Protons and deuterons from Al+Al collisions at 1.93A GeV measured by FOPI∗

Full mapping of phase space of particles emitted from the final state of heavy-ion collisions at beam energies around 2A GeV has not yet been accomplished. This paper reports the investigation of protons and deuterons emitted from central collisions of light symmetric system of Al+Al at 1.93A GeV, done by FOPI. These particles, being the most abundant subsample of Light Charged Particles (LCP, Z≤2), amount to a bulk of nuclear matter participating in the collision. Analysis of proton and deuteron phase space helps sheding light on a degree of nuclear stopping [1], and possible deviations from the ideal state of thermal equilibrium. The only published thermal model prediction for this colliding system [2] is based on an array of four yield ratios, limiting the predictive power to a coarse suggestion. Establishing the ratio of deuteron to proton yields enhances this array, which could be further on compared to the predictions of thermal models as well as the non-equilibrium transport model UrQMD [3]. The S279 experiment was carried out in August 2005 at the SIS-18. The Al ions were hitting the Al target, shifted 40 cm upstream from its nominal position. The CDC chamber was surrounded by the Plastic Barrel (PLB) in a version covering the polar angles between 27◦ and 57◦ with respect to the target position. Out of 290·106 recorded events in total, samples of 9.8·106 (29.5·106) events were selected for the analysis of protons (deuterons). For each sample, events corresponding to 20% of most central collisions were selected. Particles were identified using standard FOPI PID techniques [4] and analysed on the pt − y (rapidity) plane. For CDC tracks matching the PLB hits, the information from both detectors was used, and for the particles emitted at polar angles larger that 57◦, only the CDC information was available. The compensation of losses due to deficiency was done in two steps: