arXiv subjects

Shun Zhou

Publications and source records attributed to Shun Zhou.

At least 91 records · Page 5Linked to original sources

arXiv2018

Renormalization-Group Equations of Neutrino Masses and Flavor Mixing Parameters in Matter

We borrow the general idea of renormalization-group equations (RGEs) to understand how neutrino masses and flavor mixing parameters evolve when neutrinos propagate in a medium, highlighting a meaningful possibility that the genuine flavor quantities in vacuum can be extrapolated from their matter-corrected counterparts to be measured in some realistic neutrino oscillation experiments. Taking the matter parameter $a \equiv 2\sqrt{2} \ G^{}_{\rm F} N^{}_e E$ to be an arbitrary scale-like variable with $N^{}_e$ being the net electron number density and $E$ being the neutrino beam energy, we derive a complete set of differential equations for the effective neutrino mixing matrix $V$ and the effective neutrino masses $\widetilde{m}^{}_i$ (for $i = 1, 2, 3$). Given the standard parametrization of $V$, the RGEs for $\{\widetildeθ^{}_{12}, \widetildeθ^{}_{13}, \widetildeθ^{}_{23}, \widetildeδ\}$ in matter are formulated for the first time. We demonstrate some useful differential invariants which retain the same form from vacuum to matter, including the well-known Naumov and Toshev relations. The RGEs of the partial $μ$-$τ$ asymmetries, the off-diagonal asymmetries and the sides of unitarity triangles of $V$ are also obtained as a by-product.

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arXiv2018

Towards a complete reconstruction of supernova neutrino spectra in future large liquid-scintillator detectors

In this paper, we show how to carry out a relatively more realistic and complete reconstruction of supernova neutrino spectra in the future large liquid-scintillator detectors, by implementing the method of singular value decomposition with a proper regularization. For a core-collapse supernova at a distance of $10~{\rm kpc}$ in the Milky Way, its $\overlineν^{}_e$ spectrum can be precisely determined from the inverse beta-decay process $\overlineν^{}_e + p \to e^+ + n$, for which a $20~{\rm kiloton}$ liquid-scintillator detector with the resolution similar to the Jiangmen Underground Neutrino Observatory (JUNO) may register more than 5000 events. We have to rely predominantly on the elastic neutrino-electron scattering $ν+ e^- \to ν+ e^-$ and the elastic neutrino-proton scattering $ν+ p \to ν+ p$ for the spectra of $ν^{}_e$ and $ν^{}_x$, where $ν$ denotes collectively neutrinos and antineutrinos of all three flavors and $ν^{}_x$ for $ν^{}_μ$ and $ν^{}_τ$ as well as their antiparticles. To demonstrate the validity of our approach, we also attempt to reconstruct the neutrino spectra by using the time-integrated neutrino data from the latest numerical simulations of delayed neutrino-driven supernova explosions.

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arXiv2018

Observational Constraints on Secret Neutrino Interactions from Big Bang Nucleosynthesis

We investigate possible interactions between neutrinos and massive scalar bosons via $g^{}_ϕ \overlineν νϕ$ (or massive vector bosons via $g^{}_V \overlineν γ^μνV^{}_μ$) and explore the allowed parameter space of the coupling constant $g^{}_ϕ$ (or $g^{}_V$) and the scalar (or vector) boson mass $m^{}_ϕ$ (or $m^{}_V$) by requiring that these secret neutrino interactions (SNIs) should not spoil the success of Big Bang nucleosynthesis (BBN). Incorporating the SNIs into the evolution of the early Universe in the BBN era, we numerically solve the Boltzmann equations and compare the predictions for the abundances of light elements with observations. It turns out that the constraint on $g^{}_ϕ$ and $m^{}_ϕ$ in the scalar-boson case is rather weak, due to a small number of degrees of freedom. However, in the vector-boson case, the most stringent bound on the coupling $g^{}_V \lesssim 6\times 10^{-10}$ at $95~\%$ confidence level is obtained for $m^{}_V \simeq 1~{\rm MeV}$, while the bound becomes much weaker $g^{}_V \lesssim 8\times 10^{-6}$ for smaller masses $m^{}_V \lesssim 10^{-4}~{\rm MeV}$. Moreover, we discuss in some detail how the SNIs affect the cosmological evolution and the abundances of the lightest elements.

