arXiv subjects

Zhi-zhong Xing

Publications and source records attributed to Zhi-zhong Xing.

At least 91 records · Page 5Linked to original sources

arXiv2012

A further study of μ-τsymmetry breaking at neutrino telescopes after the Daya Bay and RENO measurements of θ_{13}

Current neutrino oscillation data indicate that θ_{13} is not strongly suppressed and θ_{23} might have an appreciable deviation from π/4, implying that the 3 \times 3 neutrino mixing matrix V does not have an exact μ-τpermutation symmetry. We make a further study of the effect of μ-τsymmetry breaking on the democratic flavor distribution of ultrahigh-energy (UHE) cosmic neutrinos at a neutrino telescope, and find that it is characterized by |V_{μi}|^2 - |V_{τi}|^2 which would vanish if either θ_{23} = π/4 and θ_{13} = 0 or θ_{23} = π/4 and δ= \pm π/2 held. We observe that the second-order μ-τsymmetry breaking term \barΔ may be numerically comparable with or even larger than the first-order term Δin the flux ratios ϕ^{T}_e : ϕ^{T}_μ: ϕ^{T}_τ\simeq (1- 2Δ) : (1 + Δ+ \barΔ) : (1 + Δ- \barΔ), if \sin (θ_{23} - π/4) and \cosδhave the same sign. The detection of the UHE \barν_e flux via the Glashow-resonance channel \barν_e e \to W^- \to anything is also discussed by taking account of the first- and second-order μ-τsymmetry breaking effects.

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arXiv2012

Enhanced Electromagnetic Transition Dipole Moments and Radiative Decays of Massive Neutrinos due to the Seesaw-induced Non-unitary Effects

In a simple extension of the standard electroweak theory where the phenomenon of lepton flavor mixing is described by a 3x3 unitary matrix V, the electric and magnetic dipole moments of three active neutrinos are suppressed not only by their tiny masses but also by the Glashow-Iliopoulos-Maiani (GIM) mechanism. We show that it is possible to lift the GIM suppression if the canonical seesaw mechanism of neutrino mass generation, which allows V to be slightly non-unitary, is taken into account. In view of current experimental constraints on the non-unitarity of V, we find that the effective electromagnetic transition dipole moments of three light Majorana neutrinos and the rates of their radiative decays can be maximally enhanced by a factor of O(10^2) and a factor of O(10^4), respectively. This important observation reveals an intrinsic and presumably significant correlation between the electromagnetic properties of massive neutrinos and the origin of their small masses.

hep-ph↗

arXiv2012

Flavor Mixing Democracy and Minimal CP Violation

We point out that there is a unique parametrization of quark flavor mixing in which every angle is close to the Cabibbo angle θ_C \simeq 13^\circ with the CP-violating phase ϕ_q around 1^\circ, implying that they might all be related to the strong hierarchy among quark masses. Applying the same parametrization to lepton flavor mixing, we find that all three mixing angles are comparably large (around π/4) and the Dirac CP-violating phase ϕ_l is also minimal as compared with its values in the other eight possible parametrizations. In this spirit, we propose a simple neutrino mixing ansatz which is equivalent to the tri-bimaximal flavor mixing pattern in the ϕ_l \to 0 limit and predicts \sinθ_13 = 1/\sqrt{2} \sin (ϕ_l/2) for reactor antineutrino oscillations. Hence the Jarlskog invariant of leptonic CP violation J_l = (\sinϕ_l)/12 can reach a few percent if θ_13 lies in the range 7^\circ \leq θ_13 \leq 10^\circ.

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arXiv2012

Impacts of the Higgs mass on vacuum stability, running fermion masses and two-body Higgs decays

The latest results of the ATLAS and CMS experiments indicate 116 GeV \lesssim M_H \lesssim 131 GeV and 115 GeV \lesssim M_H \lesssim 127 GeV, respectively, for the mass of the Higgs boson in the standard model (SM) at the 95% confidence level. In particular, both experiments point to a preferred narrow mass range M_H \simeq (124 ... 126) GeV. We examine the impact of this preliminary result of M_H on the SM vacuum stability by using the two-loop renormalization-group equations (RGEs), and arrive at the cutoff scale Λ_VS \sim 4 \times 10^{12} GeV (for M_H = 125 GeV, M_t = 172.9 GeV and α_s(M_Z) = 0.1184) where the absolute stability of the SM vacuum is lost and some kind of new physics might take effect. We update the values of running lepton and quark masses at some typical energy scales, including the ones characterized by M_H, 1 TeV and Λ_VS, with the help of the two-loop RGEs. The branching ratios of some important two-body Higgs decay modes, such as H \to b\bar{b}, H \to τ^+ τ^-, H\to γγ, H\to W^+ W^- and H \to Z Z, are also recalculated by inputting the values of relevant particle masses at M_H.

