Abstract
Non-Standard neutrino Interactions (NSI) are vector contact interactions involving two neutrinos and two first generation fermions, which can affect neutrino propagation in matter. SU(2) gauge invariance suggests that NSI should be accompanied by more observable charged lepton contact interactions. However, these can be avoided at tree level in various ways. We focus on lepton flavour-changing NSI, suppose they are generated by New Physics heavier than $m_W$ that does not induce (charged) Lepton Flavour Violation (LFV) at tree level, and show that LFV is generated at one loop in most cases. The current constraints on charged Lepton Flavour Violation therefore suggest that mu <---> e flavour-changing NSI are unobservable and tau <---> l flavour-changing NSI are an order of magnitude weaker than the weak interactions. This conclusion can be avoided if the heavy New Physics conspires to cancel the one-loop LFV, or if NSI are generated by light New Physics to which our analysis does not apply.
Highlights
AND REVIEWNonstandard neutrino interactions (NSI) are four-fermion interactions induced by physics from beyond-the-Standard Model, constructed from a vector current of two Standard Model (SM) neutrinos of flavor ρ and σ, and two first generation fermions f ∈ fe; u; dg
We focus on lepton flavour-changing NSI, suppose they are generated by new physics heavier than mW that does not induce lepton flavor violation (LFV) at tree level, and show that LFV is generated at one loop in most cases
Ref. [26] considered models with heavy new particles that induced NSI at tree level, these authors did not allow cancellations among the contributions of different mediators to LFV interactions. They showed that their allowed models induced additional, better constrained operators, so that ε ≳ 10−2 was excluded. We review this question from an EFT perspective allowing arbitrary cancellations, between operators of dimension six and eight,4 in order to find linear combinations of operators that induce NSI but not LFV at tree level
Summary
Nonstandard neutrino interactions (NSI) are four-fermion interactions induced by physics from beyond-the-Standard Model, constructed from a vector current of two Standard Model (SM) neutrinos of flavor ρ and σ, and two first generation fermions f ∈ fe; u; dg. It is clear that to obtain ε ≳ 10−3, the new physics scale ΛNP cannot be far above the weak scale and is likely to be within the reach of the LHC Models that generate such large effects in the neutrino sector, while avoiding the stringent bounds on charged lepton flavor violation(LFV) [23], have been explored by various authors.. Reference [4] considered a particular dimension eight NSI operator, and erroneously argued that the exchange of a W boson between the two neutrino legs would transform them into charged leptons, thereby inducing a contact interaction that was severely constrained by experimental bounds on charged lepton flavor violation (LFV) It was pointed out in [30], that the logenhanced, one-loop mixing of this NSI operator into LFV operators vanished.
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