Search for Light Gauge Bosons of the Dark Sector at the Mainz Microtron

TL;DR

This study searches for a light U(1) gauge boson γ' that couples to leptons via kinetic mixing ε and decays to $e^+e^-$. Using a high-luminosity fixed-target setup at the Mainz Microtron with a Ta target, the experiment achieves sub-MeV level mass resolution and applies a robust background modeling framework beyond the Weizsäcker-Williams approximation. No γ' signal is observed, allowing a 90% CL exclusion on ε^2 that improves upon BaBar and muon $a_μ$ constraints in the 210–300 MeV/$c^2$ mass window. The results illustrate the power of high-current, high-resolution fixed-target experiments for dark-sector searches and motivate further scans at MAMI and other facilities.

Abstract

A new exclusion limit for the electromagnetic production of a light U(1) gauge boson γ' decaying to e^+e^- was determined by the A1 Collaboration at the Mainz Microtron. Such light gauge bosons appear in several extensions of the standard model and are also discussed as candidates for the interaction of dark matter with standard model matter. In electron scattering from a heavy nucleus, the existing limits for a narrow state coupling to e^+e^- were reduced by nearly an order of magnitude in the range of the lepton pair mass of 210 MeV/c^2 < m_e^+e^- < 300 MeV/c^2. This experiment demonstrates the potential of high current and high resolution fixed target experiments for the search for physics beyond the standard model.

Figures (7)

• Figure 1: Electromagnetic production of the $\gamma'$ boson. The coupling of the $\gamma'$ boson is parametrized as $i\,\epsilon\,e\, \gamma^\mu$.
• Figure 2: Dominant background processes. While graphs (a) and (b) have the same structure as the signal and present an irreducible background, the contributions of graphs (c) and (d) can be suppressed by the choice of kinematic setting.
• Figure 3: Coincidence time distribution after particle identification by Čerenkov detectors (set-up 1). The events of the light shaded area were used as true coincidences, while the dark shaded area was used as an estimate of the accidental coincidences.
• Figure 4: Mass distribution of the reconstructed $e^+$-$e^-$ pair (setup 1). The dark shaded area denotes the background due to accidental coincidences (scaled to a time window of 2 ns).
• Figure 5: Upper exclusion limits with 90% confidence level determined by the Feldman-Cousins algorithm (all data). The averaged limit is included for subjective judgement only ($\approx 10\%$ of the data points should be above this line at 90% C.L.).