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Users Town Meeting

Eric Voutier. Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France. P olarized Po sitrons for Nuclear Physics at J efferson Lab. ?. Electromagnetic form factors Generalized parton distributions Polarized positrons production Polarization transfert PEPPo

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Users Town Meeting

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  1. Eric Voutier Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France Polarized Positrons for Nuclear Physics at Jefferson Lab ? • Electromagnetic form factors • Generalized parton distributions • Polarized positrons production • Polarization transfert • PEPPo • Positron beam at CEBAF • Conclusions Users Town Meeting Newport News, March 16, 2012

  2. Electromagnetic form factors Eric Voutier Two Photons Physics P.A.M. Guichon, M. Vanderhaeghen, PRL 91 (2003) 142303 • Following the very first measurements of polarization transfer observables in electron elastic scattering, the validity of the 1g exchange approximation of the electromagnetic interaction has been questioned. Within the 2g exchange hypothesis, the electromagnetic structure of the nucleon may be parametrized by 3 generalized form factors, corresponding to 8 unknow quantities. 2/18 Newport News, March 16, 2012

  3. Electromagnetic form factors Eric Voutier Experimental Observables M.P. Rekalo, E. Tomasi Gustafsson, NPA 742 (2004) 322 C. Carlson, M. Vanderhaeghen, ARNPS 57 (2007) 171 • Unpolarized e± elastic scattering and polarization transfert observables off the nucleon involve up to 5 unknown quantities. Cross Section Polarization Transfert 5 unknown contributions for 6 independent observables Combining Polarized electronsand positrons allows a model independent separation of the electromagnetic form factors of the nucleon. 3/18 Newport News, March 16, 2012

  4. Generalized parton distributions Eric Voutier Parton Imaging • 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. • GPDs = GPDs(Q2,x,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 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. 4/18 Newport News, March 16, 2012

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

  6. Generalized parton distributions Eric Voutier N(e,e′gN)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 electronsandpositrons measurements at the same kinematics. 6/18 Newport News, March 16, 2012

  7. Generalized parton distributions Eric Voutier Experimental Observables 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 positronDVCS program at CLAS12. 7/18 Newport News, March 16, 2012

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

  9. e-→g→e+ Polarized positrons production Eric Voutier Polarized Bremsstrahlung E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395 At CEBAF, we are proposing to use the bremsstrahlung of polarized electrons to produce polarized positrons. 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 9/18 Newport News, March 16, 2012

  10. Polarization transfert Eric Voutier Bremsstrahlung H. Olsen, L. Maximon, PR114 (1959) 887 • 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. • 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 Newport News, March 16, 2012

  11. Some features of O&Mcalculations are not validfor the pair creation process. Polarization transfert Eric Voutier 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. 11/18 Newport News, March 16, 2012

  12. Polarization transfert Eric Voutier Bremsstrahlung and Pair Creation Revisited… E.A. Kuraev, Y.M. Bystritskiy, M. Shatnev, E. Tomasi-Gustafsson, PRC 81 (2010) 055208 PAIR CREATION BREMSSTRAHLUNG • These calculations, developped taking into account finiteelectron 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. 12/18 Newport News, March 16, 2012

  13. March 2012 May 2011 PEPPo Eric Voutier 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 polarizationdistributions will be measured, investigating the polarization transfer puzzle. 13/18 Dubna, October 4-9, 2010

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

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

  16. Positron beam at CEBAF Eric Voutier Positron Collection Concept Quarter Wave Transformer (QWT) Solenoid Collimators e- g e- 126 MeV e+ 24 MeV W target Combined Function Magnet (QD) • 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. S. Golge, PhD Thesis, 2010 (ODU/JLab) 16/18 Newport News, March 16, 2012

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

  18. Conclusions Eric Voutier Summary Polarized and unpolarized positron beams would be an important addition to the scientific capabilities of CEBAF. 2g physics, GPDs, NP @ low energy, Material Science @ very low energy… 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. The PEPPo experiment @ the CEBAF injector is a first step in this process. 18/18 Newport News, March 16, 2012

  19. Positron beam at CEBAF Eric Voutier A. Freyberger, Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009 • 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 beamof sufficient current (µA). Newport News, March 16, 2012

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