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arXiv2017

Another look at the impact of an eV-mass sterile neutrino on the effective neutrino mass of neutrinoless double-beta decays

The possible existence of an eV-mass sterile neutrino, slightly mixing with ordinary active neutrinos, is not yet excluded by neutrino oscillation experiments. Assuming neutrinos to be Majorana particles, we explore the impact of such a sterile neutrino on the effective neutrino mass of neutrinoless double-beta decays $\langle m \rangle^\prime_{ee} \equiv m^{}_1 |V^{}_{e1}|^2 e^{{\rm i}ρ} + m^{}_2 |V^{}_{e2}|^2 + m^{}_3 |V^{}_{e3}|^2 e^{{\rm i}σ} + m^{}_4 |V^{}_{e4}|^2 e^{{\rm i}ω}$, where $m^{}_i$ and $V^{}_{ei}$ (for $i = 1, 2, 3, 4$) denote respectively the absolute masses and the first-row elements of the 4$\times$4 neutrino flavor mixing matrix $V$, for which a full parametrization involves three Majorana-type CP-violating phases $\{ρ, σ, ω\}$. A zero effective neutrino mass $|\langle m \rangle^\prime_{ee}| = 0$ is possible no matter whether three active neutrinos take the normal or inverted mass ordering, and its implications for the parameter space are examined in great detail. In particular, given the best-fit values of $m^{}_4 \approx 1.3~{\rm eV}$ and $|V^{}_{e4}|^2 \approx 0.019$ from the latest global analysis of neutrino oscillation data, a three-dimensional view of $|\langle m \rangle^\prime_{ee}|$ in the $(m^{}_1, ρ)$-plane is presented and further compared with that of the counterpart $|\langle m \rangle^{}_{ee}|$ in the absence of any sterile neutrino.

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arXiv2017

Broken $S^{}_{3{\rm L}} \times S^{}_{3{\rm R}}$ Flavor Symmetry and Leptonic CP Violation

In the framework of canonical seesaw model, we present a simple but viable scenario to explicitly break an $S^{}_{3{\rm L}} \times S^{}_{3{\rm R}}$ flavor symmetry in the leptonic sector. It turns out that the leptonic flavor mixing matrix is completely determined by the mass ratios of charged leptons (i.e., $m^{}_e/m^{}_μ$ and $m^{}_μ/m^{}_τ$) and those of light neutrinos (i.e., $m^{}_1/m^{}_2$ and $m^{}_2/m^{}_3$). The latest global-fit results of three neutrino mixing angles $\{θ^{}_{12}, θ^{}_{13}, θ^{}_{23}\}$ and two neutrino mass-squared differences $\{Δm^2_{21}, Δm^2_{31}\}$ at the $3σ$ level are used to constrain the parameter space of $\{m^{}_1/m^{}_2, m^{}_2/m^{}_3\}$. The predictions for the mass spectrum and flavor mixing are highlighted: (1) The neutrino mass spectrum shows a hierarchical pattern and a normal ordering, e.g., $m^{}_1 \approx 2.2~{\rm meV}$, $m^{}_2 \approx 8.8~{\rm meV}$ and $m^{}_3 \approx 52.7~{\rm meV}$; (2) Only the first octant of $θ^{}_{23}$ is allowed, namely, $41.8^\circ \lesssim θ^{}_{23} \lesssim 43.3^\circ$; (3) The Dirac CP-violating phase $δ\approx -22^\circ$ deviates significantly from the maximal value $-90^\circ$. All these predictions are ready to be tested in the ongoing and forthcoming neutrino oscillation experiments. Moreover, we demonstrate that the cosmological matter-antimatter asymmetry can be explained via resonant leptogenesis, including the individual lepton-flavor effects. In our scenario, the leptonic CP violation at low- and high-energy scales are closely connected.

