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 E99-114 III. Outlook & Summary

Real Compton Scattering Part of JLab program of Hard Exclusive Reactions Elastic form factors Real Compton Scattering Virtual Compton Scattering Deep 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 An extra knob….s and t dependence

Asymptotic (pQCD) Mechanism Brodsky/LePage, Kronfeld, Vanderhaeghen, Dixon, Thomson ... momentum shared through hard gluon exchange three active quarks valence configuration dominates scaling: d/dt = f(CM)/s6 soft physics in the distribution amplitude f(x1, x2, x3) dominates at “sufficiently” high energy

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  F1  q(x) RA  GA  q(x) RT  F2  2J(x)/x - q(x) RCS particularly good process to measure NFPD’s since only involves 1 hadron

Measure cross section to 5% over wide range of (s, t) in order to Goals of E99-114 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

Longitudinal Polarization of Recoil Proton - 1 . 5 4 8 2 6 A L q C M ( d e g ) point handbag pQCD

Experiment ran in Hall A Jan-Feb, 2002 Experimental Setup Kinematic Range: 6% Cu Radiator Proton Spectrometer/FPP 15 cm LH2 Deflection Magnet Veto Lead-Glass Calorimeter e- Beam  e- mixed e- beam background & calibrations good angular resolution Focal Plane Polarimeter Beam: 5-40 µA, 2.5-5.75 GeV, 75 % polarization Target: LH2 15 cm long, 3 cm diameter Luminosity: 1000 higher than Cornell Experiment ran in Hall A Jan-Feb, 2002

Separation of RCS Events Two-body kinematics Template for presentation in Two Photon Workshop ep events RCS events “pion” events

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 Template for presentation in Two Photon Workshop

Experimental results: cross section s scaling for the cross section for n = 8 pQCD prediction is n =6 use only s = 8.9 and 11 GeV2 t-independence Radyushkin Template for presentation in Two Photon Workshop

pQCD Calculations Template for presentation in Two Photon Workshop 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 <q2>gluon ≈ 0.02 s Template for presentation in Two Photon Workshop u ≈ 1 GeV2

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)

Form factors of RCS and GPDs Template for presentation in Two Photon Workshop

Polarization Transfer KLL E = 3.2 GeV, cm = 120o (s = 6.9, t = -4 GeV2) Raw asymmetry for ep and p events ep gp Template for presentation in Two Photon Workshop raw asymmetry ≈ 0.05, systematics ≤ 10-4

Longitudinal Polarization Transfer u ≈ 1.1 GeV2 KLL is the average value of the longitudinal proton spin in the p CM system for a 100% circularly polarized incident photon Conclusion: active quark polarized in same direction as proton

ALL- initial-state polarization correlation parameter Day, Wojtsekhowski ALL=KLL in pQCD (mq=0) New proposed at s = 9 GeV2 Template for presentation in Two Photon Workshop ALL- KLL difference is a measure of the quark spin flip ~ mq/Eg

Future Extensions on the Neutron Need to know the incident photon energy -> Limited luminosity Post-target tagging! For s ≈ 7, t ≈ -4 GeV2 120 hours per kinematics

RCS perspective with 12 GeV Jlab beam Template for presentation in Two Photon Workshop

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 GeV. 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 GeV2 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 GeV2 measure ALL at same kinematics as KLL in Hall C RCS on the neutron 12 GeV

E02-013 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 E02-013 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 GeV2