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Register a new userResults on angular distributions of thermal dileptons in nuclear collisions
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Gianluca Usai
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The NA60 experiment at the CERN SPS has studied dimuon production in 158 AGeV In-In collisions. The strong pair excess above the known sources found in the mass region $0.2<M<2.5$ GeV has been previously interpreted as thermal radiation. In this paper results on the associated angular distributions for $M<1$ GeV, as measured in the Collins-Soper reference frame, are presented. The structure function parameters $lambda$, $mu$, $ u$ are consistent with zero and the projected polar and azimuth angle distributions are uniform. The absence of any polarization is consistent with the interpretation of the excess dimuons as thermal radiation from a randomized system.
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The NA60 experiment at the CERN SPS has studied dimuon production in 158A GeV In-In collisions. The strong excess of pairs above the known sources found in the complete mass region 0.2<M<2.6 GeV has previously been interpreted as thermal radiation. We now present first results on the associated angular distributions. Using the Collins-Soper reference frame, the structure function parameters lambda, mu and u are measured to be zero, and the projected distributions in polar and azimuth angles are found to be uniform. The absence of any polarization is consistent with the interpretation of the excess dimuons as thermal radiation from a randomized system.
We calculate the transverse momentum and invariant mass dependence of elliptic flow of thermal dileptons for Au+Au collisions at the Relativistic Heavy Ion Collider. The system is described using hydrodynamics, with the assumption of formation of a thermalized quark-gluon plasma at some early time, followed by cooling through expansion, hadronization and decoupling. Dileptons are emitted throughout the expansion history: by annihilation of quarks and anti-quarks inthe early quark-gluon plasma stage and through a set of hadronic reactions during the late hadronic stage. The resulting differential elliptic flow exhibits a rich structure, with different dilepton mass windows providing access to different stages of the expansion history. Elliptic flow measurements for dileptons,combined with those of hadrons and direct photons, are a powerful tool for mapping the time-evolution of heavy-ion collisions.
Most recent PHENIX results on electromagnetic probes are presented including first preliminary results obtained with the Hadron Blind Detector (HBD) on e+e- invariant mass spectra from Au+Au collisions at sqrt(s_NN) = 200 GeV.
We present a subset of experimental results on charge fluctuation from the heavy-ion collisions to search for phase transition and location of critical point in the QCD phase diagram. Measurements from the heavy-ion experiments at the SPS and RHIC energies observe that total charge fluctuations increase from central to peripheral collisions. The net-charge fluctuations in terms of dynamical fluctuation measure $ u_{(+-,dyn)}$ are studied as a function of collision energy (sqsn) and centrality of the collisions. The product of $ u_{(+-,dyn)}$ and $langle N_{ch} rangle$ shows a monotonic decrease with collision energies, which indicates that at LHC energy the fluctuations have their origin in the QGP phase. The fluctuations in terms of higher moments of net-proton, net-electric charge and net-kaon have been measured for various sqsn. Deviations are observed in both $Ssigma$ and $kappasigma^2$ for net-proton multiplicity distributions from the Skellam and hadron resonance gas model for sqsn $<$ 39 GeV. Higher moment results of the net-electric charge and net-kaon do not observe any significant non-monotonic behavior as a function of collision energy. We also discuss the extraction of the freeze-out parameters using particle ratios and experimentally measured higher moments of net-charge fluctuations. The extracted freeze-out parameters from experimentally measured moments and lattice calculations, are found to be in agreement with the results obtained from the fit of particle ratios to the thermal model calculations.
This paper presents the studies on the influence of the target material impurities on physical observables registered in heavy ion collisions collected by fixed target experiments. It mainly concerns the measures of multiplicity fluctuations which can be used to searches for critical point of strongly interacting matter, e.g. in the {NA61/SHINE} fixed-target experiment at CERN SPS. The elemental composition of the targets used in the NA61/SHINE experiment was determined applying wavelength dispersive X-ray fluorescence (WDXRF) technique. The influence of measured target impurities on multiplicity distributions and scaled variance was estimated using simulation events. The modification of the standard analysis was proposed to reduce this influence.
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