We have measured the flavor dependence of multiplicities for $\\pi^+$ and $\\pi^-$ production in semi-inclusive deep-inelastic scattering (SIDIS) on proton and deuteron targets. We used a 10.6 GeV electron beam at Jefferson Lab, and 4 msr solid angle spectrometers (HMS for electrons, SHMS for pions), the lepton vertex spanned the kinematic range $0.3 < x < 0.6$, $2 < Q^2 < 5$ GeV$^2$, and $4 < W^2 < 11$ GeV$^2$. The pion fractional momentum range was $0.3 < z < 0.7$ and the small transverse momentum range was $0 < P_t < .25$ GeV. We used the multiplicities to form sum-and-difference ratios, testing the validity of factorization. We extracted two favored and two unfavored $W$ dependent fragmentation functions (FFs) from these multiplicities. Assuming factorization at low $P_t$, we find that the two ``favored" FFs allow for isospin breaking (charge symmetry violation) at low $W$, while converging to a common value at the highest $W$ of this experiment. The two unfavored FFs are consistent with each other within the experimental uncertainties.

Jointly-supervised doctoral thesis Title: Nucleon spin structure studies in Drell-Yan process at COMPASS Author: Jan Matoušek Department I: Department of Low Temperature Physics, Faculty of Mathem- atics and Physics, Charles University Department II: Department of Physics, University of Trieste Supervisor I: prof. Miroslav Finger (Department I) Supervisor II: prof. Anna Martin (Department II) Abstract: The nucleon structure is presently described by Transverse Momentum Depend- ent (TMD) Parton Distribution Functions (PDFs), which generalise the collinear PDFs, adding partonic spin and transverse momentum degrees of freedom. The recent HERMES and COMPASS data on hadron production in deep inelastic scattering (SIDIS) of leptons off transversely polarised nucleons have provided a decisive validation of this framework. Nevertheless, the TMD PDFs should be studied in complementary reactions, like pp hard scattering and Drell-Yan pro- cesses. In particular the Sivers TMD PDF, which encodes the correlation between the nucleon transverse spin and quark transverse momentum and appears in the Sivers Transverse Spin Asymmetry (TSA), is expected to have opposite sign in Drell-Yan and SIDIS. In 2015 COMPASS measured for the first time the Drell- Yan process on a transversely polarised target π− p↑ → µ− µ+ X to test...