1. (i) Physics motivations (ii) The PEPPo experiment (iii) PEPPo measurements (iv) Outlook Eric Voutier Institut de Physique Nucléaire d’Orsay Orsay, France Paphos, November 2 rd -7 th, 2015 XI th Conference on Electromagnetic Interactions with Nucleons and Nuclei

2. Physics Motivations Electromagnetic form factors (U,P) Inclusive structure functions (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) International Linear Collider Positron-ion and circular Colliders (LHeC, EIC, MEIC) Thermal positron facility Paphos, November 2 nd -7 th, 2015

3. Two Photons Physics  Following the very first measurements of polarization transfer observables in electron elastic scattering, the validity of the 1  exchange approximation of the electromagnetic interaction has been questioned. P.A.M. Guichon, M. Vanderhaeghen, PRL 91 (2003) 142303 3/17 Electromagnetic form factors Within the 2  exchange hypothesis, the electromagnetic structure of the nucleon may be parametrized by 3 generalized form factors, corresponding to 8 unknow quantities. Eric Voutier Paphos, November 2 nd -7 th, 2015

4. Electromagnetic form factors Experimental Observables M.P. Rekalo, E. Tomasi Gustafsson, NPA 742 (2004) 322 C. Carlson, M. Vanderhaeghen, ARNPS 57 (2007) 171 Combining polarized electrons and positrons allows a model independent separation of the electromagnetic form factors of the nucleon.  Unpolarized e ± elastic scattering and polarization transfert observables off the nucleon involve up to 5 unknown quantities. 5 unknown contributions for 6 independent observables Cross Section Polarization Transfert 4/17 Eric Voutier Paphos, November 2 nd -7 th, 2015

5. 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 5/17 Paphos, November 2 nd -7 th, 2015

6. PEPPo Experiment Polarized positron production Proof-of-principle experiment Electron & positron polarimetry Paphos, November 2 nd -7 th, 2015

7. 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 Polarized positron production 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. 7/17 Paphos, November 2 nd -7 th, 2015

8. 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 from 8.25 MeV/c e - to e + 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 8/17 Paphos, November 2 nd -7 th, 2015

9. Compton Transmission Polarimeter FADC 250 Hamamatsu R6236-100  CsI crystals a (6×6×28 cm 3 ) are coupled to PMTs equiped with custom amplified basis to extend the PMT life-time in the high rate environment of PEPPo.  The ~2 µs long and 2 V optimized signal is fed into the JLab custom FADC250 that sampled the signal at 250 MHz. The flexibility of the FADC250 allows for 3 data taking modes : - Sample (500 samples /detector event); - Semi-integrated (1 integral / detector event); - Integrated (1 integral /helicity gate event). Electron and positron polarimetry Eric Voutier 9/17 Paphos, November 2 nd -7 th, 2015

10. Analyzing Magnet Response  Experimental asymmetries are measured with respect to beam helicity ; they are linearly proportional to the target polarization, itself proportional to the target magnetization. IHWP inserted A. Adeyemi (Hampton University) Eric Voutier 10/17 Electron and positron polarimetry G. Alexander et al., NIMA 610 (2009) 451 Paphos, November 2 nd -7 th, 2015 P T = 7.06% ± 0.05% Stat. ± 0.07% Syst.

11. PEPPo Measurements Polarized positron production Proof-of-principle experiment Electron & positron polarimetry Paphos, November 2 nd -7 th, 2015

12. Proof-of-principle experiment Eric Voutier 12/17 Paphos, November 2 nd -7 th, 2015

13.  The location of the 511 keV peak is used for in-situ monitoring of the effective gain of the read-out chain, and link to radioactive source calibration data.  Compton physics asymmetries are obtained from the polarization sensitive energy deposit spectra in the central crystal.  The coincidence time spectra allows for accidental subtraction. Polarized positron characterization Eric Voutier 511 keV from e+ annihilation at rest Bremsstrahlung end-point Energy (FADC units) Energy Deposit Spectra 13/17 Paphos, November 2 nd -7 th, 2015

14. 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 Polarized positron characterization 14/17 Paphos, November 2 nd -7 th, 2015

15.  Significant non-zero experimental asymmetries increasing with positron momentum sign efficient polarization transfer from electrons to positrons. Positron Polarization Eric Voutier Polarized positron characterization 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). 15/17

16. D. Abbott 1,P. Adderley 1, A. Adeyemi 3, 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 4, S. Covert 1, C. Cuevas 1, O. Dadoun 5, D. Dale 4, H. Dong 1, J. Dumas 1,2, E. Fanchini 2, T. Forest 4, E. Forman 1, A. Freyberger 1, E. Froidefond 2, S. Golge 6, J. Grames 1, P. Guèye 3, J. Hansknecht 1, P. Harrell 1, J. Hoskins 14, C. Hyde 7, B. Josey 13, R. Kazimi 1, Y. Kim 1,4, D. Machie 1, K. Mahoney 1, R. Mammei 1, M. Marton 2, J. McCarter 11, M. McCaughan 1, M. McHugh 10, D. McNulty 4, T. Michaelides 1, R. Michaels 1, B. Moffit 1, C. Muñoz Camacho 8, J.-F. Muraz 2, K. Myers 9, A. Opper 14, 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 12, D. Turner 1, A. Variola 5, E. Voutier 2,8, Y. Wang 1, Y. Zhang 9 1 Jefferson Lab, Newport News, VA, US 2 LPSC, Grenoble, France 4 Hampton University, Hampton, VA, USA 5 Idaho State University & IAC, Pocatello, ID, USA 3 LAL, Orsay, France 6 North Carolina University, Durham, NC, USA 7 Old Dominion University, Norfolk, VA, US 11 IPN, Orsay, France 12 Rutgers University, Piscataway, NJ, USA 8 The College of William & Mary, Williamsburg, VA, USA 9 University of Virginia, Charlottesville, VA, USA 13 Virginia Tech, Blacksburg, VA, USA 14 University of New Mexico 10 George Washington University, Washington, DC, 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 Paphos, November 2 nd -7 th, 2015

17. Outlook 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… Eric Voutier Paphos, November 2 nd -7 th, 2015 The PEPPo experiment @ the CEBAF injector has been a first step in an R&D effort for a polarized positron beam at JLab. 17/17 PEPPo II is building-up with the aim of a polarized positron beam for CLAS12 E e+ > 4 GeV I e+ > 10 nA P e+ > 50% Please join !!

18. Outlook 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… PEPPo II is building-up with the aim of a polarized positron beam for CLAS12 E e+ > 4 GeV I e+ > 10 nA P e+ > 50% Please join !! Eric Voutier Paphos, November 2 nd -7 th, 2015 The PEPPo experiment @ the CEBAF injector has been a first step in an R&D effort for a polarized positron beam at JLab.

20. Magnetic Flux Measurement Eric Voutier P T = 7.06% ± 0.05% Stat. ± 0.07% Syst. Commissionning  The iron core target is equipped with 3 pick-up coils measuring the magnetic flux generated by the magnet current variation (ramping-up, polarity reversal).  Specific cycling procedures are used during the experiment to monitor the target polarization. E. Froidefond, E. Voutier, PEPPo TN-14-02 (2014) 22/36 Paphos, November 3 rd -7 th, 2015