1. Prospects for Positron Beams at JLab e + @ JLab (i) Physics motivations (ii) Polarized positron production (iii) PEPPo (iv) Perspectives (v) Conclusions Institut de Physique Nucléaire Orsay, France Eric Voutier Newport News, June 15-18, 2016 Summer CLAS Collaboration Meeting

2. Newport News, June 15-18, 2016 Physics Motivations Two-photon physics (U,P) Generalized parton distributions (U,P) Search for the U-boson coupling to dark matter (U,P) Charge conjugation violation to access C 3q (P) Charge current physics (U,P) Standard Model Tests (U,P) Hadronic Physics Standard Model Tests High Energy Physics 2/28

3.  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 Newport News, June 15-18, 2016 Generalized parton distributions 3/28

4. N(e,e ′  N) Differential Cross Section M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009 Polarized electrons and positrons allow to separate the unknown amplitudes of the cross section for electro-production of photons. Electron observables Electron & positron observables Generalized parton distributions Additional observables Eric Voutier Newport News, June 15-18, 2016 4/28

5. Cross Sections Experimental observables Eric Voutier H. Avakian, V. Burkert, V. Guzey, JPos09, AIP 1160 (2009) 43  Significant differences between polarized and unpolarized lepton beams supports again importance of a polarized positron beam. Newport News, June 15-18, 2016  Evaluation of experimental observables is performed considering GPD H and E only, within a dual parameterization approach. 5/28

6. Moments Experimental observables Eric Voutier  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. Newport News, June 15-18, 2016 H. Avakian, V. Burkert, V. Guzey, JPos09, AIP 1160 (2009) 43 6/28

7. Projected Data Experimental observables Eric Voutier High quality data with modest polarized positron beam current can be achieved, allowing to strictly separate the interference contribution to the (e,e’  ) process. Newport News, June 15-18, 2016 H. Avakian, V. Burkert, V. Guzey, JPos09, AIP 1160 (2009) 43  Statistics of projected data involves 1000 h data taking time at 11 GeV with e - at 10 35 cm -2.s -1, and e + at 2×10 34 cm -2.s -1 (8 nA, 10 cm LH 2 ). 7/28

8. Polarized Positron Production Sokolov-Ternov self-polarization Photon circular polarization transfer Newport News, June 15-18, 2016 8/28

9. 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 Newport News, June 15-18, 2016 9/28

10. 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 Photon circular polarization transfer 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. Newport News, June 15-18, 2016 10/28

11. PEPPo Proof-of-principle experiment Calibration Positron polarization Newport News, June 15-18, 2016 11/28

12. 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 3.1-6.3 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 Newport News, June 15-18, 2016 12/28

13. Eric Voutier Calibration Newport News, June 15-18, 2016 13/28

14. PEPPo Results  PEPPo demonstrated efficient polarization transfer from 8.2 MeV/c electrons to positrons, expanding polarized positron capabilities from GeV to MeV accelerators. (PEPPo Collaboration) D. Abbott et al., Phys. Rev. Lett. 116 (2016) 214801 Positron polarization e - beam polarization 85.2 ± 0.6 ± 0.7 % Eric Voutier Newport News, June 15-18, 2016 14/28 Whenever producing e + from e -, polarization is coming for free if initial electrons are polarized.

15. 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 15/28 Newport News, June 15-18, 2016

16. Perspectives Polarized positron beam at JLab e + @ CEBAF e + @ CLAS12 e + @ JLEIC Newport News, June 15-18, 2016 16/28

17. Polarized positron beam at JLab e + Production Scenarii CEBAF INJ (10-100 MeV) LERF (100 MeV) CEBAF (12 GeV) Eric Voutier Newport News, June 15-18, 2016 17/28

18. 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 e + @ CEBAF Newport News, June 15-18, 2016 18/28

19. e + Source Concept S. Golge, PhD Thesis, 2010 (ODU/JLab) A. Freyberger at the Town Hall Meeting, JLab, 2011 1mA I = 300 nA  p/p = 10 -2  x = 1.6 mm.mrad  y = 1.7 mm.mrad Eric Voutier e + @ CEBAF Newport News, June 15-18, 2016 19/28

