Charged lepton flavour change and Non-Standard neutrino Interactions

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.