Measurement of Beam-Spin Asymmetries for Deep Inelastic $π^+$ Electroproduction

Abstract

We report the first evidence for a non-zero beam-spin azimuthal asymmetry in the electroproduction of positive pions in the deep-inelastic region. Data have been obtained using a polarized electron beam of 4.3 GeV with the CLAS detector at the Thomas Jefferson National Accelerator Facility (JLab). The amplitude of the $\sinφ$ modulation increases with the momentum of the pion relative to the virtual photon, $z$, with an average amplitude of $0.038 \pm 0.005 \pm 0.003$ for $0.5 < z < 0.8$ range.

Figures (8)

• Figure 1: Comparison of the distributions measured with CLAS at 4.3 GeV (circles) in $x,Q^2$(GeV$^2$), missing mass $M_X$(GeV), and the transverse pion momentum $P_{\perp}$(GeV) with LUND-MC reconstructed events. The distributions are averages over the range $0.5<z<0.8$; the MC results are normalized to the same number of events.
• Figure 2: Pion multiplicities as a function of $z$ for different $x$ ranges normalized by the total number of pions in the corresponding $x$-range.
• Figure 3: The beam-spin azimuthal asymmetry as a function of azimuthal angle $\phi$, measured in the range $z$=0.5--0.8.
• Figure 4: The beam-spin azimuthal asymmetry as a function of missing mass $M_X$, in $\gamma^*p\rightarrow \pi^+X$ extracted in the range $0.5<z<0.8$. Triangles up and down are the results for positive and negative helicities, respectively, and the filled circles are for their average. Open circles show the measured $A_{LU}^{\sin\phi}$ extracted as a $\sin \phi$ moment of the spin asymmetry. Open squares show the measured $A_{LU}^{\sin\phi}$ for the sample averaged over the beam polarization. Data are slightly shifted in $M_X$ for clarity.
• Figure 5: The beam-spin azimuthal asymmetry as a function of $z$ extracted for different cuts on the missing mass $M_X$ (in GeV), $M_X > 1.1$ (circles), $M_X > 1.2$ (squares), $M_X > 1.3$ (triangles up) and $M_X > 1.4$ (triangles down).