1 / 31

The GPD program at COMPASS

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. g *. g. Factorisation:

charlottelewis
Download Presentation

The GPD program at COMPASS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. Generalized Parton Distributions and DVCS g* g Factorisation: Q2 large, -t<1 GeV2 hard x+x x-x soft GPDs Pi, hi – proton momentum P2,h2 P1,h1 t and helicity ~ ~ depending on 3 variables: x, x, t 4 Generalised Parton Distributions : H, E, H, E for each quark flavour and for gluons for DVCS gluons contribute at higher orders in αs

  3. allows separation and wrt quark flavours H E Vector mesons (ρ, ω, φ) ~ ~ H E Pseudoscalar mesons (π, η) conserve flip nucleon helicity GPDs and Hard Exclusive Meson Production • 4 Generalised Parton Distributions (GPDs) for each quark flavour and for gluons • factorisation proven only for σL σT suppressed by 1/Q2 necessary to extract longitudinal contribution to observables (σL , …) Flavour sensitivity of HEMP on the proton • quarks and gluons enter at the same order of αS • at Q2≈ few GeV2power corrections/higher order pQCD terms are essential • wave function of meson (DA Φ) additional input

  4. forP1 = P2recover usual parton densities no similar relations; these GPDs decouple forP1 = P2 needs orbital angular momentum between partons Dirac axial Pauli pseudoscalar GPDs properties, links to DIS and form factors Ji’s sum rule total angular momentum carried by quark flavour q (helicity and orbital part)

  5. 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 q q x z p p t • Contribution to the nucleon spin puzzle • E related to the angular momentum • 2Jq =  x (Hq (x,ξ,0)+Eq (x,ξ,0)) dx • ½ = ½ ΔΣ+ ΔG + < Lzq > + < Lzg > E ‘Holy Grails’ of the GPD quest

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

  7. Nucleon tomography from fits to elastic form factors from GPD fits to Diehl, Feldmann, Jakob, Kroll – (2005) unpolarized proton uV dV

  8. Nucleon tomography from fits to elastic form factors from GPD fits to Diehl, Feldmann, Jakob, Kroll – (2005) in ┴ polarized proton uV dV

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

  10. μ’  μ p’ Future GPD program @ COMPASS • The GPDs program is part of the COMPASS Phase II (2012-2016) proposal to be submitted to CERN in 2009. • The first stage of this program requires a 4 m long recoil proton detector (RPD) together with a 2.5 m long LH2 target. Upgrades of electromagnetic calorimeters to enlarge coverage at large xB and reduce bkg. • The second stage requires either a new transversely polarizedNH3 target(with a thin superconductiong coil) inserted in the RPD or a new SciFi (?) RPD inserted in the existing NH3 target system. primary physics goal is DVCS, meson production will be done at the same time Stage 1 (~2012) to constrain H dσ/dt → t-slope parameter b d(+, ) +d(-, )  Im(F1H) sin  d(+, ) -d(-, )  Re(F1H) cos  Stage 2 (~2014) to constrain E d(, S) -d(, S+π)  Im(F2H – F1E) sin (- S) cos 

  11. COMPASS kinematical coverage for DVCS CERN SPS high energy muon beam 100/190 GeV with a 2.5m long LH2 target L= 1032 cm-2 s-1 Q2→ 8 GeV2 → 12 GeV2 if luminosity increased by factor 4 ~10-2 < x <~10-1 x → 0.15 with upgrade of present calorimetry

  12. The GPDs in the next several years • H1, ZEUS, HERMES, JLab 6 GeV are providing the first results significant increase of statistics expected after full data sets analysed • The energy upgrade of the CEBAF accelerator will allow access to the high xB region which requires large luminosity. • The GPD project at COMPASS will explore intermediate xB(0.01-0.10) and large Q2 (up to ~8(12) GeV2) range COMPASS will be the only experiment in this range before availability of new colliders

  13. DVCS + BH with + and - beams and unpolarized proton target  μ’ * θ μ  p dσ(μpμp) = dσBH + dσDVCSunpol +PμdσDVCSpol + eμaBHReTDVCS +eμ PμaBHImTDVCS Beam Charge & Spin Difference DU,CS  d( +) -d( -)  2(eμaBHReTDVCS +PμdσDVCSpol) Beam Charge & Spin Sum SU,CS  d( +) +d( -)  2(dσBH + dσDVCSunpol + eμPμaBHImTDVCS)

  14. 1<Q2<8 t-slope measurement; relevant for nucleon ‘tomography’ Using SU,CS, integrating over  dDVCS/dt ~ exp(-B|t|) and subtracting BH ‘tomography’: B(x) ~ <rT2>(x) FFS model adapted for COMPASS by A.S. assumed B(x) = b0 + 2 α’ ln(x0/x) α’ =0.125 GeV-2 with 160 GeV muon beam 2.5m LH2 target global = 10%, 140 days L=1222pb-1 for gluons, from J/Ψ at HERAα’ ~ 0.164 GeV-2- photoproduction (Q2≈0) α’ ~ 0.02 GeV-2- DIS (Q2=2-80 GeV2) for valence quarks, from fits to FFα’~ 1 GeV-2

