Partonic Medium Research Articles

Review on DIS electroproduction on nuclei

The inclusive Deep-Inelastic-Scattering (DIS) of high energy leptons on free nucleons has been used from late 60s to measure the partonic distributions and, by using nuclear targets, to study the medium modification to these distributions. Semi-Inclusive DIS (SIDIS) of high energy leptons on free nucleons can be used to study another fundamental property of the quarks, the fragmentation function. This function can be also modified by the nuclear medium and can be revealed by performing SIDIS measurements on nuclear targets. These measurements have been recently performed by HERMES with a 27.5 GeV positron beam and at lower energy by CLAS. The possible interpretations in terms of medium modifications of the parton fragmentation function and the implications on the parton energy loss are discussed. Similar hadron quenching appeared from Au-Au central collisions at RHIC. Despite the two different regimes, both these observations have been interpreted in terms of in medium parton energy loss which depends on the density of the medium. A challenge in this research field, will be represented by the measurement of the jet quenching in a hot and dense medium like the Quark Gluon Plasma which is expected to be created at the LHC energy. For this purpose, an electromagnetic calorimeter to complement the excellent performance of the ALICE detector has been proposed.

Heavy quarks at RHIC from parton transport theory

There are several indications that an opaque partonic medium is created in energetic Au+Au collisions \sqrt{s} ~ 100 GeV/nucleon at the Relativistic Heavy Ion Collider (RHIC). At the extreme densities of ~ 10-100 times normal nuclear density reached even heavy-flavor hadrons are affected significantly. Heavy-quark observables are presented from the parton transport model MPC, focusing on the nuclear suppression pattern, azimuthal anisotropy ("elliptic flow"), and azimuthal correlations. Comparison with Au+Au data at top RHIC energy \sqrt{s} = 200 GeV indicates significant heavy quark rescattering, corresponding to roughly five times higher opacities than estimates based on leading-order perturbative QCD. We propose measurements of charm-anticharm, e.g., D-meson azimuthal correlations as a sensitive, independent probe to corroborate these findings.

Resonance production in RHIC collisions

Short lived resonances are sensitive to the properties of the medium produced in heavy ion collision, in particular the temperature, density and expansion velocity. Thermal models underpredict the yield of K(892) and Λ(1520) in Au+Au collisions which allows us to conclude that an extended hadronic interaction phase exists between chemical and thermal freeze-out. During this time the decay particles of resonances will re-scatter and coalesce to regenerate resonances. These mechanisms affect the resonance yield mostly in the low momentum region below 1 GeV/c. Therefore the nuclear suppression factor RAA of resonances with more re-scattering than regeneration will be suppressed compared to stable particles in that pT range. It is interesting to study the RAA of resonances at higher momenta where the spectra of non-resonant particles exhibit effects such as enhancement through constituent quark recombination and quenching in the dense partonic medium. In addition the RAA’s of strange particles show the effect of canonical suppression on the nuclear suppression factor which leads to a significant difference between RAA and RCP, in particular for strange baryons. Therefore the RAA and the elliptic flow v2 for strange resonances in comparison to strange particles are investigated.

Correlated Emission of Hadrons from Recombination of Correlated Partons

We discuss different sources of hadron correlations in relativistic heavy ion collisions. We show that correlations among partons in a quasithermal medium can lead to the correlated emission of hadrons by quark recombination and argue that this mechanism offers a plausible explanation for the dihadron correlations in the few GeV/c momentum range observed in Au+Au collisions at the BNL Relativistic Heavy Ion Collider.

High-pT hadron spectra at RHIC: an overview

Recent results on high transverse momentum (pT) hadron production in p+p, d+Au and Au+Au collisions at the relativistic heavy-ion collider (RHIC) are reviewed. Comparison of the nuclear modification factors, RdAu(pT) and RAA(pT), demonstrates that the large suppression in central Au+Au collisions is due to strong final-state effects. Theoretical models which incorporate jet quenching via gluon bremsstrahlung in the dense partonic medium that is expected in central Au+Au collisions at ultra-relativistic energies are shown to reproduce the shape and magnitude of the observed suppression over the range of collision energies so far studied at RHIC.

Strange hard probes to characterize the partonic medium at RHIC

I will show examples of high momentum identified particle measurements in heavy ion collisions at RHIC energies. Identified particles enable us to probe the existence and the hadronization mechanisms of the produced partonic medium in the case of the formation of a Quark-Gluon-Plasma (QGP). Besides probing the features of the plasma we also learn about the in-medium parton fragmentation process. I propose future measurements at RHIC-II and LHC in order to determine the dynamic evolution of the system from partonic to hadronic degrees of freedom.

