Previous Michelson Postdoctoral Prize Lectureships

Neutrino Magnetic Moments:

The detection of a neutrino magnetic moment comparable to present limits would be an unequivocal indication of physics beyond the Standard Model. However, the existence of a neutrino magnetic moment implies contributions to the neutrino mass via electroweak radiative corrections. We derive model-independent upper bounds on neutrino magnetic moments generated by physics above the electroweak scale. For Dirac neutrinos we find a bound several orders of magnitude more stringent than present experimental limits. For Majorana neutrinos the magnetic moment contribution to the mass term is suppressed by Yukawa couplings, thus we do not exclude the possibility of detecting a magnetic moment experimentally.

Galactic Positrons and Annihilating Dark Matter:

The Galactic positron flux, observed as a 0.511 MeV annihilation line, is difficult to account for with astrophysical sources. This motivates the recent suggestion that the positrons are produced instead by the annihilation of light (1-100 MeV) dark matter particles in the Galactic halo. By calculating the radiative corrections to such an annihilation process, we show that the positrons produced would necessarily be accompanied by a flux of (``internal bremsstrahlung'') gamma rays, of sufficient magnitude to exceed the observed galactic diffuse gamma ray flux, unless the dark matter mass is less than about 20 MeV.