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

A new exclusion limit for the electromagnetic production of a light $U(1)$ gauge boson ${\gamma }^{\prime }$ 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 $210\mathrm{MeV}/c^2<m_{e^{+}{e}^{-}}<300\mathrm{MeV}/c^2$. This experiment demonstrates the potential of high current and high resolution fixed target experiments for the search for physics beyond the standard model.

Figure 1

Electromagnetic production of the ${\gamma }^{\prime }$ boson. The coupling of the ${\gamma }^{\prime }$ boson is parametrized as $iϵe{\gamma }^{\mu }$.

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 (setup 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 $e^{+}\mathrm{\text{−}}{e}^{-}$ 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.).

Figure 6

Comparison of simulation with data (setup 1). As a model the coherent electroproduction from a heavy nucleus was used.

Figure 7

Exclusion limits with 90% confidence level in terms of relative coupling ${\alpha }^{\prime }/\alpha ={ϵ}^2$. Also shown are the previous results by BABAR [12] and for $a_{\mu }$ of the muon [2].