The 3rd International Conference on the Initial Stages in High-Energy Nuclear Collisions (InitialStages2016), to be held at Instituto Superior Técnico, Alameda Campus, Lisbon, Portugal, on May 23rd-27th 2016. The meeting follows up on the 2014 (Napa) and 2013 (A Toxa) editions.
ALICE is the LHC experiment devoted to the study of heavy-ion collisions. The main purpose of ALICE is to investigate the properties of the deconfined state of nuclear matter, the Quark-Gluon Plasma (QGP). Quarkonium measurements play a crucial role in this investigation. Indeed, heavy quarks are created during the initial stages of the collision, before the QGP formation, and their number is conserved throughout the partonic and hadronic phases of the collision. The sequential suppression of the quarkonium states by colour screening has long been suggested as a signature and thermometer of the QGP. However, the first results on quarkonium suppression in Pb-Pb collisions at the LHC seem to indicate that for charmonia both regeneration and colour screening mechanisms play a role, while for bottomonia the regeneration mechanism should be small. Additionally, ALICE has recently reported an excess of J/$\psi$ at very low transverse momentum (below 300 MeV/$c$), presumably of electromagnetic origin that could become a valuable probe of the system.
We will present the latest results on quarkonium production in Pb-Pb collisions at LHC energies measured by ALICE. Quarkonia are measured at mid-rapidity in the dielectron decay channel and at forward rapidity in the dimuon one. Comparisons of measurements at different collision energies and to available theoretical calculations will be discussed.
This talk summarizes the latest results on jets in pp, pPb and PbPb collisions from the CMS collaboration
Rapidity correlations are analyzed in a simple model, where in the initial stage multiple sources, extended in space-time rapidity, are formed. We show how the varying longitudinal extent of the sources generates correlations in the initial entropy deposition, which later contribute to the observed correlations in hadron production. Our analytic analysis allows to understand their structure and to identify the component due to the underlying fluctuation of the number of sources and the component from the genuine longitudinal fluctuations. Our results reproduce semiquantitatively the basic features of the recent measurements at the LHC.
We present results of a 3+1 dimensional hydrodynamic model for flow correlations in pseudorapidity. The method can be used to unfold the correlations in the initial fireball at different space-time rapidities.
We discuss possible mechanism of generating initial correlations in rapidity, both with nucleon and parton degrees of freedom. Particular sensitivity to the initial correlations is found in collisions of small asymmetric systems, such as p-Pb collisions.
Bose-Einstein correlations between identified charged pions are measured for $p$+Pb
collisions at $\sqrt{s_{\mathrm{NN}}}=5.02~\mathrm{TeV}$ with the ATLAS detector with
a total integrated luminosity of $28~\inb$. Pions are identified using ionization
energy loss measured in the pixel detector. Two-particle correlation functions and
the extracted source radii are presented as a function of average transverse pair
momentum ($k_{\mathrm{T}}$) and rapidity ($y^{*}_{k}$) as well as collision centrality.
Pairs are selected with a rapidity $-2 < y^{*}_{k} < 1$ and with an average transverse
momentum $0.1 < k_{\mathrm{T}} < 0.8 \GeV$. The effect on the two-particle correlation
function from jet fragmentation is studied, and a new method for constraining its
contributions to the measured correlations is described. The measured source sizes
are substantially larger in more central collisions and are observed to decrease with
increasing pair $k_{\mathrm{T}}$. Radii are also evaluated in intervals of $y^{*}_{k}$
and a correlation with the local multiplicity $dN/dy^{*}$ is demonstrated.
The scaling of the extracted radii with the mean number of participants is also used
to compare a selection of initial-geometry models.
Recent measurements in high-multiplicity proton-proton (pp) and proton-lead (p-Pb)
collisions show several features that are similar to those observed in heavy-ion collisions. In this respect strangeness production may provide a valuable investigative
tool.
Baryon-to-meson ratios, such as Λ/KS0, have been measured
differentially in pT and show an evolution with increasing charged particle
multiplicity in small systems similar to the one observed with centrality in heavy-ion
collisions, where this behaviour is interpreted to be strongly related to the
hydrodynamical evolution of the system. Furthermore the production rate of strange and
multi-strange hadrons relative to pions exhibits a significant increase with multiplicity
in pp collisions, similarly to that observed in p-Pb. This increase is observed to be
more pronounced for hadrons with a larger strangeness content.
