Abstract
The KOTO experiment has recently performed a search for neutral Kaons decaying into neutral pions and a pair of neutrinos. Three events were observed in the KOTO signal region, with an expected background of about 0.05. Since no clear signal of systematic errors have been found, the excess of events in the decay KL→ π0ν overline{v} is quite intriguing. One possibility to explain this anomaly would be the presence of a scalar \U0001d719 with mass of the order of the pion mass and inducing decays KL→ π0\U0001d719 which mimic the observed signal. A scalar with mass of the order of the pion mass and a coupling to muons of the order of the Standard Model Higgs coupling could also explain the muon anomalous magnetic moment anomaly (g − 2)μ. We built on these facts to show that a light singlet scalar with couplings to the leptons and quarks as the ones induced by mixing with Higgs states in two Higgs doublet models may lead to an explanation of both anomalies. More specifically, we show that this is the case in the so-called type-X models in which leptons and quarks couple to two different Higgs doublets, and for scalar masses that are in the range between 40 and 70 MeV. Due to the relatively large coupling to leptons required to fit (g − 2)μ, the scalar lifetime accidentally falls into the sub-nanosecond range which is essential to evade the severe proton beam dump experiments and astrophysical constraints, though it becomes sensitive to constraints from electron beam dump experiments. The additional phenomenological properties of this model are discussed.
Highlights
The KOTO experiment looked for the decay of neutral Kaons into neutral pions and a pair of neutrinos [16, 17]
It is known that a scalar, with mass of about the pion mass and either stable or with a lifetime lower than about a nanosecond can provide an explanation of the KOTO anomaly without violating the Grossman-Nir bound
We have explored the possible extension of the scalar sector by an additional Higgs doublet and a singlet
Summary
We discuss the ultra-violet completion of the effective model and the constraints in the subsection. We will assume that tan β = v2/v1 1, implying that the field Φ2 will have approximately standard interactions with quarks and gauge bosons, and will carry the dominant component of the SM-like Higgs in the alignment limit [50,51,52]. Our calculation in the effective model assumes that the singlet contribution is approximately given by the one coming from the mixing of the singlet with h, q = W. The contributions coming from the mixing of the singlet with H are suppresed due to the 1/ tan β suppression of the H coupling to quarks and its suppression to its coupling to the W and Goldstone bosons in the alignment limit, cos(β − α) ∼ 0. We conclude all contributions not contained in our approximate expression are suppressed by at least a 1/ tan β factor with respect to the ones we considered
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