New physics searches in the MeV-GeV mass range
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Abstract
The search for light dark-sector particles in the MeV–GeV mass range has intensified over the past decade, driven by astrophysical evidence for dark matter, the strong CP problem, and persistent anomalies in particle physics—most notably the historical tension surrounding the muon anomalous magnetic moment, (g-2)_\mu. Together, these factors have stimulated a broad experimental program aimed at probing hidden-sector scenarios and exploring Beyond the Standard Model physics in a previously underexplored MeV--GeV region of parameter space.
A particularly well-motivated class of dark-sector candidates consists of axion-like particles, which generalize the canonical QCD axion by decoupling the strict mass-coupling relationship, allowing them to appear across a broader mass spectrum without necessarily addressing the strong CP problem. Arising naturally in many extensions of the Standard Model, these pseudoscalar states serve as prominent targets for low-energy precision probes.
In this thesis, constraints on the masses and couplings of ALPs and light scalar particles to photons and leptons are systematically derived. By utilizing data from current electron-positron collider experiments, such as BESIII and Belle II, and combining it with the most recent theoretical and experimental evaluations of lepton anomalous magnetic moments, the importance of a multi-coupling framework is demonstrated. A non-trivial interplay between leptonic and photonic interactions is revealed, showing that neglecting this dynamics can significantly obscure the true experimental bounds.
Furthermore, while existing high-energy collider experiments probe large portions of the sub-GeV parameter space, a persistent sensitivity gap in the mass range from a few to several hundred MeV is identified, primarily driven by the kinematic challenges of resolving highly collimated photon pairs. It is demonstrated that this gap can be effectively addressed by near-future, high-precision facilities. Specifically, the projected sensitivity of the MAGIX@MESA experiment, which operates below the hadroproduction threshold to provide a uniquely clean and well-controlled environment for direct BSM searches, is evaluated.
Complementary to this, a proposed Jefferson Lab program utilizing polarized positron scattering is explored. By exploiting novel polarization-dependent observables, such as the beam-normal single-spin asymmetry, it is shown how the helicity-conserving Standard Model background can be efficiently suppressed and how specific mediator spin structures can be disentangled. This framework is also extended to include vector and axial vector mediators, such as dark photons, thereby establishing a robust roadmap for discovering weakly coupled dark sector particles in the MeV--GeV mass range.