Limit on the production of a new vector boson in $\mathrm{e^+ e^-}\rightarrow {\rm U}\gamma$, U$\rightarrow \pi^+\pi^-$ with the KLOE experiment

TL;DR

This paper reports a search for a hypothetical dark photon in electron-positron collisions producing a pion pair and a photon, setting new limits on the photon-dark photon mixing strength in the 527-987 MeV mass range.

Contribution

The study provides the first search for a dark photon decaying into pions in this mass range using initial-state radiation at KLOE, improving existing limits above 700 MeV.

Findings

No evidence for dark photon signal was observed.

Set new upper limits on the mixing parameter  in the 527-987 MeV range.

Limits above 700 MeV are more stringent than previous results.

Abstract

The recent interest in a light gauge boson in the framework of an extra U(1) symmetry motivates searches in the mass range below 1 GeV. We present a search for such a particle, the dark photon, in ${\rm e^+ e^-}\rightarrow {\rm U}\gamma$, U$\rightarrow \pi^+\pi^-$ based on 28 million $\mathrm{e^+ e^-} \rightarrow \pi^+ \pi^-\gamma$ events collected at DA$\Phi$NE by the KLOE experiment. The $\pi^+ \pi^-$ production by initial-state radiation compensates for a loss of sensitivity of previous KLOE ${\rm U} \rightarrow \mathrm{e^+ e^-}$, $\mu^+\mu^-$ searches due to the small branching ratios in the $\rho-\omega$ resonance region. We found no evidence for a signal and set a limit at 90\% CL on the mixing strength between the photon and the dark photon, $\varepsilon^2$, in the U mass range between $527$ and $987$~MeV. Above 700 MeV this new limit is more stringent than previous ones.