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Positrons at Jefferson Lab

Anthony W. Thomas

AIP Conf. Proc. 1160, pp. 3-7; doi:http://dx.doi.org/10.1063/1.3232031 (5 pages)

Online Publication Date: 2 September 2009

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We review the compelling case for establishing a capability to accelerate positrons at Jefferson Lab. The potential applications range from the study of two-photon exchange and deeply-virtual Compton scattering to exploiting the charged current weak interaction to probe the flavor structure of hadrons and nuclei. There are also fascinating ideas for using such a capability to discover new physics beyond the Standard Model of nuclear and particle physics.

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13.60.Fz	Elastic and Compton scattering
25.30.Hm	Positron-induced reactions
14.80.Bn	Standard-model Higgs bosons
24.60.Ky	Fluctuation phenomena
98.35.Jk	Galactic center, bar, circumnuclear matter, and bulge (including black hole and distance measurements)

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Two-photon exchange in elastic electron-proton scattering: theoretical issues

Peter G. Blunden

AIP Conf. Proc. 1160, pp. 8-12; doi:http://dx.doi.org/10.1063/1.3232040 (5 pages)

Online Publication Date: 2 September 2009

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Recent theoretical developments on selected topics in the effect of two-photon exchange on elastic electron-proton scattering are reviewed.

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13.60.Hb	Total and inclusive cross sections (including deep-inelastic processes)
25.30.Bf	Elastic electron scattering
12.40.Yx	Hadron mass models and calculations
13.88.+e	Polarization in interactions and scattering

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Two-photon exchange measurements with positrons and electrons

John Arrington

AIP Conf. Proc. 1160, pp. 13-18; doi:http://dx.doi.org/10.1063/1.3232022 (6 pages)

Online Publication Date: 2 September 2009

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Two-photon exchange contributions have potentially broad ranging impact on several charged lepton scattering measurements. Previously believed to be extremely small, based in part on comparisons of positron scattering and electron scattering in the 1950s and 1960s, recent data suggest that the corrections may be larger than expected, in particular in kinematic regions that were inaccessible in these early positron scattering measurements. Additional measurements using positron beams at Jefferson Lab would allow for a detailed investigation of these contributions in a range of reactions and observables.

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34.80.Bm	Elastic scattering
25.30.-c	Lepton-induced reactions
24.70.+s	Polarization phenomena in reactions
13.40.Gp	Electromagnetic form factors

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The OLYMPUS Experiment at DESY

M. Kohl

AIP Conf. Proc. 1160, pp. 19-23; doi:http://dx.doi.org/10.1063/1.3232027 (5 pages)

Online Publication Date: 2 September 2009

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Recent determinations of the proton electric to magnetic elastic form factor ratio from polarization transfer measurements at Jefferson Lab indicate an unexpected and dramatic discrepancy with the elastic form factor ratio obtained using the Rosenbluth separation technique in unpolarized cross section measurements. This discrepancy has been explained as the effect of two-photon exchange beyond the usual one-photon exchange approximation in the calculation of the elastic electron-proton scattering cross section. The OLYMPUS experiment at DESY, Hamburg, Germany has been proposed to definitively determine the effect of two-photon exchange in elastic lepton-proton scattering by precisely measuring the ratio of positron-proton to electron-proton elastic unpolarized cross sections.

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33.15.Kr	Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
13.40.Gp	Electromagnetic form factors
13.85.Dz	Elastic scattering
29.40.Mc	Scintillation detectors

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Electron- and positron-proton elastic scattering in CLAS

L. B. Weinstein and CLAS Collaboration

AIP Conf. Proc. 1160, pp. 24-28; doi:http://dx.doi.org/10.1063/1.3232028 (5 pages)

Online Publication Date: 2 September 2009

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There is a significant disagreement between measurements of the proton electric form factor, G^p_E, using Rosenbluth separations and polarization transfer. This disagreement, if not explained, could pose a fundamental challenge to our understanding of electron scattering or proton structure. Two-photon exchange (TPE) processes, although not fully calculable, are the most likely explanation of this disagreement. We will definitively test this assertion by comparing the electron-proton and positron-proton elastic scattering cross section in the Jefferson Lab CLAS. We will make a mixed identical electron and positron tertiary beam by passing a 5.5 GeV primary electron beam through a radiator to make a photon beam and then passing the photon beam through a converter to make electron-positron pairs. Measuring the elastic cross sections simultaneously using identical lepton beams should significantly reduce systematic uncertainties.

