Active Neutrinos Research Articles

A Right-handed neutrino portal to the hidden sector: active neutrinos and their twins in an F-theory model

We analyze the neutrino phenomenology in an SU(5) F-theory model with both a visible sector and a twin hidden sector. At low energies, the strong and weak scales of the two sectors may differ but the spectrum of states is described by the MSSM (MSSM′) in the visible (twin) sectors. What is special about the model is that there are right-handed neutrinos which couple to both sectors via Yukawa couplings. As a result, assuming 3 right-handed neutrinos with a large mass much greater than the weak scale, at tree-level the seesaw mechanism results in 3 massive Majorana neutrinos and 3 massless ones. The massless neutrinos acquire mass via radiative corrections. In our analysis, the massless neutrinos are predominantly active neutrinos, while the massive neutrinos are predominantly sterile neutrinos. We fit the active neutrino masses and mixing angles and discuss the phenomenology of the lightest sterile neutrino. Finally we consider some possible scenarios for cosmology.

Probing HNL‐ALP Couplings at Colliders

AbstractAxion‐like particles (ALPs) and heavy neutral leptons (HNLs) are both well‐motivated extensions of the Standard Model. As ALPs couple to on‐shell fermions proportionally to their masses, processes involving both types of particles may give rise, for TeV HNLs, to relevant phenomenology at colliders. In this work, we point out a particularly clean process, whose final state consists of four jets and two charged leptons, and we estimate its current and future sensitivity at the LHC. For on‐shell HNLs, i) there is no dependence on the overall scale of the mixing between HNLs and the active neutrinos, making this process sensitive down to the type‐I Seesaw line; ii) the signal strength of the process is sizable only as far as the HNL masses are below a few TeVs, contrary to what the proportionality to masses of the ALP couplings may suggest. Although ALPs and HNLs have been mainly studied independently in the literature, considering their interplay may lead to joint limits that are much stronger than those on the individual particles taken separately. This concise study paves the way for similar searches at colliders and other experiments.

A seesaw model for large neutrino masses in concordance with cosmology

Cosmological constraints on the sum of the neutrino masses can be relaxed if the number density of active neutrinos is reduced compared to the standard scenario, while at the same time keeping the effective number of neutrino species Neff ≈ 3 by introducing a new component of dark radiation. We discuss a UV complete model to realise this idea, which simultaneously provides neutrino masses via the seesaw mechanism. It is based on a U(1) symmetry in the dark sector, which can be either gauged or global. In addition to heavy seesaw neutrinos, we need to introduce \U0001d4aa(10) generations of massless sterile neutrinos providing the dark radiation. Then we can accommodate active neutrino masses with ∑mν ~ 1 eV, in the sensitivity range of the KATRIN experiment. We discuss the phenomenology of the model and identify the allowed parameter space. We argue that the gauged version of the model is preferred, and in this case the typical energy scale of the model is in the 10 MeV to few GeV range.

INFLATION AND AXION DARK MATTER IN THE 3-3-1 MODEL

We consider a renormalizable theory, which successfully explains the number of Standard Model(SM) fermion families and whose non-SM scalar sector includes an axion dark matter as well asa field responsible for cosmological inflation. In such theory, the axion gets its mass via radiativecorrections at one-loop level mediated by virtual top quark, right handed Majorana neutrinos andSM gauge bosons. Its mass is obtained in the range 4 keV÷ 40 keV, consistent with the onepredicted by XENON1T experiment, when the right handed Majorana neutrino mass is varied from100 GeV up to 350 GeV, thus implying that the light active neutrino masses are generated from a lowscale type I seesaw mechanism. Furthermore, the theory under consideration can also successfullyaccommodates the XENON1T excess provided that the PQ symmetry is spontaneously broken atthe 1010 GeV scale.PACS numbers: 11.30.Fs, 12.15.Ff, 12.60.-i

Constraining feeble neutrino interactions with ultralight dark matter

If ultralight $(\ll$ eV), bosonic dark matter couples to right handed neutrinos, active neutrino masses and mixing angles depend on the ambient dark matter density. When the neutrino Majorana mass, induced by the dark matter background, is small compared to the Dirac mass, neutrinos are "pseudo-Dirac" fermions that undergo oscillations between nearly degenerate active and sterile states. We present a complete cosmological history for such a scenario and find severe limits from a variety of terrestrial and cosmological observables. For scalar masses in the "fuzzy" dark matter regime ($\sim 10^{-20}$ eV), these limits exclude couplings of order $10^{-30}$, corresponding to Yukawa interactions comparable to the gravitational force between neutrinos and surpassing equivalent limits on time variation in scalar-induced electron and proton couplings.