In this talk strange (KS0,Λ,Λ) and multi-strange (Ξ,Ω) hadron
production measurements at mid-rapidity, in pp collisions at √s = 7 TeV, will be shown
as a function of charged-particle multiplicity. Perspectives for similar studies at √s = 13 TeV will also be discussed.
Previous measurements have demonstrated the collective nature of multiparticle correlations in high-multiplicity pPb collisions at the LHC. This collectivity, while consistent with a hydrodynamic flow origin, can also be interpreted in terms of initial state effects arising from gluon saturation. To investigate the detailed properties of this collectivity, differential Fourier coefficients in transverse momentum and pseudorapidity are presented based on a variety of analysis methods using data obtained with the CMS detector at the LHC. The multiparticle nature of the correlations is explored using 4-, 6- and 8-particle cumulant analyses as well as a Lee-Yang Zeros analysis that accounts for correlations among all particles. An event-plane analysis using the scalar product method is performed where the influence of recently demonstrated event-plane decorrelation on the pseudorapidity dependence is considered. The pPb collision results are compared to peripheral PbPb collision results obtained with comparable mid-rapidity charged particle densities. While a significant pseudorapidity dependence is observed in the azimuthal dependence of the particle yields, a decorrelation of the event-plane angle with pseudorapidity can account for much of the observed behavior.
We present our work on the simulation of the quark-gluon-plasma using the colored particle-in-cell (CPIC) method. CPIC is a generalization of particle-in-cell simulations - commonly used in plasma physics - to real-time non-abelian lattice gauge theories. Using this method we are able to model the early time dynamics of heavy-ion collisions in the laboratory frame in 3+1 dimensions. In particular we study the effects of finite pancake thickness in the McLerran-Venugopalan model. Our approach requires a consistent treatment of gauge fields and color currents, which is achieved using charge-conserving CPIC methods. We show that the description in the laboratory frame agrees with boost-invariant approaches as a limiting case and investigate collisions beyond boost-invariance.
Demonstrated that the investigations of the forward-backward correlations between intensive observables enable to obtain more clear signal about the initial stage of hadronic interaction, e.g. about the process of string fusion, compared to usual forward-backward multiplicity correlations. As an example the correlation between mean-event transverse momenta of charged particles in separated rapidity intervals is considered. The calculations are fulfilled in the model with string fusion by introducing a lattice in transverse plane. The dependence of the correlation strength on the collision centrality is obtained for different initial energies. It is shown that above RHIC energy the dependence reveals the decline of the correlation coefficient for most central collisions, reflecting the attenuation of color field fluctuations due to the string fusion at large string density. It is also found that contrary to the correlation between transverse momenta of single particles the strength of the correlation between mean-event transverse momenta of particles in two separated rapidity intervals is not decreasing with the total number of produced strings, remaining significant even in the case of Pb-Pb collisions, in which the total number of strings can reach several thousand. All this makes this type of correlation promising for the observation of the signatures of string fusion at the initial stage of hadronic interaction in relativistic heavy ion collisions at LНС energy.
The work was supported by the RFBR grant 15-02-02097 and the Saint-Petersburg State University grant 11.38.197.2014.
Initial state models translate the spacetime overlap of nuclear density between projectile and target nuclei to generate profiles of entropy (or energy) at the QGP thermalization time which are subsequently evolved by relativistic viscous fluid dynamics. Historically, these initial conditions were generated by one of two means: either from a two-component Glauber ansatz which asserts an admixture of soft and hard collision processes or from specific calculations in Color-Glass Condensate (CGC) effective field theory.
In contrast to the phenomenological approach of the two-component Glauber model where the binary collision fraction is tuned to fit experiment, CGC initial conditions represent predictions calculated from an underlying effective field theory. While modern CGC initial condition models have been highly effective in describing the wealth of bulk observables at RHIC and the LHC, multiple models exist in the literature which predict different results and describe the data with varying degrees of success.
In this talk, we use a parametric initial condition ansatz which mimics the behavior of CGC calculations and extends the coverage of existing two-component parametric mappings to span the full space of reasonable initial condition models. We show that the parameterized initial conditions are highly constrained by bulk observables utilizing Bayesian statistics and a multi-parameter model to data comparison that relies on a state-of-the-art hybrid model for calculating the time-evolution of the QGP phase and its subsequent decay into the hadronic final state. We compare these results to specific calculations in CGC effective field theory and discuss implications for the initial stages of QGP formation.