Weighted transverse spin asymmetries in 2015 COMPASS Drell-Yan data

Abstract

Muon pairs created in the Drell-Yan process of a negative pion beam of 190 GeV/c momentum impinging on a transversely polarised NH 3 target were detected with the COMPASS spectrometer at CERN in 2015. In addition to the extraction of the transverse spin asymmetries (TSAs) from these data, a complementary analysis of the TSAs weighted by powers of the dimuon momentum q T has been performed and is presented in this paper. In the transverse momentum dependent parton distribution functions (TMD PDFs) formalism, the q T -weighted TSAs can be interpreted in terms of products of the TMD PDFs of the beam pion and of the transversely polarised target proton, unlike the conventional TSAs, which are interpreted as their convolutions. This allowed us to make a straightforward comparison with the expectation based on the weighted Sivers asymmetry measured in the SIDIS process. In a similar way, the Boer-Mulders function of the pion and proton can be obtained as well. Transverse momentum weighted asymmetries in Drell-Yan processWe studied the DY reaction π − p ↑ → µ − µ + X with a 190 GeV/c π − beam and a transversely polarised proton target. At leading order (LO), the reaction proceeds via annihilation of a quark-antiquark pair into a virtual photon. The cross-section consists of five terms, each 1 arXiv:1812.08505v1 [hep-ex]

Transverse spin effects in hard semi-inclusive collisions

“…This prediction is considered as an important test of our understanding of the origin and nature of SSAs in SIDIS and Drell-Yan processes, within a QCD TMD factorisation scheme. First experimental results [62] hint at a confirmation of the sign change [39], but no definite conclusion can still be drawn [189,190]. The quark Sivers function has been extracted from SIDIS data on the weighted asymmetry A sin(φ h −φ S ) U T [17,18,19,20,21,24], interpreted through Eq.…”

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

“…Possible Q 2 evolution effects due to somewhat different hard scales of two measurements are neglected. The procedure is exactly the same followed for the 2015 data set, described in [12]. It makes use of the unpolarized PDFs from CTEQ 5D global fit [14,18] and the FFs from DSS 07 global fit [15], to determine the first transverse moments of u-and d-quark Sivers functions f The projection to the DY case is obtained using Eq.…”

“…The unpolarized PDF used, f u 1,p , is the same adopted in the SIDIS case. The result is compared to the weighted Sivers asymmetry extracted from the combined 2015 + 2018 DY sample in Fig 4a. The systematic uncertainty is discussed in detail in [12]. The significance of the result does not yet allow one to draw conclusions on the sign-change hypothesis.…”

“…2. Each weighted TSA is obtained from fits to the data using the so-called 'modified double ratio method' [12]. The results obtained from the combined 2015 and 2018 samples are presented in Fig.…”

“…The systematic uncertainty is discussed in detail in [12]. The significance of the result does not yet allow one to draw conclusions on the sign-change hypothesis.…”