Non-unitarity, sterile neutrinos, and non-standard neutrino interactions

Abstract

Abstract:The simplest Standard Model extension to explain neutrino masses involves the addition of right-handed neutrinos. At some level, this extension will impact neutrino oscillation searches. In this work we explore the differences and similarities between the case in which these neutrinos are kinematically accessible (sterile neutrinos) or not (mixing matrix non-unitarity). We clarify apparent inconsistencies in the present literature when using different parametrizations to describe these effects and recast both limits in the popular neutrino non-standard interaction (NSI) formalism. We find that, in the limit in which sterile oscillations are averaged out at the near detector, their effects at the far detector coincide with non-unitarity at leading order, even in presence of a matter potential. We also summarize the present bounds existing in both limits and compare them with the expected sensitivities of near-future facilities taking the DUNE proposal as a benchmark. We conclude that non-unitarity effects are too constrained to impact present or near future neutrino oscillation facilities but that sterile neutrinos can play an important role at long baseline experiments. The role of the near detector is also discussed in detail.

Can we probe intrinsic CP and T violations and nonunitarity at long baseline accelerator experiments?

“…Going beyond the recent studies revealing how potential new physics scenarios can hinder the clean determination of this important parameter [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], we address the issue of clean separation of the intrinsic leptonic CP phase from the extrinsic contribution arising due to SI as well as new physics 7 . We also show the impact of new physics on testing non-unitarity.…”

“…We also probe deviations due to new physics scenarios. We go beyond the SM in two respects -one in which we introduce subdominant effects due to a possible source of new physics dubbed as NSI [17][18][19][20][21][22][23][24][25][26] and another where the presence of extra sterile state can lead to non-unitarity in the 3×3 part even though the overall mixing matrix is still unitary [27][28][29][30][31][32][33][34] and examine consequences relevant to long baseline experiments (For other new physics scenarios such as non-unitarity, see [35][36][37][38][39]). We highlight the regions in L − E space where the effects due to CP and T violation are drastically modified due to new physics.…”

In-flight cLFV conversion: $${e-\mu }$$ e - μ , $${e-\tau }$$ e - τ and $${\mu -\tau }$$ μ - τ in minimal extensions of the standard model with sterile fermions

“…Observational constraints on the sterile masses and their mixings with the active states arise from an extensive number of sources. Firstly, and other than requiring compatibility between the left-handed lepton mixing matrixŨ PMNS and the corresponding best-fit intervals 3 defined from ν-oscillation data [54][55][56][57][58][59][60], we also impose, when relevant, unitarity bounds as arising from non-standard neutrino interactions with matter, on the deviation ofŨ PMNS from unitarity [61][62][63]. Further constraints on the active-sterile mixings (and on the mass regime of new states) arise from electroweak precision observables; these include new contributions to the invisible Z -decay width (addressed in [64][65][66][67]), which must comply with LEP results on (Z → νν) [68]; moreover, any contribution to cLFV Z decay modes should not exceed the present uncertainty on the total Z width [68],…”

Discriminating sterile neutrinos and unitarity violation with CP invariants

“…The corresponding constraints from rare decays and electroweak precision observables are presented in [52] ("minimal flavor violation" MUV, the non unitarity is parametrized as in scenario (iii))…”