Physics, Yale
STAR, Yale
STAR, BNL
RHIC
BNL
Jefferson
Laboratory
LBNL
Sigma(1385)
Pentaquarks
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The Relativistic Heavy Ion Collider (RHIC) is located at Brookhaven National Laboratory (BNL) on Long Island, NY, USA. The circumference of the two independent accelerator rings is 3.8 km. The first collisions occurred in the year 2000 and since then collisions of p+p, d+Au, Au+Au, and Cu+Cu at squareroot(sNN)=19, 62, 130, 200 GeV were performed. There are 6 beam interaction points at RHIC and 4 dedicated heavy-ion experiments. BRAHMS and PHOBOS are the two small experiments and the two large ones are PHENIX and STAR.
The Solenoidal Tracker At RHIC (STAR) is a large acceptance detector system that is designed to investigate the strongly interacting matter at high energy densities and search for signatures of Quark-Gluon Plasma (QGP), the hadronic deconfinement phase consisting of "free" partons, and its space time evolution. The physics program of STAR also includes the study of nucleon spin structure functions with polarized p+p collisions, the study of pomeron and photon interactions from intense electromagnetic fields of the colliding ions at RHIC and the study of initial parton distribution functions of the incident nuclei with p+p and d+Au collisions. The p+p and d+Au collisions are also essential to establish the reference data for the heavy ion collisions.
During the
expansion of the hot and dense matter (fireball) created in
heavy ion collisions, chemical freeze-out is reached when the hadrons
stop interacting inelastically. Elastic interactions
continue until thermal freeze-out. Due to the very short lifetime
(tau < tau_{fireball}~10 fm) of most resonances, a large fraction of
their decays occur before the thermal freeze-out. The elastic
interactions of decay products with the surrounding particles until the
thermal freeze-out, result in a signal loss for the reconstructed
resonances. However, secondary interactions (regeneration)
increase the resonance yield (such as Lambda +pi-->Sigma (1385)).
The contribution of re-scattering and regeneration to the total
observed yields depends on the time span between the chemical and
thermal freeze-out, the lifetime of each resonance, and the
regeneration and rescattering probabilities. Thus the study of
resonances provides an additional tool in the determination of the
hadronic expansion time between chemical and thermal freeze-out by
comparing resonance to stable particle ratios.