  15. Beam Charge and Spin Asymmetry from DU,CS/ SU,CS : Comparison to different models 160 GeV muon beam 2.5m LH2 target global = 10%, 140 days L=1222pb-1 D.Mueller-hep-ph/0904.0458 fit to world data

  16. Beam Charge and Spin Asymmetry in various kinematic bins Q2 = 12 GeV2 VGG model 4 < Q2 < 8 160 GeV muon beam 2.5m LH2 target global = 10%, 140 days L=1222pb-1 x = 0.15 0°   360° 2 < Q2 < 4 1 < Q2 < 2 0.005< x < 0.01 0.01< x < 0.02 0.02 < x < 0.03 0.03< x < 0.07 If Lumi × 4  more bins up to Q2 = 12 GeV2

  17. dσT(μpμp) = ST PμdσTBH + STdσTDVCS + ST PμdσTDVCSpol + STeμ aTBHTTDVCS +STeμ PμaTBHTTDVCSpol Single γ production withtransversely polarised target dσ(μpμpγ) = dσU(μpμp)+ dσT(μpμpγ) unpolarized target transversely polarized target 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 cf. the next slide both+ and - beams needed DT,CS ADT,CS≡ DT,CS/d σ0  dT( +) -dT( -) measure or/and AST,CS≡ ST,CS/d σ0 ST,CS  dT( +) +dT( -) dσ0is unpolarised, charge averaged cross section

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

  19. An ‘appetizer’ – HERMES measurements of DVCS from Transverse Target Spin asymmetryAUTsin(ϕ-ϕs)cosϕ c1,T-Int for proton sensitiveto Ju(not to Jd) =>allows model dependent constraints study of azimuthal asymmetries from transversely polarized NH3 target is a part of Phase 2 of COMPASSproposal, simulations are in progress

  20. TTS asymmetryAUTsin(ϕ-ϕs) for ρ0 production on protons from COMPASS 2007 data from transversely polarised NH3 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 Hq,g , Eq,g < Q2 > ≈ 2.2 (GeV/c)2 < xBj > ≈ 0.04 < pt2 > ≈ 0.18 (GeV/c)2 AUTsin(ϕ-ϕs) compatible with 0 in progress: L/T γ* separation (using ρ0decay angular distribution)

  21. ρ+ W = 10 GeV t’ integrated K*0 ρ0 ω predictions for protons AUT(ρ)≈ -0.02 AUT(ω)≈ -0.10 Comparison to a GPD model ‘Hand-bag model’; GPDs from DD using CTEQ6 • Goloskokov-Kroll power corrections due to kt of quarks included [EPJ C53 (2008) 367] both contributions of γ*L and γ*T included COMPASS p↑ preliminary

  22. Detectors to be built • Large Proton Recoil Detector and a long LH2 target (Phase 1) with dedicated read out electronics with 1 GHz sampling • Proton Recoil Detector for a transversely polarized ammonia target (Phase 2) • Large Q2 trigger • Monitoring of muon flux • ECAL1 and ECAL2 to be extended and upgraded • ECAL0 to be designed and build to increase range in xBj and to reduce background sketch, not to the scale

  23. μ’ γ proton 2008 DVCS test run Goal: evaluate feasibility to detect DVCS/BH in the COMPASS setup μ p →μ p γ Use COMPASS ‘hadron’ set-up in EMCals LH2 40 cm in the small RPD 1.5 days of 160 GeV muon beam (μ+and μ-)

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

  25. Recoil Proton Detector Small 1 m long Recoil Proton Detector and a 40cm LH2 target available in 2008/2009 Proton identification in RPD Elasticscattering with pion beam (2008)

  26. Selection of exclusive single γ events 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. GeV/c DVCS Bethe-Heitler σ ≈ 1 ns σ ≈ 4.5 cm Timing difference : z positiondifference : tμ- tRPD zμμ’- zRPD

  27. Kinematic constraints in the transverse plane μ’+γ Δp =|Pμ’+γ|-|PRPD| Δpperp< 0.2 GeV γ μ’ Δϕ proton Transverse plane Δϕ=ϕmiss- ϕRPD Δϕ< 36 deg

  28. μ’ * θ μ p φ Azimuthal distribution for single photon events After cuts All Q2 Before cuts All Q2 A flat background contribution in φsuppressed The peakatφ=0 remains =>identified as BH

  29. μ’ * θ μ p φ Azimuthal distribution for exclusive single photon events Monte-Carlo simulation of BH (dominant) and DVCS Bethe-Heitler DVCS After all cuts, Q2>1GeV2 Data & MC => global = 13%±5% Clear signature of dominant BH events

  30. 2009DVCS pilot run 2 weeks of DVCS pilotrun 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)

  31. 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 includingGPDwill be submitted to SPSC in 2009 • 2008-9: The small RPD and liquid H2 target are available for the hadron program  tests of DVCS feasibility • from 2012: The complete GPD program at COMPASS with a long RPD + liquid H2 target (2012) + transversely polarized ammonia target (2014)

More Related