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Description
Non-zero transverse momentum of partons and its possible correlation with partonic spin is reflected in the distributions of the transverse momentum $P_T$ and azimuthal angle $\phi$ of hadrons produced in deep inelastic scattering (DIS). Assuming Gaussian dependence of transverse momentum dependent (TMD) PDFs and fragmentation functions (FFs) upon quark transverse momentum, an exponential distribution of $P_T^2$ is expected. For an unpolarised nucleon, three azimuthal modulations that can be related to combinations of twist-two or higher-twist TMD PDFs and FFs arise: the $\cos\phi$ modulation related to the so-called Cahn effect, the $\cos2\phi$ term related to the Boer–Mulders PDF and the $\sin\phi$ modulation known as beam-spin asymmetry.
In 2016 and 2017, the COMPASS collaboration at CERN collected tens of millions SIDIS events using a longitudinally polarised 160 GeV/$c$ muon beam scattering off a liquid hydrogen target. The $P_T^2$ distributions and the amplitudes of the aforementioned azimuthal modulations for charged hadrons have been extracted from part of the data. Radiative effects have been investigated using Djangoh MC generator. The results qualitatively agree with earlier COMPASS measurements with an isoscalar target.
