PHYSICAL REVIEW D 102, 113005 (2020)
Pionium as a source of false events in the K → πνν¯ decays
Peter Lichard Institute of Physics and Research Centre for Computational Physics and Data Processing, Silesian University in Opava, 746 01 Opava, Czech Republic and Institute of Experimental and Applied Physics, Czech Technical University in Prague, 128 00 Prague, Czech Republic
(Received 1 June 2020; revised 3 November 2020; accepted 20 November 2020; published 18 December 2020)
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¯. In fact, a part of pioniums may escape the decay region before decaying into two π0s (or to π0π0γ 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 KL decay events that appeared in the KOTO experiment.
DOI: 10.1103/PhysRevD.102.113005
Two important experiments investigating the rare kaon this surprising result. The analysis performed in Ref. [9] decays in flight are currently running. The main aim of both shows that if the four events in the KOTO signal region were is to test the Standard Model and to constrain new physics real, it would mean a branching fraction of the underlying theories by precisely measuring the very rare kaon decays mechanism equal to into a pion and two neutrinos. The NA62 experiment at the CERN Super Proton Synchrotron [1–4] deals with pos- B K → π0νν¯ 2 1þ2.0ðþ4.1Þ 10−9 ð L ÞKOTO ¼ . −1.1 −1.7 × ð3Þ itively charged kaons Kþ and aims to collect enough ð Þ Kþ → πþνν¯ events to get a signal to background ratio at the 68(95)% confidence level. 10∶1 þ − of . The Standard Model predicts the branching The π π atom, pionium (usually denoted as A2π), was fraction [5] for this decay [6], discovered in 1993 at the Institute of High Energy Physics at Serpukhov, Russia [12] and intensively studied in the B Kþ → πþνν¯ 8 4 1 0 10−11: ð Þ¼ð . . Þ × ð1Þ Dimeson Relativistic Atomic Complex (DIRAC) experi- ment [13] at the CERN Proton Synchrotron. In these The KOTO experiment [7] is being conducted at the Hadron experiments, the pioniums were produced by the proton Experimental Facility at the Japan Proton Accelerator beam impinging on a target. In the same target, the Research Complex. It was designed to observe the decay pioniums broke-up into their constituents with approxi- K → π0νν¯ L of long-lived neutral kaons. The theoretical mately equal energies and small relative momenta. branching fraction [6] is The decay of pionium to two neutral pions is dominant 0 −11 and the measured lifetime is [13] BðKL → π νν¯Þ¼ð3.4 0.6Þ × 10 : ð2Þ þ0.28 −15 τ1 ¼ 3.15−0 26 × 10 s: ð4Þ Until recently, both rare kaon decay experiments proceeded s . as expected, slowly pushing down the upper bounds The NA48=2 Collaboration at the CERN SPS [14] studied of branching fractions. However, in September 2019, K → π π0π0 decays and found an anomaly in the π0π0 Satoshi Shinohara (on behalf of the KOTO Collaboration) invariant mass distribution in the vicinity of 2mπþ that can be [8] announced the presence of four events in the signal interpreted as the production of pioniums in the kaon decays 0 10 0 02 region in the situation where mere . . events and their subsequent two-π0 decay. For our later consid- were expected. Very soon, several papers appeared, e.g., 48=2 – erations, it is important that the NA Collaboration in Refs. [9 11], aimed at finding new physics interpretations of their seminal paper [14] determined the branching ratio