Generalized Parton Distributions Program at COMPASS. QCD Evolution 2015

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1 Generalized Parton Distributions Program at COMPASS Eric Fuchey (CEA Saclay) On behalf of COMPASS Collaboration QCD Evolution 2015 Thomas Jefferson National Accelerator Facility (May )

2 Generalized Parton Distributions (GPDs) : 3D structure of nucleon Nucleon: Wigner distributions (x, => Correlation bt xp => Quark orbital angular momentum * kt, bt) kt xp GPDs (x, b T) bt TMDs (x, kt) d2b T d2k T PDFs (x) Form Factors (t) d2k T (Fourier transform b T ) dx bt d2b T xp 2

3 Accessing GPDs : Exclusive processes DVCS: p p, HEMP ' k * Q2 = -(k-k')2 xbj = Q2/2p (k-k') k' q (Q2, xbj) DVCS: Interference with Bethe-Heitler Factorization GPDs * DVCS + * * Bethe-Heitler (BH) 4 Chiral-even GPDs: H, E, H, E + 4 chiral-odd: H T, ET, H T, E T 26 August 2014 Factorization proved for: Q2, t << Q2, xbj finite (Bjorken regime) 3

4 The COMPASS experiment: Large acceptance spectrometer for hadronic physics at CERN p, ±, pol. ±,... ~ 200 GeV 4

5 The COMPASS experiment: Experimental setup [NIM A577 (2007) 455] SM2 SM1 RICH ECAL1 ECAL2 Beam * Tracking: DCs, GEMs, MM, dipole magnets ( p ); * ECals + HCals (E); * RICH (PID); 5

6 Additional setup for DVCS (and other exclusive channels) p p ' p SM1 Target: 2.5m LH2; precoil: 4m ToF detector CAMERA; : ECAL0 (cover higher xbj), 1, GeV Pol. ± beam 6

7 Compass assets for GPD study Both and available (currently unique); GPDs in large kinematic region (0.005 < xbj < ~0.3) => Complementary of JLab (valence) DESY: ZEUS, H1, (gluons), HERMES; COMPASS + Jefferson Lab: only current facilities for GPD study before future Electron-Ion Collider; Versatile: Capable to record DVCS and DVMP ( 0,,, ) 7 xbj

8 DVCS on unpolarized p: Study of GPD H 2009: Test run 2012: Pilot run : Data run 8

9 GPD H 2012 Pilot run (4 weeks) (small angles) (intermediate angles) Partially equipped (large angles) Full Scale Oct

10 GPD H Analysis of 2012 data: selection of p p Reconstructed vertices (1, 1 ') in target volume; Only 1 high energy photon with E > 4, 5, 10 GeV in Ecal 0, 1, 2; Partially At least 1 protonequipped reconstructed in CAMERA (compatible with vertex); All particles reconstructed: exclusivity conditions to clean combinatorial: 1 - cut on MX2; 2 - cut on (next slide); 3 - cut on p (next slide); MX2 = (k+p-k'-q'-p')2 T p p ' (k') p: initial proton at rest (k) (q') p (p') 10

11 GPD H Analysis of 2012 data: selection of p p ' (k') All particles reconstructed: exclusivity conditions to clean combinatorial: 1 - cut on MX2; 2 - cut on ; 3 - cut on p ; (q') (k) Spectro+calo p p Camera p (p') pmiss=k+p-k'-q' T p T T T p = p(pmiss) p(p') T = (pmiss) (p') 11

12 GPD H Analysis of 2012 data: subtraction 0: main background source for p p. 2 possible cases: - visible 0 (both detected, easy to reject); - invisible (1 undetected, estimated with MC); Visible 0 contamination evaluated combining exclusive candidates with all low energy in the event. Invisible 0 contamination estimated with 2 MC simulations: - semi-inclusive contribution (LEPTO); - exclusive contribution (HepGen/ 0); MC normalized with visible 0 peak in DVCS. 2 extreme cases considered: Fully semi-inclusive background lower limit Fully exclusive background upper limit 12

13 GPD H Analysis of 2012 data: Preliminary results + Next steps Higher xbj: Low xbj: BetheHeitler DVCS signal Partially equipped * * Full Scale => Simultaneous ep ep 0 analysis; => 0 cross section + improved 0 subtraction; => DVCS cross section t dependence. Next steps: WORK IN PROGRESS 13

14 GPD H Compass GPD program DVCS on H2 target: Study of GPD H Spin and charge cross section Difference (Currently unique COMPASS feature) 1 CFFs H accesed through modulation of DCS, U 14

15 GPD H Compass GPD program DVCS on H2 target: Study of GPD H + Proton size Spin and charge cross section Sum (Currently unique COMPASS feature) s sin s1 Im s 2 sin B(xBj) = ½ <r2> T 4 weeks in 2012 (projection) 2 years of data ( ) 15 xbj

16 DVCS, DVMP on polarized p( ): Study of GPD E : p p > 2018: p p 16

17 GPD L p L p H <=> q (PDF) L p L p E <=> f1t Nucleon spin flip T TMD Sivers Quark kt, Transversely pol. nucleon 17

18 GPD E p pv on transversely polarized proton target > 2018 Phase I COMPASS setup; No Recoil Proton Detector => exclusivity ensured by a missing mass technique; H ( ' )X p H ( ' )X p p p p p M 2X M 2p Emiss = 2M p 18

