arXiv subjects

Zhi-zhong Xing

Publications and source records attributed to Zhi-zhong Xing.

At least 145 records · Page 8Linked to original sources

arXiv2006

Neutrino Masses, Lepton Flavor Mixing and Leptogenesis in the Minimal Seesaw Model

We present a review of neutrino phenomenology in the minimal seesaw model (MSM), an economical and intriguing extension of the Standard Model with only two heavy right-handed Majorana neutrinos. Given current neutrino oscillation data, the MSM can predict the neutrino mass spectrum and constrain the effective masses of the tritium beta decay and the neutrinoless double-beta decay. We outline five distinct schemes to parameterize the neutrino Yukawa-coupling matrix of the MSM. The lepton flavor mixing and baryogenesis via leptogenesis are investigated in some detail by taking account of possible texture zeros of the Dirac neutrino mass matrix. We derive an upper bound on the CP-violating asymmetry in the decay of the lighter right-handed Majorana neutrino. The effects of the renormalization-group evolution on the neutrino mixing parameters are analyzed, and the correlation between the CP-violating phenomena at low and high energies is highlighted. We show that the observed matter-antimatter asymmetry of the Universe can naturally be interpreted through the resonant leptogenesis mechanism at the TeV scale. The lepton-flavor-violating rare decays, such as $μ\to e + γ$, are also discussed in the supersymmetric extension of the MSM.

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arXiv2006

On the Fukugita-Tanimoto-Yanagida Ansatz with Partially Non-degenerate Right-handed Majorana Neutrinos

Taking three right-handed Majorana neutrino masses $M^{}_i$ to be partially non-degenerate, we make a new analysis of the Fukugita-Tanimoto-Yanagida ansatz and confront it with current neutrino oscillation data. We determine the parameter space for cases (A) $M^{}_3 = M^{}_2 \neq M^{}_1$, (B) $M^{}_2 = M^{}_1 \neq M^{}_3$ and (C) $M^{}_1 = M^{}_3 \neq M^{}_2$, and examine their respective deviations from the original $M^{}_1 = M^{}_2 = M^{}_3$ case. The numerical constraints on three light neutrino masses, three neutrino mixing angles and three CP-violating phases are also obtained, together with the predictions for the Jarlskog invariant of CP violation and the effective masses of the tritium beta decay and the neutrinoless double-beta decay.

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arXiv2006

Nearly Tri-bimaximal Neutrino Mixing and CP Violation from mu-tau Symmetry Breaking

Assuming the Majorana nature of massive neutrinos, we generalize the Friedberg-Lee neutrino mass model to include CP violation in the neutrino mass matrix $M^{}_ν$. We show that a favorable neutrino mixing pattern (with $θ^{}_{12} \approx 35.3^\circ$, $θ^{}_{23} = 45^\circ$, $θ^{}_{13} \neq 0^\circ$ and $δ= 90^\circ$) can naturally be derived from $M^{}_ν$, if it has an approximate or softly-broken $μ$-$τ$ symmetry. We point out a different way to obtain the nearly tri-bimaximal neutrino mixing with $δ= 0^\circ$ and non-vanishing Majorana phases. The most general case, in which all the free parameters of $M^{}_ν$ are complex and the resultant neutrino mixing matrix contains both Dirac and Majorana phases of CP violation, is also discussed.

hep-ph↗

arXiv2006

Neutrino Telescopes as a Probe of Broken $μ$-$τ$ Symmetry

It is known that neutrino oscillations may map $ϕ_e : ϕ_μ: ϕ_τ= 1 : 2 : 0$, the initial flavor ratios of ultrahigh-energy neutrino fluxes produced from a distant astrophysical source, into $ϕ^D_e : ϕ^D_μ: ϕ^D_τ= 1 : 1 : 1$ at the detector of a neutrino telescope. We remark that this naive expectation is only valid in the $μ$-$τ$ symmetry limit, in which two neutrino mixing angles satisfy $θ_{13} = 0$ and $θ_{23} = π/4$. Allowing for the slight breaking of $μ$-$τ$ symmetry, we find $ϕ^D_e : ϕ^D_μ: ϕ^D_τ= (1 -2 Δ) : (1 +Δ) : (1 +Δ)$ with $Δ$ characterizing the combined effect of $θ_{13} \neq 0$ and $θ_{23} \neq π/4$. Current neutrino oscillation data indicate $-0.1 \lesssim Δ\lesssim +0.1$. We also look at the possibility to probe $Δ$ by detecting the $\barν_e$ flux of $E_{\barν_e} \approx 6.3 {\rm PeV}$ via the Glashow resonance channel $\barν_e e \to W^- \to anything$.