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arXiv2017

Symmetric formulation of neutrino oscillations in matter and its intrinsic connection to renormalization-group equations

In this article, we point out that the effective Hamiltonian for neutrino oscillations in matter is invariant under the transformation of the mixing angle $θ^{}_{12} \to θ^{}_{12} - π/2$ and the exchange of first two neutrino masses $m^{}_1 \leftrightarrow m^{}_2$, if the standard parametrization of lepton flavor mixing matrix is adopted. To maintain this symmetry in perturbative calculations, we present a symmetric formulation of the effective Hamiltonian by introducing an $η$-gauge neutrino mass-squared difference $Δ^{}_* \equiv ηΔ^{}_{31} + (1-η)Δ^{}_{32}$ for $0 \leq η\leq 1$, where $Δ^{}_{ji} \equiv m^2_j - m^2_i$ for $ji = 21, 31, 32$, and show that only $η= 1/2$, $η= \cos^2θ^{}_{12}$ or $η= \sin^2 θ^{}_{12}$ is allowed. Furthermore, we prove that $η= \cos^2 θ^{}_{12}$ is the best choice to derive more accurate and compact neutrino oscillation probabilities, by implementing the approach of renromalization-group equations. The validity of this approach becomes transparent when an analogy is made between the parameter $η$ herein and the renormalization scale $μ$ in relativistic quantum field theories.

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arXiv2016

Overview on Neutrino Theory and Phenomenology

In this talk, I give an overview on recent theoretical and phenomenological studies of massive neutrinos. First of all, the present status of neutrino mixing parameters is summarized. The phenomenology of neutrino oscillations is then discussed, and current understanding of lepton flavor mixing is presented. Finally, I consider the seesaw models of neutrino masses and briefly mention the direct searches at the CERN Large Hadron Collider (LHC).

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arXiv2016

Relic Right-handed Dirac Neutrinos and Cosmic Neutrino Background

The PTOLEMY experiment, implementing a $100~{\rm g}$ surface-deposition tritium target, is promising to detect cosmic neutrino background via $ν^{}_e + {^3}{\rm H} \to {^3}{\rm He} + e^-$. In this talk, I consider a thermal production of right-handed Dirac neutrinos in the early Universe, and investigate their impact on the capture rate of cosmic relic neutrinos at PTOLEMY.

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arXiv2016

Looking into Analytical Approximations for Three-flavor Neutrino Oscillation Probabilities in Matter

Motivated by tremendous progress in neutrino oscillation experiments, we derive a new set of simple and compact formulas for three-flavor neutrino oscillation probabilities in matter of a constant density. A useful definition of the $η$-gauge neutrino mass-squared difference $Δ^{}_* \equiv ηΔ^{}_{31} + (1-η) Δ^{}_{32}$ is introduced, where $Δ^{}_{ji} \equiv m^2_j - m^2_i$ for $ji = 21, 31, 32$ are the ordinary neutrino mass-squared differences and $0 \leq η\leq 1$ is a real and positive parameter. Expanding neutrino oscillation probabilities in terms of $α\equiv Δ^{}_{21}/Δ^{}_*$, we demonstrate that the analytical formulas can be remarkably simplified for $η= \cos^2 θ^{}_{12}$, with $θ_{12}^{}$ being the solar mixing angle. As a by-product, the mapping from neutrino oscillation parameters in vacuum to their counterparts in matter is obtained at the order of ${\cal O}(α^2)$. Finally, we show that our approximate formulas are not only valid for an arbitrary neutrino energy and any baseline length, but also still maintaining a high level of accuracy.

