AA Collisions Research Articles

Charge Fluctuations in <i>pp</i> and <i>AA</i> Collisions at RHIC and LHC Energies

Various measures of event-by-event net charge and charge ratio fluctuations in pp and AA collisions at RHIC and LHC energies are studied using the different Monte Carlo generators: URQMD, HIJING and HIJING/ and the results are compared with the predictions for the independent emission, hadron gas and QGP phase. Values of the D-measures are observed to exhibit significant energy dependence for both pp and AA data. Furthermore, there is essentially no significant difference in the values of the D-measures predicted by the various Monte Carlo codes used in the present study. A slight centrality dependence of the D-measures in terms of net charge fluctuations is observed in the case of Au-Au data at 200A GeV/c. These findings, thus, suggest that a difference in the D-measures for pp and AA collisions either the re-scattering effect plays a predominant role or there might be some new physics present in these collisions.

Harmonic Moments of the Gluon Density Distribution in AA Collisions

By using Monte-Carlo implementations of $k_T$-factorization formula with running-coupling BK unintegrated gluon distributions for nucleus-nucleus collisions, we compute higher order harmonic moments of the initial density distribution for both RHIC(Au+Au@200GeV) and LHC(Pb+Pb@2.76TeV) collisions. We study their sensitivity to the size of the valence parton distribution in the nucleon.

Boosted Saturation Bound in Colliding Nuclei

Interaction with a nucleus in pA collisions enhances the higher Fock components in the projectile proton. Effectively, this corresponds to an increase of the hard scale of the process by the saturation momentum Q^2 -> Q^2+Q_{sA}^2, which leads to an increased gluon distribution function at small x (but suppressed at x -> 1) compared to that in pp collisions. In the case of AA collisions the gluon distributions of bound nucleons in both nuclei turn out to be enhanced, i.e. to be boosted to higher saturation scales compared to pA collisions. A set of bootstrap equations relating the saturation scales in the colliding nuclei is derived and solved. The boosting effect has a moderate magnitude at the energies of RHIC, but becomes significant at LHC.

Matter–Antimatter Asymmetry in Heavy-Ion Collisions

Implementing modified phase space and distribution function in grand-canonical ensemble and taking into account the experimental acceptance, the ratios of antiparticle-to-particle over the whole range of energies are well reproduced by hadron resonance gas (HRG) model. We introduce a systematic study for antiparticle-to-particle ratios measured in various pp and AA collisions and find that the ratios of bosons and baryons get very close to unity indicating that the matter–antimatter asymmetry nearly vanishes at LHC energy. We predict that the LHC heavy-ion program will produce the same particle ratios as the pp program implying the dynamics and evolution of the system would not depend on the initial conditions.

Heavy-flavor dynamics in nucleus–nucleus collisions: from the RHIC to the LHC

The stochastic dynamics of c and b quarks in the fireball created in nucleus–nucleus collisions at the RHIC and LHC is studied employing a relativistic Langevin equation, based on a picture of multiple uncorrelated random collisions with the medium. Heavy-quark transport coefficients are evaluated within a pQCD approach, with a proper HTL resummation of medium effects for soft scatterings. The Langevin equation is embedded in a multi-step setup developed to study heavy-flavor observables in pp and AA collisions, starting from a NLO pQCD calculation of initial heavy-quark yields, complemented in the nuclear case by shadowing corrections, kT-broadening and nuclear geometry effects. Then, only for AA collisions, the Langevin equation is solved numerically in a background medium described by relativistic hydrodynamics. Finally, the propagated heavy quarks are made hadronize and decay into electrons. Results for the nuclear modification factor RAA of heavy-flavor hadrons and electrons from their semi-leptonic decays are provided, both for RHIC and LHC beam energies.

