Nuclear Suppression Factor Research Articles

Nuclear suppression factor forPTincreasing towards the kinematic limit

The suppression of high transverse momentum P_T hadron production in ultrarelativistic heavy-ion (A-A) collisions as compared to the scaled expectation from proton-proton (p-p) collisions expressed as the nuclear modification factor R_AA is experimentally well established and can be traced back to interactions between the hard parton shower and the soft bulk matter. Physical intuition suggests that the medium modification should cease to be important when the hard scale of parton production is much larger than the typical momentum scale in the medium (e.g. the temperature T) and that consequently R_AA(P_T) -> 1 for P_T >> T. However, R_AA is not a 'simple' observable, but rather results from many different mechanisms which influence what happens when P_T increases. In particular, R_AA does not necessarily approch unity even if the hadron momentum is probed at the kinematic limit of the reaction. The aim of this work is to identify and discuss such mechanisms and to present different model expectations of what one would find if one could measure suppression out to the kinematic limit for hard hadron production and give predictions for the P_T dependence at both RHIC and LHC.

Probing quark gluon plasma properties by heavy flavors

The Fokker-Planck (FP) equation has been solved to study the interaction of nonequilibrated heavy quarks with the quark gluon plasma expected to be formed in heavy ion collisions at Relativistic Heavy Ion Collider energies. Solutions of the FP equation have been convoluted with the relevant fragmentation functions to obtain the D and B meson spectra. Results are compared with experimental data measured by the STAR Collaboration. It is found that the present experimental data cannot distinguish p{sub T} spectra obtained from the equilibrium versus the nonequilibrium charm distributions. Data at lower p{sub T} may play a crucial role in making the distinction between the two. The nuclear suppression factor R{sub AA} for nonphotonic single-electron spectra resulting from semileptonic decays of hadrons containing heavy flavors has been evaluated using the present formalism. It is observed that the experimental data on the nuclear suppression factor of nonphotonic electrons can be reproduced within this formalism by enhancing the perturbative QCD cross sections by a factor of 2, provided that the expansion of the bulk matter is governed by the velocity of sound c{sub s}{approx}1/{radical}(4). The ideal-gas equation of state fails to reproduce the data even with enhancement of the perturbative QCD cross sectionsmore » by a factor of 2.« less

Q-PYTHIA: a medium-modified implementation of final state radiation

We present a Monte Carlo implementation, within PYTHIA, of medium-induced gluon radiation in the final state branching process. Medium effects are introduced through an additive term in the splitting functions computed in the multiple-soft scattering approximation. The observable effects of this modification are studied for different quantities as fragmentation functions and the hump-backed plateau, and transverse momentum and angular distributions. The anticipated increase of intra-jet multiplicities, energy loss of the leading particle and jet broadening are observed as well as modifications of naive expectations based solely on analytical calculations. This shows the adequacy of a Monte Carlo simulator for jet analyses. Effects of hadronization are found to wash out medium effects in the soft region, while the main features remain. To show the performance of the implementation and the feasibility of our approach in realistic experimental situations we provide some examples: fragmentation functions, nuclear suppression factors, jet shapes and jet multiplicities. The package containing the modified routines is available for public use. This code, which is not an official PYTHIA release, is called Q-PYTHIA. We also include a short manual to perform the simulations of jet quenching.

Photon-hadron jet correlations inp+pandAu+Aucollisions atsNN=200GeV

We report the observation at the Relativistic Heavy Ion Collider (RHIC) of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics (pQCD), making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I_AA, in central Au+Au collisions, is 0.32 +/- 0.12(stat) +/- 0.09(syst) for hadrons of 3 < p_T < 5 in coincidence with photons of 5 < p_T < 15 GeV/c. The suppression is comparable to that observed for high-p_T single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, z_T = p_T^hadron/p_T^photon, as expected for a measure of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.

Quenching of light hadrons at RHIC in a collisional energy loss scenario

We evaluate the nuclear suppression factor, RAA(pT) for light hadrons by taking into account the collisional energy loss. Within the ambit of the present model we show that in the measured pT domain of RHIC the contribution from the elastic process has a sizable magnitude.

Mass ordering of differential elliptic flow and its violation for ϕ mesons

We simulate the dynamics of Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) with a hybrid model that treats the dense early quark-gluon plasma (QGP) stage macroscopically as an ideal fluid but models the dilute late hadron resonance gas (HG) microscopically using a hadronic cascade. By comparing with a pure hydrodynamic approach we identify effects of hadronic viscosity on the transverse momentum spectra and differential elliptic flow v{sub 2}(p{sub T}). We investigate the dynamical origins of the observed mass ordering of v{sub 2}(p{sub T}) for identified hadrons, focusing on dissipative effects during the late hadronic stage. Within our approach, we find that, at RHIC energies, much of the finally observed mass splitting is generated during the hadronic stage, due to buildup of additional radial flow. The {phi} meson, having a small interaction cross section, does not fully participate in this additional flow. As a result, it violates the mass-ordering pattern for v{sub 2}(p{sub T}) that is observed for other hadron species. We also show that the early decoupling of the {phi} meson from the hadronic rescattering dynamics leads to interesting and unambiguous features in the p{sub T} dependence of the nuclear suppression factor R{sub AA} and of the {phi}/pmore » ratio.« less

