The MiniBooNE collaboration recently released a report claiming have observed an excess of electron and anti-electron neutrino with significance of $4.8 \, σ$ C.L. corroborating, in this way, the long-standing LSND anomaly. Combined LSND and MiniBooNE analysis reach a significance of $6.0\, σ$ C.L. Such a result, if confirmed by future experiments, will cause considerable impact on particle physics since that such anomalies, when interpreted in terms of neutrino oscillation, require the existence of at least one light sterile neutrino. It happens that, on according to standard scenarios, such light sterile neutrino is incompatible with current cosmological data. In this way, understand these anomalies require an extension of the standard model capable of generating tiny masses for both active and sterile neutrinos and re-conciliates such a result with cosmology. An interesting proposal in this direction involve the existence of a secret sector interacting exclusively with sterile neutrinos. In this work we implement the canonical seesaw mechanism into the standard model in such a way that generates tiny masses to the active and sterile neutrinos and embody a secret sector capable of re-conciliating eV sterile neutrinos with cosmology. As other gains, the scalar content required by the implementation of the mechanism provides contribution to rare lepton decays, may accommodate the $g-2$ of the muon and poses a scalar singlet that may drive inflation through Higgs inflation mechanism without problem with loss of unitarity.
This talk summarizes our recent work which studied the impact of resonant $ν_e \toν_s$ and $\barν_e\to\barν_s$ ($ν_s$ is a sterile neutrino) conversions on supernova physics, under the assumption that the mass of the sterile state is in the few eV -cosmologically significant range.
A recent cosmological bound on the gravitino mass, $m_{3/2}<4.7$ eV, together with LHC results on the Higgs mass and direct searches, excludes minimal gauge mediation with high reheating temperatures. We discuss a minimal, vector-mediated model which incorporates the seesaw mechanism for neutrino masses, allows for thermal leptogenesis, ameliorates the $μ$ problem, and achieves the observed Higgs mass and a gravitino as light as $1$-$2$ eV.
Lepton flavour asymmetries generated at the onset of the oscillations of sterile neutrinos with masses above the electroweak scale can be large enough to partly survive washout and to explain the baryon asymmetry of the Universe. This opens up new regions of parameter space, where Leptogenesis is viable within the type-I seesaw framework. In particular, we find it possible that the sterile neutrinos masses are substantially below 10^9 GeV, while not being degenerate. However, the required reheat temperature that is determined by the begin of the oscillations lies some orders of magnitude above…
I show that it may be possible to explain the present evidence for a gamma-ray emission from the galactic halo as due to halo WIMP annihilations. Not only the intensity and spatial pattern of the halo emission can be matched but also the relic density of the candidate WIMP can be in the cosmologically interesting domain. After a model-independent analysis to learn about the properties of a suitable candidate, I present a working model: a sterile neutrino in a model with an extended Higgs sector. Two examples indicate the existence of an interesting region in the model parameter space where pre…
Gauge mediation provides us with a complete picture of supersymmetry breaking and its mediation within the effective field theories, and thus allows us to discuss consistencies with low-energy particle physics as well as cosmological observations. We study in detail the cosmological evolution of the pseudo-modulus field in the supersymmetry breaking sector and also the production of the gravitinos in the early Universe in a simple (but a complete) model of gauge mediation. Under fairly reasonable assumptions, it is found that there exists a parameter region where dark matter of the Universe is…
We address the impact of sterile fermion states on the anomalous magnetic moment of charged leptons, as well as their contribution to neutrinoless double beta decays. We illustrate our results in a minimal, effective extension of the Standard Model by one sterile fermion state, and in a well-motivated framework of neutrino mass generation, embedding the Inverse Seesaw into the Standard Model. The simple "3+1" effective case succeeds in alleviating the tension related to the muon anomalous magnetic moment, albeit only at the 3$σ$ level, and for light sterile states (corresponding to a }cosmolog…
We derive conditions under which f(G) gravity models, whose Lagrangian densities f are written in terms of a Gauss-Bonnet term G, are cosmologically viable. The most crucial condition to be satisfied is that f_GG, the second derivative of f with respect to G, must be positive, which is required to ensure the stability of a late-time de-Sitter solution as well as the existence of standard radiation/matter dominated epochs. We present a number of explicit f(G) models in which a cosmic acceleration is followed by the matter era. We find that the equation of state of dark energy can cross the phan…
All $f(R)$ modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a strong coupling. This coupling induces a cosmological evolution radically different from standard cosmology. We find that in all $f(R)$ theories that behave as a power of $R$ at large or small $R$ (which include most of those proposed so far in the literature) the scale factor during the matter phase grows as $t^{1/2}$ instead of the standard law $t^{2/3}$. This behaviour is grossly inconsistent with cosmological observations (e.g. WMAP), thereby ruling out …
An experimentally verifiable Higgs-triplet model of neutrino masses from large extra dimensions was recently proposed. We extend it to accomodate a light sterile neutrino which also mixes with the three active neutrinos. A previously proposed phenomenological model of four neutrinos (\underline {the only viable such model now left}, in view of the latest atmospheric and solar neutrino-oscillation data) is specifically realized.
