Small Mixing Angle Research Articles

Neutrino mixing matrix and masses from a particular two-zero texture based on A 4 symmetry

The current study aims to investigate the particular case of two zeros in a Majorana neutrino mass matrix based on A 4 symmetry, where charged lepton mass matrix is diagonal. The texture is with (μ, μ) and (τ, τ) vanishing element of the neutrino mass matrix. The texture has magic and μ − τ symmetry, with a tribimaximal form of the mixing matrix, which leads to θ 13 = 0 that it is not consistent with experimental data and at first, does not seem to be allowed. Since θ 13 a small mixing angle compared to others neutrino mixing angles justifies the use of perturbation theory. We propose that, θ 13, and the Dirac phase δ, and two Majorana phases ρ and σ could be generated by using a complex symmetric perturbation mass matrix in the mass basis and find that affect to the atmospheric mixing angle. We show that only the predictions of the case I, with Δ < 0 and , are consistent with the experimental data. Furthermore, the allowed range of our parameter space and complex elements of perturbation mass matrix are found, which led to finding the allowed region of the neutrino masses, the Majorana phases, the effective neutrino mass for the neutrinoless double beta decay, the allowed deviation of θ 23 from 45°, and to predict the normal neutrino mass hierarchy. The predicted region of and θ 23 are in line with the current experimental data which indicate the accuracy of our model and its results. The results of the case II, with Δ > 0 and , are ruled out.

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.

Sterile neutrino production at small mixing in the early universe

Sterile neutrinos can be produced in the early universe via interactions with their active counterparts. For small active-sterile mixing angles, thermal equilibrium with the standard model plasma is not reached and sterile neutrinos are only produced via flavor oscillations. We study in detail this regime, taking into account matter potentials and decoherence effects caused by elastic scatterings with the plasma. We find that resonant oscillations occurring at temperatures T≲10 GeV lead to a significant enhancement of the sterile neutrino production rate. Taking this into account, we improve constraints on the active-sterile mixing from Big Bang nucleosynthesis and the cosmic microwave background, excluding mixing angles down to θs∼10−10−10−16 for sterile neutrino masses in the 10 MeV to 10 GeV range. We observe that if sterile neutrinos predominantly decay into metastable hidden sector particles, this process provides a novel dark matter production mechanism, consistent with the sterile neutrino origin of light neutrino masses via the seesaw mechanism.

Boosted neutrinos and relativistic dark particles as messengers from reheating

Usually information from early eras such as reheating is hard to come by. In this paper we argue that, given the right circumstances, right-handed sterile neutrinos decaying to left-handed active ones at relatively late times can carry information from reheating by propagating freely over the thermal history. For not too small mixing angles, suitable right-handed neutrino masses are around \U0001d4aa MeV–GeV. We identify the typical spectra and argue that they provide information on the ratio of the inflaton mass to the reheating temperature. This primordial neutrino signal can be strong enough that it can be detected in IceCube. More speculatively, for a reheating temperature and inflaton mass satisfying TR=\U0001d4aa(1–100) MeV, and mϕ=\U0001d4aa(1016-19) GeV they may even explain the observed PeV events. Also more general relativistic dark particles can play the role of such messengers, potentially not only allowing for the PeV events but also alleviating the H0-tension.

