Impact of Baryon anti-Baryon annihilation on hyperon ($\Lambda$, $\bar\Lambda$) production and apparent strangeness enhancement in $\bar\Lambda/\bar{p}$ in heavy ion collisions at SPS energy

Abstract

A deconfined medium of quarks and gluon, called the Quark-Gluon Plasma (QGP) is produced when heavy-nuclei are collided at relativistic energies. The QGP formation is often characterized by a phenomenon called strangeness enhancement where, the relative production of strange-to-non-strange particles are enhanced in central collisions compared to peripheral or proton-proton interactions. Besides the enhancement in K/$\pi$ ratios, a non-monotonic energy dependence was also reported for $\bar{\Lambda}$ to $\bar{p}$ ratios at CERN SPS, attributed to a signature for the strangeness enhancement as well. As anti-particles are produced directly from the reaction, the $\bar{\Lambda}$/$\bar{p}$ ratios are considered as a cleaner probe for the strangeness enhancement. However, at this energy range hadronic interactions have a dominant role to play and, importantly for $\bar{\Lambda}$ and $\bar{p}$, processes like baryon-anti-baryon ($\mathrm{B\bar{B}}$) annihilation can have a significant impact. In this work, we use a hadronic transport model UrQMD, to investigate the role of baryon-anti-baryon ($\mathrm{B\bar{B}}$) annihilation on $\Lambda$, $\bar{\Lambda}$ hyperon production and its effect on $\bar{\Lambda}$/$\bar{p}$ ratios. The UrQMD calculations that include $\mathrm{B\bar{B}}$ annihilation can produce the trend of average transverse mass spectra for $\Lambda$ and $\bar{\Lambda}$, as well as, the characteristic enhancement in $\bar{\Lambda}$/$\bar{p}$ ratios in data as a function of centrality and collision energy. Furthermore, $\bar{\Lambda}$/$\bar{p}$ ratios extracted from the feed-down corrected SPS data are seen to be in good agreement with UrQMD model calculations with $\mathrm{B\bar{B}}$ annihilation. This suggests that $\bar{\Lambda}$/$\bar{p}$ enhancement is not necessarily because of strangeness enhancement and $\mathrm{B\bar{B}}$ annihilation has a significant role to play.