arXiv subjects

Zhi-zhong Xing

Publications and source records attributed to Zhi-zhong Xing.

At least 109 records · Page 6Linked to original sources

arXiv2010

Direct Detection of the Cosmic Neutrino Background Including Light Sterile Neutrinos

Current cosmological data drop an interesting hint about the existence of sub-eV sterile neutrinos, which should be a part of the cosmic neutrino background (C$ν$B). We point out that such light sterile neutrinos may leave a distinct imprint on the electron energy spectrum in the capture of relic electron neutrinos by means of radioactive beta-decaying nuclei. We examine possible signals of sterile neutrinos relative to active neutrinos, characterized by their masses and sensitive to their number densities, in the reaction $ν^{}_e + ~^3{\rm H} \to ~^3{\rm He} + e^-$ against the corresponding tritium beta decay. We stress that this kind of direct laboratory detection of the C$ν$B and its sterile component might not be hopeless in the long term.

astro-ph.CO↗

arXiv2010

Broken S_3 Flavor Symmetry of Leptons and Quarks: Mass Spectra and Flavor Mixing Patterns

We apply the discrete S_3 flavor symmetry to both lepton and quark sectors of the standard model extended by introducing one Higgs triplet and realizing the type-II seesaw mechanism for finite neutrino masses. The resultant mass matrices of charged leptons (M_l), neutrinos (M_nu), up-type quarks (M_u) and down-type quarks (M_d) have a universal form consisting of two terms: one is proportional to the identity matrix I and the other is proportional to the democracy matrix D. We argue that the textures of M_l, M_u and M_d are dominated by the D term, while that of M_nu is dominated by the I term. This hypothesis implies a near mass degeneracy of three neutrinos and can naturally explain why the mass matrices of charged fermions are strongly hierarchical, why the quark mixing matrix is close to I and why the lepton mixing matrix contains two large angles. We discuss a rather simple perturbation ansatz to break the S_3 symmetry and obtain more realistic mass spectra of leptons and quarks as well as their flavor mixing patterns. We stress that the I term, which used to be ignored from M_l, M_u and M_d, is actually important because it can significantly modify the smallest lepton flavor mixing angle theta_13 or three quark flavor mixing angles.

hep-ph↗

arXiv2010

Radiative Corrections to the CKM Unitarity Triangles

The 3 X 3 CKM matrix defines six unitarity triangles in the complex plane, which will be carefully explored in the LHCb experiment and at the Super-B factory. We calculate the running effects of nine different inner angles and eighteen different sides of the six triangles from the electroweak scale to a superhigh-energy scale by using the one-loop renormalization-group equations, and demonstrate that all the nine angles are stable against radiative corrections. In particular, we find that the CP-violating angle αis most insensitive to the changes of energy scales.

hep-ph↗

arXiv2010

Casas-Ibarra Parametrization and Unflavored Leptogenesis

The Casas-Ibarra parametrization is a description of the Dirac neutrino mass matrix M_D in terms of the neutrino mixing matrix V, an orthogonal matrix O and the diagonal mass matrices of light and heavy Majorana neutrinos in the type-I seesaw mechanism. Because M^\dagger_D M_D is apparently independent of V but dependent on O in this parametrization, a number of authors have taken it for grant that unflavored leptogenesis has nothing to do with CP violation at low energies. Here we question this observation by clarifying the physical meaning of O. We establish a clear relationship between O and the observable quantities, and find that O does depend on V. Our general conclusion is that both unflavored leptogenesis and flavored leptogenesis have no direct connection with low-energy CP violation.

hep-ph↗

arXiv2009

Relating the neutrino mixing angles to a lepton mass hierarchy

We propose two phenomenological scenarios of lepton mass matrices and show that either of them can exactly give rise to \tan^2θ_{13} = m_e/(m_e + 2m_μ), \tan^2θ_{23} = m_μ/(m_e + m_μ) and \tan^2θ_{12} = (m_e m_2 + 2m_μm_1)/(m_e m_1 + 2m_μm_2) in the standard parametrization of lepton flavor mixing. The third relation, together with current experimental data, predicts a normal but weak hierarchy for the neutrino mass spectrum. We also obtain θ_{13} \approx 2.8^\circ for the smallest neutrino mixing angle and J \approx 1.1% for the Jarlskog invariant of leptonic CP violation, which will soon be tested in the long-baseline reactor and accelerator neutrino oscillation experiments. A seesaw realization of both scenarios is briefly discussed.

hep-ph↗

arXiv2009

A Test of CPT Symmetry in K^0 vs \bar{K}^0 to π^+π^-π^0 Decays

I show that the CP-violating asymmetry in K^0 vs \bar{K}^0 \to π^+π^-π^0 decays differs from that in K_L \to π^+π^-, K_L \to π^0π^0 or the semileptonic K_L transitions, if there exists CPT violation in K^0-\bar{K}^0 mixing. A delicate measurement of this difference at a super flavor factory (e.g., the ϕfactory) will provide us with a robust test of CPT symmetry in the neutral kaon system.

