Twarde procesy ekskluzywne i partonowa struktura nukleonu

1. Twarde procesy ekskluzywne i partonowa struktura nukleonu Andrzej Sandacz Instytut Problemów Jądrowych, Warszawa • DVCS –głęboko wirtualne rozpraszanie Comptona • Ekskluzywna produkcja mezonów wektorowych na akceleratorzeHERA • Spinowa asymetria w produkcjiρ0 z poprzecznie spolaryzowaną tarczą • Ekskluzywna produkcja pionów • Projekt pomiarówDVCSi produkcji mezonów wCOMPASS-ie Seminarium Fizyki Wielkich Energii, Uniwersytet Warszawski 11 stycznia 2008

2. Generalized Parton Distributions g* g Factorisation: Q2 large, -t<1 GeV2 hard x+x x-x soft GPDs t p (P2, s’) p (P1, s) ~ ~ depending on 3 variables: x, x, t 4 Generalised Parton Distributions : H, E, H, E for each quark flavour and for gluons x≈xB/(2-xB ) for DVCS gluons contribute only at higher orders in αs

3. Dirac axial Pauli pseudoscalar GPDs properties and links to ‘standard’ physics forP1 = P2recover usual parton densities no similar relations; these GPDs decouple forP1 = P2 needs orbital angular momentum between partons

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

5. Observables and their relationship to GPDs T The imaginary part of amplitude T probes GPD at x = x The real part of amplitude T dependson the integral of GPD over x DGLAP DGLAP ERBL notation:

6.  different charges: e+ e-(only @HERA!): polarization observables: e DVCS Asymmetries measured up to now e + p p DVCS BH calculable ~ DsC~cosf∙Re{ H+ xH +… } H ~ DsLU~sinf∙Im{H+ xH+ kE} H ~ ~ DsUL~sinf∙Im{H+ xH+ …} H DsUT~sin(f-s)cos∙Im{(F2H–F1E) + … } H, E kinematically suppressed x≈xB/(2-xB ),k = t/4M2

7. <-t> = 0.18 GeV2 <-t> = 0.30 GeV2 <-t> = 0.49 GeV2 <-t> = 0.76 GeV2 CLAS: DVCS - BSA Accurate data in a large kinematical domain Integrated over t

8. Results from JLAB Hall A E00-100 Q2 = 2.3 GeV2 xB = 0.36 PRL97, 262002 (2006) Difference of polarized cross sections Twist-2 Twist-3 extracted twist-3 contribution small Unpolarized cross sections

9. s1int~ dominant term at small |t| GPD !!! No Q2 dependence: strong indication for scaling and handbag dominance

10. Hermes DVCS-TTSA: TowardsE and Ju, Jd Hall A nDVCS-BSA: • Neutron obtained combining • deuteron and proton • F1 small and u & d cancel in x=0.36 and Q2=1.9GeV2

11. Present status of the MODEL-DEPENDENT Ju-Jd extraction Lattice LHPC hep-lat 0705.4295 With VGG Code

12. quarks account for about 43% of the proton spin • for dquarks the spin and OAM (L) conributions almost cancel • LuandLdlarge, butnearly complete cancellation of quarks OAMs 2007 results from LHPC Collaboration (Lattice) μ2 = 4 GeV2 Lq = Jq – ΔΣq / 2 extrapolations tomπ,physusingChPT(numbers below for covariant barion ChPT) Ju+d = 0.213(26) Ju = 0.214(16) Jd = -0.001(16) Lu+d = 0.006(38) Lu = -0.195(38) Ld = 0.200(38)

13. Unpolarised DVCS cross sectionsfrom HERA - Wide range of Q2 - sensitivity to QCD evolution of GPDs - Difference between MRS/CTEQ due to different xG at low xB - Meaurements of b significantly constrain uncertainty of models σDVCSatsmall xB (< 0.01) mostly sensitive to Hg, Hsea Q² and W dependence: NLO predictions bands reflect experimental error on slope b: 5.26 < b < 6.40 GeV-2

14. t dependence of DVCS cross section New H1 results on slopes - DIS07 DIS06

15. Good agreement with NLO predictions GPDs ≡ PDFs at low scale; skewing generated by QCD evolution • Sensitivity to Hg ; 15% change of Hg=> 10% change of cross section • Real part of DVCS amplitude – small effect, few% • Skewing parameter R=Im DIS / Im DVCS ~ 0.5 Interplay of Rs (sea) and Rg (gluons) • Low sensitivity to b(Q2) vs. b(const.) • Color dipole phenomenology (no GPDs) also a satisfacory description Lessons from DVCS at H1/ZEUS

