Study of the Process ${e}^{{+}}{e}^{{-}}{\to}{\eta\pi}^{{0}}{\gamma}$ in the Cossision-Energy Range of $\sqrt{{s}}{=}{1.05{-}2.00}$ GeV

Abstract

The process $e^{+}e^{-}\to\eta\pi^{0}\gamma$ is studied in the range of center-of-mass (c.m.) collision energies between 1.05 and 2.00 GeV on the basis of data corresponding to an integrated luminosity of 94.5 pb${}^{-1}$ that were accumulated in an experiment with the SND detector at the VEPP-2000 $e^{+}e^{-}$ collider. The cross section for this process was measured for the first time and was found to receive a dominant contribution from the mechanism of a transition through an $\omega\eta$ intermediate state. The cross section measured for the process $e^{+}e^{-}\to\omega\eta\to\eta\pi^{0}\gamma$ agrees with the results of earlier measurements in the channel $e^{+}e^{-}\to\pi^{+}\pi^{-}\pi^{0}\eta$. At a level of 5.7 standard deviations (5.7 $\sigma$), it is established that the process $e^{+}e^{-}\to\eta\pi^{0}\gamma$ is not fully described by hadronic intermediate states of the $VP$ type. The cross section was measured for this missing contribution, which may be generated by radiative processes—for example, $e^{+}e^{-}\to a_{0}(1450)\gamma$. It ranges from 15 to 20 pb over a broad energy interval extending from 1.3 to 1.9 GeV.