Effects of Dimension-5 Dipole Interactions of Right-Handed Neutrinos on Big Bang Nucleosynthesis

The discovery of neutrino oscillations established that neutrinos are massive and mix, implying physics beyond the Standard Model (SM). A minimal and theoretically well motivated extension consists in introducing right-handed (sterile) neutrinos. These gauge singlet fermions appear naturally in seesaw mechanisms and provide explanations for neutrino masses, baryogenesis via leptogenesis, and potentially dark matter. In the simplest realization, right-handed neutrinos couple to the SM through renormalizable dimension-4 Yukawa interactions, leading to active–sterile mixing after electroweak symmetry breaking. This mixing governs their production and decay in the early Universe and determines their cosmological signatures. From the perspective of effective field theory (EFT), additional higher-dimensional operators are generically expected. Among them, a particularly interesting class is the dimension-5 dipole operator coupling right-handed neutrinos to electroweak gauge field strengths. Schematically, this operator has the structure L_{dipole} = (c/Λ) \bar{N}^c σ^{μν}N B_{μν} + h.c., where Λ denotes the scale of new physics, N^c is the charge conjugate right-handed neutrino field and B_{μν} is the field strength of the hypercharge gauge boson. The coefficient c depends on the UV origin of the operator and is generically expected to be 1-loop suppressed, since it is known that dipole operators can only be generated through loops. Such dipole operators are frequently studied in scenarios where the sterile neutrino has mass in the keV range and is a dark matter candidate. In these models, electroweak loop diagrams involving charged leptons and W bosons generate an effective transition magnetic moment. The resulting radiative decay channel N → νγ plays a central role in indirect detection constraints. Here we are interested in considering heavier right-handed neutrinos, that play a role in the generation of active neutrino masses but are not dark matter candidates. From a purely EFT standpoint, the dipole interaction need not be tied exclusively to electroweak loop effects. It may also arise from an unspecified ultraviolet (UV) completion involving new heavy particles. In that case, its coefficient is not necessarily related to the active–sterile mixing angle and may be treated as an independent parameter. The presence of such a dipole interaction can significantly modify the cosmological behavior of right-handed neutrinos. In particular, it affects (i) production rates in the early plasma, (ii) thermalization history, (iii) decay channels and lifetimes and (iv) energy injection in the primordial plasma. The primary goal of this thesis is to determine how the inclusion of a dimension-5 dipole operator modifies BBN constraints on right-handed neutrinos. We will proceed in two steps: a) we will compute the lifetime of the right-handed neutrinos –including the N_i → N_j γ and N_i → νγ decays generated by the dipole– and requiring that it is smaller than about 1s, i.e. most of the steriles decay well before BBN; b) we will study in detail the effect of photon injection from the Ni → Njγ and Ni → νγ decays in the chain of nuclear reactions of primordial nucleosynthesis. In the regime in which the dipole dominates over the mixing (i.e. for dipole generated by new-physics), its physics can drastically modify the usual properties of right-handed neutrinos and, as a consequence, the excluded parameter space. The aim of this thesis is to study this effect in detail, determining whether regions of parameter space open up in regions that are potentially testable by terrestrial experiments.