Neutrino magnetic and electric dipole moments: From measurements to parameter space

TL;DR

This paper analyzes how neutrino magnetic and electric dipole moments relate to fundamental parameters, highlighting the effects of CP violation, experimental bounds, and the differences between Majorana and Dirac neutrinos.

Contribution

It provides a detailed mapping of experimental limits onto the parameter space of neutrino couplings, considering CP phases and the distinctions between Majorana and Dirac cases.

Findings

CP violation causes misalignments in parameter space vectors.

Blind spots exist where no signal is observed despite large parameters.

Stringent bounds do not always imply small Hamiltonian couplings.

Abstract

Searches for neutrino magnetic moments/transitions in low energy neutrino scattering experiments are sensitive to effective couplings which are an intricate function of the Hamiltonian parameters. We study the parameter space dependence of these couplings in the Majorana (transitions) and Dirac (moments) cases, as well as the impact of the current most stringent experimental upper limits on the fundamental parameters. In the Majorana case we find that for reactor, short-baseline and solar neutrinos, CP violation can be understood as a measurement of parameter space vectors misalignments. The presence of nonvanishing CP phases opens a blind spot region where -- regardless of how large the parameters are -- no signal can be observed in either reactor or short-baseline experiments. Identification of these regions requires a combination of different data sets and allows for the determination of those CP phases. We point out that stringent bounds not necessarily imply suppressed Hamiltonian couplings, thus allowing for regions where disparate upper limits can be simultaneously satisfied. In contrast, in the Dirac case stringent experimental upper limits necessarily translate into tight bounds on the fundamental couplings. In terms of parameter space vectors, we provide a straightforward mapping of experimental information into parameter space.