Search

We study the decays of $\Lambda_b \to \Lambda_c(\to B_n f) \ell ^- \overline{\nu}$ with $\ell = e, \mu, \tau$, where $B_n$ and $f$ are the daughter baryons and the rest of the particles in $\Lambda_c$ cascade decays, respectively. In particular, we examine the full angular distributions with polarized $\Lambda_b$ and lepton mass effects, in which the time-reversal asymmetries are identified. We concentrate on the decay modes of $\Lambda_b \to \Lambda_c(\to p K^- \pi^+) \ell ^- \overline{\nu}$ to demonstrate their experimental feasibility. We show that the observables associated with the time-reversal asymmetries are useful to search for new physics as they vanish in the standard model. Explicitly, we find that they are sensitive to the right-handed current from new physics, and possible to be observed at LHCb., Comment: 22 pages, 3 figures, accepted by PRD

We study the spin correlations to probe time-reversal~(T) asymmetries in the decays of $\Lambda_b \to \Lambda V ~(V=\phi, \rho^0, \omega, K^{*0})$. The eigenstates of the T-odd operators are obtained along with definite angular momenta. We obtain the T-odd spin correlations from the complex phases among the helicity amplitudes. We give the angular distributions of $\Lambda_b \to \Lambda(\to p \pi^-)V(\to PP')$ and show the corresponding spin correlations, where $P^{(\prime)}$ are the pseudoscalar mesons. Due to the helicity conservation of the $s$ quark in $\Lambda$, we deduce that the polarization asymmetries of $\Lambda$ are close to $-1$. Since the decay of $\Lambda_b \to \Lambda \phi$ in the standard model~(SM) is dictated by the single weak phase from the product of CKM elements, $V_{tb}V_{ts}^*$, the true T and CP asymmetries are suppressed, providing a clean background to test the SM and search for new physics. In the factorization approach, as the helicity amplitudes in the SM share the same complex phase, T-violating effects are absent. Nonetheless, the experimental branching ratio of $Br(\Lambda_b \to \Lambda \phi) = (5.18\pm 1.29 )\times 10^{-6}$ suggests that the nonfactorizable effects or some new physics play an important role. By parametrizing the nonfactorizable contributions with the effective color number, we calculate the branching ratios and direct CP asymmetries. We also explore the possible T-violating effects from new physics., Comment: 26 pages, 1 figure, revised version accepted by JHEP

We analyze the weak processes of anti-triplet charmed baryons decaying to octet baryons and mesons with the SU(3) flavor symmetry and topological quark diagram scheme. We study the decay branching ratios without neglecting the contributions from ${\cal O}(\overline{15})$ for the first time in the SU(3) flavor symmetry approach. The fitting results for the Cabibbo allowed and suppressed decays of $\Lambda_{c}^+$ are all consistent with the experimental data. We predict all singly Cabibbo suppressed decays. In particular, we find that ${\cal B}(\Lambda_c^+\to p \pi^0)=(1.3\pm0.7)\times 10^{-4}$, which is slightly below the current experimental upper limit of $2.7\times 10^{-4}$ and can be tested by the ongoing experiment at BESIII as well as the future one at Belle-II., Comment: 11 pages, 2 figure, revised version accepted by PLB

We propose a model with flavor dependent gauged symmetries of $\displaystyle \prod_{i=1}^3 U(1)_{B_i-L_i}$ with $i$ the family indices. After formulating the renormalizable Yukawa Lagrangian, Higgs potential and kinetic term, we study the lepton sector based on a successful two-zero texture without introducing extra scalar bosons to avoid the dangerous Goldstone bosons. In particular, we discuss the muon related phenomenologies via additional neutral gauge bosons. In our numerical analysis, we explore the allowed parameter space, in which the anomaly of $B \to K^* \bar \mu \mu$ can be explained., Comment: 13 pages, 2 tables, 2 figures; the part of neutral meson mixings is modified

We propose a new model with flavor-dependent gauged symmetry in addition to the flavor-blind symmetry in the Standard Model. The model contains three right-handed neutrinos to cancel gauge anomalies and several Higgs bosons to construct the measured fermion masses. We show the generic features of the model and explore its phenomenology. In particular, we discuss the current bounds on the extra gauge bosons from the K and B meson mixings as well as the LEP and LHC data, and focus on their contributions to the lepton flavor violating processes of .

We analyze the weak processes of anti-triplet charmed baryons decaying to octet baryons and mesons with the SU(3) flavor symmetry and topological quark diagram scheme. We study the decay branching ratios without neglecting the contributions from ${\cal O}(\overline{15})$ for the first time in the SU(3) flavor symmetry approach. The fitting results for the Cabibbo allowed and suppressed decays of $��_{c}^+$ are all consistent with the experimental data. We predict all singly Cabibbo suppressed decays. In particular, we find that ${\cal B}(��_c^+\to p ��^0)=(1.3\pm0.7)\times 10^{-4}$, which is slightly below the current experimental upper limit of $2.7\times 10^{-4}$ and can be tested by the ongoing experiment at BESIII as well as the future one at Belle-II., 11 pages, 2 figure, revised version accepted by PLB

We propose a model with an $U(1)_{B-L}$ gauge symmetry, in which small neutrino masses, dark matter and the matter-antimatter asymmetry in the Universe can be simultaneously explained. In particular, the neutrino masses are generated radiatively, while the matter-antimatter asymmetry is led by the leptogenesis mechanism, at TeV scale. We also explore allowed regions of the model parameters and discuss some phenomenological effects including lepton flavor violating processes., Comment: 19 pages, 4 tables, 12 figures; accepted version for publication in Physics of the Dark Universe

