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.     

Measuring the collective flow with jets

In nucleus-nucleus collisions, high-p(T) partons interact with a dense medium, which possesses strong collective flow components. Here, we demonstrate that the resulting medium-induced gluon radiation does not depend solely on the energy density of the medium, but also on the collective flow. Both components can be disentangled by the measurement of particle production associated with high-p(T) trigger particles, jetlike correlations, and jets. In particular, we show that flow effects lead to a characteristic breaking of the rotational symmetry of the average jet energy and jet multiplicity distribution in the eta x phi plane. We argue that data on the medium-induced broadening of jetlike particle correlations in Au + Au collisions at the Relativistic Heavy-Ion Collider may provide evidence for a significant distortion of parton fragmentation due to the longitudinal collective flow.     

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     

A short review on jet identification

Jets can be used to probe the physical properties of the high energy density matter created in collisions at the Relativistic Heavy Ion Collider (RHIC). Measurements of strong suppression of inclusive hadron distributions and di-hadron correlations at high p _T have already provided evidence for partonic energy loss. However, these measurements suffer from well-known geometric biases due to the competition of energy loss and fragmentation. These biases can be avoided if the jets are reconstructed independently of their fragmentation details—quenched or unquenched. In this paper, we discuss modern jet reconstruction algorithms (cone and sequential recombination) and their corresponding background subtraction techniques required by the high multiplicities of heavy ion collisions. We review recent results from the STAR experiment at RHIC on direct jet reconstruction in central Au+Au collisions at  GeV.     

QCD resummation for jet substructures

We provide a novel development in jet physics by predicting the energy profiles of light-quark and gluon jets in the framework of perturbative QCD. Resumming large logarithmic contributions to all orders in the coupling constant, our predictions are shown to agree well with Tevatron CDF and Large Hadron Collider CMS data. We also extend our resummation formalism to the invariant mass distributions of light-quark and gluon jets produced in hadron collisions. The predicted peak positions and heights in jet mass distributions are consistent with CDF data within uncertainties induced by parton distribution functions.     

Net baryon density in Au+Au collisions at the relativistic heavy ion collider

We calculate the net-baryon rapidity distribution in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) in the framework of the parton cascade model (PCM). Parton rescattering and fragmentation leads to a substantial increase in the net-baryon density at midrapidity over the density produced by initial primary parton-parton scatterings. The PCM is able to describe the measured net-baryon density at RHIC.     

Glauber shadowing in jet and particle production in nucleus-nucleus collisions within perturbative QCD

Inclusive cross sections for jet and hadron production in nucleus-nucleus collisions are computed within perturbative QCD with allowance for multiple parton rescattering. Nuclear shadowing due to parton rescattering is negligible at the RHIC energies, but it becomes significant at the LHC energy.     

Jet hadrochemistry as a characteristic of jet quenching

Jets produced in nucleus–nucleus collisions at the LHC are expected to be strongly modified due to the interaction of the parton shower with the dense QCD matter. Here, we point out that jet quenching can leave signatures not only in the longitudinal and transverse jet energy and multiplicity distributions, but also in the hadrochemical composition of the jet fragments. In particular, we show that even in the absence of medium-effects at or after hadronization, the medium-modification of the parton shower may result in significant changes in jet hadrochemistry. We discuss how jet hadrochemistry can be studied within the high-multiplicity environment of nucleus–nucleus collisions at the LHC.     

Charged particle density distributions in Au + Au collisions at relativistic heavy-ion collider energies

Charged particle pseudorapidity distributions have been measured in Au+ Au collisions using the BRAHMS detector at RHIC. The results are presented as a function of the collision centrality and the center of mass energy. They are compared to the predictions of different parton scattering models and the important role of hard scattering processes at RHIC energies is discussed. Keywords. Relativistic heavy-ion collisions; charged hadron production; pseudorapidity distributions; centrality dependence; hard scattering processes.     

相对论重离子碰撞的输运模型研究

美国的布鲁克海文国家实验室相对论重离子对撞机（RHIC）和欧洲核子中心的大型强子对撞机（LHC）的大量实验结果表明,在相对论重离子碰撞中已经产生了一种近似完美流体的强耦合部分子物质。基于一个多相粒子输运模型（AMPT）理论工具,对RHIC和LHC实验上的一些重要结果的开展了三个方面的理论研究工作（集体流、喷注淬火、手征磁效应）,研究结果揭示了初始的夸克胶子等离子体（QGP）能量密度涨落经过部分子输运演化产生末态粒子的各阶次的集体流、喷注和部分子物质的相互作用导致喷注的能量损失、末态相互作用严重影响手征磁效应的大小等物理过程作用机制。The experimental results from the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) show that a nearly perfect fluid (i.e. strong-coupling Quark Gluon Plasma) has been created in relativistic heavy-ion collisions. I introduce our theoretical results based on a multi-phase transport (AMPT) model. Several important topics such as collective flow, jet quenching, chiral magnetic effect, are addressed. The simulation results indicate that the initial fluctuations of energy density of the QGP lead to all orders of harmonic flows of final particles via parton cascade, the strong interactions between jet and the QGP make jet lose much energy, and the final state interactions play an important role to affect the initial chiral magnetic effect in relativistic heavy-ion collisions.     

