We discuss progress in the field of MPI studies over the last ten years, focussing mainly on theoretical developments but also including experimental highlights.
The CMS collaboration presents two talks on small-system physics. This contribution will focus specifically on the dependence of particle correlations on energy and multiplicity
Measurements of anisotropic flow provide an important insight into the nature of collectively expanding hot and dense strongly coupled matter in heavy-ion collisions. Over the past years, striking similarities were revealed in numerous experimental results measured in high-multiplicity proton-proton and proton-lead interactions, where no emergence of such a medium was expected.
In this talk,...
Electron-positron pairs produced in ultra-relativistic heavy-ion collisions at the LHC carry important information about the system space-time evolution unperturbed by strong final-state interactions. The dielectron continuum is very rich in physics sources: on top of Dalitz and resonance decays of pseudo-scalar and vector mesons, thermal black-body radiation contains the information about the...
High multiplicity events of small colliding systems at high c.m.s. energies show similar collective features to heavy ion collisions with comparable multiplicities, such as long-range near-side correlations and v_n coefficients [1]. Whether this behaviour may be attributed to the presence of a deconfined state in small systems is an open question. Recent analyses of pp and p-Pb collisions also...
Abstract: We present studies shedding light on the correlated hadron production. This is an important source of information about the early stages of hadron formation and is not yet understood from first principles. In this talk, we will present the measurement of Bose-Einstein correlations using the ATLAS detector along with an analysis of the momentum difference between charged hadrons in...
We report on recent developments concerning the reunification of the two EPOS branches EPOS LHC and EPOS3, essentially concerning the hadronization of the plasma part.
The LHCEW group on Jets and EW bosons proposes to produce an LHC tune
to provide consistent parameters for the different parton shower and MPI
MC generators. The LHC tune will include data from all LHC collaborations, as well as lower energy data. The parameters to be tuned are those of the UE event and intrinsic kt, but are not connected with the hard process.
The goal is to provide a tune...
We present new developments in the Rivet framework for comparing theory to experiment, allowing for the first time for easy implementation of – and comparison against – heavy ion data. The framework has notably been extended with tools for centrality estimation and calculation of flow observables, as well as the implementation of several heavy ion analyses. In this talk we will demonstrate...
We summarize the results of the studies of double parton scattering (DPS) off nuclei in the parton
model [1] and pQCD [2]. A Monte Carlo implementation of the DPS processes is developed which
includes realistic NN correlation in nuclei [3], transverse geometry of hard and soft NN collisions
and distribution over the number of wounded nucleons. This implementation is an extension of the
Monte...
The idea behind the space-time colour reconection (CR) is to use the space-time distance between the partons to construct a colour reconnection model. We will describe how the space-time picture is incorporated into Herwig 7 and show the first results of a space-time CR model.
The LHCb experiment pioneered fixed target physics with LHC beams, thanks to the SMOG internal gas target. The forward geometry of the spectrometer are perfectly suitable for a fixed-target programme that, together with ion collisions, enables unique studies that shed light on cosmic ray physics as well as heavy ion physics. We present the first measurement of antiproton production in...
Heavy quarks are produced in hard scattering processes during the early stages of a heavy-ion collision at ultra-relativistic energies. Their annihilation rate is negligible, and they participate in the whole medium evolution losing their energy via radiative and collisional processes while traversing through the Quark-Gluon Plasma (QGP) formed in such collisions. This allows us to study the...
The experimental measurements of non-zero flow harmonic coefficients in p+A, and even in p+p, collisions at LHC energies have generated an ample debate on the heavy ion community in the last lustrum. Considered a golden probe of QGP formation in A+A collisions the interpretation of vn’s distinct from zero in small collision systems remains unclear. This is so because a reasonable description...
Since the surprising discovery of collective behavior in pp collisions at LHC, hadronization mechanisms has experienced a renewed interest. The prospect that collective behavior in pp and AA collisions might have a common origin, have also prompted efforts to extend MPI models for pp to heavy ion collisions.
In this talk I will outline the efforts of the microscopic model of collectivity...
The heavy-ion physics program at the LHC aims at characterizing the high energy density, high temperature, deconfined partonic state of matter called Quark-Gluon Plasma (QGP).
Azimuthal anisotropies of particles produced in heavy-ion collisions are used to study initial state geometry fluctuations and collective effects that arise from the long-range interactions with non-zero mean free path,...
