The Boer-Mulders and Cahn effects. Azimuthal modulations in the spin-independent SIDIS cross section at HERMES

The cos φh and cos 2φh modulations of the spin independent semi-inclusive deep inelastic scattering cross section were measured at Hermes. These modulations are sensitive to the polarization and transverse motion of quarks within the nucleon. The Cahn effect, which contributes at subleading twist (twist-3) to cos φh and twist-4 to cos 2φh, is sensitive to transverse motion of quarks. The Boer-Mulders effect, which contributes at subleading twist to cos φh and leading twist to cos 2φh, requires non-zero transverse quark motion and is sensitive to quark polarization, effectively measuring the quark spin-orbit correlation. These modulations were measured in a fully differential way, as a function of x, y, z, and Phperpendicularto for positive and negative pions, kaons, and unidentified-hadrons produced from hydrogen and deuterium targets at Hermes. This is the the most complete measurement to date, for the first time granting access to the flavor dependent distribution and fragmentation functions that generate these moments. These measurement would not have been possible without an improved hadron-type identification algorithm for the Ring Imaging Cherenkov (RICH) detector. The EVT event-level algorithm is presented and now is the primary method used at Hermes. (orig.)