The GPD program at COMPASS Andrzej Sandacz Sotan

The GPD program at COMPASS Andrzej Sandacz Sołtan Institute for Nuclear Studies, Warsaw on behalf of the COMPASS collaboration XIII Workshop on High Energy Spin Physics September 1 -5, 2009 Dubna, Russia

Generalized Parton Distributions and DVCS * Factorisation: hard Q 2 large, -t<1 Ge. V 2 x+x soft x-x GPDs P 1, h 1 t P 2, h 2 ~ ~ 4 Generalised Parton Distributions : H, E, H, E Pi, hi – proton momentum and helicity depending on 3 variables: x, x, t for each quark flavour and for gluons for DVCS gluons contribute at higher orders in αs

GPDs GPD and Hard Exclusive Meson Production Ø 4 Generalised Parton Distributions (GPDs) for each quark flavour and for gluons Ø factorisation proven only for σL σT suppressed by 1/Q 2 necessary to extract longitudinal contribution to observables (σL , …) Ø allows separation Flavour sensitivity of HEMP on the proton p 0 2 Du+Dd 2 Du-Dd h ρ0 ω ρ+ 2 u+d, 9 g/4 2 u-d, 3 g/4 s, g J/ψ g u-d H ~ H E ~ E conserve and wrt quark flavours Vector mesons (ρ, ω, φ) Pseudoscalar mesons (π, η) flip nucleon helicity Ø quarks and gluons enter at the same order of αS Ø at Q 2 ≈ few Ge. V 2 power corrections/higher order p. QCD terms are essential Ø wave function of meson (DA Φ) additional input

GPDs properties, links to DIS and form factors for P 1 = P 2 recover usual parton densities no similar relations; these GPDs decouple for P 1 = P 2 needs orbital angular momentum between partons Dirac axial Pauli pseudoscalar Ji’s sum rule total angular momentum carried by quark flavour q (helicity and orbital part)

‘Holy Grails’ of the GPD quest Contribution to the nucleon spin puzzle E related to the angular momentum 2 Jq = x (Hq (x, ξ, 0) +Eq (x, ξ, 0) ) dx q q p E p t ½ = ½ ΔΣ + ΔG + < Lzq > + < Lzg > GPD= a 3 -dimensional picture of the partonic nucleon structure or spatial parton distribution in the transverse plane y H(x, =0, t) → H(x, , rx, y ) probability interpretation Burkardt x P z r x

Structure of the Nucleon form factors parton distributions location of partons in nucleon longitudinal momentum fraction generalised parton distributions (GPDs) transverse location b and longitudinal momentum fraction T embody 3 D picture of hadrons x x

Nucleon tomography from fits to elastic form factors from GPD fits to Diehl, Feldmann, Jakob, Kroll – (2005) unpolarized proton u. V d. V

Nucleon tomography from fits to elastic form factors from GPD fits to Diehl, Feldmann, Jakob, Kroll – (2005) in ┴ polarized proton u. V d. V

Observables and their relationship to GPDs (at leading order: ) Beam charge asymmetry contains Re. T, integrated GPDs over x Beam or target spin asymmetry contains Im. T, therefore GPDs at x = x and -x

Future GPD program @ COMPASS Ø The GPDs program is part of the COMPASS Phase II (2012 -2016) 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 new transversely polarized NH 3 target (with a thin superconductiong coil) 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 will be done at the same time μ p’ μ’ Stage 1 (~2012) 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 (~2014) to constrain E d ( , S) - d ( , S+π) Im(F 2 H – F 1 E) sin ( - S) cos

COMPASS kinematical coverage for DVCS CERN SPS high energy muon beam 100/190 Ge. V with a 2. 5 m long LH 2 target L = 1032 cm-2 s-1 Q 2 → 8 Ge. V 2 → 12 Ge. V 2 if luminosity increased by factor 4 ~10 -2 < x < ~10 -1 x → 0. 15 with upgrade of present calorimetry

