Tapan Kumar Nayak

Centrality Dependence of Neutral Pion Production in 158A GeV

The production of neutral pions in 158AGeV Pb+Pb collisions has been studied in the WA98 experiment at the CERN SPS. Transverse momentum spectra are studied for the range 0.3 GeV/c<mT-m0<4.0 GeV/c. The results for central collisions are compared to various models. The centrality dependence of the neutral pion spectral shape and yield is investigated. An invariance of the spectral shape and a simple scaling of the yield with the number of participating nucleons is observed for centralities with greater than about 30 participating nucleons which is most naturally explained by assuming an equilibrated system.

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Ultra-Relativistic collisions of heavy nuclei have brought us within reach of creating and studying various aspects of bulk deconfined matter, known as quarkgluon plasma (QGP), which so far was believed to exist in the microsecond old universe or possibly in the cores of neutron or quark stars. We are at a very interesting situation in this area of research where the Super Proton Synchrotron (SPS) era has drawn to a close and the first results from the Relativistic Heavy Ion Collider (RHIC) have started to appear. Already from the results of the Pb run at the SPS quite a few of the signatures of QGP, e.g., J/Ψ suppression, strangeness enhancement etc., are reported to have “seen” unmistakable hints of the existence of QGP [1]. Electromagnetic probes, viz., photons and dileptons have long been recognized as the most direct probes of the collision [2–8]. It has also been emphasized that owing to the nature of their interaction they undergo minimal scatterings and are by far the best markers of the entire space-time evolution of the collision. The single photon data, obtained from Pb-Pb collisions at CERN SPS reported by the WA98 Collaboration [9] have been the focus of considerable interest in recent times [10–12]. In this work we will analyze this data in a detailed and realistic framework using a reasonable set of parameters. Let us first identify the possible sources of “excess” photons above those coming from the decays of pseudoscalar π and η mesons, as provided by the data. Firstly, one has the prompt photons coming from the hard collisions of initial state partons in the colliding nuclei. These populate the high transverse momentum region and can be estimated by perturbative QCD. The thermal contribution depends on the spacetime evolution scenario that one considers. In the event of a deconfinement phase transition, one first has a thermalized QGP which expands and cools, reverts back to hadronic matter, again expands and cools and eventually freezes out into hadrons which are mainly pions. Photon emission in the QGP occurs mainly due to QCD annihilation and Compton processes between quarks and gluons. In order to estimate the emission from the hadronic matter we will consider a gas of light mesons viz. π, ρ, ω and η. Reactions among these lead to the emission of photons. It has been emphasized that the properties of vector mesons may change appreciably because of interactions among the hadrons at high temperatures and/or densities (see the reviews [5,13–16]). This modifies the rate of photon emission as well as the equation of state of the evolving matter. Among various models for vector mesons available in the literature [5], we examine two possibilities for the hadronic phase in this paper: (i) no medium modifications of hadrons, and (ii) the scenario of the universal scaling hypothesis of the vector meson masses [14]. In principle, we can think of another scenario (iii) the large collisional broadening of the vector mesons [15]. Both (ii) and (iii) can reproduce the enhancement of the low-mass dileptons measured by CERES Collaboration at CERN SPS, but the scenario (iii) has been found to have a negligible effect on the emission rate for photons [5]. The effect of temperature dependent mass as described in case (ii) has also been incorporated in the equation of state (EOS) of the hadronic matter undergoing a (3+1) dimensional expansion. We will see that the resulting photon spectra reproduce the experimental data quite well. There is still substantial debate on the order of the phase transition as well as the value of the critical temperature. To address this aspect we will also consider a scenario where the system begins to evolve from a high temperature phase where all hadron masses approach zero (pion mass is fixed at its vacuum value). As the system expands and cools, the hadrons acquire masses as in case (ii) till freeze out. Incorporation of medium modified masses and the equation of state in this case also provides a reasonable explanation of the data. The paper is organized as follows. In the following section we describe the static photon emission rate and the equation state. In section 3 we discuss the results and finally in section 4 we present summary and discussions.

