First observation of the $\phi\to\pi^+\pi^-\gamma$ decay

TL;DR

This paper reports the first observation of the decay $oldsymbol{\phi o ext{ extpi}^+ ext{ extpi}^- ext{ extgamma}}$ using data from the CMD-2 detector, measuring its branching ratio and setting limits on related rare decays.

Contribution

First experimental observation of the $oldsymbol{\phi o extpi}^+ ext{ extpi}^- ext{ extgamma}$ decay and measurement of its branching ratio under the $f_0(980) ext{ extgamma}$ dominance assumption.

Findings

Observed $oldsymbol{\phi o extpi}^+ ext{ extpi}^- ext{ extgamma}$ decay for the first time.

Measured $oldsymbol{ ext{Br}(oldsymbol{\phi o f_0(980) ext{ extgamma})}}$ as $(1.93\pm 0.46 ext{ extpm} 0.50) imes 10^{-4}$.

Set upper limits on C-violating and P-, CP-violating decays.

Abstract

Radiative decays of the $\phi$ meson have been studied using a data sample of about 20 million $\phi$ decays collected by the CMD-2 detector at VEPP-2M collider in Novosibirsk. From selected $e^+e^-\to\pi^{+}\pi^{-}\gamma$ events the $\phi\to\pi^{+}\pi^{-}\gamma$ decay has been observed for the first time. \par Under the assumption that the intermediate $f_{0}(980)\gamma$ state dominates in the $\phi \to \pi^+\pi^-\gamma$ decay, the corresponding branching ratio is $Br(\phi\to f_{0}(980)\gamma)=(1.93\pm 0.46\pm 0.50)\times10^{-4}$. \par Selected $e^+e^-\to\mu^+\mu^-\gamma$ events were used to obtain $Br(\phi\to\mu^+\mu^-\gamma) =(1.43\pm 0.45\pm 0.14)\times10^{-5}$ for $E_{\gamma}>20$ MeV. \par Using the same data sample, upper limits at 90% CL have been obtained for the C-violating decay of the $\phi$: $Br(\phi\to\rho\gamma) < 1.2\times10^{-5}$; and for the P- and CP-violating decay of the $\eta$: \par $Br(\eta\to\pi^+\pi^-) < 3.3\times10^{-4}$.