arXiv subjects

Zhi-zhong Xing

Publications and source records attributed to Zhi-zhong Xing.

At least 127 records · Page 7Linked to original sources

arXiv2008

Interference bands in decays of doubly-charged Higgs bosons to dileptons in the minimal type-II seesaw model at the TeV scale

The dileptonic decays of doubly-charged Higgs bosons H^{\pm\pm} are investigated in the minimal type-II seesaw model with one Higgs triplet Δand one heavy Majorana neutrino N_1 at the TeV scale. We show that the branching ratios {\cal B}(H^{\pm\pm} \to l^\pm_αl^\pm_β) 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 200 GeV--1 TeV, we figure out the generous interference bands for the contributions of ν_i and N_1 to {\cal B}(H^{\pm\pm} \to l^\pm_αl^\pm_β): \sqrt{|\sinθ_{i4} \sinθ_{j4}|} \sim 10^{-8}--10^{-5}, where θ_{i4} and θ_{j4} measure the strength of charged-current interactions of N_1. We illustrate some salient features of the interference bands by considering three typical mass patterns of ν_i, and stress that it is very difficult to distinguish the type-II seesaw model from the triplet seesaw model in such a parameter region at the Large Hadron Collider.

hep-ph↗

arXiv2008

Updated Values of Running Quark and Lepton Masses

Reliable values of quark and lepton masses are important for model building at a fundamental energy scale, such as the Fermi scale M_Z \approx 91.2 GeV and the would-be GUT scale Λ_GUT \sim 2 \times 10^16 GeV. Using the latest data given by the Particle Data Group, we update the running quark and charged-lepton masses at a number of interesting energy scales below and above M_Z. In particular, we take into account the possible new physics scale (μ\sim 1 TeV) to be explored by the LHC and the typical seesaw scales (μ\sim 10^9 GeV and μ\sim 10^12 GeV) which might be relevant to the generation of neutrino masses. For illustration, the running masses of three light Majorana neutrinos are also calculated. Our up-to-date table of running fermion masses are expected to be very useful for the study of flavor dynamics at various energy scales.

hep-ph↗

arXiv2008

Tetra-maximal Neutrino Mixing and Its Implications on Neutrino Oscillations and Collider Signatures

We propose a novel neutrino mixing pattern in terms of only two small integers 1 and 2 together with their square roots and the imaginary number $i$. This ansatz is referred to as the "tetra-maximal" mixing because it can be expressed as a product of four rotation matrices, whose mixing angles are all π/4 in the complex plane. It predicts θ_{12} = \arctan(2-\sqrt{2}) \approx 30.4^\circ, θ_{13} = \arcsin[(\sqrt{2}-1)/(2\sqrt{2})] \approx 8.4^\circ, θ_{23} = 45^\circ and δ=90^\circ in the standard parametrization, and the Jarlskog invariant of leptonic CP violation is found to be {\cal J} = 1/32. These results are compatible with current data and can soon be tested in a variety of neutrino oscillation experiments. Implications of the tetra-maximal neutrino mixing on the decays of doubly-charged Higgs bosons H^{\pm\pm} -> l^\pm_αl^\pm_β(for α, β= e, μ, τ) are also discussed in the triplet seesaw mechanism at the TeV scale, which will be explored at the upcoming LHC.

hep-ph↗

arXiv2008

Correlation between the Charged Current Interactions of Light and Heavy Majorana Neutrinos

