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ENC Meeting

Eric Voutier. Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France. Polarized Electrons for Polarized Positrons GPDs @ JLabe +. Physics motivations CHIPS @ JLab Technical challenges Polarization calculations GEANT4 simulations Demonstration experiment

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ENC Meeting

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  1. Eric Voutier Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France Polarized Electrons for Polarized Positrons GPDs @ JLabe+ • Physics motivations • CHIPS @ JLab • Technical challenges • Polarization calculations • GEANT4 simulations • Demonstration experiment • Positron annihilation spectroscopy • Conclusions ENC Meeting July 20, 2009, Bonn

  2. Physics Motivations Eric Voutier Parton Imaging • Generalized Parton Distributions (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. • The GPDs encode the correlations between partons and contain then information about the dynamics of the system like the angular momentum or the distribution of the strong forces experienced by quarks and gluons inside nucleons and nuclei. X. Ji, PRL 78 (1997) 610 M. Polyakov, PL B555 (2003) 57 A new light on the nuclear structure M. Burkardt, PRD 62 (2000) 071503 M.Diehl, EPJC 25 (2002) 223 GPDs can be interpreted as 1/Q resolution distributions in the transverse plane of partons with longitudinal momentum x 1/6 Bonn, July 20, 2009

  3. GPDs Physics Motivations Eric Voutier Deep Exclusive Scattering J.C. Collins, L. Frankfurt, M. Strikman, PRD56 (1997) 2982 X. Ji, J. Osborne, PRD 58 (1998) 094018 J.C. Collins, A. Freund, PRD 59 (1999) 074009 GPDs can be accessed via exclusive reactionsin the Bjorken kinematic regime. • The factorization theorem allows to express the cross section for deep exclusive processes as a convolution of a knownhard scattering kernel with an unkownsoft matrix element related to the nucleon structure (GPDs). Factorization Interaction with elementary partons (Q2>>M2) Separation of perturbative and non-perturbative scales (-t<<Q2) Hardness ? GPD(Q2,x,x,t) Probe tagging (x) Production of one additional particle (D) Final stateidentification Exclusivity 2/6 Bonn, July 20, 2009

  4. GPDs Physics Motivations Eric Voutier GPDs enter the cross section of hard scattering processes via Compton form factors, that are integralsover the intermediate parton longitudinal momenta. Q2 >> M2-t << Q2 3/6 Bonn, July 20, 2009

  5. Out-of-plane angle entering the harmonic development of the reaction amplitude A.V. Belitsky, D. Müller, A. Kirchner, NPB 629 (2002) 323 hadronic plane j g e-’ • The Bethe-Heitler (BH) process where the real photon is emitted either by the incoming or outgoing electron interferes with DVCS. • DVCS & BH are indistinguishable but the BH amplitude is exactly calculable and known at low t. • The relative importance of each process is beam energy and kinematics dependent. g* e- leptonic plane p Physics Motivations Eric Voutier Photon Electroproduction P0 = 6 GeV/c Polarization observables help to single-out the DVCS amplitude. 4/6 Bonn, July 20, 2009

  6. Physics Motivations Eric Voutier • From M. Diehl (DESY) @ CLAS12 European Workshop, Genova, February 25-28, 2009 Polarized beam charge difference 5/6 Bonn, July 20, 2009

  7. Physics Motivations Eric Voutier • Generalized Parton Distributions • Investigation of 2g exchange in elastic scattering • Study of Coulomb distortion in the inelastic regime • Search for a light dark matter gauge U-boson • Measurement of the C3qneutral weak coupling • Positron Annihilation Spectroscopy Program and talks posted on the conference web site 6/6 Bonn, July 20, 2009

  8. CHIPS @ JLab Eric Voutier CEBAF High Intensity Positron Source J. Dumas, C. Hyde, W.J. Kossler, T. Forest, A. Freyberger, S. Golge, J. Grames, R. Kazimi, E. Voutier • The CHIPS community develops concepts and ideas for the construction of a high intensity polarized positron source at JLab for fixed target experiments to take place in the 12 GeV era. • Challenges • High e- beam intensity 1-10 mA together with high beam polarization ≥ 85% • High power e- target (~1 MW beam driver @ 10-100 MeV) • Efficient e+capture • Goals • e+ beam current > 100 nA in CW mode (CEBAF @ 1497 MHz) • As large as possible e+ beam polarization 1/4 Bonn, July 20, 2009

