20 min. each
We present explicit implementation of effective kinetic theory
(“bottom-up”) thermalization scenario and a smooth matching to the subsequent hydrodynamic evolution for realistic initial conditions.
The equilibration dynamics is captured by the out-of-equilibrium evolution of background energy-momentum tensor and linearised transverse energy and momentum response functions, which naturally transitions to hydrodynamic behavior at late times. We demonstrate that physical observables become insensitive to the crossover time. The presented framework of weak coupling kinetic theory is also a promising way of studying chemical equilibration and non-thermal photon production at early stages of heavy ion collisions.
Direct photons are an excellent tool to understand the properties of the quark gluon plasma created in heavy ion collisions. Since they are color-blind, photons carry directly the information of the system at their production time towards the detectors. The direct photons at mid-rapidity would carry convoluted information from the early to late stages of collisions, while those at forward rapidities could provide important constraints to the gluon PDF, as they are directly sensitive to the gluon density.
For Au+Au collisions at 200 GeV, PHENIX has measured a large yield of low
momentum direct photons which suggests early emission at high temperatures.
On the other hand, a large azimuthal anisotropy, which hints a later emission when the collective flow of the matter is fully developed, was also found. This apparent tension could be better understood by the measurements at various collision systems and energies.
In this talk, I will present recent PHENIX results on low pT direct photons
measured at mid rapidity for Au+Au at 39, 62.4 and (newest dataset)
200 GeV, as well as Cu+Cu at 200 GeV. In addition the analysis status of
the 2016 d+Au dataset over a wide rapidity range will be presented.
The shear and the bulk relaxation times are important ingredients of the second order hydrodynamics whose success in heavy ion phenomenology is unquestioned. In this work, we report two field-theoretical analyses involving the shear and the bulk relaxation time. Unlike viscosities themselves, field theoretical calculations of the relaxation times are hard to come by in literature, especially for the bulk relaxation time. Therefore, first we show, by carefully examining the analytic structure of the stress-energy tensor response functions, how to obtain a Kubo formula involving the bulk relaxation time. Second, by making use of the Kubo formulas involving shear effects we calculate the shear relaxation time within the massless scalar theory.
Relativistic hydrodynamics has been an invaluable tool for understanding the nature of many phenomena, from heavy ion collisions to astrophysical plasma. There is a mounting evidence that the Chapman-Enskog expansion has a vanishing radius of convergence but, on the other hand, the method of moments applied to weakly interacting gasses doesn't have such problems. The equations can be systematically improved and the convergence properties checked. If one introduces interactions in the form of a medium-dependent mass or a gauge field interacting with the plasma, the systematic expansion breaks down, since the sources of the successive moments become increasingly large, in fact diverging in the ultra-relativistic (mass-less) limit. We study therefore the simplest, non-trivial and exactly solvable system; namely, a plasma of mass-less particles interacting with an electromagnetic field and undergoing a boost invariant and transversely homogeneous expansion. We propose a method to re-sum the contribution of all the moments of the same rank. The resulting expansion is always well defined at any order, and we show the convergence properties compared to the exact solutions.
We review the recent results from the event-by-event NLO pQCD + saturation + viscous hydrodynamics (EbyE NLO EKRT) model [1,2,3], where we perform a simultaneous analysis of LHC and RHIC bulk observables to systematically constrain the temperature dependence of the QCD matter shear viscosity-to-entropy ratio eta/s(T), and to test the initial state computation. In particular, we study the centrality dependences of hadronic multiplicities, pT spectra, flow coefficients, probability distributions of relative elliptic flow fluctuations, and various flow-correlations in 2.76 and 5.02 TeV Pb+Pb collisions at the LHC and 200 GeV Au+Au collisions at RHIC [1,2]. Overall, our results match remarkably well with the LHC and RHIC measurements, and our predictions for the 5.02 TeV LHC run are in an excellent agreement with the latest data. We will also present our results for the various correlations, like symmetric cumulants and their pT dependence, that are or will be measured at RHIC and at the LHC. We also explore the applicability of viscous hydrodynamics by quantifying the magnitude of delta-f corrections in the studied flow observables, and by charting the space-time evolution of the Knudsen number for the studied eta/s(T) parametrizations [3].
[1] H. Niemi, K. J. Eskola and R. Paatelainen, Phys. Rev. C93 (2016) 2, 024907, arXiv:1505.02677 [hep-ph].
[2] H. Niemi, K. J. Eskola, R. Paatelainen and K. Tuominen, Phys. Rev. C93 (2016) 1, 014912, arXiv:1511.04296 [hep-ph].
[3] H. Niemi, K. J. Eskola and R. Paatelainen, work in progress
6 seminars, 20 min. each
We use the gauge/gravity duality to model the out-of-equilibrium first stage of
a heavy ion collision through the collision of gravitational shockwaves in
numerical relativity. This investigation of collisions of sheets of energy
density in a non-conformal theory with a gravity dual is the first non-conformal
holographic simulation of a heavy ion collision. I will discuss the new physics
that arises (as compared to the much simpler conformal case) such as a new
plasma relaxation channel, the equilibration of the conformal symmetry breaking
scalar condensate and the presence of a sizeable bulk viscosity. These
ingredients are crucial to make qualitative contact of the fast
hydrodynamization process of hot plasmas with real-world QCD deconfinement
matter above the critical point.
We present the derivation of second-order relativistic viscous hydrodynamics from an effective Boltzmann equation for a system consisting of quasiparticles of a single species. We consider temperature-dependent masses of the quasiparticles and devise a thermodynamically-consistent framework to formulate second-order evolution equations for shear and bulk viscous pressure corrections. The main advantage of this formulation is that one can consistently implement realistic equation of state of the medium within the framework of kinetic theory. Specializing to the case of one-dimensional purely-longitudinal boost-invariant expansion, we study the effect of this new formulation on viscous hydrodynamic evolution of strongly-interacting matter formed in relativistic heavy-ion collisions.
The analysis of forward-backward (FB) multiplicity and event-averaged transverse momentum correlations between particles produced in the forward $(\eta>0)$ and backward $(\eta<0)$ hemispheres of the nucleus-nucleus interaction brings new information, important in the context of our understanding the early dynamics of ultra-relativistic heavy ion collisions.
In this talk, we present new data on forward-backward correlations between charged particles produced in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ and $5.02$ TeV measured with the ALICE detector at the LHC. We focus on the dependence of (a) the FB multiplicity $correlation\;coefficient$ $b_{corr}$, (b) an $intensive\;observable$, namely the event-averaged transverse momentum $correlation\;coefficient$ $b_{corr}^{p_{T}}$, and (c) the $strongly-intensive\; quantity$ $\Sigma$ on (1) pseudorapidity gap (2) centrality and (3) width of the centrality bin.
The strongly-intensive observables and event-mean transverse momenta are robust against volume fluctuations and thus the centrality determination methods, which provides higher sensitivity of the FB correlation strength to the properties of the initial state and evolution of the medium created in AA collisions.