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13.40.Gp	Electromagnetic form factors
13.85.Dz	Elastic scattering
13.66.De	Lepton production in e−e+ interactions

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GPDs and DVCS with Positrons

Matthias Burkardt

AIP Conf. Proc. 1160, pp. 31-35; doi:http://dx.doi.org/10.1063/1.3232029 (5 pages)

Online Publication Date: 2 September 2009

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The beam charge asymmetry helps to isolate the real part of the deeply virtual Compton scattering (DVCS) amplitude. It is discussed what information can be gained both from the real and imaginary part of the DVCS amplitude.

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13.60.Fz	Elastic and Compton scattering
12.15.Ff	Quark and lepton masses and mixing
13.88.+e	Polarization in interactions and scattering

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Positrons and Electrons at HERA and HERMES

Caroline Riedl and HERMES collaboration

AIP Conf. Proc. 1160, pp. 36-42; doi:http://dx.doi.org/10.1063/1.3232030 (7 pages)

Online Publication Date: 2 September 2009

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The HERA electron-proton storage ring at DESY, Hamburg, provided a unique laboratory for the collection of data in deep-inelastic charged lepton-proton scattering at a center-of-mass energy of about 7 GeV for the fixed-target experiment HERMES and at 318 GeV for the collider experiments ZEUS and Hl. HERA could be operated with both electrons and positrons. The ability of the lepton beam to polarize itself was exploited.

Data taken with the HERMES spectrometer on unpolarized and transversely polarized gaseous targets are presented. Two examples involving interference processes are chosen that are sensitive to the beam charge: the measurement of azimuthal asymmtries in deeply-virtual COMPTON scattering and the search for a two-photon exchange signal at HERMES.

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13.60.Fz	Elastic and Compton scattering
13.66.De	Lepton production in e−e+ interactions
13.88.+e	Polarization in interactions and scattering
13.66.Bc	Hadron production in e−e+ interactions

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Deeply Virtual Compton Scattering with Positron Beams at Jefferson Lab

Volker D. Burkert

AIP Conf. Proc. 1160, pp. 43-48; doi:http://dx.doi.org/10.1063/1.3232032 (6 pages)

Online Publication Date: 2 September 2009

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A brief discussion of the DVCS program at the Jefferson Lab 12 GeV energy upgrade is given with emphasis on what can be learned from using both polarized electron and positron beams in measurements of deeply virtual Compton scattering on protons.

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13.40.Gp	Electromagnetic form factors
13.60.Fz	Elastic and Compton scattering
14.60.Cd	Electrons (including positrons)
13.88.+e	Polarization in interactions and scattering

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Polarized Positive and Negative Muon Beams to perform DVCS Measurements at COMPASS

Nicole d’Hose and COMPASS Collaboration

AIP Conf. Proc. 1160, pp. 49-55; doi:http://dx.doi.org/10.1063/1.3232033 (7 pages)

Online Publication Date: 2 September 2009

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The high energies available at CERN, and the option of using either positive or negative polarized muon beams, make the fixed-target COMPASS set-up a unique place for studying GPDs, through Deeply Virtual Compton Scattering (DVCS). A GPD program is part of the Medium and Long Term Plans at COMPASS [1]. This contribution presents the methodology and the goal of such experiments.