Sterile neutrinos: propagation in matter and sensitivity to sterile mass ordering

We analytically calculate the neutrino conversion probability Pμe in the presence of sterile neutrinos, with exact dependence on Delta {m}_{41}^2 and with matter effects explicitly included. Using perturbative expansion in small parameters, the terms involving the small mixing angles θ24 and θ34 can be separated out, with θ34 dependence only arising due to matter effects. We express Pμe in terms of the quantities of the form sin(x)/x, which helps in elucidating its dependence on matter effects and a wide range of Delta {m}_{41}^2 values. Our analytic expressions allow us to predict the effects of the sign of Delta {m}_{41}^2 at a long baseline experiment like DUNE. We numerically calculate the sensitivity of DUNE to the sterile mass ordering and find that this sensitivity can be significant in the range mid Delta {m}_{41}^2mid sim left({10}^{-4}-{10}^{-2}right) eV2, for either mass ordering of active neutrinos. The dependence of this sensitivity on the value of Delta {m}_{41}^2 for all mass ordering combinations can be explained by investigating the resonance-like terms appearing due to the interplay between the sterile sector and matter effects.

The dark dimension and the Swampland

Motivated by principles from the Swampland program, which characterize requirements for a consistent UV completion of quantum gravity, combined with observational data, we are led to a unique corner of the quantum gravity landscape. In particular, using the Distance/Duality conjecture and the smallness of dark energy, we predict the existence of a light tower of states and a unique extra mesoscopic dimension of length lsim {Lambda}^{-frac{1}{4}}sim {10}^{-6}m , with extra massless fermions propagating on it. This automatically leads to a candidate for a tower of sterile neutrinos, and an associated active neutrino mass scale {m}_{nu}sim {leftlangle Hrightrangle}^2{Lambda}^{-frac{1}{12}}{M}_{textrm{pl}}^{-frac{2}{3}} . Moreover, assuming the mechanism for stabilization of this dark dimension leads to similar masses for active and sterile neutrinos we are led to the prediction of a Higgs vev leftlangle Hrightrangle sim {Lambda}^{frac{1}{6}}{M}_{textrm{pl}}^{frac{1}{3}} . Another prediction of the scenario is a species scale hat{M}sim {Lambda}^{frac{1}{12}}{M}_{textrm{pl}}^{frac{2}{3}}sim {10}^9hbox{--} {10}^{10} GeV, corresponding to the higher-dimensional Planck scale. This energy scale may be related to the resolution of the instability of the Higgs effective potential present at a scale of ~1011 GeV. We also speculate about the interplay between this energy scale and the GZK limit on ultra-high energy cosmic rays.

Dark matter and radiative neutrino masses in conversion-driven scotogenesis

The scotogenic model generates Majorana neutrino masses radiatively, with dark matter particles running in the loop. We explore the parameter space in which the relic density of fermionic dark matter is generated via a conversion-driven freeze-out mechanism. The necessity for small Yukawa couplings to initiate chemical decoupling for conversion processes naturally reproduces small neutrino masses as long as the active neutrinos are hierarchical. The model can also resolve the recently reported deviation in the $W$-boson mass while satisfying constraints from direct detection, charged lepton flavor violation as well as collider bounds. Parts of the parameter space lead to long-lived particle signatures to be probed at the LHC.

