Speaker
Description
In QCD the momentum and spin structure of the nucleon explored in hard-scattering reactions is described in terms of Transverse Momentum Dependent Parton Distribution Functions (TMD PDFs). The PDFs were studied primarily in the collinear approximation, until it was demonstrated that the intrinsic transverse motion of partons plays an important role.
In Semi-Inclusive Deep Inelastic Scattering (SIDIS) the non-zero transverse momentum of partons $k_T$ induces azimuthal dependence of the cross-section and affects the transverse momentum distribution of the produced hadrons. The SIDIS cross-section contains three nucleon polarization independent azimuthal modulations that are related to different combinations of twist-2 or higher twist PDFs and fragmentation functions. Those are the so-called Cahn effect reflected in $cos(\phi_{h})$ modulation, the $cos(2\phi_{h})$ term related to the Boer-Mulders PDF and $sin(\phi_{h})$ twist-3 effect known as beam-spin asymmetry.
In 2016 and 2017, the COMPASS experiment at CERN collected a large sample of DIS events using a longitudinally polarised 160 GeV/c muon beam scattering on a liquid hydrogen target. The azimuthal asymmetries $A_{UU}^{\cos(\phi_{h})}$, $A_{UU}^{\cos(2\phi_{h})}$ and $A_{UU}^{\sin(\phi_{h})}$ have been extracted from part of the data. A new procedure has been developed to subtract a background coming from the decay of diffractively produced vector mesons. The results presented in this talk qualitatively agree with earlier COMPASS results obtained with an isoscalar target.
