DVCS with CLAS Elton S. Smith Jefferson Lab Conference on Intersections of Particle and Nuclear Physics New York, 2003 Elton S. Smith 1
Deeply Virtual Compton Scattering Inclusive Scattering Forward Compton Scattering θ = 0 γ γ Deeply Virtual Compton Scattering (DVCS) γ ξ t Probes the internal nucleon dynamics at the amplitude level: H(x, ξ, t),. 2
GPDs Contain Much More Information than DIS DIS only measures a cut at ξ=0 Quark distribution q(x) Antiquark distribution q(x) qq distribution 3
Beam Spin Asymmetry DVCS Bethe-Heitler e e γ γ e e e e + γ 2 p GPD s p p p p p dx B dσ = dydtdφ 8π e 3 α x 3 Q 2 B y 2 1 + ε ( ) 2 2 * * T + T + T T + T T BH DVCS BH DVCS DVCS BH Beam Spin Asymmetry ~ I m ( T DVCS ) T BH 4
Kinematics e p e p γ γ θ γ φ γ θ γ 5
DVCS Single-Spin Asymmetry CLAS Collaboration PRL 87, 182002 (2001) Curves from Belitsky et al NP B629 (2002) 323 Measure interference between DVCS-BH A N N + N + N = + = αsin φ + βsin 2φ α± ± β± ± 6
DVCS Single-spin Asymmetry @ 4.8 GeV α A N N + N + N = + = αsin φ + βsin 2φ 7
CLAS Kinematical coverage in Q 2, t, and x B!"#$%$&'( ()*+%, Integrated Luminosity = 2.6x10 40 cm -2 5 5 Q 2 (GeV 2 ) 4 GeV 1 1 x -t (GeV 2 ) 0.1 0.6 0 0.7 8
Goal Analysis of 5.7 GeV data Select a sample of events which are highly enriched with the DVCS reaction ep epγ Use this sample of events to extract the kinematic dependences of the beam spin asymmetry Procedure MC description of exclusive γ and π 0 s Use MC (and data) for guidance on how to determine our sample Estimate relative contributions of photons and π 0 s in all kinematic bins 9
Background: ep epπ 0 p γ (γ) γ e γ CLAS Detector MC 10
t-distribution of π 0 s: e1-6 data vs MC π 0 MC is applicable at small t and small θ πγ θ πγ θπγ <0.3 θ πγ <0.15 θ πγ <0.1 γ -t (GeV 2 ) 11
Angular and energy distribution of MC photons EC Acceptance Energy Cut DVCS γ π 0 π 0 γ s DVCS γ lost Photon Angle Photon Energy (GeV) 12
Selection of epγ sample using MC selection Counts 600 500 DVCS γ 400 300 200 100 0 π 0 γ s 0 0.02 0.04 0.06 0.08 0.1 θ γγc θ γ (measured) θ X (predicted) 13
e p M x2 distributions in 6 GeV data e p X No γ in calorimeter reconstructed π 0 γ γ 14
π 0 contamination in ep sample (0-γ) Reconstructed π 0 γγ epx 0-γ epx 1-γ θ γ*x (rad) 15
Samples used for Analysis ep epx 0-γ (Used to extract asymmetry) No photon found in Electromagnetic Calorimeter (EC) θ γ*x < 0.12 radians M X2 < 0.025 GeV 2 ep epx 1-γ (Used as a check) One photon found in Electromagnetic Calorimeter θγ(predicted) θγ(measured) < 10 mrad ep epπ 0 (Control sample for background) Two photons found in EC and reconstruct to π 0 θ π (predicted) θ π (measured) < 10 mrad 16
Methods Asymmetry A N N + + N N = + = α sin φ + β sin 2φ Fit for α and β Moments A LU = sinφ 2 sin φ 17
A LU for epx, epγ, epπ 0 DVCS dominates ep epx 0-γ epx 1-γ π 0 γγ θ γ*x (rad) 18
DVCS/BH x-dependence @ 6 GeV Moments Asymmetry GPD Model A LU 0.6 0.4 2.5 < Q 2 < 3.0 2.0 < W < 3.0 < Q 2 > = 2.7 < t > = 0.4 Q 2 = 2.5-3.0 GeV 2 <-t> = 0.4 GeV 2 very preliminary 0.2 0 0.25 0.3 0.35 0.4 0.45 x 19
DVCS/BH Q 2 -dependence @ 6 GeV Moments GPD Model A LU 0.6 0.4 A LU 0.6 0.4 < x < 0.5 0.375 < x < 0.425 x = 0.4 2.0 < -W < 0.5 3.0 x = 0.375-0.425 2.0 < W < 3.0 <-t> = 0.48 < t > = 0.48 GeV 2 <-t> = 0.48 GeV < t > = 2 0.44 0.4 0.2 0 very preliminary 0.2 0 very preliminary 2.5 3 3.5 4 Q 2 2.5 3 3.5 Q 2 20
DVCS/BH t-dependence @ 6 GeV GPD Model Q 2 = 1.5-2 GeV 2 x = 0.2-0.3 v e r y p r e l i m i n a r y Q 2 = 2. - 2.5 GeV 2 x = 0.25-0.325 Q 2 = 2.5-3 GeV 2 x = 0.275-0.325 Q 2 > 3 GeV 2 x = 0.35-0.45 21
Summary We have carried out exclusive DVCS measurements with CLAS between 4-6 GeV. At small t: Future: Developed realistic Monte Carlo, including DVCS/BH and π 0 production. At small t, in current CLAS configuration with γ detection at Θ γ,π > 10 o, ep epx(0-γ) allows better isolation of the exclusive process than the detection of epx (1-γ) and with higher statistics. Analysis of larger t kinematics requires more detailed studies. Full corrections for π 0 contamination, systematic studies. Expect factor ~2 more data from run: April - June 2003 Dedicated setup underway for CLAS and in Hall A for DVCS measurements at 6 GeV with full event reconstruction and large kinematic coverage. 22
Hall A DVCS at 6 GeV Methods and Goals 23
Complete Photon Detection in CLAS.+$$/ (0*1"2 γ γ &'-.π /$."..". 24