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XIII th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, Rome, September 30 – Octobe

Eric Voutier. Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France. Physics Perspectives at JLab with a Polarized Positron Beam PEPPo @ JLab. Physics motivations Polarized positrons production Perspectives Conclusions.

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XIII th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, Rome, September 30 – Octobe

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  1. Eric Voutier Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France Physics Perspectives at JLab with a Polarized Positron Beam PEPPo @ JLab • Physics motivations • Polarized positrons production • Perspectives • Conclusions XIIIth International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, Rome, September 30 – October 4, 2013

  2. Physics Motivations • Electromagneticformfactors (U,P) • Generalized parton distributions (U,P) • Inclusive structure functions (U,P) • Search for the U-boson of darkmatter (U) • Charge conjugation violation to access C3q (P) • International LinearCollider • Positron-ion Collider (LHeC, e-RHIC, EIC, MEIC, ENC) • Thermal positron facility Rome, September 30 – October 4, 2013

  3. Electromagnetic form factors Eric Voutier Two Photons Physics P.A.M. Guichon, M. Vanderhaeghen, PRL 91 (2003) 142303 • Following the very first measurements of polarization transfer observables in electron elastic scattering, the validity of the 1g exchange approximation of the electromagnetic interaction has been questioned. Within the 2g exchange hypothesis, the electromagnetic structure of the nucleon may be parametrized by 3 generalized form factors, corresponding to 8 unknow quantities. 3/21 Rome, September 30 – October 4, 2013

  4. Electromagnetic form factors Eric Voutier Experimental Observables M.P. Rekalo, E. TomasiGustafsson, NPA 742 (2004) 322 C. Carlson, M. Vanderhaeghen, ARNPS 57 (2007) 171 • Unpolarized e± elastic scattering and polarization transfert observables off the nucleon involve up to 5 unknown quantities. Cross Section Polarization Transfert 5 unknown contributions for 6 independent observables Combining polarized electronsand positrons allows a model independent separation of the electromagnetic form factors of the nucleon. 4/21 Rome, September 30 – October 4, 2013

  5. Generalized parton distributions Eric Voutier Parton Imaging • GPDs are the appropriate framework to deal with the partonic structure of hadrons and offer the unprecedented possibility to access the spatial distribution of partons. • GPDs = GPDs(Q2,x,x,t) whoseperpendicular component of the momentumtransfer to the nucleonisFourier conjugate to the transverse position of partons. • GPDs encode the correlationsbetween partons and contain information about the dynamics of the system like the angularmomentum or the distribution of the strong forcesexperienced by quarks and gluons inside hadrons. X. Ji, PRL 78 (1997) 610 M. Polyakov, PL B555 (2003) 57 A new light on hadron structure M. Burkardt, PRD 62 (2000) 071503 M.Diehl, EPJC 25 (2002) 223 GPDs can be interpreted as a 1/Q resolution distribution in the transverse plane of partons with longitudinal momentum x. 5/21 Rome, September 30 – October 4, 2013

  6. Generalized parton distributions Eric Voutier N(e,e′gN) Differential Cross Section M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009 Additional observables Electron observables Electron & positron observables Polarized electronsand positrons allow to separate the unknown amplitudes of the cross section for electro-production of photons. 6/21 Rome, September 30 – October 4, 2013

  7. Generalized parton distributions Eric Voutier Experimental Observables V. Burkert, V. Guzey, JPos09, AIP Conf. Proc. 1160 (2009) • Calculations of experimental observableswithin a dual parametrization of the nucleonGPDs are predictingverysignificanteffectscomparingelectrons and positrons. A beam current larger than 10 nA would be ideal for a polarized positronDVCS program at CLAS12. 7/21 Rome, September 30 – October 4, 2013

  8. Polarized Positrons Production • PEPPo concept • Proof-of-principleexperiment • Preliminaryexperimentalresults Rome, September 30 – October 4, 2013

  9. e-→g→e+ PEPPo concept Eric Voutier Polarized Bremsstrahlung E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395 Polarized Electrons for Polarized Positrons Polarized positrons can be created by materialization of polarized photons issued from the bremsstrahlung of polarized electrons. Sustainablepolarizedelectronintensities up to 4 mA have been demonstratedfrom a superlattice photocathode. R. Suleiman et al., PAC’11, New York (NJ, USA), March 28 – April 1, 2011 9/21 Rome, September 30 – October 4, 2013

  10. PEPPo concept Eric Voutier Bremsstrahlung and Pair Creation H. Olsen, L. Maximon, PR 114 (1959) 887 E.A. Kuraev, Y.M. Bystritskiy, M. Shatnev, E. Tomasi-Gustafsson, PRC 81 (2010) 055208 • Finite lepton mass calculations of the bremsstrahlung and pair creationprocessespredictvery efficient polarizationtransfers. PAIR CREATION BREMSSTRAHLUNG 10/21 Rome, September 30 – October 4, 2013