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arXiv2012

Implications of the Daya Bay observation of θ_{13} on the leptonic flavor mixing structure and CP violation

The Daya Bay Collaboration has recently reported its first \barν_e \to \barν_e oscillation result which points to θ_{13} \simeq 8.8^\circ \pm 0.8^\circ (best-fit \pm 1σrange) or θ_{13} \neq 0^\circ at the 5.2σlevel. The fact that this smallest neutrino mixing angle is not strongly suppressed motivates us to look into the underlying structure of lepton flavor mixing and CP violation. Two phenomenological strategies are outlined: (1) the lepton flavor mixing matrix U consists of a constant leading term U_0 and a small perturbation term ΔU; and (2) the mixing angles of U are associated with the lepton mass ratios. Some typical patterns of U_0 are reexamined by constraining their respective perturbations with current experimental data. We illustrate a few possible ways to minimally correct U_0 in order to fit the observed values of three mixing angles. We point out that the structure of U may exhibit an approximate μ-τpermutation symmetry in modulus, and reiterate the geometrical description of CP violation in terms of the leptonic unitarity triangles. The salient features of nine distinct parametrizations of U are summarized, and its Wolfenstein-like expansion is presented by taking U_0 to be the democratic mixing pattern.

hep-ph↗

arXiv2011

A full parametrization of the 6 X 6 flavor mixing matrix in the presence of three light or heavy sterile neutrinos

In addition to three active neutrinos, one or more light sterile neutrinos have been conjectured to account for the LSND, MiniBooNE and reactor antineutrino anomalies (at the sub-eV mass scale) or for warm dark matter in the Universe (at the keV mass scale). Heavy Majorana neutrinos at or above the TeV scale have also been assumed in some seesaw models. Such hypothetical particles can weakly mix with active neutrinos, and thus their existence can be detected at low energies. In the (3+3) scenario with three sterile neutrinos we present a full parametrization of the 6 X 6 flavor mixing matrix in terms of fifteen rotation angles and fifteen phase angles. We show that this standard parametrization allows us to clearly describe the salient features of some problems in neutrino phenomenology, such as (a) possible contributions of light sterile neutrinos to the tritium beta decay and neutrinoless double-beta decay; (b) leptonic CP violation and deformed unitarity triangles of the 3 X 3 flavor mixing matrix of three active neutrinos; (c) a reconstruction of the 6 X 6 neutrino mass matrix in the type-(I+II) seesaw mechanism; and (d) flavored and unflavored leptogenesis scenarios in the type-I seesaw mechanism with three heavy Majorana neutrinos.

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arXiv2011

Neutrinos as Hot or Warm Dark Matter

Both active and sterile sub-eV neutrinos can serve for hot dark matter (DM). On the other hand, keV sterile neutrinos could be a good candidate for warm DM. The beta-decaying (e.g., H-3 and Ru-106) and EC-decaying (e.g., Ho-163) nuclei are considered as the most promising targets to capture those extremely low energy neutrinos and antineutrinos, respectively. We calculate the capture rates of relic electron neutrinos and antineutrinos against the corresponding beta-decay or EC-decay backgrounds in different flavor mixing schemes. We stress that such direct laboratory measurements of hot or warm DM might not be hopeless in the long term.

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arXiv2011

Two-zero Textures of the Majorana Neutrino Mass Matrix and Current Experimental Tests

In view of the latest T2K and MINOS neutrino oscillation data which hint at a relatively large theta_13, we perform a systematic study of the Majorana neutrino mass matrix M_nu with two independent texture zeros. We show that three neutrino masses (m_1, m_2, m_3) and three CP-violating phases (delta, rho, sigma) can fully be determined from two neutrino mass-squared differences (delta m^2, Delta m^2) and three flavor mixing angles (theta_12, theta_23, theta_13). We find that seven patterns of M_nu (i.e., A_{1,2}, B_{1,2,3,4} and C) are compatible with current experimental data at the 3-sigma level, but the parameter space of each pattern is more strictly constrained than before. We demonstrate that the texture zeros of M_nu are stable against the one-loop quantum corrections, and there exists a permutation symmetry between Patterns A_1 and A_2, B_1 and B_2 or B_3 and B_4. Phenomenological implications of M_nu on the neutrinoless double-beta decay and leptonic CP violation are discussed, and a realization of those texture zeros by means of the Z_n flavor symmetries is illustrated.

