[Paper Review] Transverse spin azimuthal asymmetries in SIDIS at COMPASS: Multidimensional analysis

B. Parsamyan|arXiv (Cornell University)|Apr 7, 2015

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents the first multidimensional extraction of transverse spin azimuthal asymmetries in semi-inclusive deep inelastic scattering (SIDIS) using COMPASS data from 2010, analyzing dependencies on $x$, $Q^2$, $z$, and $p_T$. Key results include non-zero signals for Sivers, Collins, $A_{LT}^{ ext{cos}( heta)}$, and $A_{UT}^{ ext{sin}( heta_S)}$ asymmetries, with $Q^2$-dependence observed in Sivers and Collins effects, providing critical input for TMD evolution studies.

ABSTRACT

COMPASS is a high-energy physics experiment operating at the SPS at CERN. Wide physics program of the experiment comprises study of hadron structure and spectroscopy with high energy muon and hadrons beams. As for the muon-program, one of the important objectives of the COMPASS experiment is the exploration of the transverse spin structure of the nucleon via spin (in)dependent azimuthal asymmetries in single-hadron production in deep inelastic scattering of polarized leptons off transversely polarized target. For this purpose a series of measurements were made in COMPASS, using 160 GeV/c longitudinally polarized muon beam and transversely polarized $^{6}LiD$ (in 2002, 2003 and 2004) and $NH_{3}$ (in 2007 and 2010) targets. The experimental results obtained by COMPASS for unpolarized target azimuthal asymmetries, Sivers and Collins effects and other azimuthal observables play an important role in the general understanding of the three-dimensional nature of the nucleon. Giving access to the entire "twsit-2" set of transverse momentum dependent parton distribution functions and fragmentation functions COMPASS data triggers constant theoretical interest and is being widely used in phenomenological analyses and global data fits. In this review main focus is given to the very recent results obtained by the COMPASS collaboration from first ever multi-dimensional extraction of transverse spin asymmetries.

Motivation & Objective

• To perform the first multidimensional extraction of transverse spin azimuthal asymmetries (TSAs) in SIDIS using COMPASS data.
• To probe the full $x$-$Q^2$-$z$-$p_T$ phase-space dependence of TSAs for improved understanding of nucleon structure.
• To extract $Q^2$-dependence of TSAs as input for testing TMD evolution equations.
• To measure higher-twist and leading-twist transverse-spin asymmetries, including Sivers, Collins, and $A_{LT}^{ ext{cos}( heta)}$ effects.
• To test theoretical predictions for higher-twist asymmetries using high-statistics COMPASS data.

Proposed method

• Multi-differential analysis of SIDIS data using 160 GeV/c muon beam and transversely polarized $^6$LiD and $NH_3$ targets.
• Systematic extraction of eight TSAs across multiple kinematic configurations: 3D ($x$-$Q^2$-$z$, $x$-$z$-$p_T$, etc.) and 4D grids.
• Use of model-independent cross-section formula (Eq. 1) to decompose asymmetries into modulation terms with $ ext{cos}( heta)$, $ ext{sin}( heta)$, and $ ext{sin}(3 heta - heta_S)$ dependencies.
• Application of Wandzura-Wilczek approximation to reduce higher-twist TMDs to leading-twist level for phenomenological interpretation.
• Statistical extraction of asymmetries in 7×6 $z$-$p_T$ bins and multiple $x$-bins to resolve multivariate dependencies.
• Comparison of $A_{LT}^{ ext{cos}( heta)}$ results with theoretical predictions from Kotzinian et al. (2006).

Experimental results

Research questions

• RQ1 What is the $Q^2$-dependence of the Sivers and Collins asymmetries in multidimensional kinematic space?
• RQ2 How do the transverse spin asymmetries vary across different $x$, $z$, and $p_T$ bins?
• RQ3 Is the $A_{UT}^{ ext{sin}( heta_S)}$ asymmetry non-zero, and what does its $z$-dependence reveal about higher-twist TMDs?
• RQ4 Does the $A_{LT}^{ ext{cos}( heta)}$ double-spin asymmetry show a non-zero signal, and how does it compare to theoretical expectations?
• RQ5 Are the remaining four TSAs consistent with zero, and do they align with current higher-twist predictions?

Key findings

• The Sivers asymmetry shows a sizable signal for positive hadrons, increasing with $p_T$, and hints at $Q^2$-dependence at large $z$.
• The Collins asymmetry exhibits a clear mirrored behavior between positive and negative hadrons, with increasing magnitude in both $z$ and $p_T$.
• The $A_{UT}^{ ext{sin}( heta_S)}$ asymmetry is non-zero and significant at large $z$, indicating a measurable higher-twist effect.
• The $A_{LT}^{ ext{cos}( heta)}$ double-spin asymmetry is the only DSA observed to be non-zero, with data consistent with theoretical predictions from Kotzinian et al. (2006).
• The remaining four asymmetries ($A_{UT}^{ ext{sin}(2 heta_h - heta_S)}$, $A_{UT}^{ ext{sin}(3 heta_h - heta_S)}$, $A_{LT}^{ ext{cos}( heta_S)}$, $A_{LT}^{ ext{cos}(2 heta_h - heta_S)}$) are consistent with zero within statistical accuracy.
• The multidimensional analysis reveals non-trivial $x$- and $Q^2$-dependence in Sivers and Collins effects, providing direct input for TMD evolution studies.