Dynamical mixing between 23S1 and 13D1 charmed mesons

In charmed $D$ and $D_s$ mesons sector, the matrix of a Hamiltonian in a quark potential model is computed in the $2^3S_1$ and $1^3D_1$ subspace. The masses of four mixed states of $2^3S_1$ and $1^3D_1$ denoted with $D^*_1(2635)$, $D^*_1(2739)$, $D^*_{s1}(2715)$ and $D^*_{s1}(2805)$ are obtained. It is an off-diagonal part of the spin-orbit tensor interaction that causes the mixing between the $2^3S_1$ and $1^3D_1$ states. The mixing angles between the $2^3S_1$ and $1^3D_1$ states are tiny. Under the mixing, a $^3P_0$ model is employed to compute the hadronic decay widths of all OZI-allowed decay channels of the four mixed states. The two light mixed states $D^*_1(2635)$ and $D^*_{s1}(2715)$ are close in mass to $D^*_J(2600)$ and $D^*_{s1}(2700)$, while the two heavy mixed states $D^*_1(2739)$ and $D^*_{s1}(2805)$ are lighter in mass than $D(2750)$ and $D^*_{s1}(2860)$. The mixing angles obtained from dynamical interaction are inconsistent with the mixing angles obtained from hadronic decay. Based on mass spectra and hadronic decay analyses, $D^*_J(2600)$, $D(2750)$, $D^*_{s1}(2700)$ and $D^*_{s1}(2860)$ are impossibly the mixed states of $2^3S_1$ and $1^3D_1$ at the small mixing angles. The inconsistence implies that $D^*_1(2760)$ and $D^*_{s1}(2860)$ have not been properly resolved from present experimental data, or there exist large unknown off-diagonal interactions that result in large mixing angles.

Latest Results from Neutrino Oscillation Experiment Daya Bay

The Daya Bay Reactor Neutrino Experiment was designed to measure Θ13, the smallest mixing angle in the three-neutrino mixing framework, with unprecedented precision. The experiment consists of eight identically designed detectors placed underground at different baselines from three pairs of nuclear reactors in South China. Since Dec. 2011, the experiment has been running stably for more than 7 years, and has collected the largest reactor antineutrino sample to date. Daya Bay greatly improved the precision on Θ13 and made an independent measurement of the effective mass splitting in the electron antineutrino disappearance channel. Daya Bay also performed a number of other precise measurements such as a high-statistics determination of the absolute reactor antineutrino flux and the spectrum evolution, as well as a search for the sterile neutrino mixing, among others. The most recent results from Daya Bay are discussed in this paper, as well as the current status and future prospects of the experiment.

Induced resonance makes light sterile neutrino dark matter cool

We describe two new generation mechanisms for Dark Matter composed of sterile neutrinos with ${\cal O}(1)$ keV mass. The model contains a light scalar field which coherently oscillates in the early Universe and modulates the Majorana mass of the sterile neutrino. In a region of model parameter space, the oscillations between active and sterile neutrinos are resonantly enhanced. This mechanism allows us to produce sterile neutrino DM with small mixing angle with active neutrinos, thus evading the X-ray constraints. At the same time the spectrum of produced DM is much cooler, than in the case of ordinary oscillations in plasma, opening a window of lower mass DM, which is otherwise forbidden by structure formation considerations. In other regions of the model parameter space, where the resonance does not appear, another mechanism can operate: large field suppresses the active-sterile oscillations, but instead sterile neutrinos are produced by the oscillating scalar field when the effective fermion mass crosses zero. In this case DM component is cold, and even 1 keV neutrino is consistent with the cosmic structure formation.

A light scalar in the Minimal Dilaton Model in light of the LHC constraints * * Supported by the National Natural Science Foundation of China (NNSFC)(11605123, 11547103, 11674253, 11547310), the China Scholarship Council (CSC) (201706275160) and the US National Science Foundation (NSF) (PHY-0855561)

Whether an additional light scalar exists is an interesting topic in particle physics beyond the Standard Model (SM), as we do not know as yet the nature of physics beyond the SM in the low mass region in view of the inconsistent results of the ATLAS and CMS collaborations in their search for light resonances around 95 GeV in the diphoton channel. We study a light scalar in the Minimal Dilaton Model (MDM). Under the theoretical and the latest experimental constraints, we sort the selected data samples into two scenarios according to the diphoton rate of the light scalar: the large-diphoton scenario (with ) and the small-diphoton scenario (with ), which are favored by the CMS and ATLAS results, respectively. We compare the two scenarios, test the characteristics of the model parameters, the scalar couplings, production and decay, and consider how they could be further discerned at colliders. We draw the following conclusions for the two scenarios: (i) The large-diphoton scenario has in general a small Higgs-dilaton mixing angle ( ) and a small dilaton vacuum expectation value (VEV) ( ), and the small-diphoton scenario has large mixing ( ) or large VEV ( ). (ii) The large-diphoton scenario in general predicts small coupling ( ) and large coupling ( ), while the small-diphoton scenario predicts small coupling ( ). (iii) The large-diphoton scenario can interpret the small diphoton excess seen by CMS at its central value, when GeV, and . (iv) The large-diphoton scenario in general predicts a negative correlation between the Higgs couplings and , while the small-diphoton scenario predicts that both couplings are smaller than 1, or .

