Non-unitarity, sterile neutrinos, and non-standard neutrino interactions
Received: October
Non-unitarity, sterile neutrinos, and non-standard neutrino interactions
Mattias Blennow 1 2 4 7 8 9 10
Pilar Coloma 1 2 4 6 8 9 10
Enrique Fernandez-Martinez 1 2 3 4 8 9 10
Josu Hernandez-Garcia 1 2 3 4 8 9 10
Jacobo Lopez-Pavon 0 1 2 4 5 8 9 10
Open Access 1 2 4 8 9 10
c The Authors. 1 2 4 8 9 10
Cantoblanco E-28049 Madrid, Spain
0 CERN, Theoretical Physics Department
1 via Dodecaneso 33 , 16146 Genova , Italy
2 P. O. Box 500, Batavia, IL 60510 , U.S.A
3 Instituto de F sica Teorica UAM/CSIC
4 KTH Royal Institute of Technology, Albanova University Center , 106 91 Stockholm , Sweden
5 INFN , Sezione di Genova
6 Theoretical Physics Department, Fermi National Accelerator Laboratory
7 Department of Theoretical Physics, School of Engineering Sciences
8 nd that , in the limit
9 Calle Nicolas Cabrera 13-15 , Cantoblanco E-28049 Madrid , Spain
10 [21] G.H. Collin, C.A. Arguelles, J.M. Conrad and M.H. Shaevitz , Sterile neutrino ts to short
The simplest Standard Model extension to explain neutrino masses involves the addition of right-handed neutrinos. At some level, this extension will impact neutrino oscillation searches. In this work we explore the di erences and similarities between the case in which these neutrinos are kinematically accessible (sterile neutrinos) or not (mixing matrix non-unitarity). We clarify apparent inconsistencies in the present literature when using di erent parametrizations to describe these e ects and recast both limits in the popular neutrino non-standard interaction (NSI) formalism. We in which sterile oscillations are averaged out at the near detector, their e ects at the far detector coincide with non-unitarity at leading order, even in presence of a matter potential. We also summarize the present bounds existing in both limits and compare them with the expected sensitivities of near-future facilities taking the DUNE proposal as a benchmark. We conclude that non-unitarity e ects are too constrained to impact present or near future neutrino oscillation facilities but that sterile neutrinos can play an important role at long baseline experiments. The role of the near detector is also discussed in detail.
neutrino; interactions; cDepartamento de F sica Teorica; Universidad Autonoma de Madrid
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Non-unitarity and sterile neutrino phenomenology comparison
1 Introduction
Sterile neutrino case
Present constraints on deviations from unitarity
DUNE sensitivities
A Current constraints on sterile neutrinos
The simplest extension of the Standard Model (SM) of particle physics able to account
for the evidence for neutrino masses and mixings is the addition of right-handed neutrinos
to its particle content. A Majorana mass term for these new singlet fermions is then
allowed by all symmetries of the Lagrangian. This new mass scale at which lepton number
is violated could provide a mechanism to also explain the origin of the observed
matterantimatter asymmetry of the Universe [1] and is a necessary missing piece to solve the
mysterious avour puzzle. However, given that this new mass scale is not related to the
electroweak symmetry breaking mechanism, there is no theoretical guidance for its value.
A large Majorana scale leads to the celebrated seesaw mechanism [2{5], providing a very
natural explanation of the lightness of neutrino masses. On the other hand, it also leads
to unnaturally large contributions to the Higgs mass, worsening the hierarchy problem [6].
Conversely, a light neutrino mass could also naturally stem from a symmetry argument [7{
13]. Indeed, neutrino masses are protected by the lepton number symmetry and, if this
is an approximate symmetry of the theory, a large hierarchy of scales is not required to
naturally accommodate the lightness of neutrinos. Thus, the value of this scale of new
physics can only be probed experimentally and, depending on its value, very di erent and
interesting phenomenology would be induced in di erent observables.
In this work we analyze the phenomenological impact of these new physics in neutrino
oscillation facilities. If the new mass scale is kinematically accessible in meson decays,
the sterile states will be produced in the neutrino beam. On the other hand, if the extra
neutrinos are too heavy to be produced, the e ective three by three PMNS matrix will show
unitarity deviations. We will refer to these situations as sterile and non-unitary neutrino
oscillations, respectively. The aim of this work is to discuss in which limits these two
regimes lead to the same impact on the oscillation probabilities, and reconcile apparently
inconsistent results in previous literature.
This work is organized as follows. In section 2 we will compare the non-unitarity
and sterile neutrino phenomenology and discuss in which cases both limits are equivalent.
In section 3 we will present and solve the apparent inconsistency present in the literature
studying non-unitarity e ects in di erent parametrizations, and provide a mapping between
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