Limit on the production of a new vector boson in ${\mathrm{e}}^{+}{\mathrm{e}}^{-}\to \mathrm{U}{\hat{I}}^3$, U$\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}$ with the KLOE experiment

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 ${\mathrm{e}}^{+}{\mathrm{e}}^{-}\to \mathrm{U}{\hat{I}}^3$, U$\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}$ based on 28 million ${\mathrm{e}}^{+}{\mathrm{e}}^{-}\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}{\hat{I}}^3$ events collected at DA$\hat{I}¦$NE by the KLOE experiment. The $\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ production by initial-state radiation compensates for a loss of sensitivity of previous KLOE $\mathrm{U}\to {\mathrm{e}}^{+}{\mathrm{e}}^{-}$, $\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ searches due to the small branching ratios in the $\ddot{I}-\ddot{I}‰$ 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, ${\epsilon }^2$, in the U mass range between $527$ and $987$~MeV. Above 700 MeV this new limit is more stringent than previous ones.

6 pages, 9 figures, 1 table, submitted to Phys. Lett. B