  11. March 2012 May 2011 Proof-of-principle experiment Eric Voutier E11-105 @ CEBAF Injector J. Grames, E. Voutier et al. The proof-of-principlePEPPoexperiment, installedat the CEBAF injector, intented to demonstratethe feasibility of using bremsstrahlung of polarized electrons to produce polarized positrons. The positron yield and polarizationdistibutions have been measured. 11/21 Rome, September 30 – October 4, 2013

  12. PEPPo Proof-of-principle experiment Eric Voutier Principle of Operation G. Alexander et al, PRL 100 (2008) 210801, NIMA 610 (2009) 451 Pe- = 83.7% ± 2.8%Stat. ± 0.6%Syst. S1 Pe- Ie = 1 µA T1 = 1 mm D e- T1 • Longitudinal e- (Pe-) produce elliptical g whose circular (Pg) component is proportional to Pe-. • Pg transfers to e+ into longitudinal (Pe+) and transverse (Pt) polarization components. On the average Pt=0. S2 PT e+ D T2 Calorimeter PT = 7.53% ± 0.04%Stat. ± 0.06%Syst. PEPPodid measure the longitudinal polarization transfer from electrons to positrons in the 3.2-6.3 MeV/cmomentum range. 12/21 Rome, September 30 – October 4, 2013

  13. Preliminary experimental results Eric Voutier Data Taking Data weretakenusingtwo major sequences: Use the polarizedelectronbeamdirectlyfrom the CEBAF injector to calibrate the analyzing power of the Compton transmission polarimeter; Use the Compton transmission polarimeter to measure the polarizationtransferfromelectronsto positrons. Reconversion Target photon C A L O R I M E T E R Vacuum window • The DAQ trigger for positron measurementsis a coincidencebetween a thinscintillatorplacedprior the reconversion target, and the central crystal(PMT5). • Drasticreduction of the neutral background. e+ Trigger Scintillator 13/21 Rome, September 30 – October 4, 2013

  14. Preliminary experimental results Eric Voutier Electron Analyzing Power PEPPo On-line Analysis • A high quality measurement of the electron analyzing power has been achieved and is currently in the final analysis stage. • Experimental data are as expected selective with respect to simulations, allowing for the calibration of the polarimeter model. 14/21 Rome, September 30 – October 4, 2013

  15. Preliminary experimental results Eric Voutier Positron Polarization Set 1 (Initial Run) Set 2 (Improved Shielding) Set 1 (Initial Run) Set 2 (Improved Shielding) Electron Beam Polarization PEPPo Preliminary Pe+ = AT / k Ae- Pe- PT • Positron polarizationisdeducedusingmeasuredelectronbeam and targetpolarizations, electronanalyzing power, experimentalasymmetry, and estimated e+/e-analyzing power ratio (1.1-1.4). • Significantnon-zeroexperimentalasymmetriesincreasingwith positron momentumsign efficient polarizationtransferfromelectrons to positrons. 15/21 Rome, September 30 – October 4, 2013

  16. Perspectives • Positron beamat CEBAF • Technological challenges Rome, September 30 – October 4, 2013

  17. Positron beam at CEBAF Eric Voutier Positron Collection Concept Quarter Wave Transformer (QWT) Solenoid Collimators e- g e- 126 MeV e+ 24 MeV W target Combined Function Magnet (QD) • A collection efficiency of 3х10-4 is predicted at the maximum positron production yield, corresponding to a positron energy of 24 MeV. • The resulting beam can then be accelerated without significant loss, and injected into the CEBAF main accelerator section. S. Golge, PhD Thesis, 2010 (ODU/JLab) 17/21 Rome, September 30 – October 4, 2013

  18. Positron beam at CEBAF Eric Voutier e+ Source Concept A. Freyberger at the Town Hall Meeting, JLab, 2011 1mA I = 300 nA Pe+ > 60% Pe- dp/p = 10-2 ex = 1.6 mm.mrad ey = 1.7 mm.mrad S. Golge, PhD Thesis, 2010 (ODU/JLab) 18/21 Rome, September 30 – October 4, 2013

  19. Positron beam at CEBAF Eric Voutier e+ Production Scenarii • CEBAF INJ (10-100 MeV) • FEL (100 MeV) • CEBAF (12 GeV) 19/21 Rome, September 30 – October 4, 2013

