In the present talk we continue our discussions [1-5] on neutrino electromagnetic properties and start with a short introduction to the derivation of the general structure of the electromagnetic form factors of Dirac and Majorana neutrinos.
Then we consider experimental constraints on neutrino magnetic and electric dipole moments, electric millicharge, charge radii and anapole moments from the terrestrial laboratory experiments (the bounds obtained by the reactor MUNU, TEXONO and GEMMA experiments and the solar Super-Kamiokande and the recent Borexino experiments). A special credit is done to the most severe constraints on neutrino magnetic moments, millicharge and charge radii [6-10]. The world best reactor [6] and solar [7] neutrino and astrophysical [11,12] bounds on neutrino magnetic moments, as well as bounds on millicharge from the reactor neutrinos [8] are included in the recent issues of the Review of Particle Physics (see the latest Review: P.A. Zyla et al. (Particle Data Group), Prog. Theor. Exp. Phys. 2020, 083C01). The best astrophysical bound on neutrino millicharge was obtained in [13]. The most severe astrophysical bound on neutrino magnetic moment has been obtained recently in [14].
In the recent studies [15] it is shown that the puzzling results of the XENON1T collaboration [16] at few keV electronic recoils could be due to the scattering of solar neutrinos endowed with finite Majorana transition magnetic moments of the strengths lie within the limits set by the Borexino experiment with solar neutrinos [7]. The comprehensive analysis of the existing and new extended mechanisms for enhancing neutrino transition magnetic moments to the level appropriate for the interpretation of the XENON1T data and leaving neutrino masses within acceptable values is provided in [17].
Considering neutrinos from all known sources, as well as including all available data from XENON1T and Borexino, the strongest up-to-date exclusion limits on the active-to-sterile neutrino transition magnetic moment are derived in [18] .
A comprehensive analisys of constraints on neutrino electric millicharge from experiments of elastic neutrino-electron interaction and future prospects involving coherent elastic neutrino-nucleus scattering is presented in [19].
We also present results of the recent detailed study [20] of the electromagnetic interactions of massive neutrinos in the theoretical formulation of low-energy elastic neutrino-electron scattering. The formalism of neutrino charge, magnetic, electric, and anapole form factors defined as matrices in the mass basis with account for three-neutrino mixing is presented. Using the derived new expression for a neutrino electromagnetic scattering cross section [20], we further developed studies of neutrino electromagnetic properties using the COHERENT data [9] and obtained [10] new bounds on the neutrino charge radii from the COHERENT experiment. Worthy of note, our paper [10] has been included by the Editors Suggestion to the Phys. Rev. D “Highlights of 2018”, and the obtained constraints on the nondiagonal neutrino charge radii since 2019 has been included by the Particle Data Group to the Review of Particle Physics.
The main manifestation of neutrino electromagnetic interactions, such as: 1) the radiative decay in vacuum, in matter and in a magnetic field, 2) the neutrino Cherenkov radiation, 3) the plasmon decay to neutrino-antineutrino pair, 4) the neutrino spin light in matter, and 5) the neutrino spin and spin-flavour precession are discussed. Phenomenological consequences of neutrino electromagnetic interactions (including the spin light of neutrino [21]) in astrophysical environments are also reviewed.
The second part of the proposed talk is dedicated to results of our mostly recently performed detailed studies of new effects in neutrino spin, spin-flavour and flavor oscillations under the influence of the transversal matter currents [22, 23] and a constant magnetic field [24,25], as well as to our newly developed approach to the problem of the neutrino quantum decoherence [26] and also to our recent proposal [27] for an experimental setup to observe coherent elastic neutrino-atom scattering (CEνAS) using electron antineutrinos from tritium decay and a liquid helium target that as we have estimated opens a new frontier in constraining the neutrino magnetic moment.
The discussed in the second part of the talk new results include two new effects that can be summarized as follows:
1) as it was shown for the first time in [22] neutrino spin and spin-flavor oscillations can be engendered by weak interactions of neutrinos with the medium in the case when there are the transversal matter currents; in [23] the quantum treatment of these phenomena is presented and different possibilities for the resonance amplification of oscillations are discussed, the neutrino Standard Model and non-standard interactions are accounted for;
2) within a new treatment [24] of the neutrino flavor, spin and spin-flavour oscillations in the presence of a constant magnetic field, which is based on the use of the exact neutrino stationary states in the magnetic field, it is shown that there is an interplay of neutrino oscillations on different frequencies. In particular: a) the amplitude of the flavour oscillations νLe↔ νLμ at the vacuum frequency is modulated by the magnetic field frequency μB , and b) the neutrino spin oscillation probability (without change of the neutrino flavour) exhibits the dependence on the neutrino energy and mass square difference Δm2 .
The discovered new phenomena in neutrino oscillations should be accounted for reinterpretation of results of already performed experiments on detection of astrophysical neutrino fluxes produced in astrophysical environments with strong magnetic fields and dense matter. These new neutrino oscillation phenomena are also of interest in view of future experiments on observations of supernova neutrino fluxes with large volume detectors like DUNE, JUNO and Hyper-Kamiokande.
Three other new results discussed in the concluding part of the talk are as follows:
3) a new theoretical framework, based on the quantum field theory of open systems applied to neutrinos, has been developed [26] to describe the neutrino evolution in external environments accounting for the effect of the neutrino quantum decoherence; we have used this approach to consider a new mechanism of the neutrino quantum decoherence engendered by the neutrino radiative decay to photons and dark photons in an astrophysical environment, the corresponding new constraints on the decoherence parameter have been obtained;
4) in [27] we have proposed an experimental setup to observe coherent elastic neutrino-atom scattering (CEνAS) using electron antineutrinos from tritium decay and a liquid helium target and shown that the sensitivity of this apparatus (when using 60 g of tritium) to a possible electron neutrino magnetic moment can be of order about 7×10−13μB at 90% C.L., that is more than one order of magnitude smaller than the current experimental limit;
5) in our most recent paper [28] we investigate effects of non-zero Dirac and Majorana CP violating phases on neutrinoantineutrino oscillations in a magnetic field of astrophysical environments; it is shown that in the presence of strong magnetic fields and dense matter, non-zero CP phases can induce new resonances in the oscillations channels ν e ↔ ν¯e, νe ↔ ν¯µ and νe ↔ ν¯τ ; the resonances can potentially lead to significant phenomena in neutrino oscillations accessible for observation in experiments; the detection of supernovae neutrino fluxes in the future experiments, such as JUNO, DUNE and Hyper-Kamiokande, can give an insight into the nature of CP violation and, consequently, provides a tool for distinguishing the Dirac or Majorana nature of neutrinos.
The best world experimental bounds on neutrino electromagnetic properties are confronted with the predictions of theories beyond the Standard Model. It is shown that studies of neutrino electromagnetic properties provide a powerful tool to probe physics beyond the Standard Model.
This research has been supported by the Interdisciplinary Scientific and Educational School of Moscow University “Fundamental and Applied Space Research” and also by the Russian Foundation for Basic Research under Grant No. 20-52-53022-GFEN-a.
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