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13.60.Fz	Elastic and Compton scattering
13.35.Bv	Decays of muons
11.30.Rd	Chiral symmetries
13.88.+e	Polarization in interactions and scattering

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New measurement of charge asymmetry xF₃ from HERA

L. Schoeffel

AIP Conf. Proc. 1160, pp. 56-59; doi:http://dx.doi.org/10.1063/1.3232034 (4 pages)

Online Publication Date: 2 September 2009

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After presenting the recent measurements of neutral current cross section in DIS at HERA, we explain the effect of the γ−Z₀ interference at the electro-weak scale, visible on these data. Then, the beam charge difference xF₃ is measured and the interference itself is extracted. Results are discussed in the context of perturbative QCD.

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14.80.Bn	Standard-model Higgs bosons
14.65.Dw	Charmed quarks
13.66.De	Lepton production in e−e+ interactions

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Estimates of inclusive cross sections and structure functions at JLab using positron beams

M. Eric Christy

AIP Conf. Proc. 1160, pp. 60-63; doi:http://dx.doi.org/10.1063/1.3232035 (4 pages)

Online Publication Date: 2 September 2009

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Estimates for rates and the statistical precision for measurements of inclusive cross sections and structure functions utilizing a possible positron beam at JLab are presented.

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14.60.Cd	Electrons (including positrons)
13.88.+e	Polarization in interactions and scattering
13.60.Fz	Elastic and Compton scattering
07.81.+a	Electron and ion spectrometers

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Positron sources for Linear Colliders

Wei Gai and Wanming Liu

AIP Conf. Proc. 1160, pp. 67-73; doi:http://dx.doi.org/10.1063/1.3232036 (7 pages)

Online Publication Date: 2 September 2009

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Positron beams have many applications and there are many different concepts for positron sources. In this paper, only positron source techniques for linear colliders are covered. In order to achieve high luminosity, a linear collider positron source should have a high beam current, high beam energy, small emittance and, for some applications, a high degree of beam polarization. There are several different schemes presently being developed around the globe. Both the differences between these schemes and their common technical challenges are discussed.

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14.60.Cd	Electrons (including positrons)
23.20.Ra	Internal pair production
13.60.Fz	Elastic and Compton scattering
13.88.+e	Polarization in interactions and scattering

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E166: Polarized Positrons & Polarimetry

K. Peter Schüler and E166 Collaboration

AIP Conf. Proc. 1160, pp. 74-80; doi:http://dx.doi.org/10.1063/1.3232037 (7 pages)

Online Publication Date: 2 September 2009

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A proof-of-principle experiment has been carried out in the Final Focus Test Beam (FFTB) at Stanford Linear Accelerator Center (SLAC) to demonstrate production of polarized positrons in a manner suitable for implementation at the International Linear Collider (ILC). A helical undulator of 2.54 mm period and 1 -m length produced circularly polarized photons with a first harmonic endpoint energy of 8 MeV when traversed by a 46.6 GeV electron beam. The polarized photons were converted to polarized positrons in a 0.2-radiation-length tungsten target. The polarization of these positrons was measured using a Compton transmission polarimeter to have peak value in excess of 80%.

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13.88.+e	Polarization in interactions and scattering
07.20.Fw	Calorimeters
14.60.Cd	Electrons (including positrons)
13.60.Fz	Elastic and Compton scattering

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High Power Polarized Positron Source

Alexander Mikhailichenko

AIP Conf. Proc. 1160, pp. 81-86; doi:http://dx.doi.org/10.1063/1.3232038 (6 pages)

Online Publication Date: 2 September 2009

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We discuss the basics of polarized positron production by low energy polarized electrons. Efficiency of conversion ∼0.1–1% might be interesting for the Continuous Electron Beam Accelerator Facility (CEBAF) and the International Linear Collider (ILC).