Third family hypercharge model for {R}_{K^{left(ast right)}} and aspects of the fermion mass problem

We present a model to explain LHCb’s recent measurements of RK and {R}_{K^{left(ast right)}} based on an anomaly-free, spontaneously-broken U(1)F gauge symmetry, without any fermionic fields beyond those of the Standard Model (SM). The model explains the hierarchical heaviness of the third family and the smallness of quark mixing. The U(1)F charges of the third family of SM fields and the Higgs doublet are set equal to their respective hypercharges. A heavy Z′ particle with flavour-dependent couplings can modify the left[overline{b_L}{gamma}^{rho }{s}_Lright]left[overline{mu_L}{gamma}_{rho }{mu}_Lright] effective vertex in the desired way. The Z′ contribution to {B}_s-overline{B_s} mixing is suppressed by a small mixing angle connected to Vts, making the constraint coming from its measurement easier to satisfy. The model can explain RK and {R}_{K^{*}} whilst simultaneously passing other constraints, including measurements of the lepton flavour universality of Z couplings.

Muon g−2 and rare top decays in up-type specific variant axion models

The invisible variant axion models (VAM's) offer a very attractive solution for the strong CP problem without the domain wall problem. We consider the up-type specific variant axion models and examine their compatibility with the muon $g-2$ anomaly and the constraints from lepton flavor universality, several flavor observables, and top quark measurements. We find that the combined $\chi^2$ fit favors the parameters $m_A\sim 15$ GeV and $\tan\beta \sim 40$, the same as the type-X 2HDM. Moreover, we find that there are no conflict with any flavor observables as long as the mixing angle $\rho_u$ is sufficiently small. In particular, a small nonzero mixing angle $\rho_u \sim \pi/100$ is slightly favored by the observed $B_s \to \mu\mu$ branching ratio. The up-specific VAM predicts the flavor-violating top rare decay $t\to uA$ followed by $A \to \tau\tau$, which would provide a smoking gun signature at the LHC. We show that current searches of $A$ already impose some constraints on the parameter space but are not sensitive to the most interesting light $m_A$ region. We propose an efficient search strategy that employs di-tau tagging using jet substructure information, and demonstrate that it can enhance the sensitivity on $BR(t \to uA)$, especially in the light $m_A$ region. This model also predicts the flavor-violating decay of heavy Higgs bosons, such as $H \to t u$, that would suppress the $H \to \tau\tau/\mu\mu$ decays. We also examine the up-specific VAM with the muon-specific lepton sector and the down-type specific VAM's as interesting alternative scenarios.

Next-to-minimal dark matter at the LHC

We examine the collider signatures of a WIMP dark matter scenario comprising a singlet fermion and an SU(2) n-plet fermion, with a focus on n = 3 and n = 5. The singlet and n-plet masses are of the order of the electroweak scale. The n-plet contains new charged particles which will be copiously pair-produced at the LHC. Small mixing angles and near-degenerate masses, both of which feature naturally in these models, give rise to long-lived particles and their characteristic collider signatures. In particular, the n = 5 model can be constrained by displaced lepton searches independently of the mixing angle, generically ruling out 5-plet masses below about 280 GeV. For small mixing angles, we show that there is a parameter range for which the model reproduces the observed thermal relic density but is severely constrained by disappearing track searches in both the n = 3 and the n = 5 cases. The n = 3 model is further constrained by soft di-lepton searches irrespective of whether any of the new particles are long-lived.

A realistic U(2) model of flavor

We propose a simple U(2) model of flavor compatible with an SU(5) GUT structure. All hierarchies in fermion masses and mixings arise from powers of two small parameters that control the U(2) breaking. In contrast to previous U(2) models this setup can be realized without supersymmetry and provides an excellent fit to all SM flavor observables including neutrinos. We also consider a variant of this model based on a D6 × U(1)F flavor symmetry, which closely resembles the U(2) structure, but allows for Majorana neutrino masses from the Weinberg operator. Remarkably, in this case one naturally obtains large mixing angles in the lepton sector from small mixing angles in the quark sector. The model also offers a natural option for addressing the Strong CP Problem and Dark Matter by identifying the Goldstone boson of the U(1)F factor as the QCD axion.

