arXiv subjects

Shun Zhou

Publications and source records attributed to Shun Zhou.

At least 73 records · Page 4Linked to original sources

arXiv2021

Sufficient and Necessary Conditions for CP Conservation in the Case of Degenerate Majorana Neutrino Masses

In this paper, we carry out a systematic study of the sufficient and necessary conditions for CP conservation in the leptonic sector with massive Majorana neutrinos. In particular, the emphasis is placed on the number of CP-violating phases in the presence of a partial mass degeneracy (e.g., $m^{}_1 = m^{}_2 \neq m^{}_3$) or a complete mass degeneracy $m^{}_1 = m^{}_2 = m^{}_3$, where $m^{}_i$ (for $i = 1, 2, 3$) stand for the masses of three ordinary neutrinos. In the canonical seesaw model with three right-handed neutrino singlets, CP-violating phases in the special case of a partial (e.g., $M^{}_1 = M^{}_2 \neq M^{}_3$) or complete (i.e., $M^{}_1 = M^{}_2 = M^{}_3$) mass degeneracy of three heavy Majorana neutrinos are also examined. In addition, we derive the renormalization-group equations of the weak-basis invariants in the effective theory with a general mass spectrum of Majorana neutrinos, to which the solutions establish the direct connection between CP violation at low- and high-energy scales.

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arXiv2021

Precise Values of Running Quark and Lepton Masses in the Standard Model

The precise values of the running quark and lepton masses $m^{}_f(μ)$, which are defined in the modified minimal subtraction scheme ($\overline{\rm MS}$) with $μ$ being the renormalization scale and the subscript $f$ referring to all the charged fermions in the Standard Model (SM), are very useful for the model building of fermion masses and flavor mixing and for the precision calculations in the SM or its new-physics extensions. In this paper, we calculate the running fermion masses by taking account of the up-to-date experimental results collected by Particle Data Group and the latest theoretical higher-order calculations of relevant renormalization-group equations and matching conditions in the literature. The emphasis is placed on the quantitative estimation of current uncertainties on the running fermion masses, and the linear error propagation method is adopted to quantify the uncertainties, which has been justified by the Monte-Carlo simulations. We identify two main sources of uncertainties, i.e., one from the experimental inputs and the other from the truncations at finite-order loops. The correlations among the uncertainties of running parameters can be remarkable in some cases. The final results of running fermion masses at several representative energy scales are tabulated for further applications.

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arXiv2020

The Minimal Seesaw Model with a Modular $S_4$ Symmetry

In this paper, we incorporate the modular $S^{}_4$ flavor symmetry into the supersymmetric version of the minimal type-I seesaw model, in which only two right-handed neutrino singlets are introduced to account for tiny Majorana neutrino masses, and explore its implications for the lepton mass spectra, flavor mixing and CP violation. The basic idea is to assign two right-handed neutrino singlets into the unique two-dimensional irreducible representation of the modular $S^{}_4$ symmetry group. Moreover, we show that the matter-antimatter asymmetry in our Universe can be successfully explained via the resonant leptogenesis mechanism working at a relatively-low seesaw scale $Λ^{}_{\rm SS} \approx 10^7~{\rm GeV}$, with which the potential problem of the gravitino overproduction can be avoided. In this connection, we emphasize that the observed matter-antimatter asymmetry may lead to a stringent constraint on the parameter space and testable predictions for low-energy observables.

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arXiv2020

Weak-basis invariants and CP conservation in the leptonic sector with Majorana neutrinos

In this talk, we present a recent investigation of the sufficient and necessary conditions for CP conservation in the leptonic sector with massive Majorana neutrinos in terms of CP-odd weak-basis invariants. The number of weak-basis invariants to guarantee CP conservation in the leptonic sector is clarified and a new set of invariants are advocated for the description of CP conservation, given the physical parameters in their experimentally allowed regions.