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arXiv2016

Discriminating between Thermal and Nonthermal Cosmic Relic Neutrinos through Annual Modulation at PTOLEMY

If massive neutrinos are Dirac particles, the proposed PTOLEMY experiment will hopefully be able to discover cosmic neutrino background via $ν^{}_e + {^3}{\rm H} \to {^3}{\rm He} + e^-$ with a capture rate of $Γ^{}_{\rm D} \approx 4~{\rm yr}^{-1}$. Recently, it has been pointed out that right-handed components of Dirac neutrinos could also be copiously produced in the early Universe and become an extra thermal or nonthermal ingredient of cosmic relic neutrinos, enhancing the capture rate to $Γ^{}_{\rm D} \approx 5.1~{\rm yr}^{-1}$ or $Γ^{}_{\rm D} \approx 6.1~{\rm yr}^{-1}$. In this work, we investigate the possibility to distinguish between thermal and nonthermal spectra of cosmic relic neutrinos by measuring the annual modulation of the capture rate. For neutrino masses of $0.1~{\rm eV}$, we have found the amplitude of annual modulation in the standard case is ${\cal M} \approx 0.05\%$, which will be increased to $0.1\%$ and $0.15\%$ in the presence of additional thermal and nonthermal right-handed neutrinos, respectively. The future detection of such a modulation will be helpful in understanding the Majorana or Dirac nature of massive neutrinos.

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arXiv2016

Renormalisation Group Corrections to the Littlest Seesaw Model and Maximal Atmospheric Mixing

The Littlest Seesaw (LS) model involves two right-handed neutrinos and a very constrained Dirac neutrino mass matrix, involving one texture zero and two independent Dirac masses, leading to a highly predictive scheme in which all neutrino masses and the entire PMNS matrix is successfully predicted in terms of just two real parameters. We calculate the renormalisation group (RG) corrections to the LS predictions, with and without supersymmetry, including also the threshold effects induced by the decoupling of heavy Majorana neutrinos both analytically and numerically. We find that the predictions for neutrino mixing angles and mass ratios are rather stable under RG corrections. For example we find that the LS model with RG corrections predicts close to maximal atmospheric mixing, $θ_{23}=45^\circ \pm 1^\circ$, in most considered cases, in tension with the latest NOvA results. The techniques used here apply to other seesaw models with a strong normal mass hierarchy.

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arXiv2016

Viability of exact tri-bimaximal, golden-ratio and bimaximal mixing patterns and renormalization-group running effects

In light of the latest neutrino oscillation data, we examine whether the leptonic flavor mixing matrix can take on an exact form of tri-bimaximal (TBM), golden-ratio (GR) or bimaximal (BM) mixing pattern at a superhigh-energy scale, where such a mixing pattern could be realized by a flavor symmetry, and become compatible with experimental data at the low-energy scale. Within the framework of the Minimal Supersymmetric Standard Model (MSSM), the only hope for realizing such a possibility is to count on the corrections from the renomalization-group (RG) running. In this work we focus on these radiative corrections, and fully explore the allowed parameter space for each of these mixing patterns. We find that when the upper bound on the sum of neutrino masses $Σ^{}_ν\equiv m^{}_1 + m^{}_2 + m^{}_3 < 0.23~\text{eV}$ at the $95\%$ confidence level from Planck 2015 is taken into account, none of these mixing patterns can be identified as the leptonic mixing matrix below the seesaw threshold. If this cosmological upper bound on the sum of neutrino masses were relaxed, the TBM and GR mixing patterns would still be compatible with the latest neutrino oscillation data at the $3σ$ level, but not at the $1σ$ level. Even in this case, no such a possibility exists for the BM mixing.

