Majorana neutrinos and other Majorana particles:Theory and experiment

This is a somewhat modified version of Chapter 15 of the book "The Physics of Ettore Majorana", by Salvatore Esposito with contributions by Evgeny Akhmedov (Ch. 15) and Frank Wilczek (Ch. 14), Cambridge University Press, 2014.Comment: LaTeX, 53 pages, 4 figure

On the likely dominance of WIMP annihilation to fermion pair+W/Z (and implication for indirect detection)

Arguably, the most popular candidate for Dark Matter (DM) is a massive, stable, Majorana fermion. However, annihilation of Majorana DM to two fermions often features a helicity-suppressed s-wave rate. Radiating a gauge boson via electroweak (EW) and electromagnetic (EM) bremsstrahlung removes this s-wave suppression. The main purpose of this talk is to explain in some detail why the branching ratio to a fermion pair is likely suppressed while the decay to the pair plus a W/Z is not. In doing so, we investigate the general conditions for s-wave suppression and un-suppression using Fierz transformations and partial wave expansions. Suppression for the 2-to-2 process is sufficiently severe that the EW and EM bremsstrahlung are likely to be the dominant modes of gauge-singlet Majorana DM annihilation. We end this talk with a discussion of the challenge presented by space-based data for Majorana DM models, given that the enhanced rate to radiated W and Z gauge bosons and their dominant decay via hadronic channels tends to produce more anti-protons than are observed.Comment: 22 pages, including five sets of figures and two tables; expands upon talk presented at the CETUP* Dark Matter Workshop, Lead, South Dakota, July 201

Neutrino Helicity Reversal and Fundamental Symmetries

A rather elusive helicity reversal occurs in a gedanken experiment in which a massive left-handed Dirac neutrino, traveling at a velocity u < c, is overtaken on a highway by a speeding vehicle (traveling at velocity v with u < v < c). Namely, after passing the neutrino, looking back, one would see a right-handed neutrino (which has never been observed in nature). The Lorentz-invariant mass of the right-handed neutrino is still the same as before the passing. The gedanken experiment thus implies the existence of right-handed, light neutrinos, which are not completely sterile. Furthermore, overtaking a bunch of massive right-handed Dirac neutrinos leads to gradual de-sterilization. We discuss the helicity reversal and the concomitant sterilization and de-sterilization mechanisms by way of an illustrative example calculation, with a special emphasis on massive Dirac and Majorana neutrinos. We contrast the formalism with a modified Dirac neutrino described by a Dirac equation with a pseudoscalar mass term proportional to the fifth current.Comment: 17 pages; RevTeX; inspired in part by discussions at the First International Conference on Logic and Relativity, Alfred Renyi Institute of Mathematics (invited talk); article in pres

Neutrino-Antineutrino Oscillations

Oppgaven presenterer teorien bak massive nøytrinoer. Vi viser hvordan nøytrino masse og miksing leder til nøytrino oscillasjoner. Flere konsekvenser av slike oscillasjoner blir diskutert. Etter introduksjonen av en Majorana masse beregnes flere tverrsnitt for både Majorana og Dirac nøytrinoer. Vi viser at for prosesser i Standard Modellen er forskjellen mellom disse proporsjonal til nøytrino massen. For Majorana nøytrinoer utledes nøytrino-antinøytrino oscillasjons sannsynligheten og sammenhengen mellom slike oscillasjoner og tversnittene for Majorana nøytrinoer diskuteres.Masteroppgave i fysikkPHYS399MAMN-PHY

Neutrino-Charged Matter Interactions: a General Four-Fermion Effective Parametrization

Given the eventuality of neutrino and muon factories in the foreseeable future, all possible 2-to-2 processes involving two neutrinos, whether Dirac or Majorana ones, and two charged fermions are considered on the basis of the most general Lorentz invariant four-fermion effective interaction possible, in the limit of massless particles. Such a parametrization should enable the assessment of the sensitivity to physics beyond the Standard Model, including the eventual discrimination between the Dirac or Majorana character of neutrinos, of specific experimental beam and detector designs.Comment: 1+21 page