Can pion spectra in Pb+Pb collisions at CERN SPS energies be described by those in p+p reactions and energy-momentum conservation ? Rapidity distributions of pions in p+p and Pb+Pb collisions at CERN SPS energies

TL;DR

This paper demonstrates that pion rapidity distributions in Pb+Pb collisions at CERN SPS energies can be explained by local energy-momentum conservation applied to fire-streaks, aligning with p+p collision data without tuning.

Contribution

The study introduces a model based on local energy-momentum conservation in fire-streaks that reproduces pion rapidity distributions in heavy ion collisions without parameter tuning.

Findings

Pion rapidity distributions in Pb+Pb match p+p data when modeled with fire-streak energy-momentum conservation.

Differences in normalization are due to baryon stopping and strangeness enhancement effects.

The model explains the shape of distributions without adjusting to experimental data.

Abstract

The centrality dependence of rapidity distributions of pions in Pb+Pb reactions can be understood by imposing local energy-momentum conservation in the longitudinal "fire-streaks" of excited matter. With no tuning nor adjustment to the experimental data, the rapidity distribution of pions produced by the fire-streak which we obtained from Pb+Pb collisions reproduces the shape of the experimental pion rapidity distribution in p+p interactions, measured by the NA49 Collaboration at the same energy. The observed difference in the absolute normalization of this distribution can be explained by the difference in the overall energy balance, induced by baryon stopping and strangeness enhancement phenomena occurring in heavy ion collisions. We estimate the latter effects using a collection of SPS experimental data on $\pi^\pm$, $K^\pm$, net $p$, and $n$ production in p+p and Pb+Pb reactions. Implications of the above findings are discussed.