Our results from centrality selected Pb-Pb collisions are compared to pp collisions at the same collision energy, showing the role of the system size w.r.t. short-range and long-range correlations present in the process, and to Monte Carlo simulations using several event generators.
Recent measurements of correlations between two particles separated in pesudorapidity and azimuthal angles have shown striking similarities between results obtained in pp, and in p+A and A+A collision systems. In pp collision system, unlike in the p+A and A+A systems, the strength of the correlations quantified by the anisotropy parameter v_2 does not show any dependence on the charged-particle multiplicity. Recent theoretical models suggest that this can be due to lack of correlation between the charged-particle multiplicity and the impact parameter of the pp collision. An independent handle on the impact parameter can be obtained by requiring the presence of a hard-scattering process in the collision. This talk presents the first measurement of two-particle correlations in pp collisions with a Z boson identified via its dimuon decay channel. The analysis uses ATLAS data recorded under nominal pp luminosities, and a procedure to correct for contribution of the tracks coming from pileup vertices is used. The anisotropy parameter v_2 measured in Z-tagged events is compared to the v_2 measured in minimum-bias collisions.
Longitudinal dynamics has recently become a topic of great interest in the study of ultra-relativistic heavy ion collisions. Both the multiplicity and the azimuthal anisotropic particle distribution measured in narrow $\eta$ intervals can fluctuate from the backward to forward pseudorapidtity region event-by-event. The multiplicity fluctuation is measured in $pp$, $p$+Pb and Pb+Pb collisions. A genuine long-range correlation (LRC) is found, similar across three systems. Both PYTHIA 8 and EPOS-LHC models significantly underpredict the magnitude of LRC in $pp$ collisions. A comprehensive study of decorrelations of flow harmonics $v_n$ is done for Pb+Pb collisions at 2.76 and 5.02 TeV. The decorrelation of $v_n$ between $\eta_1$ and $\eta_2$ is found to follow a linear dependence on the pseudorapidity separation $|\eta_1-\eta_2|$ for $v_{2-5}$, and shows a small but a measurable variation with the collision energy. Furthermore, the higher order moments of flow decorrelations are measured. The first measurement of non-linear mode-mixing effects between lower and higher order flow harmonics as a function of pseudorapidity is also presented. These results will help to constrain initial conditions along longitudinal direction and also help to understand the longitudinal evolution of the fireball. In addition, differential measurement of v_n is presented in broad ranges of $p_T$ (0.5-40 GeV), pseudorapidity ($|\eta|$<2.5) and the collision centrality. The first measurement of $v_6$ and $v_7$, as well as flow harmonics in ultra-central collisions are discussed. A procedure of removing correlations arising from back-to-back jets is implemented in the two-particle correlation method to evaluate $v_n$ free of a jet-bias. The scaling relations between the $v_n$ harmonics are also discussed.
Measurement of the longitudinal decorrelation of the harmonic flow event plane can improve our understanding of the longitudinal fluctuation of the QGP at the initial state, and provide important constraints for (3+1)D hydrodynamical models. Recent calculations indicate a stronger longitudinal decorrelation at RHIC energies than at the LHC. In this presentation, we will report the decorrelation measurements in Au+Au collisions at 200 GeV from STAR collaboration. By using the Forward Meson Spectrometer (FMS, $2.5 < |\eta| <4.5 $) and Time Projection Chamber (TPC, $|\eta |<1.0 $) detectors, the flow decorrelation can be measured in a rapidity gap around 2, and the non-flow effect is significantly suppressed. The results will be presented for different harmonic orders and centralities. The comparison with the measurement from Pb+Pb at 2.76 TeV and calculations from different models will also be discussed.
6 seminars, 20 min. each
In this talk we present the consequences of considering correlated constituents inside the proton on the initial state properties of p+p interactions at LHC energies [1,2]. The proton is modeled as a system of gluonic hot spots whose transverse positions are subjected to short-range repulsive correlations. We rely on a Monte Carlo Glauber approach with event-by-event fluctuations in the transverse positions of the hot spots and their entropy deposition. In fact the inclusion of non-trivial spatial correlations leads to substantial differences in the results for initial state properties, the spatial eccentricities. Further we show the centrality dependence of the (anti)-correlation of $\varepsilon_2$ and $\varepsilon_3$ in terms of symmetric cumulants. This study is particularly relevant and timely as the recent measurement of the symmetric cumulants by the CMS Collaboration has shown, once again, a similar pattern across different interaction systems from high-multiplicity p+p to p+Pb and Pb+Pb adding evidence on the possibility of having collective effects in small systems.
[1] Physics Letters B 770, 149 (2017)
[2] arXiv: 1612.06274 [hep-ph] (to appear in PRC).
In late 2015, the ALICE collaboration recorded data from Pb-Pb collisions at the unprecedented energy of $\sqrt{s_{NN}}$ = 5.02 TeV as well as data from pp collisions
at the same energy, to be used as a reference for heavy-ions. In Pb-Pb collisions a strongly coupled Quark Gluon Plasma (QGP) is produced which gives rise to collective
phenomena whose signatures can be retrieved in final state hadronic observables. Recent observations in small systems, such as pp and p-Pb collisions, show remarkable
similarities among these systems, which are highly suggestive of the presence of collectivity. Current research therefore tries to identify whether a unified description
of the pp, p-A and A-A data can be established.
Hydrodynamic and recombination models are tested against the measured spectral shapes at low and intermediate transverse momenta. In particular, the Boltzmann-Gibbs blast-wave
is a three-parameter simplified hydrodynamical model whose parameters (kinetic freeze-out temperature, average transverse velocity, exponent of the velocity profile) can be studied
for various collision systems and energies.
In this talk, an overview of the new ALICE results which contribute to the understanding of collective phenomena will be presented. Pion, kaon, proton $p_T$-spectra
and the result of a blast-wave fit are presented for pp collisions at $\sqrt{s}$ = 7 TeV, Pb-Pb at $\sqrt{s_{NN}}$ = 2.76 TeV, p-Pb and Pb-Pb at $\sqrt{s_{NN}}$ = 5.02 TeV.
The dependence of the blast-wave model parameters on the event multiplicity is also discussed.
The measurement of the four-particle cumulant and anisotropic flow coefficient for the second and third flow harmonic, $c_{2}\{4\}$ and $c_{3}\{4\}$, are presented using $pp$ data at $\sqrt{s}=5.02$ and $\sqrt{s}=13$ TeV, and $p+$Pb data at $\sqrt{s_{\text{NN}}}=5.02$ TeV. These measurements aim to assess collective nature of multi-particle production. While collectivity is well established in $p$+Pb and Pb+Pb collisions, its evidence in $pp$ collisions is contested because of larger non-flow contributions. The values of $c_{n}\{4\}$ are calculated using the standard cumulant method and the recently proposed two and three subevent methods, which can further suppress the non-flow contributions in small systems. In these collision systems, the three subevent method gives a negative $c_{2}\{4\}$, and thus a well-defined $v_{2}\{4\}$. The magnitude of $c_{2}\{4\}$ is found to be nearly independent of $\langle N_{\text{ch}} \rangle$ in $pp$ and third harmonic $c_{3}\{4\}$ is consistent with 0 for all systems. The $v_{2}\{4\}$ is found to be smaller than $v_{2}\{2\}$ measured using the two-particle correlation method, as expected for a collective behavior. Following a recent model framework, the measured values of $v_{2}\{4\}$ and $v_{2}\{2\}$ are used to probe the number of sources in the initial state collision geometry.
