Study of jet fragmentation in p+p collisions at 200 GeV in the STAR experiment

The measurement of jet fragmentation functions in p+p collisions at 200 GeV is of great interest because it provides a baseline to study jet quenching in heavy-ion collisions. It is expected that jet quenching in nuclear matter modifies the jet energy and multiplicity distributions, as well as the jet hadrochemical composition. Therefore, a systematic study of the fragmentation functions for charged hadrons and identified particles is a goal both in p+p and Au+Au collisions at RHIC. Studying fragmentation functions for identified particles is interesting in p+p by itself because it provides a test of NLO calculations at RHIC energies. We present a systematic comparison of jet energy spectra and fragment distributions using different jet-finding algorithms in p+p collisions in STAR. Fragmentation functions of charged and neutral strange particles are also reported for different jet energies.     

What more can be learned from high p<Subscript>T</Subscript> probes at RHIC?

The current status of the understanding of jet quenching in nuclear collisions at RHIC is reviewed. The experimentally large level of suppression of jets in Au+Au collisions at RHIC is a success, but also introduces a challenge in terms of quantitative understanding of the properties of the collision zone. The medium appears to be equally black to all interacting probes utilized to date, limiting the amount of tomographic information that can be obtained from quenching phenomena. In order to recover this information, a probe to which the medium is gray needs to be found. PACS 25.75.-q     

Diagnostics of hot dense matter produced in Relativistic Heavy Ion Collisions

We study, in a pQCD calculation augmented by nuclear effects, the jet energy loss needed to reproduce the measured π ⁰ spectra in Au+Au collisions at large p _T , measured by PHENIX at RHIC at $\sqrt s = 200$ AGeV. Averaged energy loss obtained in the GLV formalism is applied in our recent calculation based on NLO pQCD (including shadowing and multiscattering). The method of jet tomography is capable to measure the opacity of the produced hot dense matter at RHIC energy in heavy ion collisions.     

Cold Nuclear Matter Effects on Isolated Prompt Photon and Isolated Prompt Photon+Jet Productions in Relativistic Heavy-Ion Collisions

We investigate the cold nuclear matter(CNM) effects on isolated prompt photon and isolated prompt photon associated jet productions in nuclear collisions at the NLO accuracy by using the EPS09 NLO nuclear parton distribution functions and their error sets.Nuclear modification factors of isolated prompt photon and isolated prompt photon+jet productions due to CNM effects in p+A and A+A reactions at the RHIC and the LHC are provided with varying rapidity and transverse momentum of the final state photon.It is shown that the CNM effects on isolated prompt photon and photon+jet are modest,which give a small enhancement at low pT region and a more obvious suppression at large pT at central rapidity.At forward rapidity a pronounced suppression of γ as well as γ+jet is always observed.     

Jet flavor tomography of quark gluon plasmas at RHIC and LHC

A new Monte Carlo model of jet quenching in nuclear collisions, CUJET1.0, is applied to predict the jet flavor dependence of the nuclear modification factor for fragments f=π,D,B,e(-) from quenched jet flavors g,u,c,b in central collisions at RHIC and LHC. The nuclear modification factors for different flavors are predicted to exhibit a novel level crossing pattern over a transverse momentum range 5相似文献   

Identification of exotic jet topologies via three-particle correlations in PHENIX

Modifications of jet properties resulting from the coupling of jets to the strongly interacting matter produced in RHIC collisions are of great current interest. In recent work, the PHENIX collaboration has applied a novel technique to the analysis of two-particle azimuthal correlations which extinguishes the harmonic part of the underlying event revealing the true jet shape. Recent extensions of the method to three-particle correlations allow for a more revealing study of jet topologies in Au+Au collisions at (√s _NN=200 GeV).     

Azimuthal anisotropy and formation of an extreme state of strongly interacting matter at the relativistic heavy-ion collider (RHIC)

Experimental results obtained by studying the azimuthal anisotropy of final states in nucleus-nucleus interactions at the energies of the relativistic heavy-ion collider (RHIC) are systematized. The medium is found to exhibit a pronounced collective behavior, which is likely to be formed at an early, parton, stage of the spacetime evolution of product hot and dense matter. Experimental data on the azimuthal anisotropy indicate that strongly interacting matter produced in the final state under extreme conditions behaves as a nearly ideal liquid rather than an ideal gas of quarks and gluons. The experimentally observed suppression of high-transverse-momentum jets and substantial modification of jetlike azimuthal correlations in heavy-ion collisions suggest that the energy loss of partons propagating in high-temperature matter featuring a high density of color charges is extremely large. The dependence of the amount of hardjet suppression in nucleus-nucleus collisions on the orientation of a jet with respect to the reaction plane was first discovered experimentally at RHIC. A strong suppression of the production of high-transverse-momentum particles and jets at RHIC is a unique phenomenon, which was discovered experimentally at lower energies.     

