Transversity measurements at HERMES

Abstract

Azimuthal single-spin asymmetries (SSA) in semi-inclusive electroproduction of charged pions in deep-inelastic scattering (DIS) of positrons on a transversely polarised hydrogen target are presented. Azimuthal moments for both the Collins and the Sivers mechanism are extracted. In addition the subleading-twist contribution due to the transverse spin component from SSA on a longitudinally polarised hydrogen target is evaluated.

Figures (2)

• Figure 1: Collins moments (upper panel) and Sivers moments (middle panel) for charged pions (as labelled) as a function of $x$, $z$ and $\boldsymbol{P}_{\,\text{h}\perp}$, multiplied by two to have the possible range $\pm1$. The error bands represent the maximal systematic uncertainty due to acceptance and detector smearing effects and due to a possible contribution from the $\left<\cos{\phi}\right>_{\text{UU}}$ moment in the spin-independent cross section. The common overall $6.6\%$ scaling uncertainty is due to the target polarisation uncertainty. The lower panel shows the fraction of charged pions produced in vector meson decay simulated by PHYTHIA6Sjostrand:2000wi (tuned for HERMES kinematics).
• Figure 2: The azimuthal moment $\left<\sin\phi\right>_{\text{UL}}^q$ ($\bullet$, multiplied by two) shows the subleading-twist contribution to the measured asymmetries on a longitudinally polarised hydrogen target for charged pions as a function of $x$ and $z$. In addition the measured lepton-axis azimuthal moments are plotted ($\triangle$ and $\Box$). There is an overall systematic error of $0.003$. The superscript "q" and "l" distinguishes between moments with respect to the photon-axis and lepton-axis taking into account that the measured asymmetries contain contributions from both transverse and longitudinal polarisation components with respect to the virtual photon direction Airapetian:2005jc.