The evidence for neutrino oscillations implies that three neutrino flavors (ν_e, ν_μ, ν_τ) must have different mass states (ν_1, ν_2, ν_3). The most popular idea of generating tiny masses of ν_i is to introduce three heavy Majorana neutrinos N_i (for i = 1, 2, 3) into the standard model and implement the seesaw mechanism. In this approach the neutrino mixing matrix V appearing in the charged current interactions of ν_i is not unitary, and the strength of unitarity violation of V is associated with the matrix R which describes the strength of charged current interactions of N_i. We present an explicit parametrization of the correlation between V and R in terms of nine rotation angles and nine phase angles, which can be measured or constrained in the precision neutrino oscillation experiments and by exploring possible signatures of N_i at the LHC and ILC. Two special but viable scenarios, the Type-I seesaw model with two heavy Majorana neutrinos and the Type-II seesaw model with one heavy Majorana neutrino and one Higgs triplet, are taken into account to illustrate the simplified V-R correlation. The implications of R \neq 0 on the low-energy neutrino phenomenology are also discussed. In particular, we demonstrate that the non-unitarity of V is possible to give rise to an appreciable CP-violating asymmetry between ν_μ-> ν_τand \barν_μ-> \barν_τoscillations with short or medium baselines.

hep-ph↗

arXiv2007

Generalized Friedberg-Lee model for neutrino masses and leptonic 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. The most general case with all the free parameters of M_νbeing complex is discussed. 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-τsymmetry. We also point out a different way to obtain the nearly tri-bimaximal neutrino mixing pattern with δ=0^\circ and non-vanishing Majorana phases.

hep-ph↗

arXiv2007

The Minimal Type-I Seesaw Model and Flavor-dependent Leptogenesis

In this talk, we first give a brief review of the so-called minimal seesaw models and then concentrate on the minimal type-I seesaw model with two almost degenerate right-handed Majorana neutrinos of ${\cal O}(1 {\rm TeV})$. A specific texture of the neutrino Yukawa coupling matrix is proposed to achieve the nearly tri-bimaximal neutrino mixing pattern. This ansatz predicts (1) $θ^{}_{23} = π/4$, $|δ| = π/2$ and $\sin^2 θ^{}_{12} = (1 - 2 \tan^2 θ^{}_{13})/3$ in the $m^{}_1 = 0$ case; and (2) $θ^{}_{23} =π/4$ and $θ^{}_{13} = δ= 0$ in the $m^{}_3 = 0$ case. In both cases, the cosmological baryon number asymmetry can be explained via the resonant leptogenesis mechanism. Finally, we demonstrate the significance of flavor-dependent effects in our leptogenesis scenario.

hep-ph↗

arXiv2007

The Minimal Type-II Seesaw Model and Flavor-dependent Leptogenesis

Current experimental data allow the zero value for one neutrino mass, either m_1 =0 or m_3 =0. This observation implies that a realistic neutrino mass texture can be established by starting from the limit (a) m_1 = m_2 =0 and m_3 \neq 0 or (b) m_1 = m_2 \neq 0 and m_3 =0. In both cases, we may introduce a particular perturbation which ensures the resultant neutrino mixing matrix to be the tri-bimaximal mixing pattern or its viable variations. We find that it is natural to incorporate this kind of neutrino mass matrix in the minimal Type-II seesaw model with only one heavy right-handed Majorana neutrino N. We show that it is possible to account for the cosmological baryon number asymmetry in the m_3 =0 case via thermal leptogenesis, in which the CP-violating asymmetry of N decays is attributed to the electron flavor.

hep-ph↗

arXiv2007

TeV-scale Type-II Seesaw Models and Possible Collider Signatures

A natural extension of the standard model to accommodate massive neutrinos is to introduce one Higgs triplet and three right-handed Majorana neutrinos, leading to a 6 \times6 neutrino mass matrix. We show that three light Majorana neutrinos (i.e., the mass eigenstates of ν_e, ν_μand ν_τ) are exactly massless, if and only if M_L = M_D M^{-1}_R M^T_D exactly holds in this seesaw model. We propose three simple Type-II seesaw scenarios with broken A_4 \times U(1)_X flavor symmetry to interpret the observed neutrino mass spectrum and neutrino mixing pattern. Such a TeV-scale neutrino model can be tested in two complementary ways: (1) searching for possible collider signatures of lepton number violation induced by the right-handed Majorana neutrinos and doubly-charged Higgs particles; and (2) searching for possible consequences of unitarity violation of the 3\times 3 neutrino mixing matrix in the future long-baseline neutrino oscillation experiments.

