During the first 10 microseconds following the Big Bang, the temperature of the universe was so high that ordinary hadrons such as protons and neutrons could not form. Instead, the dominant form of matter was unbound quarks and gluons in a state referred to as quark-gluon plasma. The extraordinarily high temperature of the epoch just after the Big Bang, approximately 2 x 10**12 K (200 MeV in energy units), is achievable today only via accelerator-based experiments colliding heavy nuclei at very high energy at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab. Analysis of available experimental data indicate that the energy density reached is more than 10**30 J/cm**3, exceeding that of ordinary nuclear matter by two orders of magnitude, and more than a factor of 10 above the threshold estimated for quark gluon plasma formation. However, there is as yet no experimental information whatsoever about the temperature reached by the plasma.
Measuring the temperature requires detecting thermal radiation emitted by the plasma. This radiation consists of photons also pairs of electrons and positrons from decays of virtual photons. Unfortunately, enormous backgrounds from decays of the thousands of other particles formed in the collision have prevented reliable measurement of the thermal radiation to date. This proposal aims to solve this problem by acquisition of a detector using novel technology to directly measure and reject the otherwise overwhelming background of electrons and positrons from those decays. The energy distribution of the remaining electron-positron pairs reflects the temperature of the quark gluon plasma created at RHIC. Furthermore, good background rejection will allow a search for evidence of restoration of chiral symmetry. As the universe cooled down from the quark gluon plasma state, condensation of the quarks spontaneously broke the chiral symmetry present at high temperature. This step gave rise to the masses of the particles in normal matter, which are large compared to the masses of the light quarks inside the particles. Extensive theoretical study of chiral symmetry predicts that the symmetry should be restored at the high temperatures reached by RHIC and that the properties of particles can be significantly different. We will measure the decays of rho, omega and phi particles into electron-positron pairs and look for evidence of changes in their masses and widths.
