arXiv subjects

Jiajun Liao

Publications and source records attributed to Jiajun Liao.

54 records · Page 3Linked to original sources

arXiv2018

IceCube's astrophysical neutrino energy spectrum from CPT violation

The 6-year dataset of high-energy starting events (HESE) at IceCube indicates a spectrum of astrophysical neutrinos much softer than expected from the Fermi shock acceleration mechanism. On the other hand, IceCube's up-going muon neutrino dataset and Fermi-LAT's gamma-ray spectrum point to an $E^{-2}$ neutrino spectrum. If the HESE data above 200 TeV are fit with the latter flux, an excess at lower energies ensues, which then suggests a multicomponent spectrum. We show that the HESE dataset can be explained by a single $E^{-2}$ power-law neutrino flux from a muon-damped $pγ$ source if neutrino interactions are modified by CPT violation. The low-energy excess is naturally explained by the pileup of events from superluminal neutrino decay, and there is no cutoff at high energies due to the contribution of subluminal antineutrinos. The best-fit scenario with CPT violation also predicts the observation of Glashow resonance events in the near future.

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arXiv2017

COHERENT constraints on nonstandard neutrino interactions

Coherent elastic neutrino-nucleus scattering consistent with the standard model has been observed by the COHERENT experiment. We study nonstandard neutrino interactions using the detected spectrum. For the case in which the nonstandard interactions (NSI) are induced by a vector mediator lighter than 50 MeV, we obtain constraints on the coupling of the mediator. For a heavier mediator, we find that degeneracies between the NSI parameters severely weaken the constraints. However, these degeneracies do not affect COHERENT constraints on the effective NSI parameters for matter propagation in the Earth.

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arXiv2017

A light $Z^\prime$ for the $R_K$ puzzle and nonstandard neutrino interactions

We show that the $R_K$ puzzle in LHCb data and the discrepancy in the anomalous magnetic moment of the muon can be simultaneously explained if a 10 MeV mass $Z^\prime$ boson couples to the muon but not the electron, and that clear evidence of the nonstandard matter interactions of neutrinos induced by this coupling may be found at DUNE.

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arXiv2017

Nonstandard interactions in solar neutrino oscillations with Hyper-Kamiokande and JUNO

Measurements of the solar neutrino mass-squared difference from KamLAND and solar neutrino data are somewhat discrepant, perhaps due to nonstandard neutrino interactions in matter. We show that the zenith angle distribution of solar neutrinos at Hyper-Kamiokande and the energy spectrum of reactor antineutrinos at JUNO can conclusively confirm the discrepancy and detect new neutrino interactions.

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arXiv2017

Nonmaximal neutrino mixing at NO$ν$A from nonstandard interactions

Muon neutrino disappearance measurements at NO$ν$A suggest that maximal $θ_{23}$ is excluded at the 2.6$σ$ CL. This is in mild tension with T2K data which prefer maximal mixing. Considering that NO$ν$A has a much longer baseline than T2K, we point out that the apparent departure from maximal mixing in NO$ν$A may be a consequence of nonstandard neutrino propagation in matter.

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arXiv2016

Nonstandard neutrino interactions at DUNE, T2HK and T2HKK

We study the matter effect caused by nonstandard neutrino interactions (NSI) in the next generation long-baseline neutrino experiments, DUNE, T2HK and T2HKK. If multiple NSI parameters are nonzero, the potential of these experiments to detect CP violation, determine the mass hierarchy and constrain NSI is severely impaired by degeneracies between the NSI parameters and by the generalized mass hierarchy degeneracy. In particular, a cancellation between leading order terms in the appearance channels when $ε_{eτ} = \cotθ_{23} ε_{eμ}$, strongly affects the sensitivities to these two NSI parameters at T2HK and T2HKK. We also study the dependence of the sensitivities on the true CP phase $δ$ and the true mass hierarchy, and find that overall DUNE has the best sensitivity to the magnitude of the NSI parameters, while T2HKK has the best sensitivity to CP violation whether or not there are NSI. Furthermore, for T2HKK a smaller off-axis angle for the Korean detector is better overall. We find that due to the structure of the leading order terms in the appearance channel probabilities, the NSI sensitivities in a given experiment are similar for both mass hierarchies, modulo the phase change $δ\to δ+ 180^\circ$.

