Abstract
Positron beams, both polarized and unpolarized, are identified as essential ingredients for the experimental programs at the next generation of lepton accelerators. In the context of the hadronic physics program at Jefferson Lab (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of nucleons and nuclei, in both the elastic and deep-inelastic regimes. For instance, elastic scattering of polarized and unpolarized electrons and positrons from the nucleon enables a model independent determination of its electromagnetic form factors. Also, the deeply-virtual scattering of polarized and unpolarized electrons and positrons allows unambiguous separation of the different contributions to the cross section of the lepto-production of photons and of lepton-pairs, enabling an accurate determination of the nucleons and nuclei generalized parton distributions, and providing an access to the gravitational form factors. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model of particle physics through the search of a dark photon, the precise measurement of electroweak couplings, and the investigation of charged lepton flavor violation. This document discusses the perspectives of an experimental program with high duty-cycle positron beams at JLab.
Highlights
Quantum Electrodynamics (QED) is an outstanding example of the power of quantum theory
In the following we focus on three specific examples: the measurement of a new set of EW neutral current (NC) couplings, the investigation of charged lepton flavor violation (CLFV), and the search for BSM dark photons
This document discussed the main physics reach of positron beams at JLab, which is further detailed in the contributions to the Topical Issue of the European Physics Journal A about Positron beams and physics at Jefferson Lab (e+@JLab)
Summary
Quantum Electrodynamics (QED) is an outstanding example of the power of quantum theory. This is of particular interest for studying limitations of the one-photon exchange Born approximation in elastic and inelastic scatterings [21, 22] It is essential for the experimental determination of the GPDs where the interference between the known Bethe-Heitler (BH) process and the unknown DVCS requires polarized and unpolarized electron and positron beams for a model independent extraction of the different contributions to the cross section [20]. Such polarized lepton beams provide the ability to test new physics beyond the frontiers of the Standard Model via a precise measurement of the electroweak coupling parameters [23], the investigation of charged lepton flavor violation [24], and the search for new particles linked to dark matter [25, 26]. The last section addresses the production and implementation of polarized and unpolarized positron beams at JLab
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