Study the Low Mass Dielectron Production in Relativistic Heavy-Ion
Collisions at RHIC-STAR
Principal Investigator: Wei Xie
The objective of this proposal is to measure the enhancement of the dielectron production in
the low mass region (mee < 1.2 GeV/c2) in heavy-ion collisions and find out where the
enhancement starts to appear as a function of the collision energies using the STAR detector at
the Relativistic Heavy Ion Collider (RHIC). Dielectron production is not affected by strong
interaction and is therefore considered as an ideal probe to study the chiral symmetry restoration
and the properties of the QCD medium during its spacetime evolution. The research will include
measuring dielectron yield in Au+Au collisions at √𝑠𝑛𝑛 = 200 𝐺𝑒𝑉 and lower energies.
At very high temperature (T > Tc, where Tc ~ 170MeV), quarks and gluons are expected to
no longer be confined inside the hadrons and form a color-charged plasma, i.e. “Quark Gluon
Plasma” (QGP). At around the same time when the phase transition from hadron gas to QGP
happens, chiral asymmetry is expected to be restored as a result of the significant reduction of
quark condensate masses or broadening of the vector mass spectra. Dielectron is one of the ideal
probes to study QGP and the chiral symmetry restoration. Compared to hadrons, electrons have
little interaction with the medium and can therefore travel through the medium while keeping
most of the original information untouched. This feature enables us to study the chiral symmetry
restoration as well as the properties of the medium during space-time evolution of the system.
Large enhancement of the low mass dilepton production in heavy-ion collisions was first
discovered by the CERES and HELIOS/3 experiments at the CERN Super Proton Synchrotron
(SPS) and was attributed mainly to the in-medium modification of the ρ meson production in the
hadron gas due to the chrial symmetry restoration. This understanding was further strengthened
by the new SPS measurements, especially the high precision measurements of dimuon
production in In+In collisions at √𝑠𝑛𝑛 = ~20 𝐺𝑒𝑉 from NA60 experiment. At the Relativist
Heavy-ion Collider (RHIC), with about ten times higher collision energy, the measurements of
dielectron production in √𝑠𝑛𝑛 = 200 𝐺𝑒𝑉 Au+Au collision from PHENIX shows a large
enhancement in the broad region of mee = 0.15-0.75 GeV/c2. Unlike the results from CERN SPS,
this enhancement seems to come from the QGP thermal radiation. The result, however, cannot be
understood by any available theoretical models since the QGP thermal radiation is expected to
dominate the region of 1.2<mee<2.9GeV/c2 instead.
Since the large enhancement at RHIC is not observed at SPS, it would be essential to find
out where it starts to appear as a function of collision energies. To reach this goal, we propose to
study low mass dielectron production using the STAR detector at RHIC at √𝑠𝑛𝑛 = 20 −
200 𝐺𝑒𝑉, i.e. between the SPS and RHIC peak collision energies. In the mean time, we plan to
carry out the same study at √𝑠𝑛𝑛 = 200 𝐺𝑒𝑉 to cross check PHENIX result. We expect that the
outcome of these studies will provide crucial input to the understanding of QGP and the chiral
symmetry restoration.