hep-ph↗

arXiv2009

Friedberg-Lee symmetry and tri-bimaximal neutrino mixing in the inverse seesaw mechanism

The inverse seesaw mechanism with three pairs of gauge-singlet neutrinos offers a natural interpretation of the tiny masses of three active neutrinos at the TeV scale. We combine this picture with the newly-proposed Friedberg-Lee (FL) symmetry in order to understand the observed pattern of neutrino mixing. We show that the FL symmetry requires only two pairs of the gauge-singlet neutrinos to be massive, implying that one active neutrino must be massless. We propose a phenomenological ansatz with broken FL symmetry and exact μ-τsymmetry in the gauge-singlet neutrino sector and obtain the tri-bimaximal neutrino mixing pattern by means of the inverse seesaw relation. We demonstrate that non-unitary corrections to this result are possible to reach the percent level and a soft breaking of μ-τsymmetry can give rise to CP violation in such a TeV-scale seesaw scenario.

hep-ph↗

arXiv2009

Non-unitary neutrino mixing and CP violation in the minimal inverse seesaw model

We propose a simplified version of the inverse seesaw model, in which only two pairs of the gauge-singlet neutrinos are introduced, to interpret the observed neutrino mass hierarchy and lepton flavor mixing at or below the TeV scale. This minimal inverse seesaw scenario (MISS) is technically natural and experimentally testable. In particular, we show that the effective parameters describing the non-unitary neutrino mixing matrix are strongly correlated in the MISS, and thus, their upper bounds can be constrained by current experimental data in a more restrictive way. The Jarlskog invariants of non-unitary CP violation are calculated, and the discovery potential of such new CP-violating effects in the near detector of a neutrino factory is discussed.

hep-ph↗

arXiv2009

Multiple seesaw mechanisms of neutrino masses at the TeV scale

In pursuit of a balance between theoretical naturalness and experimental testability, we propose two classes of multiple seesaw mechanisms at the TeV scale to understand the origin of tiny neutrino masses. They are novel extensions of the canonical and double seesaw mechanisms, respectively, by introducing even and odd numbers of gauge-singlet fermions and scalars. It is thanks to a proper implementation of the global U(1)xZ_2N symmetry that the overall neutrino mass matrix in either class has a suggestive nearest-neighbor-interaction pattern. We briefly discuss possible consequences of these TeV-scale seesaw scenarios, which can hopefully be explored in the upcoming Large Hadron Collider and precision neutrino experiments, and present a simple but instructive example of model building.

hep-ph↗

arXiv2009

Low-energy limits on heavy Majorana neutrino masses from the neutrinoless double-beta decay and non-unitary neutrino mixing

In the type-I seesaw mechanism, both the light Majorana neutrinos (ν_1, ν_2, ν_3) and the heavy Majorana neutrinos (N_1, ..., N_n) can mediate the neutrinoless double-beta (0νββ) decay. We point out that the contribution of ν_i to this 0νββprocess is also dependent on the masses M_k and the mixing parameters R_{ek} of N_k as a direct consequence of the exact seesaw relation, and the effective mass term of ν_i is in most cases dominant over that of N_i. We obtain a new bound |\sum R^2_{ek} M_k| < 0.23 eV (or < 0.85 eV as a more conservative limit) at the 2σlevel, which is much stronger than |\sum R^2_{ek} M^{-1}_k| < 5 \times 10^{-8} GeV^{-1} used in some literature, from current experimental constraints on the 0νββdecay. Taking the minimal type-I seesaw scenario for example, we illustrate the possibility of determining or constraining two heavy Majorana neutrino masses by using more accurate low-energy data on lepton number violation and non-unitarity of neutrino mixing.

hep-ph↗

arXiv2009

Right Unitarity Triangles, Stable CP-violating Phases and Approximate Quark-Lepton Complementarity

Current experimental data indicate that two unitarity triangles of the CKM quark flavor mixing matrix V are almost the right triangles with α\approx 90^\circ. We find that α= 90^\circ actually points to {\rm Re}(V_{tb}V_{ud}V^*_{td}V^*_{ub}) = 0. Considering a very suggestive parametrization of V, we show that its CP-violating phase ϕis nearly equal to α(i.e., ϕ- α\approx 1.1^\circ). Both ϕand αare stable against the renormalizaton-group evolution from the electroweak scale M_Z to a superhigh energy scale M_X or vice versa, and thus it is impossible to obtain α= 90^\circ at M_Z from ϕ= 90^\circ at M_X. We conjecture that there might also exist a maximal CP-violating phase ϕ\approx 90^\circ in the MNS lepton flavor mixing matrix U. The approximate quark-lepton complementarity relations, which hold in the standard parametrizations of V and U, can also hold in our particular parametrizations of V and U simply due to the smallness of |V_{ub}| and |V_{e3}|.

hep-ph↗

arXiv2009

Naturalness and Testability of TeV Seesaw Mechanisms

After outlining some popular ways to go beyond the standard model so as to generate non-zero but tiny neutrino masses, I focus on several typical seesaw mechanisms and discuss how to get a balance between their theoretical naturalness and their experimental testability. Besides possible collider signatures at the Large Hadron Collider, new and non-unitary CP-violating effects are also expected to show up in neutrino oscillations for type-I, type-(I+II), type-III and double seesaws at the TeV scale.

