Beyond the Born Approximation

Transcription

1 Beyond the Born Approximation Measuring the Two Photon Exchange Correction in Hall D Robert Paul Bennett Old Dominion University D. Adikaram, D. Rimal, P. Khetharpal, B. Raue, L. Weinstein Hall D PWG Newport News, VA October 22, 2012 Robert Paul Bennett Beyond the Born Approximation 1

2 Start with some conclusions Large discrepancy in G p E measurements that grows with Q2 Resolvable by considering σ(e + p)/σ(e p) CLAS eg5 experiment Produced simultaneous e + /e ( 100 pa each) Continuous beam energy distribution (Brem. beam) Wide Q 2 and angle (ε) coverage Control over systematics Extensive beam profiling Simultaneous e + /e measurements Reversed magnets to remove acceptance and beam asymmetries Initial Hall B results consistent with e + /e ratio needed to resolve G p E discrepancy. This experiment can be done much better in Hall D! Discrepancy grows with Q 2 Can select high energy photons in Hall D Tagger is in separate building Install chicane magnet in Hall D alcove (install hermetic shielding wall) Robert Paul Bennett Beyond the Born Approximation 2

3 Purpose of presenting today Show you some preliminary results from the Hall B experiment Let you know that we are interested in doing Hall B-like TPE experiment in Hall D Why we think Hall D can do it better. Who else is interested? Are there any show stoppers? Robert Paul Bennett Beyond the Born Approximation 3

4 1 Physics Motivation 2 TPE 3 Experiment 4 Analysis overview 5 Results 6 Hall D Robert Paul Bennett Beyond the Born Approximation 4

5 The Proton Formfactor Puzzle Rosenbluth Separation: (SLAC, MIT BATES, JLab et al.) ( ) [ ] dσ ε(1 + τ) σ r = = τg 2 M dω σ + ɛg2 E mott [ ε = 1 + 2(1 + τ) tan 2 ] 1 Q 2 θ e/2 τ = 4M 2 Separate G E and G M contributions at a particular Q 2 using different beam energies and scattered electron angles G M measurement dominates at high Q 2, G E is suppressed Polarization Transfer: (Hall A & C) G E G M = P t (E e + E e ) tan θe P l 2M 2 Longitudinal polarized electrons incident on proton target Measure transverse and longitudinal polarization of recoiled proton Robert Paul Bennett Beyond the Born Approximation 5

6 The Proton Formfactor Puzzle Rosenbluth Separation: (SLAC, MIT BATES, JLab et al.) ( ) [ ] dσ ε(1 + τ) σ r = = τg 2 M dω σ + ɛg2 E mott [ ε = 1 + 2(1 + τ) tan 2 ] 1 Q 2 θ e/2 τ = 4M 2 Separate G E and G M contributions at a particular Q 2 using different beam energies and scattered electron angles G M measurement dominates at high Q 2, G E is suppressed Polarization Transfer: (Hall A & C) G E G M = P t (E e + E e ) tan θe P l 2M 2 Longitudinal polarized electrons incident on proton target Measure transverse and longitudinal polarization of recoiled proton Robert Paul Bennett Beyond the Born Approximation 6

7 Beyond the Born Approximation Use G M from Rosenbluth Separation and G E from Polarization Transfer To account for the difference we need a ε dependent correction to the cross section on the order of a few percent Robert Paul Bennett Beyond the Born Approximation 7

8 TPE Contribution The general 1 γ and 2 γ exchange amplitudes A = e2 Q 2 ū(k )γ µ u(k) 1 : ū(p ) [G M γ µ P F µ 2 M 2 : ū(p ) ] u(p) [ GM γ µ F P µ 2 M + F ] γkp µ 3 M 2 u(p) Modified G E and G M New ε dependent term The general 1 γ and 2 γ exchange cross section 1 : dσ [ εg 2 dω E + ] τg2 M [ 2 : dσ ε G 2 dω E + τ G ] 2 M [ ( + 2ε τ G M + G ) ] E GM Y 2γ ( ) Y 2γ Re F3 G M Guichon and Vanderhaeghen, PRL 91 (03) Robert Paul Bennett Beyond the Born Approximation 8