  9. CHIPS @ JLab Eric Voutier Undulator Source A. Mikhailichenko et al., AIP CP 915 (2007) 1095 • A high energy e- beam travelling through an helical undulator emits circularly polarized g’s. • The polarized g’s create longitudinally polarized e+’s by interaction in a Ti-alloy target. Simulations for a 50 GeV beam E166 @ SLAC scheme 2/4 Bonn, July 20, 2009

  10. e+Diag. Bending magnets Spectrometer 46.6 GeV e- γ Diag. 10 MeV γ Undulator e-Dump W Target e- to Dump CHIPS @ JLab Eric Voutier E166 @ SLAC reconversion target magnetized iron detector e+ spectrometer G. Alexander et al, PRL 100 (2008) 210801 G. Alexander et al, physics.ins-det:0905.3066 3/4 Bonn, July 20, 2009

  11. CHIPS @ JLab Eric Voutier Polarized Bremsstrahlung E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395 • Within a high Z target, longitudinally polarized e-’s radiate circularlypolarized g’s. • Within the same/different target, circularly polarized g’s create longitudinally polarized e+’s. bulk GaAs Strained GaAs Superlattice GaAs Pe- 35% 85% 35% 75% 75% 85% 85% 1995 1998 1999 2000 2004 2007 2010 30 mA 150 mA 1 mA 100 mA 50 mA 100 mA 180 mA Ie- All operating with suitable photocathode lifetime to sustain weeks of operation Evolution of CEBAF polarized electron source 4/4 Bonn, July 20, 2009

  12. Technical Challenges Eric Voutier Polarized e- @ JLab State of the Art @ JLab • Strained single-layer and multi-layer (superlattice) GaAs are used to achieve polarization greater than 50%. • Strained superlattice photocathodes provide the largest polarization (P~85%). • They operate at JLab since 09/2005 delivering a constant polarization over photocathode charge lifetime. 1/7 Bonn, July 20, 2009

  13. 100 nm 14 pairs chekc Technical Challenges Eric Voutier The determinant elements for the achievement of a high intensity high polarization e- beam are: a performant electron gun, a very good photocathode, and a powerful laser. QE ~ 1% & P ~ 85% @ 780nm Strained Superlattice GaAs Load-lock gun Fiber-based laser system Improved electron gun with ultra-high vaccuum J. Grames et al., AIP CP 915 (2007) 1037 Pw ~ 2W @ 780 nm & dt ~ 40 ps @ 499 MHz J. Hansknecht, M. Poelker, PRST-AB 9 (2006) 063501 2/7 Bonn, July 20, 2009

  14. Technical Challenges Eric Voutier J. Grames et al., Proc. of XXIInd Particle Accelerator Conference, Albuquerque (NM, USA), June 25-29, 2007 100 kV Load Lock Polarized e- Photogun RF Fiber Laser 1 mA e- beam Load & Prepare Superlattice Photocathode An accelerator quality beam of 1 mA with a charge lifetime of ~200 C was demonstrated with a superlattice photocathode. Beam polarization was not measured 3/7 Bonn, July 20, 2009

  15. Technical Challenges Eric Voutier e+ Beam Concept Geometric Emittance < 5 mm-mrad Absolute Energy Spread < 1 MeV Beam Current > 50 nABunch Length < 2 ps Duty Factor=100 %Frequency=1497 MHz 4/7 Bonn, July 20, 2009

  16. Technical Challenges Eric Voutier S. Golge et al., Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009 • A possible concept involves the construction of a dedicatede+ tunnelat the end of the injector and parallel to the north linac. • Positrons would be produced with 120 MeV e- (JLab 12 GeV) incident on a tungsten target. • e+’s are selected with a quadrupole triplet and transported to the accelerator section. G4beamline simulations indicate a global efficiency of 10-5 e+/e- for 120 MeV e- off a 3 mm W target. 10 mA e-→ 100 nA e+ 5/7 Bonn, July 20, 2009

  17. g e+ e- Technical Challenges Eric Voutier S. Golge et al., Proc. of XXIInd Particle Accelerator Conference, Albuquerque (NM, USA), June 25-29, 2007 • Previous studies of the collection system (10 MeV e- beam) indicate that a large fraction of the beam powerdeposits in the conversion target. Possible solutions are a rotating target or a liquid metal target but have not yet been investigated in the JLab context. 6/7 Bonn, July 20, 2009

