JLab PAC33, January 16, 2008 Polarization transfer in WACS 1  p   p Polarization transfer in Wide-Angle Compton Scattering Proposal D. Hamilton, R. Gilman, A. Nathan, and B. Wojtsekhowski, co-spokespersons Outline Mechanism of the reaction is a key question Compton  e - >  e process and polarization observables WACS present results and experimental program Proposed experiment and expected results

JLab PAC33, January 16, 2008 Polarization transfer in WACS 2  p   p Two basic options for mechanism: Collective response - several partons involved in high momentum interaction with the photons Individual response - one quark absorbs incident photon and the same quark emits scattered photon Mechanism of the process

JLab PAC33, January 16, 2008 Polarization transfer in WACS 3  p   p Regge poles - VMD - since 1960’s …, Laget pQCD - two-gluon- Brodsky, …, Guichon&Vanderhaeghen, Brooks&Dixon, Thomson et al. Diquark model - Guichon&Kroll, 1996 Leading quark- Brodsky et al., 1972 GPDs (handbag)- Radyushkin, Kroll et al. CQM- G.Miller Main issues: Competing mechanisms Interplay between hard and soft processes Threshold for onset of asymptotic regime Role of hadron helicity flip Studies of the RCS process

JLab PAC33, January 16, 2008 Polarization transfer in WACS 4  p   p Exact formalism for CS off electron

JLab PAC33, January 16, 2008 Polarization transfer in WACS 5  p   p Compton in 1925 Directed Quanta of Scattered X-Rays Arthur Compton and Alfred Simon, PR 26, 3 (1925) recoil electron photo electron

JLab PAC33, January 16, 2008 Polarization transfer in WACS 6  p   p Cross section of WACS vs pQCD calculations normalization problems, calculations are 3-10 times below data Calculation by Thomson et al Calculation by Brooks & Dixon Cornell dataJLab data

JLab PAC33, January 16, 2008 Polarization transfer in WACS 7  p   p Single-quark mechanism “handbag” diagram accounts for scattering from quark and introduces the FFs for ++,+-,.. Form factors: R V, R T, R A KN-like polarization observables quark coupling is small compare t WACS in GPD approach Form factors allows to fit cross section

JLab PAC33, January 16, 2008 Polarization transfer in WACS 8  p   p The plan of proposed experiment proposed -u = 3.6 -u = 2.3  p p

JLab PAC33, January 16, 2008 Polarization transfer in WACS 9  p   p Result: test of reaction mechanism Proposed for E  ~ 4.3 GeV for cm angles 90 o and 110 o ( a 70 o cm data point will be obtained in E07-002)

JLab PAC33, January 16, 2008 Polarization transfer in WACS 10  p   p Result: test of reaction mechanism Proposed for E  ~ 4.3 GeV for cm angles 90 o and 110 o ( a 70 o cm data point will be obtained in E07-002)

JLab PAC33, January 16, 2008 Polarization transfer in WACS 11  p   p Experiment concept: similar to E Beam 80% polarized, 80 μA E e = 4.8, ~ 4.3 GeV Photon flux ~ eqv.  /s; in this experiment it will be boosted by an additional factor 5 with larger calorimeter

JLab PAC33, January 16, 2008 Polarization transfer in WACS 12  p   p Selection of the WACS events Magnet separates e'/γ, no veto needed, allows higher luminosity

JLab PAC33, January 16, 2008 Polarization transfer in WACS 13  p   p Focal Plane Polarimeter FPP used in many Hall A experiments FOM = εA y 2 ~ 0.02, so we need ≈1-2 M RCS events Calibrated with ep elastic scattering

JLab PAC33, January 16, 2008 Polarization transfer in WACS 14  p   p Present proposal (K LL,LT ) has photon energy 4.3 GeV, large s, -t, -u E Results vs new plan s = 6.9 GeV 2 -t = 4.0 -u = 1.13 PRL 98, PRL 94,

JLab PAC33, January 16, 2008 Polarization transfer in WACS 15  p   p WACS research program CS research has significant chance to discover the basic reaction mechanism CS research would be extended for cross sections up to to s = 20 GeV 2 with the 12 GeV upgrade, could possibly find onset of pQCD in exclusive reactions

JLab PAC33, January 16, 2008 Polarization transfer in WACS 16  p   p Proposed Kinematics E γ = 4.3 GeV, s = 8.9 GeV 2 Large -t,-u Precision data for K LL, and K LT

JLab PAC33, January 16, 2008 Polarization transfer in WACS 17  p   p Beam Time Request 21 days beam on target in Hall A

JLab PAC33, January 16, 2008 Polarization transfer in WACS 18  p   p Summary  Mechanism of Real Compton Scattering could be found from proposed experiment - need two data points  WACS has largely the same physics as FFs and DVCS  Experimental technique is well understood and tested  This polarization experiment belongs to 6-GeV program  Beam time request: 21 days beam on target in Hall A

JLab PAC33, January 16, 2008 Polarization transfer in WACS 19  p   p Backup slides

JLab PAC33, January 16, 2008 Polarization transfer in WACS 20  p   p Beam Time Request - by hours 21 days of beam in Hall A

JLab PAC33, January 16, 2008 Polarization transfer in WACS 21  p   p Reply to TAC comments Installation time ~ < 1 month in Hall A. As suggested by RadCon group the photon radiator will be made from Pb, for which case rad-budget is twice lower than for the standard Cu radiator.

JLab PAC33, January 16, 2008 Polarization transfer in WACS 22  p   p Polarization observables in QED

JLab PAC33, January 16, 2008 Polarization transfer in WACS 23  p   p GPD-based prediction for K LS

JLab PAC33, January 16, 2008 Polarization transfer in WACS 24  p   p NLO GPD-based calculations NLO corrections to the cross section are 10-20% Corrections to polarization observables ~ 1-2% at  cm =90 o ~ 10-15% at 120 o

JLab PAC33, January 16, 2008 Polarization transfer in WACS 25  p   p GPDs and form factors of WACS G A at  w    

JLab PAC33, January 16, 2008 Polarization transfer in WACS 26  p   p Systematic Uncertainties By far the largest contribution to systematic uncertainty from dilution factor/background asymmetry stability. In previous analysis, background was fitted with polynomial distribution. In future analysis, a dedicated Monte Carlo (already developed) will be used to better understand background. Expected improvement in systematic uncertainty by factor two or more - dKLL syst ~ 0.01