16. 4 Generalised Parton Distributions (GPDs) for each quark flavour and for gluons GPDs depend on 3 variables: x, x, t H E Vector mesons (ρ, ω, φ) ~ ~ H E Pseudoscalar mesons (π, η) Hard exclusive meson production • factorisation proven only for σL σT suppressed of by 1/Q2 necessary to extract longitudinal contribution to observables (σL , …) • allows separation and wrt quark flavours Flavour sensitivity of DVMP on the proton conserve flip nucleon helicity • quarks and gluons enter at the same order of αS • wave function of meson (DA Φ) additional information/complication

17. Dipole models for exclusive VM production at small x • at small x sensitivity mostly to gluons • at very small x huge NLO corrections, large ln(1/x) terms (BFKL type logs) • pQCD models to describe colour dipole-nucleon cross sections and meson WF dipole transv. size W-dep. t-dep. large weak steep small strong shallow • Frankfurt-Koepf-Strikman (FKS) Phys.Rev. D57 (1998) 512 sensitivity to different gluon density distibutions • Martin-Ryskin-Teubner (MRT) Phys.Rev. D62 (2000) 014022 • Farshaw-Sandapen-Shaw (FSS) Phys.Rev. D69 (2004) 094013 • Kowalski-Motyka-Watt (KMW) Phys.Rev. D74 (2006) 074016 sensitivity to ρ0 wave function • Dosch-Fereira (DF) hep-ph/0610311 (2006)

18. Recent ZEUS results on exclusive ρ0 production 2 < Q2 < 160 GeV2 32 < W < 180 GeV 2∙10-4 < xBj < 10-2 | t | < 1 GeV2 significant increase of precision + extended Q2 range 96-00 data: 120 pb-1 arXiv:0708.1478[hep-ex] W-dependenceσ∞Wδ • steeper energy dependence with increasing Q2

19. W-dependence for hard exclusive processes at small x ϕ ϒ J/ψ • steep energy dependence for all vector mesons in presence of hard scale Q2 and/or M2 • ‘universality’ of energy dependence at small x? at Q2+M2≈ 10 GeV2 still significant difference between ρ and J/ψ

20. dσ / dt - ρ0 example (1 out of 6) ZEUS Fit • shallower t-dependence with increasing Q2

21. Q2 and/or M2 • b-slopes decrease with increasing scale approaching a limit ≈ 5 GeV-2 at large scales • ‘universality’ of slopes as a function of (Q2+ M2) ? t-dependence for hard exclusive processes at small x recent data suggestive of possible ≈ 15% difference between ρ and J/ψ

22. Selected results onR = σL/σTforρ0 production the same W- and t-dependence for σLand σT δL ≈ δT bL ≈ bT the same size of the longitudinal and transverse γ*involved in hard ρ0 production i.e. contribution of large qqbar fluctuations of transverse γ* suppressed

23. Comparison to ‘dipole models’ extensive comparison of the models to recent ZEUS ρ0 data in arXiv:0708.1478[hep-ex] below just selected examples • considered models describe qualitatively all features of the data reasonably well • recent ZEUS data are a challenge; none of the models gives at the moment satisfactory quantitative description of all features of the data

24. Comparison to GPD model ‘Hand-bag model’; GPDs from DD using CTEQ6 • Goloskokov-Kroll power corrections due to kt of quarks included arXiv:0708,3569[hep-ph] both contributions of γ*L and γ*T calculated W=90 GeV ■ H1 □ZEUS W=90 GeV ZEUS (recent) σ/10 W=75 GeV complete calculation leading twist only (in collinear approx.) • leading twist prediction above full calculation, even at Q2= 100 GeV2 ≈ 20% • contribution of σTdecreases with Q2, but does not vanish even at Q2= 100 GeV2 ≈ 10% • sea quark contribution, including interference with gluons, non-negligible 25% at Q2 = 4 GeV2

