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Diffractive ρ ° production at COMPASS

Diffractive ρ ° production at COMPASS. Nicole d’Hose, CEA-Saclay On behalf of the COMPASS collaboration. Results on spin dependence for exclusive ρ ° production  ρ ° spin density matrix elements and R= σ L / σ T  longitudinal double-spin asymmetry A 1 ρ.

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Diffractive ρ ° production at COMPASS

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  1. Diffractive ρ° production at COMPASS Nicole d’Hose, CEA-Saclay On behalf of the COMPASS collaboration Results on spin dependence for exclusive ρ° production ρ° spin density matrix elements and R= σL/σT  longitudinal double-spin asymmetry A1ρ DIS 2006, Tsukuba, Japan, April 20-24, 2006

  2. μN μ’N’ρ * π+π- Q 2 W N’ N t Description of the diffractive ρ° production Regge theory, pQCD calculations at large Q2… At low energy W < 5 GeV: exchange of 2 quarks or of Reggeon ρ,ω (JP=1-), a2, f2 (JP=2+), a3, f3 (JP=3-), … At higher energy : exchange of 2 gluons or of Pomeron COMPASS: 3. 10-3< Q2 < 7 GeV2, <W>=10 GeV, t small Experimental observations (NMC, E665, ZEUS, H1, HERMES): the helicity of γ* is approximatively retained by theρ° meson  SCHC the exchange object has natural parity P=(-1)J NPE

  3. Spin properties of the production amplitudes Angular Distribution of the production and decay ofρπ+π- Spin density matrix elements bilinear combinations of the helicity amplitudesA( γ*(λγ)ρ(λρ) ) Tλρλγ λγ=  1, 0 λρ=  1, 0 if NPE T- λρ -λγ= (-1)λρ-λγTλρλγ 9helicity amplitudes reduce to five5 independent amplitudes : A(L L), A(TT) >>A(T L) >A(L T) >A(T -T) i.e. T00,T11 >>T01>T10>T-11 SCHC>> single helicity flip > double helicity flip SCHNC

  4. μ COMPASSexperiment trigger-hodoscopes ~ 250 physicists from 28 institutes DW45 straws SM2 dipole Muon-filter2,MW2 HCAL1 RICH_1 Gem_11 SM1 dipole ECAL2,HCAL2 MWPC Gems Scifi Polarised Target 6LiD Muon-filter1,MW1 Physics from COMPASS: F. Kunne +ΔG/G : K. Kurek A1dand g1d : M. Stolarski Transversity: H. Fischer ΛPolarisation: B. Grube Prospects for GPDs: F-H Heinsius straws,MWPC,Gems,SciFi Veto Gems,SciFi,DCs,straws Silicon SciFi Micromegas,DC,SciFi 160 GeV from CERN SPS Beam Polarization ~ -76%

  5. The polarized 6LiD-Target 3He – 4He dilution refrigerator (T~50mK) Superconducting Solenoid (2.5T) + Dipole(0.5T) Target Polarization ~ 50% Dilution factor f ~ 0.36 μ Two 60cm long target cells with opposite polarization 4 possible spin combinations: longitudinal transverse       Reversed once a week •   •   Reversed every 8 hours

  6. Selection of Incoherent exclusive ρ0 production Assuming both hadrons are p 0.5 < Mpp< 1 GeV Mpp * Q 2 W N’ N Emiss Exclusivity of the reaction Emiss=(M²X-M²N) /2MN -2.5 < Emiss < 2.5 GeV t quasi-free nucleons in 6LiD polarized target • Kinematics: • ν > 30 GeV • Eμ’ > 20 GeV pt² Incoherent production 0.15 < pt²< 0.5 GeV² scattering off a quasi-free nucleon Background ~12%

  7. Kinematical domain of the final sample 2002 : 800,000 evts 2003 : 1,600,000 evts + 2004 : not yet analyzed ~ same statistics as in 2003 Q²> 1GeV² ~60,000 evts <W> = 10.2 GeV

  8. ρ° spin density matrix elements ρ° angular distributions W(cosθ, φ, Φ) depends on theSpin density matrix elements 23 (15) observables with polarized (unpolarized) beam φ This analysis yet: - only 2002 data - only one-dimensional angular distribution

