Progress on the DVCS program at Compass E. Burtin CEA Saclay Irfu/SPhN GDR nucléon, Ecole Polytechnique, 12 décembre 2008

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1 Progress on the DVCS program at Compass E. Burtin CEA Saclay Irfu/SPhN GDR nucléon, Ecole Polytechnique, 12 décembre 2008 Today and tomorrow 2010 and beyond

2 2008 (Today) : Target region Ring B Ring A

3 2008 : Recoil Proton Detector

4 2008 : Recoil Proton Detector Work in progress Hadron beam: Elastically scattered Protons

5 Hadron beam : Elastic events (all A i B j )

6 2008 & 2009 Hadron running «Diffractive production» «Central Production» Search for Hybrid Search for Glueballs π π fast π 1 (1600) slow p slow Data taking completed in 2008 Data taking in 2009

7 2008 : muon beam Goal : Evaluate feasibility of DVCS detection in the Compass environment 160 GeV μ on a 40cm long Liquid H2 target 5 days plan with the Beam Momentum Station reinstalled But LHC accident. Test shorten to 1 ½ day (without BMS) Data taken with μ+ and μ beams reduced beam intensity (1/3) Work in progress RPD and extrapolation to a larger size proton signal observed calibation done with elastic events with hadron beam Calorimetry for exclusive measurement

8 GPD program : experimental set up DVCS µp µ p γ µ 2.5m cryogenic target to be designed and built - Phase 1: long H2 target - Phase 2: transversely polarized µ γ ECal1 + ECal2 θ γ 10 p Nµ= /SPS cycle (duration 9.6s, every 48s) 4m long Recoil proton detector to insure exclusivity to be designed and built

9 2009 : muon beam Exclusive production of photons Wish: 6 weeks in GeV μ+ & μ- 40 cm H2 target and RPD Goal: Evaluate feasibility of further experiments Identify exclusive production of photons Evaluate background Check Bethe-Heitler contribution Evaluate μ+/μ- systematic effects

10 Comparison 160 GeV E beam =160 GeV Q 2 =1.5 x= <t< >t>-0.7

11 Beyond 2010 : Compass_2 proposal A proposal is being written by the collaboration: Polarized pdf with a proton target Transversity through SIDIS Transversity through the «Drell-Yan» process Generalized Parton Distributions

12 Beyond 2010 : «Longitudinal» Goals : Measurement of g 1 p at low x Extraction of Δu, Δd, Δubar, Δdbar, Δs, g 1 NS Additional constrain on QCD fits (ΔG)

13 Beyond 2010 : Transversity Goal : Precision measurement on proton transversity => 160 GeV NH3 polarized target

14 Physics content of Drell Yan d d u u Target p Polarised DY π - p SSA ~ sin(φ Φ s ) + sin(φ+φ s ) Sensitive to Beam: S= GeV 2 Polarized target: NH 3 / 6 LiD Polarization: >80% / >40% Dilution factor: 0.15 / 0.35 Sivers h 1 (x 1,k T 2 ) h 1 (x 2,k T 2 ) Boer-Mulders Transversity Luminosity ~10 31 cm -2 s -1

15 GPD program : kinematic domain HERA COMPASS at CERN-SPS Compass Acceptance High energy muon beam GeV Unique availability of + and polar( +)=-0.80 polar( -)=+0.80 change once per day Gluons valence quarks valence quarks and sea quarks and gluons 250 cm LH 2 target L= cm -2 s -1 COMPASS JLab 12 GeV, FAIR,

16 Tomography of the nucleon GPDs : phase 1 Sea quarks Valence quarks 0.6fm 0.4fm H1 Measure DVCS cross setion : Compass dσ(x)/dt nucleon size vs x

17 from Belitsky, Kirchner, Müller : polarized beam off unpolarized target Harmonic structure dσ (μp μpγ) = dσ BH + dσ DVCS unpol + P μ dσ DVCS pol + e μ a BH Re A DVCS + e μ P μ a BH Im A DVCS µ µ γ* φ γ p θ Known expression P μ e μ e μ P μ Twist-2 M 11 >> Twist-3 M 01 Twist-2 gluon M -11

18 Harmonic structure Case of COMPASS : μ+(p=-0.8) and μ-(p=+0.8) µ µ γ* φ p γ θ dσ μ+ dσ μ = 2 P μ dσ DVCS pol + e μ a BH Re A DVCS s 1 DVCS sinϕ c 0 Int + c 1 Int cosϕ + c 2 Int cos2ϕ + c 3 Int cos3ϕ dσ μ+ + dσ μ- = 2(dσ BH +dσ DVCS unpol ) + 2 e μ P μ a BH Im A DVCS c 0 DVCS +BH + c 1 DVCS +BH cosϕ + c 2 DVCS +BH cos2ϕ s 1 Int sinϕ + s 2 Int sin2ϕ

19 GPDs : phase 1 Constraining the GPD H through beam charge difference : dσ(µ + ) - dσ(µ - ) Re(F 1 H) Comparison with other experiments (Compass : 6 month, ε=10%) beam charge asymmetry => more complex harmonic structure Acosφ H1 Acosφ Hermes Acosφ Compass Hermes published Identification Recoil Detector Missing mass Change beam charge daily yearly Observable Beam charge Difference Beam charge Asymmetry

20 GPDs : phase 2 Constraining the GPD E: Transversaly polarized target and recoil detector dσ(φ S ) - dσ(φ S + ) Im(F 2 H F 1 E)

21 Conclusions The recoil proton detector is operational DVCS measurements at Compass are becoming reality! 6 weeks pilot run in 2009 Wide future physics program at Compass

22 Remark on systematic errors Beam charge difference : BCD = N + Γ + N Γ Counting rates Flux*Accepance*efficiency Modelisation : ΔΓ + Γ + ΔΓ Γ = a + δ = a δ a : charge independent δ : charge independent Impact on BCD : Impact on BC Sum : BCD'= BCD (1 a + δ /BCA) BCS'= BCS (1 a δ BCA)