arXiv subjects

Zhi-zhong Xing

Publications and source records attributed to Zhi-zhong Xing.

At least 73 records · Page 4Linked to original sources

arXiv2016

A review of mu-tau flavor symmetry in neutrino physics

Behind the observed pattern of lepton flavor mixing is a partial or approximate mu-tau flavor symmetry --- a milestone on our road to the true origin of neutrino masses and flavor structures. In this review article we first describe the features of mu-tau permutation and reflection symmetries, and then explore their various consequences on model building and neutrino phenomenology. We pay particular attention to soft mu-tau symmetry breaking, which is crucial for our deeper understanding of the fine effects of flavor mixing and CP violation.

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arXiv2016

Leptonic Unitarity Triangles and Effective Mass Triangles of the Majorana Neutrinos

Given the best-fit results of six neutrino oscillation parameters, we plot the Dirac and Majorana unitarity triangles (UTs) of the 3\times 3 lepton flavor mixing matrix to show their real shapes for the first time. The connections of the Majorana UTs with neutrino-antineutrino oscillations and neutrino decays are explored, and the possibilities of right or isosceles UTs are discussed. In the neutrino mass limit of m_1 \to 0 or m_3 \to 0, which is allowed by current experimental data, we show how the six triangles formed by the effective Majorana neutrino masses \langle m\rangle_{αβ} (for α, β= e, μ, τ) and their corresponding component vectors look like in the complex plane. The relations of such triangles to the Majorana phases and to the lepton-number-violating decays H^{++} \to α^+ β^+ in the type-II seesaw mechanism are also illustrated.

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arXiv2015

On the four-zero texture of quark mass matrices and its stability

We carry out a new study of quark mass matrices $M^{}_{\rm u}$ (up-type) and $M^{}_{\rm d}$ (down-type) which are Hermitian and have four zero entries, and find a new part of the parameter space which was missed in the previous works. We identify two more specific four-zero patterns of $M^{}_{\rm u}$ and $M^{}_{\rm d}$ with fewer free parameters, and present two toy flavor-symmetry models which can help realize such special and interesting quark flavor structures. We also show that the texture zeros of $M^{}_{\rm u}$ and $M^{}_{\rm d}$ are essentially stable against the evolution of energy scales in an analytical way by using the one-loop renormalization-group equations.

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arXiv2015

How to interpret a discovery or null result of the $0ν2β$ decay

The Majorana nature of massive neutrinos will be crucially probed in the next-generation experiments of the neutrinoless double-beta ($0ν2β$) decay. The effective mass term of this process, $\langle m\rangle^{}_{ee}$, may be contaminated by new physics. So how to interpret a discovery or null result of the $0ν2β$ decay in the foreseeable future is highly nontrivial. In this paper we introduce a novel three-dimensional description of $|\langle m\rangle_{ee}^{}|$, which allows us to see its sensitivity to the lightest neutrino mass and two Majorana phases in a transparent way. We take a look at to what extent the free parameters of $|\langle m\rangle_{ee}^{}|$ can be well constrained provided a signal of the $0ν2β$ decay is observed someday. To fully explore lepton number violation, all the six effective Majorana mass terms $\langle m\rangle_{αβ}^{}$ (for $α, β= e, μ, τ$) are calculated and their lower bounds are illustrated with the two-dimensional contour figures. The effect of possible new physics on the $0ν2β$ decay is also discussed in a model-independent way. We find that the result of $|\langle m\rangle_{ee}^{}|$ in the normal (or inverted) neutrino mass ordering case modified by the new physics effect may somewhat mimic that in the inverted (or normal) mass ordering case in the standard three-flavor scheme. Hence a proper interpretation of a discovery or null result of the $0ν2β$ decay may demand extra information from some other measurements.

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arXiv2015

Neutrino Masses and Flavor Oscillations

This essay is intended to provide a brief description of the peculiar properties of neutrinos within and beyond the standard theory of weak interactions. The focus is on the flavor oscillations of massive neutrinos, from which one has achieved some striking knowledge about their mass spectrum and flavor mixing pattern. The experimental prospects towards probing the absolute neutrino mass scale, possible Majorana nature and CP-violating effects will also be addressed.

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arXiv2014

Quark Mass Hierarchy and Flavor Mixing Puzzles

The fact that quarks of the same electric charge possess a mass hierarchy is a big puzzle in particle physics, and it must be highly correlated with the hierarchy of quark flavor mixing. This review article is intended to provide a brief description of some important issues regarding quark masses, flavor mixing and CP violation. A comparison between the salient features of quark and lepton flavor mixing structures is also made.

