1. Positron Beam for Measurements of Generalized Parton Distributions Generalized Parton Distributions e + @ JLab (i) Generalized parton distributions (ii) Polarized positron production (iii) Polarization transfer physics (iv) PEPPo (v) Perspectives (vi) Conclusions Institut de Physique Nucléaire Orsay, France Eric Voutier Trento, June 8-12, 2015 New Directions in Nuclear Deep Inelastic Scattering ?

2. Generalized Parton Distributions GPD concept Electro-production of photons Experimental observables Trento, June 8-12, 2015

3. GPD concept  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. Parton Imaging 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.  GPDs = GPDs(Q 2,x, ,t) whose perpendicular component of the momentum transfer to the nucleon is Fourier conjugate to the transverse position of partons.  GPDs encode the correlations between partons and contain information about the dynamics of the system like the angular momentum or the distribution of the strong forces experienced by quarks and gluons inside hadrons. X. Ji, PRL 78 (1997) 610 M. Polyakov, PL B555 (2003) 57 A new light on hadron structure Eric Voutier Trento, June 8-12, 2015 3/31

4. GPDs enter the cross section of hard scattering processes via an integral over the intermediate quarks longitudinal momenta GPDs Deep Exclusive Scattering GPDs can be accessed via exclusive reactions in the Bjorken kinematic regime Q 2 >> M 2 -t << Q 2 GPD concept Eric Voutier Trento, June 8-12, 2015 4/31

5. Photon Electroproduction  The Bethe-Heitler (BH) process where the real photon is emitted either by the incoming or outgoing electron interferes with DVCS.  DVCS & BH are indistinguishable but the BH amplitude is exactly calculable and known at low t.  The relative importance of each process is beam energy and kinematics dependent. Electro-production of photons leptonic plane e-’e-’  p e-e- ** hadronic plane  Out-of-plane angle entering the harmonic development of the reaction amplitude A.V. Belitsky, D. Müller, A. Kirchner, NPB 629 (2002) 323 Polarization observables help to single-out the DVCS amplitude. P 0 = 6 GeV/c Eric Voutier Trento, June 8-12, 2015 5/31

6. N(e,e ′  N) Differential Cross Section Unpolarized Target M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009 Even in  Odd in  Polarized electrons and positrons allow to separate the 4 unknown components of the  electro-production cross section. Electron observables Electron & positron observables Eric Voutier Trento, June 8-12, 2015 Electro-production of photons 6/31

7. N(e,e ′  N) Differential Cross Section Polarized Target M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009 Polarized targets allow to access other GPD combinations Additional observables  Four new cross section components that may be separated from Rosenbluth-like experiments, or the combination of polarized electrons and positrons measurements at the same kinematics. Trento, June 8-12, 2015 Electro-production of photons 7/31 Eric Voutier

8. Model Calculations Experimental observables Trento, June 8-12, 2015 Eric Voutier 8/31  Evaluation of cross section and asymmetries is performed considering GPD H and E only, within a dual parametrization.  The experimental configuration assume the detection of the full ep  final state in CLAS12 with an 11 GeV electron/positron beam.  Statistics of projected data involve 1000 h data taking time at 10 35 cm -2.s -1 for electrons, and 2×10 34 cm -2.s -1 for positrons (8 nA, 10 cm LH 2 ). H. Avakian, V. Burkert, V. Guzey, JPos09 (2009)  x ×  y < 2×2 mm 2  E /E < 10 -3 Polarized positrons

9. Cross Sections Experimental observables Trento, June 8-12, 2015 Eric Voutier 9/31 H. Avakian, V. Burkert, V. Guzey, JPos09 (2009)  Significant differences between polarized and unpolarized lepton beams supports again importance of a polarized positron beam.

10. Moments Experimental observables Trento, June 8-12, 2015 Eric Voutier 10/31 H. Avakian, V. Burkert, V. Guzey, JPos09 (2009)  The sin(  ) moments linked to the imaginary part of the interference amplitude express the important benefit of a polarized positron beam for GPD physics at JLab.

11. Projected Data Experimental observables Trento, June 8-12, 2015 Eric Voutier 11/31 H. Avakian, V. Burkert, V. Guzey, JPos09 (2009) High quality data with modest polarized positron beam current can be achieved, allowing to separate the pure interference contribution to the photon electro-production process.