hep-ph↗

arXiv2009

Theoretical Overview of Neutrino Properties

I give an overview of some basic properties of massive neutrinos. The first part of this talk is devoted to three fundamental questions about three known neutrinos and to their flavor issues -- the mass spectrum, mixing pattern and CP violation. The second part of this talk is to highlight a few hot topics at the frontiers of neutrino physics and neutrino astrophysics, including the naturalness and testability of TeV seesaw mechanisms at the LHC, effects of non-standard interactions on neutrino oscillations, flavor distributions of ultrahigh-energy cosmic neutrinos at neutrino telescopes, collective flavor oscillations of supernova neutrinos, flavor effects in thermal leptogenesis, the GSI anomaly and Moessbauer neutrino oscillations, and so on. I finally make some concluding remarks for the road ahead.

hep-ph↗

arXiv2009

Tev Neutrino Physics at the Large Hadron Collider

I argue that TeV neutrino physics might become an exciting frontier of particle physics in the era of the Large Hadron Collider (LHC). The origin of non-zero but tiny masses of three known neutrinos is probably related to the existence of some heavy degrees of freedom, such as heavy Majorana neutrinos or heavy Higgs bosons, via a TeV-scale seesaw mechanism. I take a few examples to illustrate how to get a balance between theoretical naturalness and experimental testability of TeV seesaws. Besides possible collider signatures at the LHC, new and non-unitary CP-violating effects are also expected to show up in neutrino oscillations for type-I, type-(I+II) and type-III seesaws at the TeV scale.

hep-ph↗

arXiv2008

Lepton-number-violating decays of singly-charged Higgs bosons in the minimal type-(I+II) seesaw model at the TeV scale

The lepton-number-violating decays of singly-charged Higgs bosons H^\pm are investigated in the minimal type-(I+II) seesaw model with one SU(2)_L Higgs triplet Δand one heavy Majorana neutrino N_1 at the TeV scale. We find that the branching ratios {\cal B}(H^+ \to l^+_α{\bar ν}) (for α= e, μ, τ) depend not only on the mass and mixing parameters of three light neutrinos ν_i (for i = 1, 2, 3) but also on those of N_1. Assuming the mass of N_1 to lie in the range of 200 GeV to 1 TeV, we figure out the generous interference bands for the contributions of ν_i and N_1 to {\cal B}(H^+ \to l^+_α{\bar ν}). We illustrate some salient features of such interference effects by considering three typical mass patterns of ν_i, and show that the relevant Majorana CP-violating phases can affect the magnitudes of {\cal B}(H^+ \to l^+_α{\bar ν}) in this parameter region.

hep-ph↗

arXiv2008

Generic Friedberg-Lee Symmetry of Dirac Neutrinos

We write out the generic Dirac neutrino mass operator which possesses the Friedberg-Lee (FL) symmetry and find that its corresponding neutrino mass matrix is asymmetric. Following a simple way to break the FL symmetry, we calculate the neutrino mass eigenvalues and show that the resultant neutrino mixing pattern is nearly tri-bimaximal. Imposing the Hermitian condition on the neutrino mass matrix, we also show that the simplified ansatz is consistent with current experimental data and favors the normal neutrino mass hierarchy.

hep-ph↗

arXiv2008

Correlative signatures of heavy Majorana neutrinos and doubly-charged Higgs bosons at the Large Hadron Collider

We explore an intriguing possibility to test the type-II seesaw mechanism of neutrino mass generation at the Large Hadron Collider (LHC). We show that the lepton-number-violating signatures of heavy Majorana neutrinos N_i (for i=1, 2, 3) and doubly-charged Higgs bosons H^++(H^--) can be closely correlated with each other in a class of TeV-scale type-II seesaw models. Taking the minimal version of such models for example, we calculate the cross sections of pp -> l^+_αl^+_βX (l^-_αl^-_βX) processes mediated separately by N_1 and H^++(H^--), and illustrate their nontrivial correlation at the LHC.

hep-ph↗

arXiv2008

Implications of Leptonic Unitarity Violation at Neutrino Telescopes

A measurement of the ultrahigh-energy (UHE) cosmic neutrinos at a km^3-size neutrino telescope will open a new window to constrain the 3 x 3 neutrino mixing matrix V and probe possible new physics. We point out that it is in principle possible to examine the non-unitarity of V, which is naturally expected in a class of seesaw models with one or more TeV-scale Majorana neutrinos, by using neutrino telescopes. Considering the UHE neutrinos produced from the decays of charged pions arising from proton-proton and (or) proton-gamma collisions at a distant astrophysical source, we show that their flavor ratios at a terrestrial neutrino telescope may deviate from the democratic flavor distribution ϕ^T_e : ϕ^T_μ: ϕ^T_τ= 1 : 1 : 1 due to the seesaw-induced unitarity violation of V. Its effect can be as large as several percent and can serve for an illustration of how sensitive a neutrino telescope should be to this kind of new physics.

hep-ph↗