The role of in-medium color flow for high-pT particle production in AA collisions

The suppression of high-pT single-hadron spectra in heavy-ion collisions is usually interpreted as due to parton energy loss of high-momentum quarks and gluons propagating in the plasma. Here, we discuss to what extent this partonic picture must be complemented by a picture of medium-modified hadronization. In particular, we show how color exchange with the medium modifies the properties of color-singlet clusters arising from the parton branchings, producing a softening of the hadron spectra.

Nuclear suppression of : From RHIC to the LHC

A parameter-free calculation for J/Psi suppression in pA collisions, based on the dipole description, is confronted with the new data from the PHENIX experiment. Achieving good agreement, we employed this model predicting the contribution of initial state interactions (ISI) to J/Psi suppression in AA collisions. Such a transition from pA to AA is not straightforward, since involves specific effects of double color filtering and boosting of the saturation scale. Relying on this refined ISI contribution, we updated the previous analysis of RHIC data on J/Psi production in Cu-Cu and Au-Au collisions at \sqrt{s}=200 GeV, and determined the transport coefficient of the created dense medium at q_0=0.6 GeV^2/fm. Nuclear effects for J/Psi production at the LHC are predicted using the transport coefficient q_0=0.8 GeV^2/fm, extracted from data on suppression of high-p_T hadrons in central lead-lead collisions at \sqrt{s}=2.76 TeV. Our analysis covers only direct J/Psi production, while data may also include the feed-down from decay of heavier states and B-mesons.

Energy dependence of the saturation scale and the charged multiplicity in pp and AA collisions

A natural framework to understand the energy dependence of bulk observables from lower energy experiments to the LHC is provided by the Color Glass Condensate, which leads to a "geometrical scaling" in terms of an energy dependent saturation scale Q_s. The measured charged multiplicity, however, seems to grow faster (~\sqrt{s}^0.3) in nucleus-nucleus collisions than it does for protons (~\sqrt{s}^0.2), violating the expectation from geometric scaling. We argue that this difference between pp and AA collisions can be understood from the effect of DGLAP evolution on the value of the saturation scale, and is consistent with gluon saturation observations at HERA.

Charged particle identification with PHOS and central tracking of ALICE

The ALICE Photon Spectrometer (PHOS) has been designed to measure the photon spectrum produced in pp and AA collisions at LHC over a broad transverse momentum range from 0.5–100 GeV/c. We present a study on the matching of charged tracks, detected by the ALICE central tracking detectors, with clusters of hit PHOS elements. Matching efficiency and contamination due to false matches have been deduced for charged pions, muons and electrons. The study indicates also that material from other detectors in front of PHOS disturbs the photon detection.

A simplified model for calculating R AA and R CP of cold nuclear matter effects

An incoherent binary nucleon-nucleon collision model of AA collisions is presented for simulating particle production in cold nuclear matter. With a simple phenomenological parameter, the mean nucleon energy loss fraction, this model yields pseudorapidity density distributions that are comparable to those of experiment, as well as those of HIJING. Particle production data for a given binary collision is extracted from the PYTHIA event generator. The nuclear geometry is described by the Glauber model. The preliminary RAA and RCP results are also presented and discussed, with a proposal that RAA be redefined.

Photon — jet correlations measurement with ALICE at LHC: a feasibility study in proton-proton collisions

The feasibility to measure the imbalance distribution between direct isolated photons and charged hadrons is discussed within the context of the ALICE experiment at LHC. The case of proton-proton collisions at \(\sqrt s \) TeV is studied with Monte-Carlo simulations. Performances for prompt photon identification and rejection of the π0 decay photons are presented. Such a measurement will offer the possibilty to probe the medium formed in AA collisions with jets of energies below 50 GeV, which cannot be reconstructed in the heavy-ion environment.