Expectations for dihadron correlation measurements extrapolated to5.5ATeV

The suppression of high transverse momentum (${P}_{T}$) inclusive hadron spectra in heavy-ion collisions as compared to the scaled expectation of high ${P}_{T}$ hadron production in $p\text{\ensuremath{-}}p$ collisions is the most direct manifestation of the interaction of hard partons with the soft bulk medium produced in heavy-ion collisions which is absent in $p\text{\ensuremath{-}}p$ collisions. Yet the measured nuclear suppression factor ${R}_{\mathit{AA}}$ is a very averaged quantity and hence only a limited amount of information about the medium evolution and the nature of the interaction with the medium can be deduced from ${R}_{\mathit{AA}}$. Measurements of hard back-to-back hadron correlations in 200$A$ GeV Au-Au collisions at RHIC have opened a new window to study the energy loss of partons in a medium in a more differential way and for a different distribution of in-medium path lengths than in the case of ${R}_{\mathit{AA}}$. In this work, we present an extrapolation of our results for back-to-back yields at RHIC energies to 5.5 TeV Pb-Pb collisions at the CERN LHC. We also discuss differences and similarities between the measurement at RHIC.

Recent results from the NA49 experiment

Recent results from the NA49 experiment at the CERN-SPS are discussed. These include new data on proton and (anti)-proton production at different beam energies, which allow us to study the energy dependence of stopping. Also, new data on the system size dependence of hyperon production at 40A and 158, on global Λ polarization, on elliptic flow of strange particles and on high- nuclear suppression factors are presented.

Hard and soft probe–medium interactions in a 3D hydro+micro approach at RHIC

We utilize a 3D hybrid hydro+micro model for a comprehensive and consistent description of soft and hard particle production in ultra-relativistic heavy-ion collisions at RHIC. In the soft sector we focus on the dynamics of (multi-) strange baryons, where a clear strangeness dependence of their collision rates and freeze-out is observed. In the hard sector we study the radiative energy loss of hard partons in a soft medium in the multiple soft scattering approximation. While the nuclear suppression factor RAA does not reflect the high quality of the medium description (except in a reduced systematic uncertainty in extracting the quenching power of the medium), the hydrodynamical model also allows us to study different centralities and in particular the angular variation of RAA with respect to the reaction plane, allowing for a controlled variation of the in-medium path length.

Angular hadron correlations probing the early medium evolution

Hard processes are a well-calibrated probe to study heavy-ion collisions. However, the information to be gained from the nuclear suppression factor RAA is limited, hence one has to study more differential observables to do medium tomography. The angular correlations of hadrons associated with a hard trigger appear suitable as they show a rich pattern when going from low pT to high pT. Of prime interest is the fate of away-side partons with an in-medium pathlength O (several fm). At high pT, the correlations become dominated by the punchtrough of the away-side parton with subsequent fragmentation. We discuss what information about the medium density can be gained from the data.

Prospects of medium tomography using back-to-back hadron correlations

We discuss the prospects of extracting information on bulk QCD matter distribution and evolution on the basis of hard hadronic back-to-back correlations in ultrarelativistic heavy-ion collisions. Using both hydrodynamic and parametrized evolution models for the space-time evolution of produced matter, which have been tested against data from the BNL Relativistic Heavy-Ion Collider, we study six different setups for the space-time dependence of hard-parton energy losses. Assuming that the energy loss of hard partons traversing the medium is radiative and calculable in the Baier-Dokshitzer-Mueller-Peign\'e-Schiff formalism, we adjust one parameter, the quenching power scale, to the measured nuclear suppression factor ${R}_{\mathit{AA}}$ in each of the setups and study the systematic variations of the back-to-back yield as a function of transverse momentum ${p}_{T}$. We show which space-time regions are probed by one- and two-particle observables and study in some detail the role of longitudinal and transverse expansion. We also comment on the importance of considering fluctuations around the average energy loss. We conclude that while current data are too limited in momentum coverage, future data for higher trigger energy might provide the lever arm in away-side hadron momentum necessary to perform medium tomography, provided that sufficient precision can be achieved.

Jet quenching in a three-dimensional hydrodynamic medium

We study the radiative energy loss of hard partons in a soft medium in the multiple soft scattering approximation. The soft medium is described by a 3D hydrodynamical model and we treat the averaging over all possible parton paths through the medium without approximation. While the nuclear suppression factor ${R}_{AA}$ does not reflect the high quality of the medium description (except in a reduced systematic uncertainty in extracting the quenching power of the medium), the hydrodynamical model also allows to study different centralities and in particular the angular variation of ${R}_{AA}$ with respect to the reaction plane, allowing for a controlled variation of the in-medium path-length. We study the angular dependence of ${R}_{AA}$ for different centralities, discuss the influence of hydrodynamical expansion and flow and comment on the comparison with preliminary data.

