Lambda polarization measured at HERMES and lambda spin structure

Abstract

Polarizations of δ and $\bar \Lambda$ hyperons have been measured in leptoproduction by the HER-MES Experiment. Data were accumulated in years 1996 and 1997 using the 27.5 GeV polarized HERA positron beam and unpolarized gas targets. In semi-inclusive reactions with Q ²>1 GeV², the preliminary result is P _δ=0.03±0.06 (stat) ±0.03 (syst) at an average δ fractional energy 〈z〉=0.446 and Feynman scaling variable 〈x _F 〉=0.3. The spin transfer coefficient from the virtual photon to the δ was found to be D ^δ _LL′ =0.11±0.17 (stat) ±0.09 (syst). This result is consistent with both the naive expectation of zero polarization of the up quark in the δ (D ^δ_LL′ =0), and the Burkardt-Jaffe prediction of D ^δ_LL′ =−0.2 for the up quark contribution. Inclusive reactions with only the δ or $\bar \Lambda$ detected in the final state were also studied. In this case most δ's and $\bar \Lambda$'s are produced in the photoproduction regime with Q ²<0.08 (GeV/c)². Transverse δ and $\bar \Lambda$ polarizations perpendicular to the production plane have been measured. Averaged over the spectrometer acceptance for the transverse δ and $\bar \Lambda$ momentum 0<pt<1.7 GeV, the preliminary results are P _n =0.066±0.011 (stat)±0.025 (syst) for δ and P _n =−0.066±0.037 (stat)±0.025 (syst) for $\bar \Lambda$. This is the first clear indication of transverse δ polarization in leptoproduction. Theoretical analysis of these results is underway.