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arXiv2006

Towards Determination of the Initial Flavor Composition of Ultrahigh-energy Neutrino Fluxes with Neutrino Telescopes

We propose a simple but useful parametrization of the flavor composition of ultrahigh-energy neutrino fluxes produced from distant astrophysical sources: $ϕ^{}_e : ϕ^{}_μ: ϕ^{}_τ= \sin^2 ξ\cos^2 ζ: \cos^2 ξ\cos^2 ζ: \sin^2 ζ$. We show that it is possible to determine or constrain $ξ$ and $ζ$ by observing two independent neutrino flux ratios at the second-generation neutrino telescopes, provided three neutrino mixing angles and the Dirac CP-violating phase have been well measured in neutrino oscillations. Any deviation of $ζ$ from zero will signify the existence of cosmic $ν^{}_τ$ and $\barν^{}_τ$ neutrinos at the source, and an accurate value of $ξ$ can be used to test both the conventional mechanism and the postulated scenarios for cosmic neutrino production.

astro-ph↗

arXiv2006

On the Quasi-fixed Point in the Running of CP-violating Phases of Majorana Neutrinos

Taking the standard parametrization of three-flavor neutrino mixing, we carefully examine the evolution of three CP-violating phases $(δ, α^{}_1, α^{}_2)$ with energy scales in the realistic limit $θ^{}_{13} \to 0$. If $m^{}_3$ vanishes, we find that the one-loop renormalization-group equation (RGE) of $δ$ does not diverge and its running has no quasi-fixed point. When $m^{}_3 \neq 0$ holds, we show that the continuity condition derived by Antusch {\it et al} is always valid, no matter whether the $τ$-dominance approximation is taken or not. The RGE running of $δ$ undergoes a quasi-fixed point determined by a nontrivial input of $α^{}_2$ in the limit $m^{}_1 \to 0$. If three neutrino masses are nearly degenerate, it is also possible to arrive at a quasi-fixed point in the RGE evolution of $δ$ from the electroweak scale to the seesaw scale or vice versa. Furthermore, the continuity condition and the quasi-fixed point of CP-violating phases in another useful parametrization are briefly discussed.

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arXiv2006

On the Koide-like Relations for the Running Masses of Charged Leptons, Neutrinos and Quarks

Current experimental data indicate that the Koide relation for the pole masses of charged leptons, which can be parametrized as $Q^{pole}_l = 2/3$, is valid up to the accuracy of $O(10^{-5})$. We show that the running masses of charged leptons fail in satisfying the Koide relation (i.e., $Q_l(μ) \neq 2/3$), but the discrepancy between $Q_l (μ)$ and $Q^{pole}_l$ is only about 0.2% at $μ=M_Z$. The Koide-like relations for the running masses of neutrinos ($1/3 < Q_ν(M_Z) < 0.6$), up-type quarks ($Q_{U}(M_Z) \sim 0.89$) and down-type quarks ($Q_{D}(M_Z) \sim 0.74$) are also examined from $M_Z$ up to the typical seesaw scale $M_R \sim 10^{14}$ GeV, and they are found to be nearly stable against radiative corrections. The approximate stability of $Q_{U}(μ)$ and $Q_{D}(μ)$ is mainly attributed to the strong mass hierarchy of quarks, while that of $Q_l(μ)$ and $Q_ν(μ)$ is essentially for the reason that the lepton mass ratios are rather insensitive to radiative corrections.

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arXiv2006

Lepton Mass Hierarchy and Neutrino Mixing

We speculate that the mass spectrum of three neutrinos might have a normal hierarchy as that of three charged leptons or that of three up-type (or down-type) quarks. In this spirit, we propose a novel parametrization of the $3\times 3$ lepton flavor mixing matrix. Its mixing angles $θ_l$ and $θ_ν$ can be related to the mass ratios $m_e/m_μ$ and $m_1/m_2$ in a specific texture of lepton mass matrices with vanishing (1,1) elements: $\tanθ_l = \sqrt{m_e/m_μ}$ and $\tanθ_ν= \sqrt{m_1/m_2}$. The latter relation, together with solar and atmospheric neutrino oscillation data, predicts 0.0030 eV $\lesssim m_1 \lesssim$ 0.0073 eV, 0.009 eV $\lesssim m_2 \lesssim $ 0.012 eV and 0.042 eV $\lesssim m_3 \lesssim$ 0.058 eV. The smallest neutrino mixing angle is found to be $θ_{13} \approx θ_l/\sqrt{2} \approx 3^\circ$, which is experimentally accessible in the near future.

