1,805 research outputs found

    Top-quark flavor-changing tqZtqZ couplings and rare ΔF=1\Delta F=1 processes

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    We model-independently study the impacts of anomalous tqZtqZ couplings (q=u,cq=u,c), which lead to the t→qZt\to q Z decays, on low energy flavor physics. It is found that the tuZtuZ-coupling effect can significantly affect the rare KK and BB decays, whereas the tcZtcZ-coupling effect is small. Using the ATLAS's branching ratio (BR) upper bound of BR(t→uZ)<1.7×10−4BR(t\to uZ) < 1.7\times 10^{-4}, the influence of the anomalous tuZtuZ-coupling on the rare decays can be found as follows: (a) The contribution to the Kaon direct CP violation can be up to Re(ϵ′/ϵ)≲0.8×10−3Re(\epsilon'/\epsilon) \lesssim 0.8 \times 10^{-3}; (b) BR(K+→π+ννˉ)≲12×10−11BR(K^+\to \pi^+ \nu \bar \nu) \lesssim 12 \times 10^{-11} and BR(KL→π0ννˉ)≲7.9×10−11BR(K_L \to \pi^0 \nu\bar \nu)\lesssim 7.9 \times 10^{-11}; (c) the BR for KS→μ+μ−K_S \to \mu^+ \mu^- including the long-distance effect can be enhanced by 11%11\% with respect to the standard model result, and (d) BR(Bd→μ+μ−)≲1.97×10−10BR(B_d \to \mu^+ \mu^-) \lesssim 1.97 \times 10^{-10}. In addition, although Re(ϵ′/ϵ)Re(\epsilon'/\epsilon) cannot be synchronously enhanced with BR(KL→π0ννˉ)BR(K_L\to \pi^0 \nu \bar\nu) and BR(KS→μ+μ−)BR(K_S\to \mu^+ \mu^-) in the same region of the CP-violating phase, the values of Re(ϵ′/ϵ)Re(\epsilon'/\epsilon), BR(K+→π+ννˉ)BR(K^+ \to \pi^+ \nu \bar\nu), and BR(Bd→μ+μ−)BR(B_d \to \mu^+ \mu^-) can be simultaneously increased.Comment: 19 pages, 3 figures, version to be published in the European Physical Journal

    Penguin b→sℓ′+ℓ′−b \to s\ell'^+ \ell'^- and BB-meson anomalies in a gauged Lμ−Lτ{L_\mu -L_\tau}

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    The Z′Z'-gauge boson in an U(1)Lμ−LτU(1)_{L_\mu-L_\tau} gauge symmetry has two interesting features: one is its vector couplings to the charged leptons, and the other is the decoupling from the electron. Based on these properties, we investigate the feasibility to simultaneously resolve the RK(∗)=BR(B→K(∗)μ+μ−)/BR(B→K(∗)e+e−)R_{K^{(*)}} = BR(B\to K^{(*)} \mu^+ \mu^-)/BR(B\to K^{(*)} e^+ e^-) and RD(∗)=BR(Bˉ→D(∗)τνˉτ)/BR(Bˉ→D(∗)ℓνˉℓ)R_{D^{(*)}} = BR(\bar B\to D^{(*)} \tau \bar\nu_\tau)/BR(\bar B\to D^{(*)} \ell \bar\nu_\ell) anomalies in an U(1)Lμ−LτU(1)_{L_\mu-L_\tau} model, where the former is expected to arise from the Z′Z'-penguin-induced b→sμ+μ−b\to s \mu^+ \mu^- process and the latter from the tree-level b→cτνˉτb\to c \tau \bar\nu_\tau decay. In order to achieve the intended purpose, we employ one vector-like doublet lepton and one singlet scalar leptoquark (LQ), in which the new particles all carry the U(1)Lμ−LτU(1)_{L_\mu-L_\tau} charges; the b→sZ′b\to sZ' effective interaction is generated from the vector-like lepton and LQ loop, and the b→cτνˉτb\to c \tau \bar\nu_\tau decay is induced from the LQ. When the constraints from the b→sγb\to s \gamma, B+→K+ννˉB^+\to K^+ \nu \bar\nu, Bc−→τνˉτB^-_c\to \tau \bar \nu_\tau, ΔF=2\Delta F=2, and τ→μℓℓˉ\tau\to \mu \ell \bar \ell processes are included, it is found that RDR_D and RD∗R_{D^*} can be enhanced to fit the experimental data, and the Wilson coefficient C9C_9 from the LQ-loop can reach C9LQ,μ∼−1C^{LQ,\mu}_9\sim -1, which can explain the RKR_{K} and RK∗R_{K^*} anomalies. In addition, in this simple model, the Higgs lepton-flavor violating h→μτh\to \mu \tau decay can occur at the tree level, and its branching ratio can be as large as the current experimental upper limit.Comment: 21 pages, 6 figures, 1 table, version published in PL

