Σ⁰ production in p+Nb reactions at E=3.5 GeV

This work describes the experimental search for uncharged Σ⁰-Hyperons via their decay channels Σ⁰→Λe⁺e^- (BR=5 x 10^-3) and Σ⁰→Λγ (BR=100 %) in proton induced reactions on a nuclear target (Nb) at an incident kinetic beam energy of E=3.5 GeV. The HADES experiment is located at the Gesellschaft fuer Schwerionenforschung (GSI), a heavy ion research facility in Darmstadt. The experiment is dedicated to the investigation of e⁺/e^- pair and strangeness production in pion, proton and heavy ion induced nuclear reactions. Its prior goal is to study in-medium modifications of hadrons in baryonic matter. Due to the capability of measurements in a low energy regime as compared to other experiments, the ability to handle highest event rates and its high momentum resolution and angular acceptance, a unique access to hyperon and dielectron production in baryon dominated matter is offered. After a major electronic upgrade on the ring imaging Cherenkov detector the behavior of the photon detector is investigated for the impact of single photons. A dedicated lamp has been used to generate photons in the wavelength area of λ∼170 nm (E∼6.6 eV), which are sent to the multiwire proportional chamber, where they were detected. The digitisation parameters of the HADES simulation and analysis framework have been adapted and compared to signals fromdielectrons stemming from π⁰ Dalitz decays (π⁰→γe⁺e^-) produced in Ni+Au and Au+Au collisions at E=1.23 AGeV. For the identification of the Σ⁰ hyperons Λ hyperons habe been identified via their decay Λ→pπ^-. The coincidently emitted γ could not be measured due to the lack of an electromagnetic calorimeter but have been identified using external conversion γ→e⁺e^- in the same way as the dielectron pair stemming from the Σ⁰ Dalitz decay (Σ⁰→Λe⁺e^-). The relatively small mass difference of Σ⁰ and Λ (m_Σ⁰-Λ∼77 MeV/c²) led to small momenta for the conversion partners which therefore could not be fully reconstructed. This work presents a method to reconstruct those γ particles. Using a dedicated sideband technique the background is modelled and subtracted. A total of ∼220 Σ⁰ candidates could be identified. After acceptance and efficiency correction the differential cross section of (dσ)/(dΩ)(Σ⁰)=2.3±(0.2)^stat±(^+0.6_-0.6)^sys±(0.2)^norm mb has been extrated.With an extrapolation to the whole phase space also the (Λ)/(Σ⁰) ratio has been calculated to (Λ)/(Σ⁰)=1.6±0.7. The total cross section for the inclusive production of Σ⁰ hyperons in p+Nb reactions has been calculated to σ^Σ0_total=(5.8±(0.5)^stat±(^+1.4_-1.4)^sys)±(0.6)^norm±(1.7)^extrapol mb. A comparison with transport and thermal production models shows a reasonable agreement within the uncertainties and gives first hints of nuclear medium effects.