Resummed pQCD for <Emphasis Type="Italic">W</Emphasis><Superscript><Emphasis Type="Italic">±</Emphasis></Superscript> and <Emphasis Type="Italic">Z</Emphasis><Superscript>0</Superscript> transverse momentum spectra at RHIC and LHC

The transverse momentum distributions of W_± and Z₀ are predicted at RHIC and LHC. A resummation formalism with power corrections to the renormalization group equations is used. Shadowing effects are discussed and found to be unimportant at RHIC, but important for LHC. We study the enhancement of power corrections due to multiple scattering in nuclear collisions and numerically illustrate the weak effects of the dependence on the nuclear mass.     

First fragmentation function measurements from full jet reconstruction in heavy-ion collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV by STAR

Measurements of inclusive hadron suppression and di-hadron azimuthal correlations in ultra-relativistic nuclear collisions at RHIC have provided important insights into jet quenching in hot QCD matter, but are limited in their sensitivity due to well-known biases. Full jet reconstruction in heavy-ion collisions would conceptually provide a direct measurement of the energy of the scattered parton before energy loss, alleviating such biases and allowing a measurement of the energy loss probability distribution in a model-independent way from hard probes. In these proceedings we utilize recent progress in the reconstruction of jets in the heavy ion environment and present the first measurement of the fragmentation function from fully reconstructed jets in heavy ion collisions. The fragmentation function measured in central Au + Au collisions at  GeV will be presented and discussed with respect to p + p reference measurements.     

Heavy quark results from STAR

I report the most recent measurements on open heavy flavor production at RHIC on behalf of the STAR collaboration. The total charm production cross section in midrapidity at RHIC energy is found to approximately scale by number of binary collisions in d + Au, Cu + Cu and Au + Au collisions. The nuclear modification factor of non-photonic electrons is strongly suppressed in central Au + Au collisions, suggesting substantial heavy quark energy loss at RHIC. The bottom decay contribution to non-photonic electrons was studied via the e–h and e–D ⁰ azimuthal angular correlations. The bottom contribution is found to be important at p _T >5 GeV/c, and is consistent with the FONLL calculation within uncertainties. Charm production through gluon jet splitting was measured by studying the D ^*± contents in the fully reconstructed jets in p+p collisions. This rate is consistent with pQCD evaluation of gluon splitting into a pair of charm quarks and subsequent hadronization.     

Dynamical magnetic enhancement of light and heavy quark jet quenching at RHIC

A new Monte Carlo implementation of Djordjevic?s dynamical scattering generalization of the DGLV radiative energy loss opacity series is used with a hybrid interpolation scheme to compute both light and heavy quark jet quenching up to third order in opacity. The enhancement of the ratio of bottom to charm quark energy loss due to perturbative long range color magnetic effects in nonuniform Bjorken expanding geometries is found to reduce the significance of the heavy quark jet puzzle posed by the observed near equality (within sizeable errors) of pion and nonphotonic electron nuclear modification at RHIC. Jet Flavor Spectroscopy discussed below will be a powerful tool to differentiate competing dynamical models of the QGP produced in ultra-relativistic nuclear collisions.     

Multiplicity distributions inside parton cascades developing in a medium

The explanation of the suppression of high-p_T hadron yields at RHIC in terms of jet-quenching implies that the multiplicity distributions of particles inside a jet and jet-like particle correlations differ strongly in nucleus–nucleus collisions at RHIC or at the LHC from those observed at e⁺e^- or hadron colliders. We present a framework for describing the medium-induced modification, which has a direct interpretation in terms of a probabilistic medium-modified parton cascade, and which treats leading and subleading partons on an equal footing. We show that our approach can account for the strong suppression of single inclusive hadron spectra measured in Au–Au collisions at RHIC, and that this implies a characteristic distortion of the single inclusive distribution of soft partons inside the jet. We determine, as a function of the jet energy, to what extent the soft fragments within a jet can be measured above some momentum cut. PACS 12.38.Mh; 25.75.-q     

J/ψ-N center of mass energy dependence of nuclear absorption effect on J/ψ production

In this paper, the J/ψ nuclear absorption effect is studied at RHIC and LHC energies with the EKS98 shadowing parameterizations. By assuming that the J/ψ absorption cross section, σ abs , increases with the charmonium-nucleon (J/ψ-N) center of mass energy, s J/ψN , it is found that σ abs should depend on x F (or y) at a certain center of mass energy per nucleon pair,s , especially at LHC energies. The theoretical results with the x F (or y)-dependence of the absorption effect are in good agreement with the experiment data from PHENIX in d-Au collisions and the predicted results will be examined by the forthcoming experimental data from LHC in d-Pb collisions. Finally, we also present baseline calculations of cold nuclear matter effects on J/ψ production in nucleus-nucleus (A-A) collisions and find that the x F (or y)-dependence of absorption effect is very small at both RHIC and LHC energies in A-A collisions.     

