Wide-Angle Compton Scattering up to 10 GeV

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1 γp -> γp Wide-Angle Compton Scattering up to 10 GeV B. Wojtsekhowski Outline WACS physics WACS method and results Next WACS measurements Proposed measurements with NPD/HMS JLab, January 24, 2013 WACS in 12 GeV era 1

2 WACS: Introduction Part of JLab program of Hard Exclusive Reactions Elastic Form Factors: GM P,GE P,GM N,GE N,FF π WACS: high t in two-photon reaction Deeply Virtual Compton Scattering (DVCS) Deeply Virtual Meson Production Common issues: Handbag diagram Interplay between hard and soft processes Threshold for onset of asymptotic regime Role of hadron helicity flip JLab, January 24, 2013 WACS in 12 GeV era 2

3 Unification of nucleon structure within DIS Form factors DVMP N -> N* DVCS WACS x ξ t JLab, January 24, 2013 WACS in 12 GeV era 3

4 P. Kroll, analysis JLab, January 24, 2013 WACS in 12 GeV era 4

5 and form factors of WACS JLab, January 24, 2013 WACS in 12 GeV era 5

6 Cross section of Wide-Angle Compton Scattering Three-quark mechanism dominates at asymptopia 2 hard gluon exchanges. Constituent counting rules: dσ/dt = f(θ CM )/s 6 complicated polarization observables Single-quark mechanism handbag diagram dominates. Form factors: simple polarization observables JLab, January 24, 2013 WACS in 12 GeV era 6

7 hard soft Polarization observables of WACS in handbag calculations photon helicity and P L of the recoil proton LO: LO + R T : JLab, January 24, 2013 WACS in 12 GeV era 7

8 P. Kroll 2005 WACS cross section JLab, January 24, 2013 WACS in 12 GeV era 8

9 The experiment provides the answer Test of the reaction mechanism in the cloud chamber. Arthur Compton photon detected beam dump Physical Review (1925) electron detected

10 The 6 GeV WACS experiment 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(θ cm ) JLab, January 24, 2013 WACS in 12 GeV era 10

11 Mixed e/γ beam > productivity 1300 higher than clean γ photons/sec Two body kinematics ep events pion events RCS events

12 Results of 6 GeV WACS experiment PRL 98, (2007) 9 8 n (θ cm ) Hard-gluon, n= θ cm (deg) n = 8.0 +/- 0.2 JLab, January 24, 2013 WACS in 12 GeV era 12

13 Form factors of RCS and partonic structure of the nucleon JLab, January 24, 2013 WACS in 12 GeV era 13

14 Polarization transfer in Wide Angle Compton Scattering Very clean data Calibration to elastic e-p polarization data taken parallel to WACS Expected small systematic uncertainty Result is closer to CQM ep JLab, January 24, 2013 WACS in 12 GeV era 14

15 RCS perspective with 12-GeV JLab JLab, January 24, 2013 WACS in 12 GeV era 15

16 Experimental Setup in Hall C Beam current, Je = 40 µa Beam energy, Ee = 6.6, 8.8, and 11 GeV Target, LH2, 15 cm long Cu radiator 6% rad. length HMS detects proton, Ωp = 6+ msr Calorimeter for the photon 6-55 deg. proton proton photon photon

17 WACS counting rate cross section solid angle photon flux Pion/RCS ratio and cross section for the 4.3 GeV photon energy JLab, January 24, 2013 WACS in 12 GeV era 17

18 Photon energy range Photon fraction JLab, January 24, 2013 WACS in 12 GeV era 18

19 Experimental Setup in Hall C Photon energy, E g = 10 GeV Target, LH2, 15 cm long Photon flux, 2.2 x per sec (inside 10% energy interval) HMS detects proton, Ω p = 6 msr E γ = 10 GeV, s = GeV 2 θγ, [deg] Eγ, [GeV ] θ p, [deg] J h J v P p, [GeV/c] θ cm, [deg] t, [GeV 2 ] Time, (1200 ev.), 40µA hours NA NA NA Total (1.5-2 factor for calibration) NA NA NA Beam time (40 µa) request is 309 hours A full experiment (3 beam energies) will take about 30 days ( 40 µa )

20 RCS perspective with 12-GeV JLab E γ = 8 GeV, s = 15.9 GeV 2 θγ, [deg] Eγ, [GeV ] θ p, [deg] J h J v Ω γ, [mrad] Distance to the calo, [m] P p, [GeV/c] θ cm, [deg] t, [GeV 2 ] JLab, January 24, 2013 WACS in 12 GeV era 20

21 RCS perspective with 12-GeV JLab E γ = 10 GeV, s = GeV 2 θγ, [deg] Eγ, [GeV ] θ p, [deg] J h J v Ω γ, [mrad] Distance to 10msr/3m, [m] P p, [GeV/c] θ cm, [deg] t, [GeV 2 ] Time, [hour] NA NA NA JLab, January 24, 2013 WACS in 12 GeV era 21

22 Experimental Setup in Hall C Radiation budget for a calorimeter using E experience Low energy e/γ background is dominant (10-15 cm of lead-glass) 1.05 Relative transparency Distance from the calorimeter face [cm] The red line is for the blocks after experiment: 29 Coulomb ~ 500 hours at 15 µa The blue is after annealing

23 Experimental Setup in Hall C Radiation budget for a calorimeter from the E The total energy flow obtained from the pedestal shift and from the rate vs. threshold data like these: Counting rate [Hz] 10 Beam energy: 3.3 GeV 2 10 Beam current: 10 µ A 2 10 Target: 15 cm LH 2 O Calorimeter angle: 35 E Calorimeter to target: 10.6 m elastic 10 Solid angle: 0.36 msr Threshold [GeV] Threshold [mv] 3 10 Counts counts Beam energy: Beam current: Target: Calorimeter angle: Calorimeter to target: Solid angle: 5.76 GeV 10 µa 15 cm LH m 3.0 msr energy (MeV) Energy [MeV]

24 Experimental Setup in Hall C Radiation budget for a calorimeter using E experience A total energy flow estimate obtained from the pedestal shift and from the rate vs. threshold data is: df dω = A exp ( 9E γ/e max ) Energy flow = [MeV/msr/s/(1µA)] For the 1000 hour run with a beam of 40 µa on 15 cm LH2 (+6% Cu radiator) and a 10 msr solid angle calorimeter at 5 meters we found the specific energy density deposited in the calorimeter will be 1.5 J/g, which corresponds to 150 krad The 10 cm plastic shield will reduce the load by a factor of 4 Projected radiation dose for a calorimeter is 36 krad

25 Summary WACS would be able to cover most of the s/t range Projected parameters of NPD (10 msr at 3 m) are adequate (assume wide angle/distance range) Experiment will need about 1000 hours