Differential Cross Section Distributions and Polarization Observables in the Reaction p⃗p →pK⁰Σ⁺ at p_⃗p = 2.95 GeV/c

Abstract

The reaction p⃗p → pK⁰S⁺ was studied with the upgraded COSY-TOF detector. A polarized proton beam with momentum 2.95 CeV/c of the Cooler Synchrotron at Forschungzentrum Jülich was focused on a liquid hydrogen target. The pK⁰Σ⁺ final state was identified based on the analysis of the delayed decays of the strange hadrons K_s → π⁺ π^- and Σ⁺ → pπ⁰, nπ⁺. The azimuthal symmetry and large acceptance of the detector as well as the excellent tracking capability provided by the silicon quirl and straw tube tracking subdetectors allows measuring the complete pK⁰Σ⁺ final state distribution.<br /> To achieve the best possible precision in the reconstruction of the track parameters the whole detector setup was calibrated in iterative procedure. The experiment conditions, namely the target dimension and beam polarization where determined based on the analysis of pp elastic scattering events.<br /> Two new reconstruction algorithms of the reaction p⃗p → pK⁰Σ⁺ were developed. The combination of these two algorithms yield a reconstruction effciency ε_reco = 0.027 ± 0.001 which is more than two times better than that of previous COSY-TOF measurements.<br /> All reconstruction and analysis algorithms were controlled by Monte Carlo simulations. The existing simulation software was extended by implementing the new subdetectors and by adding detector ineffciencies to obtain a more realistic detector response. An interface for EvtGen as an alternative particle generator was added to gain more flexibility in the generation of angular distributions.<br /> A new COSY-TOF analysis framework was developed. It incorporates the best features of modern analysis frameworks and allows for straightforward implementation of reconstruction and analysis algorithms. <br /> From about one week of data taking 905 events of the reaction p⃗p → pK⁰Σ⁺ were reconstructed. The fraction of background in this data sample is estimated to be at most 5.9%. Based on this, the total cross section of the reaction p⃗p → pK⁰Σ⁺ was determined to be σ = (2.95 ± 0.1_stat ± 0.22_syst) μb. <br /> From the analysis of the pK⁰Σ⁺ Dalitz plot it is concluded that the N(1710) P₁₁ resonance significantly contributes to the production mechanism. A strongly attractive pΣ⁺ final state interaction is excluded.<br /> The polarization of the Σ⁺ hyperon is determined to be positive as a function of the transverse momentum p_t. For the first time the analyzing power A_N and the spin transfer coefficient D_NN of the Σ⁺ hyperon are determined. The analyzing power A_N has sine-like dependence of cos θ^*_Σ⁺ with an amplitude of (34 ± 13)%. The spin transfer coefficient is found to be compatible with zero at large Σ⁺ backward center-of-mass momentum x_f < 0 while at large Σ⁺ forward center-of-mass momentum x_f > 0 it is +1 within the errors.