This thesis address theoretical and phenomenological aspects of active and sterile mixing pattern within minimal extended seesaw frameworks. It consists of six chapters, where chapters one and six are dedicated to introduction and conclusion chapters, respectively. In chapter two, we study active-sterile phenomenology with a single generation of sterile neutrino ($m_S\sim \mathcal{O}$(eV)) along with the three active neutrinos. Three independent cases for sterile mass matrices are studied in both normal and inverted hierarchy mass ordering. Chapter three is an extension of the previous chapter…
We perform a thorough study of the theoretical consistency of recently proposed, viable, quadratic modifications of gravity that are functions of the the Gauss-Bonnet invariant, regarding the stability of their perturbations around vacuum, maximally symmetric spaces of constant curvature. We pay special attention, in particular, to the investigation of pathological instabilities associated with the occurrence of propagating spin-0 tachyon modes, and with the development of a graviton ghost. The latter effect is associated with the known "Ricci stability" issue, well studied in $f(R)$-theories.…
Even after the negative result by the MiniBooNE experiment, we can still have a (3+1)-scheme with one sterile neutrino whose mixing lies within the allowed region by MiniBooNE. In this note we discuss the possible effects of such a scheme on the flavor ratio of high-energy cosmic neutrinos from cosmologically distant astrophysical sources. It is shown that in principle there is still a chance to observe deviation from the standard three flavor scenario in the flavor ratio of the high-energy cosmic neutrinos. It is proposed to see the energy spectrum to cope with the theoretical uncertainties w…
It has been recently suggested \cite{Bezrukov:2017ike,Bezrukov:2018wvd} that a cosmic scalar field can completely change the keV-scale sterile neutrino production in the early Universe. Its effect may, for various parameter choices, either suppress sterile neutrino production and make moderate active-sterile mixing cosmologically acceptable, or increase the production and generate considerable dark matter component out of sterile neutrino with otherwise negligible mixing with SM. In this paper we provide analytic estimates complementing and providing details of the numerical calculations perfo…
Standard simple assumptions are usually made about the pre-Big Bang Nucleosynthesis epoch, from which we do not have observations. Modifying these assumptions, the predicted density of relic particles such as neutralinos and sterile neutrinos can be very different from that in the standard case. For example, neutralinos could have the dark matter density in (almost) any supersymmetric model, and sterile neutrinos with mixings large enough to be soon detected in neutrino experiments would become cosmologically acceptable. These possibilities are important in view of what the LHC, and neutrino e…
We investigate how sterile neutrinos with a range of masses influence cosmology in MeV-scale reheating temperature scenarios. By computing the production of sterile neutrinos through the combination of mixing and scattering in the early Universe, we find that light sterile neutrinos, with masses and mixings as inferred from short-baseline neutrino oscillation experiments, are consistent with big-bang nucleosynthesis (BBN) and cosmic microwave background (CMB) radiation for the reheating temperature of ${\cal O}(1)$ MeV if the parent particle responsible for reheating decays into electromagneti…
We investigate observable cosmological aspects of sterile neutrino dark matter produced via the freeze-in mechanism. The study is performed in a framework that admits many cosmologically interesting variations: high temperature production via annihilation processes from higher dimensional operators or low temperature production from decays of a scalar, with the decaying scalar in or out of equilibrium with the thermal bath, in supersymmetric or non-supersymmetric setups, thus allowing us to both extract generic properties and highlight features unique to particular variations. We find that whi…