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arXiv2016

Impact of nonstandard interactions on sterile neutrino searches at IceCube

We analyze the energy and zenith angle distributions of the latest 2-year IceCube dataset of upward going atmospheric neutrinos to constrain sterile neutrinos at the eV scale in the 3+1 scenario. We find that the parameters favored by a combination of LSND and MiniBooNE data are excluded at more than the 99% C.L. We explore the impact of nonstandard matter interactions on this exclusion, and find that the exclusion holds for nonstandard interactions (NSI) that are within the stringent model-dependent bounds set by collider and neutrino scattering experiments. However, for large NSI parameters subject only to model-independent bounds from neutrino oscillation experiments, the LSND and MiniBooNE data are consistent with IceCube.

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arXiv2016

Degeneracies in long-baseline neutrino experiments from nonstandard interactions

We study parameter degeneracies that can occur in long-baseline neutrino appearance experiments due to nonstandard interactions (NSI) in neutrino propagation. For a single off-diagonal NSI parameter, and neutrino and antineutrino measurements at a single L/E, there exists a continuous four-fold degeneracy (related to the mass hierarchy and $θ_{23}$ octant) that renders the mass hierarchy, octant, and CP phase unknowable. Even with a combination of NO$ν$A and T2K data, which in principle can resolve the degeneracy, both NSI and the CP phase remain unconstrained because of experimental uncertainties. A wide-band beam experiment like DUNE will resolve this degeneracy if the nonzero off-diagonal NSI parameter is $ε_{eμ}$. If $ε_{eτ}$ is nonzero, or the diagonal NSI parameter $ε_{ee}$ is O(1), a wrong determination of the mass hierarchy and of CP violation can occur at DUNE. The octant degeneracy can be further complicated by $ε_{eτ}$, but is not affected by $ε_{ee}$.

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arXiv2016

Dark matter annihilation with s-channel internal Higgsstrahlung

We study the scenario of fermionic dark matter that annihilates to standard model fermions through an s-channel axial vector mediator. We point out that the well-known chirality suppression of the annihilation cross section can be alleviated by s-channel internal Higgsstrahlung. The shapes of the cosmic ray spectra are identical to that of t-channel internal Higgsstrahlung in the limit of a heavy mediating particle. Unlike the general case of t-channel bremsstrahlung, s-channel Higgsstrahlung can be the dominant annihilation process even for Dirac dark matter. Since the s-channel mediator can be a standard model singlet, collider searches for the mediator are easily circumvented.

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arXiv2015

Generalized perturbations in neutrino mixing

We derive expressions for the neutrino mixing parameters that result from complex perturbations on (1) the Majorana neutrino mass matrix (in the basis of charged lepton mass eigenstates) and on (2) the charged lepton mass matrix, for arbitrary initial (unperturbed) mixing matrices. In the first case, we find that the phases of the elements of the perturbation matrix, and the initial values of the Dirac and Majorana phases, strongly impact the leading order corrections to the neutrino mixing parameters and phases. For experimentally compatible scenarios wherein the initial neutrino mass matrix has $μ-τ$ symmetry, we find that the Dirac phase can take any value under small perturbations. Similarly, in the second case, perturbations to the charged lepton mass matrix can generate large corrections to the mixing angles and phases of the PMNS matrix. As an illustration of our generalized procedure, we apply it to a situation in which nonstandard scalar and nonstandard vector interactions simultaneously affect neutrino oscillations.

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arXiv2015

Neutrino seesaw mechanism with texture zeros

In the context of the Type I seesaw mechanism, we carry out a systematic study of the constraints that result from zeros in both the Dirac and right-handed Majorana neutrino mass matrices. We find that most constraints can be expressed in the standard form with one or two element/cofactor zeros alone, while there are 9 classes of nonstandard constraints. We show that all the constraints are stable under one-loop renormalization group running from the lightest right-handed neutrino mass scale to the electroweak scale. We study the predictions of the nonstandard constraints for the lightest neutrino mass, Dirac CP phase and neutrinoless double beta decay.

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arXiv2015

Partial quark-lepton universality and neutrino CP violation

We study a model with partial quark-lepton universality that can naturally arise in grand unified theories. We find that constraints on the model can be reduced to a single condition on the Dirac CP phase $δ$ in the neutrino sector. Using our current knowledge of the CKM and PMNS mixing matrices, we predict $-32.4^\circ < δ< 32.0^\circ$ at $2σ$.

