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

Abstract

Experimental data from the NA49 collaboration show an unexpectedly steep rise of the rapidity width of the $\varphi$ 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 $\varphi$ increases drastically for the highest beam energy. We conclude that the observed steep increase of the $\varphi$ rapidity width is a problem of limited detector acceptance and the simplified Gaussian fit approximation.

Comments: 5 pages, 4 figures