Constraints on active and sterile neutrinos in an interacting dark energy cosmology

We investigate the impacts of dark energy on constraining massive(active/sterile)neutrinos in interacting dark energy(IDE)models by using the current observations.We employ two typical IDE models,the interacting w cold dark matter(IwCDM)model and the interacting holographic dark energy(IHDE)model,to make an analysis.To avoid large-scale instability,we use the parameterized post-Friedmann approach to calculate the cosmological perturbations in the IDE models.The cosmological observational data used in this work include the Planck cosmic microwave background(CMB)anisotropies data,the baryon acoustic oscillation data,the type Ia supernovae data,the direct measurement of the Hubble constant,the weak lensing data,the redshift-space distortion data,and the CMB lensing data.We find that the dark energy properties could influence the constraint limits of active neutrino mass and sterile neutrino parameters in the IDE models.We also find that the dark energy properties could influence the constraints on the coupling strength parameterβ,and a positive coupling constant,β>0,can be detected at the 2.5σstatistical significance for the IHDE+νs model by using the all-data combination.In addition,we also discuss the"Hubble tension"issue in these scenarios.We find that the H0 tension can be effectively relieved by considering massive sterile neutrinos,and in particular in the IHDE+νsmodel the H0 tension can be reduced to be at the 1.28σlevel.

Search for heavy sterile neutrinos in trileptons at the LHC

We present a search strategy for both Dirac and Majorana sterile neutrinos from the purely leptonic decays of W^±→e^±e^±μ ν and μ^±^±e ν at the 14 TeV LHC. The discovery and exclusion limits for sterile neutrinos are shown using both the Cut-and-Count（CC） and Multi-Variate Analysis（MVA） methods. We also discriminate between Dirac and Majorana sterile neutrinos by exploiting a set of kinematic observables which differ between the Dirac and Majorana cases. We find that the MVA method, compared to the more common CC method, can greatly enhance the discovery and discrimination limits. Two benchmark points with sterile neutrino mass m N =20 GeV and 50 GeV are tested. For an integrated luminosity of 3000 fb^-1, sterile neutrinos can be found with 5σ significance if heavy-to-light neutrino mixings ｜U_（Ne）｜^2~｜U_（Nμ）｜^2~10^-6, while Majorana vs. Dirac discrimination can be reached if at least one of the mixings is of order 10^-5.

Searching for sterile neutrinos in dynamical dark energy cosmologies

We investigate how the dark energy properties change the cosmological limits on sterile neutrino parameters by using recent cosmological observations. We consider the simplest dynamical dark energy models, the wCDM model and the holographic dark energy（HDE） model, to make an analysis. The cosmological observations used in this work include the Planck 2015 CMB temperature and polarization data, the baryon acoustic oscillation data, the type Ia supernova data, the Hubble constant direct measurement data, and the Planck CMB lensing data. We find that, mν,sterileff〈 0.2675 eV and Neff〈 3.5718 for ΛCDM cosmology, mν,sterileff〈 0.5313 eV and Neff〈 3.5008 for wCDM cosmology, and mν,sterileff〈 0.1989 eV and Neff〈 3.6701 for HDE cosmology, from the constraints of the combination of these data. Thus, without the addition of measurements of growth of structure, only upper limits on both mν,sterileff and Neff can be derived, indicating that no evidence of the existence of a sterile neutrino species with e V-scale mass is found in this analysis. Moreover, compared to the ΛCDM model, in the wCDM model the limit on mν,sterileff becomes much looser, but in the HDE model the limit becomes much tighter. Therefore, the dark energy properties could significantly influence the constraint limits of sterile neutrino parameters.