The GPDs in the next several years v H 1, ZEUS, HERMES, JLab 6 Ge. V are providing the first results significant increase of statistics expected after full data sets analysed v. The energy upgrade of the CEBAF accelerator will allow access to the high x. B region which requires large luminosity. v The GPD project at COMPASS will explore intermediate x. B (0. 01 -0. 10) and large Q 2 (up to ~8(12) Ge. V 2) range COMPASS will be the only experiment in this range before availability of new colliders

DVCS + BH with + and - beams and unpolarized proton target dσ(μp μp ) = dσBH + dσDVCSunpol + eμ a. BH Re. TDVCS μ μ’ * + Pμ dσDVCSpol θ p + eμ Pμ a. BH Im. TDVCS Beam Charge & Spin Difference DU, CS d ( + ) - d ( - ) 2(eμ a. BH Re. TDVCS + Pμ dσDVCSpol ) Beam Charge & Spin Sum SU, CS d ( + ) + d ( - ) 2(dσBH + dσDVCSunpol + eμPμ a. BH Im. TDVCS)

t-slope measurement; relevant for nucleon ‘tomography’ Using SU, CS , integrating over and subtracting BH d DVCS/dt ~ exp(-B|t|) ‘tomography’: B(x) ~ <r. T 2>(x) FFS model adapted for COMPASS by A. S. assumed B(x) = b 0 + 2 α’ ln(x 0/x) 1<Q 2<8 with α’ =0. 125 Ge. V-2 160 Ge. V muon beam 2. 5 m LH 2 target global = 10%, 140 days L = 1222 pb-1 for gluons, from J/Ψ at HERA for valence quarks, from fits to FF α’ ~ 0. 164 Ge. V-2 α’ ~ 0. 02 Ge. V-2 α’ ~ 1 Ge. V-2 - photoproduction (Q 2 ≈0) - DIS (Q 2=2 -80 Ge. V 2)

Beam Charge and Spin Asymmetry from DU, CS / SU, CS : Comparison to different models 160 Ge. V muon beam 2. 5 m LH 2 target global = 10%, 140 days L = 1222 pb-1 D. Mueller-hep-ph/0904. 0458 fit to world data

Beam Charge and Spin Asymmetry in various kinematic bins Q 2 = 12 Ge. V 2 VGG model 160 Ge. V muon beam 2. 5 m LH 2 target global = 10%, 140 days L = 1222 pb-1 4 < Q 2 < 8 x = 0. 15 0° 360° 2 < Q 2 < 4 1 < Q 2 < 2 0. 005< x < 0. 01< x < 0. 02 < x < 0. 03< x < 0. 07 If Lumi × 4 more bins up to Q 2 = 12 Ge. V 2

Single γ production with transversely polarised target dσ(μp μpγ) = dσU(μp μp ) + dσT(μp μpγ) unpolarized target transversely polarized target dσT(μp μp ) = ST Pμ dσTBH + ST dσTDVCS + ST Pμ dσTDVCSpol + ST eμ a. TBH TTDVCS + ST eμ Pμ a. TBH TTDVCSpol to isolate TTS part measurements at opposite target polarisations needed dσT = ½ {dσ (ST=+PT) - dσ(ST=−PT)} to disentangle DVCS and Interference terms having the same azimuthal dependence both + and - beams needed measure DT, CS ST, CS d T( + ) - d T( - ) d T( + ) + d T( - ) or/and cf. the next slide ADT, CS ≡ DT, CS/d σ0 AST, CS ≡ ST, CS/d σ0 dσ0 is unpolarised, charge averaged cross section

Harmonics decomposition of TTS-dependent 1 γ production cross section Belitsky, Müller, Kirchner ST Pμ ST e μ Pμ twist-2 terms not shown are terms with sin(kφ) and cos(kφ) (k=2, 3) dependence those are twist-3 and NLO twist-2 gluon helicity flip terms

