Large Mixing of Light and Heavy Neutrinos in Seesaw Models and the LHC

TL;DR

This paper investigates the potential for larger-than-expected mixing between light and heavy neutrinos in seesaw models, exploring their detectability at the LHC and implications for flavor-changing processes.

Contribution

It demonstrates that multiple neutrino generations can lead to significantly larger mixing, enhancing the prospects for observing heavy neutrinos at the LHC and affecting flavor physics.

Findings

Larger mixing angles are possible with multiple neutrino generations.

Heavy neutrino production at the LHC can be significant with enhanced mixing.

Flavor-changing neutral currents may be observable due to sizable mixing.

Abstract

In the type-I seesaw model the size of mixing between light and heavy neutrinos, nu and N, respectively, is of order the square root of their mass ratio, (m_nu/m_N)^(1/2), with only one generation of the neutrinos. Since the light-neutrino mass must be less than an eV or so, the mixing would be very small, even for a heavy-neutrino mass of order a few hundred GeV. This would make it unlikely to test the model directly at the LHC, as the amplitude for producing the heavy neutrino is proportional to the mixing size. However, it has been realized for some time that, with more than one generation of light and heavy neutrinos, the mixing can be significantly larger in certain situations. In this paper we explore this possibility further and consider specific examples in detail in the context of type-I seesaw. We study its implications for the single production of the heavy neutrinos at the LHC via the main channel q qbar' -> W^* -> l N involving an ordinary charged lepton l. We then extend the discussion to the type-III seesaw model, which has richer phenomenology due to presence of the charged partners of the heavy neutrinos, and examine the implications for the single production of these heavy leptons at the LHC. In the latter model the new kinds of solutions that we find also make it possible to have sizable flavor-changing neutral-current effects in processes involving ordinary charged leptons.