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14.60.Cd	Electrons (including positrons)
13.88.+e	Polarization in interactions and scattering
29.20.-c	Accelerators
13.60.Fz	Elastic and Compton scattering
84.32.Hh	Inductors and coils; wiring

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Generation of High Intensity Thermal Positron Beams Using a 20 MeV Electron Linac

S. Chemerisov and Charles D. Jonah

AIP Conf. Proc. 1160, pp. 87-93; doi:http://dx.doi.org/10.1063/1.3232039 (7 pages)

Online Publication Date: 2 September 2009

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We present an update on the development of a positron-facility at Argonne National Laboratory. We discuss the advantages of using a low energy electron accelerator, present our latest results on slow positron production simulations, and describe our plans for further development of the facility. We have installed a new converter/moderator assembly that is appropriate for our electron energy and increases the yield about an order of magnitude. We have obtained a Penning trap and buncher from Lawrence Livermore National Laboratory (LLNL) that we plan to install. We have simulated the relative yields of thermalized positrons as a function of incident positron energy on the moderator. We use these data to calculate positron yields that we compare with our experimental data as well as with available literature data. We will discuss the new design of the next generation positron front end utilizing reflection moderation geometry.

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14.60.Cd	Electrons (including positrons)
29.20.-c	Accelerators
24.10.Lx	Monte Carlo simulations (including hadron and parton cascades and string breaking models)
82.80.Pv	Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)

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A Mini Linac Based Positron Source at CEA-Saclay

P. Debu, L. Liszkay, P. Pérez, J.-M. Rey, Y. Sacquin, V. Blideanu, A. Curtoni, O. Delferriere, P. Dupré, T. Muranaka, and N. Ruiz

AIP Conf. Proc. 1160, pp. 94-98; doi:http://dx.doi.org/10.1063/1.3232041 (5 pages)

Online Publication Date: 2 September 2009

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We are installing at CEA-Saclay a demonstration setup for an intense positron source. It is based on a compact 5.5 MeV electron linac used to produce positrons via pair production on a tungsten target. A relatively high current of 0.15 mA compensates for low positron efficiencies at low energy, which is below the neutron activation threshold. The expected production rate is 5⋅10¹¹ fast positrons per second. A set of coils is arranged to select the fast positrons from the diffracted electron beam in order to study the possibility of using a rare gas cryogenic moderator away from the main flux of particles. The commissioning of the linac is under way. This setup is part of a project to demonstrate the feasibility of an experiment to produce the H⁺ ions for a free fall measurement of neutral antihydrogen (H). Its small size and cost could be of interest for material science applications, after adaptation of the time structure.

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14.60.Cd	Electrons (including positrons)
29.20.-c	Accelerators
23.20.Ra	Internal pair production

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The CEBAF e⁺ Footprint

Arne P. Freyberger

AIP Conf. Proc. 1160, pp. 101-108; doi:http://dx.doi.org/10.1063/1.3232015 (8 pages)

Online Publication Date: 2 September 2009

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The Continuous Electron Beam Accelerator Facility (CEBAF) at the Jefferson Laboratory (JLAB) is capable of accelerating e⁻ to 6 GeV in energy. Presently CEBAF is being upgraded to a maximum energy of 12 GeV. In addition to e⁻ scattering, the user community has expressed interest in performing e⁺ scattering experiments with the upgraded CEBAF accelerator. This paper describes the existing and planned CEBAF accelerator complex, possible e⁺ production locations and the expected e⁺ beam qualities. Possibilities for production of e⁺ at the JLAB free electron laser (FEL) is also briefly described.

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14.60.Cd	Electrons (including positrons)
85.60.Ha	Photomultipliers; phototubes and photocathodes
29.20.dk	Synchrotrons
32.30.-r	Atomic spectra

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Admittance Test and Conceptual Study of a CW Positron Source for CEBAF

Serkan Golge, Charles E. Hyde, and Arne Freyberger

AIP Conf. Proc. 1160, pp. 109-114; doi:http://dx.doi.org/10.1063/1.3232016 (6 pages)

Online Publication Date: 2 September 2009

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A conceptual study of a Continuous Wave (CW) positron production is presented in this paper. The Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab (JLAB) operates with a CW electron beam with a well-defined emittance, time structure and energy spread. Positrons created via bremsstrahlung photons in a high-Z target emerge with a large emittance compared to incoming electron beam. An admittance study has been performed at CEBAF to estimate the maximum beam phase space area that can be transported in the LINAC and in the Arcs. A positron source is described utilizing the CEBAF injector electron beam, and directly injecting the positrons into the CEBAF LINAC.

