# Exclusive processes in leptoproduction at COMPASS Andrzej Sandacz

- Slides: 22

Exclusive processes in leptoproduction at COMPASS Andrzej Sandacz Sołtan Institute for Nuclear Studies, Warsaw on behalf of the COMPASS collaboration Photon 2009 Int. Conference on the Structure and the Interactions of the Photon 11 -15 May 2009, DESY, Hamburg

Present analyses of exclusive channels in leptoproduction at COMPASS Physics analyses for ρ0 and φ channels v transverse target spin asymmetry ρ0 on p, d v double spin asymmetry ρ0 , φ on d v SDMEs ρ0 , φ on p, d v cross sections, R(=σL/σT), t-slopes ρ0 , φ on p, d Searches for signals of exclusive J/ψ(→μ+μ-), ω0, π0 production Feasibility study to detect exclusive single photon events from ‘DVCS 2008 test run’

as in run NIM A 577(2007) 455 COMPASS setup • high energy beam • large angular acceptance • broad kinematical range variety of tracking detectors to cope with different particle flux from θ = 0 to θ ≈ 200 mrad two stages spectrometer Large Angle Spectrometer (SM 1) Small Angle Spectrometer (SM 2) Luminosity 5· 1032 cm-2 s-1 Sci. Fi Straws Silicon SDC Micromegas MWPC GEMs W 45 Muon. Wall SM 2 E/HCAL 50 SM 1 Muon. Wall Polarised Target μ e. V G 60 PKU-RBRC 1 Workshop on Transverse Spin Physics, June 30, 2008 m RICH calorimetry, PID RICH detector F. Bradamante

COMPASS polarised ammonia target (2007) polarised material - protons in NH 3 molecule transverse/longitudinal polarisation ~90% dilution factor f ~ 0. 14 Microwave reversal every week

Q 2 > 1 Ge. V 2 Selections of exclusive events W > 5 Ge. V 0. 005 < x. Bj < 0. 1 Transversely polarised proton target (NH 3), PT ≈ 90%, 2007 data 0. 05 < pt 2 < 0. 5 Ge. V 2 ρ0 • recoil proton (recoiling system) not detected • charged pion mass assumed for h+(h-) -0. 3 < Mππ – Mρ(PDG) < 0. 3 Ge. V/c 2 • cuts on pt 2 to remove coherent production from N and further suppress non-exclusive background

Observables in hard exclusive meson production relevant for GPDs for vector mesons unpolarised cross section transverse target spin asymmetry access to GPD E related to orbital momentum HM , EM are weighted sums of integrals of the GPDs Hq, g , Eq, g weights depend on contributions of various quark flavours and of gluons to the production of meson M (large Q 2 approximation) Give access to the orbital angular momentum of quarks q q Ji’s sum rule So far GPD E poorly constrained by data (mostly by Pauli form factors)

Transverse target spin asymmetry for exclusive ρ0 production definitions to disentangle contributions from γL and γT the distribution of ρ0 decay polar angle needed in addition Diehl and Sapeta (2005) • Spin-dependent photoabsorption cross sections and interference terms σijmn Aim amplitudes for subprocess γ* p → ρ0 p with proton polarisation i and photon polarisation m • Virtual photon polarisation parameter (if ml can be neglected)

Cross sections in terms of target polarisation wrt virtual photon (in deep inelastic kinematics)

Extraction of transverse target spin asymmetry Flux Acceptance Dilution factor Mean target polarisation Numer of target nucleons Asymmetry extraction from double ratio method using 3 targets with two polarisations each in the double ratio Flux and σ0 cancel also Acceptance cancels provided no changes between spin reversals AUT from a fit to DR (ϕ-ϕs)

Transverse target spin asymmetry: polarised protons (2007) NH 3 target new < Q 2 > ≈ 2. 2 (Ge. V/c)2 < x. Bj > ≈ 0. 04 < pt 2 > ≈ 0. 18 (Ge. V/c)2 In progress: L/T γ* separation (using ρ0 decay angular distribution)

Transverse target spin asymmetry: polarised deuterons (2002 -2004) 6 Li. D < Q 2 > ≈ 2. 0 (Ge. V/c)2 < x. Bj > ≈ 0. 03 target < pt > ≈ 0. 11 Ge. V/c In progress: L/T γ* separation (using ρ0 decay angular distribution) and coherent / incoherent separation for deuteron

Comparison to a GPD model and to HERMES COMPASS p↑ ρ+ GPD model: Goloskokov, Kroll 2008 W = 10 Ge. V t’ integrated K*0 ρ0 ω predictions for protons AUT(ρ) ≈ -0. 02 AUT(ω) ≈ -0. 10 similar for both experiments HERMES extracted also ρ TTSA separately for 0 γ* L and γ* T ! compatible with 0