Tilt of primordial gravitational wave spectrum in a universe with sterile neutrinos

In this work, we constrain the spectral index nt of the primordial gravitational wave power spectrum in a universe with sterile neutrinos by using the Planck temperature data, the WMAP 9-year polarization data, the baryon acoustic oscillation data, and the BICEP2 data. We call this model the ACDM＋r＋vs＋nt model. The additional massive sterile neutrino species can significantly relieve the tension between the Planck and BICEP2 data, and thus can reduce the possible effects of this tension on the fit results of nt. To constrain the parameters of sterile neutrino, we also utilize the Hubble constant direct measurement data, the Planck Sunyaev-Zeldovich cluster counts data, the Planck CMB lensing data, and the cosmic shear data. We find that due to the fact that the BICEP2 data are most sensitive to the multipole （ - 150 corresponding to k - 0.01 Mpc^-1, there exists a strong anticorrelation between nt and r0.0o2 in the BICEP2 data, and this further results in a strongly blue-tilt spectrum. However, a slightly red-tilt tensor power spectrum is also allowed by the BICEP2 data in the region with larger value of r0.00z. By using the full data sets, we obtain mvisterile ^eff = 0.48 -0.13^＋0.11 eV, Near = 3.73 -0.37^＋0.34, and nt = 0.96 -0.63^ ＋0.48 for the ACDM＋r＋vs＋nt model.

A compact analytical approximation for a light sterile neutrino oscillation in matter

The existence of light sterile neutrinos is a long-standing question in particle physics.Several experimental“anomalies”might be explained by introducing eV mass scaled light sterile neutrinos.Many experiments are actively searching for such light sterile neutrinos through neutrino oscillation.For long baseline experiments,the matter effect should be treated carefully for precise calculation of the neutrino oscillation probabilities.However,this is usually time-consuming or analytically complex.In this manuscript,we adopt a Jacobi-like method to diagonalize the Hermitian Hamiltonian matrix and derive analytically simplified neutrino oscillation probabilities for 3(active)+1(sterile)-neutrino mixing for a constant matter density.These approximations can reach a considerably high numerical accuracy while retaining their analytical simplicity and fast computing speed.This would be useful for current and future long baseline neutrino oscillation experiments.

Anomalies in(semi)-leptonic B decays B~±→τ~±ν,B~±→Dτ~±ν and B~±→D~*τ~±ν,and possible resolution with sterile neutrino

The universality of the weak interactions can be tested in semileptonic b→c transitions,and in particular in the ratios R（D＊） ≡ Γ（B → D（＊）τν）/Γ（B → D＊lν）（where 1 = μ or e）.Due to the recent differences between the experimental measurements of these observables by Ba Bar,Belle and LHCb on the one hand and the Standard Model predicted values on the other hand,we study the predicted ratios R（D＊）=Γ（B→D＊τ＋＂missing＂）/Γ（B→D（＊）lν）in scenarios with an additional sterile heavy neutrino of mass ～1 Ge V.Further,we evaluate the newly defined ratio R（0）≡Γ（B→τ＋＂missing＂）/Γ（B→μν） in such scenarios,in view of the future possibilities of measuring the quantity at Belle-Ⅱ.

Testing lepton f lavor universality in terms of BESⅢ and charm-tau factory data

The recent measurements on Rκ and Rπ imply that there exists a possible violation of the leptonic flavor universality which is one of the cornerstones of the Standard Model. It is suggested that a mixing between sterile and active neutrinos might induce such a violation. In this work we consider the scenarios with one or two sterile neutrinos to explicitly realize the data while the constraints from the available experiments have been taken into account. Moreover, as indicated in literature, the deviation of the real PMNS matrix from the symmetric patterns may be due to a μ-τ asymmetry, therefore the measurements on RD（Ds）eμ=F（D（Ds）→e＋νe）/Г（D（Ds）→μ＋νμ） and RD（Ds）μτ=Г（D（Ds）→μ＋νμ）Г（D（Ds）→ι＋ντ） （and for some other heavy mesons B± and Bc etc.） may shed more light on the physics responsible for the violation of the leptonic flavor universality. The data of BESⅢ are available to test the universality and that of future charm-tau factories will provide more accurate information. In this work, we will discuss RD（Ds）eμ and RD（Ds）μτ in detail and also briefly consider the cases for B± and Bc.