Pb+

The variation of average transverse mass of identified hadrons with charge multiplicity have been studied for AGS, SPS and RHIC energies. The observation of a plateau in the average transverse mass for multiplicities corresponding to SPS energies is attributed to the formation of a co-existence phase of quark gluon plasma and hadrons. A subsequent rise for RHIC energies may indicate a deconfined phase in the initial state. Several possibilities which can affect the average transverse mass are discussed. Constraints on the initial temperature and thermalization time have been put from the various experimental data available at SPS energies.

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Abstract Two-particle correlations offer the best chance to view the space-time evolution of relativistic heavy-ion collisions. After reviewing motivations for measuring two-particle correlation functions, predictions of two-particle correlations using full-event simulations are compared to experimental results. Several conclusions regarding space-time evolution of a reaction are reached.

Pb Collisions

Abstract We present the STAR preliminary results on mid-rapidity and low transverse momentum mean, standard deviation, skewness, and kurtosis of net-charge and net-proton distributions in Au+Au and Cu+Cu collisions at s N N = 200 GeV for various collision centralities. All the measured high moments of these distributions can be scaled by the number of participating nucleons, consistent with the soft process emissions. The ratios of fourth to second order cumulants of both the net-charge and net-proton distributions are consistent with models without QCD critical point.

Photons from Pb-Pb collisions at ultrarelativistic energies

Two-pion correlation functions are analyzed at mid-rapidity for three systems (14.6 A-GeV Si+Al, Si+Au, and 11.6 A-GeV Au+Au), seven distinct centrality conditions, and different kT bins in the range 0.1--0.5 GeV/c. Source reference frames are determined from fits to the Yano-Koonin source parameterization. Bertsch-Pratt radius parameters are shown to scale linearly with both number of projectile and total participants as obtained from a Glauber model calculation. A finite emission duration that increases linearly with system/centrality is also reported. The mT dependence of the Bertsch-Pratt radii is measured for the central Si+Au and Au+Au systems. The system/centrality dependence is investigated separately for both high and low mT regions.

Indication of a coexisting phase of quarks and hadrons in nucleus-nucleus collisions

Abstract Recent single-particle inclusive measurements and two-particle correlation data from E802 and its succesor, E859, are presented. The K + / π + ratio observed for collisions of 14.6 A·GeV 28 Si ions on various targets is presented as a function of both forward and transverse energy. Source sizes are determined via interferometry using π + , π − , K + and proton pairs.

Testing transport theories with correlation measurements

Abstract Details concerning the design, fabrication and performance of STAR Photon Multiplicity Detector (PMD) are presented. The PMD will cover the forward region, within the pseudorapidity range 2.3–3.5, behind the forward time projection chamber. It will measure the spatial distribution of photons in order to study collective flow, fluctuation and chiral symmetry restoration.

Study of the Fluctuations of Net-charge and Net-protons Using Higher Order Moments

Abstract We have studied the timing and light yield characteristics of long and thin scintillator samples of various sizes. Both the light yield and the time resolution degrade exponentially with respect to the distance from one end; the inverse slope parameters for the time resolutions are found to be twice those of the light yield. This is interpreted in terms of a photoelectron statistics picture, which can also explain the observed position dependence of time resolution for time-of-flight counters.

System, centrality, and transverse mass dependence of two-pion correlation radii in heavy ion collisions at 11.6A and 14.6A GeV/c

Lattice QCD models predict the presence of a critical point in the QCD phase diagram where the first order phase transition between the hadron gas and Quark-Gluon Plasma ceases to exist. Higher moments of conserved quantities, such as net–charge, net–baryon number and net–strangeness, are proposed to be sensitive probes for locating the critical point. The moments of net–charge distributions have been studied as a function of centrality for Au+Au collisions at √