Abstract
Rare decays of $K$ and $B$ mesons provide a powerful probe of dark sectors with light new particles. We show that the pair production of $O(100\,{\rm MeV})$ dark states can be probed with the decays of $K_L$ mesons, owing to the enhanced two-body kinematics, $K_L\to X_1X_2$ or $X_2X_2$. If either or these two particles is unstable, e.g. $X_2\to X_1\pi^0$, $X_2\to X_1\gamma$ or $X_{1,2}\to \gamma\gamma$, such decays could easily mimic $K_L\to \pi^0 \nu\overline{\nu}$ signatures, while not being ruled out by the decays of charged kaons. We construct explicit models that have enhanced $K_L$ decay signatures, and are constrained by the results of the KOTO experiment. We note that recently reported excess events can also be accommodated while satisfying all other constraints ($B$ decays, colliders, beam dumps). These models are based on the extensions of the gauge and/or scalar sector of the theory. The lightest of $X_{1,2}$ particles, if stable, could constitute the entirety of dark matter.
Highlights
Long-lived mesons such as neutral and charged K and B mesons have long been used as sensitive probes of new physics
The missing energy decays of B mesons have been used in the past to set limits on the pair production of dark matter states and the single production of the Higgs-like particles
We have addressed a possibility that KL decays can provide an additional probe
Summary
Long-lived mesons such as neutral and charged K and B mesons have long been used as sensitive probes of new physics. Taking this bound and the existing measurement of the charged mode into account, one should not expect any signal in the neutral mode at KOTO’s current level of sensitivity, regardless of whether short-distance new physics exists Given this consideration, the recent KOTO report of an anomalously high number of KL → π0ννevents [37] stands out. The current best limits on these decays come from Belle [41], and are given at 90% C.L. as BRðB → KννÞ < 1.6 × 10−5 at Belle ð11Þ from a combination of neutral and charged B decays Whether such limits constrain the production of new light particles in kaon decays is a highly model-dependent question, but for models with SM-like FCNCs B decays present somewhat weaker but still competitive bounds. Belle II will be able to measure the missing energy B decays to within 10% precision [29], which together with new dedicated searches will bring about significant improvements in dark sector limits below ∼5 GeV
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have