Andrzej Rybicki
IFJ PAN Kraków, POLAND
Abstract
Precise experimental data on identified charged pion production have been obtained for peripheral Pb+Pb collisions at $\sqrt{s_{NN}}=17.3$ GeV. These double differential data points cover a very wide range both in pion rapidity $y$ and in transverse momentum $p_T$, extending from the central "mid-rapidity" region up to and beyond beam rapidity. Specifically, the region corresponding to the fragmentation of the "spectator system" - the nuclear remnant surviving the collision - is also covered by the present measurement, starting from $p_T = 0$.
A conglomerate of different phenomena becomes apparent from this double differential study of charged pion ($\pi^+/\pi^-$) ratios. Strong Coulomb effects are evidently present near spectator rapidity, bringing new information on the space-time scenario of pion emission as demonstrated in our earlier works including also pion directed flow [1, 2]. What is more, $p_T$-integrated $\pi^+/\pi^-$ ratios deviate from simple predictions based on the $p/n$ content of the Pb nucleus [3] suggesting additional sources of pion emission presumably originating from the spectator system. To support this, a very steep peak in the rapidity dependence of the ratio of pion production in Pb+Pb with respect to p+p collisions, $\frac{Pb+Pb}{p+p}$, suggests a powerful amplification of pion production at spectator rapidity [4].
The aim of this talk is to discuss these experimental data in view of their possible application as a new source of information on the nuclear EoS and as a tool for testing the existing transport models. We also plan to address the future perspectives in view of our new research programme involving also the NA61/SHINE experiment at the CERN SPS.