Long range azimuthal correlations are usually regarded as a signature of collectivity, and their discovery in high multiplicity pp collisions has
raised many questions concerning the possible mechanisms behind their creation.
In this talk, we will present the latest results of di-hadron correlations
obtained from pp collisions at $\sqrt{s} = 7$ TeV. We will show per trigger
yields of the near-side with a large $\Delta \eta$ separation as a function of multiplicity, and discuss suitable ways to obtain such yields and their accuracy. We will also show two-particle azimuthal cumulants with a large $\Delta \eta$ separation from pp collisions at $\sqrt{s} = 13$ TeV. The two-particle cumulants evaluate the Fourier components of the long range yields, and their multiplicity dependence can help decipher whether they arise from collective or few particle correlations. We will show four-particle cumulants with varying $\Delta \eta$ separations at the same energy, and discuss how such separations impact the contributions from non-flow for multi-particle measurements. Comparisons to PYTHIA will be made, in order to estimate the non-flow contributions. Finally, we will also compare the same observables measured in p-Pb and Pb-Pb collisions, where the existence of collectivity is much better established.
Small system (p+p, p+A, d/He+A) collisions at RHIC and LHC
exhibit interesting azimuthal anisotropies, with explanations varying
from geometry coupled to final state interactions to glasma diagrams to
color recombination. The PHENIX experiment has extended many of these
observables, including multi-particle cumulants, to lower energies with
the d+Au beam energy scan at 19.6, 39.0, 62.4, and 200 GeV. We present
new results on v2 anisotropies as a function of transverse momentum and
pseudorapidity and for various centrality selections. We compare the
results with calculations within the framework of viscous hydrodynamics
and parton scattering models (with and without final state
interactions). We present new results on multi-particle cumulants from
the d+Au beam energy scan at 19.6, 39.0, 62.4, and 200 GeV. All energies
result in real valued v_2{4}, in contrast to p+Au collisions at 200 GeV
where the v_2{4} is imaginary.
New multiparticle observables, which are called "Symmetric 2-harmonic 4-particle cumulants"(SC), were recently introduced in [1]. These quantify the correlation between event-by-event fluctuation of two different flow harmonics. Because the correlation between different order flow harmonics responds differently to the initial conditions or(and) $\eta/s$, SC provide strong constraints for the QGP properties in heavy-ion collisions. The higher harmonic flow (n > 3) can be understood as superpositions of linear and nonlinear responses to lower order initial anisotropy coefficients. Nonlinear mode contributions can be quantified based on the newly proposed observables, so called nonlinear coefficients ($\chi_{m,n}$) [2]. The higher order to lower order harmonic correlations can be used to understand viscous corrections to the momentum distribution at freeze-out, which is probably the least understood part of hydrodynamic calculations [3, 4]. These recently developed flow observables, SC and nonlinear coefficients, show a great potential to investigate the various stages of the expanding medium in heavy-ion collisions. In this talk, SC results of higher order harmonics(up to $v_5$), their transverse-momentum dependence and the nonlinear coefficients of higher order flow harmonics will presented. The results will be compared to calculations from AMPT and hydrodynamic models.
[1] ALICE, Phys.Rev.Lett. 117 (2016) 182301
[2] L. Yan and J. Y. Ollitrault, Phys. Lett. B 744 , 82 (2015)
[3] D. Teaney and L. Yan, Phys. Rev. C 86, 044908 (2012)
[4] H. Niemi,K.J. Eskola, and R. Paatelainen, Phys. Rev. C 93, 024907 (2016)
3 talks
Having a thermalized system is the textbook requirement for the
applicability of hydrodynamics. However, there is mounting evidence
that hydrodynamics offers a good quantitative description even in
off-equilibrium situations. Recent applications of resurgence to
relativistic fluid dynamics in the context of conformal systems is
able to put the new, generalized theory of off-equilibrium fluid
dynamics on solid theoretical footing, without requiring near local
equilibrium or even isotropy. This potentially explains the
'unreasonable success' of hydrodynamics in describing experimental
data for p+p and p+A collisions.
In this talk I present our recent work on studying nucleus-nucleus collisions in perturbation theory. Classical field approximation has been broadly used to study the early stages of heavy ion collisions. Extrapolating the classical field dynamics to late times does not give the expected pressure isotropization. Including vacuum fluctuations into the classical field approach may lead to
isotropization, but the resulting formalism appears to be non-renormalizable. An alternative approach is to use the Boltzmann equation at later times when the classical field theory may break down. Whether and how such a transition from classical fields to the Boltzmann equation occurs has not been explicitly shown in the literature. We use a perturbative approach in the Schwinger-Keldysh formalism to study this issue by including a single-rescattering correction to the classical fields. Specifically, we calculate the two-point Green function and the energy-momentum tensor due to a single 2 -> 2 scattering process involving two classical fields in the scalar \phi^4 theory. We compare our exact diagrammatic results for these quantities to those in kinetic theory to conclude whether a transition from classical fields to the Boltzmann equation occurs or not.
We investigate the real time dynamics of the Chiral Magnetic Effect and anomalous transport in ultra-relativistic heavy ion collisions. To this end we perform real-time lattice simulations with dynamical (Wilson and Overlap) fermions simultaneously coupled to color and electro-magnetic fields and we show that for light quarks the transport of electric and axial charges is essentially dissipation-less and can be understood from a simple hydrodynamic picture, while for heavier quarks significant derivations arise.
Further we discuss how our studies can be extended to compute the Chiral Magnetic Effect during the pre-equilibrium stage of a heavy-ion collision in order to provide initial conditions for the subsequent kinetic descriptions and anomalous hydrodynamic evolution. We illustrate the need of a consistent formulation of chiral kinetic theory, which we show can be derived from a first-principle world-line approach to quantum field theory. Our work ties in with recent progress towards a unified understanding of the pre-equilibrium dynamics of anomalous and topological effects.
4 talks
We describe the dynamical model of antishadowing in nuclei based on our theory of the leading twist shadowing. We also demonstrate that the data on the centrality dependence of the forward (large x) jet production off nuclei at the LHC and RHIC are consistently described as a consequence of the x-dependent color fluctuations in nucleons.
Collective behavior is often described in a hydrodynamic picture of a thermalized quark gluon plasma, created in collisions of heavy ions. Recent results from LHC regarding collisions in small collision systems, have raised the question to what degree existing models for multi parton interactions and hadronization can describe collectivity, if they are corrected to the dense environment of the LHC.