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arXiv2014

Resolving the octant of theta_{23} via radiative mu-tau symmetry breaking

We point out that the observed neutrino mixing pattern at low energies is very likely to originate from the 3 times 3 lepton flavor mixing matrix U which possesses the exact mu-tau permutation symmetry |U_{mu i}| = |U_{tau i}| (for i=1,2,3) at a superhigh energy scale Lambda_{mu tau} \sim 10^{14} GeV. The deviation of theta_{23} from 45^\circ and that of delta from 270^\circ in the standard parametrization of U are therefore a natural consequence of small mu-tau symmetry breaking via the renormalization-group equations (RGEs) running from Lambda_{mu tau} down to the electroweak scale Lambda_{EW} \sim 10^2 GeV. In fitting current experimental data we find that the RGE-corrected value of theta_{23} is uniquely correlated with the neutrino mass ordering: theta_{23} \simeq 42.4^\circ reported by Capozzi et al (or theta_{23} \simeq 48.9^\circ reported by Forero et al) at Lambda_{EW} can arise from theta_{23} = 45^\circ at Lambda_{mu tau} in the minimal supersymmetric standard model if the neutrino mass ordering is inverted (or normal). Accordingly, the preliminary best-fit results of delta at Lambda_{EW} can also evolve from delta = 270^\circ at Lambda_{mu tau} no matter whether the massive neutrinos are Dirac or Majorana particles.

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arXiv2014

A partial $μ$-$τ$ symmetry and its prediction for leptonic CP violation

We find that the lepton flavor mixing matrix $U$ should possess a partial $μ$-$τ$ permutation symmetry $|U^{}_{μ1}| = |U^{}_{τ1}|$, and the latter predicts a novel correlation between the Dirac CP-violating phase $δ$ and three flavor mixing angles $θ^{}_{12}$, $θ^{}_{13}$ and $θ^{}_{23}$ in the standard parametrization. Inputting the best-fit values of these angles reported by Capozzi {\it et al}, we obtain the prediction $δ\simeq 255^\circ$ in the normal neutrino mass ordering, which is in good agreement with the best-fit result $δ\simeq 250^\circ$. In this connection the inverted neutrino mass ordering is slightly disfavored. If this partial $μ$-$τ$ symmetry is specified to be $|U^{}_{μ1}| = |U^{}_{τ1}| =1/\sqrt{6}~$, one can reproduce the phenomenologically-favored relation $\sin^2θ^{}_{12} = \left(1 - 2\tan^2θ^{}_{13}\right)/3$ and a viable two-parameter description of $U$ which were first uncovered in 2006. Moreover, we point out that the octant of $θ^{}_{23}$ and the quadrant of $δ$ can be resolved thanks to the slight violation of $|U^{}_{μ2}| = |U^{}_{τ2}|$ and $|U^{}_{μ3}| = |U^{}_{τ3}|$ either at the tree level or from radiative corrections.

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arXiv2014

Geometry of the effective Majorana neutrino mass in the neutrinoless double-beta decay

The neutrinoless double-beta ($0νββ$) decay is a unique process to identify the Majorana nature of massive neutrinos, and its rate depends on the size of the effective Majorana neutrino mass $\langle m\rangle_{ee}$. We put forward a novel "coupling-rod" diagram to describe $\langle m\rangle_{ee}$ in the complex plane, by which the effects of the neutrino mass ordering and CP-violating phases on $\langle m\rangle_{ee}$ are intuitively understood. We show that this geometric language allows us to easily obtain the maximum and minimum of $|\langle m\rangle_{ee}|$. It remains usable even if there is a kind of new physics contributing to $\langle m\rangle_{ee}$, and it can also be extended to describe the effective Majorana masses $\langle m\rangle_{eμ}$, $\langle m\rangle_{eτ}$, $\langle m\rangle_{μμ}$, $\langle m\rangle_{μτ}$ and $\langle m\rangle_{ττ}$ which may appear in some other lepton-number-violating processes.