hep-ph↗

arXiv2007

Neutrino telescopes as a probe of active and sterile neutrino mixings

If the ultrahigh-energy (UHE) neutrino fluxes produced from a distant astrophysical source can be measured at a km^3-size neutrino telescope, they will provide a promising way to help determine the flavor mixing pattern of three active neutrinos. Considering the conventional UHE neutrino source with the flavor ratio ϕ_e : ϕ_μ: ϕ_τ= 1 : 2 : 0, I show that ϕ^D_e : ϕ^D_μ: ϕ^D_τ= (1 -2 Δ) : (1 +Δ) : (1 +Δ) holds at the detector of a neutrino telescope, where Δcharacterizes the effect of μ-τsymmetry breaking (i.e., θ_13 \neq 0 and θ_23 \neq π/4). Current experimental data yield -0.1 \leq Δ\leq +0.1. It is also possible to probe Δby detecting the \barν_e flux of E_\barν_e \approx 6.3 PeV via the Glashow resonance channel \barν_e e \to W^- \to anything. Finally, I give some brief comments on the possibility to constrain the mixing between active and sterile neutrinos by using the UHE neutrino telescopes.

astro-ph↗

arXiv2007

Neutrino Mixing and Leptogenesis in Type-II Seesaw Scenarios with Left-Right Symmetry

We propose two Type-II seesaw scenarios for the neutrino mass matrix in the left-right symmetric model, in which the Higgs triplet Yukawa coupling matrix takes the appealing Friedberg-Lee texture. We show that the nearly tri-bimaximal neutrino mixing pattern, which is especially favored by current neutrino oscillation data, can be obtained from both scenarios. We also show that the cosmological baryon number asymmetry can naturally be interpreted in these two scenarios via the flavor-independent leptogenesis mechanism.

hep-ph↗

arXiv2007

Testing CPT Invariance in B^0_d-\bar{B}^0_d and B^0_s-\bar{B}^0_s Oscillations

Recent CDF and D0 measurements of B^0_s-\bar{B}^0_s mixing make it possible to search for CP violation and test CPT symmetry in a variety of B_s decays. Considering both coherent B^0_d\bar{B}^0_d decays at the Υ(4S) resonance and coherent B^0_s\bar{B}^0_s decays at the Υ(5S) resonance, we formulate their time-dependent and time-integrated rates by postulating small CPT violation in B^0_d-\bar{B}^0_d and B^0_s-\bar{B}^0_s oscillations. We show that the opposite-sign dilepton events from either C-odd or C-even B^0_q\bar{B}^0_q states (for q = d or s) can be used to determine or constrain the CPT-violating parameter at a super-B factory. The possibility of distinguishing between the effect of CPT violation and that of ΔB = -ΔQ transitions is also discussed.

hep-ph↗

arXiv2007

A Compromise between Neutrino Masses and Collider Signatures in the Type-II Seesaw Model