In this talk I will present some of the attempts to include such corrections in the Lund Monte Carlo programs Pythia8, DIPSY and Ariadne. I will describe how the Rope Hadronization model correctly predicts the rise of strange/non-strange hadrons with multiplicity, as well as how interacting Lund strings can dynamically generate a pressure, giving rise to a ridge structure. Finally I will discuss how such models developed for pp collisions, can convincingly be extrapolated to collisions of nuclei.
Mapping the wave functions of heavy nuclei (and protons) over a very wide range of x and Q^2 is a major goal of heavy ion physics. Not only is the average wave function of interest but the spectrum of fluctuations is important as a boundary condition for nuclear-nuclear collisions. CMS has measured transverse energy production from 5.02 TeV p-Pb collisions over 13 units of rapidity as a function of several different centrality estimators. These data show very strong auto-correlations between dEt/deta and the eta range used to define centrality that are not well produced by current event generators. The collaboration has also studied ultra-peripheral proton-lead and lead-lead collisions. For ultra-peripheral pPb collisions CMS has measured the production of exclusive rho and upsilon mesons over a wide range of rapidity and transverse momenta. These data complement and extend earlier e-p measurements at HERA and LHC measurements on coherent vector meson photoproduction by ALICE and CMS. Finally the collaboration has recorded a significant sample of photonuclear di-jet events from ultra-peripheral PbPb collisions. These are sensitive to the gluon density in the nucleus at low x and moderate Q^2.
6 seminars, 20 min. each
Single inclusive particle production cross sections in high energy hadron collisions at forward rapidity are an important benchmark process for the CGC picture of small x QCD. Recent calculations of this process have not led to a stable perturbative expansion for this quantity at high transverse momenta. We consider the quark channel production cross section using the new rapidity factorization procedure proposed by Iancu et al. We show that for fixed coupling one does indeed obtain a physically meaningful cross section which is positive and reduces in a controlled way to previous leading order calculations. We then discuss why it is difficult to use a running coupling prescription consistent with existing fits of the initial condition for the BK evolution of the target.
One of the striking results from the LHC has been the observation of collective behavior in high-multiplicity proton-proton and proton-nucleus collisions. The characteristic features of multi-particle correlation measurements are similar to those previously observed in collisions of heavy nuclei, and have been interpreted as a result of the hydrodynamic evolution of the produced QCD matter.
In order to study the applicability of the hydrodynamical description in these smaller systems, it becomes crucial to have a good control of the fluctuations of the proton wave function which drives the initial state geometry in pA and pp collisions. This input we obtain by studying the HERA diffractive vector meson production data, that can be used to determine both the average shape and the amount of shape fluctuations in the proton.
We use the constrained fluctuating proton shape in hydrodynamical simulations of proton-nucleus collisions at LHC energies. We find that while pure color charge fluctuations do not give large enough initial state eccentricities to describe the large observed flow harmonics, the simulations using fluctuating proton shapes are compatible with the LHC data. In particular, we find a good description of the mean transverse momentum and the elliptic and triangular flow down to multiplicities ~2 times the mean multiplicity. The description of the HBT radii is also found to be reasonable.
We also study the effect of subnucleon scale fluctuations in ultraperipheral heavy ion collisions, which are basically just photon-nucleus collisions. We find that the inclusion of the proton and neutron shape fluctuations clearly changes the transverse momentum spectrum of the diffractively produced J/Psi mesons in incoherent events. This happens at a scale realted to the size scale of the sub-nucleoninc fluctuations and should be observable at the LHC. We present predictions for the LHC Run-2 measurements at 5 TeV, and show that the previously measured incoherent/coherent cross section ratio at 2.76 TeV prefers the inclusion of the nucleon shape fluctuations that significantly increase the incoherent cross section. Finally, we discuss how to obtain the energy (Bjorken-x) dependence of the proton shape by solving the JIMWLK equation with an initial condition compatible with the HERA DIS measurements.
References:
H. M, B. Schenke, Phys. Rev. D94, 2016, 034042, arXiv:1607.01711
H. M, B. Schenke, Phys. Rev. Lett. 117, 2016, arXiv:1603.04349
H. M., B. Schenke, C. Shen, P. Tribedy, arXiv: 1705.03177
H. M., B. Schenke, arXiv: 1703.09256
For ultrarelativistic proton-proton and proton-nucleus collisions, we perform an exploratory study of the contribution to the elliptic flow v2 coming from the orientation of the momentum of the produced particles with respect to the reaction plane. Via the CGC factorization valid at high energies, this contribution is related to the orientation of a color dipole with respect to its impact parameter, which in turn probes the transverse inhomogeneity in the target. We show that the angular dependence is controlled by soft gluon exchanges and hence is genuinely non-perturbative. The effects of multiple scattering turn out to be essential (in particular, they change the sign of v2). We show that sizable values for v2, comparable to those observed in the LHC data and having a similar dependence upon the transverse momenta of the produced particles, can be easily generated via peripheral collisions. In particular, v2 develops a peak at a transverse momentum which scales with the saturation momentum in the target.
This talk is based on: arXiv:1702.03943.
We consider correlations between produced quarks in p-A collisions in the framework of the Color Glass Condensate. We show that a quark-quark ridge that shows a dip at ∆η ∼ 2 relative to the gluon-gluon ridge. The origin of this dip is the short range (in rapidity) Pauli blocking experienced by quarks in the wave function of the incoming projectile. We observe that these correlations, present in the initial state, survive the scattering process. We also discuss the correlations between the produced quarks and antiquarks in p-A collisions.
In this talk, I will discuss recent progress in understanding
multiparticle correlations in small systems from an initial state model.
In this model of eikonal quarks scattering off of a dense nuclear target
with localized domains of color charge, we are able to reproduce many of
the qualitative features observed in light-heavy ion collisions at RHIC
and the LHC which are often ascribed to collective behavior. These
include the ordering of n-th moment two particle Fourier azimuthal
anisotropy coefficient, $v_n\{2\}$; a negative four particle second
Fourier cumulant $c_2\{4\}$, giving rise to a real $v_2\{4\}$; the
similarity for multiparticle second Fourier harmonics $v_2\{4\} \approx
v_2\{6\} \approx v_2\{8\}\$; and the energy dependence of the four
particle symmetric cumulants. I will also contrast this model with the
Glasma graph approximation, where some of these feature cannot be
reproduced, leading to the conclusion that multiple scattering is a key
ingredient for the observed multiparticle correlations from the initial
state.
I present the latest results from our work on simulating the early stages of heavy-ion collisions within the CGC picture in three dimensions. In our simulation we introduce an extra parameter for the longitudinal thickness of the nuclei and thereby explicitly break the boost invariance of the system. Consequently the full 3+1 dimensional classical Yang-Mills equations have to be solved numerically. The Glasma resulting from the collision of two such nuclei with longitudinal extent exhibits a Gaussian rapidity profile in its rest-frame energy density. We compare our results to experimental data from RHIC and find reasonable agreement.
