Impact parameter dependence of K±, p, pbar, d and dbar production in fixed-target Pb + Pb collisions at 158 GeV/nucleon

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Abstract
Nuclear matter is expected to undergo a phase transition to quark matter in ultrarelativistic heavy-ion collisions, possibly showing up as a discontinuity in the impact parameter dependence of relevant observables. Following this expectation, we have investigated the impact parameter dependence of the invariant yields of K±, p, d, and in the range ~ 2 - 12 fm in fixed-target Pb+Pb collisions at 158 GeV/nucleon incident energy at the CERN SPS. The particles have been measured near zero transverse momentum and in the rapidity range y = 3.1 - 4.4. In addition, the centrality dependence of the baryon chemical potential, the effective temperature and the size of the particle emitting source at freeze-out were studied. No dramatic change in the distribution of any of these variables is observed as a function of the impact parameter. The same is found for the particle yields, with the exception of the yield of charged kaons per number of nucleons participating in the collision (Np), where there is an indication of a threshold behaviour at Np~80.