This talk summarize the results on two-particle angular correlations for charged particles emitted in $e^+e^−$ collisions data collected at 91 GeV with the ALEPH detector at LEP. With the archived data, the correlation functions are studied over a broad range of pseudorapidity $\eta$ and azimuthal angle $\phi$ with respect to the electron-positron beam axis and the event thrust axis....
We propose and carry a detailed study of an observable sensitive to different mechanisms of minijet production. The class of observables measures how the transverse momenta of hadrons produced in association with various trigger objects are balanced as a function of rapidity. It is shown that the observables are sensitive to the model parameters relevant for the minijet production mechanisms:...
Quarkonium (a bound state of $Q\bar{Q}$ pair) production as a function of charged-particle multiplicity has been measured in pp collisions at the LHC. They exhibit a non-trivial correlation that can lead to a better understanding of the multiple-parton interaction mechanism in the initial state of the collision as well as possible collective effects in small systems. The ALICE detector at the...
One of the key results of the LHC Run 1 was the observation of an enhanced production
of strange particles in high multiplicity pp and p-Pb collisions at 7 and 5.02 TeV, respectively.
In addition, the multiplicity dependent results on particle production in pp collisions allowed the discovery of collective-like behavior in small systems at the LHC.
In order to provide further insights into the...
The correlation of charmonium (and also of heavy quarks in general) production with the charged particle multiplicity is of high interest, as it could give new insight into the interplay between hard and soft mechanisms in particle production, both at parton level and at hadronization.
Experimental data shows an intriguing increase of the self-normalized J/ψ yield with multiplicity.
In this...
We discuss the computation of the 1->2 splitting functions and matching coefficients at two loops, which are required for the computation of DPS cross sections at NLO.
The contributions to the Double Parton Scattering cross section, due to pairs of interacting partons generated by perturbative splitting is enhanced at short relative transverse distances, between the centers of mass of the two partonic collisions. One my thus foresee the interesting possibility of discriminating experimentally between contributions to the double parton scattering cross...
Double parton scatterings (DPS) provide vital information on the parton-parton correlations and parton distributions in a hadron. DPS also constitutes as a background to new physics searches. The measurement of DPS in Vector Boson (V) + jets processes is important because of clean experimental signature and large production cross-section. The available DPS measurements, with V + jets, are...
We will present an estimate of the effective cross-section in double parton scattering based on the eikonal minijet model with soft gluon resummation, which was recently applied to the calculation of the total and inelastic non-diffractive proton proton cross-section at LHC and to the estimate of survival probabilities at LHC. Comparison with results from an empirical model modified to take...
Though already thirty years have passed since the double parton scattering (DPS) effects have been observed for the first time, a strong interest to this topic still persists. In spite of large theoretical and experimental progress, a lot of space is left for improvement of our understanding of the DPS mechanisms. Certain advantages are provided by studies performed in various final states, in...
Prospects and results from the AFP detector in ATLAS
In my talk I will highlight a number of recent developments in particle astrophysics that interface with high-energy physics. A particular focus lies on the importance of hadronic interaction models for the interpretation of Ultra-High Energy Cosmic Ray observations.
The description of collisions involving hadrons has always been a challenging task, especially at nonperturbative scales and the forward phase space. Being at the end of LHC Run 2 an overview of recent measurements and developments in soft QCD and small-x physics at the LHC is presented, together with remaining open questions and an outlook for future studies that are still missing to improve...
I will discuss about the impact of open heavy-flavour data on the gluon and sea quark PDFs in the low x/low Q2 region and the still open problems in the fitting procedure, with examples/considerations from the experience with the PROSA and the ABMP PDF fits.
Jet veto cross section measurements in ATLAS
Multiple Partonic Interactions (MPI) refer to those cases where more than one semi-hard partonic scattering occurs within the same pp collision. Their relevance increases at high collision energies. Therefore, it is crucial to deepen the understanding of MPI contributions to particle production in hadronic collisions at the LHC.
ALICE has performed several measurements in small collision...
A two-component (soft + hard) model (TCM) of hadron production in high-energy nuclear collisions has been applied to $p_t$ spectra, particle densities on $\eta$, 2D angular correlations and total yields from p-p, p-A and A-A collisions and provides a self-consistent description of charge-multiplicity and collision-energy dependence for a broad array of data. The TCM provides alternative...
Particles carrying heavy flavor are important probes to study the properties of the Quark-Gluon Plasma (QGP) created in relativistic heavy-ion collisions at RHIC. They are produced in hard scattering during the earliest stages of nuclear collisions and, because of their high mass, theoretical calculations are more tractable than those involving light quarks. The RHIC experiments have collected...
PHENIX has measured low pT direct photon yields and elliptic flow in 200 GeV Au+Au collisions with three different methods and the results are consistent. The yield measurements have been extended to lower collision energies and different colliding systems. It has been found that the integrated yields scale with the respective charged particle density, independent of system size, collision...
The talk will present the most recent results obtained by the CMS collaboration on minimum bias processes and the so-called underlying event.
Abstract: Gluon splitting to b-quark pairs is a unique probe into the properties of gluon fragmentation, as identified b-tagged jets provide a proxy for the quark daughters of the initial gluon. If available, we present a measurement of key differential distributions related to g→b bbar using data collected with the ATLAS detector at √s=13 TeV. Track jets are used to probe angular scales below...
In this work we study the underlying event (UE) activity as a function of the highest jet transverse momentum ($p_{\rm T}^{\rm jet}$) in terms of the number and summed $p_{\rm T}$ densities of charged particles in the azimuthal region transverse to the $p_{\rm T}^{\rm jet}$ direction. The UE activity considering charged particles within different $p_{\rm T}$ intervals in $pp$ collisions at...
Abstract: Calculations of jet substructure observables which are accurate beyond leading-logarithmic accuracy have recently become available. Such observables are significant not only for probing a new regime of QCD at a hadron collider, but also for improving the understanding of jet substructure properties that are used in many studies at the Large Hadron Collider. In this talk, we discuss...
Practical solutions to the evolution equations for double-parton distributions (DPDs) that are accurate but feasible in terms of computing resources are a challenging task in the study of double-parton scattering phenomenology in perturbative QCD. Due to the higher dimensionality of DPDs with respect to single PDFs, a simple extension of the available methods for PDF evolution is...
We consider soft gluon evolution of a system of clusters forming the initial state of the cluster hadronziation model, in order to constrain colour reconnection models from a perturbative point of view.
We show that this ansatz produces clusters with properties attributed to
a colour pre-confined state and find strong evidence for formerly investigated colour reconnection models based on...
Double parton distributions (DPDs) are not only an essential building block
needed in factorization theorems to calculate double parton scattering (DPS)
cross sections in perturbation theory, but they also contain very detailed
information about hadronic structure. However, as presently an extraction of
DPDs from experimental data or lattice calculations is not yet feasible, it is
common...
Double parton distributions (DPDs) are an important piece in the description of double hard interactions. On the lattice we calculate correlation functions of two local quark currents, which can be related to Mellin moments of DPDs. For the first moment we calculate all contributing Wick contractions for the pion, considering several channels corresponding to the quark polarization....
The thirty-year long history of the studies of the double parton scattering (DPS) lead to a large number of measurements in various channels and wide range of the collisions energies. At the LHC era the role of the DPS in the production processes becomes more and more significant as its contribution is expected to increase at higher energies. Estimates of this contribution provide an important...
The talk will present prospects and results from the precise proton-spectrometer (PPS), a new detector born from the collaboration between the CMS and TOTEM experiments at the LHC
The TOTEM experiment, located at the interaction point 5 of the LHC, has measured the total, elastic and inelastic proton-proton cross sections in a centre-of-mass energy range from 2.76 to 13 TeV, mostly in dedicated fills with special beam optics. Most recently, TOTEM has performed a series of detailed measurements at √s = 13 TeV. The total, elastic and inelastic proton-proton cross-sections...
The talk will present an overview of the results on diffraction and exclusive production obtained by the CMS experiment at the LHC
The unintegrated gluon distribution (UGD) encodes the gluon content in the proton in the high-energy domain and gives the probability that a gluon can be emitted by a colliding proton, with given longitudinal momentum fraction and transverse momentum. The UGD, being a nonperturbative quantity, it is not well known and several models for it have been introduced so far.
The diffractive...
With installation of new shower counters (HERSCHEL) along the beampipe the sensitivity of the LHCb experiment to the central exclusive production (CEP) has significantly increased. And the dedicated triggers introduced in the early period of Run 2 allowed to extended dataset for the CEP measurements.
