Prospects for GPD studies at COMPASS E Burtin

Prospects for GPD studies at COMPASS E. Burtin CEA-Saclay Irfu/SPh. N On behalf of the COMPASS Collaboration CERN , March 4 th, 2010 • • Physics Motivations Sensitivity of observables Experimental issues Beam tests results

Generalized Parton Distributions * , p, r hard x+ soft Factorisation: Q 2 large, -t<1 Ge. V 2 x- GPDs P t Generalized Parton Distributions for quarks : 4 functions H, E, H, E(x, , t) P’ contains pdf H(x, 0, 0) = q(x) measured in DIS contains form factors measured in elastic scattering contains information on the nucleon spin : Ji’s sum rule :

3 -D partonic structure of the nucleon (Pz, ry, z) Hard Exclusive Scattering Deeply Virtual Compton Scattering ep ep * Q² x+ p x- GPDs t x. P p z r x boost y GPDs : H( x, , t ) Fourier ( =0) access to correlations : ( Px, ry, z ) Burkardt, Belitsky, Müller, Ralston, Pire

What makes Compass unique ? CERN High energy muon beam • 100 - 190 Ge. V • 80% Polarisation • μ+ and μ- available üOpposite polarization Luminosity lim it Foreseen program : DVCS and meson production off a liquid H 2 target (unpolarized) Will explore the intermediate x. Bj region Uncovered region between ZEUS+H 1 and HERMES+Jlab Gluon, sea and valence quarks x. Bj

Comparison of BH and DVCS at 160 Ge. V DVCS : Bethe-Heitler : μ x=0. 01 excellent reference yield x=0. 1 BH and DVCS at the same level access DVCS amplitude through the interference θ p x=0. 04 BH dominates μ’ * DVCS dominates study of d DVCS/dt

Azimuthal angular dependence analysis μ from Belitsky, Kirchner, Müller : polarized beam off unpolarized target μ’ * φ p dσ(μp μp ) = dσBH + dσDVCSunpol + Pμ dσDVCSpol + eμ a. BH Re ADVCS + eμ Pμ a. BH Im ADVCS Known expression Pμ e μ Pμ Twist-2 M 11 >> Twist-3 M 01 Twist-2 gluon M-11 θ

Angular dependence analysis Case of COMPASS : μ+(P=-0. 8) and μ-(P=+0. 8) unpolarized H 2 target μ φ SU, CS : dσμ++ dσμ-= 2(dσBH+dσDVCSunpol ) + 2 eμ Pμ a. BH Im ADVCS => dσ/dt DU, CS : dσμ+- dσμ-= 2 Pμ dσDVCSpol => Im (F 1 H) + eμ a. BH Re ADVCS => Re (F 1 H) μ’ * p θ

From SU, CS : t-slope measurement Assuming 3% systematic error on BH Using SU, CS : d DVCS / dt ~ exp(-Bt) B ~ ½ <r 2> Ansatz at small x : B(x) = b 0 + 2 α’ ln(x 0/x) α’ =0. 125 Ge. V-2 (FFS) 1 < Q 2 < 8 Ge. V 2 160 Ge. V muon beam 2. 5 m LH 2 target 2 years L = 1222 pb-1 εglobal = 10 % 3 σ slope measurement for : α’ > 0. 30 (ECAL 1+2 ) α’ > 0. 16 (ECAL 0+1+2)

Angular dependence analysis Case of COMPASS : μ+(P=-0. 8) and μ-(P=+0. 8) unpolarized H 2 target μ φ SU, CS : dσμ++ dσμ-= 2(dσBH+dσDVCSunpol ) + 2 eμ Pμ a. BH Im ADVCS => dσ/dt DU, CS : dσμ+- dσμ-= 2 Pμ dσDVCSpol => Im (F 1 H) + eμ a. BH Re ADVCS => Re (F 1 H) μ’ * p θ

DU, CS : Beam Charge & Spin Difference DU, CS : dσμ+- dσμ-= 2 Pμ dσDVCSpol + eμ a. BH Re ADVCS μ φ Bj μ’ * θ p 160 Ge. V muon beam 2. 5 m LH 2 target 2 years L = 1222 pb-1 εglobal = 10 % => Re (F 1 H)

BCSA(�) over the kinematical domain BCSA = DU, CS /SU, CS 4 < Q 2 < 8 Points: VGG prediction Phys. Rev. D 60: 094017, 1999 Statistical errors only Curves: FFS prediction Phys. Rev. D 59: 119901, 1999 160 Ge. V muon beam 2. 5 m LH 2 target 2 years L = 1222 pb-1 εglobal = 10 % (deg) 2 < Q 2 < 4 1 < Q 2 < 2 0. 005< x < 0. 01< x < 0. 02 < x < 0. 03< x < 0. 07

statistical precision of the cosϕ modulation BCSA = DU, CS /SU, CS = A 0 + ACS, U cos ϕ + A 2 cos 2ϕ B B B