Longitudinal double-spin asymmetry for exclusive ρ0 production < PB > = -0. 76 longitudinally polarised deuteron target (6 Li. D) wide range of Q 2 and x , PT ≈ 50% EPJ C 52 (2007) f ≈ 0. 37 W > 7. 5 Ge. V , 0. 15 < pt 2 < 0. 5 Ge. V 2 curve: where A 1 – inclusive asymmetry (d) A 1ρ on polarised deuterons consistent with 0

Longitudinal double-spin asymmetry for exclusive ρ0 production (cont. d) estimate of contribution of unnatural exchanges (π, a 1) | | | | if SCHC The asymmetry is a sensitive probe of unnatural parity exchanges Ø at small Q 2 and x data provides precise limits on their contribution ~ Ø at large Q 2 A 1ρ related to GPDs (higher-twist) ∞ k. T 2 Hg(sea) /(Q 2 Hg(sea)) Goloskokov, Kroll (2007) W = 5 Ge. V HERMES W = 10 Ge. V COMPASS small value of A 1ρ at W ≥ 10 Ge. V due to approximate cancellation of gluon and sea contributions

Spin Density Matrix Elements VM angular distributions W(cosθ, φπ, ϕ) depend on the spin density matrix elements (SDME) 23 (15) observables with polarized (unpolarized) beam SDMEs are bilinear combinations of the helicity amplitudes Tλm λγ (γ*N → m. N) λγ = ± 1, 0 λm = ± 1, 0 (averaged over nucleon spins) v describe helicity transfer from γ* to VM s-channel helicity conservation (SCHC) v describe parity of t-channel exchange (NPE vs. UPE) v impact on GPD studies – determination of σL SDME

Measurement of r 04 00 and determination of R = 0. 01 < Q² < 0. 05 < Q² < 0. 3 < Q² < 0. 6 < Q² < σL/σT 2. 0 < Q² < W(cos θ) Tλρ λγ helicity amplitudes meson photon 2002 Ø High statistics from quasi-photoproduction to hard production Ø Impact on GPD studies; determination of σL 10 Ge. V 2

Measurement of r 0. 01 < Q² < 0. 05 < Q² < 0. 3 04 1 -1 and Im r < Q² < 0. 6 3 1 -1 < Q² < 2. 0 < Q² <10 Ge. V 2 W(φ) (φ≡φπ) 2002 beam polarisation if SCHC holds weak violation of SCHC

Future GPD program @ COMPASS Ø The GPDs program is part of the COMPASS Phase II (2010 -2015) proposal to be submitted to CERN in 2009. Ø The first stage of this program requires a 4 m long recoil proton detector (RPD) together with a 2. 5 m long LH 2 target. Upgrades of electromagnetic calorimeters to enlarge coverage at large x. B and reduce bkg. Ø The second stage requires either a transversely polarized NH 3 target inserted in the RPD or a new Sci. Fi (? ) RPD inserted in the existing NH 3 target system. primary physics goal is DVCS meson production also planned μ’ cf. Laurent Schoeffel talk μ p’ Stage 1 (~2011) to constrain H dσ/dt → t-slope parameter b d ( + , ) + d ( - , ) Im(F 1 H) sin d ( + , ) - d ( - , ) Re(F 1 H) cos Stage 2 (~2013) to constrain E d ( , S) - d ( , S+π) Im(F 2 H – F 1 E) sin ( - S) cos

2008 DVCS test run Goal: evaluate feasibility to detect DVCS/BH in the COMPASS setup Use COMPASS ‘hadron’ set-up μp→μpγ LH 2 40 cm in EMCals in the small RPD μ’ γ proton 2 days of 160 Ge. V muon beam (μ+ and μ-)

Kinematic constraints in the transverse plane μ’+γ γ μ’ proton Δp =|P μ’+γ|-|P RPD| Δpperp< 0. 2 Ge. V Δϕ Transverse plane Δϕ=ϕmiss- ϕRPD Δϕ< 36 deg

Azimuthal distribution for exclusive single photon events μ DVCS Bethe-Heitler Monte-Carlo simulation of BH (dominant) and DVCS After all cuts, Q 2>1 Ge. V 2 Clear signature of dominant BH events μ’ * φ p θ

Conclusions and outlook New results on transverse target spin asymmetries for ρ0 production compatible with 0 both for the proton and the deuteron targets ongoing work on L/T separation, and coh. /incoh. separation for d Published results on double spin asymmetry for ρ0 production on d compatible with 0 in a wide x and Q 2 range precise upper limits on contribution of unnatural parity exchanges Expected high precision results on ρ0 and φ SDMEs and cross sections In preparation proposal aiming at the GPD physics equipment needed: 4 m long RPD, 2. 5 m LH 2 target, extented calorimetry RPD with polarised target ‘DVCS test’ runs: 2 days in 2008, 2 weeks in 2009 muon beam and ‘hadron setup’ including 40 cm LH 2 and the small RPD

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