Pion spectra and the geometry of nuclear collisions

The ability of low-energy charged pion spectra to probe the space-time evolution of high-energy nucleus-nucleus collisions is investigated in a classical picture. The time-dependent nuclear charge density is calculated with a Monte Carlo model of the collision. For the mass-symmetric systems studied here, it is describable by a sum of three expanding gaussian functions, corresponding to one participant and two spectator components. Characteristic expansion velocities are ∼0.4c and ∼0.16c for the participants and spectators, respectively, and the participant charge for central collisions is less than that expected from a clean-cut geometry. For pions emitted in both thermal and direct processes, we calculate the electromagnetic distortion due to the time-dependent nuclear charge distribution by solving classical equations of motion. Many features of the available π-inclusive data can be understood in this model; in particular, the 0π π^- π⁺ ratio for Ne + NaF collisions is very well reproduced and is insensitive to the details of pion source. Mid-rapidity spectra for p_⊥ ≲ m_πc are calculated for several types of sources. The magnitude of the electromagnetic distortion of the protons emitted in Ne + U collisions is estimated.