Measurement of $q_T$-weighted transverse-spin-dependent azimuthal asymmetries at COMPASS

Abstract

COMPASS is a fixed-target experiment in operation at the CERN North Area (SPS, M2 beamline) since 2002. An important part of the broad physics programme of the experiment is dedicated to the exploration of the transverse spin-structure of the nucleon studying target transverse spin dependent azimuthal asymmetries (TSAs) arising in the Semi-Inclusive DIS (SIDIS) and Drell-Yan (DY) cross-sections. In addition to those measurements, COMPASS has recently studied also the TSAs weighted by powers of the hadron transverse momentum (in SIDIS) and virtual photon transverse momentum, q T (in DY). In the transverse momentum dependent (TMD) QCD approach, the conventional DY TSAs are interpreted as convolutions of the beam pion and of the transversely polarized target proton TMD parton distribution functions (PDFs), while the q Tweighted TSAs can be interpreted as simple products of transverse moments of the TMD PDFs. In 2015 and 2018 COMPASS performed two years of Drell-Yan data taking with a 190 GeV/c π − beam impinging on a transversely polarized NH 3 target. The analysis of the q T weighted TSAs performed on these two data sets is presented in this paper. The results for DY Sivers q T weighted TSA are compared with the expectations based on the studies of the weighted Sivers asymmetry measured in the SIDIS process. Combining the information from SIDIS and DY measurements, the pion Boer-Mulders TMD PDF is also studied.

Boer–Mulders function of the pion and pretzelosity distribution of the proton in the polarized pion-proton Drell–Yan process at COMPASS

“…As we can see, in the case of the Boer-Mulders-transversity asymmetry A sin(2ϕ−ϕ S )q T /M π T , shown in Fig. 3, the preliminary results obtained with model calculation are found to be in agreement with the published data [7]. In the case of the Boer-Mulders-pretzelosity asymmetry A sin(2ϕ+ϕ S )q 3 T /2M π M 2 P T , shown in Fig.…”

“…Then, we use the DY type parametrization as adopted in Ref. 9 GeV/c 2 [7]. Feynman-x or x F , is a variable of interest that sheds light on the longitudinal structure of the initial state of the interacting quark.…”

“…Recently, COMPASS published the first measurement of the transverse spin dependent azimuthal asymmetries in the polarized Drell-Yan process, (π − p ↑ → μ − μ + X ), induced by a 190 GeV/c π − beam scattering off a transversely polarized NH 3 target [7]. The measurement of the Sivers and other azimuthal asymmetries at the same hard scale in polarized SIDIS and Drell-Yan provides a unique possibility to test prediction in QCD universal features of transverse momentum dependent parton distribution functions.…”

“…[Authors' comment: The experimental data used in this work can be taken from experimental paper Ref. [7].] Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.…”

Working Group 6 Summary: Spin and 3D Structure

“…The positive sign and the amplitude of the asymmetry is consistent with the predicted change of sign for the Sivers function between SIDIS and Drell-Yan measurements and is found to be compatible with various model predictions [31]. In addition to the studies carried out for canonical Drell-Yan TSAs, COMPASS has extracted also the so-called virtual photon transverse momentum, q T -weighted TSAs from the same data-sample [32]. At variance to the TSAs, the q T -weighted TSAs are interpreted not in terms of convolutions of TMD PDFs, but as products of k 2 T -moments of the TMD PDFs.…”