Study of the decay $K^{+}\to\pi^{+}\pi^{-}\pi^{+}\gamma$ in the OKA experiment

TL;DR

This paper reports on the measurement of the decay $K^{+} o ext{pi}^{+} ext{pi}^{-} ext{pi}^{+} ext{gamma}$, including its branching fraction and photon energy spectrum, using data from the OKA experiment, and compares results with theoretical predictions.

Contribution

First measurement of the branching fraction and photon spectrum for $K^{+} o ext{pi}^{+} ext{pi}^{-} ext{pi}^{+} ext{gamma}$ decays at high statistics, testing chiral perturbation theory predictions.

Findings

Branching fraction measured as (0.71 ± 0.05) × 10^{-5}.

Photon energy spectrum consistent with chiral perturbation theory predictions.

No significant up-down asymmetry observed.

Abstract

A high statistics data sample of the decays of $K^+$ mesons to three charged particles was accumulated by the OKA experiment in 2012 and 2013. This allowed to select a clean sample of about 450 events with $K^{+}\to\pi^{+}\pi^{-}\pi^{+}\gamma$ decays with the energy of the photon in the kaon rest frame greater than 30 MeV. The measured branching fraction of the $K^{+}\to\pi^{+}\pi^{-}\pi^{+}\gamma$, with $E_{\gamma}^{*}$ > 30 MeV is $(0.71 \pm 0.05) \times 10^{-5}$. The measured energy spectrum of the decay photon is compared with the prediction of the chiral perturbation theory to O$(p^{4})$. A search for an up-down asymmetry of the photon with respect to the hadronic system decay plane is also performed.