6 seminars, 20 min. each
Beams of relativistic heavy ions are accompanied by a large flux of equivalent photons, and photon-induced reactions are the dominant interaction mechanism in heavy-ion collisions when the colliding nuclei have transverse separation larger that the nuclear diameter. In these ultra-peripheral collisions (UPC) the photon can provide a clean probe of the partonic structure of the nucleus analogous with deep inelastic scattering. This talk presents measurements of jet production and electromagnetic processes in UPC in association with Pb+Pb collisions performed with the ATLAS detector. Dijet events are selected using requirements on rapidity gaps and forward neutron production to identify the photo-nuclear processes. The relatively clean environment of these events allows for measurements in a region of x and Q^2 where significant nuclear PDF modifications are expected to be present and not strongly constrained by previous measurements. High-mass dilepton pair continuum rates for Pb+Pb collisions are also presented to test expectations for two-photon interactions. Finally, evidence for the elastic scattering of photons ("light-by-light" scattering) is reported, a previously unobserved process made possible by the high photon flux and low event pileup provided by the LHC. While of intrinsic interest as a heretofore-unobserved standard model process, it has also been proposed as a clean channel for searches for beyond the standard model (BSM) physics.
Photoproduction of dijets in ultra-peripheral heavy-ion collisions is expected to probe nuclear structure and provide additional constraints for nuclear PDFs. The first data for these processes in Pb+Pb collisions at the LHC will soon be published by ATLAS and due to the growing interest for ultra-peripheral collisions, more will likely follow in near future. In this talk I will introduce our recent implementation of photoproduction processes for Pythia 8 Monte-Carlo event generator. In particular I will discuss how the direct and resolved components are generated and quantify the relative contributions in different kinematical regions. I will also discuss how well the partonic content of resolved photons is known and how the photon flux from different beam particles is modeled. To validate our framework, I present comparisons to charged-hadron and dijet photoproduction data measured in e+p collisions at HERA by H1 and ZEUS experiments. Then I will quantify the sensitivity of photo-nuclear dijet production to nuclear PDFs and discuss about current theoretical uncertainties.
The electromagnetic fields of lead nuclei at the LHC are an intense source of quasi-real photons. The coherent photonuclear production of J/$\psi$ provides information about the intial state of nuclei at a perturbative scale related to the mass of the charm quark. Using data from Run 1 at the LHC, the ALICE Collaboration has published cross section measurements of coherent photoproduction of J/$\psi$ in peripheral (with nuclear overlap) and ultra-peripheral (without nuclear overlap) Pb-Pb collisions. Coherent photoproduction is characterized by very low $p_{\rm T}$ of the produced J/$\psi$ ($p_{\rm T} \sim 1/R = 30$ MeV/$c$), and in this region of phase space photoproduction dominates over hadroproduction in collisions down to at least 50% centrality. These results will be reviewed here. In addition, new results from Run 2 data for peripheral collisions at mid rapidity, as well as for ultra-peripheral collisions at forward rapidities will be presented.
The availability of the largest collision energy in pp collisions at the LHC allows a significant advance in the measurement of J/$\psi$ production as a function of event multiplicity. The interesting relative increase measured in pp collisions at the LHC at $\sqrt{s} = 7$ TeV and at RHIC at $\sqrt{s} = 200$ GeV is studied now at unprecedented multiplicities for pp collisions. The newest measurement performed at mid-rapidity in pp collisions at $\sqrt{s} = 13$ TeV, facilitated by triggering on high-multiplicity events, imposes strong constraints to theoretical models, such as those implementing multiple partonic interactions. Also, it allows the comparison with the J/$\psi$ production studied in p-Pb collisions at similar event multiplicities. We will show our newest measurements of J/$\psi$ yields as a function of transverse momentum and event multiplicity in pp collisions at $\sqrt{s} =$ 2.76, 5.02 and 13 TeV at mid- and forward-rapidity together with those obtained in p-Pb collisions at $\sqrt{s_{\rm NN}} =$ 5.02 and 8.16 TeV.The discussion will include comparisons with recent theoretical calculations.
We present measurements of the charmonium nuclear modification factor and the second-order azimuthal anisotropy Fourier coefficient $v_2$ of inclusive J/$\psi$ in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 and 8.16 TeV.
The J/$\psi$ nuclear modification factor, measured as a function of rapidity, transverse momentum and event activity in p-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV has shown a sizeable kinematic dependence within a fair agreement with theoretical models including nuclear shadowing and energy loss. On the contrary, additional mechanisms, related to final state interactions, are needed to describe the stronger suppression observed in the production of the loosely bound $\psi$(2S) state in Run-1 data. Preliminary Run-2 results on the charmonia production in p-Pb collisions will be presented and compared to Run-1 results and to theoretical calculations.
The measurement of the inclusice J/$\psi$ $v_2$ is done via a study of the angular correlations between forward and backward J/$\psi$ and mid-rapidity charged particles. We observe a strong indication of long-range correlations,
reminiscent to the double-ridge structures previously discovered in two-particle correlations at mid and forward rapidity. The corresponding $v_2$ in the transverse momentum range between 3 and 6 GeV/$c$ are found to be non-zero with various degrees of significance depending on the rapidity and the collision energy. The sizeable measured $v_2$ at high transverse momentum is comparable to the one already observed in Pb-Pb collisions, suggesting a common mechanism which will be discussed in view of possible implications for the theoretical models of J/$\psi$ production.
The quark-gluon plasma, which is produced at an early stage of ultrarelativistic heavy-ion collisions, is expected to be initially strongly populated with chromodynamic fields. We address the question how heavy quarks interact with such a turbulent plasma in comparison with an equilibrated one of the same energy density. For this purpose we derive a Fokker-Planck transport equation of heavy quarks embedded in a plasma of light quarks and gluons. We first discuss the equilibrium plasma and then the turbulent one applying the same approach, where the heavy quarks interact not with the plasma constituents but rather with the long wavelength classical fields. The momentum broadening and collisional energy loss of a test heavy quark are computed and analyzed.
6 seminars, 20 min. each
Hamiltonian perturbation theory in the light-cone form (LCPT) has become a standard tool in understanding hadronic scattering processes within a first principles QCD approach. The calculational inconvenience of LCPT compared to standard covariant perturbation theory is balanced by several features that make its physical interpretation more transparent. Due to the more complicated mathematical structure resulting from the breaking of explicit rotational symmetry, LCPT has never been the formulation of choice for higher-order loop calculations. More recently, however, calculations of small-x dilute-dense scattering processes in the Color Glass Condensate (CGC) picture have been approaching the NLO level, for example in the small-x evolution equation and inclusive deep inelastic scattering cross sections. In this talk I will discuss our latest work [1,2], in which we formulate the LCPT rules in helicity space, independently of the spinor representation. Using the helicity space approach, one-loop computations can be done by using the four-dimensional helicity (FDH) scheme. This presents a major technological improvement for LCPT loop calculations. Demonstrating the effectiveness of our approach, we compute the full NLO correction for DIS cross section in the dipole factorization formalism, and also compare our results to the conventional dimensional regularization (CDR) scheme [3].
