Limit on the production of a low-mass vector boson in $\mathrm{e}^{+}\mathrm{e}^{-} \to \mathrm{U}\gamma$, $\mathrm{U} \to \mathrm{e}^{+}\mathrm{e}^{-}$ with the KLOE experiment

TL;DR

This study searched for a hypothetical light vector boson, called the U boson, in electron-positron collisions but found no evidence, setting upper limits on its possible interaction strength across a mass range of 5 to 520 MeV/c².

Contribution

First experimental search for the U boson in e+e- collisions at KLOE, establishing new upper limits on its coupling strength in the 5-520 MeV/c² mass range.

Findings

No evidence of the U boson was observed.

Set upper limits on the mixing parameter ε² from 10^{-6} to 10^{-4}.

Excluded certain parameter space for the dark photon model.

Abstract

The existence of a new force beyond the Standard Model is compelling because it could explain several striking astrophysical observations which fail standard interpretations. We searched for the light vector mediator of this dark force, the $\mathrm{U}$ boson, with the KLOE detector at the DA$\Phi$NE $\mathrm{e}^{+}\mathrm{e}^{-}$ collider. Using an integrated luminosity of 1.54 fb$^{-1}$, we studied the process $\mathrm{e}^{+}\mathrm{e}^{-} \to \mathrm{U}\gamma$, with $\mathrm{U} \to \mathrm{e}^{+}\mathrm{e}^{-}$, using radiative-return to search for a resonant peak in the dielectron invariant-mass distribution. We did not find evidence for a signal, and set a 90\%~CL upper limit on the mixing strength between the Standard Model photon and the dark photon, $\varepsilon^2$, at $10^{-6}$--$10^{-4}$ in the 5--520~MeV/c$^2$ mass range.