Neff from an excited dark matter state

For a cold dark matter (DM) originating from the coannihilation processes, there will be excited state(s) in the dark sector that may decay or annihilate away shortly after their freeze-out. In this paper, we investigate the impact of the decay of these excited states on the effective number of neutrino species, ${N}_{\mathrm{eff}}$, which is an important cosmological parameter and will be tested by the future CMB-S4 project. We work in the framework of pseudo-Dirac DM mode via the Higgs portal. The relic density and the direct detection signal of the DM are calculated, which gives the available parameter space. Impacts of the excited state (the heavy component of the pseudo-Dirac fermion) on the decoupling of active neutrinos are investigated by solving Boltzmann equations of photon and neutrinos with a collision term induced by the decay of the excited state. The numerical result shows that the maximum value of $|\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}|$ can reach the order $\mathcal{O}(0.1)$ with suitable mass and lifetime of the excited DM state, which may be probed by future CMB-S4 and SPT/SO projects.

Addressing the short-baseline neutrino anomalies with energy-dependent mixing parameters

Several neutrino experiments have reported results that are potentially inconsistent with our current understanding of the lepton sector. A candidate solution to these so-called short-baseline anomalies is postulating the existence of new, eV-scale, mostly sterile neutrinos that mix with the active neutrinos. This hypothesis, however, is strongly disfavored once one considers all neutrino data, especially those that constrain the disappearance of muon and electron neutrinos at short baselines. Here, we show that if the sterile-active mixing parameters depend on the energy scales that characterize neutrino production and detection, then the sterile-neutrino hypothesis may provide a reasonable fit to all neutrino data. The reason for the improved fit is that the stringent disappearance constraints on the different elements of the extended neutrino mixing matrix are associated to production and detection energy scales that are different from those that characterize the anomalous appearance data at MiniBooNE. We show, via a concrete example, that secret interactions among the sterile neutrinos can lead to the results of interest.

Comprehensive analysis of charged lepton flavour violation in the symmetry protected type-I seesaw

The type-I seesaw model is probably the most straightforward and best studied extension of the Standard Model that can account for the tiny active neutrino masses determined from neutrino oscillation data. In this article, we calculate the complete set of one-loop corrections to charged lepton flavour violating processes within this model. We give the results both using exact diagonalisation of the neutrino mass matrix, and at leading order in the seesaw expansion (i.e. mathcal{O} (v2/ {M}_R^2 )). Furthermore, we perform the matching onto the SU(2)L invariant Standard Model Effective Field Theory at the dimension 6 level. These results can be used as initial conditions for the renormalisation group evolution from the right-handed neutrino scale down to the scale of the physical processes, which resums large logarithms. In our numerical analysis, we study the inverse seesaw limit, i.e. the symmetry protected type-I seesaw, where the Wilson coefficient of the Weinberg operator is zero such that sizeable neutrino Yukawas are permissible and relevant effects in charged lepton flavour violating observables are possible. We correlate the different charged lepton flavour violating processes, e.g. ℓ → ℓ′γ, ℓ → 3ℓ′, μ → e conversion and Z → ℓℓ′, taking into account the constraints from electroweak precision observables and tests of lepton flavour universality.

The Influence of the Effective Number of Active and Sterile Neutrinos on the Determination of the Values of Cosmological Parameters

