Pionium as a source of false events in the K→πνν¯ decays

Peter Lichard

doi:10.1103/physrevd.102.113005

Abstract

We suggest that the decay modes of kaons with a pion and a pionium ($\pi^+\pi^-$ atom) in the final state can constitute a not yet considered background to the very rare decay $K\to\pi\nu\bar\nu$. In fact, a part of pioniums may escape the decay region before decaying into two $\pi^0$s (or to $\pi^0\pi^0\gamma$ in the case of excited pionium). To illustrate the importance of this background, we show that it may even explain, under some assumptions, the unexpected $K_L$ decay events that appeared in the KOTO experiment.

Highlights

We suggest that the decay modes of kaons with a pion and a pionium in the final state can constitute a not yet considered background to the very rare decay K → πνν
B02pðCoul:Þ 1⁄4 0.34þ−00..2833 × 10−9: A comparison with (3) shows that if the experimental cut on transverse momentum of π0 is taken into account, the 2p pioniums with the Coulombic binding energy cannot be a source of the unexpected events in the KOTO experiment
We have suggested that events in which the kaon decays into pionium, which leaves the decay region without decaying, may contribute to the background in the K → πννexperiments

Summary

Peter Lichard

We suggest that the decay modes of kaons with a pion and a pionium (πþπ− atom) in the final state can constitute a not yet considered background to the very rare decay K → πνν. To illustrate the importance of this background, we show that it may even explain, under some assumptions, the unexpected KL decay events that appeared in the KOTO experiment. Two important experiments investigating the rare kaon decays in flight are currently running. The main aim of both is to test the Standard Model and to constrain new physics theories by precisely measuring the very rare kaon decays into a pion and two neutrinos. The NA62 experiment at the CERN Super Proton Synchrotron [1,2,3,4] deals with positively charged kaons Kþ and aims to collect enough Kþ → πþννevents to get a signal to background ratio of 10∶1.

Experimental Facility at the Japan Proton Accelerator

PETER LICHARD