One-hadron transverse spin effects on a proton target at COMPASS

The nucleon spin structure on quark level can be described at leading twist by three quark distribution functions, when the contribution of the transverse momentum of the quarks is ignored. The unpolarized distributon function f ( x ) describes the probability of finding a quark with a momentum fraction x of the nucleon momentum. The helicity distributon function g ( x ) gives the difference in probability of finding quarks with momentum fraction x with spins parallel and antiparallel to the nucleon spin inside a longitudinally polarized nucleon. The last one, the so-called transversity distributon function h ( x ) , describes the difference in probability of finding quarks with momentum fraction x with spins parallel and antiparallel to the nucleon spin inside a transversely polarized nucleon. The distribution functions f ( x ) and g ( x ) have been investigated for almost four decades, while h ( x ) is still mostly unknown. Due to its chiral-odd nature, it cannot be accessed in inclusive deep-inelastic scattering (DIS), but it can be measured in semi inclusive deep inelastic scattering (SIDIS) of leptons off a transversely polarized nucleon target, where it leads in combination with the so-called Collins fragmentation function H ? 1 to an azimuthal asymmetry in the distribution of the hadrons produced. If additionally the transverse momentum of the quarks is taken into account, eight distribution functions are needed at leading order to describe the structure of the nucleon. For a transversely polarized nucleon target the Sivers effect is of special interest, as it describes the fragmentation of an unpolarized quark inside a transversely polarized target nucleon, which can be measured as an asymmetry in the azimuthal distribution of the hadrons produced. Parameterizing the SIDIS cross section up to twist-three leads to 18 structure functions of which eight depend on a transversely polarized target. Four of the eight are connected to leading order distribution functions, the aforementioned transversity and Sivers functions, the worm-gear 2 function and the pretzelosity function. The other four are of subleading order. The investigation of the structure of the nucleon spin is one of the main goals of the COMPASS experiment at CERN. For the measurement of transverse spin effects a 160 GeV =c polarized + beam is scattered off a polarized nucleon target. In the years 2002–2004 a deuterium ( 6 LiD) target was used, while in 2007 and 2010 the measurement was done on a proton (NH 3 ) target. In this thesis the methods of analysis and the results for the eight transverse spin dependend distribution functions from the 2010 data-taking period will be shown for unidentified hadrons as well as for identified pions and kaons. Furthermore the Collins and Sivers asymmetries for the production of K 0 are presented. The work is concluded by a comparison of the measured asymmetries with the results of the HERMES experiment at DESY and existing model predictions. A short interpretation of the results for the Collins and Sivers asymmetries is also given.