Jet quenching: the medium modification of the single and double fragmentation functions

The physics of the quenching of hard jets in dense matter is briefly reviewed. This is presented within the framework of the partonic medium modification of the fragmentation functions. Modifications in both deeply inelastic scattering (DIS) off large nuclei and high-energy heavy-ion collisions are presented.

Medium Density and Parton Energy Loss in Gold–Gold Collisionsfrom the Recombination Model

The pion production at mid-rapidity in gold–gold collisions at(s)1/2 = 200 GeV is investigated by the parton recombination model.The density of the hot medium produced in the collisions and the hardparton energy loss effect are obtained by fitting the experimental data.Both of them are found to have power-law dependence on the number ofbinary collisions. The implication of such dependence on the nuclearmodification factor is discussed.

Recombination of shower partons at highpTin heavy-ion collisions

A formalism for hadron production at high \pt in heavy-ion collisions has been developed such that all partons hadronize by recombination. The fragmentation of a hard parton is accounted for by the recombination of shower partons that it creates. Such shower partons can also recombine with the thermal partons to form particles that dominate over all other possible modes of hadronization in the $3<p_T<8$ GeV range. The results for the high \pt spectra of pion, kaon, and proton agree well with experiments. Energy loss of partons in the dense medium is taken into account on the average by an effective parameter by fitting data, and is found to be universal independent of the type of particles produced, as it should. Due to the recombination of thermal and shower partons, the structure of jets produced in nuclear collisions is different from that in $pp$ collisions. The consequence on same-side correlations is discussed.

RELATIVISTIC HEAVY-ION COLLISIONS: RECENT RESULTS FROM RHIC

High energy collisions of heavy nuclei at the Relativistic Heavy-Ion Collider permit the study of nuclear matter at extreme densities and temperatures. Selected experimental highlights from the early RHIC program are presented. Measurements of the total multiplicity in heavy-ion collisions show a surprising similarity to measurements in e+e- collisions after nuclear geometry is taken into account. RHIC has sufficient center-of-mass energy to use large transverse momentum particles and jets as a probe of the nuclear medium. Signatures of "jet quenching" due to radiative gluon energy loss of fast partons in a dense medium are observed for the first time at RHIC. In order to account for this energy loss, initial energy densities of 30-100 times normal nuclear matter density are required.

Propagation of fast partons in the nuclear medium

The color dipole approach has been applied in the target rest frame to address the issues of transverse momentum broadening and energy loss of a fast quark propagating in the nuclear medium. A recent application of the theory to the FermiLab E772/E866 experimental data, determining the rate of energy loss of a quark propagating in the medium to be 2 to 3 GeV/fm, will be reviewed. Calculations for the transverse momentum distribution will be presented, and the results will be compared to the E866 data. The theory will be shown to compare favorably to the data, and these results will be shown to suggest that the momentum broadening of a quark is about twice the generally accepted size.

Finding the remnants of lost jets at RHIC

A fast parton propagating through partonic matter loses energy by medium-induced gluon radiation. We propose a method to detect the energy loss of jets and the final-state interaction of the radiated gluons and the jets with the partonic medium. By embedding jets and radiated energy into a parton cascade, we find that the fate of lost jets can be studied by analyzing the momentum distribution of soft hadrons relative to the jet axis.

Azimuthal asymmetry of J / ψ suppression in non-central heavy-ion collisions

The azimuthal asymmetry of J / ψ suppression in non-central heavy-ion collisions is studied within a dynamic model of J / ψ suppression in a deconfined partonic medium. Within this model, J / ψ suppression in heavy-ion collisions is caused mainly by the initial state nuclear absorption and dissociation via gluon- J / ψ scattering in deconfined partonic medium. Only the second mechanism gives arise to azimuthal asymmetry of the final J / ψ production. We demonstrate that if there is an onset of suppression by quark–gluon plasma (QGP) in the NA50 data, it must be accompanied by the non-vanishing azimuthal asymmetry. Using the same critical density above which the QGP effect enters, we predict the azimuthal asymmetric coefficient v 2 as well as the survival probability for J / ψ at the RHIC energy.