  20. Technological challenges Eric Voutier Towards a Polarized Positron Beam • The pathfrom a sketchedconcept to a full designis a • correlated multi-parameterproblemrequiring to adress and solveseveral • technical/technological challenges. • High IntensityPolarized Electron Source • Polarizedelectron gun atJLabdiddemonstratehighintensitycapabilities, though the polarizationathighcurrentwas not measured. • High Power Production Target • High power absorbers (10-100 kW) are challenging for heat dissipation, radiation management, and acceleratorintegration. • Optimized Positron Collection • Properties of usable beam for Physics (intensity, polarization, momentumspread, emittance) are stronglyrelated to the positron production and collection schemesand strategies (1 or 2 targets). • Positron Beam «Shaping» • Pre-acceleration(HadronicPhysics) or deccelaration (Material Science) are required to produce a beamsuitable for Phyiscs. 20/21 Rome, September 30 – October 4, 2013

  21. Conclusions Eric Voutier Summary Themeritsof polarizedand/or unpolarized positronbeams for the Physics program atJLabiscomparable to the benefits of polarizedwith respect to unpolarizedelectrons. 2g physics, GPDs… NP @ lowenergy, Material Science @ verylowenergy… An R&D effort is necessary to resolve the several technical and technological challenges raised by the development of a (un)polarized positron beam for Physics. The PEPPoexperiment @ the CEBAF injector has been a first step in thisprocess. 21/21 Rome, September 30 – October 4, 2013

  22. Viewers Beam position monitors Solenoid ELEGANT beam optics Viewers Collimator Polarized Analyzing Target (7.5 cm Fe) Solenoid Dipoles Corrector magnets Quadrupoles PEPPo branch jonction Faraday Cup G4PEPPo experiment model Production Target T1 (0.1-1 mm W) Annihilation detector Viewer Positron Selection Device Corrector magnets ReconversionTarget T2 (2 mm W) Compton Transmission Polarimeter Proof-of-principle experiment Eric Voutier Rome, September 30 – October 4, 2013

  23. Proof-of-principle experiment Eric Voutier DAQ Components • CsIcrystalsare coupled to PMTequipedwith LPSC custom amplified basis to comfort the PMT life-time (high rate environment). • The ~2 µs long and 2 Voptimized signal isfedinto the JLab custom FADC 250 thatsampled the signal at250 MHz. Hamamatsu R6236-100 • The flexibility of the FADC250 allows for 3 data taking modes : • Sample (500samples /detector event); • Semi-integrated (1 integral / detector event); • Integrated (1 integral /helicity gate event). FADC 250 Rome, September 30 – October 4, 2013

  24. Proof-of-principle experiment Eric Voutier Ry 4p spin rotator determines e- polarization orientation Rz P = 83.7% ± 2.8%Stat. ± 0.6%Syst. Rx (P, 0, 0) (0, 0, P) (0, P, 0) • Fast reversal of electron beam polarization at 30 Hz through experiment. • Slow systematic reversal of electron polarization with optical wave-plate (source) and target polarization(Compton). • Orient polarization in x,y,z direction using 4p spin rotator. Reverse beam helicity Reverse magnet polarity Rome, September 30 – October 4, 2013

  25. Proof-of-principle experiment Eric Voutier Magnetic Flux Measurement • The iron core target is equipped with 3 pick-up coils measuring the magnetic flux generated upon magnet current variation. • Specific cycling procedures are used during the experiment to monitor the target polarization. < PT > = 7.53% ± 0.06%Opera ± 0.04%Coil Rome, September 30 – October 4, 2013

  26. Preliminary experimental results Eric Voutier Energy Dependent Analysis Eg 200ns Set 1 Set 2 Set 1+2 • Accidentalsubtractionisdone per helicity state for eachenergy bin. • Positron polarizationwillbededuced for eachenergy bin usingsimulatedanalyzing powerconstrained by electron calibration data. Under progress… Rome, September 30 – October 4, 2013

  27. Positron beam at CEBAF Eric Voutier e+ Figure of Merit • The Figure of Merit is the quantity of interest for the accuracy of a measurement which combines the incident flux of particles and its polarisation. (GEANT4 simulations based on the full screening case of O&M) Optimum FoM Optimum energy J. Dumas, Doctorate Thesis, 2011 (LPSC Grenoble/JLab) Rome, September 30 – October 4, 2013

  28. Positron beam at CEBAF Eric Voutier Optimized Polarized Positron Source • Polarization transfer up to 75%may be expected over the full energy range of the CEBAF injector. • Production efficiencies up to 10-4-10-3may be expected, depending on the electron beam energy. These figures are linearly sensitive to the acceptance of the positron collection system. J. Dumas, Doctorate Thesis, 2011 (LPSC Grenoble/JLab) Rome, September 30 – October 4, 2013

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