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arXiv2014

Texture and Cofactor Zeros of the Neutrino Mass Matrix

We study Majorana neutrino mass matrices that have two texture zeros, or two cofactor zeros, or one texture zero and one cofactor zero. The two texture/cofactor zero conditions give four constraints, which in conjunction with the five measured oscillation parameters completely determine the nine independent real parameters of the neutrino mass matrix. We also study the implications that future measurements of neutrinoless double beta decay and the Dirac CP phase will have on these cases.

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arXiv2014

Dual models of the neutrino mass spectrum

We show that any model with a homogeneous relationship among elements of the neutrino mass matrix with one mass hierarchy yields predictions for the oscillation parameters and Majorana phases similar to those given by a model with the same homogeneous relationship among cofactors of the neutrino mass matrix with the opposite mass hierarchy, except when the lightest mass is of order 20 meV or less.

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arXiv2013

One diagonal texture or cofactor zero of the neutrino mass matrix

In view of the recent measurement of nonzero $θ_{13}$, we carry out a systematic study of a simple class of neutrino models that has one diagonal texture or cofactor zero in the mass matrix. There are seven free parameters in the model and five of them are already measured by neutrino oscillation experiments; some cases for the normal or inverted hierarchy are excluded and for the rest we obtain the preferred values for the lightest neutrino mass and Dirac CP phase. We find that there are strong similarities between one diagonal texture zero models with one mass hierarchy and one diagonal cofactor zero models with the opposite mass hierarchy. We also make predictions for neutrinoless double beta decay for these models. For the one cofactor zero models, we present a simple realization based on a new U(1) gauge symmetry.

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arXiv2013

Seesaw mechanism with four texture zeros in the neutrino Yukawa matrix

With the recent observation of nonzero θ_{13}, five neutrino oscillation parameters are now known. By imposing four zeros in the Yukawa coupling matrix of the type I seesaw model, the number of parameters in the neutrino mass matrix is reduced to seven, and we are able to make predictions for the lightest neutrino mass, Dirac CP phase, and neutrinoless double beta decay. Four texture zeros in the Yukawa coupling matrix is equivalent to either a single texture zero or a single cofactor zero for an off-diagonal element of the light neutrino mass matrix. We find strong similarities between single texture zero models with one mass ordering and single cofactor zero models with the opposite mass ordering. In the context of a specific class of single-flavor leptogenesis models, we find additional constraints on the parameter space.

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arXiv2012

Perturbations to μ-τ symmetry in neutrino mixing

Many neutrino mixing scenarios that have μ-τsymmetry with θ_{13}=0 are in disagreement with recent experimental results that indicate a nonzero value for θ_{13}. We investigate the effect of small perturbations on Majorana mass matrices with μ-τsymmetry and derive analytic formulae for the corrections to the mixing angles. We find that since m_1 and m_2 are nearly degenerate, μ-τsymmetry mixing scenarios are able to explain the experimental data with about the same size perturbation for most values of θ_{12}. This suggests that the underlying unperturbed mixing need not have θ_{12} close to the experimentally preferred value. One consequence of this is that a new class of models with μ-τsymmetry is possible, with unperturbed θ_{12} equal to zero or 90 degrees for arbitrary unperturbed θ_{13}.

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arXiv2011

Lorentz noninvariant oscillations of massless neutrinos are excluded

The bicycle model of Lorentz noninvariant neutrino oscillations without neutrino masses naturally predicts maximal mixing and a 1/E dependence of the oscillation argument for muon-neutrino to tau-neutrino oscillations of atmospheric and long-baseline neutrinos, but cannot also simultaneously fit the data for solar neutrinos and KamLAND. Within the Standard Model Extension, we examine all 19 possible structures of the effective Hamiltonian for Lorentz noninvariant oscillations of massless neutrinos that naturally have a 1/E dependence at high neutrino energy. Due to the lack of any evidence for direction dependence, we consider only direction-independent oscillations. Although we find a number of models with a 1/E dependence for atmospheric and long-baseline neutrinos, none can also simultaneously fit solar and KamLAND data.

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