A natural extension of the standard $SU(2)_{\rm L} \times U(1)_{\rm Y}$ gauge model to accommodate massive neutrinos is to introduce one Higgs triplet and three right-handed Majorana neutrinos, leading to a $6\times 6$ neutrino mass matrix which contains three $3\times 3$ sub-matrices $M_{\rm L}$, $M_{\rm D}$ and $M_{\rm R}$. We show that three light Majorana neutrinos (i.e., the mass eigenstates of $ν_e$, $ν_μ$ and $ν_τ$) are exactly massless in this model, if and only if $M_{\rm L} = M_{\rm D} M_{\rm R}^{-1} M_{\rm D}^T$ exactly holds. This no-go theorem implies that small but non-vanishing neutrino masses may result from a significant but incomplete cancellation between $M_{\rm L}$ and $M_{\rm D} M_{\rm R}^{-1} M_{\rm D}^T$ terms in the Type-II seesaw formula, provided three right-handed Majorana neutrinos are of ${\cal O}(1)$ TeV and experimentally detectable at the LHC. We propose three simple Type-II seesaw scenarios with the $A_4 \times U(1)_{\rm X}$ flavor symmetry to interpret the observed neutrino mass spectrum and neutrino mixing pattern. Such a TeV-scale neutrino model can be tested in two complementary ways: (1) searching for possible collider signatures of lepton number violation induced by the right-handed Majorana neutrinos and doubly-charged Higgs particles; and (2) searching for possible consequences of unitarity violation of the $3\times 3$ neutrino mixing matrix in the future long-baseline neutrino oscillation experiments.

hep-ph↗

arXiv2007

Deviations from Tri-bimaximal Neutrino Mixing in Type-II Seesaw and Leptogenesis

Current experimental data allow the zero value for one neutrino mass, either m_1 = 0 or m_3 = 0. This observation implies that a realistic neutrino mass texture can be established by starting from the limit (a) m_1 = m_2 = 0 and m_3 \neq 0 or (b) m_1 = m_2 \neq 0 and m_3 = 0. In both cases, we may introduce a particular perturbation which ensures the resultant neutrino mixing matrix to be the tri-bimaximal mixing pattern or its viable variations with all entries being formed from small integers and their square roots. We find that it is natural to incorporate this kind of neutrino mass matrix in the minimal Type-II seesaw model with only one heavy right-handed Majorana neutrino N in addition to the SU(2)_L Higgs triplet Δ_L. We show that it is possible to account for the cosmological baryon number asymmetry in the m_3 =0 case via thermal leptogenesis, in which the one-loop vertex correction to N decays is mediated by Δ_L and the CP-violating asymmetry of N decays is attributed to the electron flavor.

hep-ph↗

arXiv2007

Tri-bimaximal Neutrino Mixing and Flavor-dependent Resonant Leptogenesis

We propose a particularly economical neutrino mass model, in which there are only two right-handed Majorana neutrinos of ${\cal O}(1)$ TeV and their masses are highly degenerate. Its novel Yukawa-coupling texture together with the seesaw mechanism allows us to achieve the normal neutrino mass hierarchy with $m^{}_1 = 0$ and a nearly tri-bimaximal neutrino mixing pattern with the maximal CP-violating phase: $θ^{}_{23} = π/4$, $|δ| = π/2$ and $\sin^2 θ^{}_{12} = (1 - 2 \tan^2 θ^{}_{13})/3$. One may also obtain the inverted neutrino mass hierarchy with $m^{}_3 = 0$ and the corresponding neutrino mixing pattern with $θ^{}_{23} =π/4$ and $θ^{}_{13} = δ= 0$. In both cases, it is possible to interpret the cosmological baryon number asymmetry $η^{}_{\rm B} \approx 6.1 \times 10^{-10}$ through the resonant leptogenesis mechanism. We demonstrate the significance of flavor-dependent effects in this leptogenesis scenario: they can either flip the sign of the flavor-independent prediction for $η^{}_{\rm B}$ in the $m^{}_1 = 0$ case or magnify the magnitude of the flavor-independent prediction for $η^{}_{\rm B}$ about 50 times in the $m^{}_3 = 0$ case.

