Why the dilepton temperatures at the relativistic heavy ion colliders are constant, T ~ 290 MeV?

Abstract

The STAR collaboration at RHIC (BNL) and the ALICE collaboration at LHC (CERN) published recently dielectron ($e^+e^-$ pair) spectra in the intermediate mass region (IMR), $M_{e^+e^-}$ = (1-3) GeV, which show a constant, i.e. energy-independent, emission temperature $T_{IMR}\simeq$ 287+-27 MeV, at all bombarding energies $\sqrt{s_{NN}}$ from 27 to 200 GeV. What causes this strange 'Thermostat' behaviour? Why the temperature is so small and constant, although the bombarding energy is increased by orders of magnitude, so the early temperatures of the created parton plasma ought to rise?

Figures (1)

• Figure 1: The five experimentally measured thermal temperatures of IMR dielectrons are shown. Here IMR denotes the intermediate mass region, $1~\textrm{GeV}<M_{IMR}<3~\textrm{GeV}$. For clarity, the STAR 2024 point is shifted to the right from its actual position by 25 GeV along the horizontal axis. The observed temperatures are constant within $\sim 10\%$ for all nuclear collisions' experimental data at the various bombarding energies at BNL and CERN. The theoretically determined temperature of the deconfinement phase transition of pure gluon matter (see text) is shown by the shaded horizontal band. The dashed line marks the temperature, obtained by arithmetic averaging of the four experimentally determined dilepton temperatures from STAR (RHIC). The arrow indicates that only the upper temperature limit can be extracted from the ALICE experiment at CERN.