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Electron- and Positron-proton elastic scattering in CLAS

Electron- and Positron-proton elastic scattering in CLAS. The G E p problem, Two Photon Exchange and Positrons Making Positrons in Hall B Test run results Projected improvements Data analysis Summary. Larry Weinstein Old Dominion University.

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Electron- and Positron-proton elastic scattering in CLAS

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  1. Electron- and Positron-proton elastic scattering in CLAS • The GEp problem, Two Photon Exchange and Positrons • Making Positrons in Hall B • Test run results • Projected improvements • Data analysis • Summary Larry Weinstein Old Dominion University Spokespeople: Will Brooks, Brian Raue, John Arrington, Kyungseon Joo, Andrei Afanasev, LW Students: Megh Niroula, Maryam Moteabbed

  2. The Proton Charge Form Factor Super-Rosenbluth Separations Rosenbluth Separations OOPS!! Polarization Measurements

  3. How to measure Two Photon Exchange ABorn ~ e± = ±1 R measures the real part of the two photon amplitude A2γ ~ (e±)2 = +1

  4. Recap: The Problem: Rosenbluth/Polarization Transfer discrepancy in GE The Probable Cause: Two-Photon Exchange contributions of a few percent How to test it: Compare electron and positron elastic scattering from the proton to better than a few percent Now where did I leave those positrons …

  5. Existing e+/e- cross section ratios Positrons to the rescue? mixed Q2 (Q2 > 1) 1.2 R (e+/e- cross section ratio) mixedε 1 0 1 epsilon Doesn’t constrain much Data: Mar et al, PRL 21 (1968) 482

  6. How to make positrons in Hall B: • 1 µA 5.7 GeV e− beam hits 5% radiator, makes photons • Electrons dumped in tagger dump • Photon beam hits 2% converter, makes e+/e− pairs • e+/e− beams split by 3-dipole chicane • Photon beam blocked • Low energy leptons blocked • Lepton beams recombined • Simultaneous identical e+/e− beams hit hydrogen target in CLAS CLAS

  7. CAL TOF CER DC2 DC1 JLab CLAS Midplane slice 3D view DC3

  8. CLAS in Maintenance Position

  9. Experiment Features/Bugs • Identical mixed e+/e− beam (250 pA each) • Continuous beam energy distribution • Wide Q2 and angle (ε) coverage • Simultaneous cross section measurements • Minimize systematic uncertainty • Allows 1-2% measurement of e+/e− cross section ratio • Reverse magnetic fields (chicane/Torus) to reduce acceptance effects • Overdetermined ep kinematics allows background rejection • Unprecedented photon luminosity (1000 times!) But can we use a tertiary beam with an unshielded detector?? Find the background sources Eliminate them or shield them

  10. TPE Simulation Tagger Configuration CLAS Tagger Electron beam Tagger Dump

  11. GEANT4 simulation – vertex origin of detector hitsOld (2005) Test Run Photon blocker CLAS Beamline Ouch! Tagger exit & dump

  12. GEANT4 simulation – tagger shielding bunker design

  13. Oct 2006 test run • 3-Dipole chicane with photon blocker • Remove material from tagger exit line • Shield tagger exit line (bunker) • Tagger dump cooling • Lepton-beam fiber monitor (FIU) • LOTS of new shielding and collimation • Collimate after chicane • Collimate before CLAS • LOTS of modifications during the test run Chicane and shielding Old tagger dump exit New tagger dump exit

  14. Collimator CLAS Collimator Chicane Dipoles Fiber BPM Collimator Convertor Beam line Shielding added during test Tagger Target Shielding Tagger Dump

  15. We really do see positrons! • Block one lepton beam • Scan chicane dipoles 1&3 • Watch the beam move • Repeat for the other beam Beam Profile Fiber Monitor (FIU) Beam position (mm) 1st and 3rd Dipole Current (A)

  16. 2006 Test Run Results: Luminosity Maximum luminosity achieved: • 80 nA 3.2 GeV electrons • 0.5% radiator, 5% convertor • Figure of Merit: 20 pA (80nA*0.5%*5%) • Limited by drift chamber occupancies (3% limit) • Region 1 DC occupancy 2.3% • Occupancy entirely lepton-beam related • Lepton beam too large - 80% of hits not target related • Region 3 DC occupancy 0.7% • Test run achieved 4% of proposal luminosity Luminosity and backgrounds agreed with simulations. Factor of 10-20 improvement on previous test runs!