We point out a correlation between the effective number of relativistic degrees of species $ΔN_\text{eff}$, the cosmologically measured $m_{ν,\text{sterile}}^{\text{eff}}$, and the terrestrially measured neutrino mass sum and effective electron neutrino mass, $Σm_ν$ and $m_{ν_e}$, which arises in the Dirac neutrino hypothesis. If the neutrinos are Dirac particles, and if the active neutrinos' sterile partners were once thermalized in the early universe, then this new cosmological relic would simultaneously contribute to the effective number of relativistic species, $ΔN_\text{eff}$, and also le…
It is known that a keV scale sterile neutrino is a good warm dark matter candidate. We study how this possibility could be realized in the context of gauge extensions of the standard model. The naïve expectation leads to large thermal overproduction of sterile neutrinos in this setup. However, we find that it is possible to use out-of-equilibrium decay of the other right-handed neutrinos of the model to dilute the present density of the keV sterile neutrinos and achieve the observed dark matter density. We present the universal requirements that should be satisfied by the gauge extensions of t…
In a recent paper [1] (PRL 98,131302,2007) we have shown that f(R)=R + mu R^{n} modified gravity dark energy models are not cosmologically viable because during the matter era that precedes the accelerated stage the cosmic expansion is given by a sim t^{1/2} rather than a sim t^{2/3}, where a is a scale factor and t is the cosmic time. A recent work [2] (PLB 639,135-143,2006) by Capozziello et al. criticised our results presenting some apparent counter-examples to our claim in f(R)= mu R^{n} models. We show here that those particular R^{n} models can produce an expansion as a sim t^{2/3} but t…
We reexamine sterile neutrino dark matter in gauged $U(1)_{B-L}$ model. Improvements have been made by tracing and careful evaluation of the evolution of the number densities of sterile neutrinos $N$ and extra neutral gauge bosons $Z'$. As a result, the cosmologically-interesting gauge coupling of $U(1)_{B-L}$ for freeze-in sterile neutrinos turns out to be smaller than the values reported in the literature. This avoids the overproduction of $Z'$ so that it is consistent with the big bang nucleosynthesis and the cosmic microwave background constraints on the effective number of neutrino specie…
We show how active-sterile neutrino oscillations in the early Universe can play an interesting role in explaining the current observations of CMBR anisotropies and light element abundances. We describe different possible phenomenological scenarios in the interpretation of present data and how active-sterile neutrino oscillations can provide a viable theoretical framework.
A number of experimental anomalies involving neutrinos hint towards the existence of at least an extra (a very light) sterile neutrino. However, such a species, appreciably mixing with the active neutrinos, is disfavored by different cosmological observations like Big Bang Nucleosynthesis (BBN), Cosmic Microwave Background (CMB) and Large Scale Structure (LSS). Recently, it was shown that the presence of additional interactions in the sterile neutrino sector via light bosonic mediators can make the scenario cosmologically viable by suppressing the production of the sterile neutrinos from activ…
Recently an inflationary model with a vector field coupled to the inflaton was proposed and the phenomenology studied for the Bianchi type I spacetime. It was found that the model demonstrates a counter-example to the cosmic no-hair theorem since there exists a stable anisotropically inflationary fix-point. One of the great triumphs of inflation, however, is that it explains the observed flatness and isotropy of the universe today without requiring special initial conditions. Any acceptable model for inflation should thus explain these observations in a satisfactory way. To check whether the m…
Sterile neutrinos with masses in the keV range are viable candidates for the warm dark matter. We analyze existing data for the extragalactic diffuse X-ray background for signatures of sterile neutrino decay. The absence of detectable signal within current uncertainties of background measurements puts model-independent constraints on allowed values of sterile neutrino mass and mixing angle, which we present in this work.