Neutrino masses in the minimal gauged ( B−L ) supersymmetry

We present the radiative corrections to neutrino masses in a minimal supersymmetric extension of the standard model with local $U(1)_{B-L}$ symmetry. At tree level, three tiny active neutrinos and two nearly massless sterile neutrinos can be obtained through the seesaw mechanism. Considering the one-loop corrections to the neutrino masses, the numerical results indicate that two sterile neutrinos obtain ${\rm KeV}$ masses and the small active-sterile neutrino mixing angles. The lighter sterile neutrino is a very interesting dark matter candidate in cosmology. Meanwhile the active neutrinos mixing angles and mass squared differences agree with present experimental data.

BBN for the LHC: Constraints on lifetimes of the Higgs portal scalars

LHC experiments can provide a remarkable sensitivity to exotic metastable massive particles, decaying with significant displacement from the interaction point. The best sensitivity is achieved to models where the production and decay occur due to different coupling constants, and the lifetime of exotic particles determines the probability of decay within a detector. The lifetimes of such particles can be independently limited from standard cosmology, in particular the Big Bang Nucleosynthesis. In this paper, we analyze the constraints on the simplest scalar model coupled through the Higgs portal, where the production occurs via $h\to SS$, and the decay is induced by the small mixing angle of the Higgs field $h$ and scalar $S$. We find that throughout the most part of the parameter space, $2 m_\mu < m_S < m_h/2$, the lifetimes of exotic particle has to be less than 0.1 seconds, while below $2m_\mu$ it could grow to about a second. The strong constraints on lifetimes are induced by the nucleonic and mesonic decays of scalars that tend to raise the $n/p$ ratio. Strong constraints on lifetimes of the minimal singlet extensions of the Higgs potential is a welcome news for the MATHUSLA proposal that seeks to detect displaced decays of exotic particles produced in the LHC collisions. We also point out how more complicated exotic sectors could evade the BBN lifetime constraints.

Results from the search for eV-sterile neutrinos with IceCube

The IceCube neutrino telescope at the South Pole has measured the atmospheric muon neutrino spectrum as a function of zenith angle and energy. Using IceCubes full detector configuration we have performed searches for eV-scale sterile neutrinos. Such a sterile neutrino, motivated by the anomalies observed in short-baseline experiments, is expected to have a significant effect on survival probability due to matter-induced resonant effects for energies of order 1 TeV. This effect makes this search unique and sensitive to small sterile mixing angle values. This work comprises results obtained using up-going muon neutrinos taken with one year of full detector configuration.

Latest results from Daya Bay

The Daya Bay Reactor Neutrino Experiment was designed to measure θ13, the smallest mixing angle in the three-neutrino mixing framework, with unprecedented precision. The experiment consists of eight functionally identical detectors placed underground at different baselines from three pairs of nuclear reactors in South China. Since Dec. 2011, the experiment has been running stably for more than 4 years, and has collected the largest reactor anti-neutrino sample to date. Daya Bay is able to greatly improve the precision on θ13 and to make an independent measurement of the effective mass splitting in the electron antineutrino disappearance channel. Daya Bay can also perform a number of other precise measurements, such as a high-statistics determination of the absolute reactor antineutrino flux and spectrum, as well as a search for sterile neutrino mixing, among others. The most recent results from Daya Bay are discussed in this paper, as well as the current status and future prospects of the experiment.

On the IceCube result on ν̄μ →ν̄s oscillations

We elucidate the mechanism of enhancement of the [Formula: see text] transitions at small mixing angles of Earth-core-crossing neutrinos, which is at the basis of the recent result of the IceCube experiment on atmospheric muon antineutrino oscillations into sterile antineutrino.

