Measurement of the azimuthal modulations of hadrons in unpolarised SIDIS

Abstract

In 2016 and 2017 COMPASS has collected a considerable amount of deep inelastic scattering events with a 160 GeV/c muon beam and a liquid hydrogen target. An analysis of 4 % of these data has allowed to extract preliminary results on the amplitudes of three azimuthal modulations, $A_{UU}^{\cos {\varphi }_h}$, $A_{UU}^{\cos 2{\varphi }_h}$ and $A_{LU}^{\sin {\varphi }_h}$. The first two are particularly important since they carry information on the intrinsic transverse momentum ${\vec{k}}_T$ of the quarks and on the correlations between the quark spin and ${\vec{k}}_T$, expressed by the Boer-Mulders TMD PDFs. They show strong kinematic dependence as a function of the Bjorken variable $x$, of the fraction of virtual photon energy carried by the hadron $z$ and of the component $P_{hT}$ of the hadron momentum orthogonal to the virtual photon direction, thus confirming the results of previous measurements. Also, as was recently shown by COMPASS, hadrons coming from decay of diffractively produced vector mesons give significant contribution to the amplitudes in certain kinematic regions, explaining in part the kinematic dependencies and helping the interpretation of the measurements. The effect on the published COMPASS ${}^6$LiD data is discussed.

Comments: Presented on behalf of the COMPASS Collaboration at XXVII International workshop on deep-inelastic scattering and related subjects - DIS2019, 8-12 April, 2019, Torino, Italy To be published in the Proceedings of Science