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arXiv2006

Distinguishable RGE running effects between Dirac neutrinos and Majorana neutrinos with vanishing Majorana CP-violating phases

In a novel parametrization of neutrino mixing and in the approximation of $τ$-lepton dominance, we show that the one-loop renormalization-group equations (RGEs) of Dirac neutrinos are different from those of Majorana neutrinos even if two Majorana CP-violating phases vanish. As the latter can keep vanishing from the electroweak scale to the typical seesaw scale, it makes sense to distinguish between the RGE running effects of neutrino mixing parameters in Dirac and Majorana cases. The differences are found to be quite large in the minimal supersymmetric standard model with sizable $\tanβ$, provided the masses of three neutrinos are nearly degenerate or have an inverted hierarchy.

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arXiv2005

Why is the $3\times 3$ neutrino mixing matrix almost unitary in realistic seesaw models?

A simple extension of the standard model is to introduce $n$ heavy right-handed Majorana neutrinos and preserve its $\rm SU(2)^{}_L \times U(1)^{}_Y$ gauge symmetry. Diagonalizing the $(3+n) \times (3+n)$ neutrino mass matrix, we obtain an exact analytical expression for the effective mass matrix of $ν^{}_e$, $ν^{}_μ$ and $ν^{}_τ$. It turns out that the $3\times 3$ neutrino mixing matrix $V$, which appears in the leptonic charged-current weak interactions, must not be exactly unitary. The unitarity violation of $V$ is negligibly tiny, however, if the canonical seesaw mechanism works to reproduce the correct mass scale of light Majorana neutrinos. A similar conclusion can be drawn in the realistic Type-II seesaw models.

hep-ph↗

arXiv2005

Possible Deviation from the Tri-bimaximal Neutrino Mixing in a Seesaw Model

We propose a simple but suggestive seesaw model with two phenomenological conjectures: three heavy (right-handed) Majorana neutrinos are degenerate in mass in the symmetry limit and three light Majorana neutrinos have the tri-bimaximal mixing pattern $V^{}_0$. We show that a small mass splitting between the first generation and the other two generations of heavy Majorana neutrinos is responsible for the deviation of the solar neutrino mixing angle $θ^{}_{12}$ from its initial value $35.3^\circ$ given by $V^{}_0$, and the slight breaking of the mass degeneracy between the second and third generations of heavy Majorana neutrinos results in a small mixing angle $θ^{}_{13}$ and a tiny departure of the atmospheric neutrino mixing angle $θ^{}_{23}$ from $45^\circ$. It turns out that a normal hierarchy of the light neutrino mass spectrum is favored in this seesaw scenario.

hep-ph↗

arXiv2005

Running CP-violating Phases of Majorana Neutrinos

Three CP-violating phases of the $3\times 3$ lepton flavor mixing matrix $V$, denoted as $δ$, $ρ$ and $σ$ in a useful phase convention, are entangled with one another in the one-loop renormalization-group equations. We show that $δ=90^\circ$ at the electroweak scale ($Λ_{\rm EW} \sim 10^{2}$ GeV) can be radiatively generated from $δ=0^\circ$ at the seesaw scale ($Λ_{\rm SS} \sim 10^{14}$ GeV) in the minimal supersymmetric standard model, provided three neutrino masses are nearly degenerate. As for the Majorana phases $ρ$ and $σ$, it is also possible to radiatively generate $ρ=90^\circ$ or $σ= 90^\circ$ at $Λ_{\rm EW}$ from $ρ=0^\circ$ or $σ= 0^\circ$ at $Λ_{\rm SS}$. This observation opens a new and interesting window for model building, in order to understand possible connection between the phenomena of CP violation at low- and high-energy scales.

hep-ph↗

arXiv2005

A Novel Parametrization of tau-lepton Dominance and Simplified One-loop Renormalization-group Equations of Neutrino Mixing Angles and CP-violating Phases

We point out that the $τ$-lepton dominance in the one-loop renormalization-group (RG) equations of neutrino mixing quantities allows us to set a criterion to choose the most suitable parametrization of the lepton flavor mixing matrix $U$: its elements $U^{}_{3i}$ (for $i=1,2,3$) should be as simple as possible. Such a novel parametrization is quite different from the "standard" one used in the literature and can lead to greatly simplified RG equations for three mixing angles and the physical CP-violating phase(s) in the standard model or its minimal supersymmetric extension, no matter whether neutrinos are Dirac or Majorana particles. Some important features of our analytical results and their phenomenological consequences are also discussed.

