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 DIS 2006, Tsukuba, Japan, April 20-24, 2006

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

SCHNC Spin properties of the production amplitudes Angular Distribution of the production and decay of ρ  π + π -  Spin density matrix elements  bilinear combinations of the helicity amplitudes A( γ*(λ γ )  ρ (λ ρ ) )  T λ ρ λγ λ γ =  1, 0 λ ρ =  1, 0 if NPE T - λ ρ -λγ = (-1) λ ρ- λγ T λ ρ λγ 9 helicity amplitudes reduce to five 5 independent amplitudes : A(L  L), A(T  T) >> A(T  L) > A(L  T) > A(T  -T) i.e. T 00, T 11 >> T 01 > T 10 > T -11 SCHC >> single helicity flip > double helicity flip

Physics from COMPASS: F. Kunne + Δ G/G : K. Kurek A 1 d and g 1 d : M. Stolarski Transversity: H. Fischer Λ Polarisation: B. Grube Prospects for GPDs: F-H Heinsius COMPASS experiment Veto SM1 dipole SM2 dipole HCAL1 Muon-filter1,MW1 Micromegas,DC,SciFi Gems,SciFi,DCs,straws MWPC Gems Scifi trigger-hodoscopes Silicon RICH_1Gem_11 ECAL2,HCAL2 straws,MWPC,Gems,SciFi straws Muon-filter2,MW2 DW45 SciFi Polarised Target 6 LiD ~ 250 physicists from 28 institutes μ 160 GeV from CERN SPS Beam P olarization ~ -76%

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

Selection of Incoherent exclusive ρ 0 production Incoherent production 0.15 < p t ²< 0.5 GeV² scattering off a quasi-free nucleon Assuming both hadrons are  0.5 < M  < 1 GeV  M  E miss pt²pt² Exclusivity of the reaction E miss =(M² X -M² N ) /2M N -2.5 < E miss < 2.5 GeV W t Q 2 * N N’ Kinematics: ν > 30 GeV Eμ’ > 20 GeV quasi-free nucleons in 6 LiD polarized target Background ~12%

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

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

Measurement of r Distribution : Spin density matrix elements: T λ ρ λγ are helicity amplitudes meson photon 0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV 2

2002 Determination of R ρ ° =  L /  T If SCHC holds : only T 00 ≠0 T 11 ≠0 Then : Impact on GPDs study: - easy determination of  L dominant at Q 2 >2 GeV 2 -factorisation well esta- blished for  L High statistics from γ-production to hard regime - Better coverage at high Q 2 with 2003 and 2004 data

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

A measurable asymmetry can arise from: - an interference between a tiny Unnatural Parity Exchange contribution ( π, a 1 ( 1260 ) Reggeon) with a dominant NPE contribution (ρ, ω, a 2 ( 1320 ), f 2 Reggeon or a Pomeron ) - in Generalized Vector Meson Dominance Model: Fraas, NPB113 (1976) if x Bj <0.2 + some assumptions - in pQCD with gluon GPDs at small x Bj 1 GeV 2 Goloskokov,Kroll, EPJC42 (2005) and Martin,Ryskin,Teubner, PRD62 (2000) Ryskin, Phys.Atom.Nucl.62 (1999) parton-hadron duality A 1 ρ related to generalized spin-dependent gluon distributions  longitudinal double-spin asymmetry A 1 ρ transverse photons and longitudinally polarized nucleons if SCHC

Experimental method ( 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  in COMPASS kinematics  η (effects in syst. err.)

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

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

A 1 ρ (deuteron) from COMPASS data Vertical bars: statistical errors Blue bands: total systematic errors including false asymmetries, uncertainties on P t, P b, parametrization of D, f effect of η Background corrections presently studied and indicating tot. syst. err < stat err.

COMPASS results =10 GeV A 1 ρ (deuteron) consistent with 0 extention of the kinematical range by almost 2 decades both in Q 2 and x Bj HERMES EPJC29 (2003) = 4 GeV A 1 ρ (proton) exhibits a positive tendency A 1 ρ (deuteron) consistent with 0 COMPASS preliminary and HERMES results for A 1 ρ (d) at small x Bj : small UNPE

Conclusions and perspectives High statistics data for SDME and R for incoherent exclusive ρ° prod. in a wide Q 2 range from photo-production to 7 GeV 2 weak violation of SCHC observed Double spin asymmetry for longitudinally polarized target A 1 ρ (deuteron) consistent with zero over a wide Q 2 and x Bj range Spin asymmetries for transversally polarized target (also for GPD study) Exclusive production of  and J/  also observed and are studied

0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² Statistical error only, limited by MC Preliminary : - Corrected for acceptance, smearing and efficiency (MC:DIPSI gen) - Background not subtracted 0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV 2 Angular distributions for 2002 data φ

 0 mass distribution Söding parametrization: No acceptance corrections yet ! (subtracted)