hep-ph↗

arXiv2007

Massive and Massless Neutrinos on Unbalanced Seesaws

The observation of neutrino oscillations requires new physics beyond the standard model (SM). A SM-like gauge theory with p lepton families can be extended by introducing q heavy right-handed Majorana neutrinos but preserving its SU(2)_L x U(1)_Y gauge symmetry. The overall neutrino mass matrix M turns out to be a symmetric (p+q) x (p+q) matrix. Given p>q, the rank of M is in general equal to 2q, corresponding to 2q non-zero mass eigenvalues. The existence of (p-q) massless left-handed Majorana neutrinos is an exact consequence of the model, independent of the usual approximation made in deriving the Type-I seesaw relation between the effective p x p light Majorana neutrino mass matrix M_νand the q x q heavy Majorana neutrino mass matrix M_R. In other words, the numbers of massive left- and right-handed neutrinos are fairly matched. A good example to illustrate this seesaw fair play rule is the minimal seesaw model with p=3 and q=2, in which one massless neutrino sits on the unbalanced seesaw.

hep-ph↗

arXiv2007

D0-anti-D0 Mixing and CP Violation in D0 vs anti-D0 to K*(+-) K(-+) Decays

The noteworthy BaBar and Belle evidence for $D^0$-$\bar{D}^0$ mixing motivates us to study its impact on $D^0\to K^{*\pm} K^\mp$ decays and their CP-conjugate processes. We show that both the $D^0$-$\bar{D}^0$ mixing parameters ($x$ and $y$) and the strong phase difference between $\bar{D}^0\to K^{*\pm}K^\mp$ and $D^0\to K^{*\pm}K^\mp$ transitions ($δ$) can be determined or constrained from the time-dependent measurements of these decay modes. On the $ψ(3770)$ and $ψ(4140)$ resonances at a $τ$-charm factory, it is even possible to determine or constrain $x$, $y$ and $δ$ from the time-independent measurements of coherent $(D^0\bar{D}^0) \to (K^{*\pm} K^\mp)(K^{*\pm} K^\mp)$ decays. If the CP-violating phase of $D^0$-$\bar{D}^0$ mixing is significant in a scenario beyond the standard model, it can also be extracted from the $K^{*\pm} K^\mp$ events.

hep-ph↗

arXiv2007

Neutrino Mixing: from the Broken μ-τSymmetry to the Broken Friedberg-Lee Symmetry

I argue that the observed flavor structures of leptons and quarks might imply the existence of certain flavor symmetries. The latter should be a good starting point to build realistic models towards deeper understanding of the fermion mass spectra and flavor mixing patterns. The μ-τpermutation symmetry serves for such an example to interpret the almost maximal atmospheric neutrino mixing angle (θ_23 \sim 45^\circ) and the strongly suppressed CHOOZ neutrino mixing angle (θ_13 < 10^\circ). In this talk I like to highlight a new kind of flavor symmetry, the Friedberg-Lee symmetry, for the effective Majorana neutrino mass operator. Luo and I have shown that this symmetry can be broken in an oblique way, such that the lightest neutrino remains massless but an experimentally-favored neutrino mixing pattern is achievable. We get a novel prediction for θ_13 in the CP-conserving case: \sinθ_13 = \tanθ_12 |(1- \tanθ_23)/ (1+ \tanθ_{23})|. Our scenario can simply be generalized to accommodate CP violation and be combined with the seesaw mechanism. Finally I stress the importance of probing possible effects of μ-τsymmetry breaking either in terrestrial neutrino oscillation experiments or with ultrahigh-energy cosmic neutrino telescopes.

hep-ph↗

arXiv2007

Friedberg-Lee Symmetry Breaking and Its Prediction for theta_13

We consider an effective Majorana neutrino mass operator with the Friedberg-Lee symmetry; i.e., it is invariant under the transformation ν_α-> ν_α+ z (for α= e, μ, τ) with z being a space-time independent constant element of the Grassmann algebra. We show that this new flavor symmetry can be broken in such a nontrivial way that the lightest neutrino remains massless but an experimentally-favored neutrino mixing pattern is achievable. In particular, we get a novel prediction for the unknown neutrino mixing angle θ_13 in terms of two known angles: \sinθ_13 = \tanθ_12 |(1- \tanθ_23)/ (1+\tanθ_23)|. The model can simply be generalized to accommodate CP violation and be combined with the seesaw mechanism.

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