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arXiv2020

Cosmic Flavor Hexagon for Ultrahigh-energy Neutrinos and Antineutrinos at Neutrino Telescopes

In this paper, we propose a hexagonal description for the flavor composition of ultrahigh-energy (UHE) neutrinos and antineutrinos, which will hopefully be determined at the future large neutrino telescopes. With such a geometrical description, we are able to clearly separate the individual flavor composition of neutrinos from that of antineutrinos in one single regular hexagon, which can be regarded as a natural generalization of the widely-used ternary plot. For illustration, we consider the $pp$ or $pγ$ collisions as the dominant production mechanism for UHE neutrinos and antineutrinos in the cosmic accelerator, and investigate how neutrino oscillations in the standard picture and in the presence of Lindblad decoherence could change the flavor composition of neutrinos and antineutrinos at neutrino telescopes.

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arXiv2020

Transition Probabilities in the Two-Level Quantum System with PT-Symmetric Non-Hermitian Hamiltonians

We investigate how to define in a consistent way the probabilities of the transitions between the "flavor" states of the two-level quantum system, which is described by a non-Hermitian but parity and time-reversal (PT) symmetric Hamiltonian. Explicit calculations are carried out to demonstrate the conservation of probability if a proper definition of the final state is adopted. Finally, this formalism is applied to two-flavor neutrino oscillations $ν^{}_μ\to ν^{}_μ$ and $ν^{}_μ\to ν^{}_τ$ in vacuum, where the exact PT symmetry requires the vacuum mixing angle to be maximal, which is compatible with current neutrino oscillation experiments. A possible generalization to the three-flavor case is briefly discussed.

quant-ph↗

arXiv2020

Prospects for Pre-supernova Neutrino Observation in Future Large Liquid-scintillator Detectors

Before massive stars heavier than $(8 \cdots 10)$ solar masses evolve to the phase of a gravitational core collapse, they will emit a huge number of MeV-energy neutrinos that are mainly produced in the thermal processes and nuclear weak interactions. The detection of such pre-supernova (pre-SN) neutrinos could provide an important and independent early warning for the optical observations of core-collapse SNe. In this paper, we investigate the prospects of future large liquid-scintillator detectors for the observation of pre-SN neutrinos in both $\barν^{}_e + p \to e^+ + n$ and $ν(\barν) + e^- \to ν(\barν) + e^-$ reaction channels, where $ν$ ($\barν$) denotes neutrinos (antineutrinos) of all three flavors. We propose a quantitative assessment of the capability in terms of three working criteria, namely, how far the SN distance can be covered, how long the early warning before the core collapse can be sent out, and how well the direction pointing to the SN can be determined. The dependence of the final results on the different models of progenitor stars, neutrino flavor conversions and the relevant backgrounds is also discussed.

astro-ph.HE↗

arXiv2020

Effective neutrino masses in KATRIN and future tritium beta-decay experiments

Past and current direct neutrino mass experiments set limits on the so-called effective neutrino mass, which is an incoherent sum of neutrino masses and lepton mixing matrix elements. The electron energy spectrum which neglects the relativistic and nuclear recoil effects is often assumed. Alternative definitions of effective masses exist, and an exact relativistic spectrum is calculable. We quantitatively compare the validity of those different approximations as function of energy resolution and exposure in view of tritium beta decays in the KATRIN, Project 8 and PTOLEMY experiments. Furthermore, adopting the Bayesian approach, we present the posterior distributions of the effective neutrino mass by including current experimental information from neutrino oscillations, beta decay, neutrinoless double-beta decay and cosmological observations. Both linear and logarithmic priors for the smallest neutrino mass are assumed.