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arXiv2011

Captures of Hot and Warm Sterile Antineutrino Dark Matter on EC-decaying Ho-163 Nuclei

Capturing low-energy electron antineutrinos on radioactive Ho-163 nuclei, which decay into Dy-163 via electron capture (EC), is a noteworthy opportunity to detect relic sterile antineutrinos. Such hypothetical particles are more or less implied by current experimental and cosmological data, and they might be a part of hot dark matter or a candidate for warm dark matter in the Universe. Using the isotope Ho-163 as a target and assuming reasonable active-sterile antineutrino mixing angles, we calculate the capture rate of relic electron antineutrinos against the corresponding EC-decay background in the presence of sterile antineutrinos at the sub-eV or keV mass scale. We show that the signature of hot or warm sterile antineutrino dark matter should in principle be observable, provided the target is big enough and the energy resolution is good enough.

astro-ph.CO↗

arXiv2011

The Glashow resonance as a discriminator of UHE cosmic neutrinos originating from p-gamma and p-p collisions

We re-examine the interesting possibility of utilizing the Glashow resonance (GR) channel nu_ebar + e^- to W^- to anything to discriminate between the UHE cosmic neutrinos originating from p-gamma and pp collisions in an optically thin source of cosmic rays. We propose a general parametrization of the initial neutrino flavor composition by allowing the ratios Phi^{p gamma}_{pi^-}/Phi^{p gamma}_{pi^+} and Phi^{pp}_{pi^-}/Phi^{pp}_{pi^+} to slightly deviate from their conventional values. A relationship between the typical source parameter kappa = (Phi^{p gamma}_{pi^+} + Phi^{p gamma}_{pi^-})/(Phi^{pp}_{pi^+} + Phi^{pp}_{pi^-} + Phi^{p gamma}_{pi^+} + Phi^{p gamma}_{pi^-}) and the working observable of the GR R_0 = Phi^T_{nu_ebar}/ (Phi^T_{nu_mu} + Phi^T_{nu_mu}) at a neutrino telescope is derived, and the numerical dependence of R_0 on kappa is illustrated by taking account of the latest experimental data on three neutrino mixing angles. It is shown that a measurement of R_0 is in principle possible to identify the pure p-gamma interaction (kappa =1), the pure pp interaction (kappa =0) or a mixture of both of them (0 < kappa < 1) at a given source of UHE cosmic neutrinos. The event rate of the GR signal against the background is also estimated.

hep-ph↗

arXiv2011

The T2K Indication of Relatively Large theta_13 and a Natural Perturbation to the Democratic Neutrino Mixing Pattern

The T2K Collaboration has recently reported a remarkable indication of the ν_μ-> ν_e oscillation which is consistent with a relatively large value of θ_{13} in the three-flavor neutrino mixing scheme. We show that it is possible to account for such a result of θ_{13} by introducing a natural perturbation to the democratic neutrino mixing pattern, without or with CP violation. A testable correlation between θ_{13} and θ_{23} is predicted in this ansatz. We also discuss the Wolfenstein-like parametrization of neutrino mixing, and comment on other possibilities of generating sufficiently large θ_{13} at the electroweak scale.

hep-ph↗

arXiv2011

Finite quantum corrections to the tribimaximal neutrino mixing

We calculate finite quantum corrections to the tribimaximal neutrino mixing pattern V_TB in three generic classes of neutrino mass models. We show that three flavor mixing angles can all depart from their tree-level results described by V_TB, among which θ_12 is most sensitive to such quantum effects, and the Dirac CP-violating phase can radiatively arise from two Majorana CP-violating phases. This theoretical scheme offers a new way to understand why θ_13 is naturally small and how three CP-violating phases are presumably correlated.

hep-ph↗

arXiv2011

A Possible Detection of the Cosmic Antineutrino Background in the Presence of Flavor Effects

Lusignoli and Vignati have recently pointed out that it is in principle possible to directly detect the cosmic antineutrino background by using the rather stable isotope holmium-163 as a target, which can decay into dysprosium-163 via electron capture (EC) with a very small energy release. In this paper we calculate the rate of the relic antineutrino capture on holmium-163 nuclei against the corresponding EC decay rate by taking account of different neutrino mass hierarchies and reasonable values of theta_13. We show that such flavor effects are appreciable and even important in some cases, and stress that a calorimetric measurement of the cosmic antineutrino background might be feasible in the far future.

astro-ph.CO↗

arXiv2011

Non-Hermitian Perturbations to the Fritzsch Textures of Lepton and Quark Mass Matrices