Meson production : filter of GPDs Cross section measurement : Vector meson : ρ, ω, … H & E ~ Pseudo-scalar : π, η… H Would allow for flavor separation : Hρ0 = 1/ 2 (2/3 Hu + 1/3 Hd + 3/8 Hg) Hω = 1/ 2 (2/3 Hu – 1/3 Hd + 1/8 Hg) H = -1/3 Hs - 1/8 Hg ρ : ω : 9 : 1 : 2 at large Q 2 Transversely polarized target asymmetry on vector meson : E/H ( studied at COMPASS without RPD )

with Continuation of the GPD program : constrain the GPD E + , - beam and transversely polarized NH 3 (proton) target DT, CS d T ( + ) - d T ( - ) Im(F 2 H – F 1 E) sin( - S) cos 160 Ge. V muon beam 1. 2 m polarized NH 3 target (f=0. 26) 2 years - εglobal = 10 %

GPD program : new equipments DVCS μp μ’p’ μ’ 2. 5 m liquid H 2 target μ 4 m long Recoil Proton Detector p’ ECal 1 + ECal 2 10° Hermetic calorimetry new ECAL 0 Later stage… Transversely polarized target Associated RPD

Recoil Proton Detector • 4 m long scintillator slabs • ~ 300 ps timing resolution • Full scale prototype tested successfully Gandalf Project: 1 GHz digitalisation of the PMT signal to cope for high rate

Requirements - Photon energy range 0. 2 - 30 Ge. V - Size: 320 cm x 320 cm ; - Granularity 4 x 4 – 6 x 6 cm 2 - Energy resolution < 10. 0%/√E (Ge. V) - Thickness < 50 cm, - Insensitive to the magnetic field. Prototype under studies • Shaschlyk module with AMPD readout • Tested ECAL 1&2 ECAL 0 DVCS+BH

2008 beam test Compass hadron run Target region Ring B Ring A DVCS Bethe-Heitler Selection of events : - one vertex with μ and μ’ - no other charged tracks - only 1 high energy photon (Δt<5 ns) - 1 proton in RPD with p < 1. Ge. V/c

2008 beam test : exclusivity cuts μ’+γ γ μ’ Δp =|P μ’+γ|-|P RPD| Δpperp < 0. 2 Ge. V miss proton Δϕ Transverse plane Δϕ=ϕmiss - ϕRPD Δϕ < 36 deg

2008 beam test : Bethe-Heitler signal Monte-Carlo simulation of BH (dominant) and DVCS Deep VCS Bethe-Heitler => Bethe-Heitler observed Detection efficiency : ε μ+p->μ+p+γ = 0. 32 +/- 0. 13 After all cuts, Q 2>1 Ge. V 2 Global efficiency : - μ+p->μ+p+γ efficiency - SPS & COMPASS availability - Dead time - trigger efficiency εglobal = 0. 13 +/- 0. 05 ~ 10 times more data taken in 2009 Projections of errors are realistic

Conclusions & perspectives COMPASS has a great potential in GPDs physics • Study of the GPD H with a LH 2 target : measurement ot t-slopes – transverse partonic structure of the nucleon measurement of Beam Charge and Spin differences & asymmetries • Equipements needed : 4 m long RPD, 2. 5 m LH 2 target, Extended & improved calorimetry • at a later stage : study of the GPD E with a transversely polarized target

DU, CS /SU, CS : Beam Charge & Spin Asymetry μ BCSA = DU, CS /SU, CS φ Bj μ’ * θ p 160 Ge. V muon beam 2. 5 m LH 2 target 2 years L = 1222 pb-1 εglobal = 10 %

Experimental realisation 2008 DVCS beam test Compass hadron set-up μ’ γ proton • New « super. RPD » and H 2 target • Hermetic calorimetry : Move ECALs upstream and/or complete ECAL 2 • New ECAL 0 upstream of SM 1

ECAL 0 andbeam ECAL 1 2008 DVCS test Horizontal proton γ μ’ “super. RPD” SM 1 ECAL 0 Vertical ECAL 1 ECAL 2

Hadron program RPD Proton identification in RPD Elastic scattering (hadron beam)

Kinematical consistency : ϑγ*γ μ’ With μ, μ’ and γ : μ γ* γ With μ, μ’ and proton : proton μ+p->μ’+γ+p Events rejected

Exclusive photon production signal μ μ’ * φ Before cuts All Q 2 θ p After cuts All Q 2 The peak at φ=0 remains => caracteristic of BH

Liquid H 2 Target & RPD Ring B Ring A

Measurements and Estimations for resolution RPD(2008) tmin=-0. 06 Ge. V 2 Good resolution in t Importance for the transverse imaging Mu. Rex (2006) B L=1 m; th=1 cm Atten length = 0. 7 m σB = 300 ps L=4 m; th=5 cm Atten length = 4 m σB = 200 ps A L=50 cm; th=5 mm σA = 180 ps L=2. 83 m; th=4 mm σA = 270 ps To. F σTo. F = 350 ps RB-RA= 85 -12 = 63 cm σTo. F = 310 ps RB-RA= 110 -25 = 85 cm