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43.20.Mv	Waveguides, wave propagation in tubes and ducts
14.60.Cd	Electrons (including positrons)
29.20.-c	Accelerators
84.37.+q	Measurements in electric variables (including voltage, current, resistance, capacitance, inductance, impedance, and admittance, etc.)

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Positron program at the Idaho Accelerator Center

Giulio Stancari

AIP Conf. Proc. 1160, pp. 115-119; doi:http://dx.doi.org/10.1063/1.3232017 (5 pages)

Online Publication Date: 2 September 2009

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Positron physics is an important part of the research activities at the Idaho Accelerator Center (IAC). With positron annihilation spectroscopy, maps of nanodefects in materials have been obtained. For this purpose, positrons are generated by radioactive decay, photoactivation, or pair production. Preliminary tests of positron sources in the MeV range based on electron linacs have also been carried out at the IAC, and an expansion of this program is planned. A similar positron beam at Jefferson Lab would greatly improve our knowledge of the inner structure of the proton. In this paper, research with positrons at the IAC is reviewed. After a description of the Center’s facilities, results from positron annihilation spectroscopy are discussed, together with future plans for testing a prototype positron source for CEBAF.

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14.60.Cd	Electrons (including positrons)
29.20.-c	Accelerators
07.20.Fw	Calorimeters
13.60.Fz	Elastic and Compton scattering
13.88.+e	Polarization in interactions and scattering

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A Polarized Positron Source for CEBAF

J. Dumas, J. Grames, and E. Voutier

AIP Conf. Proc. 1160, pp. 120-125; doi:http://dx.doi.org/10.1063/1.3232018 (6 pages)

Online Publication Date: 2 September 2009

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A compact polarized positron source for Jefferson Lab is discussed. This scheme relies upon a high polarization (85%), high current (1 mA), low-energy (<100 MeV) electron beam to generate polarized positrons in a conversion target via polarized bremsstrahlung and pair creation. GEANT4 is used to simulate source distributions suitable for a CEBAF-like injector with positron polarization ∼60% and nano-Ampere intensity. An experiment to test this scheme is outlined.

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14.60.Cd	Electrons (including positrons)
13.60.Fz	Elastic and Compton scattering
13.88.+e	Polarization in interactions and scattering
23.40.Bw	Weak-interaction and lepton (including neutrino) aspects
23.20.Ra	Internal pair production

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JLAB Electron Driver Capabilities

Reza Kazimi

AIP Conf. Proc. 1160, pp. 126-132; doi:http://dx.doi.org/10.1063/1.3232019 (7 pages)

Online Publication Date: 2 September 2009

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Several schemes have been proposed for adding a positron beam option at the Continuous Electron Beam Facility (CEBAF) at Jefferson Laboratory (JLAB). They involve using a primary beam of electrons or gamma rays striking a target to produce a positron beam. At JLAB electron beams are produced and used in two different accelerators, CEBAF and the JLAB FEL (Free Electron Laser). Both have low emittance and energy spread. The CEBAF beam is polarized. The FEL beam is unpolarized but the injector can produce a higher current electron beam. In this paper we describe the characteristics of these beams and the parameters relevant for positron production.

