Electromagnetic Probes at RHIC-II

Abstract

We summarize how future measurements of electromagnetic (e.m.) probes at the Relativistic Heavy Ion Collider (RHIC), in connection with theoretical analysis, can advance our understanding of strongly interacting matter at high energy densities and temperatures. After a brief survey of the important role that e.m. probes data have played at the Super Proton Synchrotron (SPS, CERN) and RHIC to date, we identify key physics objectives and observables that remain to be addressed to characterize the (strongly interacting) Quark-Gluon Plasma (sQGP) and associated transition properties at RHIC. These include medium modifications of vector mesons via low-mass dileptons, a temperature measurement of the hot phases via continuum radiation, as well as gamma-gamma correlations to characterize early source sizes. We outline strategies to establish microscopic matter and transition properties such as the number of degrees of freedom in the sQGP, the origin of the hadron masses and manifestations of chiral symmetry restoration, which will require accompanying but rather well-defined advances in theory. Increased experimental precision, order of magnitude higher statistics than currently achievable, as well as a detailed scan of colliding species and energies are then mandatory to achieve sufficient discrimination power in theoretical interpretations. This increased precision can be achieved with hardware upgrades to the large RHIC detectors (PHENIX and STAR) along with at least a factor of ten as increase in luminosity over the next few years as envisioned for RHIC-II.

72 pages, 42 figures

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• Low-momentum direct photon measurement in Cu$+$Cu collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV
• Spectral and transport properties from lattice QCD
• The low and intermediate mass dilepton and photon results
• Dielectron Azimuthal Anisotropy at mid-rapidity in Au+Au collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV
• Direct Photons in Hydrodynamic Modeling of Relativistic Nuclear Collisions
• The di-lepton physics program at STAR
• Perspectives of a Midrapidity Dimuon Program at RHIC: A Novel and Compact Muon Telescope Detector
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• Hydrodynamic model of heavy-ion collisions with low momentum components