Wide Angle Compton Scattering Kees de Jager for

Wide Angle Compton Scattering Kees de Jager for the Hall A RCS Collaboration QCD-N’ 06 Workshop June 13, 2006 I. Compton Scattering off the Nucleon at Large p • Reaction Mechanisms • Nucleon Structure II. JLab E 99 -114 III. Outlook & Summary Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 1

Real Compton Scattering • Part of JLab program of Hard Exclusive Reactions – Elastic form factors – Real Compton Scattering – Virtual Compton Scattering – Deep Virtual Meson Production • Common issues: – Interplay between hard and soft processes – Threshold for onset of asymptotic region • RCS: a possible clean way to distinguish between competing reaction mechanisms Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 2

Asymptotic (p. QCD) Mechanism Brodsky/Le. Page, Kronfeld, Vanderhaeghen, Dixon, Thomson. . . • momentum shared through hard gluon exchange • three active quarks • valence configuration dominates • scaling: d /dt = f( CM)/s 6 • soft physics in the distribution amplitude f(x 1, x 2, x 3) • dominates at “sufficiently” high energy Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 3

Soft Overlap Mechanism (Radyushkin; Diehl, Kroll, et al. ) • Handbag diagram dominates • hard scattering from single quark • momentum shared by soft overlap 1 -body form factor • soft gluon exchange neglected • Structure contained in GPDs: • Links between diverse processes: DIS, (e, e), RCS, ( , ), DVCS, … • New RCS form factors: RV F 1 q(x) RA GA q(x) RT F 2 2 J(x)/x - q(x) Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 4

Goals of E 99 -114 Hyde-Wright, Nathan, Wojtsekhowski Measure cross section to 5% over wide range of (s, t) in order to • Study reaction mechanism: • t dependence at fixed s • s-independence at fixed t • polarization transfer • scaling power n( cm) • Extract form factors: • RV from unpolarized cross section • RA, RT from polarization transfer • For p( , 0) • cross section • polarization Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 5

Longitudinal Polarization of Recoil Proton 1 point 0. 5 ALL handbag 0 -0. 5 p. QCD -1 0 40 80 120 160 CM (deg) Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 6

Experimental Setup Kinematic Range: Proton Spectrometer/FPP 6% Cu Radiator 15 cm LH 2 e- Beam Deflection Magnet e- Veto Lead-Glass Calorimeter • mixed e- beam Þ background & calibrations • good angular resolution • Focal Plane Polarimeter Beam: 5 -40 µA, 2. 5 -5. 75 Ge. V, 75 % polarization Target: LH 2 15 cm long, 3 cm diameter Luminosity: 1000 higher than Cornell Experiment ran in Hall A Jan-Feb, 2002 Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 7

Separation of RCS Events Two-body kinematics ep events RCS events “pion” events Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 8

Experimental results: cross section Identify elastic photon-proton scattering events p --> p through correlation of: • in-plane angles • out-off-plane angles • angle vs. energy Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 9

Experimental results: cross section s scaling for the cross section for n = 8 p. QCD prediction is n =6 use only s = 8. 9 and 11 Ge. V 2 t-independence Radyushkin Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 10

p. QCD Calculations ● ● Kronfeld and Nizic, PRD 44, 3445 (1991) Vanderhaeghen, EPJ A 8, 455 (2000) Brooks and Dixon, PRD 62, 114021 (2000) Thomson, Pang and C. R. Ji, PRD 73, 045023 (2006) + 335 other diagrams <q 2>gluon ≈ 0. 02 s u ≈ 1 Ge. V 2 Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 11

Regge (VM) Calculations by Laget In present kinematical region coherence length of hadronic component of virtual photon is long enough that intermediate vector meson dominates in interaction. Hence, photon production amplitude can be obtained from VM production amplitude, modified by the VM decay constant. Interaction ≈ exchange of VMs (Regge trajectories) and two non-perturbative gluons Cano and Laget, PLB 551, 317 (2003) Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 12

Form factors of RCS and GPDs Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 13

Polarization Transfer KLL E = 3. 2 Ge. V, cm = 120 o (s = 6. 9, t = -4 Ge. V 2) Raw asymmetry for ep and p events ep p raw asymmetry ≈ 0. 05, systematics ≤ 10 -4 Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 14

Longitudinal Polarization Transfer KLL is the average value of the u ≈ 1. 1 Ge. V 2 longitudinal proton spin in the p CM system for a 100% circularly polarized incident photon Conclusion: active quark polarized in same direction as proton Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 15

ALL- initial-state polarization correlation parameter Day, Wojtsekhowski ALL=KLL in p. QCD (mq=0) New proposed at s = 9 Ge. V 2 ALL- KLL difference is a measure of the quark spin flip ~ mq/E Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 16

Future Extensions on the Neutron Need to know the incident photon energy -> Limited luminosity Post-target tagging! For s ≈ 7, t ≈ -4 Ge. V 2 120 hours per kinematics Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 17

RCS perspective with 12 Ge. V Jlab beam Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 18

Summary • Photon scattering from a single quark, without a hard gluon exchange, appears to be the dominant mechanism in the RCS process on the proton in an energy range of a few Ge. V. • The agreement between the present experimental results and hand-bag based calculations allows the use of the RCS results to constrain GPD models. Wide-angle CS at s ≈ 10 -20 Ge. V 2 provides a powerful tool to study the partonic structure at large -t, complimentary to elastic form factor measurements. • How will these RCS results constrain a global “form factor” fit ? • Planned extensions angular distribution of KLL and KLT at s = 7 Ge. V 2 measure ALL at same kinematics as KLL in Hall C RCS on the neutron 12 Ge. V Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 19

E 02 -013 results are coming Cates, Liyanage, Wojtsekhowski • Nucleon form-factor data provide a constraint for the GPD integral • Data on both the proton and the neutron form factors are needed to constrain the u and d quark GPDs • Experiment E 02 -013 in Hall A just completed data taking for the neutron electrical form factor at momentum transfers of 1. 2, 1. 8, 2. 5 and 3. 5 Ge. V 2 Thomas Jefferson National Accelerator Facility Operated by the Jefferson Science Associates for the U. S. Department Of Energy Wide-Angle Compton Scattering, June 13, 2006 20