The HERMES experiment has measured the transverse polarization of Lambda and (Lambda) over bar hyperons produced inclusively in quasireal photoproduction at a positron beam energy of 27.6 GeV. The transverse polarization P-n(Lambda) of the Lambda hyperon is found to be positive while the observed (Lambda) over bar polarization is compatible with zero. The values averaged over the kinematic acceptance of HERMES are P-n(Lambda)=0.078 +/- 0.006(stat)+/- 0.012(syst) and P-n((Lambda) over bar)=-0.025 +/- 0.015(stat)+/- 0.018(syst) for Lambda and (Lambda) over bar, respectively. The dependences of P-n(Lambda) and P-n((Lambda) over bar) on the fraction zeta of the beam's light-cone momentum carried by the hyperon and on the hyperon's transverse momentum p(T) were investigated. The measured Lambda polarization rises linearly with p(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.
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