Resonance particles in heavy-ion collisions

Wada, Masayuki
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Heavy ions are collided at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) in an effort to create a unique state of nuclear matter, where quarks and gluons can freely move over volumes larger than the typical size of a nucleon (typical scale of Quantum Chromodynamics, QCD). In this state, called a "Quark Gluon Plasma" (QGP), it is proposed Chiral symmetry is restored. The fact that Chiral symmetry is a symmetry of the Standard model and is broken at low energy (current energy scale of universe) makes the study of its possible very interesting. The analysis in this dissertation searches for signatures of chiral symmetry restoration at the phase transition between the QGP and the hadronic gas phase by using resonance particles as probes. Resonances may decay inside of hot dense matter due to their short lifetimes, and therefore their decay daughters carry away dynamical information such as the mass and decay width. Mass shift and width broadening are predicted signatures of chiral symmetry restoration. The phi resonances reconstructed from the dielectron decay channel are investigated in this dissertation. This decay channel does not suffer scattering from the late hadronic medium due to the relatively small interaction cross section of leptons with hadrons. The disadvantage of this channel comes from the small branching ratio. Therefore, large statistics and clean Particle IDentification (PID) are necessary for this analysis. Those requirements were fulfilled with high luminosity beams at RHIC and the newly developed and installed Time Of Flight (TOF) detectors, which provide clear particle identification up to momentum of 2-3 GeV/c, as well as the large acceptance of the Solenoidal Tracker At RHIC (STAR) detector. In this dissertation, measurements of mass, width, transverse momentum spectrum, and yields of [phi][right arrow] e⁺e⁻ at mid-rapidity [mathematical symbols] from the STAR experiment in Au+Au collisions at [mathematical symbols]=200 GeV are presented and compared to a previously measured [phi] meson result from a hadronic decay channel. The possibility of medium modification which implies Chiral symmetry restoration is discussed.