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arXiv2016

Radiative Corrections to the Solar Lepton Mixing Sum Rule

The simple correlation among three lepton flavor mixing angles $(θ^{}_{12}, θ^{}_{13}, θ^{}_{23})$ and the leptonic Dirac CP-violating phase $δ$ is conventionally called a sum rule of lepton flavor mixing, which may be derived from a class of neutrino mass models with flavor symmetries. In this paper, we consider the solar lepton mixing sum rule $θ^{}_{12} \approx θ^ν_{12} + θ^{}_{13} \cos δ$, where $θ^ν_{12}$ stems from a constant mixing pattern in the neutrino sector and takes the value of $θ^ν_{12} = 45^\circ$ for the bi-maximal mixing (BM), $θ^ν_{12} = \tan^{-1}(1/\sqrt{2}) \approx 35.3^\circ$ for the tri-bimaximal mixing (TBM) or $θ^ν_{12} = \tan^{-1}\left[2/(\sqrt{5} + 1)\right] \approx 31.7^\circ$ for the golden-ratio mixing (GR), and investigate the renormalization-group (RG) running effects on lepton flavor mixing parameters when this sum rule is assumed at a superhigh-energy scale. For illustration, we work within the framework of the minimal supersymmetric standard model (MSSM), and implement the Bayesian approach to explore the posterior distribution of $δ$ at the low-energy scale, which becomes quite broad when the RG running effects are significant. Moreover, we also discuss the compatibility of the above three mixing scenarios with current neutrino oscillation data, and observe that radiative corrections can increase such a compatibility for the BM scenario, resulting in a weaker preference for the TBM and GR ones.

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arXiv2016

Majorana Neutrino Masses from Neutrinoless Double-Beta Decays and Lepton-Number-Violating Meson Decays

The Schechter-Valle theorem states that a positive observation of neutrinoless double-beta ($0νββ$) decays implies a finite Majorana mass term for neutrinos when any unlikely fine-tuning or cancellation is absent. In this note, we reexamine the quantitative impact of the Schechter-Valle theorem, and find that current experimental lower limits on the half-lives of $0νββ$-decaying nuclei have placed a restrictive upper bound on the Majorana neutrino mass $|δm^{ee}_ν| < 7.43 \times 10^{-29}~{\rm eV}$ radiatively generated at the four-loop level. Furthermore, we generalize this quantitative analysis of $0νββ$ decays to that of the lepton-number-violating (LNV) meson decays $M^- \to {M^\prime}^+ + \ell^-_α+ \ell^-_β$ (for $α$, $β$ = $e$ or $μ$). Given the present upper limits on these rare LNV decays, we have derived the loop-induced Majorana neutrino masses $|δm^{ee}_ν| < 9.7 \times 10^{-18}~{\rm eV}$, $|δm^{eμ}_ν| < 1.6 \times 10^{-15}~{\rm eV}$ and $|δm^{μμ}_ν| < 1.0 \times 10^{-12}~{\rm eV}$ from $K^- \to π^+ + e^- + e^-$, $K^- \to π^+ + e^- + μ^-$ and $K^- \to π^+ + μ^- + μ^-$, respectively. A partial list of radiative neutrino masses from the LNV decays of $D$, $D_s^{}$ and $B$ mesons is also given.

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arXiv2016

Getting the Most from Detection of Galactic Supernova Neutrinos in Future Large Liquid-Scintillator Detectors

Future large liquid-scintillator detectors can be implemented to observe neutrinos from a core-collapse supernova (SN) in our galaxy in various reaction channels: (1) The inverse beta decay $\overlineν^{}_e + p \to n + e^+$; (2) The elastic neutrino-proton scattering $ν+ p \to ν+ p$; (3) The elastic neutrino-electron scattering $ν+ e^- \to ν+ e^-$; (4) The charged-current $ν^{}_e$ interaction $ν^{}_e + {^{12}}{\rm C} \to e^- + {^{12}}{\rm N}$; (5) The charged-current $\overlineν^{}_e$ interaction $\overlineν^{}_e + {^{12}}{\rm C} \to e^+ + {^{12}}{\rm B}$; (6) The neutral-current interaction $ν+ {^{12}}{\rm C} \to ν+ {^{12}}{\rm C}^*$. The less abundant $^{13}{\rm C}$ atoms in the liquid scintillator are also considered as a target, and both the charged-current interaction $ν^{}_e + {^{13}}{\rm C} \to e^- + {^{13}}{\rm N}$ and the neutral-current interaction $ν+ {^{13}}{\rm C} \to ν+ {^{13}}{\rm C}^*$ are taken into account. In this work, we show for the first time that a global analysis of all these channels at a single {liquid-}scintillator detector, such as Jiangmen Underground Neutrino Observatory (JUNO), is very important to test the average-energy hierarchy of SN neutrinos and how the total energy is partitioned among neutrino flavors. In addition, the dominant channels for reconstructing neutrino spectra and the impact of other channels are discussed in great detail.