25. The asymmetry defined as to disentangle contributions fromγLandγTthe distribution of ρ0decay polar angle needed in addition Diehl and Sapeta Transverse target spin asymmetry for exclusive ρ0production Give access to GPD E related to the orbital angular momentum of quarks Ji’s sum rule q q So far GPDE poorly constrained by data; mostly by Pauli form factors The asymmetry defined as to disentangle contributions fromγLandγTthe distribution of ρ0decay polar angle needed in addition Diehl and Sapeta Eur. Phys. J.C 41, 515 (2005)

26. Assuming SCHC Simultaneous fit of 12 parameters of azimuthal distributions Im (E*H) / |H|2 Method forL/T separationused by HERMES A. Rostomyan and J. Dreschler arXiv:0707.2486 Angular distribution W(cos θ, φ, φs) and Unbinned Maximum Likelihood fit a prerequisite for the method: determination of acceptance correction as a function ofcos θ, φandφs

27. ρ0transverse target spin asymmetry from HERMES Transversely polarised proton target, PT≈ 75% 2002-2005 data, 171.6 pb-1 • for the first time ρ0TTSA extracted separately for γ*Land γ*T

28. ρ0transverse target spin asymmetry from HERMES • in a model dependent analysis data favours positive Ju in agreement with DVCS results from HERMES

29. obtained using Double Ratio Method ρ0transverse target spin asymmetry from COMPASS Transversely polarised deuteron target (6LiD), PT≈ 50% 2002-2004 data obtained using Double Ratio Method in bins ofϑ = φ – φS: u (d) are for upstream (downstream) cell of polarised target arrows indicate transverse polarisation of corresponding cells raw asymmetry εfrom the fitto DR(η) dilution factor f≈ 0.38

30. ρ0transverse target spin asymmetry from COMPASS new • asymmetry for deuteron target consistent with zero ongoing work on: • longitudinal/transverse separation • separation of incoherent/coherent in 2007 data taken with transversely polarised proton target (NH3)

31. ~ • at small |t’| E dominates as it contains t-channel pion-pole σLVGG LO σLVGGLO+power corrections σT+εσLRegge model (Laget) • LO calculations strongly underestimate the data • data support magnitude of the power corrections(kt and soft overlap) • Regge calculations provides good description of the magnitude of σtot and of t’ and Q2 dependences Exclusive π+ production from HERMES e p → e n π+ ~ ~ • at leading twist σLsensitive to GPDs H and E • L/T separation not possible at HERMES, σTexpected to be supressed as 1/Q2

32. Beam spin asymmetry in exclusive π0 production from CLAS a fit α sinφ Regge model (Laget) pole terms (ω, ρ, b1)+ boxdiagrams (cuts) • first measurement of BSA for exclusive π0 production above resonance region • sizeable BSA (0.04 – 0.11) indicate that both T and L amplitudes contribute • necessity for L/T separation and measurements at higher Q2 ~ • at leading twist σLsensitive to GPD H e p → e p π0 • no t-channel pion-pole (in contrast to exclusive π+ production) • any non-zero BSA would indicate L-T interference, i.e. contribution not described in terms of GPD’s preliminary

33. Cross sections for exclusive π0 production from JLAB HALL A DVCS Collab. E00-110 h = ±1 is the beam helicity from P. Bertin, Baryon-07 t-slope close to 0, maybe even small negative

34. dashed – prediction for σL from VGG model using GPDs Vanderhaeghen, Guichon, Guidal

35. Summary for existing measurements • New precise data on cross sections and asymmetries → significantly more stringent constraints on the models for GPDs • Results on DVCS promissing; indication of scaling and handbag dominance in valence region good agreement with NLO for HERA data • To describe present data on DVMP, both at large and small x, including power corrections (or higher order pQCD terms) is essential • First experimental efforts to constrain GPD E and quark orbital momentum

36. Generalized Parton Distributions @ « Expression of Interest » SPSC-EOI-005 and presentation to SPSC writing of the proposal, preparation of the future GPD program ~2010 Roadmap: Now with 6LiD or NH3 polarized target and no recoil detector After 2010 with H2 or D2 target with a recoil detector and an additional calorimetry

37. Competition in the world and COMPASS role E=190, 100GeV HERA Ix2 COMPASS at CERN-SPS High energy muon beam 100/190 GeV μ+ or μ- change once per day polar(μ+)=-0.80 polar(μ-)=+0.80 2.108 μper SPS cycle in 2010 ? new Linac4 (high intensity H- source) as injector for the PSB + improvements on the muon line Gluons valence quarks valence quarks and sea quarks and gluons COMPASS JLab 12 GeV, FAIR,… 2010 2014