[1] T. Lappi, R. Paatelainen Annals Phys. 379 (2017) 34-66
[2] T. Lappi, R. Paatelainen work in progress
[3] G. Beuf Phys. Rev. D 94 (2016) 054016
We present a new framework to match parton-level all-legs High Energy Factorization calculations in proton-proton collisions to the CASCADE parton showers.
We then discuss its application to the phenomenology of inclusive four-jet production at the LHC, including the case of 2 jets + 2 b-tagged jets, discussing the contributions from Multi Parton Interactions.
We compute the angular momentum and flow carried by the system of classical gluon fields of colliding color charges, in an attempt to study the initial angular momentum and vorticity in high energy nuclear collisions. We discuss how to match our results to viscous fluid dynamics in order to conserve angular momentum. We show results for the subsequent evolution of the fluid system. Partly based on arXiv:1705.10779.
In hydrodynamic approach to heavy ion collisions, hadrons with nonzero spin produced out of the fluid can acquire polarization via spin-vorticity thermodynamic coupling mechanism [1]. The hydrodynamical quantity steering the polarization is the thermal vorticity, that is minus the antisymmetric part of the gradient of four-temperature field $\beta^\mu = u^\mu/T$.
Based on this idea, it has been shown in the framework of cascade+viscous hydro model, UrQMD+vHLLE [2] that in Au-Au collisions at RHIC Beam Energy Scan (BES) the mean polarization of Lambda hyperons grows with decreasing collision energy up to 1.5% at $\sqrt{s}=7.7$ GeV RHIC Au-Au collisions. This goes in line with recent measurements of Lambda polarization by STAR experiment [3].
We show how the excitation function of the mean polarization is related to vorticity in the initial state and explore effects of initial state fluctuations on it. Finally, we propose a new polarization observable which can be probed in heavy ion collisions at LHC energies.
[1] F. Becattini, V. Chandra, L. Del Zanna, E. Grossi, Ann. Phys. 338 (2013) 32.
[2] I. Karpenko, F. Becattini, arXiv:1610.04717, to be published in EPJC.
[3] STAR collaboration, arXiv:1701.06657
We present a general framework for studying the angular anisotropy
of dileptons produced from polarized virtual photons in relativistic
heavy-ion collisions. The spin-anisotropy coefficients
characterizing the angular distribution of the dilepton final state
are introduced and their dependence on the medium evolution via flow
velocity and temperature profiles is obtained.
We illustrate these effects in dilepton production from
quark-antiquark annihilation in the QGP phase and $\pi^+\pi^-$
annihilation in the hadronic phase for a static
uniform medium and for a longitudinally expanding system.
It is shown that the anisotropy coefficients are in general non-zero
in a thermalized medium, and depend on the flow of the medium as
well as on the transverse momentum and invariant mass of the virtual
photon. The present framework can be easily implemented in a
realistic hydrodynamic simulation of relativistic heavy-ion
collisions in order to study the effect of non-trivial medium
properties on dilepton anisotropy.
Our presentation will be based on our recent paper [1].
We construct a new, simple model of the heavy ion collision, local in the impact parameter plane, and appropriate for the SPS energy range. This model can be regarded as a new realization of the ``fire-streak'' approach, originally applied to studies of lower energy nucleus-nucleus reactions.
Starting from local energy and momentum conservation, we nicely describe the
centrality dependence of the pion rapidity distribution in Pb+Pb collisions at $\sqrt{s_{NN}}=17.3$ GeV. In particular we also explain the broadening of
this distribution when going from central to peripheral collisions.
The results of our calculations are compared with SPS experimental data.
We discuss the resulting implications on the role of energy and momentum conservation for the early stages of the A+A collision, and for the dynamics of subsequent particle production.
A specific space-time picture emerges, where the longitudinal evolution of the system strongly depends on the position in the impact parameter (${b_x}$, ${b_y}$) plane. In non-central collisions we predict the existance of ``streams'' of excited matter moving very close to the spectator system in configuration $(x,y,z)$ space.
This picture is consistent with our earlier findings on the longitudinal evolution of the system as deduced from electromagnetic effects on charged pion directed flow [2], and can provide an explanation for specific low-$p_T$ phenomena seen in the fragmentation region of Pb+Pb collisions which we also address in this talk. We present our conclusions on the link between the initial stages of the A+A collision and the final state observables connected to strong and electromagnetic phenomena.
[1] A. Szczurek, M. Kiełbowicz and A. Rybicki,
Phys. Rev. C 95 (2017), 024908.
[2] A. Rybicki and A. Szczurek,
Phys. Rev. C 87 (2013), 054909.
6 seminars 20 min. each
The measurement of heavy flavour production is a powerful tool to study the properties of the high-density QCD medium created in heavy-ion collisions as heavy quarks are sensitive to the transport properties of the medium and may interact with the QCD matter differently from light quarks. In particular, the comparison between the nuclear modification factors (RAA) of light- and heavy-flavour particles provides insights into the expected flavour dependence of in-medium parton energy loss. Furthermore, azimuthal anisotropy coefficient (vn) of heavy-flavor particles provide insights into the degree of the thermalization of the bulk medium at low pT, and unique information about the path length dependence of heavy quark energy loss at high pT. In proton-nucleus collisions, precise measurements of the RpA of D and B mesons down to very low pT are considered unique tools to investigate the role of PDF modifications at very low values of x in an environment that is less affected by non-perturbative hadronization effects. Using the large statistics proton-proton, PbPb and pPb samples collected at 5.02 TeV during the 2015 and 2016 LHC run, high precision open charm and beauty measurements are performed with the CMS detector in a wide transverse momentum range. This allows us to set an important milestone in our understanding of the interactions between heavy quarkas and the medium. In this talk, the most recent results of RAA, v2 and v3 of D0 mesons, and the RAA of the B mesons in PbPb collisions at 5.02 TeV are presented and compared to the same results for charged hadrons at the same energy. The latest results for heavy-flavour production in proton-nucleus collisions at 5 and 8 TeV using the recent high-statistics data collected in the 2016 run will also be presented.
Charm and bottom quarks provide a powerful tool to study the properties of the hot, dense medium created in heavy ion collisions, and in particular may help differentiate between initial and final state effects in large collision systems. Measurements of open heavy flavour particle and quarkonia production, including their prompt and non-prompt components, and their correlations with light hadrons, build a path to understanding how heavy quarks propagate through the quark-gluon plasma. Additionally, an important component in these studies is the comparison between large and small collision systems. In this talk, ATLAS presents results on measurements of quarkonia production in PbPb collisions at 5.02 TeV, including separated prompt and non-prompt particle yields and a new measurement of the anisotropic flow of the J/Psi. The flow measurement provides information on the stage at which charmonium states are formed during the system evolution, thus giving insights on the effects that modify their production. Additionally, first measurements of open and bound heavy quark states are available in p+Pb collisions at 8.16 TeV. Results include yields measured differentially in particle momentum and rapidity and, where available, are compared to the baseline pp measurement at the same energy.
