${\Sigma }^0$ production in proton nucleus collisions near threshold

Abstract

The production of ${\Sigma }^0$ baryons in the nuclear reaction p (3.5 GeV) + Nb (corresponding to $\sqrt{s_{NN}}=3.18$ GeV) is studied with the detector set-up HADES at GSI, Darmstadt. ${\Sigma }^0$s were identified via the decay ${\Sigma }^0\to \Lambda \gamma$ with subsequent decays $\Lambda \to p{\pi }^{-}$ in coincidence with a $e^{+}{e}^{-}$ pair from either external ($\gamma \to e^{+}{e}^{-}$) or internal (Dalitz decay ${\gamma }^{\ast }\to e^{+}{e}^{-}$) gamma conversions. The differential ${\Sigma }^0$ cross section integrated over the detector acceptance, i.e. the rapidity interval $0.5<y<1.1$, has been extracted as $\Delta {\sigma }_{{\Sigma }^0}=2.3\pm (0.2{)}^{stat}\pm {\left({}_{-0.6}^{+0.6}\right)}^{sys}\pm (0.2{)}^{norm}$ mb, yielding the inclusive production cross section in full phase space ${\sigma }_{{\Sigma }^0}^{total}=5.8\pm (0.5{)}^{stat}\pm {\left({}_{-1.4}^{+1.4}\right)}^{sys}\pm (0.6{)}^{norm}\pm (1.7{)}^{extrapol}$ mb by averaging over different extrapolation methods. The ${\Lambda }_{all}$/${\Sigma }^0$ ratio within the HADES acceptance is equal to 2.3 $\pm$ $(0.2{)}^{stat}$ $\pm$ ${(}_{-0.6}^{+0.6}{)}^{sys}$. The obtained rapidity and momentum distributions are compared to transport model calculations. The ${\Sigma }^0$ yield agrees with the statistical model of particle production in nuclear reactions.

arXiv comment7 pages, 5 figures

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• Strong absorption of hadrons with hidden and open strangeness in nuclear matter
• Production and electromagnetic decay of hyperons: a feasibility study with HADES as a Phase-0 experiment at FAIR