Here we give a brief review on the current bounds on the general Majorana transition neutrino magnetic moments (TNMM) which cover also the conventional neutrino magnetic moments (NMM). Leptonic CP phases play a key role in constraining TNMMs. While the Borexino experiment is the most sensitive to the TNMM magnitudes, one needs complementary information from reactor and accelerator experiments in order to probe the complex CP phases.
Recent experiment proposed to observe induced radiative neutrino transitions are confronted to existing bounds on neutrino magnetic moments from earth-based experiments. These are found to exclude any observation by several orders of magnitude, unless the magnetic moments are assumed to be strongly momentum dependent. This possibility is discussed in some generality, and we find that nontrivial dependence of the neutrino form factor may indeed occur, leading to quite unexpected effects, although this is insufficient by orders of magnitude to justify the experiments.
The recent puzzling results of the XENON1T collaboration at few keV electronic recoils could be due to the scattering of solar neutrinos endowed with finite Majorana transition magnetic moments (TMMs). Within such general formalism, we find that the observed excess in the XENON1T data agrees well with this interpretation. The required TMM strengths lie within the limits set by current experiments, such as Borexino, specially when one takes into account a possible tritium contamination.
We explore the effects of nonstandard neutrino interactions in the lower components of the solar neutrino spectrum which are predominant by the vacuum oscillations. The recent measurements of Borexino experiment between 2011 and 2015 provide a clean test to study the nonstandard neutrino interactions at the source (sun) and the at solar detector. In this work, first the possible standard model parameters are estimated from the combined data of the low energy regime and then the nonstandard effects at the source, at the detector, and from the interplay between source and detector parameters are bounded. The same effects are also investigated for the proposed experiments like LENA and Jinpin Neutrino Experiment with their projected sensitivities.
A search for the solar neutrino effective magnetic moment has been performed using data from 1291.5 days exposure during the second phase of the Borexino experiment. No significant deviations from the expected shape of the electron recoil spectrum from solar neutrinos have been found, and a new upper limit on the effective neutrino magnetic moment of $mu_{ u}^{eff}$ $<$ 2.8$cdot$10$^{-11}$ $mu_{B}$ at 90% c.l. has been set using constraints on the sum of the solar neutrino fluxes implied by the radiochemical gallium experiments.Using the limit for the effective neutrino moment, new limits for the magnetic moments of the neutrino flavor states, and for the elements of the neutrino magnetic moments matrix for Dirac and Majorana neutrinos, are derived.
We study the effects induced by new neutral fermions below their mass threshold, due to their possible mixing with the standard neutrinos. We use as experimental constraints the recent results on lepton universality, together with the measurement of the $mu$ decay rate and the updated LEP data. In particular, the inclusion in our data set of the most recent determinations of the $tau$ branching fractions, mass and lifetime implies that a previous indication of a non-vanishing mixing for $ u_tau$ is no longer present. We obtain new stringent limits on the mixing parameters between $ u_e$, $ u_mu$, $ u_tau$ and heavy neutral states of different weak isospin. If no assumption on the type of neutrinos involved in the mixing is made, we find $snue^2<0.0071$, $snumu^2<0.0014$ and $snutau^2<0.033$.