Abstract
Seesaw mechanism constrains from below mixing between active and sterile neutrinos for fixed sterile neutrino masses. Signal events associated with sterile neutrino decays inside a detector at fixed target experiment are suppressed by the mixing angle to the power of four. Therefore sensitivity of experiments such as SHiP and DUNE should take into account minimal possible values of the mixing angles. We extend the previous study of this subject [1] to a more general case of non-zero CP-violating phases in the neutrino sector. Namely, we provide numerical estimate of minimal value of mixing angles between active neutrinos and two sterile neutrinos with the third sterile neutrino playing no noticeable role in the mixing. Thus we obtain a sensitivity needed to fully explore the seesaw type I mechanism for sterile neutrinos with masses below 2 GeV, and one undetectable sterile neutrino that is relevant for the fixedtarget experiments. Remarkably, we observe a strong dependence of this result on the lightest active neutrino mass and the neutrino mass hierarchy, not only on the values of CP-violating phases themselves. All these effects sum up to push the limit of experimental confirmation of sterile-active neutrino mixing by several orders of magnitude below the results of [1] from 10-10 - 10-11 down to 10-12 and even to 10-20 in parts of parameter space; non-zero CP-violating phases are responsible for that.
Highlights
Neutrino oscillations clearly call for an extension of the Standard Model of particle physics (SM)
Direct observations imply limits on how much new physics can affect these abundances. These limitations are mainly independent from the seesaw constraint, and can significantly change with the introduction of some new physics affecting active-sterile neutrino mixing in the early Universe
Big Bang Nucleosynthesis (BBN) doesn’t constrain too weak mixings, which we show might be allowed by seesaw constraint
Summary
Neutrino oscillations clearly call for an extension of the Standard Model of particle physics (SM). If active-sterile mixing angles are small, active neutrino masses are double suppressed, m ∼ U2 M This is the standard seesaw type I mechanism, for more details one can address [2]. The minimal values of mixing between sterile and active neutrinos, consistent with the type I seesaw mechanism, have been estimated in [1] for the case when some of sterile neutrinos are lighter than 2 GeV and CP-violating phases are set to zero. The obtained results can be used to estimate the sensitivity of future experiments required to fully explore the parameter space of type I seesaw models with sterile neutrinos in the interesting mass range
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