Type-I Seesaw with eV-Scale Neutrinos

Article
Report number	arXiv:1912.05875
Title	Type-I Seesaw with eV-Scale Neutrinos
Author(s)	Branco, G.C. (Lisbon, CFTP) ; Penedo, J.T. (Lisbon, CFTP) ; Pereira, Pedro M.F. (Lisbon, CFTP) ; Rebelo, M.N. (Lisbon, CFTP ; CERN) ; Silva-Marcos, J.I. (Lisbon, CFTP)
Publication	2020-07-23
Imprint	2020-01-13
Number of pages	43
In:	JHEP 2007 (2020) 164
DOI	10.1007/JHEP07(2020)164 (publication)
Subject category	hep-ph ; Particle Physics - Phenomenology
Abstract	We consider seesaw type-I models including at least one (mostly-)sterile neutrino with mass at the eV scale. Three distinct situations are found, where the presence of light extra neutrinos is naturally justified by an approximately conserved lepton number symmetry. To analyse these scenarios consistently, it is crucial to employ an exact parametrisation of the full mixing matrix. We provide additional exact results, including generalised versions of the seesaw relation and of the Casas-Ibarra parametrisation, valid for every scale of seesaw. We find that the existence of a light sterile neutrino imposes an upper bound on the lightest neutrino mass. We further assess the impact of light sterile states on short- and long-baseline neutrino oscillation experiments, emphasise future detection prospects, and address CP Violation in this framework via the analysis of CP asymmetries and construction of weak basis invariants. The proposed models can accommodate enough active-sterile mixing to play a role in the explanation of short-baseline anomalies.
Copyright/License	publication: © 2020-2024 The Authors (License: CC-BY-4.0) preprint: (License: arXiv nonexclusive-distrib 1.0)

$Active-sterile mixing measure $\sin^2 2\vartheta_{\mu e}^{(4)}$ {\it versus} the lightest neutrino mass $m_\text{min}$ from a scan of the case-Ia parameter space, with NO ($m_\text{min} = m_1$). The heavy spectrum at tree level has $M_1 = 1$ eV and $M_2 = 1$ keV, while three values of the heaviest mass are considered, $M_3 = 10^{13}\, (10^{14})\, [5\times 10^{14}]$ GeV, corresponding to the black (dark blue) [light blue] points in the scatter plot. The horizontal green band shows the $99.7\%$ CL interval of Ref.~\cite{Gariazzo:2015rra}, while the vertical red exclusion band is obtained by combining the most stringent bound on the sum of light neutrino masses from cosmology, $\sum_i m_i < 0.12$ eV ($95\%$ CL)~\cite{Vagnozzi:2017ovm}\cite{Aghanim:2018eyx}, with the $3\sigma$ ranges of mass-squared differences. The dark green contour delimits the region inside which loop-stable points have been found (see text), while the benchmark of Table~\ref{tab:Ia} is marked in yellow.$ $The average of $\sin^2 2\vartheta_{\mu e}^{(4)}$ and $\sin^2 2\vartheta_{\mu e}^{(5)}$ {\it versus} the lightest neutrino mass $m_\text{min}$, from a scan of the case-Ib parameter space, with NO ($m_\text{min} = m_1$). The heavy spectrum at tree level has $M_1 = 3.00$ eV and $M_2 = 3.16$ eV, while three values of the heaviest mass are considered, $M_3 = 10^{13}\, (10^{14})\, [5\times 10^{14}]$ GeV, corresponding to the black (dark blue) [light blue] points in the scatter plot. The vertical red band corresponds to the cosmological constraint, as in Figure~\ref{fig:Ia}. The dark green contour delimits the region inside which loop-stable points have been found, while the benchmark of Table~\ref{tab:Ib} is marked in yellow.$ $Active-sterile mixing measure $\sin^2 2\vartheta_{\mu e}^{(4)}$ {\it versus} the lightest neutrino mass $m_\text{min}$ from a scan of the case-II parameter space, with NO ($m_\text{min} = m_1$). The heavy spectrum at tree level has $M_1 = 1$ eV, while three values of the heaviest quasi-degenerate masses are considered, $M_2 \simeq M_3 = 8\,v\, (14\,v)\, [20\,v]$, corresponding to the black (dark blue) [light blue] points in the scatter plot. Here, $v \simeq 174$ GeV is the Higgs VEV. The horizontal green band shows the $99.7\%$ CL interval of Ref.~\cite{Gariazzo:2015rra}, and the vertical red band corresponds to the cosmological constraint, as in Figure~\ref{fig:Ia}. The dark green contour delimits the region inside which loop-stable points have been found, while the benchmark of Table~\ref{tab:II} is marked in yellow.$ Show more plots

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