9 Positrons to the rescue! The Born amplitude changes sign as the the charge of the incident beam. The leading TPE terms of the elastic scattering cross section are sensitive to the lepton charge The elastic e ± p e ± p scattering contribution: σ(e ± ) A born + ± A 2γ 2 σ(e ± ) A born (α) 2 ± 2A born (α)re(a 2γ ) The ratio of the cross sections isolates the TPE correction term R = σ(e+ ) σ(e ) = 1 2δ 2γ δ 2γ = 2Re(A 2γ) A born We can calculate this very well (QED) Theoretical calculation of the diagram is hard : Need to integrate over all baryon states The e p/e + p ratio measures the real part of the TPE contribution Robert Paul Bennett Beyond the Born Approximation 9

10 Phenomenology Robert Paul Bennett Beyond the Born Approximation 10

11 Making Positrons at CLAS Primary electron beam: 5.5 GeV and na Radiator: 0.9% of primary electrons radiate high energy photons Tagger magnet: Transport electrons tagger dump Converter: 9% of photons are converted to electron/positron pairs Chicane: separate the lepton beams Remaining photons are stopped at the photon blocker e + and e beams are then recombined and continue to the target Target: liquid hydrogen: length = 18cm (30 cm) & diameter = 6cm (6 cm) Detector: CLAS (DC, TOF) Robert Paul Bennett Beyond the Born Approximation 11

12 Making Positrons at CLAS Primary electron beam: 5.5 GeV and 100 na Radiator: 0.9% of primary electrons radiate high energy photons Tagger magnet: Transport electrons tagger dump Converter: 9% of photons are converted to electron/positron pairs Chicane: separate the lepton beams Remaining photons are stopped at the photon blocker e + and e beams are then recombined and continue to the target Target: liquid hydrogen: length = 18cm (30 cm) & diameter = 6cm (6 cm) Detector: CLAS (DC, TOF) Robert Paul Bennett Beyond the Born Approximation 12

13 Making Positrons at CLAS Primary electron beam: 5.5 GeV and 100 na Radiator: 0.9% of primary electrons radiate high energy photons Tagger magnet: Transport electrons tagger dump Converter: 9% of photons are converted to electron/positron pairs Chicane: separate the lepton beams Remaining photons are stopped at the photon blocker e + and e beams are then recombined and continue to the target Target: liquid hydrogen: length = 18cm (30 cm) & diameter = 6cm (6 cm) Detector: CLAS (DC, TOF) Robert Paul Bennett Beyond the Born Approximation 13

14 Making Positrons at CLAS Primary electron beam: 5.5 GeV and 100 na Radiator: 0.9% of primary electrons radiate high energy photons Tagger magnet: Transport electrons tagger dump Converter: 9% of photons are converted to electron/positron pairs Chicane: separate the lepton beams Remaining photons are stopped at the photon blocker e + and e beams are then recombined and continue to the target Target: liquid hydrogen: length = 18cm (30 cm) & diameter = 6cm (6 cm) Detector: CLAS (DC, TOF) Robert Paul Bennett Beyond the Born Approximation 14

15 Making Positrons at CLAS Primary electron beam: 5.5 GeV and 100 na Radiator: 0.9% of primary electrons radiate high energy photons Tagger magnet: Transport electrons tagger dump Converter: 9% of photons are converted to electron/positron pairs Chicane: separate the lepton beams Remaining photons are stopped at the photon blocker e + and e beams are then recombined and continue to the target Target: liquid hydrogen: length = 18cm (30 cm) & diameter = 6cm (6 cm) Detector: CLAS (DC, TOF) Robert Paul Bennett Beyond the Born Approximation 15

