Eur. Phys. J. A (2012) 48: 100
https://doi.org/10.1140/epja/i2012-12100-0

Regular Article - Theoretical Physics

Extraction of the sound velocity from rapidity spectra: Evidence for QGP formation at FAIR/RHIC-BES energies

¹ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, 94720, Berkeley, CA, USA
² Frankfurt Institute for Advanced Studies (FIAS), Ruth-Moufang-Str. 1, D-60438, Frankfurt am Main, Germany
³ Institut für Theoretische Physik, Goethe Universität Frankfurt, Max-von-Laue Str. 1, D-60438, Frankfurt Main, Germany

^* e-mail: jsfroschauer@lbl.gov

Received: 20 April 2012
Accepted: 11 July 2012
Published online: 26 July 2012

Abstract

We analyze longitudinal pion spectra from GeV to GeV within Landau’s hydrodynamical model and the UrQMD hybrid approach. From the measured data on the widths of the pion rapidity spectra, we extract the sound velocity c _s ² in the dense stage of the reactions. It is found that the sound velocity has a local minimum (indicating the softest point in the equation of state, EoS) at GeV, an energy range accessible at the Facility for Antiproton and Ion Research (FAIR) as well as the RHIC-Beam Energy Scan (RHIC-BES). This softening of the EoS is compatible with the formation of a QGP at the onset of deconfinement. The extracted sound velocities are then used to calculate an excitation function for the mean transverse mass of pions from the hybrid model. We find that, above GeV, even the lowest c _s ² gives a considerably larger of pions compared to data.