Wide-Angle Compton Scattering and Pion photo-production

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1 γp -> γp Wide-Angle Compton Scattering and Pion photo-production B. Wojtsekhowski, for the NPS/WACS/Pion collaboration Outline WACS proposal update (David&Simon) Pion proposal formulation (Dipangkar) Experimental technique. How will we do these experiments? Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 1

2 Unification of EM reactions Longitudinal Transverse DIS Form factors DVMP GPDs Pion DVCS WACS x ξ t Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 2

3 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 Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 3

4 and form factors of WACS GPDs Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 4

5 FF F1 at large-q 2 hard and soft spectator contributions Nucleon GPDs FF F 1 Chernyak, Zhitnitsky 1977 Brodsky, Lepage 1979 N.Kivel & M.Vanderhaeghen Duncan, Mueller 1980 Fadin, Milshtein 1981,82 NK, Vanderhaeghen 2010 NK, /Q 4 4 /Q 4 hard- and soft-spectator contributions have the same power large rapidity log s from the soft-collinear overlap this estimates are also true for WACS amplitudes Soft spectator scattering is especially important in processes with baryons Can one observe and to study the soft-overlap mechanism? The best opportunity is provided by WACS N.Kivel s talk 11/13 6 Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 5

6 WACS phenomenology GPDs T 2 (s, t) =C 2 (s, t)r(s, t) T i (s 0,t)=C i (s 0,t)R(s, t)+ H i (s 0,t) C i (s 0,t) H 2(s, t) C 2 (s, t) i =4, 6 s 0 6= s! Assume that the hard-spectator corrections are small ) R(s, t) = T 2(s, t) C 2 (s, t) R(t) dominates by the soft-spectator contribution this can be checked experimentally d dt = 2 1 s 2 R(s, t) 2 ( su) 2 C 2(s, t) C 4(s, t) 2 + C 6 (s, t) 2 m=0 To the leading order accuracy d dt 2 2 s s 2 R(s, t) 2 u + u s Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 6 m=0 C i = Ci LO + s 4 C F Ci NLO + N.Kivel s... talk 11/13 = d KN 0 dt R(s, t) 2 17

7 WACS phenomenology with NLO corrections GPDs R(s, t) s d exp /dt d KN 0 /dt s 4 C F C2 LO Re [C2 NLO C4 NLO ]+C6 LO Re [C NLO C2 LO 2 + C6 LO 2 6 ] used data: JLab/Hall-A, 2007 R #$'# #$&% #$&# #$#%!"# "1$7+,-. ' 534+6"7$8+,-. ' 534+6"&#$8+,-. ' ' / 0 % 1 2 ()*+,-. ' NK, Vanderhaeghen 2013 (in preparation) all power corrections m/q are neglected empirical fit: R(s, t) = 2 =0.95 ± 0.02 N.Kivel s =1.67 ± 0.05 talk 11/13 t 2 /dof = Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 7

8 WACS phenomenology with NLO corrections & kinematical power corrections GPDs Next-to-Leading-order NK, Vanderhaeghen, to appear R s, GeV !"%! 123()45! )95/":)*+, %!"$# )95:";)*+, % )95$!";)*+, %!"$!!"!#! % -. # / 0 &'()*+, % empirical fit: R(s, t) = 2 t,gev 2 /d.o.f R, NLO 0.95 ± ± R, LO 1.0 ± ± R, NLO 0.98 ± ± N.Kivel s talk 11/13 21 Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 8

9 New experiment at Jlab in unexplored range of s and t GPDs Cornell, 1977 Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 9

10 New experiment at Jlab in unexplored range of s and t GPDs Table 1: Kinematics variables for WACS in five settings with a 4-pass beam (4A 4E) and five settings with a 5-pass beam (5A 5E). Kin E in E p p p cm s -t -u [GeV] [ ] [GeV] [ ] [GeV/c] [ ] [GeV 2 ] [GeV 2 ] [GeV 2 ] 4A B C D E A B C D E Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 10

