A framework for testing leptonic unitarity by neutrino oscillation experiments

Abstract

If leptonic unitarity is violated by new physics at an energy scale much lower than the electroweak scale, which we call low-scale unitarity violation, it has different characteristic features from those expected in unitarity violation at high-energy scales. They include maintaining flavor universality and absence of zero-distance flavor transition. We present a framework for testing such unitarity violation at low energies by neutrino oscillation experiments. Starting from the unitary 3 active plus N (arbitrary positive integer) sterile neutrino model we show that by restricting the active-sterile and sterilesterile neutrino mass squared differences to 0.1 eV 2 the oscillation probability in the (3 + N ) model becomes insensitive to details of the sterile sector, providing a nearly modelindependent framework for testing low-scale unitarity violation. Yet, the presence of the sterile sector leaves trace as a constant probability leaking term, which distinguishes lowscale unitarity violation from the high-scale one. The non-unitary mixing matrix in the active neutrino subspace is common for the both cases. We analyze how severely the unitarity violation can be constrained in ν e -row by taking a JUNO-like setting to simulate medium baseline reactor experiments. Possible modification of the features of the (3 + N ) model due to matter effect is discussed to first order in the matter potential.

IceCube bounds on sterile neutrinos above 10 eV

“…where the last term is a constant leaking term [60]. Note that, except for the leaking term, all the sterile neutrino effects are encoded in the matrix α, in particular in the elements α µµ , α τ τ , and α τ µ , regardless of how many sterile neutrinos are considered as long as they are all in the averaged out regime [59].…”

Indirect unitarity violation entangled with matter effects in reactor antineutrino oscillations

“…If these additional oscillations are averaged out, it will be similar to the indirect UV effect, but with an additional constant term appeared as shown in Ref. [33,44]. According to Ref.…”

“…A typical and conservative expectation is that the magnitude of κ ij should be smaller than 0.05, which indicates that the active-sterile mixing angles θ ij (for i = 1, 2, 3 and j = 4, 5, 6) can be taken as large as 7.5 • . So far a lot of attention has been paid to possible effects of indirect UV in the acceleratorbased long-baseline neutrino oscillation experiments [19,23,24,25,26,27,28,29,30], and limited attention has also been given to this kind of effect in a reactor-based antineutrino oscillation experiment [31,32,33]. It is already known that the UV-induced "zero-distance effect" must appear in the "disappearance" oscillation probability P (ν α → ν α ) (for α = e, µ, τ ) [31,34], for example,…”

White Paper on New Opportunities at the Next-Generation Neutrino Experiments (Part 1: BSM Neutrino Physics and Dark Matter)

“…The subject of neutrino oscillations with a non-unitarity neutrino mixing matrix have recently received some attention, see for instance Refs. [326][327][328][329][330][331] for the main different approaches and views of the matter. It is worth to mention that electroweak precision tests and charge lepton flavor violation (cLFV) constraints reduce the margin for unitarity deviation [332][333][334][335][336][337] although these are highly model dependent.…”