Robert Szafron

Lepton number violation and ‘Diracness’ of massive neutrinos composed of Majorana states

Majorana neutrinos naturally lead to a Lepton Number Violation (LNV). A superposition of Majorana states can mimic Dirac-type neutrinos, leading to its conservation (LNC). We demonstrate on the example of specific observables related to high and low energy processes how a strength of LNV correlates with neutrino parameters such as CP-phases, flavor mixings, mass ratios. We stress the coaction between low and high energy studies for putting phenomenological models to a quantitative test. Secondly, we conclude that in order to fully study the role of heavy neutrinos in search for New Physics (NP) signals, a departure from trivial scenarios assuming degeneracy in mass and no flavor mixing or CP-phases becomes necessary for a proper physical analysis.

Light-by-light scattering in the Lamb shift and the bound electron g factor

We compute an

Michel decay spectrum for a muon bound to a nucleus

\mathcal{O}\left(\alpha^2(Z\alpha)^6\right)

Real and complex random neutrino mass matrices andθ13

contribution to the hydrogen-atom Lamb shift arising from the light-by-light scattering. Analogous diagrams, with one atomic electric field insertion replaced by an external magnetic field, contribute to the gyromagnetic factor of the bound electron at

High-energy electrons from the muon decay in orbit: Radiative corrections

\mathcal{O}\left(\alpha^2(Z\alpha)^4\right)

Muon decay spin asymmetry

. We also calculate the contribution to the gyromagnetic factor from the muon magnetic loop.

Shape function in QED and bound muon decays

The spectrum of electrons from muons decaying in an atomic bound state is significantly modified by their interaction with the nucleus. Somewhat unexpectedly, its first measurement, at the Canadian laboratory TRIUMF, differed from basic theory. We show, using a combination of techniques developed in atomic, nuclear, and high-energy physics, that radiative corrections eliminate the discrepancy. In addition to solving that outstanding problem, our more precise predictions are potentially useful for interpreting future high-statistics muon experiments that aim to search for exotic interactions at 10−16 sensitivity.

Bound muon decay spectrum in the leading logarithmic accuracy

Recently it has been shown that one of the basic parameters of the neutrino sector, so called theta13 angle is very small, but quite probably non-zero. We argue that the small value of theta13 can still be reproduced easily by a wide spectrum of randomly generated models of neutrino masses. For that we consider real and complex neutrino mass matrices, also including sterile neutrinos. A qualitative difference between results for real and complex mass matrices in the region of small theta13 values is observed. We show that statistically the present experimental data prefers random models of neutrino masses with sterile neutrinos.

Higgs and EW symmetry breaking studies

Abstract We determine the O ( α ) correction to the energy spectrum of electrons produced in the decay of muons bound in atoms. We focus on the high-energy end of the spectrum that constitutes a background for the muon–electron conversion and will be precisely measured by the upcoming experiments Mu2e and COMET. The correction suppresses the background by about 15%.

Search for doubly charged Higgs bosons through vector boson fusion at the LHC and beyond

We compute the spin asymmetry of the muon decay through O(alpha^2) in perturbative QED. These two-loop corrections are about a factor five (twenty) smaller than the current statistical (systematic) uncertainty of the most precise measurement, performed by the TWIST collaboration. We point out that at O(alpha^2) the asymmetry requires a careful definition due to multi-lepton final states and suggest to use familiar QCD techniques to define it in an infra-red safe way. We find that the TWIST measurement of the asymmetry is in excellent agreement with the Standard Model.