We study the possibility of simultaneously addressing neutrino phenomenology and the dark matter in the framework of inverse seesaw. The model is the extension of the standard model by the addition of two right handed neutrinos and three sterile fermions which leads to a light sterile state with the mass in the keV range along with three light active neutrino states. The lightest sterile neutrino can account for a feasible dark matter(DM) candidate. We present a $S_{4}$ flavor symmetric model which is further augmented by $Z_{4}\times Z_{3}$ symmetry to constrain the Yukawa Lagrangian. The str…
In view of the recent results from the MiniBooNE experiment we revisit the global neutrino oscillation fit to short-baseline neutrino data by adding one, two or three sterile neutrinos with eV-scale masses to the three Standard Model neutrinos. We find that four-neutrino oscillations of the (3+1) type, which have been only marginally allowed before the recent MiniBooNE results, become even more disfavored with the new data. In the framework of so-called (3+2) five-neutrino mass schemes the MiniBooNE results can be nicely reconciled with the LSND appearance evidence thanks to the possibility of…
New parameter region in sterile neutrino searches: a scenario to alleviate cosmological neutrino mass bound and its testability at oscillation experiments
We formulate a set of naturalness criteria for sterile neutrinos (ν') to be light, needed for reconciling the LSND neutrino anomaly with the other neutrino data. A light sterile neutrino becomes as natural as the light active neutrinos if it carries quantum numbers of a chiral gauge symmetry broken at the TeV scale. The simplest such theory is shown to be an SU(2) gauge theory with the ν' transforming as a spin 3/2 multiplet. We develop this model and show that it leads naturally to the phenomenologically viable (3+2) neutrino oscillation scheme. We also present next-to-minimal models for ligh…
Several anomalies in neutrino oscillation experiments point to the existence of a $\sim 1\,$eV sterile neutrino $ν_s$ mixing with $ν_e$ at the level of $U_{e4}\cong 0.1$, but such a neutrino is strongly disfavored by constraints on additional light degrees of freedom ($δN_{\rm eff}$) and total neutrino mass ($\sum_νm_ν$) from cosmology. "Secret neutrino interactions" that have been invoked to suppress the cosmological production of $ν_s$ typically falter, but recently it was pointed out that $ν_s$ could get a large mass in the early universe by coupling to ultralight dark matter $ϕ$, which can…
Sterile neutrinos produced through oscillations are a well motivated dark matter candidate, but recent constraints from observations have ruled out most of the parameter space. We analyze the impact of new interactions on the evolution of keV sterile neutrino dark matter in the early Universe. Based on general considerations we find a mechanism which thermalizes the sterile neutrinos after an initial production by oscillations. The thermalization of sterile neutrinos is accompanied by dark entropy production which increases the yield of dark matter and leads to a lower characteristic momentum.…
We investigate the enhancement of lepton number, energy, and entropy transport resulting from active-sterile neutrino conversion $ν_e\toν_s$ deep in the post-bounce supernova core followed by re-conversion $ν_s\toν_e$ further out, near the neutrino sphere. We explicitly take account of shock wave and neutrino heating modification of the active neutrino forward scattering potential which governs sterile neutrino production. We find that the $ν_e$ luminosity at the neutrino sphere could be increased by between $\sim 10 %$ and $\sim 100 %$ during the crucial shock re-heating epoch if the sterile …
Sterile neutrinos with masses in the keV range are considered to be a viable candidate for warm dark matter. The rate of their production through active-sterile neutrino transitions peaks, however, at temperatures of the order of the QCD scale, which makes it difficult to estimate their relic abundance quantitatively, even if the mass of the sterile neutrino and its mixing angle were known. We derive here a relation, valid to all orders in the strong coupling constant, which expresses the production rate in terms of the spectral function associated with active neutrinos. The latter can in turn…
We show that the canonical oscillation-based (non-resonant) production of sterile neutrino dark matter is inconsistent at $>99$% confidence with observations of galaxies in the Local Group. We set lower limits on the non-resonant sterile neutrino mass of $2.5$ keV (equivalent to $0.7$ keV thermal mass) using phase-space densities derived for dwarf satellite galaxies of the Milky Way, as well as limits of $8.8$ keV (equivalent to $1.8$ keV thermal mass) based on subhalo counts of $N$-body simulations of M 31 analogues. Combined with improved upper mass limits derived from significantly deeper X…
We study the zero textures of the Yukawa matrices in the minimal extended type-I seesaw (MES) model which can give rise to $\sim$ eV scale sterile neutrinos. In this model, three right handed neutrinos and one extra singlet $S$ are added to generate a light sterile neutrino. The light neutrino mass matrix for the active neutrinos, $ m_ν$, depends on the Dirac neutrino mass matrix ($ M_{D} $), Majorana neutrino mass matrix ($ M_{R} $) and the mass matrix ($ M_{S} $) coupling the right handed neutrinos and the singlet. The model predicts one of the light neutrino masses to vanish. We systematica…
We for the first time map the range of active-sterile neutrino mixing angles in which leptogenesis is possible in the type I seesaw model with three heavy neutrinos with Majorana masses between 50 MeV and 100 TeV, covering the entire experimentally accessible mass range. Our study includes both,the asymmetry generation during freeze-in (ARS mechanism) and freeze-out (resonant leptotenesis) of the heavy neutrinos. The range of mixings for which leptogenesis is feasible is considerably larger than in the minimal model with only two heavy neutrinos and extends all the way up to the current experi…