20. Figure-of-Merit R. Dollan, K. Laihem, A. Schälicke, NIM A 559 (2006) 185 J. Dumas, J. Grames, E. Voutier, JPos09, AIP 1160 (2009) 120 J. Dumas, Doctorate Thesis (2011) Optimum energy Optimum FoM  The polarization distribution of generated positrons is typical of bremsstrahlung induced pair creation with a production rate dominated by low-energy particles. The positron energy at optimum FoM is about half of the electron beam energy. e + @ CEBAF Eric Voutier Newport News, June 15-18, 2016 20/28

21. Expected Performances J. Dumas, Doctorate Thesis (2011)  The optimized FoM at each electron beam energy defined the « operational conditions »; simplistic cuts mimic a capture system and/or an accelerator acceptance, and define the quantitative source performances.  p/p = 10%  = 10° In the 100 MeV energy range, one can reasonably expect to optimaly achieve 75% electron polarization transfer and 10 -4 -10 -3 e + /e -. e + @ CEBAF Eric Voutier Newport News, June 15-18, 2016 21/28

22. 10 6 electrons (P b =+1) E(e - ) varying 6-11 GeV Positrons θ Tungsten target Thickness : varying from 0.1 mm to 10 mm E(e + ) z-axis No magnetic field Conceptual Simulations e + @ CLAS12 M. Mazouz (2015)  GEANT4 simulation of a generic first order production scheme in absence of magnetic collection of positrons, and with high energy polarized electrons. Eric Voutier Newport News, June 15-18, 2016 22/28 0.1 mm 0.2 0.7 2.0 10 mm

23. Basic Characteristics e + @ CLAS12 M. Mazouz (2015) Eric Voutier 11 GeV electrons Tungsten target 5mm 6 GeV ( ± 2‰) positrons 50 μA beam 10 nA beam 80% longitudinal polarization 65% longitudinal polarization 3 kW (heat)  mrd  High power target (~ 3 kW)  High power beam dump (~ 0.5 MW)  Significant radiation impact Newport News, June 15-18, 2016 23/28

24. Implementation e + @ CLAS12 A new tunnel parallel to the Hall B ones is required, long enough to allow for production and collection of positrons as well as dump of the electron beam. Eric Voutier Newport News, June 15-18, 2016 24/28

25. Expected Performances e + @ JLEIC Eric Voutier  A group (J. Grames, J. Guo, F. Lin, V. Morozov, E. Voutier) is exploring a proposal to test a prototype polarized positron injector for JLEIC. L ≥10 33 cm -2 s -1 P e+ ≥40% Instead of using a GeV electron beam for positron production, a novel approach being considered is to use and accumulate an MeV electron beam to then directly produce the positron bunch train. Positron polarization is produced by the PEPPo method. A pulsed beam with low average current (~10 nA) but significant bunch charge (~2pC) and high macro-pulse peak current (~50µA) is required fo r injection into JLEIC. Newport News, June 15-18, 2016 25/28 20 times CEBAF

26. Conceptual Design 26/28 e + @ JLEIC Eric Voutier Newport News, June 15-18, 2016 J. Grames, J. Guo, F. Lin, V. Morozov (2016) Polarized e - Source CEBAF like polarized electron source, with low-frequency laser. Higher voltage gun to provide higher e - bunch charge. Polarized e - Accumulator High gain (>100) accumulation of e - in a spin preserving ring Fast injection/extraction (<100 ns) Polarized e + Source High polarization transfer by PEPPo method ~nC e - /bunch->~pC e + /bunch e + CEBAF direct acceleration

27. Positron Working Group 27/28 Eric Voutier Newport News, June 15-18, 2016 Positron beam at JLab  Together with the technological effort in the design and testing of a PEPPo based polarized positron source for Jlab, it is time to revisit the physics reach of such a beam. We wish to call for the creation of a Positron Working Group whose charge will be to develop the Physics Case for Polarized Positron at Jlab. JLab12 & JLEIC We are expecting to have several Working Sessions and a final Workshop to deliver a White Paper by the end of 2017. Anybody interested to contribute please email Voutier@jlab.org and Grames@jlab.org

28. Conclusions Summary The PEPPo technique provides an efficient and reliable method for producing a polarized positron beam, particularly adapted to CEBAF. JLab12, JLEIC 28/28 Eric Voutier Newport News, June 15-18, 2016 Please consider joining this effort !!  The next step in e + source R&D is maximizing the beam intensity and the beam quality.  A polarized positron experimental program at JLab12 and JLEIC became realistically possible and needs to be developed further.