COMPASS plans to measure GPDs

Transcription

1 COMPASS plans to measure GPDs Jean-Marc Le Goff DAPNIA/CEA-Saclay 24 Feb 2006, Albuquerque workshop on Orbital Angular Momentum The COMPASS experiment GPDs at COMPASS now: deep ρ production 2010: DVCS and HEMP

2 The COMPASS experiment

3 The COMPASS Collaboration (230 Physicists from 12 Countries) Dubna (LPP and LNP), Moscow (INR, LPI, State University), Protvino CERN Bielefeld, Bochum, Bonn (ISKP & PI), Erlangen, Freiburg, Heidelberg, Mainz, München (LMU & TU) Warsaw (SINS), Warsaw (TU) Prag Helsinki Nagoya CEA-Saclay Torino(University,INFN), Trieste(University,INFN) Lisboa Burdwan, Calcutta Tel Aviv

4 COMPASS fixed target experiment at CERN muons hadrons π/k, p Beam: µ + / spill (4.8s / 16.2s) h/spill Beam polarisation: 80% Beam momentum: 160 GeV/c GeV/c COMPASS SPS LHC COMPASS : COmmon Muon and Proton Apparatus for Structure and Spectroscopy

5 Physics goals muon beam nucleon spin structure Quark and Gluon Polarization in (longitudinally) polarized nucleons transverse spin distribution function Δ T q(x) Flavor dependent polarized quark helicity densities Δq(x) Lambda polarisation hadron beams nucleon spectroscopy Primakoff-Reactions - polarizability of π and K Exotics : glueballs and hybrids charmed mesons and baryons - semi-leptonic decays - double-charmed baryons Diffractive vector-meson production

6 Spectrometer trigger-hodoscopes DW45 Polarised Target RICH_1 SM1 dipole SM2 dipole HCAL1 straws Muon-filter2,MW2 Gem_11 ECAL2,HCAL2 MWPC Gems Scifi Muon-filter1,MW1 Veto straws,mwpc,gems,scifi BMS Silicon SciFi Gems,SciFi,DCs,straws Micromegas,DC,SciFi

7 COMPASS and GPDs

8 measurement of GPDs Deeply Virtual Compton Scattering (DVCS): γ* γ* Q 2 Q 2 γ γ Collins et al. hard x + ξ x - ξ x + ξ x - ξ soft GPDs p p t =Δ 2 GPDs p p t =Δ 2 Q 2 large t << Q 2 Hard Exclusive Meson Production (HEMP): hard soft γ* L x + ξ Q 2 GPDs p p t =Δ 2 meson x - ξ γ* L x + ξ Q 2 GPDs p p t =Δ 2 meson x - ξ + γ* L Quark contribution Gluon contribution

9 Complementarity of experiments E=190, 100GeV At fixed x Bj, study in Q < x Bj < 0.01 Gluons H1 and ZEUS PLB517(2001) PLB573(2003) Valence and sea quarks and Gluons Hermes PRL87(2001) COMPASS plans Valence quarks JLab PRL87(2001)

10 if Nμ 5 Q 2 < 17 GeV 2 Benefit of a higher muon intensity for GPDs study if Nμ 2 Q 2 < 11 GeV 2 E=190, 100GeV DVCS limited by luminosity now N μ = μ /SPS spill Q 2 < 7.5 GeV 2 At fixed x Bj, study in Q 2

11 μ flux at COMPASS in 2010? sharing CNGS/FT COMPASS new Linac 4 up to 10 times more p +improve μ line >>LINAC4 From Lau Gatignon

12 What can we measure now?

13 DVCS, HEMP? Vanderhagen et al.: σ DVCS small, bckg 1/Q 4 1/Q 6 HEMP factorization: γ L vector meson decay R=σ T /σ L ρ 0 largest σ vector mesons pseudo-scalar ρ 0 π + π -, charged particules E miss = M 2 X M 2M N 2 N

