1. PAC38, Newport News, August 22-26, 2011 PEPPo a Proof-of-Principle Experiment Polarized Electrons for Polarized Positrons A. Adeyemi 4, P. Aguilera 1, H. Areti 1, M. Baylac 2, G. Bosson 2, A. Camsonne 1, L. Cardman 1, S. Covrig 1, C. Cuevas 1, O. Dadoun 3, J. Dumas 1,2, E. Fanchini 2, T. Forest 5, E. Forman 1, A. Freyberger 1, S. Golge 6, J. Grames 1, P. Guèye 4, C. Hyde 7, Y. Kim 1,5, J.-F. Muraz 2, M. Marton 2, M. Poelker 1, S. Setiniyaz 5, J.-S. Réal 2, R. Suleiman 1, A. Variola 3, E. Voutier 2 1 Jefferson Lab, Newport News, VA, USA 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, USA

2. Newport News, August 22-26, 2011 This proposal (PR12-11-105) aims at a demonstration of a new technique for producing polarized positrons, and constitutes a first step towards the development of a polarized positron source for Physics experiments at JLab, and other potential applications. Objectives It follows PAC35’s enthusiastic endorsement of LOI-10-010 which noted that “Any accelerator facility, like JLab, using polarized electrons for its physics program would like an intense beam of polarized positrons. This Letter marks a proof of principle experiment that should become a full proposal.” 2/17

3. Why polarized positrons at JLab ?  The investigation of the partonic structure of nucleons and nuclei via Generalized Parton Distributions (GPDs) is a major goal of the JLab and JLab12 experimental programs. Parton Imaging M. Burkardt, PRD 62 (2000) 071503 M.Diehl, EPJC 25 (2002) 223 Newport News, August 22-26, 2011 The 4 quark-helicity conserving GPDs (H q, H q, E q, E q ) of the nucleon are extracted from the DVCS reaction off proton and neutron targets, combining experimental observables from polarized beam and targets. These observables involve different linear and bilinear combinations of Compton form factors whose real and imaginary parts are linked to GPDs. Other reactions ( DVMP, TCS …) are expected to bring additional information. A large set of different observables is required to unravel the partonic structure of hadronic matter and extract unambiguously the flavored nucleon GPDs. 3/17

4. (e,e ′  ) Differential Cross Section M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009 Why polarized positrons at JLab ? Newport News, August 22-26, 2011 Electron observables Electron & positron observables Polarized electrons and positrons provide the separation of the different reaction amplitudes of the (e,e'  ) cross section allowing for an experimentally complete determination of the GPDs. 4/17

5. Why polarized positrons at JLab ? Two Photons Physics The availability of a 100 nA e + beam (unpolarized or polarized) would permit an order of magnitude improvement in the statistical accuracy of e + e - comparisons.  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 J. Guttmann, N. Kivel, M. Meziane, M. Vanderhaeghen, arXiv:hep-ph:1012.0564 The precision of Coulomb corrections in the study of the EMC effect at JLab energies is also a concern. 5/17  e + p) /  (e - p)

6. 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. Why PEPPo ? Newport News, August 22-26, 2011 Polarization builds up exponentially with a time constant characteristic of the energy and the curvature of the positrons  ~ 20 mn @ HERA 6/17 Not compatible with CW beam delivery at JLab.

7. 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 % Newport News, August 22-26, 2011 7/17 Why PEPPo ? Compton Backscattering Undulator 1.3 GeV Require independent ~GeV to multi-GeV electron beam.

8. Polarized Bremsstrahlung E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395 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 PEPPo intends to demonstrate the feasibility of using bremsstrahlung of polarized electrons for the production of polarized positrons. Newport News, August 22-26, 2011 8/17 e - →  → e + Why PEPPo ?