Neutrino, as the second most abundant known particle in the Universe, has a significant impact on its expansion rate during the radiation- and matter-dominated eras. For this reason, a change in the number of neutrino species can lead to substantial changes in the estimates of cosmological parameters, the most accurate values of which, at the moment, are obtained by analysing the anisotropy of the CMB. In the presented work we consider the influence of a hypothetical sterile neutrino (with eV-scale mass) on the determination of cosmological parameters. The possible existence of such neutrino is suggested by the analysis of a series of different experiments. If it is detected, it will be necessary to include it into the $$\Lambda\rm CDM$$ model with the fixed values of its mass $$m_{\rm s}$$ and mixing angle $$\theta_{s}$$ , which is the main method used through this paper. Apart from that, the seesaw mechanism requires there to be at least two sterile states, one of them being much heavier than the active ones. The heavier sterile state ( $$m_{s}\sim 1$$ keV) would decay and increase the effective number of active neutrinos. Therefore, the influence of a change in the effective number of relativistic neutrino species $$N_{\textrm{eff}}$$ was studied as well, which could be caused by various reasons, for example, by the decay processes of dark matter particles or the above-mentioned sterile neutrinos, as well as processes leading to an increase in the temperature of relic neutrinos $$T_{\textrm{C}\nu\textrm{B}}$$ . The effects studied in this work lead to a significant change in the estimates of the cosmological parameters, including the value of $$H_{0}$$ . It has been discovered that the accounting of the sterile neutrino with masses $$m=1$$ and $$2.7$$ eV leads to a decrease in the estimate of the current Hubble parameter value $$H_{0}$$ and, therefore, exacerbates the ‘‘ $$H_{0}$$ -tension’’ problem. An increase in the value of the effective number of relativistic neutrino species leads, on the contrary, to an increase in the $$H_{0}$$ estimate, resolving the above-mentioned problem at $$N_{\textrm{eff}}=3.0+0.9$$ , which is equivalent to an increase of the neutrino temperature up to $$T^{0}_{\textrm{C}\nu\textrm{B}}=1.95+0.14$$ K. At the same time, the rest of the cosmological parameters do not change significantly, leaving us within the framework of the standard $$\Lambda$$ CDM model.

Probing neutrino magnetic moments and the Xenon1T excess with coherent elastic solar neutrino scattering

The detection potential of the active neutrino magnetic moment ($\nu_{a}$MM) and the active sterile transition magnetic moment ($\nu_{s}$MM) of the solar neutrino CE$\nu$NS process in future Dark Matter direct detection experiments is studied and compared with the respective allowed range to explain the Xenon1T excess. We find that the sensitivity to the $\nu_{a}$MM approaches to the level between $10^{-10}\mu_B$ and $10^{-11}\mu_B$, which is dominantly limited by the detection threshold. On the other hand, the future solar neutrino CE$\nu$NS detection would be powerful in probing the $\nu_{s}$MM for the sterile neutrino mass below 10 MeV, which could unambiguously test the $\nu_{s}$MM explanation of the Xenon1T excess. The sensitivity in the general framework with both $\nu_{a}$MM and $\nu_{s}$MM contributions is also derived.

Fermion masses and mixings, dark matter, leptogenesis and $$g-2$$ muon anomaly in an extended 2HDM with inverse seesaw

We propose a predictive \(Q_4\) flavored 2HDM model, where the scalar sector is enlarged by the inclusion of several gauge singlet scalars and the fermion sector by the inclusion of right-handed Majorana neutrinos. In our model, the \(Q_4\) family symmetry is supplemented by several auxiliary cyclic symmetries, whose spontaneous breaking produces the observed pattern of SM charged fermion masses and quark mixing angles. The light active neutrino masses are generated from an inverse seesaw mechanism at one loop level thanks to a remnant preserved \(Z_2\) symmetry. Our model successfully reproduces the measured dark matter relic abundance and is consistent with direct detection constraints for masses of the DM candidate around \(\sim\) 6.3 TeV. Furthermore, our model is also consistent with the lepton and baryon asymmetries of the Universe as well as with the muon anomalous magnetic moment.

Constraints on Sterile Neutrino Models from Strong Gravitational Lensing, MilkyWay Satellites, and the Lyman-α Forest.

The nature of dark matter is one of the most important unsolved questions in science. Some darkf matter candidates do not have sufficient nongravitational interactions to be probed in laboratory or accelerator experiments. It is thus important to develop astrophysical probes which can constrain or lead to a discovery of such candidates. We illustrate this using state-of-the-art measurements of strong gravitationally lensed quasars to constrain four of the most popular sterile neutrino models, and also report the constraints for other independent methods that are comparable in procedure. First, we derive effective relations to describe the correspondence between the mass of a thermal relic warm dark matter particle and the mass of sterile neutrinos produced via Higgs decay and grand unified theory (GUT)-scale scenarios, in terms of large-scale structure and galaxy formation astrophysical effects. Second, we show that sterile neutrinos produced through the Higgs decay mechanism are allowed only for mass >26 keV, and GUT-scale scenario >5.3 keV. Third, we show that the single sterile neutrino model produced through active neutrino oscillations is allowed for mass >92 keV, and the three sterile neutrino minimal standard model (νMSM) for mass >16 keV. These are the most stringent experimental limits on these models.