  18. Technical Challenges 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). 7/7 Bonn, July 20, 2009

  19. D < 0.5 → No screening 0.5 ≤ D < 120 → Intermediate screening 120 ≤ D → Complete screening Polarization Calculations Eric Voutier Polarization Transfert H. Olsen, L. Maximon, PR114 (1959) 887 • Polarization transfert in polarized bremsstrahlung and pair creation processes have been investigated by O&M within the Born approximation, for relativistic particles and small scattering angles. Unpolarized cross section — for bremsstrahlung +for pair creation • Screening effects have been evaluated within the Molière model Coulomb correction Screening correction 1/4 Bonn, July 20, 2009

  20. Polarization Calculations Eric Voutier • Calculations at a 60 MeV incident energy and a 0.41 mrd scattering angle PAIR CREATION BREMSSTRAHLUNG Singularities originating from a nul cross section (tip region problem) Selecting complete screening removes singularities 2/4 Bonn, July 20, 2009

  21. Polarization Calculations Eric Voutier • Calculations at a 3 MeV incident energy and a 0.41 mrd scattering angle PAIR CREATION BREMSSTRAHLUNG Unphysical results over the whole phase space Some features of O&Mcalculations are not valid Coulomb corrections ? Screening corrections ? Relativistic approximation ? … 3/4 Bonn, July 20, 2009

  22. A possible explanation X. Artru (IPN Lyon) @ PosiPol 2009 http://indico.cern.ch/conferenceOtherViews.py?view=nicecompact&confId=53079 In the O&M calculations, Coulomb corrections and screening effects are implemented at the level of the cross section and not at the level of the amplitude, as should be. • End point problem R.T. Deck, C.J. Mullin, C.L. Hammer, NC 32 (1964) 180 J.L. Matthews, R.O. Owens, NIM 111 (1973) 157 There exists in the litterature solutions to correct for the unphysical behaviour of the cross section near the end point region, based on specific calculations at this point and a derivative continuation. • New calculations V. Strakhovenko (Budker INP) @ PosiPol 2009http://indico.cern.ch/conferenceOtherViews.py?view=nicecompact&confId=53079New expressions for polarization transferts in bremsstrahlung and pair creation have been obtained that do not exhibit the O&M singular features, and provide an almost universal energy depedence spectra. E.A. Kuraev, Y.M. Bystritskiy, M. Shatnev, E. Tomasi Gustafsson New calculations of the bremsstrahlung and pair creation processes are worked out in a peripheral kinematics approach with no relativistic approximation… work in progress Polarization Calculations Eric Voutier 4/4 Bonn, July 20, 2009

  23. GEANT4 Simulations Eric Voutier e+ Source Polarization • Simulation are performed within the GEANT4 framework, taking advantage of the polarization capabilities developed by the E166 Collaboration. • The source files are modified to select complete screening, independently of D. (a cooking recipe while waiting for a better calculation) R. Dollan, K. Laihem. A. Schälicke, NIM A559 (2006) 185 1/3

  24. GEANT4 Simulations 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. Optimum FoM Optimum energy 2/3 Bonn, July 20, 2009

  25. GEANT4 Simulations Eric Voutier • Simplistic cuts are applied to mimic a capture system and the accelerator acceptance. Thickness sensitivity is under study… 3/3 Bonn, July 20, 2009

  26. Demonstration Experiment Eric Voutier Conceptual Design • An experiment to test the production of polarized positrons is currently designed. The goals are to measure the yield and polarizationdistributrions as a function of the beam energy. • The ATF/KEK and E166 devices serve as a conceptual guidance towards the final design. • Polarimetry would consist of a Compton transmission polarimeter. • Calibration and check of the analyzing power can be performed with the polarized electron beam. • The Compton asymmetry would be measured by reversing the beam (up to 1 kHz frequency of random change) and/or the target polarization. M. Fukuda et al, PRL 91 (2003) 164801 T. Omori et al, PRL 96 (2006) 114801 G. Alexander et al, PRL 100 (2008) 210801 G. Alexander et al, physics.ins-det:0905.3066 1/1 Bonn, July 20, 2009

