Azimuthal Asymmetries in Unpolarised Semi-Inclusive DIS at COMPASS

TL;DR

This paper reports on measurements of azimuthal asymmetries in unpolarised SIDIS at COMPASS, revealing insights into transverse momentum dependent PDFs and fragmentation functions through analysis of azimuthal modulations.

Contribution

It introduces a new background subtraction procedure for azimuthal asymmetry measurements in SIDIS and provides updated results consistent with previous findings.

Findings

Azimuthal modulations are extracted from COMPASS data.

A new background subtraction method improves measurement accuracy.

Results agree qualitatively with earlier COMPASS measurements.

Abstract

In semi-inclusive deep inelastic scattering (SIDIS) the non-zero transverse momentum of partons induces azimuthal dependence of the cross-section. For unpolarised nucleon, three azimuthal modulations that can be related to different combinations of twist-two or higher twist transverse momentum dependent PDFs and fragmentation functions arise: the so-called Cahn effect reflected in $\cos\phi_\mathrm{h}$ modulation, the $\cos2\phi_\mathrm{h}$ term related to the Boer--Mulders PDF and $\sin\phi_\mathrm{h}$ twist-three effect known as beam-spin asymmetry. In 2016 and 2017, the COMPASS experiment at CERN collected a large sample of SIDIS events using a longitudinally polarised 160~GeV/$c$ muon beam scattering on a liquid hydrogen target. Amplitudes of the aforementioned azimuthal modulations 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.