1. ? Users Town MeetingNewport News, March 16, 2012 Polarized Positrons for Nuclear Physics at Jefferson Lab (i) Electromagnetic form factors (ii) Generalized parton distributions (iii) Polarized positrons production (iv) Polarization transfert (v) PEPPo (vi) Positron beam at CEBAF (vii) Conclusions Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France Eric Voutier

2. 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 2/18 Electromagnetic form factors Eric Voutier Newport News, March 16, 2012 Within the 2  exchange hypothesis, the electromagnetic structure of the nucleon may be parametrized by 3 generalized form factors, corresponding to 8 unknow quantities.

3. Eric Voutier 3/18 Electromagnetic form factors Newport News, March 16, 2012 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. Generalized parton distributions Eric Voutier  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 4/18 Newport News, March 16, 2012

5. N(e,e ′  N) Differential Cross Section Unpolarized Target Eric Voutier M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009 Even in  Odd in  Polarized electrons and positrons allow to separate the four unknown components of the cross section for electro-production of photons. Electron observables Electron & positron observables 5/18 Generalized parton distributions Newport News, March 16, 2012

6. N(e,e ′  N) Differential Cross Section Polarized Target Eric Voutier 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. 6/18 Generalized parton distributions Newport News, March 16, 2012

7. Experimental Observables Eric Voutier 7/18 Generalized parton distributions Newport News, March 16, 2012 V. Burkert, V. Guzey, Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009  Calculations of experimental observables within a dual parametrization of the nucleon GPDs are predicting very significant effects comparing electrons and positrons. A beam current larger than 10 nA would be ideal for a polarized positron DVCS program at CLAS12.

8. 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, March 16, 2012 8/18 Compton Backscattering Undulator 1.3 GeV Require independent ~GeV to multi-GeV electron beam. Polarized positrons production Eric Voutier

9. 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 At CEBAF, we are proposing to use the bremsstrahlung of polarized electrons to produce polarized positrons. 9/18 e - →  → e + Polarized positrons production Eric Voutier Newport News, March 16, 2012

10. Eric Voutier Polarization transfert Bremsstrahlung H. Olsen, L. Maximon, PR114 (1959) 887  The observed singularity reflects the known problem of unpolarized cross sections in the tip region: Coulomb corrections appear too strong for heavy nuclei, leading to negative cross sections. 10/18  The most currently used framework to evaluate polarization transferts for polarized bremsstrahlung and pair creation processes is the O&M work developped in the Born approximation for relativistic particles and small scattering angles. Newport News, March 16, 2012

11. Eric Voutier Polarization transfert Pair Creation H. Olsen, L. Maximon, PR114 (1959) 887  Pair creation is obtained from bremsstrahlung expressions by kinematical substitutions.  Unphysical polarization transferts are observed at high energy in specific kinematic regions.  Unphysical polarization transferts are observed at small energy over the full kinematic range. Some features of O&M calculations are not valid for the pair creation process. 11/18 Newport News, March 16, 2012

12. Eric Voutier BREMSSTRAHLUNG PAIR CREATION Polarization transfert Bremsstrahlung and Pair Creation Revisited… E.A. Kuraev, Y.M. Bystritskiy, M. Shatnev, E. Tomasi-Gustafsson, PRC 81 (2010) 055208 Newport News, March 16, 2012 12/18  These calculations, developped taking into account finite electron mass effects, do not exhibit any unphysical features.  These calculations show a remarkable kinematic symmetry.  Significant differences with O&M persist at high photon energies.

13. March 2012 May 2011 Eric Voutier PEPPo E11-105 @ CEBAF Injector J. Grames, E. Voutier et al. The Polarized Electrons for Polarized Positrons experiment will test the production of polarized positrons from the bremsstralhung of polarized electrons. The positron yield and polarization distributions will be measured, investigating the polarization transfer puzzle. Dubna, October 4-9, 2010 13/18

14. Eric Voutier Positron beam at CEBAF 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 energy Optimum FoM Dubna, October 4-9, 2010 14/18 J. Dumas, Doctorate Thesis, 2011 (LPSC Grenoble/JLab)

15. Eric Voutier Optimized Polarized Positron Source Newport News, March 16, 2012 15/18 Positron beam at CEBAF J. Dumas, Doctorate Thesis, 2011 (LPSC Grenoble/JLab)  Polarization transfer up to 75% may be expected over the full energy range of the CEBAF injector.  Production efficiencies up to 10 -4 -10 -3 may be expected, depending on the electron beam energy. These figures are linearly sensitive to the acceptance of the positron collection system.

16. Eric Voutier Newport News, March 16, 2012 16/18 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.

17. Eric Voutier 17/18 Positron beam at CEBAF e + Source Concept Newport News, March 16, 2012 S. Golge, PhD Thesis, 2010 (ODU/JLab) A. Freyberger at the Town Hall Meeting, JLab, 2011 1mA I = 300 nA P e+ = 75% P e- ?  p/p = 10 -2  x = 1.6 mm.mrad  y = 1.7 mm.mrad

18. Conclusions 18/18 Eric Voutier Summary Polarized and unpolarized positron beams would be an important addition to the scientific capabilities of CEBAF. 2  physics, GPDs, NP @ low energy, Material Science @ very low energy… The PEPPo experiment @ the CEBAF injector is a first step in this process. Newport News, March 16, 2012 The User Community has an important role to play in developing a strong physics case for positrons @JLab and contributing to the design/construction of a positron line.

20. Eric Voutier  Accelerator magnets Most magnet power supplies are reversible except the arc dipoles which requires a manual action. The e - to e + switching time will limit the precision on a charge asymmetry measurement.  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 Each pass 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 tune mode beam of sufficient current (µA). A. Freyberger, Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009 Newport News, March 16, 2012 Positron beam at CEBAF