  27. Positron Annihilation Spectroscopy Eric Voutier 2D/3D ACAR The motion of atomic electrons results in a Dopler distorsion of the energy of g’sfrom e+e- annihilation, and an angular deviation breaking g’s colinearity. Surface and purity studies of semiconductors Technological (production and thermalisation of e+) and instrumental (sQ = 1 mrd, sk ≤ 1 keV) challenges. 1/1 Bonn, July 20, 2009

  28. Conclusions Eric Voutier Summary The possibilities of developing a CW « low cost » polarized positron source at JLab look promising. Technical challenges concern the electron beam driver (10 mA @ 85%) and the high power conversion target (optimum positron capture, accelerator acceptance, damping/accumulator ring ?…) Calculations of O&M appear to be of limited application in the low energy domain. The longitudinal polarization transfert from electrons to positrons was neverexperimentally investigated. Need for experimental data Proposal expected by end of 2009 – Collaborators are welcome Such a new type of polarizedpositron source is of interest not only for JLab physics but also for the Super B project and the next generation of e±N colliders (EIC, eRHIC, LHeC, ENC), and would be a breakthrough for solid state physics. 1/1 Bonn, July 20, 2009

  29. DVCSDeeply Virtual Compton Scattering DVMPDeeply Virtual Meson Production DA Hard gluon GPDs GPDs Physics Motivations Eric Voutier GPDs can be accessed via exclusive reactionsin the Bjorken kinematic regime. Longitudinal response only • The identification of the DVMP process requires the detection of the meson disintegration products in large acceptance detectors. • Factorisation applies only to longitudinally polarized virtual photons whose contribution to the electroproduction cross section must be isolated. • Meson production acts as a GPD and a flavor filter. • The DVCS process is identified via double (eg) or triple (egN) coincidences, allowing for small scale detectors and large luminosities. • Additionalamplitudes contribute to the reaction process and interfere with the DVCS amplitude. Bonn, July 20, 2009

  30. GPDs Physics Motivations Eric Voutier The GPDs Framework D. Müller et al., FP 42 (1994) 101 A.V. Radyushkin, PRD 56 (1997) 5524 X. Ji, PRL 78 (1997) 610 Coherence between quantum states of different helicity, longitudinal momentum and transverse position. • At leading twist, the partonic structure of the nucleon is described by 4 quark helicity conserving and chiral even GPDs and 4 quark helicity flipping and chiral odd GPDs (+8gluon GPDs). P. Hoodbhoy, X. Ji, PRD 58 (1998) 054006 M. Diehl, EPJC 19 (2001) 485 x Initial longitudinal momentum fraction -2x Transferred longitudinal momentum fraction (skewness) (-t)1/2 Momentum transfer to the nucleon • In the forward limit (t  0, x  0), H’s reduce to the forward parton distributions (density, helicity, transversity). • E’s, which involve nucleon helicity flip, do not have a DIS equivalent. E’s are new and unknown distributions Bonn, July 20, 2009

  31. GPDs Physics Motivations Eric Voutier Form Factors • The first Mellin moments relate GPDs to Dirac (Hq), Pauli (Eq), axial ( ), and pseudo-scalar ( ) nucleon form factors. • Similar relations relate chiral odd GPDs to tensor form factors. x independence from Lorentz invariance Transverse spin-flavor dipole moment in an unpolarized nucleon Energy Momentum Tensor X. Ji, PRL 78 (1997) 610 M. Polyakov, PLB 555 (2003) 57 M. Burkardt, PRD 72 (2005) 094020 • The second Mellin moments relate GPDs to the nucleon dynamics, i.e. parton angular momentum and strong forces distributions. Correlation between quark spin and angular momentum in an unpolarized nucleon GPDsunify in the same universal framework parton distributions, form factors, and the spin of the nucleon. Bonn, July 20, 2009

  32. Physics Motivations Eric Voutier E03-106 n-DVCS P.Y. Bertin, C.E. Hyde, F. Sabatié, E. Voutier et al. d A. Airapetian et al., arXiv/hep-ex:0802.2499 Measurements off protonare sensitive toJu (uquark in theproton) M. Mazouz et al., PRL 99 (2007) 242501 Measurements off neutron are sensitive to Jd (uquark in theneutron) A.W. Thomas, arXiv/hep-ph:0803.2775 u Neutrons are a mandatory step in the hunt for the quark orbital momentum. Bonn, July 20, 2009

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