Collisional energy loss and the suppression of high pT hadrons

We calculate nuclear suppression factor ( R AA ) for light hadrons by taking only the elastic processes and argue that in the measured p T domain of RHIC, collisional rather than the radiative processes is the dominant mechanism for partonic energy loss.

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.

Heavy-quark diffusion, flow and recombination at RHIC

We discuss recent developments in assessing heavy-quark interactions in the quark–gluon plasma (QGP). While induced gluon radiation is expected to be the main energy-loss mechanism for fast-moving quarks, we focus on elastic scattering which prevails towards lower energies, evaluating both perturbative (gluon-exchange) and nonperturbative (resonance formation) interactions in the QGP. The latter are treated within an effective model for D- and B-meson resonances above Tc as motivated by current QCD lattice calculations. Pertinent diffusion and drag constants, following from a Fokker–Planck equation, are implemented into an expanding fireball model for Au–Au collisions at RHIC using relativistic Langevin simulations. Heavy quarks are hadronized in a combined fragmentation and coalescence framework, and resulting electron-decay spectra are compared to recent RHIC data. A reasonable description of both nuclear suppression factors and elliptic flow up to momenta of ∼5 GeV supports the notion of a strongly interacting QGP created at RHIC. Consequences and further tests of the proposed resonance interactions are discussed.

Towards jet tomography: γ-hadron correlations

Hard pQCD processes taking place in ultrarelativistic heavy-ion collisions are a well-calibrated probe. It is believed that the interaction with the surrounding medium of outgoing partons from a hard vertex is capable of revealing details of the medium. We demonstrate that correlation measurements of hard photon-hadron back-to-back coincidences are a tool suitable for extracting such tomographic information. Introducing the concept of averaged energy loss probability distributions, we first argue that almost no information about details of the medium evolution is reflected in the nuclear suppression factor R{sub AA}. Thus, a wide variety of jet quenching scenarios and geometries are compatible with the measured data. This problem can be overcome by a {gamma}-hadron correlation measurement. We show that averaged probability distributions for quarks are accessible experimentally, and we sketch an analysis procedure capable of distinguishing different energy loss scenarios leading to the same nuclear suppression factor.

Strangeness at RHIC: recent results from STAR

In this article I will summarize the intermediate- to high-momentum strangeness measurements from STAR since SQM03. Some of the results shown here have been previously published in the QM04 proceedings. The focus is on dynamic measurements such as resonance production, elliptic flow and momentum-dependent particle suppression in central AA collisions. Resonance scattering and regeneration in the hadronic phase between chemical and kinetic freeze-out might yield a scale for the lifetime of this phase. I will show differences in the baryon to meson behaviour at intermediate transverse momentum that can be attributed to quark coalescence in AA and the Cronin effect in dA collisions. The canonical yield suppression, which is unique to strange particles when comparing pp to AA collisions, affects the compatibility of the nuclear suppression factors RCP and RAA. Particle identified two particle correlations in AA collisions show that at high transverse momentum the measurement might be dependent on the species of the trigger particle.

Strange Particle Production at the Intersection of Soft and Hard Processes at RHIC

Nuclear suppression factor and u2 measurements as a function of transverse momentum for neutral strange particles are shown and compared to particle identified spectra from PHENIX and WA98 and to models that attempt to describe the suppression of particles at moderate pt in central RHIC Au−Au collisions. It is shown that identified spectra carry additional information to the published charged particle spectra. In particular, dependencies of the RAA and u2 values on the measured particle species will be discussed in the context of several models.

Hadron production in heavy ion collisions: Fragmentation and recombination from a dense parton phase

We discuss hadron production in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). We argue that hadrons at transverse momenta ${P}_{T}l5\phantom{\rule{0.3em}{0ex}}\text{GeV}$ are formed by recombination of partons from the dense parton phase created in central collisions at RHIC. We provide a theoretical description of the recombination process for ${P}_{T}g2\phantom{\rule{0.3em}{0ex}}\text{GeV}$. Below ${P}_{T}=2\phantom{\rule{0.3em}{0ex}}\text{GeV}$ our results smoothly match a purely statistical description. At high transverse momentum hadron production is well described in the language of perturbative QCD by the fragmentation of partons. We give numerical results for a variety of hadron spectra, ratios, and nuclear suppression factors. We also discuss the anisotropic flow ${v}_{2}$ and give results based on a flow in the parton phase. Our results are consistent with the existence of a parton phase at RHIC hadronizing at a temperature of $175\phantom{\rule{0.3em}{0ex}}\text{MeV}$ and a radial flow velocity of $0.55c$.