Harnessing the power of atoms as electron accelerators

Atoms can act as electron accelerators, effectively providing a scan in
energy when fixed-energy positrons from a beam collide with them. This
talk explores the potential of this phenomenon for new physics searches,
particularly for new vector bosons or pseudoscalars coupling to
fermions, such as Dark Photons and ALPs. We propose using a positron
beam directed at a fixed high-Z target, such as 92U, at available
facilities like JLab, PADME or the H3 beam at CERN. We find that the
spread in electron momentum significantly enhances the experimental
reach when compared to the electron-at-rest approximation, probing parts
of the parameter space currently unexplored. Additionally, we show how
this effect, using a 12 GeV beam as foreseen at JLab, can achieve high
statistical accuracy in measuring the hadronic cross section for
electron-positron annihilation across the relevant range of
center-of-mass energies. This provides a new method to study the
hadronic vacuum polarization contribution to (g−2)μ, adding a new piece
of information to the existing puzzle.