Two-particle correlation functions for positive and negative pions have been measured in Au+Au collisions at 10.8~GeV/c per nucleon. The data were analyzed using one- and three-dimensional correlation functions. From the results of the three-dimensional fit the phase space density of pions was calculated. It is consistent with local thermal equilibrium.
The combination of the ionization energy loss (dE/dx) from Time Projection Chamber (TPC) at $simeq8$% resolution and multi-gap resistive plate chamber time-of-flight (TOF) at 85$ps$ provides powerful particle identification. We present spectra of ide
ntified charged pions from transverse momentum $p_Tsimeq0.2$ GeV/c to ~7-8 GeV/c in Au+Au collisions at $sqrt{s_{_{NN}}}=62.4$ GeV. Physics implications will be discussed.
We present the combined results on electron-pair production in 158 GeV/n {Pb-Au} ($sqrt{s}$= 17.2 GeV) collisions taken at the CERN SPS in 1995 and 1996, and give a detailed account of the data analysis. The enhancement over the reference of neutral
meson decays amounts to a factor of 2.31$pm0.19 (stat.)pm0.55 (syst.)pm0.69 (decays)$ for semi-central collisions (28% $sigma/sigma_{geo}$) when yields are integrated over $m>$ 200 MeV/$c^2$ in invariant mass. The measured yield, its stronger-than-linear scaling with $N_{ch}$, and the dominance of low pair $p_t$ strongly suggest an interpretation as {it thermal radiation} from pion annihilation in the hadronic fireball. The shape of the excess centring at $mapprox$ 500 MeV/$c^2$, however, cannot be described without strong medium modifications of the $rho$ meson. The results are put into perspective by comparison to predictions from Brown-Rho scaling governed by chiral symmetry restoration, and from the spectral-function many-body treatment in which the approach to the phase boundary is less explicit.
We present a detailed measurement of charged two-pion correlation functions in 0%-30% centrality $sqrt{s_{_{NN}}}=200$ GeV Au$+$Au collisions by the PHENIX experiment at the Relativistic Heavy Ion Collider. The data are well described by Bose-Einstei
n correlation functions stemming from Levy-stable source distributions. Using a fine transverse momentum binning, we extract the correlation strength parameter $lambda$, the Levy index of stability $alpha$ and the Levy length scale parameter $R$ as a function of average transverse mass of the pair $m_T$. We find that the positively and the negatively charged pion pairs yield consistent results, and their correlation functions are represented, within uncertainties, by the same Levy-stable source functions. The $lambda(m_T)$ measurements indicate a decrease of the strength of the correlations at low $m_T$. The Levy length scale parameter $R(m_T)$ decreases with increasing $m_T$, following a hydrodynamically predicted type of scaling behavior. The values of the Levy index of stability $alpha$ are found to be significantly lower than the Gaussian case of $alpha=2$, but also significantly larger than the conjectured value that may characterize the critical point of a second-order quark-hadron phase transition.
We investigate identical pion HBT intensity interferometry for central Au+Au collisions at 1.23A GeV. High-statistics $pi^-pi^-$ and $pi^+pi^+$ data are measured with HADES at SIS18/GSI. The radius parameters, derived from the correlation function de
pending on relative momenta in the longitudinal-comoving system and parametrized as three-dimensional Gaussian distribution, are studied as function of transverse momentum. A substantial charge-sign difference of the source radii is found, particularly pronounced at low transverse momentum. The extracted Coulomb-corrected source parameters agree well with a smooth extrapolation of the center-of-mass energy dependence established at higher energies, extending the corresponding excitation functions down towards a very low energy. Our data would thus rather disfavour any strong energy dependence of the radius parameters in the low energy region.
We present results from Experiment E917 for antilambda and antiproton production in Au+Au collisions at 11.7 A GeV. We have measured invariant spectra and yields for both species in central and peripheral collisions. We find that the antilambda/antip
roton ratio near mid-rapidity increases from 0.26+0.19-0.15 in peripheral collisions to 3.6+4.7-1.8 in central collisions, a value that is substantially larger than current theoretical estimates.