Open and hidden charm production in nucleus-nucleus collisions is considered as a key signature of Quark Gluon Plasma formation, but is also affected by cold nuclear matter effects, that can be studied by observing heavy flavour production in systems of different size and at different energy scale.
We present the latest LHCb production measurements of exclusive charmed and strange hadrons in p-p and p-Pb collisions at forward and backward rapidity. Cold nuclear matter effects are tested with accurate measurements of nuclear modification factors, forward-backward production asymmetry and ratios of open to hidden charm states and meson to baryon states. The results include the first measurement of charmed baryon production in pA collisions, and quarkonia production in the p-Pb collisions at 8 TeV centre-of-mass energies collected in November 2016.
Electroweak bosons produced in p+Pb and Pb+Pb collisions are excellent tools to probe a variety of initial state effects on hard processes involving nuclei. These effects include the nuclear modification of the partonic momentum structure and the energy loss of the incoming partons participating in a hard scattering. These probes are also important for serving as a standard candle against which to calibrate centrality in Pb+Pb collisions, thus putting measurements of jet suppression in these collisions into a proper context. In this talk, we present two new results from the ATLAS detector at the LHC: (1) Prompt photon production in the recently collected high-statistics 8.16 TeV p+Pb collision data. Photon yields are reported over a broad kinematic range, 25 < pT^gamma < 500 GeV and |eta^lab| < 2.37, and the production rates are compared to an extrapolated pp reference based on existing 8 TeV collision data. (2) High-statistics measurement of W boson production, new at this collision energy, both in 5.02 TeV Pb+Pb collisions.
The top quark pair production cross section is measured for the first time in heavy ion collisions, using proton-lead (pPb) data corresponding to an integrated luminosity of 180 $\text{nb}^{-1}$ at $\sqrt{s_{\text{NN}}}$ = 8.16 TeV. The measurement is performed by analyzing final states with exactly one isolated charged lepton (electron or muon) and at least four jets. The measured cross section is compared to expectation from the standard model using proton and nuclear parton distribution functions. The result is expected to be useful as an additional constraint to the gluon distribution inside the nucleus at large longitudinal parton momentum fractions. Furthermore, it paves the way to establish the measurement of top quark production in nucleus-nucleus collisions.
A typical approach to study the medium produced in heavy ion collisions is to understand the passage of elementary particles through it. As Photons and Z bosons do not participate in the strong interaction, their correlation with jets within the same event is a clean probe of the medium-induced energy loss of (predominantly) quark jets. In this talk, Z+jet and photon+jet correlations are studied using the high statistics PbPb and pp data taken at a center-of-mass energy of $\sqrt{s_{NN}}$ = 5.02 TeV with the CMS detector. The evolution of azimuthal angular distributions and average momentum imbalance between the jet and Z or photon as a function of the transverse momentum of the color neutral probe will be presented. In addition the jet $I_{AA}$, as a function of photon $p_{T}$ and collision centrality is studied. With the high statistics photon-jet sample collected from pp and PbPb collisions, the first measurement of photon-tagged jet shape and fragmentation function is reported in various of classes of event collision centrality.
3 talks
The hot and dense matter created in non-central heavy-ion collisions is supposed to have the initial angular momentum carried by two colliding nuclei. Some fraction of the angular momentum might be transferred to the global polarization of spin of particles composing the medium due to spin-orbit coupling. The STAR experiment has made the first observation of \Lambda hyperon polarization in Au+Au collisions at \sqrt{s_{NN}} = 7.7 — 39 GeV, showing a possible difference between \Lambda and \bar{\Lambda}. The difference might be the effect of a magnetic coupling to spin and thus could be a possible constraint on the magnitude and life time of the initial magnetic field. These results are quite important to further understand the role of the thermal vorticity in heavy-ion collisions. It is closely related to the chiral phenomena like the Chiral Magnetic Effect and Chiral Vortical Effect, and also likely related to other observables such as the directed flow and elliptic flow.
We present recent experimental results on the global polarization and discuss their implications. New results for 200 GeV and LHC energies will be also discussed. Furthermore we present results of the directed flow to discuss the initial tilt of the source and density fluctuations with a possible connection to the vorticity.
The hydrodynamical model has by now become a paradigm for the study of the QCD plasma formed in nuclear collisions at ultra-relativistic energies. The possibility of new collective phenomena in heavy-ion collisions, such as vorticity and turbulence, have gained widespread attention in recent times. It was proposed that the rotation caused in the medium formed in non-central collisions may have possible observable consequences due to spin polarization of the hot and dense matter. Consequently, much effort has recently been invested in studies of polarization and spin dynamics of particles produced in high-energy nuclear collisions, both from the experimental and theoretical point of view. I review the important advancements on this topic and present our recent work on the formulation of relativistic fluid dynamics for a system of spin-1/2 particles [1].
[1] W. Florkowski, B. Friman, A. Jaiswal and E. Speranza, arXiv:1705.00587 [nucl-th].
4 talks
In this talk, I am going to address three questions about collectivity in heavy ion collisions:
1) What do we mean by collectivity?
2) What distinguish between flow & non-flow?
3) How to interpret the sign of flow cumulants?
The standard picture of heavy ion collisions is that large systems (collisions of large nuclei like Au+Au and Pb+Pb) create a quark-gluon plasma that exhibits collective behavior indicative of nearly inviscid hydrodynamical evolution. Recently, data from small systems (collisions of a small projectile and a large target like d+Au and p+Pb) have been found to exhibit strikingly similar evidence for collective behavior. Asymmetric systems, by their nature, provide unique insight on the relation between geometry, transverse expansion, and longitudinal dynamics. In 2016, RHIC delivered a beam energy scan of d+Au collisions at 4 different energies: 200, 62.4, 39, and 19.6 GeV. In this talk we present a wide array of results from the Run16 d+Au BES and discuss the implications for collectivity and longitudinal dynamics, and we put these results in context with other measurements over a range of energies in p+p, p/d/3He+A, and A+A collisions.
In this talk, I will give an overview of recent experimental
measurements of mixed harmonic azimuthal correlations and its rapidity
dependence at RHIC. I will discuss how such measurements over a wide
range of energy can help us explore the longitudinal structure of the
initial state and constrain the temperature dependent transport
properties of the matter formed in relativistic heavy ion collisions.
I will discuss recent experimental and theoretical developments in the subject of two- and multi-particle rapidity correlations in proton-proton, proton-nucleus and nucleus-nucleus collisions. In particular, I will focus on (i) a mysterious baryon-baryon rapidity anti-correlation found at both RHIC and the LHC and (ii) a peculiar scaling in all colliding systems at the LHC of the long-range rapidity correlations originating from the fluctuating fireball shape.
