Publication : t17/132

Abstract:
We show that in a central nucleus-nucleus collision, the variation of the mean transverse mass with the multiplicity is determined, up to a rescaling, by the variation of the energy over entropy ratio as a function of the entropy density, thus providing a direct link between experimental data and the equation of state. Each colliding energy thus probes the equation of state at an effective entropy density, whose approximate value is $19$~fm$^{-3}$ for Au+Au collisions at 200~GeV and $41$~fm$^{-3}$ for Pb+Pb collisions at 2.76~TeV, corresponding to temperatures of $227$~MeV and $279$~MeV if the equation of state is taken from lattice calculations. The relative change of the mean transverse mass as a function of the colliding energy gives a direct measure of the pressure over energy density ratio $P/epsilon$, at the corresponding effective density. Using RHIC and LHC data, we obtain $P/epsilon=0.21pm 0.10$, in agreement with the lattice value $P/epsilon=0.23$ in the corresponding temperature range. Measurements over a wide range of colliding energies using a single detector with good particle identification would help reducing the error.

publi.pdf