Possible explanation of the irregular energy dependence of the rapidity width of $\phi$ mesons observed in Pb+Pb collisions

TL;DR

The paper explains the steep increase in the rapidity width of $$ mesons observed in Pb+Pb collisions as a result of detector acceptance and analysis methods, rather than new physics.

Contribution

It demonstrates that the observed rapidity width increase can be reproduced by the UrQMD model and is due to experimental limitations, not novel phenomena.

Findings

UrQMD reproduces rapidity distribution shapes of most hadrons.

The apparent steep rise in $$ width is due to limited detector acceptance.

Analysis method influences the extracted rapidity width values.

Abstract

Experimental data from the NA49 collaboration show an unexpectedly steep rise of the rapidity width of the $\phi$ meson as function of beam energy, which was suggested as possible interesting signal for novel physics. In this work we show that the Ultra-relativistic Quantum-Molecular-Dynamics (UrQMD) model is able to reproduce the shapes of the rapidity distributions of most measured hadrons and predicts a common linear increase of the width for all hadrons. Only when following the exact same analysis technique and experimental acceptance of the NA49 and NA61/SHINE collaborations, we find that the extracted value of the rapidity width of the $\phi$ increases drastically for the highest beam energy. We conclude that the observed steep increase of the $\phi$ rapidity width is a problem of limited detector acceptance and the simplified Gaussian fit approximation.