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arXiv2019

On the Properties of the Effective Jarlskog Invariant for Three-flavor Neutrino Oscillations in Matter

In this paper, we show that the ratio of the effective Jarlskog invariant $\widetilde{\cal J}$ for leptonic CP violation in three-flavor neutrino oscillations in matter to its counterpart ${\cal J}$ in vacuum $\widetilde{\cal J}/{\cal J} \approx 1/(\hat{C}^{}_{12} \hat{C}^{}_{13})$ holds as an excellent approximation, where $\hat{C}^{}_{12} \equiv \sqrt{1 - 2 \hat{A}^{}_* \cos 2θ^{}_{12} + \hat{A}^2_*}$ with $\hat{A}^{}_* \equiv a\cos^2 θ^{}_{13}/Δ^{}_{21}$ and $\hat{C}^{}_{13} \equiv \sqrt{1 - 2 A^{}_{\rm c} \cos 2θ^{}_{13} + A^2_{\rm c}}$ with $A^{}_{\rm c} \equiv a/Δ^{}_{\rm c}$. Here $Δ^{}_{ij} \equiv m^2_i - m^2_j$ (for $ij = 21, 31, 32$) stand for the neutrino mass-squared differences in vacuum and $θ^{}_{ij}$ (for $ij = 12, 13, 23$) are the neutrino mixing angles in vacuum, while $Δ^{}_{\rm c} \equiv Δ^{}_{31}\cos^2θ^{}_{12} + Δ^{}_{32} \sin^2 θ^{}_{12}$ and the matter parameter $a \equiv 2\sqrt{2}G^{}_{\rm F} N^{}_e E$ are defined. This result has been explicitly derived by improving the previous analytical solutions to the renormalization-group equations of effective neutrino masses and mixing parameters in matter. Furthermore, as a practical application, such a simple analytical formula has been implemented to understand the existence and location of the extrema of $\widetilde{\cal J}$.

hep-ph↗

arXiv2019

Towards the meV limit of the effective neutrino mass in neutrinoless double-beta decays

In this paper, we emphasize why it is important for future neutrinoless double-beta ($0νββ$) decay experiments to reach the sensitivity to the effective neutrino mass $|m^{}_{ββ}| \approx 1~{\rm meV}$. Assuming such a sensitivity and the precisions on neutrino oscillation parameters after the JUNO experiment, we fully explore the constrained regions of the lightest neutrino mass $m^{}_1$ and two Majorana-type CP-violating phases $\{ρ, σ\}$. The implications for the neutrino mass spectrum, the effective neutrino mass $m^{}_β$ in beta decays and the sum of three neutrino masses $Σ\equiv m^{}_1 + m^{}_2 + m^{}_3$ relevant for cosmological observations are also discussed.

hep-ph↗

arXiv2019

Probing Cosmic Axions through Resonant Emission and Absorption in Atomic Systems with Superradiance

The $μ$eV-mass axion is one of the most promising candidates for cold dark matter, and remains to be a well-motivated solution to the CP problem of Quantum Chromodynamics (QCD) via the Peccei-Quinn mechanism. In this paper, we propose a novel method to detect the dark-matter axions in our galaxy via the resonant emission $|{\rm e}\rangle \to |{\rm g}\rangle + γ+ γ^{\prime}_{} + a$ (or absorption $a + |{\rm e}\rangle \to |{\rm g}\rangle + γ+ γ^{\prime}_{}$) in an atomic system with superradiance, where $|{\rm e}\rangle$ and $|{\rm g}\rangle$ stand for the excited and ground energy levels of atoms, respectively. A similar process via $|{\rm e}\rangle \to |{\rm g}\rangle + γ+ a$ (or $a + |{\rm e}\rangle \to |{\rm g}\rangle + γ$) is also put forward to probe the axion-electron coupling. For the nominal experimental setup assuming a background-free environment, most of the parameter space for typical QCD axion models can be covered with parahydrogen molecules or ytterbium atoms. However, the background in a realistic experimental setup remains to be a major issue that needs to be solved in future studies. Searching for better atomic or molecular candidates may be required for a bigger signal-to-noise ratio.

hep-ph↗

arXiv2019

On the Possibility to Determine Neutrino Mass Hierarchy via Supernova Neutrinos with Short-Time Characteristics