Probing TMDs through azimuthal distributions of pions inside a jet in hadronic collisions

The azimuthal distributions around the jet axis of leading pions produced in the jet fragmentation process in pp collisions are studied within the framework of the so-called generalized parton model. The observable leading-twist azimuthal asymmetries are estimated in kinematic configurations presently investigated at RHIC. It is shown how the main contributions coming from the Collins and Sivers effects can be disentangled. In addition, a test of the process dependence of the Sivers function is provided.     

High-p t and jet physics from RHIC to LHC

The observation of the strong suppression of high-p t hadrons in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) at BNL has motivated a large experimental program using hard probes to characterize the deconfined medium created. However, what can be denoted as “leading particle” physics accessible at RHIC presents some limitations which motivate at higher energy the study of much more penetrating objects: jets. The gain in center-of-mass energy expected at the Large Hadron Collider (LHC) at CERN will definitively improve our understanding on how the energy is lost in the system, opening a major new window of study: the physics of jets on an event-by-event basis. We will concentrate on the expected performance for jet reconstruction in ALICE using the EMCal calorimeter. (for the ALICE Collaboration) The text was submitted by the author in English.     

Physics of collisions between ultrarelativistic nuclei: Manifestation of collective effects

Experimental results concerning heavy-ion collisions studied by means of the Relativistic Heavy-Ion Collider (RHIC) at an energy of 100 GeV per nucleon and showing manifestations of collective effects are surveyed. These effects are interpreted as a consequence of the formation of a dense thermalized medium referred to as quark-gluon plasma (strongly interacting quark-gluon plasma, quark-gluon matter, parton medium). An azimuthal anisotropy of particles, the suppression of the particle yield at high transverse momenta in relation to proton-proton collisions, and a change in the shape of the peak from a hadron jet in nucleus-nucleus collisions are spectacular manifestations of collective effects.     

Measurement of direct photon production in p+p collisions at sqrt[s] = 200 GeV

Cross sections for midrapidity production of direct photons in p+p collisions at the Relativistic Heavy Ion Collider (RHIC) are reported for transverse momenta of 3 < pT < 16 GeV/c. Next-to-leading order perturbative QCD (pQCD) describes the data well for pT >5 GeV/c, where the uncertainties of the measurement and theory are comparable. We also report on the effect of requiring the photons to be isolated from parton jet energy. The observed fraction of isolated photons is well described by pQCD for pT >7 GeV/c.     

From leading hadron suppression to jet quenching at RHIC and at the LHC

In nucleus-nucleus collisions at the Relativistic Heavy Ion Collider (RHIC), one generically observes a strong medium-induced suppression of high- p_T hadron production. This suppression is accounted for in models which assume a significant medium-induced radiative energy loss of high- p_T parent partons produced in the collision. How can we further test the microscopic dynamics conjectured to underlie this abundant high- p_T phenomenon? What can we learn about the dynamics of parton fragmentation, and what can we learn about the properties of the medium which modifies it? Given that inelastic parton scattering is expected to be the dominant source of partonic equilibration processes, can we use hard processes as an experimentally well-controlled window into QCD non-equilibrium dynamics? Here I review what has been achieved so far, and which novel opportunities open up with higher luminosity at RHIC, and with the wider kinematical range accessible soon at the LHC.Received: 15 February 2005, Published online: 3 June 2005PACS: 12.38.Mh, 24.85. + p     

Identified particle distributions in pp and Au+Au collisions at square root of (sNN)=200 GeV

Transverse mass and rapidity distributions for charged pions, charged kaons, protons, and antiprotons are reported for square root of [sNN]=200 GeV pp and Au+Au collisions at Relativistic Heary Ion Collider (RHIC). Chemical and kinetic equilibrium model fits to our data reveal strong radial flow and long duration from chemical to kinetic freeze-out in central Au+Au collisions. The chemical freeze-out temperature appears to be independent of initial conditions at RHIC energies.     

Variation of jet quenching from RHIC to LHC and thermal suppression of the QCD coupling constant

We perform a joint jet tomographic analysis of the data on the nuclear modification factor R _AA from PHENIX at RHIC and ALICE at LHC. The computations are performed accounting for radiative and collisional parton energy loss with running coupling constant. Our results show that the observed slow variation of R _AA from RHIC to LHC indicates that the QCD coupling constant is suppressed in the quark-gluon plasma produced at LHC.