On the Implications of Energy and Momentum Conservation for Particle Emission in A+A Collisions at SPS Energies

TL;DR

This paper presents a simple, impact-parameter-local model for heavy ion collisions at SPS energies, emphasizing energy and momentum conservation, and explains pion distribution broadening and longitudinal evolution in Pb+Pb collisions.

Contribution

It introduces a new impact-parameter-local 'fire-streak' model that incorporates energy and momentum conservation to describe particle production at SPS energies.

Findings

Successfully describes pion rapidity distribution broadening from central to peripheral collisions.

Provides a space-time picture of longitudinal evolution dependent on impact parameter.

Aligns with electromagnetic effects on pion flow and explains low-$p_T$ phenomena in fragmentation regions.

Abstract

We construct a simple model of heavy ion collisions, local in the impact parameter plane, and appropriate for the SPS energy range. This model can be regarded as a new realization of the "fire-streak" approach, originally applied to studies of lower energy nucleus-nucleus reactions. Starting from local energy and momentum conservation, we nicely describe the broadening of the pion rapidity distribution when going from central to peripheral Pb+Pb collisions at $\sqrt{s_{NN}}$=17.3 GeV. The results of our calculations are compared with SPS experimental data. We discuss the resulting implications on the role of energy and momentum conservation for the dynamics of particle production in heavy ion collisions. A specific space-time picture emerges, where the longitudinal evolution of the system strongly depends on the position in the impact parameter (${b_x}$, ${b_y}$) plane. This picture is consistent with our earlier findings on the longitudinal evolution of the system as deduced from electromagnetic effects on charged pion directed flow, and can provide an explanation for specific low-$p_T$ phenomena seen in the fragmentation region of Pb+Pb collisions.