Electromagnetic Probes at Rhic-Ii

Abstract

We summarize how future measurements of electromagnetic (EM) 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 EM 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 γ-γ 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 hadron masses and manifestations of chiral symmetry restoration, which will require accompanying but rather well-defined advances in theory. Increased experimental precision, an order of magnitude higher statistics than currently achievable, as well as a detailed scan of colliding species and energies are mandatory to discriminate between theoretical interpretations. This increased precision can be achieved through hardware upgrades to the large RHIC detectors (PHENIX and STAR) along with at least a factor of ten increase in luminosity over the next few years, as envisioned for RHIC-II.

Direct real photons in relativistic heavy ion collisions

“…The ∆Q inv region where the correlation is enhanced (width of the Gaussian) is inversely proportional to the source size R (∆Q inv ∼h/R). That means that HBT at very low ∆Q inv can in principle differentiate between photons from the collision itself (length scale is fm, ∆Q inv ∼h/f m ∼100 MeV) and final state hadron decay (length scale is nm, ∆Q inv ∼h/f m ∼100 eV) 16 . Furthermore, if f is the fraction of photons from a particular source, the λ strength parameter of the respective correlation is proportional to f 2 [110].…”

“…The first results on inclusive photon production and its comparison to the expected hadron decay yield in p+A and A+A collisions were published by the HELIOS/NA34 collaboration [262]. Protons, 16 O and 32 S beams were used on W and Pt targets, and photons were measured with the (external) conversion technique (Sec. III B).…”

“…The idea that photons can test the most diverse manifestations of the strong interaction took hold. The rapid growth of the field has been documented in several reviews in the past (see for instance [11][12][13][14][15][16][17]). By now, photon physics became mature, but by no means a closed chapter -just the opposite.This review was written with the following goals in mind.…”

Relativistic Heavy-Ion Collider (RHIC) physics overview

“…With several detector upgrades, heavy flavor collectivity and energy loss, color screening effects, QGP thermal radiation [115][116][117] and jet quenching will be studied with better precision at RHIC.…”

“…The dilepton spectra at intermediate mass range are directly related to thermal radiation of the QGP [115,116]. At low mass range, we can study the vector meson inmedium properties through their dilepton decays, the observable of possible chiral symmetry restoration.…”