    Charged-Higgs on RD(∗)R_{D^{(*)}}, τ\tau polarization, and FBA

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    We study the influence of a charged-Higgs on the excess of branching fraction ratio, RM=BR(Bˉ→Mτνˉτ)/BR(Bˉ→Mℓνˉℓ)R_M = BR(\bar B \to M \tau \bar\nu_\tau)/BR(\bar B \to M \ell \bar \nu_\ell) (M=D,D∗)(M=D, D^*), in a generic two-Higgs-doublet model. In order to investigate the lepton polarization, the detailed decay amplitudes with lepton helicity are given. When the charged-Higgs is used to resolve excesses, it is found that two independent Yukawa couplings are needed to explain the RDR_D and RD∗R_{D^*} anomalies. We show that when the upper limit of BR(Bc→τνˉτ)<30%BR(B_c \to \tau \bar \nu_\tau)<30\% is included, RDR_D can be significantly enhanced while RD∗<0.27R_{D^*}<0.27. With the BR(Bc→τνˉτ)BR(B_c\to \tau \bar \nu_\tau) constraint, we find that the τ\tau-lepton polarizations can be still affected by the charged-Higgs effects, where the standard model (SM) predictions are obtained as: PDτ≈0.324P^\tau_{D} \approx 0.324 and PD∗τ≈−0.500P^\tau_{D^*}\approx -0.500, and they can be enhanced to be PDτ≈0.5P^\tau_{D} \approx 0.5 and PD∗τ≈−0.41P^\tau_{D^*} \approx -0.41 by the charged-Higgs. The integrated lepton froward-backward asymmetry (FBA) is also studied, where the SM result is AˉFBD(∗),τ≈−0.359(0.064)\bar A^{D^{(*)},\tau}_{FB} \approx -0.359(0.064), and they can be enhanced (decreased) to be AˉFBD(∗),τ≈−0.33(0.02)\bar A^{D^{(*)},\tau}_{FB} \approx -0.33 (0.02).Comment: 22 pages, 4 figures, version to be published in EPJ

    2 TeV Higgs boson and diboson excess at the LHC

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    Diboson resonance with mass around 2 TeV in the dijet invariant mass spectrum is reported by ATLAS experiment in proton-proton collisions at s=8\sqrt{s}=8 TeV. We propose that the candidate of resonance is a heavy neutral Higgs H0H^0 or charged Higgs H±H^\pm and use the extended two-Higgs-doublet (THD) to demonstrate the potentiality. We find that the large Yukawa coupling to the first generation of quarks can be realized in THD and the required value for producing the right resonance production cross section is of O(0.06−0.2){\cal O}(0.06-0.2). Besides WW/ZZWW/ZZ channels, we find that if the mass of pseudoscalar A0A^0 satisfies the jet mass tagging condition ∣mj−mZ/W∣<13|m_j - m_{Z/W}|< 13 GeV, the diboson excess could be also caused by ZA0ZA^0 or WA0WA^0 channel.Comment: 12 pages, 7 figures, title changed, Fig.2 corrected, comments and references added, version to appear in PL

    ϵK\epsilon_K and ϵ′/ϵ\epsilon'/\epsilon in a diquark model

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    Based on the calculations using the lattice QCD by RBC-UKQCD collaboration and a large NcN_c dual QCD, the resulted ϵ′/ϵ\epsilon'/\epsilon, which is less than the experimental data by more than a 2σ2\sigma in the standard model (SM), suggests the necessity of a new physics effect. In order to complement the insufficient ϵ′/ϵ\epsilon'/\epsilon, we study the extension of the SM with a colored scalar in a diquark model. In addition to the pure diquark box diagrams, it is found that the box diagrams with one WW-boson and one diquark, ignored in the literature, play an important role in the ΔS=2\Delta S=2 process. The mass difference between KLK_L and KSK_S in the diquark model is well below the current data, whereas the Kaon indirect CP violation ϵK\epsilon_K gives a strict constraint on the new parameters. Three mechanisms are classified in the study of ϵ′/ϵ\epsilon'/\epsilon, and they include: tree-level diagram, QCD and electroweak penguins, and chromomagnetic dipole. Taking the Kobayashi-Maskawa phase as the unique CP source, we analyze each contribution of the three mechanisms in detail and conclude that these diquark-mediated effects can singly enhance ϵ′/ϵ\epsilon'/\epsilon to be of O(10−3){\cal O}(10^{-3}), depending on the values of free parameters, when the bound from ϵK\epsilon_K is satisfied.Comment: 44 pages, 10 figures, typos corrected, references added, and charged-Goldstone-boson effects include
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