  9. 04 00 Measurement of r 0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV2 Distribution : Spin density matrix elements: Tλρλγare helicity amplitudes meson photon

  10. 2002 2003 + 2004 Determination of Rρ° =sL/sT If SCHC holds : only T00≠0 T11≠0 Then : • Impact on GPDs study: • -easy determination of sL • dominant at Q2>2 GeV2 • factorisation well esta- • blished for sL - High statistics from γ-production to hard regime - Better coverage at high Q2 with 2003 and 2004 data

  11. weak violation ? further study with 2003 data Measurement of r and Im r 04 1-1 3 1-1 φ Distribution : 2002 beam polarisation Spin density matrices: 2002 If SCHC holds

  12. longitudinal double-spin asymmetry A1ρ transverse photons and longitudinally polarized nucleons if SCHC A measurable asymmetry can arise from: - an interference between a tiny Unnatural Parity Exchange contribution(π, a1(1260) Reggeon) with a dominant NPE contribution (ρ, ω, a2(1320), f2Reggeonor a Pomeron) - in Generalized Vector Meson Dominance Model: Fraas, NPB113 (1976) if xBj<0.2 + some assumptions - in pQCD with gluon GPDs at small xBj < 0.01 and large Q2 > 1 GeV2 Goloskokov,Kroll, EPJC42 (2005) and Martin,Ryskin,Teubner, PRD62 (2000) Ryskin, Phys.Atom.Nucl.62 (1999) parton-hadron duality A1ρ related to generalized spin-dependent gluon distributions

  13. Experimental method (2002+2003 data) Polarized muon beam and target (with 2 cells with opposite spin configuration) Relation between muon-N cross asymmetry and virtual photon-N asymmetry: D depolarization factor Positivity limit: η ~10-410-2in COMPASS kinematics  η (effects in syst. err.)

  14. Depolarization factor D for incoherent exclusive ρ° production R: E665 parametrization

  15. Dilution factor f for incoherent exclusive ρ° production nD, nA: numbers of nucleons in deuteron and nucleus A in the target : cross sections per nucleon for exclusive incoherent ρ° production on deuteron and nucleus A

  16. A1ρ(deuteron)from 2002+2003 COMPASS data Vertical bars: statistical errors Blue bands: total systematic errors including false asymmetries, uncertainties on Pt, Pb, parametrization of D, f effect of η Background corrections presently studied and indicating tot. syst. err < stat err.

  17. COMPASS preliminary and HERMES results for A1ρ(d) HERMES EPJC29 (2003) <W>= 4 GeV A1ρ(proton) exhibits a positive tendency A1ρ(deuteron) consistent with 0 COMPASS results <W>=10 GeV A1ρ(deuteron) consistent with 0 extention of the kinematical range by almost 2 decades both in Q2 and xBj at small xBj : small UNPE

  18. Conclusions and perspectives High statistics data for SDME and R for incoherent exclusive ρ° prod. in a wide Q2 range from photo-production to 7 GeV2 weak violation of SCHC observed Double spin asymmetry for longitudinally polarized target A1ρ(deuteron)consistent with zero over a wide Q2 and xBj range Spin asymmetries for transversally polarized target (also for GPD study) Exclusive production off and J/Y also observed and are studied

  19. longitudinal double-spin asymmetry A1ρ in exclusive ρ° production at COMPASS transverse photons and longitudinally polarized nucleons A measurable asymmetry can arise from: - an interference between a tiny Unnatural Parity Exchange contribution(π, a1(1260) Reggeon) with a dominant NPE contribution (ρ, ω, a2(1320), f2 Reggeon or a Pomeron) - in Generalized Vector Meson Dominance Model: Fraas, NPB113 (1976) if xBj<0.2 + some assumptions - in pQCD with gluon GPDs at small xBj < 0.01 and large Q2 > 1 GeV2 Goloskokov,Kroll, EPJC42 (2005) and Ryskin, Phys.Atom.Nucl.62 (1999) A1ρ related to spin-dependent gluon generalized parton distributions

  20. 0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² Angular distributions for 2002 data 0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV2 φ Preliminary : - Corrected for acceptance, smearing and efficiency (MC:DIPSI gen) - Background not subtracted Statistical error only, limited by MC

  21. r0 mass distribution Söding parametrization: No acceptance corrections yet ! (subtracted)

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