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arXiv2016

Improved Statistical Determination of Absolute Neutrino Masses via Radiative Emission of Neutrino Pairs from Atoms

The atomic transition from an excited state $|{\rm e}\rangle$ to the ground state $|{\rm g}\rangle$ by emitting a neutrino pair and a photon, i.e., $|{\rm e}\rangle \to |{\rm g}\rangle + |γ\rangle + |ν^{}_i\rangle + |\barν^{}_j\rangle$ with $i, j = 1, 2, 3$, has been proposed by Yoshimura and his collaborators as an alternative way to determine the absolute scale $m^{}_0$ of neutrino masses. More recently, a statistical analysis of the fine structure of the photon spectrum from this atomic process has been performed [N. Song {\it et al.}, Phys.\ Rev.\ D {\bf 93}, 013020 (2016)] to quantitatively examine the experimental requirements for a realistic determination of absolute neutrino masses. In this paper, we show how to improve the statistical analysis and demonstrate that the previously required detection time can be reduced by one order of magnitude for the case of a $3σ$ determination of $m_0^{} \sim 0.01~\text{eV}$ with an accuracy better than $10\%$. Such an improvement is very encouraging for further investigations on measuring absolute neutrino masses through atomic processes.

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arXiv2016

Electroweak Vacuum Stability and Diphoton Excess at 750 GeV

Recently, both ATLAS and CMS collaborations at the CERN Large Hadron Collider (LHC) have announced their observations of an excess of diphoton events around the invariant mass of $750~{\rm GeV}$ with a local significance of $3.6σ$ and $2.6σ$, respectively. In this paper, we interpret the diphoton excess as the on-shell production of a real singlet scalar in the $pp \to S \to γγ$ channel. To accommodate the observed production rate, we further introduce a vector-like fermion $F$, which is carrying both color and electric charges. The viable regions of model parameters are explored for this simple extension of the Standard Model (SM). Moreover, we revisit the problem of electroweak vacuum stability in the same scenario, and find that the requirement for the electroweak vacuum stability up to high energy scales imposes serious constraints on the Yukawa coupling of the vector-like fermion and the quartic couplings of the SM Higgs boson and the new singlet scalar. Consequently, a successful explanation for the diphoton excess and the absolute stability of electroweak vacuum cannot be achieved simultaneously in this economical setup.

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arXiv2016

Quark Flavor Mixings from Hierarchical Mass Matrices

In this paper, we extend the Fritzsch ansatz of quark mass matrices while retaining their hierarchical structures and show that the main features of the Cabibbo-Kobayashi-Maskawa (CKM) matrix $V$, including $|V^{}_{us}| \simeq |V^{}_{cd}|$, $|V^{}_{cb}| \simeq |V^{}_{ts}|$ and $|V^{}_{ub}|/|V^{}_{cb}| < |V^{}_{td}|/|V^{}_{ts}|$, can be well understood. This agreement is observed especially when the mass matrices have non-vanishing $(1,3)$ and $(3,1)$ off-diagonal elements. The phenomenological consequences of these for the allowed texture content and gross structural features of `hierarchical' quark mass matrices are addressed from a model independent prospective under the assumption of factorizable phases in these. The approximate and analytical expressions of the CKM matrix elements are derived, and a detailed analysis reveals that such structures are in good agreement with the observed quark flavor mixing angles and the CP-violating phase at the $1σ$ level and call upon a further investigation of the realization of these structures from a top-down prospective.

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