An alternative model of jet suppression at RHIC energies

We propose a simple Glauber-type mechanism for the suppression of jet production up to transverse momenta of about 10 at RHIC. For processes in this kinematic region, the formation time is smaller than the interval between two successive hard partonic collisions and the subsequent collision influences the jet production. The number of jets then roughly scales with the number of participants. Proportionality to the number of binary collisions is reco vered for very high transverse momenta. The model predicts suppression of jet production in collisions at RHIC. Received: 20 January 2003 / Published online: 7 March 2003     

Evaluating results from the Relativistic Heavy Ion Collider with perturbative QCD and hydrodynamics

We review the basic concepts of perturbative quantum chromodynamics (QCD) and relativistic hydrodynamics, and their applications to hadron production in high energy nuclear collisions. We discuss results from the Relativistic Heavy Ion Collider (RHIC) in light of these theoretical approaches. Perturbative QCD and hydrodynamics together explain a large amount of experimental data gathered during the first decade of RHIC running, although some questions remain open. We focus primarily on practical aspects of the calculations, covering basic topics like perturbation theory, initial state nuclear effects, jet quenching models, ideal hydrodynamics, dissipative corrections, freeze-out and initial conditions. We conclude by comparing key results from RHIC to calculations.     

Parton energy loss at strong coupling and the universal bound

The apparent universality of jet quenching observed in heavy-ion collisions at RHIC for light and heavy quarks, as well as for quarks and gluons, is very puzzling and calls for a theoretical explanation. Recently, it has been proposed that synchrotron-like radiation at strong coupling gives rise to a universal bound on the energy of a parton escaping from the medium. Since this bound appears to be quite low, almost all of the observed particles at high transverse momentum have to originate from the surface of the hot fireball. Here I make a first attempt of checking this scenario against the RHIC data and formulate a “universal-bound model” of jet quenching that can be further tested at RHIC and LHC.     

Feasibility study of heavy ion physics program at NICA

There are strong experimental and theoretical evidences that in collisions of heavy ions at relativistic energies nuclear matter undergoes a phase transition to the deconfined state—Quark Gluon Plasma. The caused energy region of such transition was not found at high energy at SPS and RHIC and search for this energy is shifted to lower energies, which will be covered by the future NICA (Dubna), FAIR (Darmstadt) facilities and BES II at RHIC. Fixed target and collider experiments at the NICA facility will work at the energy range from a few AGeV up to \(\sqrt {\;{S_{NN}}} \; = \;11\;GeV\) GeV and will study the most interesting area on the nuclear matter phase diagram. The most remarkable results were observed in the study of collective phenomena occurring in the early stage of nuclear collisions. Investigation of the collective flow will provide information on Equation of State (EoS) for nuclear matter. Study of the Event-by-Event fluctuations and correlations can give us signals of critical behavior of the system. Femtoscopy analysis provides the space-time history of the collisions. Also, it was found that baryon stopping power revealing itself as a “wiggle” in excitation function of curvature of the (net)proton rapidity spectrum relates to the order of the phase transition. The available observations of an enhancement of dilepton rates at low invariant masses may serve as a signal of the chiral symmetry restoration in hot and dense matter. Due to this fact, measurements of the dilepton spectra are considered to be an important part of the NICA physics program. The study of strange particles and hypernuclei production gives additional information on the EoS and “strange” axis of the QCD phase diagram. In this paper a feasibility of the considered investigations is shown by the detailed Monte Carlo simulations applied to the planned experiments (BM@N, MPD) at NICA.     

Fluctuations and correlations as a signal of deconfinement

Event-by-event fluctuations of the K/??, K/p, and p/?? ratio in central AA collisions have been studied for SPS and RHIC energies. The Hadron-String-Dynamical transport approach (HSD) can qualitatively reproduce the measured excitation function for the K/?? ratio fluctuations. The di-jet azimuthal correlations also have been investigated within the HSD model. We found that the suppression of the away-side jet in the hadronic mediumis not enough to explain the experimental data from RHIC. The additional suppression should be attributed to a quark-gluon plasma produced in heavy-ion collisions.