Formula omitted] production in an equilibrating partonic system

Any color singlet or octet c c pair is created at short distances and then expands to a full size of J ψ . Such a dynamical evolution process is included here in calculations for the J ψ number distribution as a function of transverse momentum and rapidity in central AuAu collisions at both RHIC and LHC energies. The c c pairs are produced in the initial collision and in the partonic system during the prethermal and thermal stages through the partonic channels ab → c c[ 2S+1L J] and ab → c c[ 2S+1L J] x , and then they dissociate in the latter two stages. Dissociation of c c in the medium occurs via two reactions: (a) color singlet c c plus a gluon turns to color octet c c , (b) color octet c c plus a gluon persists as color octet. There are modest yields of c c in the prethermal stage at RHIC energy and through the reactions ab → c c[ 2S+1L J] at LHC energy for partons with large average momentum in the prethermal stage at both collider energies and in the thermal stage at LHC energy. Production from the partonic system competes with the suppression of the initial yield in the deconfined medium. Consequently, a bulge within −1.5 < y < 1.5 has been found for the J ψ number distribution and the ratio of J ψ number distribution for AuAu collisions to nucleon-nucleon collisions. This bulge is caused by the partonic system and is thus an indicator of a deconfined partonic medium. Based on this result we suggest the rapidity region worth measuring in future experiments at RHIC and LHC to be −3 < y < 3.

Thermal dilepton signal versus dileptons from open charm and bottom decays in heavy-ion collisions

We analyze the opportunity to observe thermal dileptons emitted off deconfined matter resulting in heavy-ion collisions at RHIC and LHC energies. Special kinematical conditions provided by the detector systems PHENIX and ALICE, and the so-called ${M}_{\ensuremath{\perp}}$ scaling behavior of thermal dilepton spectra are taken into account. Our considerations include energy loss effects of the fast heavy quarks in deconfined matter, which for themselves can help to identify the creation of a hot and dense parton medium. Due to a thresholdlike effect for decay leptons we find a window at large transverse pair momentum and fixed transverse mass where the thermal signal can exceed the background of dileptons from correlated semileptonic decays of charm and bottom mesons.

J/ψ supressions in a thermally equilibrating quark-gluon plasma at RHIC

We estimate the survival probability of $J/{\psi}$ in relativistic heavy-ion collisions, using both short-distance QCD and nuclear absorption mechanism. The suppression is found to be almost 100 percent for a thermally equilibrating quark-gluon plasma. The measurement of such a huge suppression of $J/{\psi}$ may suggest the existence of a deconfined partonic medium, possibly a thermalised quark-gluon plasma, at RHIC.

Estimates of dilepton spectra from open charm and bottom decays in relativistic heavy-ion collisions

The spectra of lepton pairs from correlated open charm and bottom decays in ultrarelativistic heavy-ion collisions are calculated. Our approach includes energy loss effects of heavy quarks in deconfined matter which are determined by temperature and density of the expanding parton medium. We find a noticeable suppression of the initial transverse momentum spectrum of heavy quarks due to the energy loss at LHC conditions. Within the central rapidity covered by the ALICE detector system, the dominant contribution from bottom decays to the high invariant mass dielectron spectrum is predicted.

Energy loss and p⊥-broadening of high energy partons in hot and cold QCD matter

Recent work on jet p ⊥-broadening and induced gluon radiation from jets passing through hot and cold QCD matter is reviewed. The basic assumption used is that multiple scattering of a high energy parton in a QCD medium can be viewed as a series of independent scatterings, exactly as in the standard Glauber theory. We show that p ⊥ w 2 grows as L, the length of the nuclear matter, as does − dE dz the energy loss of the parton per unit length. We find − dE dz = − αN c 8 p ⊥w 2 , a generalization of an earlier result of Brodsky and Hoyer. Numerical estimates suggest that p ⊥-broadening and induced energy loss may be significantly enhanced in hot matter as compared to cold matter.

Induced gluon radiation in a QCD medium

Soft gluon radiation induced by multiple scattering of a fast quark or gluon propagating through QCD matter is discussed. After revisiting the Landau-Pomeranchuk-Migdal effect in QED we show that large formation times of bremsstrahlung quanta determine the QCD radiation intensity (in analogy to QED) and derive the gluon energy spectrum. Coherent suppression takes place as compared to the Bethe-Heither situation of independent emissions. As a result the energy loss of fast partons in a QCD medium depends on the incident energy E similarly to QED, - dE dz ∞ E .

Collision dissociation of heavy quarkonia in dense partonic systems

The influence of collisions in a hot partonic medium on the formation of heavy quarkonia is investigated. We find that the initial color octet propagation of\(Q\bar Q\) pairs created in nuclear collisions leads to a large cross section for collision dissociation even before Debye screening of the quarkonium potential in the color singlet final state becomes effective. This leads to a sizeable suppression of quarkonium states in addition to previously considered effects. The concept of a quarkonium “formation time”, during which the\(Q\bar Q\) pair is not affected by the surrounding medium, is shown to be incorrect. In particular, we argue that the υ resonance will always be suppressed in nuclear collisions, contrary to recent suggestions that an unsuppressed υ state might signal quarkgluon plasma formation.