Transverse polarization ofΛandΛ¯hyperons in quasireal photoproduction

The HERMES experiment has measured the transverse polarization of $\Lambda$ and $\overline{\Lambda }$ hyperons produced inclusively in quasireal photoproduction at a positron beam energy of 27.6 GeV. The transverse polarization $P_{\mathrm{n}}^{\Lambda }$ of the $\Lambda$ hyperon is found to be positive while the observed $\overline{\Lambda }$ polarization is compatible with zero. The values averaged over the kinematic acceptance of HERMES are $P_{\mathrm{n}}^{\Lambda }=0.078\pm 0.006(\mathrm{stat})\pm 0.012(\mathrm{syst})$ and $P_{\mathrm{n}}^{\overline{\Lambda }}=-0.025\pm 0.015(\mathrm{stat})\pm 0.018(\mathrm{syst})$ for $\Lambda$ and $\overline{\Lambda }$, respectively. The dependences of $P_{\mathrm{n}}^{\Lambda }$ and $P_{\mathrm{n}}^{\overline{\Lambda }}$ on the fraction $\zeta$ of the beam’s light-cone momentum carried by the hyperon and on the hyperon’s transverse momentum $p_{\mathrm{T}}$ were investigated. The measured $\Lambda$ polarization rises linearly with $p_{\mathrm{T}}$ and exhibits a different behavior for low and high values of $\zeta$, which approximately correspond to the backward and forward regions in the center-of-mass frame of the ${\gamma }^{*}N$ reaction.