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This paper presents the analysis and preliminary results of the diffractive ρ° production at COMPASS experiment, focusing on the spin density matrix elements and their implications for GPD studies. The weak violation of SCHC and the double spin longitudinal asymmetry for ρ° production are also discussed.
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Diffractive ρ° production at COMPASS Nicole d’Hose, CEA-Saclay On behalf of the COMPASS collaboration Physics motivations COMPASS experiment Ongoing analysis Preliminary results Prospects for the future DIS 2005, Madison, USA, April 27-May 1, 2005
μN μ’N’ρ * π+π- Q 2 W N’ N t Description of the diffractive ρ° production Regge theory, pQCD calculations (for σL ), … 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: 10-2<Q2<10 GeV2, <W>=10GeV, 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
Spin properties of the production amplitudes Angular Distribution of the production and decay ofρπ+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(TT) >>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
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 straws,MWPC,Gems,SciFi Veto Gems,SciFi,DCs,straws Silicon First results @ DIS2005: ΔG/G : C. Bernet Longitudinalspin depdt g1d : J. Hannappel Transversity : 1) Collins and Sivers asym: P. Pagano 2) 2 hadrons correlation: R. Joosten SciFi Micromegas,DC,SciFi 160 GeV from CERN SPS
Selection of the sample * Q 2 • Topology : • 1 incoming μ and 1 scattered μ • only 2 hadrons of opposite charge • vertex in the 6LiD polarized target • Kinematical conditions: • ν > 30 GeV • Eμ’ > 20 GeV • Q² > 0.01 GeV² Focus on Incoherent Exclusive ρ0 production... W N’ N t
Background proc. rejected : 6LiD Incoherent exclusive ρ0 production Assuming both hadron are p 0.5 < Mpp< 1 GeV Mpp Exclusivity of the reaction Emiss=(M²X-M²N) /2MN -2.5 < Emiss < 2.5 GeV Emiss Incoherent production 0.15 < pt²< 0.5 GeV² scattering off a quasi-free nucleon pt²
Kinematical domain of the final sample after event selection • 2002 : this analysis • ~ 700,000 evts (whole range) • ~ 20,000 evts (Q²>1GeV²) • 2003 and 2004: • - trigger coverage improved • enlarged acceptance at high Q² • (up to 20 GeV) • - ~ 4 times more statistics 10-4 1 10 10-3 10-2 10-1 Q2 [GeV2] W [GeV]
ρ° angular distributions W(cosθ, φ, Φ) depends on theSpin density matrix elements 23 (15) observables with polarized (unpolarized) beam φ This analysis: only one-dimensional angular distribution We will use also: ψ= φ - Φ
0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² Angular distributions 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
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
2002 2003 + 2004 Determination of Rρ° =sL/sT If SCHC holds : only T00≠0 T11≠0 Then : Impact on GPD study: easy determination of sL factorisation only valid for sL
Measurement of r 1 1-1 0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV2 Distribution, if SCHC holds: with if in addition, NPE holds:
Measurement of r and Im r 04 1-1 3 1-1 φ Distribution : beam polarisation weak violation Spin density matrices: If SCHC holds
Conclusions and perspectives Incoherent exclusive ρ° production under investigation: - high statistics from photo-production to the hard regime - larger acceptance at high Q2 in 2003 and 2004 (up to 20 GeV2) Preliminary results on a few Spin Density Matrix Elements: - only statistical errors are shown (dominated by MC) - systematics to be evaluated (background contamination) -SDME in agreement with other experiments - weak violation of SCHC observed Double spin longitudinal asymmetry for ρ° production (NPE control) Spin asymmetries for transversally polarised target (GPD study) Exclusive production off and J/Y also observed and will be studied
r0 mass distribution Söding parametrization: No acceptance corrections yet ! (subtracted)