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Interference Physics
Coordinators: John Arrington (johna@anl.org), Charles Hyde (hyde@jlab.org)
The effort of this sub-group is to consider the benefits of the application of polarized positrons in the electromagnetic physics sector. In the energy range currently available at JLab, ther is no specific difference with respect to the scientific information obtained with an electron or a positron probe. However, when more than one QED-based mechanism contributes to a reaction process, the comparison between lepton beams of opposite charge allows one to uniquely distinguish the quantum interference between these mechanisms. This feature is expressed in the experimental measurement of the electromagnetic form factors of the nucleon where two-photon exchange mechanisms may reconcile cross section and polarization data, and also in the experimental measurement of the generalized parton distributions of the nucleon where the interference between the Bethe-Heitler and virtual Compton amplitudes is a key observable.
Charged Current Physics
Coordinators: Yulia Furletova (yulia@jlab.org), Wally Melnitchouk (wmelnitc@jlab.org)
The focus of this sub-group is the evaluation of the merits of polarized positron beams for electroweak physics, most likely accessible at EIC energies. Charged W ^{\pm} currents (CC) differentiate electron and positron beams as essentially different experimental probes, able to uniquely isolate positively or negatively charged quarks. CC DIS access combinations of quark flavors different from those measured with purely electromagnetic DIS, providing an alternative and novel source of information about PDFs, particularly for the unpolarized and polarized strange and to some extent charm PDFs. For example, the availability of polarized electron and positron beams would provide the necessary tools to measure the difference between the strange and anti-strange quark distributions as well as to investigate the isovector EMC effect.
Test of the Standard Model
Coordinators: Marco Battaglieri (battaglieri@ge.infn.it), Xiaochao Zheng (xiaochao@jlab.org)
Electromagnetic and electroweak interaction with polarized electron and positron beams may provide mew possibilities to probe the existence of physics beyond the Standard Model. Comparison between a left-handed electron beam and a right-handed positron beam will provide the first measurement of a charge-conjugation violation observable, the effective electron-quark coupling C3q. The e+e- annihilation, on the other hand, is a promising channel in search of a U-boson or heavy photon, candidate of SM-Dark Matter interaction maediator. The combination of high energy and high intensity positron beam would provide the best reach achievable in terms of mass range and coupling constant in the invisible decay channel e+e- to \gamma U</math>. Polarization observables are here expected to leverage a significant role for suppressing background to identify the experimental physics signal of interest. The absence of right-handed CC within the SM forces the CC DIS cross section to be zero at maximum beam polarization +(-)1 for e^{-(+)}. Measuring the beam longitudinal polarization sensitivity of the total CC cross section allows a natural SM test through searches for right-handed CC and setting limits on the right-handed W-boson exchange.
Positron Applications
Coordinators: Tony Forest (foretony@isu.edu), Farida Selim (faselim@bgsu.edu)
Positron annihilation spectroscopy (PAS) is a well-known technique for investigating the structural properties of materials. because of the purity of the 2 gamma signal produced from the annihilation of positrons with atomic electrons, this technique is a very sensitive probe of material defects and constitutes an accurate method for the measurement of the momentum distribution of electrons. Nevertheless, the globally poor availability of intense positron beams at low energies (1-1000 keV) percludes efficient use of PAS. An MeV electron accelerator production of positrons, like that used in the PEPPo experiment, can easily provide two orders of magnitude greater beam intensities than the most powerful nuclear reactor based facility. Adding controlled and flexible polarization capabilities with the PEPPo technique at accelerator facilities, may constitute a technological breakthrough for PAS and help address the lack of low energy positron research facilities world-wide.
Positron Source and Beam Physics
Coordinators: Joe Grames (grames@jlab.org), Vasiliy Morozov (morozov@jlab.org)
The efficient transfer of polarization from electrons to positrons (>80%) has been demonstrated by the PEPPo experiment and offers a new pathway to use low energy polarized electron beams (10-100 MeV) to promptly produce polarized positrons suitable for acceleration. A challenging aspect of the positron injector is the optimization required to achieve the desired beam characteristics, such as beam intensity, transverse emittance (size), bunch length and energy spread (longitudinal emittance) necessary and be well-mated to the accelerator design (e.g. 12 GeV CEBAF, LERF, JLEIC). A core activity of this sub-group is to evaluate the user requirements, accelerator integration and assess the merits and risks of different schemes appropriate for JLab, in order to summarize and recommend the R&D needed for a successful positron physics program.