12. Polarized Positron Production Sokolov-Ternov self-polarization Photon circular polarization transfer Trento, June 8-12, 2015

13. Storage Ring Scheme  Polarized positrons have been obtained in high energy storage ring taking advantage of the Sokolov-Ternov effect which leads to positrons polarized parallel to the magnetic field. Sokolov-Ternov self-polarization Polarization builds up exponentially with a time constant characteristic of the energy and the curvature of the positrons t ~ 20 mn @ HERA Not compatible with CW beam delivery at JLab. Eric Voutier Trento, June 8-12, 2015 13/31

14. Fixed Target Schemes  The principle of polarization transfer from circular photons to longitudinal positrons has been demonstrated in the context of the ILC project. T. Omori et al, PRL 96 (2006) 114801 G. Alexander et al, PRL 100 (2008) 210801 P(e + ) = 73 ± 15 ± 19 % Compton Backscattering Undulator 1.3 GeV Require independent ~GeV to multi-GeV electron beam. Photon circular polarization transfer Eric Voutier Trento, June 8-12, 2015 14/31

15. E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395 e - →  → e + J. Grames, E. Voutier et al., JLab Experiment E12-11-105, 2011 Polarized Bremsstrahlung Eric Voutier Trento, June 8-12, 2015 Photon circular polarization transfer 15/31 Sustainable polarized electron intensities up to 4 mA have been demonstrated from a superlattice photocathode. R. Suleiman et al., PAC’11, New York (NJ, USA), March 28 – April 1, 2011 The purpose of the PEPPo (Polarized Electrons for Polarized Positrons) experiment at the CEBAF injector was to demonstrate feasibility of using bremsstrahlung radiation of polarized electrons for the production of polarized positrons.

16. Polarization Transfer Physics Ultra-relativistic approximation Finite lepton mass calculations Trento, June 8-12, 2015

17. Ultra-relativistic approximation H. Olsen, L. Maximon, PR114 (1959) 887  The most currently used framework to evaluate polarization transfer for polarized bremsstrahlung and pair creation processes is the O&M work developped in the Born approximation for relativistic particles and small scattering angles. Bremsstrahlung and Pair Creation BREMSSTRAHLUNG PAIR CREATION Eric Voutier Trento, June 8-12, 2015 17/31

18. E.A. Kuraev, Y.M. Bystritskiy, M. Shatnev, E. Tomasi-Gustafsson, PRC 81 (2010) 055208 Bremsstrahlung and Pair Creation Finite lepton mass calculations BREMSSTRAHLUNG PAIR CREATION  These reference processes have been revisited considering the finite mass of the leptons, generating additional terms to the leptonic tensor. Eric Voutier Trento, June 8-12, 2015 18/31

19. PEPPo Proof-of-principle experiment Commissionning Polarized positrons characterization Trento, June 8-12, 2015

20. P e- e-e- T1T1 Polarized Electrons (< 10 MeV/c) strike production target BREMSSTRAHLUNG Longitudinal e - (P e- ) produce elliptical  whose circular (P  ) component is proportional to P e- S1S1 D D S2S2 P e+ Positron Transverse and Momentum Phase Space Selection e+e+  PAIR PRODUCTION  produce e + e - pairs and transfer P  into longitudinal (P e+ ) and transverse polarization averages to zero PEPPo measured the longitudinal polarization transfer in the 3.07-6.25 MeV/c momentum range.  COMPTON TRANSMISSION Polarized e + convert into polarized  (P  ) whose transmission through a polarized iron target (P T ) depends on P .P T Principle of Operation E e = 6.3 MeV I e = 1 µA T 1 = 1 mm W Geant4 PEPPo J. Dumas, PhD Thesis (2011) T2T2 PTPT Calorimeter Compton Transmission Polarimeter Proof-of-principle experiment Eric Voutier Trento, June 8-12, 2015 20/31

21. Eric Voutier Trento, June 8-12, 2015 Commissionning 21/31

22. A. Adeyemi Trento, June 8-12, 2015 Eric Voutier Commissionning Electron Analyzing Power  A high quality measurement of the electron analyzing power has been achieved.  Experimental data are as expected selective with respect to simulations, allowing for the calibration of the polarimeter model. 22/31

23. Positron Analyzing Power  GEANT4 simulations allow to link the measured electron analyzing power to the expected positron analyzing power of the PEPPo Compton transmission polarimeter. Eric Voutier Trento, June 8-12, 2015 23/31 Polarized positron characterization

24.  Significant non-zero experimental asymmetries increasing with positron momentum sign efficient polarization transfer from electrons to positrons. Positron Polarization Eric Voutier Trento, June 8-12, 2015 Polarized positron characterization 24/31 PEPPo Preliminary  Positron polarization is deduced using measured electron beam and target polarizations, electron analyzing power, experimental asymmetry, and estimated e + /e - analyzing power ratio (1.1-1.4).