Flipped versions of the universal 3-3-1 and the left-right symmetric models in [SU(3)]3 : A comprehensive approach

By considering the 3-3-1 and the left-right symmetric models as low-energy effective theories of the $SU(3{)}_{C}\ensuremath{\bigotimes}SU(3{)}_{L}\ensuremath{\bigotimes}SU(3{)}_{R}$ (for short $[SU(3){]}^{3}$) gauge group, alternative versions of these models are found. The new neutral gauge bosons of the universal 3-3-1 model and its flipped versions are presented; also, the left-right symmetric model and its flipped variants are studied. Our analysis shows that there are two flipped versions of the universal 3-3-1 model, with the particularity that both of them have the same weak charges. For the left-right symmetric model, we also found two flipped versions; one of them is new in the literature and, unlike those of the 3-3-1, requires a dedicated study of its electroweak properties. For all the models analyzed, the couplings of the ${Z}^{\ensuremath{'}}$ bosons to the standard model fermions are reported. The explicit form of the null space of the vector boson mass matrix for an arbitrary Higgs tensor and gauge group is also presented. In the general framework of the $[SU(3){]}^{3}$ gauge group, and by using the LHC experimental results and EW precision data, limits on the ${Z}^{\ensuremath{'}}$ mass and the mixing angle between $Z$ and the new gauge bosons ${Z}^{\ensuremath{'}}$ are obtained. The general results call for very small mixing angles in the range $1{0}^{\ensuremath{-}3}$ radians and ${M}_{{Z}^{\ensuremath{'}}}&gt;2.5\text{ }\text{ }\mathrm{TeV}$.

The case for mixed dark matter from sterile neutrinos

Sterile neutrinos are SU(2) singlets that mix with active neutrinos via a mass matrix, its diagonalization leads to mass eigenstates that couple via standard model vertices. We study the cosmological production of heavy neutrinos via standard model charged and neutral current vertices under a minimal set of assumptions: i) the mass basis contains a hierarchy of heavy neutrinos, ii) these have very small mixing angles with the active (flavor) neutrinos, iii) standard model particles, including light (active-like) neutrinos are in thermal equilibrium. If kinematically allowed, the same weak interaction processes that produce active-like neutrinos also produce the heavier species. We introduce the quantum kinetic equations that describe their production, freeze out and decay and discuss the various processes that lead to their production in a wide range of temperatures assessing their feasibility as dark matter candidates. The final distribution function at freeze-out is a mixture of the result of the various production processes. We identify processes in which finite temperature collective excitations may lead to the production of the heavy species. As a specific example, we consider the production of heavy neutrinos in the mass range Mh ≲ 140 MeV from pion decay shortly after the QCD crossover including finite temperature corrections to the pion form factors and mass. We consider the different decay channels that allow for the production of heavy neutrinos showing that their frozen distribution functions exhibit effects from ``kinematic entanglement'' and argue for their viability as mixed dark matter candidates. We discuss abundance, phase space density and stability constraints and argue that heavy neutrinos with lifetime τ> 1/H0 freeze out of local thermal equilibrium, and conjecture that those with lifetimes τ ≪ 1/H0 may undergo cascade decay into lighter DM candidates and/or inject non-LTE neutrinos into the cosmic neutrino background. We provide a comparison with non-resonant production via active-sterile mixing.

Search for Heavy Right Handed Neutrinos at the FCC-ee

The Standard Model of particle physics is still lacking an understanding of the generation and nature of neutrino masses. A favorite theoretical scenario (the see-saw mechanism) is that both Dirac and Majorana mass terms are present, leading to the existence of heavy partners of the light neutrinos, presumably massive and nearly sterile. These heavy neutrinos can be searched for at high energy lepton colliders of very high luminosity, such as the Future electron-positron e+e− Circular Collider, FCC-ee (TLEP), presently studied within the Future Circular Collider design study at CERN, as a possible first step. A first look at sensitivities, both from neutrino counting and from direct search for heavy neutrino decay, are presented. The number of neutrinos Nν should be measurable with a precision of ΔNν≈±(0.0004–0.0010), while the direct search appears very promising due to the long lifetime of heavy neutrinos for small mixing angles. A sensitivity down to a heavy-light mixing of |U|2≃10−12 is obtained, covering a large phase space for heavy neutrino masses between 10 and 80 GeV/c2.