16 Making Positrons at CLAS Primary electron beam: 5.5 GeV and 100 na Radiator: 0.9% of primary electrons radiate high energy photons Tagger magnet: Transport electrons tagger dump Converter: 9% of photons are converted to electron/positron pairs Chicane: separate the lepton beams Remaining photons are stopped at the photon blocker e + and e beams are then recombined and continue to the target Target: liquid hydrogen: length = 18cm (30 cm) & diameter = 6cm (6 cm) Detector: CLAS (DC, TOF) Robert Paul Bennett Beyond the Born Approximation 16

17 Beam Line Modification for TPE Extensive GEANT simulations of detector backgrounds. Confirmed simulation with test run data A lot of shielding added on tagger, tagger dump and chicane. Improved luminosity by a factor 100 Robert Paul Bennett Beyond the Born Approximation 17

18 Beam Line Modification for TPE Robert Paul Bennett Beyond the Born Approximation 18

19 Outline Physics Motivation TPE Experiment Analysis overview Results Hall D Beam Profiling TPE Calorimeter Measure beam energy vs position during low luminosity run 30 module Shashlik (Pb/Scint) calorimeter Located directly downstream of CLAS on the Fiber Monitors forward carriage 16x16 Sparse fiber monitor continually monitoring beam profile before the target 64x64 Dense fiber monitor mounted on TPE Calorimeter face for beam profiling during low luminosity runs Bicron fibers spaced 5 mm (1mm) apart glued to a Hamamatsu PMT Beam size 15 mm radius Robert Paul Bennett Beyond the Born Approximation 19

20 Systematic Beam Checks Flipped chicane polarity about once a week Check for geometric alignment of e /e + on target Varied steering magnet currents and measured individual beam positions at sparse fiber monitor Reproducible crossing for all chicane flips Robert Paul Bennett Beyond the Born Approximation 20

21 Triggering, Cuts and Corrections 1 Trigger on particle in forward 45 0 and anything in opposite sector 2 Target vertex cut ( 45 cm V z 15 cm) 3 Momentum Corrections 4 Proton energy loss corrections 5 Fiducial Cuts 6 Swimming Acceptance matching ++ and + events Robert Paul Bennett Beyond the Born Approximation 21

22 Non-Standard PID & Elastic Event Selection 1 Select ++ and + track pairs 2 Coplanarity cut (φ proton φ lepton ) 3 Reconstructed Beam Energy: [ ] 1 E 1 = M P tan(θ e/2) tan(θ P ) 1.0 E 2 = P e cos(θ e) + P p cos(θ P ) E Beam = E 1 E 2 4 Scattered lepton Energy: E e = Emeasured e Ee (θ e, θ p) 5 Proton Momentum: P (p) = P p Pe sin(θe) sin(θ p) (1) Robert Paul Bennett Beyond the Born Approximation 22

23 Q 2 vs ε (TPE II ) Robert Paul Bennett Preliminary Beyond the Born Approximation 23

24 Q 2 vs ε (TPE II ) Robert Paul Bennett Preliminary Beyond the Born Approximation 24

25 Ratios 1 Apply fiducial cuts to select regions where both e and e + can both be detected Robert Paul Bennett Beyond the Born Approximation 25

26 Ratios 1 Apply fiducial cuts to select regions where both e and e + can both be detected 2 Measure Elastic Scattering Ratio : Proton acceptance cancels in the ratio R = Y (e+ P ) Y (e P ) Robert Paul Bennett Beyond the Born Approximation 25

27 Ratios 1 Apply fiducial cuts to select regions where both e and e + can both be detected 2 Measure Elastic Scattering Ratio : Proton acceptance cancels in the ratio R = Y (e+ P ) Y (e P ) 3 Flip torus polarity : Lepton acceptance cancels in double ratio [ ] + [ ] Ye R 2 = +P Ye+ P Y e P Y e P Robert Paul Bennett Beyond the Born Approximation 25

