Abstract
Anomalies in several short-baseline neutrino oscillation experiments suggest the possible existence of sterile neutrinos at about eV scale having appreciable mixing with the already known three neutrinos. We find that if such a light sterile neutrino exists, through a combined study of the leptonic decays of $\mu^-$, $\tau^-$, $\pi^-$ and $K^-$, some semi-leptonic decays of $\tau^-$ and the invisible decay width of the $Z$ boson, it is possible to constrain the relevant mixing matrix elements. Furthermore, we compare the constraints, derived by using the method presented here, with the experimental results obtained from short-baseline neutrino oscillation experiments. We find that a single light sterile neutrino cannot satisfy the existing short-baseline neutrino oscillation constraints and explain the anomalies mentioned above. Along the way we provide a number of experimentally clean observables which can be used to directly study the light sterile neutrino independently of the neutrino oscillation experiments.
Highlights
Sterile neutrinos, first hypothesized by Pontecorvo [1], are electrically neutral fermions of either Dirac or Majorana nature with no standard weak interaction albeit mixing with the existing active neutrinos
The theoretical studies of sterile neutrinos deal with many diverse new physics scenarios which may include a multitude of sterile neutrinos with masses ranging from below the eV scale to close to the Planck mass scale
We would like to emphasize that the approach we elaborated in this paper can provide an independent and robust probe of active-sterile neutrino mixing in addition to the traditional approach of using short-baseline neutrino oscillation experiments
Summary
First hypothesized by Pontecorvo [1], are electrically neutral fermions of either Dirac or Majorana nature with no standard weak interaction albeit mixing with the existing active neutrinos. We assume that the Pontecorvo-MakiNakagawa-Sakata (PMNS) matrix [1,12], the 3 × 3 matrix that deals with the mixing of νe, νμ, ντ with ν1, ν2, ν3, remains unitary in the presence of the sterile neutrino νs, while the 4 × 4 mixing matrix V (which might be unrelated to any seesaw mechanism for generating neutrino mass) can be, in general, nonunitary [13] In this case, the effects of sterile neutrinos will become manifest in the observables associated to charged-current interactions of leptons [here repeated labels indicate summation, l 1⁄4 e, μ, τ, and γμ ≡ γμð1 − γ5Þ], 2It should be noted that the previously known reactor neutrino anomaly [7] may not require any explanation in terms of light sterile neutrinos in view of the recent paper from the Daya Bay collaboration [8].
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