Charged pion production in fixed-target Pb + Pb collisions at 158 GeV/nucleon

Changes in pion production as a function of the impact parameter of the collision or the incident energy, may reveal characteristics of a possible first-order phase transition from nuclear to quark matter, as predicted by lattice quantum chromodynamics. In this paper we investigate charged pion production in Pb+Pb collisions at 158 GeV/nucleon near 0° production angle and at forward rapidity (4:3 ≤ y ≤ 6:3). The centrality dependence of pion production is shown in the impact parameter range ∼2-12 fm at the rapidities y = 5:7 and 6.3. An enhancement in the π ^-/π⁺ ratio has been measured near beam rapidity, indicating Coulomb interaction of charged pions with the spectator protons. The charged pion yield per nucleon participating in the collision (N_p) at y = 5:7 increases faster than linearly with N_p, up to N_p ∼ 100 and then it saturates, while at y = 6:3 it does not exhibit any sudden change as a function of N_p.