Beyond Collins and Sivers: further measurements of the target transverse spin-dependent azimuthal asymmetries in semi-inclusive DIS from COMPASS

TL;DR

This paper expands the experimental map of the nucleon’s transverse-spin structure in SIDIS by reporting the first COMPASS measurements of six target-transverse spin azimuthal asymmetries beyond the Sivers and Collins effects. Using a 160 GeV/c longitudinally polarized muon beam on a transversely polarized deuterium target and a defined kinematic selection, the authors extract A_{LT}^{cos(phi_h-phi_s)}, A_{UT}^{sin(3phi_h-phi_s)}, A_{LT}^{cosphi_s}, A_{LT}^{cos(2phi_h-phi_s)}, A_{UT}^{sinphi_s}, and A_{UT}^{sin(2phi_h-phi_s)}. The extracted asymmetries, with systematic uncertainties smaller than statistics, are consistent with zero within errors, constraining the size of the corresponding twist-2 and twist-3 distributions. The work demonstrates the viability of accessing g_{1T}^q, h_{1T}^{⊥ q}, and related TMDs through SIDIS and provides crucial input for global analyses of nucleon spin structure.

Abstract

In semi-inclusive DIS of polarized leptons on a transversely polarized target eight azimuthal modulations appear in the cross-section. Within QCD parton model four azimuthal asymmetries can be interpreted at leading order, two of them being the already measured Collins and Sivers asymmetries. The other two leading twist asymmetries, related to different transverse momentum dependent quark distribution functions, and also additional four asymmetries which can be interpreted as twist-three contributions have been measured for the first time at COMPASS, using a 160 GeV/c longitudinally polarized ($P_{beam}\simeq -0.8$) muon beam and a transversely polarized $^6LiD$ target. We present here the preliminary results from the 2002-2004 data.

Figures (2)

• Figure 1: The asymmetries $A_{LT}^{\cos (\phi _h -\phi _s )}$ and $A_{UT}^{\sin (3\phi _h -\phi _s )}$ as a function of $x$, $z$ and $P_T^h$.
• Figure 2: The asymmetries $A_{LT}^{\cos \phi _s}$, $A_{LT}^{\cos (2\phi _h -\phi _s )}$, $A_{UT}^{\sin \phi _s }$ and $A_{UT}^{\sin (2\phi _h -\phi _s )}$ as a function of $x$, $z$ and $P_T^h$.