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arXiv2014

Commutators of lepton mass matrices associated with seesaw and leptogenesis

The origin of tiny neutrino masses and the baryon number asymmetry of the Universe are naturally interpreted by the canonical seesaw and leptogenesis mechanisms, in which there are the heavy Majorana neutrino mass matrix $M_{\rm R}$, the Dirac neutrino mass matrix $M_{\rm D}$, the charged-lepton mass matrix $M_\ell$ and the effective (light) neutrino mass matrix $M_ν$. We find that ${\rm Im}\left(\det\left[ M^\dagger_{\rm D} M_{\rm D}, M^\dagger_{\rm R} M_{\rm R} \right]\right)$, ${\rm Im}\left(\det\left[ M_\ell M^\dagger_\ell, M_νM^\dagger_ν\right]\right)$ and ${\rm Im}\left(\det\left[ M_\ell M^\dagger_\ell, M_{\rm D} M^\dagger_{\rm D} \right]\right)$ can serve for a basis-independent measure of CP violation associated with lepton-number-violating decays of heavy neutrinos, flavor oscillations of light neutrinos and lepton-flavor-violating decays of charged leptons, respectively. We first calculate these quantities with the help of a standard parametrization of the $6\times 6$ flavor mixing matrix, and then discuss their implications on both leptogenesis and CP violation at low energy scales. A comparison with the weak-basis invariants of leptogenesis as proposed by Branco {\it et al} is also made.

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arXiv2013

On the neutrino mass ordering and flavor mixing structure

Whether the neutrino mass spectrum is normal (m_1 < m_2 < m_3) or abnormal (m_3 < m_1 < m_2) remains an open question, but we show that the latter possibility looks quite unnatural when it is related to the lepton flavor mixing matrix U in a way similar to the reasonable correlation between the quark mass spectrum and the quark flavor mixing matrix. Taking into account the freedom in choosing the basis of weak interactions, we make a novel prediction for |U_{tau 1}|/|U_{tau 2}| in terms of the three neutrino masses in the large tau mass limit. This result is testable, and it implies that the normal neutrino mass ordering is more likely to coincide with the observed structure of U.

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arXiv2013

Majorana CP-violating phases in neutrino-antineutrino oscillations and other lepton-number-violating processes

If the massive neutrinos are identified to be the Majorana particles via a convincing measurement of the neutrinoless double beta (0νββ) decay, how to determine the Majorana CP-violating phases in the 3 x 3 lepton flavor mixing matrix U will become a desirable experimental question. The answer to this question is to explore all the possible lepton-number-violating (LNV) processes in which the Majorana phases really matter. In this paper we carry out a systematic study of CP violation in neutrino-antineutrino oscillations, whose CP-conserving parts involve six independent 0νββ-like mass terms _{αβ} and CP-violating parts are associated with nine independent Jarlskog-like parameters V^{ij}_{αβ} (for α, β= e, μ, τand i, j = 1, 2,3). With the help of current neutrino oscillation data, we analyze the sensitivities of | _{αβ}| and V^{ij}_{αβ} to the three CP-violating phases of U, and illustrate the salient features of six independent CP-violating asymmetries between ν_α\to \barν_βand \barν_α\to ν_βoscillations. As a by-product, the effects of the CP-violating phases on the LNV decays of doubly- and singly-charged Higgs bosons are reexamined by taking account of the unsuppressed value of θ_{13}. Such CP-conserving LNV processes can be complementary to the possible measurements of neutrino-antineutrino oscillations in the distant future.

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arXiv2013

Leptonic commutators and clean T violation in neutrino oscillations

A realistic medium- or long-baseline neutrino experiment may suffer from terrestrial matter effects which are likely to contaminate the genuine T-violating asymmetry between ν_α\to ν_βand ν_β\to ν_αoscillations. With the help of the commutators of lepton mass matrices in matter, we show that this kind of contamination is negligible for a variety of experiments provided the neutrino beam energy E and the baseline length L satisfy the condition 10^{-7} (L/km)^2 (GeV/E) << 1. The same observation is true for the CP-violating term of the asymmetry between ν_α\to ν_βand \barν_α\to \barν_βoscillations in matter.

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arXiv2013

Properties of CP Violation in Neutrino-Antineutrino Oscillations

If the massive neutrinos are the Majorana particles, how to pin down the Majorana CP-violating phases will eventually become an unavoidable question relevant to the future neutrino experiments. We argue that a study of neutrino-antineutrino oscillations will greatly help in this regard, although the issue remains purely academic at present. In this work we first derive the probabilities and CP-violating asymmetries of neutrino-antineutrino oscillations in the three-flavor framework, and then illustrate their properties in two special cases: the normal neutrino mass hierarchy with m_1 = 0 and the inverted neutrino mass hierarchy with m_3 = 0. We demonstrate the significant contributions of the Majorana phases to the CP-violating asymmetries, even in the absence of the Dirac phase.

