How Magnetic is the Dirac Neutrino?

Abstract

We derive model-independent, ''naturalness'' upper bounds on the magnetic moments of Dirac neutrinos generated by physics above the scale of electroweak symmetry breaking. In the absence of finetuning of effective operator coefficients, we find that current information on neutrino mass implies that j j & 10 ÿ14 bohr magnetons. This bound is several orders of magnitude stronger than those obtained from analyses of solar and reactor neutrino data and astrophysical observations. DOI: 10.1103/PhysRevLett.95.151802 PACS numbers: 13.15.+g, 13.40.Em, 14.60.Lm With the current emphasis on understanding the pattern of neutrino mass and mixing and the corresponding implications for cosmology and astrophysics, it is also of interest to consider the electromagnetic properties of the neutrino. The leading coupling of the neutrino to the photon is the magnetic moment, . The chiral symmetry obeyed by the massless neutrinos of the standard model (SM) requires that 0. Now that we know that m Þ 0, however, it is interesting to ask how large one might expect the neutrino magnetic moment to be. In the minimally extended SM containing gauge-singlet right-handed neutrinos, one finds that is nonvanishing, but unobservably small:. Current experimental limits are several orders of magnitude larger. Those obtained from laboratory experiments are based on analyses of the recoiling electron kinetic energy T in neutrino-electron scattering. The effect of a nonvanishing will be recognizable only if the corresponding electromagnetic cross section is comparable in magnitude with the well-understood weak interaction cross section. The magnitude of which can be probed in this way is then given by j exp

Neutrino mass implications for muon decay parameters

“…We obtain 7 7 The term in 33 proportional to differs from that of Ref. [10], which contains an error. However, this change does not affect the bounds on the neutrino magnetic moments obtained in that work.…”

“…However, as discussed in Ref. [10], they can either be related to O 6 B;AD and O 6 W;AD through the equations of motion or contain derivatives acting on the R fields so that they do not contribute to the neutrino-mass operator. Consequently, we need not consider them here.…”

“…As in Ref. [10], we work with the background field gauge [20] in d 4 ÿ 2 spacetime dimensions. We renormalize the operators using minimal subtraction, wherein counterterms simply remove the divergent, 1= terms from the one-loop amplitudes.…”

“…Twenty-two of these graphs were computed by the authors of Ref. [10] in their analysis of the mixing between O 6 M;AD and the magnetic moment operators. Here, we compute the remaining 37.…”

MSW Oscillations – LMA and Subdominant Effects

“…Requiring that the corresponding contribution to the neutrino masses be not much greater than their observed values, one obtains a model-independent "naturalness" bound on µ ab . For Dirac neutrinos, the bound obtained in this way turns out to be very stringent, µ ab 10 −14 µ B [45]. However, very importantly, for Majorana neutrinos the bound is much weaker, only µ ab 10 −10 µ B [46,47], owing to the different flavor symmetry properties of the mass and the transition moment operators [48].…”