We show that non-Hermitian and nearest-neighbor-interacting perturbations to the Fritzsch textures of lepton and quark mass matrices can make both of them fit current experimental data very well. In particular, we obtain θ_{23} \simeq 45^\circ for the atmospheric neutrino mixing angle and predict θ_{13} \simeq 3^\circ to 6^\circ for the smallest neutrino mixing angle when the perturbations in the lepton sector are at the 20% level. The same level of perturbations is required in the quark sector, where the Jarlskog invariant of CP violation is about 3.7 \times 10^{-5}. In comparison, the strength of leptonic CP violation is possible to reach about 1.5 \times 10^{-2} in neutrino oscillations.

hep-ph↗

arXiv2010

A Shift from Democratic to Tri-bimaximal Neutrino Mixing with Relatively Large theta_{13}

Recent neutrino oscillation data hint that the smallest neutrino mixing angle theta_{13} is possible to lie in the range 5^\circ \lesssim θ_{13} \lesssim 12^\circ. We show that reasonable perturbations to the democratic mixing pattern, which is geometrically related to the tri-bimaximal mixing pattern through an equal shift θ_* \simeq 9.7^\circ of two large mixing angles, can naturally produce a nearly tri-bimaximal neutrino mixing matrix V with sufficiently large theta_{13}. Two especially simple but viable scenarios of V are proposed and their phenomenological consequences are discussed.

hep-ph↗

arXiv2010

Possible Capture of keV Sterile Neutrino Dark Matter on Radioactive beta-decaying Nuclei

There exists an observed "desert" spanning six orders of magnitude between O(0.5) eV and O(0.5) MeV in the fermion mass spectrum. We argue that it might accommodate one or more keV sterile neutrinos as a natural candidate for warm dark matter. To illustrate this point of view, we simply assume that there is one keV sterile neutrino nu_4 and its flavor eigenstate nu_s weakly mixes with three active neutrinos. We clarify different active-sterile neutrino mixing factors for the radiative decay of nu_4 and beta decays in a self-consistent parametrization. A direct detection of this keV sterile neutrino dark matter in the laboratory is in principle possible since the nu_4 component of nu_e can leave a distinct imprint on the electron energy spectrum when it is captured on radioactive beta-decaying nuclei. We carry out an analysis of its signatures in the capture reactions nu_e + ^{3}H \to ^{3}He + e^- and nu_e + ^{106}Ru \to ^{106}Rh + e^- against the beta-decay backgrounds, and conclude that this experimental approach might not be hopeless in the long run.

hep-ph↗

arXiv2010

Intrinsic Deviation from the Tri-bimaximal Neutrino Mixing in a Class of A_4 Flavor Models

It is well known that the tri-bimaximal neutrino mixing pattern V_0 can be derived from a class of flavor models with the non-Abelian A_4 symmetry. We point out that small corrections to V_0, which are inherent in the A_4 models and arise from both the charged-lepton and neutrino sectors, have been omitted in the previous works. We show that such corrections may lead the 3 \times 3 neutrino mixing matrix V to a non-unitary deviation from V_0, but they cannot result in a nonzero value of θ_13 or any new CP-violating phases. Current experimental constraints on the unitarity of V allow us to constrain the model parameters to some extent.

hep-ph↗

arXiv2010

A Generic Diagonalization of the 3X3 Neutrino Mass Matrix and Its Implications on the mu-tau Flavor Symmetry and Maximal CP Violation

In the flavor basis where the mass eigenstates of three charged leptons are identified with their flavor eigenstates, one may diagonalize a 3 X 3 Majorana neutrino mass matrix M_nu by means of the standard parametrization of the 3 X 3 neutrino mixing matrix V. In this treatment the unphysical phases of M_nu have to be carefully factored out, unless a special phase convention for neutrino fields is chosen so as to simplify M_nu to M'_nu without any unphysical phases. We choose this special flavor basis and establish some exact analytical relations between the matrix elements of M'_nu M'_nu^dag and seven physical parameters --- three neutrino masses (m_1, m_2, m_3), three flavor mixing angles (theta_12, theta_13, theta_23) and the Dirac CP-violating phase (delta). Such results allow us to derive the conditions for the mu-tau flavor symmetry with theta_23 = pi/4 and maximal CP violation with delta = +/- pi/2, which should be useful for discussing specific neutrino mass models. In particular, we show that theta_23 = pi/4 and delta = +/- pi/2 keep unchanged when constant matter effects are taken into account for a long-baseline neutrino oscillation experiment.

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