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

The production of ${\mathrm{\Sigma }}^0$ 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. ${\mathrm{\Sigma }}^0$s were identified via the decay ${\mathrm{\Sigma }}^0\to \mathrm{\Lambda }\gamma$ with subsequent decays $\Lambda \rightarrow p \pi^{-}$incoincidencewitha$e^{+}e^{-}$ pair from either external ($\gamma \to e^{+}{e}^{-}$) or internal (Dalitz decay ${\gamma }^{\ast }\to e^{+}{e}^{-}$) gamma conversions. The differential ${\mathrm{\Sigma }}^0$ 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 $\sigma^{total}{\Sigma^{0}} = 5.8 \pm (0.5)^{stat} \pm \left(^{+1.4}{-1.4}\right)^{sys} \pm (0.6)^{norm} \pm (1.7)^{extrapol}$mbbyaveragingoverdifferentextrapolationmethods\mathrm{.}The$\Lambda{all}$\mathrm{/}$\Sigma^{0}$ratiowithintheHADESacceptanceisequalto2.3$\pm$ $(0.2{)}^{stat}$ $\pm$ ${(}_{-0.6}^{+0.6}{)}^{sys}$. The obtained rapidity and momentum distributions are compared to transport model calculations. The ${\mathrm{\Sigma }}^0$ yield agrees with the statistical model of particle production in nuclear reactions.