Abstract
Recently, the KOTO experiment at J-PARC has observed three anomalous events in the flavor-changing rare decay $K_L \to \pi^0 \nu \bar\nu$, which indicates that the corresponding branching ratio is almost two orders of magnitude larger than the Standard Model (SM) prediction. Taking this intriguing result at face value, we explore model implications of its viable explanation by a long-lived light SM-singlet scalar ($S$) emission, i.e. $K_L \to \pi^0 S$, with $S$ decaying outside the KOTO detector. We derive constraints on the parameter space of such a light scalar in the context of three simple models: (i) a real singlet scalar extension of the SM; (ii) a $B-L$ extension where neutrino masses arise via type-I seesaw mechanism from $B-L$ breaking; and (iii) a TeV-scale left-right symmetric model. The flavor-changing couplings needed to explain the KOTO excess in models (i) and (ii) originate from tree-level mixing of the scalar with SM Higgs field ($h$), and in model (iii), from the mixing of $S$ and $h$ with the neutral component of the heavy bidoublet Higgs field. After taking into account the stringent constraints from high-precision searches for flavor-changing charged and neutral kaon decays at NA62, E949, KOTO and CHARM experiments, as well as the astrophysical and cosmological constraints on a light scalar, such as those from supernova energy loss, big bang nucleosynthesis and relativistic degrees of freedom, we find that the light scalar interpretation of the KOTO excess is allowed in all these models. Parts of the parameter range can be tested in future NA62 and DUNE experiments.
Highlights
In the Standard Model (SM) of particle physics, flavorchanging neutral currents (FCNCs) are absent at tree-level and are predicted to be small at loop level, suppressed by the Glashow-Iliopoulos-Maiani (GIM) mechanism and the small off diagonal Cabibbo-Kobayashi-Maskawa (CKM) matrix elements in the quark sector [1]
We find the following three beyond the SM (BSM) scenarios which fall into this category: (i) Scalar singlet model: Here the FCNC couplings of S arise from its mixing with the SM Higgs field h, which has loop-induced FCNC couplings with SM quarks [13,14,15]
We have evaluated the effective branching ratio in Eq (13) for different vR values and found that it is almost independent of the vR value, as in the parameter space of interest the typical lifetime of S is much longer than the KOTO detector size of 3 m
Summary
In the Standard Model (SM) of particle physics, flavorchanging neutral currents (FCNCs) are absent at tree-level and are predicted to be small at loop level, suppressed by the Glashow-Iliopoulos-Maiani (GIM) mechanism and the small off diagonal Cabibbo-Kobayashi-Maskawa (CKM) matrix elements in the quark sector (or by the tiny neutrino masses in the lepton sector, if we allow nonzero neutrino masses to be part of the “new” SM) [1]. In contrast with the previous two models, the FCNC couplings of S in this case arise at tree-level, due to its mixing with the heavy scalar H1 from the bidoublet Φ (and the SM Higgs field) Another special feature of the light scalar S in the LRSM is that even for small mixing angles, it can still decay into two photons through the WR loop and the heavy charged scalar loops. The rest of the paper is organized as follows: In Sec. II, we discuss the simplest real scalar extension of the SM in light of the KOTO excess vis-á-vis other laboratory and astrophysical/cosmological constraints.
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