Quarkonium production in high energy proton-proton and proton-nucleus collisions

We present a brief overview of the most relevant current issues related to quarkonium production in high energy proton-proton and proton-nucleus collisions along with some perspectives. After reviewing recent experimental and theoretical results on quarkonium production in pp and pA collisions, we discuss the emerging field of polarisation studies. Thereafter, we report on issues related to heavy-quark production, both in pp and pA collisions, complemented by AA collisions. To put the work in a broader perspective, we emphasize the need for new observables to investigate quarkonium production mechanisms and reiterate the qualities that make quarkonia a unique tool for many investigations in particle and nuclear physics.

Two particle correlations: A probe of the LHC QCD medium

The properties of γ-jet pairs emitted in heavy-ion collisions provide an accurate mean to perform a tomographic measurement of the medium created in the collision through the study of the medium modified jet properties. The idea is to measure the distribution of hadrons emitted on the opposite side of the direct photon. The feasibility of such measurements is studied by applying the approach on the simulation data, we have demonstrated that this method allows us to measure, with a good approximation, both the jet fragmentation and the back-to-back azimuthal alignment of the direct photon and the jet. Comparing these two observables measured in pp collisions with the ones measured in AA collisions reveals the modifications induced by the medium on the jet structure and consequently allows us to infer the medium properties. In this contribution, we discuss a first attempt of such measurements applied to real proton-proton data from the ALICE experiment.

SMALL x PHYSICS AND RHIC DATA

This is a review of applications of the Color Glass Condensate (CGC) to the phenomenology of relativistic heavy-ion collisions. The initial stages of the collision can be understood in terms of the nonperturbatively strong nonlinear glasma color fields. We discuss how the CGC framework can and has been used to compute properties of the initial conditions of AA collisions. In particular this has led to recent progress in understanding multiparticle correlations, which can provide a directly observable signal of the properties of the initial stage of the collision process.

QGP tomography with photon tagged jets in ALICE

γ+jet events provide a tomographic measurement of the medium formed in heavy ion collisions at LHC energies. Tagging events with a well identified high pT direct photon and measuring the correlation distribution of hadrons emitted oppositely to the photon in ALICE, allows us to determine, with a good approximation, both the jet fragmentation function and the back-to-back azimuthal alignment of the direct photon and the jet. Comparing these two observables measured in pp collisions with the ones measured in AA collisions will reveal the modifications of the jet structure induced by the medium formed in AA collisions and consequently will infer the medium properties.

Puzzles of [formula omitted] production off nuclei

Nuclear effects for J / Ψ production in pA collisions are controlled by the coherence and color transparency effects. Color transparency onsets when the time of formation of the charmonium wave function becomes longer than the inter-nucleon spacing. In this energy regime the effective break-up cross section for a c ¯ c dipole depends on energy and nuclear path length, and agrees well with data from fixed target experiments, both in magnitude and energy dependence. At higher energies of RHIC and LHC coherence in c ¯ c pair production leads to charm quark shadowing which is a complement to the high twist break up cross section. These two effects explain well with no adjusted parameters the magnitude and rapidity dependence of nuclear suppression of J / Ψ observed at RHIC in dAu collisions, while the contribution of leading twist gluon shadowing is found to be vanishingly small. A novel mechanism of double color filtering for c ¯ c dipoles makes nuclei significantly more transparent in AA compared to pA collisions. This is one of the mechanisms which make impossible a model independent “data driven” extrapolation from pA to AA. This effect also explains the enhancement of nuclear suppression observed at forward rapidities in AA collisions at RHIC, what hardly can be related to the produced dense medium. J / Ψ is found to be a clean and sensitive tool measuring the transport coefficient characterizing the dense matter created in AA collisions. RHIC data for p T dependence of J / Ψ production in nuclear collisions are well explained with the low value of the transport coefficient q ˆ 0 < 0.5 GeV 2 / fm .