14 Diffractive ρ 0 production μn μ N ρ π+ π - Q 2 * W N N Exp (NMC, E665, Zeus, H1, Hermes): λ ρ λ γ S-channel helicity conservation SCHC exchanged object has natural parity : P=(-1) J NPE t

15 Spin properties of amplitudes angular dist. of ρ π + π - : spin density matrix el t they are bilinear combinations of helicity amplitudes : T λ ρ λ γ = A ( * γ ( λ ) ρ( λ )) γ ρ λ γ = ± 1, 0 λ ρ = ± 1, 0 9 amplitudes λρ λγ if NPE T = ( 1 T 5 amplitudes λ λ ) ρ γ λ ρ λ γ T 00, T 11 >> T 01, T 10 >> T -11 SCHC 1 helicity flip 2 flips

16 r spin density matrix el t 0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV 2 Distribution : 3 W(cosθ ) = [(1 r ) + (3r 1)cos θ] r in terms of amplitudes: ~ T (ε + δ) T σ Tot 00 2 SCHC σ L σ Tot

17 Determination of R=σ L /σ T If SCHC holds: σl 1 R = = σ (ε + δ) T r r σ L is dominant at Q 2 >2 2002: high stat in large Q 2 range 2003 and 2004 data : -much more stat - better high Q 2 coverage

18 Conclusions on rho SCHC σ tot + R R σ L when Q 2 > 2 R > 1 : accurate σ L we have transv. target spin asym E/H important for Ji sum rule ( E+H) exploratory measurement (no exclusivity, nuclear target)

19 Towards a dedicated experiment for DVCS and HEMP

20 Additions to COMPASS setup 2.5m liquid H2 target to be designed and built L = cm -2 s -1 Exclusivity: at high energy δδm 2 = δ[(m p +m π ) 2 -m p2 ] > 1 GeV 2 need 0.25 GeV 2 for ΔM cut hermetic detector γ ECal 1 or 2 θ γ 12 μ + additional calorimeter at larger angle μ p Recoil detector to insure exclusivity to be designed and built

21 A possible solution ECAL m : Funding by European Union (Bonn-Mainz-Warsaw-Saclay) 45 sector recoil detector - scintillating material studies (200ps ToF Resolution over 4m) - fast triggering and multi-hit ADC/TDC system

22 DVCS background DVCS: μp μpγ with PYTHIA 6.1 simulate: HEπ P: μp μpπ γγ Dissociation of the proton: μp μn*π Nπ DIS: μp μpx with 1γ, 1π, 2π,η not included: Beam halo with hadronic contamination Beam pile-up Secondary interactions External Bremsstrahlung Acceptance cuts: θ γ max = 30 E γ min = 50 MeV θ charged max = 30

23 GPD measurement γ* x + ξ γ x - ξ T DVCS H( x, ξ, t) dx x ξ + iε + L T DVCS : GPDs p p t =Δ 2 P H ( x, ξ, t) dx x ξ + i π H( ξξ,, t) + L ξ x B /(2-x B ) and t are fixed loop over x

24 μ p DVCS + Bethe-heitler μ p BH calculable μ μ γ* γ θ φ p Higher energy: DVCS>>BH DVCS Cross section Smaller energy: DVCS~BH Interference term will provide the DVCS amplitude

25 DVCS + BH with and μ r - μ s +. P μ+ =-0.8 P μ- =+0.8 A DVCS + 1 H(x, ξ, t) + 1 H(x, ξ, t) dx = = P ( μp μpγ ) dx - i π H(x = 1 x ξ + iε 1 x ξ t, ξ~x Bj/2 fixed ξ, ξ, t) 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 cos nφ sin nφ μ μ φ γ* p γ θ σ μ σ s + μ + σ r ~ H (x = ξ,ξ, t) s + r μ + 1 H(x, ξ, σ ~ P dx 1 x ξ μ t)