9. The PEPPo experiment @ JLab Experiment Layout  PEPPo is proposed to run at the injector, taking advantage of the existing experimental capabilities that uniquely define with precision the CEBAF polarized electron beam at creation. Newport News, August 22-26, 2011 Polarization controls Variable energy 2-7.5 MeV Polarization measurement (1.5%) Energy measurement (10 -2 ) PEPPo 9/17 Laser Intensity controls

10. Production Target T 1 (0.1-1 mm W) The PEPPo experiment @ JLab Newport News, August 22-26, 2011 10/17 Beam position monitors Viewers Positron Selection Device Dipoles Solenoid Viewer Collimator Corrector magnets Solenoid Faraday Cup Annihilation detector Beam Profiler Viewers Reconversion Target T 2 (2 mm W) Polarized Analyzing Target (7.5 cm Fe) Compton Transmission Polarimeter Corrector magnets Quadrupoles ELEGANT beam optics PEPPo branch jonction G4PEPPo experiment model

11. The PEPPo experiment @ JLab T1T1 T2T2 S1S1 S2S2 D D PtPt P e- Newport News, August 22-26, 2011 Calorimeter 11/17 e+e+ e-e- PEPPo Principle of Operation PEPPo is proposing to measure the polarization transfer from longitudinal electrons to longitudinal positrons in the 2-5 MeV/c momentum range. o Longitudinal e - (P e- ) produce elliptical  whose circular (P  ) component is proportional to P e-. o P  transfers to e + into longitudinal (P e+ ) and transverse (P t ) polarization components. On the average P t =0. I e = 1 µA T 1 = 1 mm

12. The PEPPo experiment @ JLab Compton Transmission Polarimetry 12/17 Integrated Method Semi-Integrated Method  PEPPo polarimetry relies on the sensitivity of the Compton process to the polarization of the photons generated in the T 2 target by the interaction of incoming electrons/positrons. Data acquisition based on the JLab FADC250 allows implementation of 2 data taking modes. Expected experimental asymmetries are small (1-8х10 -3 ) and we will take advantage of the current JLab practices for the control of helicity correlated systematics. Newport News, August 22-26, 2011 Detector non-linearities, polarized background PMT gain, Crystal response, pile-up

13. The PEPPo experiment @ JLab Proposed Measurements Electron Measurements Positron Measurements p(e - ) (MeV/c) A T (10 -3 ) A(e - ) (%)  P e- /P e- (%) 21.01.63.4 33.55.93.0 46.110.43.0 57.112.03.0 67.813.23.0 p(e + ) (MeV/c) T 1 (mm) A(e + ) (%) A T (10 -3 ) P(e + ) (%)  P e+ /P e+ (%) 2 1.0 4.6 1.6505.9 0.11.34016.2 3 1.0 8.7 4.0674.1 0.13.5585.7 4 1.0 12.3 6.4753.9 0.16.2734.3 5 1.0 13.8 7.7814.8 0.17.5795.2  Up to 4 different production (T1) and 2 reconversion (T2) targets configurations will be measured.  Background data will be acquired for each configuration.  Measurements will be done for + and - analyzing magnet polarities. 1 h/e - setting 2 h/e + setting Newport News, August 22-26, 2011 13/17 Use measured polarization of the e - beam to calibrate the polarimeter.

14. The PEPPo experiment @ JLab Beam Time Request p(e - ) (MeV/c) I(e - ) P(e - ) (%) Time (h) Commissioning2 – 61 nA – 1 µA≥ 85 105 Mott Measurements 61 – 5 µA≥ 85 10 e - Calibration2 – 630 pA – 10 nA≥ 85 85 Detector Measurements 6.81 – 4 µA≥ 85 36 e + Measurements6.81 – 4 µA≥ 85 100 Total 336 The PEPPo experiment is asking for 2 weeks of beam time, including the commissioning of the new beam line, detector calibration, and measurements of the polarization transfer to positrons. PEPPo operation requires the injector and the PSS north linac locked. Newport News, August 22-26, 2011 14/17

15. The Collaboration is in a great position to finish PEPPo installation before the end of the 6MSD. The PEPPo experiment @ JLab Instrumentation Status  The management of JLab Accelerator Division (Letter from A. Hutton in appendix) and Collaborating Institutions supported the development of the PEPPo project up to its today status. Polarimeter calorimeter and table PEPPo branch S 1 Solenoid DD Spectrometer Analyzing magnet DAQ, Power Supplies…. Newport News, August 22-26, 2011 15/17 Our pathway on going forward depends on the recommendation of PAC38.

