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DIS 2012

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DIS 2012

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  1. DIS 2012 MayyaGolembiovskaya (on behalf of HERMES collaboration) DESY Zeuthen Hard exclusive  meson leptoproductionat HERMES

  2. Outline • Introduction • HERMES experiment • Data sample • φmesonSDMEs • Comparison of φ and 0 SDMEs • Summary

  3. Cross section and decay angular distribution , - azimuthal angle of the K+ decay in the meson rest frame  polar angle of the K+ decay in the  meson rest frame angle between  meson-production plane and the lepton scattering plane

  4. Spin density matrix elements & helicity amplitudes can be parameterized by: helicity amplitudes connected with SDMEs; calculated from GPDs spin density matrix (V) – spin density matrix of the vector meson () - spin density matrix of the virtual photon are normalization factors - helicityamplitudes, - hermitian matrices with  08 - virtual photon polarization Schilling, Wolf

  5. The angular distribution For unpolarized target and beam: For unpolarized target and longitudinally polarized beam: • the ratio of virtual photon fluxes for longitudinal and transverse polarization

  6. Vectormeson production , , VMD model GPD model Four leading-twist GPDsfor spin-½ targets and correspond to the nucleon helicity conservation and correspond to the nucleon helicity flip Factorization theorem proved only for transition can be calculated Quarks: Ji relation: Gluons:

  7. Properties of vectormeson production • S‐channel helicityconservation (SCHC) S‐channel helicitynon-conservation • Helicityconserving amplitudes : Helicity flip amplitudes: • , V=>> , V • Theoretically predicted amplitudes hierarchy for HEMES kinematics for  • |T00| ∼ |T11|>>|T01|,|T10| ≈ |T−11| ≈ 0. • Theoretically predicted amplitudes hierarchy for HEMES kinematics for 0 • |T00|2 ∼ |T11|2 ≫ |T01|2 >|T10|2 ∼ |T−11|2

  8. HERMES at HERA Beam Longitudinally polarized leptonbeam with energy 27.6 GeV, Pbeam40 – 60% Target Internal gas target: • Unpolarized H, D, 4He, N, Ne, Kr, Xe • Polarized: longitudinally H, D, transversely H

  9. The HERMES spectrometer • Acceptance 40 <  < 220 mrad, |x| < 170 mrad, 40 < |y| < 140 mrad • Momentum resolution ≤ 1%, angular resolution ≤ 0.6 mrad

  10. mesonevent selection • Exclusive region: , Eur. Phys. J. C 29, 171 - 179 (2003),hep-ex/0302012 • Kinematics: • 1 < Q2 < 7 GeV2,<Q2> = 1.95GeV2 • W2>9 GeV2 , <W 2> = 21.89 GeV2 • 1.012 GeV<<1.028 GeV • -t < 0.4 GeV2 • E < 0.6 GeV • <xB> = 0.088

  11. SDMEs for  meson production Unpolarized (white areas) and beam-polarized (green areas) SDMEs Are shown for the first time for the whole RICH data set

  12. SDMEs for  meson production Unpolarized (white areas) and beam-polarized (green areas) SDMEs Are shown for the first time for the whole RICH data set • no statistically significant diffe- rence between proton and deuteron • s-channel helicity conservation, ‐fulfilled ‐fulfilled,‐large uncertainties • s-channel helicityviolation SDMEs classes C, D, E are compatible with 0 supporting SCHC, except from • Amplitudes hierarchy for  meson: • |T00| ∼ |T11|>>|T01|,|T10| ≈ |T−11| ≈ 0.

  13. SDMEs for 0 meson production  no statistically significant diffe- rencebetween proton and deuterons-channel helicity conservation (conservation the helicity of * in and ) – non-zero SDMEs of classes A,B, - fulfilled s-channel helicity violation significant - non-zero elements of class C, not so significant and - non-zero elements of classes D,E Unpolarized (white areas) and beam-polarized (green areas) SDMEs EPJC 62 (2009) 659-694,arXiv:0901.0701 Hierarchy of amplitudes at HERMES kinematics for : |T00|2 ∼ |T11|2 ≫ |T01|2 >|T10|2 ∼ |T−11|2

  14. Comparison of and 0 SDMEs Unpolarized (white areas) and beam-polarized (green areas) SDMEs • is 10‐20% larger for  than for • SDMEs of class B are compatible for and • SDMEs of class C shows pronounced differences between  and • For classes D and E no significant differences are seen.

  15. Summary • Unpolarized and beam-polarized SDMEs are extracted onproton and deuteron targets for φ(preliminary result)andρ0 (published result) • Compatible results on proton and deuteron targets for φandρ0 • Helicity amplitudes hierarchy for φand ρ0 mesons tested • Pronounced s-channel helicityviolation for ρ0 • Less pronounced s-channel helicityviolation for φ