In this paper, we investigate whether it is possible to determine the neutrino mass hierarchy via a high-statistics and real-time observation of supernova neutrinos with short-time characteristics. The essential idea is to utilize distinct times-of-flight for different neutrino mass eigenstates from a core-collapse supernova to the Earth, which may significantly change the time distribution of neutrino events in the future huge water-Cherenkov and liquid-scintillator detectors. For illustration, we consider two different scenarios. The first case is the neutronization burst of $ν^{}_e$ emitted in the first tens of milliseconds of a core-collapse supernova, while the second case is the black hole formation during the accretion phase for which neutrino signals are expected to be abruptly terminated. In the latter scenario, it turns out only when the supernova is at a distance of a few Mpc and the fiducial mass of the detector is at the level of gigaton, might we be able to discriminate between normal and inverted neutrino mass hierarchies. In the former scenario, the probability for such a discrimination is even less due to a poor statistics.

hep-ph↗

arXiv2019

Analytical solutions to renormalization-group equations of effective neutrino masses and mixing parameters in matter

Recently, a complete set of differential equations for the effective neutrino masses and mixing parameters in matter have been derived to characterize their evolution with respect to the ordinary matter term $a \equiv 2\sqrt{2}G^{}_{\rm F} N^{}_e E$, in analogy with the renormalization-group equations (RGEs) for running parameters. Via series expansion in terms of the small ratio $α^{}_{\rm c} \equiv Δ^{}_{21}/Δ^{}_{\rm c}$, we obtain approximate analytical solutions to the RGEs of the effective neutrino parameters and make several interesting observations. First, at the leading order, $\widetildeθ^{}_{12}$ and $\widetildeθ^{}_{13}$ are given by the simple formulas in the two-flavor mixing limit, while $\widetildeθ^{}_{23} \approx θ^{}_{23}$ and $\widetildeδ \approx δ$ are not changed by matter effects. Second, the ratio of the matter-corrected Jarlskog invariant $\widetilde{\cal J}$ to its counterpart in vacuum ${\cal J}$ approximates to $\widetilde{\cal J}/{\cal J} \approx 1/(\widehat{C}^{}_{12} \widehat{C}^{}_{13})$, where $\widehat{C}^{}_{12} \equiv \sqrt{1 - 2 A^{}_* \cos 2θ^{}_{12} + A^2_*}$ with $A^{}_* \equiv a/Δ^{}_{21}$ and $\widehat{C}^{}_{13} \equiv \sqrt{1 - 2 A^{}_{\rm c} \cos 2θ^{}_{13} + A^2_{\rm c}}$ with $A^{}_{\rm c} \equiv a/Δ^{}_{\rm c}$. Finally, after taking higher-order corrections into account, we find compact and simple expressions of all the effective parameters.

hep-ph↗

arXiv2019

A model-independent approach to the reconstruction of multi-flavor supernova neutrino energy spectra

The model-independent reconstruction of the energy spectra of $\overlineν^{}_e$, $ν^{}_e$ and $ν^{}_x$ (i.e., $ν^{}_μ$, $ν^{}_τ$ and their antiparticles) from the future observation of a galactic core-collapse supernova (SN) is of crucial importance to understand the microscopic physics of SN explosions. To this end, we propose a practically useful method to combine the multi-channel detection of SN neutrinos in a large liquid-scintillator detector (e.g., JUNO), namely, the inverse beta decay $\overlineν^{}_e + p \to e^+ + n$, the elastic neutrino-proton scattering $ν+ p \to ν+ p$ and the elastic neutrino-electron scattering $ν+ e^- \to ν+ e^-$, and reconstruct the energy spectra of $\overlineν^{}_e$, $ν^{}_e$ and $ν^{}_x$ by making the best use of the observational data in those three channels. In addition, the neutrino energy spectra from the numerical simulations of the delayed neutrino-driven SN explosions are implemented to demonstrate the robustness of our method. Taking the ordinary matter effects into account, we also show how to extract the initial neutrino energy spectra in the presence of neutrino flavor conversions.

hep-ph↗

arXiv2019

Constraining Neutrino Lifetimes and Magnetic Moments via Solar Neutrinos in the Large Xenon Detectors