An ‘appetizer’ – HERMES measurements of DVCS from Transverse Target Spin asymmetry AUTsin(ϕ-ϕs)cosϕ c 1, T-Int for proton sensitive to Ju (not to Jd) => allows model dependent constraints study of azimuthal asymmetries from transversely polarized NH 3 target is a part of Phase 2 of COMPASS proposal, simulations are in progress

TTS asymmetry AUTsin(ϕ-ϕs) for ρ0 production on protons from COMPASS 2007 data from transversely polarised NH 3 COMPASS target transverse spin dep. VM cross section access to GPD E related to orbital momentum HM , EM are weighted sums of integrals of the GPDs < Q 2 > ≈ 2. 2 (Ge. V/c)2 < x. Bj > ≈ 0. 04 AUTsin(ϕ-ϕs) Hq, g , Eq, g < pt 2 > ≈ 0. 18 (Ge. V/c)2 compatible with 0 in progress: L/T γ* separation (using ρ0 decay angular distribution)

Comparison to a GPD model • Goloskokov-Kroll [EPJ C 53 (2008) 367] ‘Hand-bag model’; GPDs from DD using CTEQ 6 power corrections due to kt of quarks included both contributions of γ*L and γ*T included COMPASS p↑ preliminary ρ+ W = 10 Ge. V t’ integrated K*0 ρ0 ω predictions for protons AUT(ρ) ≈ -0. 02 AUT(ω) ≈ -0. 10

Detectors to be built Large Proton Recoil Detector and a long LH 2 target (Phase 1) with dedicated read out electronics with 1 GHz sampling Proton Recoil Detector for a transversely polarized ammonia target (Phase 2) Large Q 2 trigger Monitoring of muon flux ECAL 1 and ECAL 2 to be extended and upgraded ECAL 0 to be designed and build to increase range in x. Bj and to reduce background sketch, not to the scale

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 1. 5 days of 160 Ge. V muon beam (μ+ and μ-)

LH target region in 2008 DVCS test run Ring B Ring A

Recoil Proton Detector Small 1 m long Recoil Proton Detector and a 40 cm LH 2 target available in 2008/2009 Proton identification in RPD Elastic scattering with pion beam (2008)

Selection of exclusive single γ events DVCS Bethe-Heitler σ ≈ 1 ns Timing difference : tμ- t. RPD Selection of events : - one vertex with μ and μ’ - no other charged tracks - only 1 high energy photon - 1 proton in RPD with p< 1. Ge. V/c σ ≈ 4. 5 cm z position difference : zμμ’- z. RPD

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 single photon events μ φ Before cuts All Q 2 After cuts All Q 2 A flat background contribution in φ suppressed The peak at φ=0 remains =>identified as BH μ’ * p θ

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 Data & MC => global = 13%± 5% θ

2009 DVCS pilot run 2 weeks of DVCS pilot run in 2009 approved by SPSC to start September 17 ‘Hadron setup’ as in 2008 with the small RPD and 40 cm LH target + operational BMS for momentum measurements of beam μ’s + beam flux measurement Both μ+ and μ- beams Goals : observe DVCS (~100 ev. ) measure BH (~1000 ev. ) to precisely verify global efficiency observe exclusive π0 events, estimate background to DVCS demonstrate feasibility of beam flux measurements at a few % level measure other channels of exclusive meson prod. (ρ0)

Conclusion & prospects • Possible physics ouput – Sensitivity to transverse size of parton distributions inside the nucleon – Sensitivity to the GPD E and total angular momentum – Working on a variety of models to quantify the physics impact of GPD measurements at COMPASS • Experimental requirements – Recoil detection with long LH target or polarized target – Good calorimetry and Extension at larger angles • Roadmap – A global COMPASS proposal for the period 2012 -2016/2017 including GPD will be submitted to SPSC in 2009 – 2008 -9: The small RPD and liquid H 2 target are available for the hadron program tests of DVCS feasibility – from 2012: The complete GPD program at COMPASS with a long RPD + liquid H 2 target (2012) + transversely polarized ammonia target (2014)