Subcritical regime of hybrid inflation with modular A4 symmetry

We consider a supergravity model that has the modular A4 symmetry and discuss the interplay between the neutrino mixing and inflation. The model contains right- handed neutrinos that have the Majorana masses and additional Yukawa couplings to the waterfall field. In the model an active neutrino is massless and we find that only the inverted hierarchy is allowed and the Majorana phase is predicted to be around ±(120–180)° from the observed neutrino mixing data. In the early universe, one of right-handed sneutrinos plays the role of the inflaton field. Focusing on the subcritical regime of the hybrid inflation that is consistent with the cosmic microwave background data, we analyze the dynamics of the scalar sector and derive an upper bound (1010) GeV on the scale of the Majorana mass.

Sterile neutrino from D-brane models

We discuss the appearance of sterile neutrino in D-brane models. In the simplest case, the sterile neutrino (SN) appears from a 5-stack intersecting D6-brane model with a gauged baryon number. SM is mixed with a single generation of left handed active neutrinos and a right handed neutrino.

Search for Light Fermionic Dark Matter Absorption on Electrons in PandaX-4T.

We report a search on sub-MeV fermionic dark matter absorbed by electrons with an outgoing active neutrino using the 0.63 tonne year exposure collected by the PandaX-4T liquid xenon experiment. No significant signals are observed over the expected background. The data are interpreted into limits to the effective couplings between such dark matter and the electron. For axial-vector or vector interactions, our sensitivity is competitive in comparison to existing astrophysical bounds on the decay of such a dark matter candidate into photon final states. In particular, we present the first direct detection limits for a vector (axial-vector) interaction which are the strongest in the mass range from 35 to 55 (25 to 45) keV/c^{2} in comparison to other astrophysical and cosmological constraints.

Neutrino astronomy as a probe of physics beyond the Standard Model: Decay of sub-MeV B−L gauge boson dark matter

The $U(1)_{B\textrm{--}L}$ symmetry, the essential component in the seesaw mechanism and leptogenesis, is naturally equipped with a massive gauge boson. If this gauge boson is the dark matter, the scenario consistent with the seesaw mechanism predicts the gauge coupling to be of the order of $\mathcal{O}(10^{-19})$ for masses $\lesssim1$ MeV, dominantly decaying into active neutrinos. We stress and explore the important role of astrophysical neutrinos of energies from $\mathcal{O}(1)$ keV to $\sim1$ MeV in testing the well-motivated $B$-$L$ symmetry extension to the Standard Model, which has been missed in the literature to date. Compared to other dark matter models, the neutrino flux in the sub-MeV energy range is a unique prediction in our setup and, once detected, would serve as a smoking gun for the existence of this $B$-$L$ gauge boson and its role as the dark matter particle, opening new windows to tackle cosmological and astrophysical conundra.

Reviving keV sterile neutrino dark matter

We propose a new production mechanism for keV sterile neutrino dark matter which relies neither on the oscillations between sterile and active neutrinos nor on the decay of additional heavier particles. The dark matter neutrinos are instead produced by thermal freeze-out, much like a typical WIMP. The challenge consists in balancing a large Yukawa coupling so that the sterile neutrinos thermalize in the early universe on the one hand, and a small enough Yukawa coupling such that they are stable on cosmological scales on the other. We solve this problem by implementing varying Yukawa couplings. We achieve this by using a three-sterile neutrino seesaw extension to the SM and embedding it in a Froggatt-Nielsen model with a single flavon. Because the vev of the flavon changes during the electroweak phase transition, the effective Yukawa couplings of the fermions have different values before and after the phase transition, thus allowing for successful dark matter genesis. Additionally, the hierarchy in the flavour structure is alleviated and the origin of the light neutrino masses is explained by the interplay of the seesaw and Froggatt-Nielsen mechanisms.