26 s + μ r σ μ σ at 100 GeV σ s μ + σ Model 1: H(x,0,t) ~ q(x) F(t) Model 2: r μ ~ P H(x,ξ, t) dx x ξ H(x,0,t) = q(x) e t <b 2 > assuming: L = cm -2 s days efficiency=25% = q(x) / x α t 2 bins shown out of 18: 3 bins in x Bj = 0.05, 0.1, bins in Q 2 from 2 to 7 GeV 2 BCA BCA Q 2 =4±0.5 GeV 2 x = 0.05 ± 0.02 x = 0.10 ± 0.03 φ φ

27 advantage of COMPASS kinematics σ s μ + σ r μ ~ P H(x,ξ, t) dx x ξ COMPASS model 1 model 2 Model 1: H(x,0,t) ~ q(x) F(t) Model 2: H(x,0,t) = q(x) e t <b 2 > = q(x) / x α t sensitive to different spatial distributions at different x

28 HEMP: filter of GPDs Cross section: Vector meson production (ρ,ω,φ ) H & E Pseudo-scalar production (π,η ) H ~ & E ~ Hρ 0 = 1/ 2 (2/3 H u + 1/3 H d + 3/8 H g ) Hω = 1/ 2 (2/3 H u 1/3 H d + 1/8 H g ) Hφ = -1/3 H s - 1/8 H g Transverse single spin asymmetry : ~ E/H

29 HEMP in 2010 HEMP requires higher Q2 than DVCS with liq H target and same μ flux as now: ρ Q 2 = 20 GeV 2 ω, π, η, φ Q 2 = 7 GeV 2 if more μ higher Q 2

30 Roadmap for DVCS at COMPASS : ρ production: σ L and transverse spin asym 2005 : "Expression of Interest": SPSC-E0I : recoil detector prototype 2008? : proposal : construction of the recoil detector cryogenic target, ECal0 2010: dedicated GPD measurement

31 Measure OAM? Jisum rulerelates total quark spin to GPDs H and E : J q = 1 2 ΔΣ + L q = xdx [ H ( x, ξ,0) + E ( x, ξ,0)] q q similar sum rule for gluons To do : 1. Measure H and E 2. Perform flavor decomposition 3. Extrapolate to t=0 4. Integrate over x at fixed ξ Sivers asymmetries are also measured at COMPASS

32 SPARE

33 Key role of calorimetry ECAL2 from 0.4 to 2 : mainly lead glass GAMS ECAL1 from 2 to 12 : good energy and position resolution for 2-γ separation in a high rate environment ECAL0 from 12 to 30 : to be designed for background rejection Careful study of γ and π 0 production COMPASS program with hadron beam

34 ρ angular distributions W(cosθ, φ, Φ) depends on the Spin density matrix elements 23 (15) observables with polarized (unpolarized) beam φ This analysis: only one-dimensional angular distribution We will use also: ψ= φ - Φ

35 Angular distributions < Q² < Q² < 0.05 < 0.05 < < Q² Q² < < < < Q² Q² < < Q² < 2.0 < Q² Q² < 10 GeV 2 φ Preliminary : - Corrected for Acceptance, smearing and efficiency (MC:DIPSI gen) Statistical error only, limited by MC -Background not subtracted

36 Measurement of r 04 and Im r φ Distribution : W ( φ) = [ 1 2r1 1 cos2φ 2 π 3 2 2Im r P 1 sin2φ] r Im ~ r Re(T μ ε beam polarisation Spin density matrices: 11 T * -11 ) (ε + δ)t σ Tot 10 = 0 =... = 0 If SCHC holds 2 weak violation

37 COMPASS 6 angular distributions among 18: 3 bins in x Bj =0.05, 0.1, bins in Q 2 from 2 to 7 GeV 2 BCA in DVCS projections for 1 year HERMES e ± p e ± pγ one single bin <x>=0.11 <Q 2 >=2.6