The talk will cover the CEP results of LHCb experiment during Run1 and Run2
During last thirty years presence of the double parton scattering (DPS) in the hadronic collisions has been demonstrated by a large number of experiments. Though the underlying mechanisms are still poorly understood. Studies of the events involving heavy flavour production allow to reach very small transverse momentum regions and to test the factorization approach to the DPS description with...
We discuss double parton distribution functions (dPDFs), the main non perturbative ingredients appearing in the double parton scattering cross section (DPS) formula in hadronic collisions. By using recent calculations of dPDFs within Light-Front constituent quark models [1], we investigate the role of correlations induced by relativistic effects on dPDF evaluations [2]. Such distributions...
In spite of the recent progress in both theoretical and experimental studies many aspects of multiple parton interactions (MPI) still require a detail investigation. In particular, double parton scattering (DPS) processes can play a dominant role for some specific kinematic regions of multi-jet production, especially in proton-nucleus (pA) collisions where the total DPS cross section...
We will discuss the measurements of exclusive di-photon events at the LHC in pp and Pb Pb runs. these events lead to the best possible sensitivities to quartic anomalous couplings between photons, Z and W bosons. We will also give the sensitivities to the search for axion like particles at high mass.
A future electron-ion collider (EIC) will be a dedicated high-luminosity machine with the capability of unraveling many issues in QCD, including a systematic tomographic mapping of the nucleon's internal structure, investigations of the quark-hadron transition, the onset and dynamics of gluon saturation at low x, and the nature of the nuclear medium. In addition to the wealth of information...
One of the largest uncertainty in many top quark measurements is the one on the modelling of colour reconnection (CR) between top quark decay products. There have been several experimental measurements, notably of the pull angle in $t\overline{t}$ events, which were intended to provide useful input to CR modelling, but we have not seen strong sensitivity. In this presentation, we propose a...
A new set of CMS underlying-event tunes is presented for the PYTHIA 8 event generator. The tunes use the NNPDF3.1 parton distribution functions at leading (LO), next-to-leading (NLO), or next-to-next-to-leading (NNLO) order in perturbative quantum chromodynamics, and the strong coupling evolution at LO or NLO. Comparisons of the predictions of the new tunes are provided for observables...
Multiple experimental measurements have shown strong indications of flow in small collision systems. In this talk, I will examine the selected theoretical models with latest as well as well hidden experimental results in small collisions systems. I will discuss which experimental ''evidence" is solid and which might be misinterpreted. In addition, I will show the potential of deep learning in...
One of the key signatures of collectivity in heavy-ion collisions is the appearance of a ridge structure over wide pseudorapidity interval. Recently it was also found in small collision systems such as proton-proton or proton-ion collisions which origin is still on debate. In this work, contributions from the geometry fluctuations in the initial-state in pp collisions to the ridge structure...
We develop a MPI based theory of ridge phenomena in `MPI, the origin of collectivity comes out to be quantum interference of different MPIs. We show that
in this approach we obtain a picture of ridge phenomena in pp collisions compatible both qualitatively and qualitatively with experimental data.
the talk is based on:
Collectivity from interference
Boris Blok (Technion), Christian D. Jäkel...
Recently there has been much interest in the pair production of
quarkonia. The production of quarkonium pairs is expected to receive
an important contribution from double parton scattering (DPS) processes.
There remain a number of open problems, especially with the CMS and ATLAS data.
The effective cross sections $\sigma_{\rm eff}$ found
from empirical analyses are about a factor $2.5$ smaller...
The origin of long-range rapidity correlations observed in p-p and p-Pb collisions at LHC is one of the outstanding questions of strong interactions. The leading theoretical explanations of the so-called "ridge" effect are hydrodynamics and the initial state dynamics, with the latter often described in the Color Glass Condensate (CGC) framework. CGC-based calculations have successfully...
The talk will report on the most recent results obtained by the CMS experiment on multiple partonic interactions in heavy-ion collisions
In this talk I will summarize results of our studies of hard MPI effects in heavy flavour sector in pp-collisions at the LHC. During last two years we extend our previous studies of double-parton scattering (DPS) in double charm production to simultaneous production of charm and bottom as well as to double bottom production. We have also analyzed in this context associated production of charm...
The talk will present an overview of the most recent results obtained by the TOTEM and CMS experiments on the so-called forward physics