16. Towards a CEBAF Polarized Positron Beam Beyond PEPPo… 16/17 Newport News, August 22-26, 2011 PEPPo will demonstrate the principle of a polarized positron source based on the bremsstrahlung of low energy (5-100 MeV) polarized electrons. PEPPo will provide quantitative information that will further our understanding of polarization transfer in the pair creation process, and that will help to optimize the design of a polarized positron source for JLab physics. Information will also be provided for potential applications in a low energy source for solid state physics. o A natural “next step” after PEPPo would study the optimization of a positron collection system and a scheme to define the phase space of the positron beam for CEBAF acceleration. R&D efforts developed within the context of PhDs told that e + currents up to 1 µA and polarizations larger than 50% may be expected. S. Golge, PhD, ODU, Norfolk (2010) J. Dumas, PhD, UJF, Grenoble (2011) Details of future efforts will be guided by JLab management and the accelerator R&D required to complete a full design.

17. Summary Proposal PR12-11-105 There is a strong interest from the User Community to use positrons for nuclear physics experiments. The PEPPo Collaboration gathers a group of physicists willing to develop a polarized positron source. The PEPPo experiment proposes to measure the polarization transfer from longitudinally polarized electrons to longitudinally polarized positrons as a proof-of-principle for a new technique for polarized positron source. Newport News, August 22-26, 2011 17/17 2 weeks of beam data taking at the injector are requested to complete PEPPo. Possible customers : JLab12, SuperB, EIC, ENC, ILC(?), Solid State Physics…

19. Newport News, August 22-26, 2011 Proposal PR12-11-105

21. Why PEPPo ? Newport News, August 22-26, 2011 Polarized Electron Source 200 kV Inverted Gun + SSL GaAs/GaAsP + RF-Fiber Laser  4 mA Test ParameterValue Laser Rep Rate1500 MHz Laser Pulselength50 ps Laser Wavelength780 nm Laser Spot Size350 µm FWHM High Polarization Photocathode SSL GaAs/GaAsP Gun Voltage200kV DC CW Beam Current4 mA Run Duration1.4 hr Extracted Charge20 C 1/e Charge Lifetime85 C R. Suleiman et al., PAC’11, New York (NJ, USA), March 28 – April 1, 2011 QE(q) = QE 0 * e –(q / 84.8) High QE ~ 1.5% ( ~6 mA/W/%). Current-limited by available laser power. Higher 200kV voltage => supersede 1mA demo (PAC’07). Pushes technology in support of Electron Ion Colliders > 50 mA, High-P e - Drivers.

22. Annihilation Detector Newport News, August 22-26, 2011 The PEPPo experiment @ JLab Signal integral Co 60 Signal integral Na 22  Two NaI detectors are used to measure the back-to-back photons emitted by the annihilation of positrons in an insertable target. Radioactive source measurements with the PEPPo DAQ system have been worked out to determine optimum operating conditions and set a coincidence trigger.

23. Fiber Array Detector Newport News, August 22-26, 2011 The PEPPo experiment @ JLab Detector (FIU) used for the TPE experiment  The beam profiler at T 2 consists of a 2 layers fiber array detector read by a multi-anode PMT. The fibers are currently changed to recover a linear detector response. 90 Sr response profile of the present fiber array showing radiation damages after the TPE run.

24. Polarimeter Calorimeter Newport News, August 22-26, 2011 The PEPPo experiment @ JLab The original E166 calorimeter has been modified for PMT readout for an optimum energy resolution. New measurements of the energy resolution with the PEPPo DAQ are on-going.

25. e + Beam Concept Geometric Emittance < 5 mm-mrad Absolute Energy Spread < 1 MeV Beam Current > 50 nA Bunch Length < 2 ps Duty Factor = 100 % Frequency = 1497 MHz Beyond PEPPo… Newport News, August 22-26, 2011 e + Chicane

26.  Accelerator magnets Most magnet power supplies are reversible except the arc dipoles which requires a manual action.  Beam diagnostics Beam position monitors and viewers will work as long as the e + current is ≥ 50 nA.  Beam modes The tune mode based on a pulsed beam about tens of µA and 250 µs long and used for beam steering will need to be redefined because of the small e + current.  RF system All passes in the linac are adjusted in phase with each other via the adjustement of their pathlength with the arc dogleg sections. The diagnostic that measures the phase difference between passes require an upgrade to the system. A. Freyberger, Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009 Beyond PEPPo… Newport News, August 22-26, 2011