hep-ph↗

arXiv2005

Generalized Tri-bimaximal Neutrino Mixing and Its Sensitivity to Radiative Corrections

We argue that the tri-bimaximal neutrino mixing pattern $V_0$ or its generalized form $V'_0$, which includes two arbitrary Majorana phases of CP violation, may result from an underlying flavor symmetry at a superhigh energy scale close to the seesaw scale ($\sim 10^{14}$ GeV). Taking the working assumption that three neutrino masses are nearly degenerate, we calculate radiative corrections to $V_0$ and $V'_0$ in their evolution down to the electroweak scale ($\sim 10^2$ GeV). Three mixing angles of $V_0$ or $V'_0$ are essentially stable against radiative corrections in the standard model (SM). In the minimal supersymmetric standard model (MSSM), however, $V_0$ is in general disfavored and $V'_0$ can be compatible with current neutrino oscillation data if its two Majorana phases $α^{}_1$ and $α^{}_2$ are properly fine-tuned. We also find that it is possible to radiatively generate the CP-violating phase $δ$ from $α^{}_1$ and $α^{}_2$, and δmay keep on staying at its fixed point in either the SM or the MSSM.

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arXiv2005

Radiative Generation of Leptonic CP Violation

Three CP-violating phases of the $3\times 3$ lepton flavor mixing matrix $V$ are entangled with one another in the renormalization-group evolution from the seesaw scale ($Λ_{\rm SS} \sim 10^{14}$ GeV) to the electroweak scale ($Λ_{\rm EW} \sim 10^2$ GeV). Concerning the Dirac phase $δ$, we show that $δ=90^\circ$ at $Λ_{\rm EW}$ can be radiatively generated from $δ=0^\circ$ at $Λ_{\rm SS}$ in the minimal supersymmetric standard model, if three neutrino masses are nearly degenerate. As for the Majorana phases $ρ$ and $σ$, it is also possible to radiatively generate $ρ=90^\circ$ or $σ= 90^\circ$ at $Λ_{\rm EW}$ from $ρ=0^\circ$ or $σ= 0^\circ$ at $Λ_{\rm SS}$. The one-loop renormalization-group equations for the Jarlskog invariant and two off-diagonal asymmetries of $V$ are derived, and their running behaviors from $Λ_{\rm SS}$ to $Λ_{\rm EW}$ are numerically illustrated.

hep-ph↗

arXiv2005

Radiative Corrections to Democratic Lepton Mixing

A new ansatz of democratic lepton mixing is proposed at the GUT scale and the radiative corrections to its phenomenological consequences are calculated at the electroweak scale. We demonstrate that it is possible to obtain the experimentally favored results for both neutrino masses and lepton flavor mixing angles from this ansatz, provided the neutrino Yukawa coupling matrix takes a specific nontrivial pattern. The seesaw threshold effects play a significant role in the running of relevant physical quantities.

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arXiv2005

Nontrivial Correlation between the CKM and MNS Matrices

We point out that the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix $V_{\rm CKM}$ and the Maki-Nakagawa-Sakata (MNS) lepton mixing matrix $V_{\rm MNS}$ can naturally be correlated in a class of seesaw models with grand unification, but the texture of their correlation matrix ${\cal F}_ν$ is rather nontrivial. The bimaximal mixing pattern of ${\cal F}_ν$ is disfavored by current data, and other special forms of ${\cal F}_ν$ may suffer from fine-tuning of the free phase parameters in fitting the so-called quark-lepton complementarity relation. A straightforward calculation of ${\cal F}_ν$ in terms of $V_{\rm CKM}$ and $V_{\rm MNS}$ reveals a striking feature of ${\cal F}_ν$: its (1,3) element cannot be zero or too small, no matter whether the (1,3) elements of $V_{\rm CKM}$ and $V_{\rm MNS}$ are vanishing or not. We also add some brief comments on possible radiative corrections to $V_{\rm CKM}$ and $V_{\rm MNS}$.

hep-ph↗

arXiv2005

Reconstruction of the Neutrino Mixing Matrix and Leptonic Unitarity Triangles from Long-baseline Neutrino Oscillations

We derive a new set of sum rules between the neutrino mass and mixing parameters in vacuum and their effective counterparts in matter. The moduli of nine genuine lepton mixing matrix elements can then be calculated in terms of the matter-corrected ones, and the latter can directly be determined from a variety of long-baseline neutrino oscillations. We show that it is possible to reconstruct the leptonic unitarity triangles and CP violation in a similar parametrization-independent way.

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