Heavy sterile neutrinos with masses ${\mathcal O}(100)$ MeV mixing with active neutrinos can be produced in the core of a collapsing supernova (SN). In order to avoid an excessive energy loss, shortening the observed duration of the SN 1987A neutrino burst, we show that the active-sterile neutrino mixing angle should satisfy $\sin^2 θ\lesssim 5 \times 10^{-7}$. For a mixing with tau flavour, this bound is much stronger than the ones from laboratory searches. Moreover, we show that in the viable parameter space the decay of such "heavy" sterile neutrinos in the SN envelope would lead to a very …
We discuss solar system constraints on f(G) gravity models, where f is a function of the Gauss-Bonnet term G. We focus on cosmologically viable f(G) models that can be responsible for late-time cosmic acceleration. These models generally give rise to corrections of the form epsilon*(r/rs)^p to the vacuum Schwarzschild solution, where epsilon = H^2 rs^2 << 1, rs is the Schwarzschild radius of Sun, and H is the Hubble parameter today. We generally estimate the strength of modifications to General Relativity in order to confront models with a number of experiments such as the deflection of light …
Opening up New Parameter Space for Sterile Neutrino Dark Matter
Washington University, Saint Louis（圣路易斯华盛顿大学）；Johannes Gutenberg-Universität Mainz（美因茨约翰内斯古腾堡大学）；Texas A&M University（德克萨斯农工大学）；Physical Research Laboratory（物理研究实验室）
We studied nonequilibrium oscillations between left-handed electron neutrinos and nonthermalized sterile neutrinos. The exact kinetic equations for neutrinos, written in terms of neutrino density matrix in momentum space were analyzed. The evolution of neutrino density matrix was numerically calculated. This allowed to study precisely the evolution of the neutrino number densities, energy spectrum distortion and the asymmetry between neutrinos and antineutrinos for each momentum mode. Both effects of distortion and asymmetry, which cannot be accounted for correctly when working in terms of par…
Non-thermalized dark matter is a cosmologically viable alternative to the widely studied weakly interacting massive particle. We study the evolution of the dark matter phase-space distributions arising from freeze-in and superWIMP production as well as the combination of both. Utilizing our implementation in CLASS, we investigate the cosmological imprints on the matter power spectrum, constrained by Lyman-$α$ forest observations. For the explicit example of a colored $t$-channel mediator model, we explore the cosmologically allowed parameter space highlighting the interplay of Lyman-$α$ constr…
We point out that the relic densities of singlet (sterile) neutrinos of interest in viable warm and cold dark matter scenarios, depend on the characteristics of the QCD transition in the early universe. In the most promising of these dark matter scenarios the production of the singlets occurs at or near the QCD transition. Since production of the singlets, their dilution, and the disappearance of weak scatterers occur simultaneously, we calculate these processes contemporaneously to obtain accurate predictions of relic sterile neutrino mass density. Therefore, a determination of the mass and s…
Knowledge of the mechanism of neutrino mass generation would help understand a lot more about Lepton Number Violation (LNV), the cosmological evolution of the Universe, or the evolu tion of astronomical objects. Here we propose a verifiable and viable extension of the Standard model for neutrino mass generation, with a low-scale seesaw mechanism via LNV condensation in the sector of sterile neutrinos. To prove the concept, we analyze a simplified model of just one single family of elementary particles and check it against a set of phenomenological constraints coming from electroweak symmetry b…
The recently proposed "mini-seesaw mechanism" combines naturally suppressed Dirac and Majorana masses to achieve light Standard Model neutrinos via a low-scale seesaw. A key feature of this approach is the presence of multiple light (order GeV) sterile-neutrinos that mix with the Standard Model. In this work we study the bounds on these light sterile-neutrinos from processes like μ---> e + γ, invisible Z-decays, and neutrinoless double beta-decay. We show that viable parameter space exists and that, interestingly, key observables can lie just below current experimental sensitivities. In partic…
Neutrinos, being the only fermions in the Standard Model of Particle Physics that do not possess electromagnetic or color charges, have the unique opportunity to communicate with fermions outside the Standard Model through mass mixing. Such Standard Model-singlet fermions are generally referred to as "sterile neutrinos''. In this review article, we discuss the theoretical and experimental motivation for sterile neutrinos, as well as their phenomenological consequences. With the benefit of hindsight in 2020, we point out potentially viable and interesting ideas. We focus in particular on steril…
A convincing explanation for the observed pulsar large peculiar velocities is still missing. We argue that any viable particle physics solution would most likely involve the resonant production of a non-interacting neutrino $ν_s$ of mass $m_{ν_s}\sim 20$--50 keV. We propose a model where anisotropic magnetic field configurations strongly bias the resonant spin flavour precession of tau antineutrinos into $ν_s$. For internal magnetic fields $B_{int} \gsim 10^{15}$ G a $\barν_τ$-$ν_s$ transition magnetic moment of the order of $10^{-12}$ Bohr magnetons is required. The asymmetric emission of $ν_…