Breakdown of [formula omitted]-factorization and [formula omitted] production in dA collisions

In spite of the sweeping coherence effects in high energy hadron and nuclei collisions, k T -factorization can be recovered for the inclusive gluon production in pA collisions at the leading logarithmic order. In open charm production at RHIC k T -factorization holds numerically with about 10–20% accuracy. This allows to extrapolate the cold nuclear matter effect observed in light and charm meson production in dA collisions to that in AA ones. Unlike the open charm, the breakdown of factorization in J / ψ production is severe. Indeed, already at the lowest order in gluon density the main contribution to the inclusive cross section is proportional to the square of gluon density in the nucleus. As a consequence, one cannot infer the cold nuclear matter effect on J / ψ production in AA collisions from that in dA. We present the calculation of J / ψ multiplicity in the framework of the CGC (color glass condensate)/saturation and show that it agrees with the experimental data.

Nonlinear waves in a quark gluon plasma

Recent measurements at RHIC suggest that a nearly perfect fluid of quarks and gluons is produced in AA collisions. Moreover the passage of supersonic partons through this medium seems to produce waves. These waves might pile up and form Mach cones, which would manifest themselves in the so called away-side jets, forming a broad structure in the angular distribution of the particles recoiling against a trigger jet of moderate energy. In most of the theoretical descriptions of these phenomena, the hydrodynamic equations are linearized for simplicity. We propose an alternative explanation for the observed broadening of the away-side peak. It is based on hydrodynamics but it is a consequence of the non-linearities of the equations, which instead of simple waves may lead to localized waves or even solitons. We investigate in detail the consequences of including the non-linear terms. We use a simple equation of state for the QGP and expand the hydrodynamic equations around equilibrium configurations. The resulting differential equations describe the propagation of perturbations in the energy density. We solve them numerically and find that localized perturbations can propagate for long distances in the plasma. Under certain conditions our solutions mimick the propagation of Korteweg - de Vries solitons.

Self-similarity of pion production in AA collisions at RHIC

Experimental data on inclusive spectra of pions produced in heavy-ion collisions at RHIC are analyzed in the framework of z-scaling. The data indicate similarity as a characteristic feature of the mechanism of pion production at high energies. It is argued that this property includes structure of the colliding objects, interaction of their constituents, and character of the fragmentation process. A microscopic scenario of nucleus interactions at a constituent level in terms of momentum fractions is developed. The centrality dependence of the shape of the scaling function Ψ(z) and the fractal dimention ɛ AA of the fragmentation process is studied. Energy losses of particles in the final state as a function of the collision energy, transverse momentum, and centrality are estimated. The scale dependence of the energy losses is discussed. A decreasing tendency of specific heat of the produced medium with the system size is established. The obtained results may be exploited to search for and study new physics phenomena in pion production in pp and AA collisions at high multiplicities.

Study of Higgs boson production and itsbb¯decay inγ−γprocesses in proton-nucleus collisions at the LHC

We explore for the first time the possibilities to measure an intermediate-mass (mH = 115-140 GeV/c^2) Standard-Model Higgs boson in electromagnetic proton-lead (pPb) interactions at the CERN Large Hadron Collider (LHC) via its b-bbar decay. Using equivalent Weizsaecker-Williams photon fluxes and Higgs effective field theory for the coupling gamma-gamma --> H, we obtain a leading-order cross section of the order of 0.3 pb for exclusive Higgs production in elastic (pPb --> gamma-gamma --> p H Pb) and semielastic (pPb --> gamma-gamma --> Pb H X) processes at sqrt(s) = 8.8 TeV. After applying various kinematics cuts to remove the main backgrounds (gamma-gamma --> b-bbar and misidentified gamma-gamma-->q-qbar events), we find that a Higgs boson with mH = 120 GeV/c^2 could be observed in the b-bbar channel with a 3sigma-significance integrating 300 pb^-1 with an upgraded pA luminosity of 10^31 cm^-2s^-1. We also provide for the first time semielastic Higgs cross sections, along with elastic t-tbar cross sections, for electromagnetic pp, pA and AA collisions at the LHC.