Neutrino Mass Constraints on Electroweak Parameters

Author: Erwin, Rebecca Joan

Year: 2007

Degree: Dissertation (Ph.D.)

Advisor: Ramsey-Musolf, Michael J.

Committee Members: Filippone, Bradley W.; Wise, Mark B.; Ramsey-Musolf, Michael J.; Vogel, Petr

Option: Physics

DOI: 10.7907/E3FG-2H43

Abstract

We use the scale of neutrino mass and naturalness considerations to obtain model-independent expectations for the magnitude of possible contributions to muon decay Michel parameters from new physics above the electroweak symmetry-breaking scale. Focusing on Dirac neutrinos, we obtain a complete basis of effective dimension four and dimension six operators that are invariant under the gauge symmetry of the Standard Model and that contribute to both muon decay and neutrino mass. We show that - in the absence of fine tuning - the most stringent neutrino mass naturalness bounds on chirality-changing vector operators relevant to muon decay arise from one-loop operator mixing. The bounds we obtain on their contributions to the Michel parameters are two orders of magnitude stronger than bounds previously obtained in the literature. In addition, we analyze the implications of one-loop matching considerations and find that the expectations for the size of various scalar and tensor contributions to the Michel parameters are considerably smaller than those derived from previous estimates of two-loop operator mixing. We also show, however, that there exist gauge-invariant operators that generate scalar and tensor contributions to muon decay but whose flavor structure allows them to evade neutrino mass naturalness bounds. We discuss the implications of our analysis for the interpretation of muon decay experiments.

We then repeat the analysis with Majorana neutrinos. Since the lowest dimension mass operator in this case is a five-dimensional operator, we start with a new basis of effective dimension five and dimension seven operators that contribute to muon decay and neutrino mass through radiative corrections. In contrast to similar studies of magnetic moments and masses using Dirac and Majorana neutrinos, which found substantially weaker bounds for Majorana magnetic moments, we find that the limits on muon decay Michel parameters from Majorana neutrinos are similar in magnitude to the limits from Dirac neutrinos. We also find, similar to the Dirac case, that there are operators in our basis whose coefficients are not bound by neutrino mass.

Finally, we calculate one-loop renormalization factors of twist-two operators in massless QCD with domain-wall fermions. The Shamir type domain-wall fermion, with an infinitely large extra dimension to describe the massless fermion, is used.

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