28 Ratios 1 Apply fiducial cuts to select regions where both e and e + can both be detected 2 Measure Elastic Scattering Ratio : Proton acceptance cancels in the ratio R = Y (e+ P ) Y (e P ) 3 Flip torus polarity : Lepton acceptance cancels in double ratio [ ] + [ ] Ye R 2 = +P Ye+ P Y e P Y e P 4 Flip chicane polarity: Beam asymmetries cancel in quadruple ratio R 4 = R 2 + R 2 Robert Paul Bennett Beyond the Born Approximation 25

29 Preliminary Results Binning 75% of total data set Robert Paul Bennett Beyond the Born Approximation 26

30 Comparison to World Data Q 2 > 1 Robert Paul Bennett Beyond the Born Approximation 27

31 Hall D Floorplan Robert Paul Bennett Beyond the Born Approximation 28

32 Advantages of Hall D TPE 1 12(9) GeV upgrade will provide much larger phase space The G P E discrepancy grows with Q2 ( factor of 3 at Q 2 = 6GeV ) 2 Hall D can select high energy photon beam 3 Photon beam is created in a separate building Tracking efficiency suffered from high background rates Limited our ability to push beam luminosity 4 There looks to be room to install the chicane in the alcove Lots of shielding wall off the alcove 5 Symmetric tracking of positively and negatively charged particles 6 Other programs interested in e + beam can piggy-back (e.g. DVCS) Robert Paul Bennett Beyond the Born Approximation 29

33 Outstanding Hall D TPE Questions 1 Is the aperture at the target is wide enough. The tertiary beam we produced in Hall B was 5cm wide. The start counter might be in the way Can the start counter be removed and reinstalled without causing too much pain to Hall D? 2 Can the Hall-D solenoid magnetic field be reversed easily and in a repeatable way? Unexpected detector asymmetries can cancel 3 Radiator thickness? We used a 0.9% radiator in Hall B Can we go thicker in Hall D? 4 What kind of dose can the tagger detectors handle? We calculated that we would deposit several mega-rad on the Hall B taggers, so we removed the plastic scintilators. 5 What p, θ and φ resolution can we acheive for e + /e? We will have to rely on over constrained kinematics to reconstruct the beam energy, since we would using brem beam 6 Any show stoppers I missed? Robert Paul Bennett Beyond the Born Approximation 30

34 Thank you Robert Paul Bennett Beyond the Born Approximation 31

35 Thank you Robert Paul Bennett Beyond the Born Approximation 31

36 Beam Asymmetry Robert Paul Bennett Beyond the Born Approximation 32

37 Magnet Cycle Dependence Robert Paul Bennett Beyond the Born Approximation 33

38 Comarison of Kinematic Coverage Robert Paul Bennett Beyond the Born Approximation 34

39 E Beam vs E e E and E e are correlated, so we cut on the sum ( E+) and difference ( E ) Robert Paul Bennett Beyond the Born Approximation 35

40 Kinematic Cuts No cuts Apply other 3 kinematic cuts Robert Paul Bennett Beyond the Born Approximation 36

41 E : ε Dependence Robert Paul Bennett Beyond the Born Approximation 37

42 E +: ε Dependence Robert Paul Bennett Beyond the Born Approximation 38

43 P p : ε Dependence Robert Paul Bennett Beyond the Born Approximation 39

44 Personnel 1 Spokes Persons Larry Weinstein, Brian Raue, Will Brooks, John Arrington, Andrei Afanasev & Kyungseon Joo 2 Post Docs Puneet Khetarpal Mauri Ungaro Robert Bennett 3 Graduate Students Dasuni Adikaram Dipak Rimal Cristian Peña Hashir Rashad Robert Paul Bennett Beyond the Born Approximation 40