The multi-ton-scale liquid xenon detectors, with an excellent energy resolution of a few keV, will be constructed to probe the dark-matter particles. In this paper, we show that precision measurements of the low-energy solar neutrinos via the elastic neutrino-electron scattering in this kind of detectors are able to improve the present limits on neutrino lifetimes and neutrino magnetic moments by about one order of magnitude. We carefully study the impact of the unknown neutrino mass spectrum on the ultimate limits in the case of non-radiative visible neutrino decays. In the case of invisible neutrino decays, the lower bounds $τ^{}_1/m^{}_1 \gtrsim 3 \times 10^{-2}~{\rm s}/{\rm eV}$ and $τ^{}_2/m^{}_2 \gtrsim 8\times 10^{-3}~{\rm s}/{\rm eV}$ at the $2σ$ level can be obtained for a total exposure of $70~{\rm ton}\cdot {\rm year}$. Furthermore, a restrictive constraint on the effective magnetic moment of neutrinos $μ^{}_{\rm eff} \lesssim 2.6\times 10^{-12}~μ^{}_{\rm B}$, with $μ^{}_{\rm B}$ being the Bohr magneton, can be achieved. This is among the best results that will be available in the laboratory experiments in the near future.

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arXiv2019

Impact of an eV-mass sterile neutrino on the neutrinoless double-beta decays: a Bayesian analysis

To quantitatively assess the impact of an eV-mass sterile neutrino on the neutrinoless double-beta ($0νββ$) decays, we calculate the posterior probability distribution of the relevant effective neutrino mass $|m^\prime_{ee}|$ in the (3+1)$ν$ mixing scenario, following the Bayesian statistical approach. The latest global-fit analysis of neutrino oscillation data, the cosmological bound on the sum of three active neutrino masses from {\it Planck}, and the constraints from current $0νββ$ decay experiments are taken into account in our calculations. Based on the resultant posterior distributions, we find that the average value of the effective neutrino mass is shifted from $\overline{|m^{}_{ee}|} = 3.37\times 10^{-3}~{\rm eV}$ (or $7.71\times 10^{-3}~{\rm eV}$) in the standard 3$ν$ mixing scenario to $\overline{|m^{\prime}_{ee}|}=2.54\times 10^{-2}~{\rm eV}$ (or $2.56\times 10^{-2}~{\rm eV}$) in the (3+1)$ν$ mixing scenario, with the logarithmically uniform prior on the lightest neutrino mass (or on the sum of three active neutrino masses). Therefore, a null signal from the future $0νββ$ decay experiment with a sensitivity to $|m^{}_{ee}| \approx \mathcal{O}(10^{-2}_{})~{\rm eV}$ will be able to set a very stringent constraint on the sterile neutrino mass and the active-sterile mixing angle.

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arXiv2018

Naumov- and Toshev-like relations in the renormalization-group evolution of quarks and Dirac neutrinos

In an analytical way of studying matter effects on neutrino oscillations, the Naumov and Toshev relations have been derived to respectively link the Jarlskog invariant of CP violation and the Dirac phase in the standard parametrization of the $3\times 3$ flavor mixing matrix to their matter-corrected counterparts. Here we show that there exist similar relations for Dirac neutrinos and charged leptons evolving with energy scales via the one-loop renormalization-group (RG) equations in the tau-dominance approximation, and for the running behaviors of up- and down-type quarks in the top-dominance approximation, provided a different parametrization is taken into account.

hep-ph↗

arXiv2018

Matter effects on the flavor conversions of solar neutrinos and high-energy astrophysical neutrinos

Can we observe the solar eclipses in the neutrino light? In principle, this is possible by identifying the lunar matter effects on the flavor conversions of solar neutrinos when they traverse the Moon before reaching the detectors at the Earth. Unfortunately, we show that the lunar matter effects on the survival probability of solar $^8{\rm B}$ neutrinos are suppressed by an additional factor of $1.2\%$, compared to the day-night asymmetry. However, we point out that the matter effects on the flavor conversions of high-energy astrophysical neutrinos, when they propagate through the Sun, can be significant. Though the flavor composition of high-energy neutrinos can be remarkably modified, it is quite challenging to observe such effects even in the next-generation of neutrino telescopes.

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