Search for the Rare Decay K_{L}\to\pi^{0}\pi^{0}\gamma

TL;DR

This paper reports a search for the rare decay $K_{L} o ext{pi}^0 ext{pi}^0 ext{gamma}$, setting a new upper limit that challenges theoretical predictions and probes higher-order chiral perturbation theory effects.

Contribution

First experimental upper limit on the decay $K_{L} o ext{pi}^0 ext{pi}^0 ext{gamma}$ at $2.43\times 10^{-7}$, significantly lower than previous bounds, testing chiral perturbation theory at $O(p^6)$.

Findings

Set an upper limit of $2.43\times 10^{-7}$ (90% CL) for the decay.

Improved sensitivity by a factor of 20 over previous results.

Provides experimental constraints on higher-order chiral perturbation theory predictions.

Abstract

The KTeV E799 experiment has conducted a search for the rare decay $K_{L}\to\pi^{0}\pi^{0}\gamma$ via the topology $K_{L}\to\pi^{0}\pi^{0}_D\gamma$ (where $\pi^0_D\to\gamma e^+e^-$). Due to Bose statistics of the $\pi^0$ pair and the real nature of the photon, the $K_{L}\to\pi^{0}\pi^{0}\gamma$ decay is restricted to proceed at lowest order by the CP conserving direct emission (DE) of an E2 electric quadrupole photon. The rate of this decay is interesting theoretically since chiral perturbation theory predicts that this process vanishes at level $O(p^4)$. Therefore, this mode probes chiral perturbation theory at $O(p^6)$. In this paper we report a determination of an upper limit of $2.43\times 10^{-7}$ (90% CL) for $K_{L}\to\pi^{0}\pi^{0}\gamma$. This is approximately a factor of 20 lower than previous results.