19 GPD E Transverse Target spin asymmetries for p p > 2018 [COMPASS Coll., PLB731, (2014)] W = 8.1 GeV, pt2 = 0.2 (GeV/c)2, Q2 = 2.2 (GeV/c)2 Leading twist => small - E(,p) 2/3 Eu+ 1/3 Ed +3/8 Eg; - Cancellation between gluon and sea quark contributions: E(,p) 2/3 Eu val+ 1/3 Ed val => Eu val ~ -Ed val. [COMPASS Coll., NPB865 1, 20 (2012)] 19 should be more promising

20 GPD E Transverse Target spin asymmetries for p p > 2018 [COMPASS Coll., PLB731, (2014)] W = 8.1 GeV, pt2 = 0.2 (GeV/c)2, Q2 = 2.2 (GeV/c)2 Leading twist => HT not small 20

21 GPD E Transverse Target spin asymmetries for p p > 2018 W = 8.1 GeV [COMPASS Coll., PLB731, (2014)] W = 8.1 GeV, p 2 T 2 2 = 0.2 (GeV/c), Q = 2.2 (GeV/c) 2 2 T p = 0.2 (GeV/c) 2 pt2 = 0.2 (GeV/c)2 Q2 = 2.2 (GeV/c)2 Leading twist => HT not small described by GPDs: E, HT Model: [Goloskokov-Kroll: Eur.Phys.J. C74, 2725 (2014)] 21

22 GPD E Transverse Target spin asymmetries for p p > 2018 W = 7.1 GeV, p 2 T 2 2 = 0.17 (GeV/c), Q = 2.2 (GeV/c) W = 7.1 GeV 2 pt2 = 0.17 (GeV/c)2 pt2 = 0.17 (GeV/c)2 Q2 = 2.2 (GeV/c)2 Leading twist - no pion pole contribution - positive - form factor - negative - form factor -pole GPD effects embedded with pion pole Dominated by pion pole contribution Model: 22 [Goloskokov-Kroll: Eur.Phys.J. A50, 146 (2014)]

23 GPD E > 2018 DVCS on transversely polarized proton target (with recoil proton detection) Projections for a future addendum of COMPASS physics program 23

24 Conclusion and outlook COMPASS offers unique features for the study of GPDs: * Both and beams; * Large kinematic range (complementary JLab / DESY / EIC); Very encouraging existing results; : DVCS test run; : p p ; : DVCS pilot run => promising DVCS signal; Two years DVCS run ( ) => Two years for DVCS and HEMP measurement 24

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26 GPD H Analysis of 2012 data: selection of ep ep Proton selection Ring B (good resolution) Proton signal in ring B ZA measured p' Z Ring A (poor resolution) ZA interpolated k LH2 target v Protons stopped in B k' Protons escaping ring B 26

27 GPD H Analysis of 2012 data: Kinematic coverage Full Scale y = (E -E ')/E 27

28 GPD H 2009 DVCS Test run (10 days, 40 cm LH2 target, short RPD, No Ecal0) * σ ep ep γ 2 2 BH + DVCS +2 BH DVCS + DVCS (0.005 < xbj < 0.01) (0.01 < xbj < 0.03) BH dominant, DVCS negligible DVCS-BH interference * * Bethe-Heitler (BH) (xbj > 0.03 ) DVCS significant 28

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31 GPD H Compass GPD program DVCS on H2 target: Proton size; Study of GPD H Spin and charge cross section Sum (Currently unique COMPASS feature) sin s s1 s 2 sin Im 1 31

32 Spin and charge cross section Sum (Currently unique capability of COMPASS) SCS, U =2 + Γ (x Bj, Q2, t) P 1 (φ)p 2 (φ) - BH DVCS =2(d σ +d σunpol +eμ Pμ ℑ m I ) BH BH +c cos(φ)+c ( c BH cos(2 φ) ) e 6 DVCS DVCS ( c 0 +c 1 cos(φ)+c 2DVCS cos(2 φ) ) y Q e6 I I +2 eμ Pμ s sin(φ)+s ( 1 2 sin (2 φ) ) 3 x Bj y t P 1 (φ)p 2 (φ) can be extracted t s ℑ m F 1 H +ξ(f 1 +F 2) H 2 F 2 E 4M dominant I 1 ( ) LO ℑm H (ξ,t,q 2)= π e 2f ( H f (ξ, ξ, t, Q2 ) H f ( ξ, ξ, t,q 2) ) f 32

33 Spin and charge cross section Difference (Currently unique capability of COMPASS) + - DVCS =2(Pμ d σ pol +eμ ℜe I) e 6 DVCS DCS, U =+2Pμ 2 2 ( s1 sin (φ) ) yq 6 e I I I I +2e μ c +c cos(φ)+ c cos(2 φ)+c { ( cos(3 φ) }) 3 x Bj y t P 1 (φ)p 2 (φ) can be extracted t I c 1 ℜ e F 1 H +ξ(f 1+ F2 ) H 2 F 2 E 4M dominant ( [ ) ℜe H (ξ, t, Q )= π e P dx H ( x, ξ, t, Q ) x ξ x+ξ f 1 2 LO 2 f f 2 ( )] 33

34 Towards GPD E: p p on transversely polarized proton target 34

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