3 talks
Studies of microscopic models using holography as well as
kinetic theory have lead to the conclusion that an effective description
in terms of hydrodynamics can give an accurate description of the
dynamics under conditions much further from equilibrium than would have
seemed reasonable. I will summarize these developments and their
implications for the applicability of hydrodynamics to small systems.
Azimuthal momentum anisotropy measurements are pervasive at RHIC and the LHC. However, there are widespread misconceptions as to the mechanistic origin of this anisotropy in both small and large systems.
In this talk, I will demonstrate how recent momentum anisotropy measurements for a broad range of systems, can be leveraged to gain new mechanistic insights and to constrain the properties of the medium produced in these collisions. In particular, the respective roles of final state effects versus initial state momentum domain effects in explanations of the measurements, will be addressed.
2 talks
Hadrons carrying heavy flavour, i.e. charm or beauty quarks, are unique probes in hadronic collisions at high energies. Due to their large masses, heavy quarks are predominantly produced in hard parton scattering processes in the initial phase of the collisions. Therefore, measurements of heavy-flavour hadron production in pp collisions provide tests of perturbative QCD and constitute a crucial baseline for the study of heavy-flavour production in heavy-ion collisions. In particular, the differential measurements of heavy-flavour particle multiplicity dependence or angular correlations with charged particles provide insight into their production mechanisms, fragmentation properties, as well as into the interplay between the hard and soft processes. At the LHC energies, multiple parton interactions may also influence heavy-flavour production. In addition, the charmed baryon-to-meson ratio is sensitive to hadronisation mechanisms.
In p-Pb collisions, heavy-flavour production is affected by cold nuclear matter effects, such as the modification of the parton density distribution of nucleons bound in nuclei with respect to those of free nucleons, multiple soft scatterings of partons in the initial state ($k_\{\\rm T\}$ broadening), initial geometry fluctuations or gluon radiation (energy loss). The modelisation of those effects usually considers a dependence on the collision geometry. Heavy-flavour measurements in p-Pb collisions help to characterise the initial state of heavy-ion collisions and are the mandatory reference for the interpretation of the corresponding studies in Pb-Pb collisions, where heavy quarks interact with the medium constituents providing information on the transport properties of the medium.
ALICE capabilities allow for full reconstruction of the hadronic decays of open-charmed D mesons ($D^\{0\}$, $D^\{+\}$, $D^\{*+\}$ and $D_\{s\}^\{+\}$) and baryons ($\\Lambda_\{c\}^\{+\}$) at central rapidity, studies of leptons from charm and beauty decays at central and forward rapidity, partial $\\Lambda_\{c\}^\{+\}$ and $\\Xi_\{c\}^\{0\}$ reconstruction of their electronic decay channel at central rapidity, as well as reconstruction of jets with D mesons at central rapidity. An overview of the heavy-flavour measurements in pp, p-Pb and Pb-Pb collisions with ALICE will be given. Special attention will be put on the production cross sections, baryon-to-meson ratio, nuclear modification factors, angular correlations between heavy flavours and charged particles, heavy flavour $v_2$, and the studies of the multiplicity dependence of their yields, which will be discussed in terms of collision centrality or as a function of charged-particle multiplicity. Results will be compared with model calculations when possible.
Proton-nucleus collisions have been studied to provide baseline measurements for hard processes in heavy ion collisions. Extensive measurements have been made of the jet production rates, the properties of the jets and the correlation between pairs of jets and other related observables. One motivation for these measurements is to quantify the modification of the parton distribution functions inside nuclei from those of the free proton. In addition, these measurements have also put constraints on modifications to jet properties in proton-nucleus collisions compared to proton-proton collisions and are potentially sensitive to novel effects such as gluon saturation at very low momentum fractions in the nucleus. This talk will provide an overview and interpretation of the latest results on jet measurements in small collision systems at the LHC.
2 talks
We present the first out-off-equilbirum analysis of relativistic
collision dynamics in the vicinity of a critical point. We numerically
solve shock wave collisions in a one parameter family of holographic
models with phase transitions of different orders. For a unique value of
the parameter, the model exhibits a second order phase transition which
connects a region of first order transition with an analytic cross-over.
We study the post-collision dynamics in the vicinity of that critical
point and analyse the out-of-equilibrium stress tensor in the aftermath
of those collisions. We observe that in the vicinity of the critical
point , independently of the nature of the transition, almost all the
energy of the projectiles ends up in a quasi-static, slowly evolving
blob of energy, as also observed in collisions with a strong first order
phase transition. We discuss the applicability of hydrodynamics for
those collisions and put our results into the context of searches for
the critical point in QCD.
This talk will give an overview of recent efforts to understand hydrodynamisation of the quark-gluon plasma away from the infinite coupling approximation. This is done using Einstein-Gauss-Bonnet theory, which reproduces several features of finite coupling physics, such as a larger \eta/s of 0.15. We find that collisions become more transparant, have a slightly wider rapidity profile and hydrodynamise more slowly. We hope to comment on the evolution of fluctuations in the transverse plane and compare this with results at weak coupling.
In this talk, I plan to discuss the exploration of the gluon saturation
phenomenon in pA collisions and at the future EIC. The proposed
cutting-edge EIC can lead us to answers to many fundamental questions
about the physical role and 3D image of gluons in nucleons and nuclei
with unprecedented precision, and also has the potential to discover a
form of ultra-dense gluonic matter. Complementary measurements in pA
collisions and at the EIC can help us measure various small-x gluon
distributions and study the onset of gluon saturation. I will also
briefly discuss the recent progress including the measurement of the
Wigner gluon distribution at low-x.
In this talk I will review the existing proposal of
energy-frontier deep-inelastic scattering at CERN, the Large
Hadron-electron Collider LHeC. I will present the proposal, consisting
in colliding 60 GeV electrons from an energy recovery linac with the LHC
beams or, in the further future, with the HE-LHC or FCC proton and
nuclear beams. Then I will discuss the physics case, focusing on the
possibilities for QCD: parton densities in proton and nuclei, imaging of
hadrons an nuclei through diffraction, investigations of the non-linear
saturation regime, and studies of nuclear modifications of parton
branching and hadronisation. I will emphasise those aspects of these
studies of relevance for the understanding of the initial stages of
high-energy heavy-ion collisions.
The 2015 Long Range Plan for Nuclear Science in the US recommends a
high-energy, high-luminosity polarized Electron-Ion Collider (EIC) as
the highest priority for new facility construction after the completion
of FRIC. The project is currently reviewed by the National Academy of
Science. The EIC will, for the first time, precisely image gluons in
nucleons and nuclei. It will reveal the origin of the nucleon spin and
will explore a new quantum chromodynamics (QCD) frontier of ultra-dense
gluon fields. This science will be made possible
by the EIC’s unique capabilities for collisions of polarized electrons
with polarized protons, polarized light ions, and heavy nuclei at high
luminosity.
In my talk I will give an overview of the physics motivation and program
of an EIC with a focus on the opportunities for small-x physics in e+A
collisions and how an EIC will affect our understanding of of heavy-ion
collisions at RHIC and the LHC.