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

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 may be a problem of limited detector acceptance and the simplified Gaussian fit approximation.