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Study of Λ hyperon production in the HERMES experiment

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Semi-inclusive A hyperon production has been studied in the HERMES experiment using the Ee = 27.5 GeV polarized positron beam incident on polarized or unpolarized internal gas target. The multiplicity distribution over fractional A energy z has been investigated and the fragmentation function DuΛ(z) has been extracted. The DuΛ(z) found by HERMES is well described with the help of the parametrization based on the experiments on A production in high energy proton-proton collisions, but it is substantially smaller then that deduced from the e+e- annihilation data. The distribution over A transverse momentum squared pT2 has also been analyzed. The results show that at z > 0.2 the pT2 distribution is dominated by the quark transverse motion in the fragmentation process. The longitudinal spin transfer from the beam to the A has been measured. The systematic uncertainty of the spin transfer has been reduced by reversal of the beam polarization several times during the data taking period. The spin transfer coefficient is found to be SΛ = 0.04±0.08(stat)±0.03(syst) at an average fractional energy of the produced A hyperons of 〈z〉 = 0.45. The dependence of the longitudinal spin transfer on z is presented and compared with recent theoretical calculations.

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