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14.60.Cd	Electrons (including positrons)
52.59.Rz	Free-electron devices
13.88.+e	Polarization in interactions and scattering
85.60.Ha	Photomultipliers; phototubes and photocathodes

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Positron Production at JLab Simulated Using Geant4

W. J. Kossler and S. S. Long

AIP Conf. Proc. 1160, pp. 133-137; doi:http://dx.doi.org/10.1063/1.3232020 (5 pages)

Online Publication Date: 2 September 2009

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The results of a Geant4 Monte-Carlo study of the production of slow positrons using a 140 MeV electron beam which might be available at Jefferson Lab are presented. Positrons are produced by pair production for the gamma-rays produced by bremsstrahlung on the target which is also the stopping medium for the positrons. Positrons which diffuse to the surface of the stopping medium are assumed to be ejected due to a negative work function. Here the target and moderator are combined into one piece. For an osmium target/moderator 3 cm long with transverse dimensions of 1 cm by 1 mm, we obtain a slow positron yield of about 8.5⋅10¹⁰/(s⋅mA) If these positrons were remoderated and re-emitted with a 23% probability we would obtain 2⋅10¹⁰/(s⋅mA) in a micro-beam.

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23.20.Ra	Internal pair production
29.25.Rm	Sources of radioactive nuclei
14.60.Cd	Electrons (including positrons)
24.10.Lx	Monte Carlo simulations (including hadron and parton cascades and string breaking models)

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Physics with Cold Polarized Positronium

David B. Cassidy

AIP Conf. Proc. 1160, pp. 138-145; doi:http://dx.doi.org/10.1063/1.3232021 (8 pages)

Online Publication Date: 2 September 2009

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Positronium (Ps) atoms exist in different spin configurations that determine their fundamental properties, such as energy level structure, decay rate and response to a magnetic field. The nature of interactions between Ps atoms is also strongly influenced by their relative spin states; oppositely polarized Ps atoms may exchange particles and scatter into different states, or join together to form molecular positronium. Conversely, interactions between spin aligned Ps atoms cannot lead directly to changes of spin states or molecule formation, but at high densities/low temperatures a collection of such atoms may undergo a phase transition to form a Bose-Einstein condensate. Here I consider some of the physics involved in such interactions and the positron beam parameters required to conduct such experiments.

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14.60.Cd	Electrons (including positrons)
42.62.Fi	Laser spectroscopy
13.60.Fz	Elastic and Compton scattering
29.25.Lg	Ion sources: polarized

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Searching for a U-boson with a positron beam

B. Wojtsekhowski

AIP Conf. Proc. 1160, pp. 149-154; doi:http://dx.doi.org/10.1063/1.3232023 (6 pages)

Online Publication Date: 2 September 2009

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A high sensitivity search for a light U-boson by means of a positron beam incident on a hydrogen target is proposed. We described a concept of the experiment and two possible realizations. The projected result of this experiment corresponds to an upper limit on the square of coupling constant |f_eU|² = 3×10⁻⁹ with a signal to noise ratio of five.

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14.60.Cd	Electrons (including positrons)
14.70.Pw	Other gauge bosons
23.20.Ra	Internal pair production
25.30.Hm	Positron-induced reactions
13.85.Qk	Inclusive production with identified leptons, photons, or other nonhadronic particles

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Coulomb distortion in the inelastic regime

P. Solvignon, D. Gaskell, and J. Arrington

AIP Conf. Proc. 1160, pp. 155-159; doi:http://dx.doi.org/10.1063/1.3232024 (5 pages)

Online Publication Date: 2 September 2009

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The Coulomb distortion effects have been for a long time neglected in deep inelastic scattering for the good reason that the incident energies were very high. But for energies in the range of earlier data from SLAC or at JLab, the Coulomb distortion could have the potential consequence of affecting the A-dependence of the EMC effect and of the longitudinal to transverse virtual photon absorption cross section ratio R(x,Q²).

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13.85.Fb	Inelastic scattering: two-particle final states
14.60.Cd	Electrons (including positrons)
24.10.Eq	Coupled-channel and distorted-wave models
13.66.Lm	Processes in other lepton-lepton interactions

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