25. P. Aderley 1, A. Adeyemi 4, P. Aguilera 1, M. Ali 1, H. Areti 1, M. Baylac 2, J. Benesch 1, G. Bosson 2, B. Cade 1, A. Camsonne 1, L. Cardman 1, J. Clark 1, P. Cole 5, S. Covert 1, C. Cuevas 1, O. Dadoun 3, D. Dale 5, J. Dumas 1,2, E. Fanchini 2, T. Forest 5, E. Forman 1, A.Freyberger 1, E. Froidefond 2, S. Golge 6, J. Grames 1, P. Guèye 4, J. Hansknecht 1, P. Harrell 1, J. Hoskins 8, C. Hyde 7, R. Kazimi 1, Y. Kim 1,5, D. Machie 1, K. Mahoney 1, R. Mammei 1, M. Marton 2, J. McCarter 9, M. McCaughan 1, M. McHugh 10, D. McNulty 5, T. Michaelides 1, R. Michaels 1, C. Muñoz Camacho 11, J.-F. Muraz 2, K. Myers 12, A.Opper 10, M. Poelker 1, J.-S. Réal 2, L. Richardson 1, S. Setiniyazi 5, M. Stutzman 1, R. Suleiman 1, C. Tennant 1, C.-Y. Tsai 13, D. Turner 1, A. Variola 3, E. Voutier 2,11, Y. Wang 1, Y. Zhang 12 1 Jefferson Lab, Newport News, VA, US 2 LPSC, Grenoble, France 3 LAL, Orsay, France 4 Hampton University, Hampton, VA, USA 5 Idaho State University & IAC, Pocatello, ID, USA 6 North Carolina University, Durham, NC, USA 7 Old Dominion University, Norfolk, VA, US 8 The College of William & Mary, Williamsburg, VA, USA 9 University of Virginia, Charlottesville, VA, USA 10 George Washington University, Washington, DC, USA 11 IPN, Orsay, France 12 Rutgers University, Piscataway, NJ, USA 13 Virginia Tech, Blacksburg, VA, USA PEPPo Collaboration Many thanks for advice, equipment loan, GEANT4 modeling support, and funding to SLAC E-166 Collaboration International Linear Collider Project Jefferson Science Association Initiatives Award Trento, June 8-12, 2015 25/31

26. Perspectives Positron beam at CEBAF Technological challenges Trento, June 8-12, 2015

27. Positron beam at CEBAF Positron Collection Concept W target Quarter Wave Transformer (QWT) Solenoid Combined Function Magnet (QD) Collimators e-e- e+e+  S. Golge, PhD Thesis, 2010 (ODU/JLab) 24 MeV 126 MeV e-e-  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. Eric Voutier Trento, June 8-12, 2015 27/31

28. Positron beam at CEBAF e + Source Concept S. Golge, PhD Thesis, 2010 (ODU/JLab) A. Freyberger at the Town Hall Meeting, JLab, 2011 1mA I = 300 nA P e+ > 50%  p/p = 10 -2  x = 1.6 mm.mrad  y = 1.7 mm.mrad Eric Voutier Trento, June 8-12, 2015 28/31

29. Positron beam at CEBAF e + Production Scenarii CEBAF INJ (10-100 MeV) FEL (100 MeV) CEBAF (12 GeV) Eric Voutier Trento, June 8-12, 2015 29/31

30. Technological challenges Towards a Polarized Positron Beam  High Intensity Polarized Electron Source Polarized electron gun at JLab did demonstrate high intensity capabilities, though the polarization at high current was not measured.  The path from a sketched concept to a full design is a correlated multi-parameter problem requiring to adress and solve several technical/technological challenges.  High Power Production Target High power absorbers (10-100 kW) are challenging for heat dissipation, radiation management, and accelerator integration.  Optimized Positron Collection Properties of usable beam for Physics (intensity, polarization, momentum spread, emittance) are strongly related to the positron production and collection schemes and strategies (1 or 2 targets).  Positron Beam «Shaping» Pre-acceleration (Hadronic Physics) or deccelaration (Material Science) are required to produce a beam suitable for Phyiscs. Eric Voutier Trento, June 8-12, 2015 30/31

31. Conclusions Summary The merits of polarized and/or unpolarized positron beams for the Physics program at JLab is comparable to the benefits of polarized with respect to unpolarized electrons. 2  physics, GPDs… NP @ low energy, Material Science @ very low energy… The PEPPo experiment @ the CEBAF injector has been a first step in this process, opening a world-wide access to polarized positrons. 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. 31/31 Eric Voutier Trento, June 8-12, 2015