  17. How to do better Limitations: • Target and heat exchanger apertures • Moller electrons trapped by the CLAS magnetic field • Background from the tagger vacuum box • Background from other (diffuse) sources • Limited beam monitoring Improvements: • Reduce pre-CLAS collimator aperture from 6 to 4 cm • Add minitorus and Moller catcher • Shield tagger vacuum box (6 mm Pb) • Large concrete pipe and disk shield • Double target length • Build a position sensitive removable beam-line calorimeter for beam characterization

  18. New beamline shielding CLAS Chicane Tagger Bunker Tagger dump

  19. Target shielding and Moller electrons Moller electrons Moller background with minitorus only (mollers bottled by torus field) target Target cell and shielding target Shielding (W and Pb) Moller background with minitorus plus shielding

  20. tomorrow? 100% today 10% 2006 test run 1% 0.1% 2004 2006 2008 2010 Luminosity vs time Luminosity (% of proposal) Date

  21. 2006 Test Run Data Analysis Challenges: Unknown incident particle energy. No particle ID (other than relative timing) 3.2 GeV incident electron beam (too low) About two days of production data taking Solutions: Look for co-planar particle pairs (opposite sectors). Identify ++ and -+ pairs. Exploit restricted kinematics to identify elastic-scattering events.

  22. Kinematic cuts: Opposite sectors (in trigger) Target vertex cut 3) Coplanarity cut: uncut All other cuts 150 180 -10 10 Df Vertex along beam line (cm)

  23. Kinematic cuts II: Beam-energy difference cut: 5) Beam angle cut (angle of pe + pp) uncut All other cuts 0 2 0 10 beam E = Eangle - Emomentum (GeV)

  24. Kinematic Cuts III: 6) lepton-proton DOCA (distance of closest approach) uncut All other cuts 0 10 DOCA (cm)

  25. e+ e- p Data Analysis: Acceptance Fiducial cuts to select regions of overlap between electrons and positrons for different torus settings. Acceptance matching (swimming) If e- is detected, would e+ be accepted? 20 e 0 -20 20 40 e Analysis limited to forward angle due to statistics

  26. Data Analysis: Kinematics positrons Positive torus polarity Q2 Q2 angle e+p electrons Q2 Q2 angle e-p epsilon momentum momentum

  27. Data Analysis: Binning 7000 Q2 1.0 0.5 0.75 0.85 0.95 epsilon

  28. Q2=0.47 1.15 Results: e+p/e-p doubleratios 0.85 Q2=0.25 Preliminary e+p/e-p (+torus) Q2=0.15 e-p/e+p (-torus) 1.15 Q2=0.10 Scatter appears consistent with statistical errors (black bars) 0.85 0.8 0.9 epsilon

  29. Test Run vs the World! Preliminary epsilon Not too bad

  30. 1% What next? • Optimize shielding • Run the experiment Anticipated uncertainties

  31. Summary: • GEp measured by Rosenbluth and Polarization transfer expts differ by a factor of 3 at Q2 = 6 • Two Photon Exchange might explain the discrepancy • The e+p/e-p ratio is the only way to measure the real part of the TPE amplitude

  32. Summary: • The TPE Engineering Test Run • Produced a mixed identical electron/positron beam • Validated beam line simulations • Measured the e+p to e-p elastic scattering ratio at low Q2 • small systematic uncertainties • The TPE experiment • Will be able to achieve the proposed luminosity • Will cover a wide kinematic range • Will determine precisely the TPE contribution and resolve the GE problem • Will be ready to run in 2010 • Anti-matter beams are cool!

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