In-Medium Modifications of Low-Mass Vector Mesons in PHENIX at RHIC

Abstract

Measurements at RHIC have established the creation of a Quark Gluon Plasma (QGP) in most central heavy-ion collisions. An important tool to understand properties of the QGP is study of the spectral shapes of low-mass vector mesons (LVM's), ρ, ω and ϕ, which can be modified in the medium by partial restoration of chiral symmetry. This modification may be accessed directly by measuring low-momentum LVM's via their decays into lepton pairs inside the hot matter. Since leptons are not subject to the strong interaction, they do not rescatter on their way out of the medium. The PHENIX experiment at RHIC has measured LVM production at mid-rapidity in p+ p, d+Au and Au+Au collisions at s N N = 200 GeV . Mass peaks for the LVM's have been observed in the di-electron invariant mass spectra with a resolution of 10 MeV/ c 2 in all of the three collision systems. The extracted spectra, mass and width of ω and ϕ in p+ p, d+Au and Au+Au, in the leptonic and hadronic decay channels are reviewed. As the widths of the mesons may be affected in the medium, the branching ratios of various decay modes may also be modified from the values in vacuum. The relative branching ratio is compared between ϕ → e + e − and ϕ → K + K − , which may be sensitive to the mass modification due to the small Q-value of ϕ → K + K − .