Spin transfer coefficient K LL’ in  photoproduction at HERMES D. Veretennikov On behalf of the HERMES collaboration DIS08, London

Λ decay and spin transfer coefficient  is “self-analyzing” particle due to its parity violation   p  - decay Spin transfer coefficient S LL’ K LL’  from longitudinally polarized target K NN’  from transversely polarized target D LL’  from longitudinally polarized beam D NN’  from transversely polarized beam L is primary axis along target or beam polarization L’ is secondary axis along  momentum Unpolarized cross-section angle between proton momentum in the  rest frame and the  spin decay constant, (  ), (  ) Angular distribution of protons (in  rest frame)

Study of Λ polarization at HERMES  DIS, e + p  e’ +  + X  D LL’  spin transfer from longitudinally polarized e + /e - beam in semi- inclusive reaction / Published P.R. D 64 (2001), P.R. D 74 (2006) /  Quasi-real photoproduction, e + p → Λ+ X  K LL’  spin transfer from longitudinally polarized target  D LL’  spin transfer from longitudinally polarized e + /e - beam  K NN’  spin transfer from transversely polarized target  Pn  transverse (spontaneous) Λ polarization / Publish P.R. D 76 (2007) / e’ +  * ( Q 2  0 GeV 2 )

Λ photoproduction mechanism in PYTHIA According to PYTHIA  is produced due to diquark fragmentation or due to quark fragmentation Most of events are within photoproduction peak (for  80% events Q 2 < 0.05 GeV 2,  15 GeV) Such that photon is quasi-real

HERMES experiment Polarized lepton (e - /e + ) beam E e = 27.5 GeV, monthly spin flip  ( D LL’ ) Long. / trans. polarized gas targets (H, D), spin flip every 90 s  ( K LL’, K NN’ ) Detector is up / down symmetric  ( P n )

Background suppression cuts for  leading  rejection ( in HERMES acceptance proton is always leading ) :  Threshold Cherenkov det  Ring imaging Cherenkov  h + h - pair background rejection :  Vertex separation d(V 1,V 2 ) > 15 cm  (  ) event reconstruction Data sample for longitudinally polarized target ( )

Formalism extraction of K LL’ No MC simulation needed  Helicity balanced data sample  Moment method  Background subtraction

Photoproduction kinematics Better separate  production channels Value of t allows for easy comparison with other experiments

K LL’ seems to be increasing at small t (diquark fragmentation?) K LL’ is p t independent (0 < p t < 1.2 GeV) Kinematical dependences of K LL’ for 

Statistics for  is not enough to see dependences on t or p t Kinematical dependences of K LL’ for 

Spin transfer for  and , world data FNAL, E704 pp collisions with transversely polarized beam RICH, STAR pp collisions with longitudinally polarized beam

World data compilation for spin transfer FNAL result confirms a trend to increase spin transfer at small t

Conclusion Longitudinal spin transfer from polarized target to the  is measured for the first time in quasi-real photoproduction K LL’ = ± stat ± syst Spin transfer is increasing at small t K LL’ is p t independent Longitudinal spin transfer to  is compatible with zero for available statistics K LL’ = ± stat ± syst