38.  μ’ * θ μ p φ DVCS + BH with polarized and charged leptons and unpolarized target μ μ + p p DVCS BH calculable dσ(μpμp) = dσBH + dσDVCSunpol + PμdσDVCSpol + eμ aBHRe ADVCS + eμ PμaBHIm ADVCS  known Pμ eμ eμ Pμ Twist-2 M11 >> Twist-3 M01 Twist-2 gluon M-11 Belitsky,Müller,Kirchner

39. Both c1Intand s1Int accessible at COMPASS with + and - with F1H dominance with a proton target F2E dominance with a neutron target (F1<<) very attractive for Ji’s sum rule study

40. μ’  * Q2 7 6 5 4 3 2 0.05 0.1 0.2 xBj Muon Beam Asymmetry at E = 100 GeV COMPASS prediction With a 2.5m H2 target μ p  6 month data taking in 2010 25 %global efficiency

41. μ’  * Muon Beam Asymmetry at E = 100 GeV COMPASS prediction μ p  VGG: double-distribution in x, model 1:H(x,ξ,t) ~ q(x) F(t) model 2 and 2*: correl x and t <b2> = α’ln 1/x H(x,0,t) = q(x) e t <b2> = q(x) / xα’t α’ slope of Regge traject. α’=0.8 α’=1.1 Guzey: Dual parametrization model 3: also Regge-motivated t-dependence withα’=1.1

42. Recoil detector design Goals:Detect protons of 250-750 MeV/c t resolution => sTOF < 300 ps exclusivity => Hermetic detector Design : 2 concentric barrels of 24 scintillators counters read at both sides European funding through a JRA for studies and construction of a prototype ( Bonn, Mainz, Saclay, Warsaw )

43. Outer Layer Inner Layer 15° CH Target B1 A2 A1 B0 i 25cm A0 110cm Recoil Detector Prototype Tests (2006) All scintillators are BC 408 A: 284cm x 6.5cm x 0.4cm Equiped with XP20H0 (screening grid) B: 400cm x 29cm x 5cm Equiped with XP4512 Use 1GHz sampler (300ns window) Design by CEA-Saclay/LAL-Orsay Installed downstream of COMPASS Trigger: A&B coincidences finger pairs Resolution on TOF Center 340ps HV low Center 310ps HV high Expected resolution 280 ps

44. Studies for a new ECAL0 (Dubna,…) Light brought by LS fibers to Avalanche Micro-Pixel Photodiode Very Challenging development for new and cheap AMPDs - magnetic field, low threshold detection, high rate environment ECAL0 modules dimesions: ~ 50x50 mm2 (inner), ~ 70x70 mm2(middle and outer) Inner part - 1200 modules (7200 AMPDs) Middle part - 980 modules (3920 AMPDs) Outer part - 1372 modules (4116 AMPDs) The first scint – 3552 AMPDs Total 3552 modules (15236+3552=18788 AMPDs) requiremens on ECAL0 geometry large transverse size: ~ 400 x 400 cm2 small depth: ~ 25 cm

45. ~ 1 mm Depletion region ~ 2 mm Substrate Si Resistor Vbias Guard ring n- p+ n+ Al conductor p- substrate p+ The AMPD (1) • High Gain (105~106) • Good Photon Detection • Efficiency (15~65%) • Compact • (package size ~ a few mm) • Relatively Low Cost • Magnetic-field tolerant • High dark noise • (order of 100-1000 kHz) • Response against input • light yield is non-linear

46. Conclusions & prospects for future COMPASS measurements • Possible physics ouput • Sensitivity to total spin of partons : Ju & Jd • Sensitivity to spatial distribution of partons • Testing a variety of GPD models (VGG, Müller, Guzey, FFS-Sch) toquantify the Physics potentialof DVCS and HEMP • Experimental challenges • Recoil Detectionfor proton (and neutron) • High performance and extension of the EM calorimetry • Roadmap • Now with the transversely polarized targets: 6LiD ( 2006) and NH3 (2007) • 2008-9: A small RPD and a liquid H2 target will be available for the hadron program (ask for 2 shifts + and -) • > 2010: A complete GPD program at COMPASS with a long RPD + large liquid H2 target before the availability of JLab 12 GeV, FAIR, EIC…