Transverse Λ polarization from a transversely polarized proton target at the COMPASS experiment

Abstract

The measurement of the transverse spin quark distribution functions Δ_Tq(x_Bj) is an important part of the physics program of the COMPASS experiment at CERN. These transversity distributions, being chiral-odd objects, are not accessible in inclusive deep-inelastic scattering (DIS), requiring the presence of another chiral-odd object. At COMPASS, Δ_Tq(x_Bj) can be measured in semi-inclusive deep-inelastic scattering (SIDIS), where a promising channel is the spin transfer to the Λ hyperons. Here, the fragmentation functions Δ_TD^Λ_q(z) provide the other chiral-odd object. In this thesis, the semi-inclusive Λ production mechanism is introduced to show the connection between a potentially measured Λ polarization P_Λ and the Δ_Tq(x_Bj) functions. An expression for the angular distribution of the weak decay Λ → pπ^- is used which allows to extract P_Λ independent of acceptance effects of the spectrometer. In 2007 a 160 GeV/c longitudinally polarized muon beam and a transversely polarized NH₃ target were employed in the COMPASS experiment. Based on the full 2007 statistics with transverse target spin configuration, the Λ and anti-Λ polarizations are analyzed as a function of x_Bj and z. The Λ and anti-Λ hyperons are unpolarized within their statistical errors and show no dependence on either x_Bj or z.<br />