Azimuthal single spin asymmetries in SIDIS in the light of chiral symmetry breaking

TL;DR

The paper investigates azimuthal single-spin asymmetries in SIDIS by combining a Collins fragmentation mechanism with chirally odd distribution functions from chiral models, examining both vanishing and Gaussian intrinsic transverse momenta. It finds that Collins effects can account for the HERMES A_UL^{sinφ} data within current uncertainties, though a sizable Sivers contribution cannot be ruled out. The authors predict large A_{UT}^{sin(φ+φ_S)} asymmetries for transverse-target kinematics, providing a clear experimental test to distinguish Collins and Sivers mechanisms and to probe chiral symmetry breaking in nonperturbative QCD. The work thus links chiral dynamics to observable SIDIS spin asymmetries and guides future measurements at HERMES and COMPASS to disentangle underlying spin-orbit correlations.

Abstract

An attempt is made to understand the z-dependence of the azimuthal single spin asymmetries observed by the HERMES collaboration in terms of chiral models based on effective quark and Goldstone boson degrees of freedom. The effects of respectively neglecting and considering Gaussian intrinsic parton transverse momenta and the Sivers effect are explored. Predictions for the transverse target polarization experiment at HERMES are presented.

Figures (6)

• Figure 1: Kinematics of the process $lp\rightarrow l'\pi X$ in the lab frame.
• Figure 2: The azimuthal single spin asymmetries $A_{UL}^{\sin\phi}(z)$ from a proton and deuteron target for $\pi^+$, $\pi^0$ and $\pi^-$ in comparison to the HERMES data Airapetian:1999tvAirapetian:2001eg.
• Figure 3: The average transverse momentum of the produced hadrons $\langle|{\bf P}_{h\perp}|(z)\rangle$ as function of $z$. The dots are the $\langle|{\bf P}_{h\perp}|(z)\rangle$ HERMES data ($h=$ pion) from Ref. Airapetian:2002mf. The curves follow from the model calculation of Ref. Bacchetta:2002tk if one assumes the parton transverse momenta in the target to be Gaussian (solid line) or to vanish (dashed line).
• Figure 4: Rough estimate of the possible magnitude of the Sivers effect in the azimuthal single spin asymmetries $A_{UL}^{\sin\phi}(z)$ from a proton target for ( a) $\pi^+$ and ( b) $\pi^0$. The thin solid lines are the central values shown in Figs. 2a and 2b. The dashed lines are the shifts of the central values due to the Sivers effect according to Eqs. (\ref{['Eq:AUL-z-final-with-Sivers']}, \ref{['Eq:B-Siv-range']}).
• Figure 5: The asymmetries $A_{UT}^{\sin(\phi+\phi_S)}(z)$ for pion production from a transversely polarized proton target at HERMES for the assumptions that intrinsic transverse parton momenta in the target are follow a Gaussian distribution (solid lines) and are negligible (dashed lines).