11 Some estimates for new kinematics Table 5: Approximate widths of 0 -decay photon distributions ( ), out-of-plane angular resolutions for protons in HMS ( p, interpolated from 2.7 mrad at p p = 2.4 GeV/c to 1.6 mrad at p p = 6.7 GeV/c by the function p 1 + 4/p p ), the out-of-plane angle Jacobian Jac = d p 0/d 0, and the corresponding ratio that gives an estimate of how well the RCS peak can be separated from the 0 background (large ratio = better separation). π Kin E p p p Jac /(Jac p ) [GeV] [GeV/c] [mrad] [mrad] [1] [1] 4A B C D E π Difference in RCS and Pion event distributions 5A B C D E Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 11

12 Selection of the kinematics WACS at 8 and 10 GeV: New Kinematic Settings Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 12

13 Selection of the kinematics WACS at 8 and 10 GeV: New Kinematic Settings E New Proposal 5 points with an 8.8 GeV beam 5 points with an 11 GeV beam Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 13

14 expectation: R is a function of t, independent of s Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 14

15 Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 15

16 Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 16

17 Total beam time is 22 days (instead of 42 in PAC40 proposal) Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 17

18 The concept of the WACS experiment Mixed e/γ beam => productivity 1300x higher than with clean γ beam Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 18

19 Selection of WACS events GPDs + epγ Magnet separates e'/γ, no veto needed, allows higher luminosity Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 19

20 Single pion production is the main background in RCS Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 20

21 Another background in RCS Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 21

22 Another background in RCS superior energy resolution of NPS will be helpful Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 22

23 How to measure π 0 production? GPDs It is the main background in RCS δy (cm) RCS π 0 ep δx (cm) Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 23

24 Pion Photo-production GPDs γ (L.Y. Zhu et al., PRL 91, (2003)) Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 24

25 Neutral Pion Photo-production GPDs + p 0 + p ( cm = 90 o ) s 7 d /dt(10 7 GeV 14 nb/gev 2 ) Bolan et al., PRL 18, 926 (1967) Anderson et al., PRD 14, 679 (1976) Shupe et al., PRD 19, 1921 (1979) Alvarez et al., PRL 12, 707 (1964) The neutral pion photoproduction data have HUGE discrepancy between the various data sets The new measurements must be done at JLab s(gev) Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 25 25

26 Pion Photo-production GPDs prediction of the ratio π 0 /π + by P. Kroll for the 90 degree CM 0 / Shupe et al. and + Anderson et al., Zu et al. CM =90 o 0 and + Anderson et al. 0 Bolan et al. and + Zu et al. 0 Alverez et al. and + Anderson et al., Zu et al. Unlike in the charged pion ratio, the GPD contributions do not cancel in the ratio for the neutral/charge: π 0 /π + 2 Moreover, the cross sections disagree with measurements by orders of magnitude. 1 However, the inconsistency between the data sets makes it impossible to compare with the predictions of Kroll et al s (GeV 2 ) Additional measurements over a wide kinematic range are needed to resolve the the inconsistency between the data sets. Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 26

27 Proposed kinematics GPDs Pion needs 3, 4, 5 pass beam (~40 hrs of beamtime) RCS needs only 4 and 5 t (GeV 2 ) 10 3-pass CM = CM = CM = s (GeV 2 ) Range in s and -t as defined by the HMS acceptance Including time for configuration change this would need 4 extra PAC days. Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 27 27

28 π + photo-production in Hall-A GPDs The GE n and GM n experiments in Hall A using the SBS/BigBite and HCAL-J will use π + photo-production for detectors calibration. Projections based on combining data from Hall-A and Hall-C 10% uncertainty assumed for both neutral and positive pions 0 / CM =90 o 0 Shupe et al. and + Anderson et al., Zu et al. 0 and + Anderson et al. 0 Bolan et al. and + Zu et al. 0 Alverez et al. and + Anderson et al., Zu et al. Projection 0 / + 2 Anderson et al. E = 5 GeV E = 7 GeV E = 9 GeV E = 11 GeV Projection pass 3-pass s (GeV 2 ) t (GeV 2 ) Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 28 28

29 Summary 6-GeV WACS observables are in agreement with the handbag model: photon scattering from a single quark New experiment will explore never investigated energy regime It will provide accurate inputs to the GPDs and SCET (and 2γ ) WACS form factor has been predicted in both approaches Pion cross section ratio has been predicted in handbag (GPD) Pion data would be 85% free: background of the WACS runs High energy data for charge pion production are also free: would be obtained from the GEN&GEM calibrations Energy resolution of NPS is essential for high quality results Hall C collaboration, February 21, 2014 WACS/Pion with HMS-NPS 29