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arXiv2012

Towards testing the unitarity of the 3X3 lepton flavor mixing matrix in a precision reactor antineutrino oscillation experiment

The 3X3 Maki-Nakagawa-Sakata-Pontecorvo (MNSP) lepton flavor mixing matrix may be slightly non-unitary if the three active neutrinos are coupled with sterile neutrinos. We show that it is in principle possible to test whether the relation |V_{e1}|^2 + |V_{e2}|^2 + |V_{e3}|^2 = 1 holds or not in a precision reactor antineutrino oscillation experiment, such as the recently proposed Daya Bay II experiment. We explore three categories of non-unitary effects on the 3X3 MNSP matrix: 1) the indirect effect in the (3+ 3) flavor mixing scenario where the three heavy sterile neutrinos do not take part in neutrino oscillations; 2) the direct effect in the (3+\mathbbm{1}) scenario where the light sterile neutrino can oscillate into the active ones; and 3) the interplay of both of them in the (3+\mathbbm{1}+ 2) scenario. We find that both the zero-distance effect and flavor mixing factors of different oscillation modes can be used to determine or constrain the sum of |V_{e1}|^2, |V_{e2}|^2 and |V_{e3}|^2 and its possible deviation from one, and the active neutrino mixing angles θ_{12} and θ_{13} can be cleanly extracted even in the presence of light or heavy sterile neutrinos. Some useful analytical results are obtained for each of the three scenarios.

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arXiv2012

Model-independent access to the structure of quark flavor mixing

We show that the observed pattern of quark flavor mixing, such as |V_{us}| \simeq |V_{cd}|, |V_{cb}| \simeq |V_{ts}|, |V_{cd}/V_{td}| \simeq |V_{cs}/V_{ts}| \simeq |V_{tb}/V_{cb}| and |V_{ub}/V_{cb}| < |V_{td}/V_{ts}|, can essentially be understood in the chiral and heavy quark mass limits. In particular, the phenomenologically successful relations |V_{ub}/V_{cb}| \sim \sqrt{m_u/m_c} and |V_{td}/V_{ts}| \simeq \sqrt{m_d/m_s} can be reasonably conjectured in the m_b \to \infty and m_t \to \infty limits, respectively. We stress that the strength of weak CP violation in the quark sector is determined by the moduli of the four corner elements of the Cabibbo-Kobayashi-Maskawa matrix. A comparison between strong and weak CP-violating effects in the standard model is also made.

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arXiv2012

Theoretical Overview on the Flavor Issues of Massive Neutrinos

We present an overview on some basic properties of massive neutrinos and focus on their flavor issues, including the mass spectrum, flavor mixing pattern and CP violation. The lepton flavor structures are explored by taking account of the observed value of the smallest neutrino mixing angle θ_{13}. The impact of θ_{13} on the running behaviors of other flavor mixing parameters is discussed in some detail. The seesaw-induced enhancement of the electromagnetic dipole moments for three Majorana neutrinos is also discussed in a TeV seesaw scenario.

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arXiv2012

Impacts of the observed theta_{13} on the running behaviors of Dirac and Majorana neutrino mixing angles and CP-violating phases

The recent observation of the smallest neutrino mixing angle $θ_{13}$ in the Daya Bay and RENO experiments motivates us to examine whether $θ_{13} \simeq 9^\circ$ at the electroweak scale can be generated from $θ_{13} = 0^\circ$ at a superhigh-energy scale via the radiative corrections. We find that it is difficult but not impossible in the minimal supersymmetric standard model (MSSM), and a relatively large $θ_{13}$ may have some nontrivial impacts on the running behaviors of the other two mixing angles and CP-violating phases. In particular, we demonstrate that the CP-violating phases play a crucial role in the evolution of the mixing angles by using the one-loop renormalization-group equations of the Dirac or Majorana neutrinos in the MSSM. We also take the "correlative" neutrino mixing pattern with $θ_{12} \simeq 35.3^\circ$, $θ_{23} = 45^\circ$ and $θ_{13} \simeq 9.7^\circ$ at a presumable flavor symmetry scale as an example to illustrate that the three mixing angles can receive comparably small radiative corrections and thus evolve to their best-fit values at the electroweak scale if the CP-violating phases are properly adjusted.

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