It is usually believed that the observation of the neutrino-antineutrino (${\nu}$-$\bar{\nu}$) oscillations is almost impossible since the oscillation probabilities are expected to be greatly suppressed by the square of tiny ratio of neutrino masses to energies. Such an argument is applicable to most models for neutrino mass generation based on the Weinberg operator, including the seesaw models. However, in the present paper, we shall give a counterexample to this argument, and show that large $\nu$-$\bar{\nu}$ oscillation probabilities can be obtained in a class of models in which both neutrino masses and neutrinoless double beta ($0\nu\beta\beta$) decays are induced by the high-dimensional lepton number violating operator ${\cal O}_7 = \bar{u}_R l^c_R \bar{L}_L H^*d_R + {\rm H.c.}$ with $u$ and $d$ representing the first two generations of quarks. In particular, we find that the predicted $0\nu\beta\beta$ decay rates have already placed interesting constraints on the $\nu_e \leftrightarrow \bar{\nu}_e$ oscillation. Moreover, we provide an UV-complete model to realize this scenario, in which a dark matter candidate naturally appears due to the new $U(1)_d$ symmetry., Comment: 21 pages, 6 figures

We study the charged scalar contributions to the Higgs decay channels of $h\rightarrow \gamma\gamma$ and $h\rightarrow Z\gamma$ in the Type-II seesaw neutrino model. In most of the allowed parameter space in the model, the new contribution to $h\rightarrow Z\gamma$ is positively correlated with that to $h\rightarrow\gamma\gamma$. If the current excess of the $h\rightarrow \gamma\gamma$ rate measured by the ATLAS Collaboration persists, the $h\rightarrow Z\gamma$ rate should be also larger than the corresponding standard model prediction. We demonstrate that the anti-correlation between $h\rightarrow\gamma\gamma$ and $h\rightarrow Z\gamma$ only exists in some special region., Comment: 11 pages, 3 figures, Title changed, Revised version published in PLB723 (2013) 156

We calculate the Higgs decay rate of $h\to Z��$ by including the contributions from new scalars with arbitrary quantum numbers of the weak isospin ($T$) and hypercharge ($Y$) in the standard model. We find that our general formula for the decay rate of $h\to Z��$ matches with that for $h\to ����$ in the limit of $m_Z=0$, but it is different from those in the literature. To illustrate our result, by taking the current $2��$ excess of the $h\to ����$ rate measured by the LHC, we examine the corresponding shift for the $Z��$ decay channel due to the new scalar. We show that the enhancement or reduction of the $h\to Z��$ rate only depends on the relative size of $T$ and the absolute value of $Y$. Explicitly, we predict $0.76, 16 pages, 7 figures, revised version accepted for publication in PRD

We study the charged scalar contributions to the Higgs decay channels of $h\rightarrow ����$ and $h\rightarrow Z��$ in the Type-II seesaw neutrino model. In most of the allowed parameter space in the model, the new contribution to $h\rightarrow Z��$ is positively correlated with that to $h\rightarrow����$. If the current excess of the $h\rightarrow ����$ rate measured by the ATLAS Collaboration persists, the $h\rightarrow Z��$ rate should be also larger than the corresponding standard model prediction. We demonstrate that the anti-correlation between $h\rightarrow����$ and $h\rightarrow Z��$ only exists in some special region., 11 pages, 3 figures, Title changed, Revised version published in PLB723 (2013) 156

We study the transition form factors of the pesudoscalar mesons ($\pi,\eta$ and $\eta^{\prime}$) as functions of the momentum transfer $Q^2$ within the light-front quark model. We compare our results with the recent experimental data by CELLO, CLEO, BaBar and Belle. By considering the possible uncertainties from the quark masses, we illustrate that our predicted form factors can fit with all the data, including those at the large $Q^2$ regions., Comment: 10 pages, 4 figures, accepted for publication in Phys. Rev. C

The explanation of the large CP asymmetries in $D^0\to (\pi^+\pi^-, K^+K^-)$ decays observed by the LHCb collaboration is likely to call for new physics beyond the CKM paradigm. We explore new contributions caused by the color-sextet scalar diquark, and demonstrate that the diquark with the mass of order 1 TeV and nominal couplings with quarks can generate the CP asymmetries at the percent level. Using the experimental data on branching ratios and CP asymmetries of $D^0\to (\pi^+\pi^-, K^+K^-)$, we derive the constraints on the diquark mass and couplings, which can be further examined on hadron colliders in the dijet final states., Comment: 9 pages, 3 figures, reference added

The explanation of the large CP asymmetries in $D^0\to (��^+��^-, K^+K^-)$ decays observed by the LHCb collaboration is likely to call for new physics beyond the CKM paradigm. We explore new contributions caused by the color-sextet scalar diquark, and demonstrate that the diquark with the mass of order 1 TeV and nominal couplings with quarks can generate the CP asymmetries at the percent level. Using the experimental data on branching ratios and CP asymmetries of $D^0\to (��^+��^-, K^+K^-)$, we derive the constraints on the diquark mass and couplings, which can be further examined on hadron colliders in the dijet final states., 9 pages, 3 figures, reference added

We study the low energy phenomena induced by the lightest charged Higgs in the private Higgs (PH) model, in which each quark flavor is associated with a Higgs doublet. We show that the couplings of the charged Higgs scalars to fermions are fixed and the unknown parameters are only the masses and mixing elements of the charged Higgs scalars. As the charged Higgs masses satisfy with $M_b, Comment: 25 pages, 10 figures, final version to appear in PRD

We study the contributions of the non-standard neutrino interactions (NSIs) to the rare meson decays of $K^{+}\to \pi^{+} \nu\bar{\nu}$ and $D^{+}\to \pi^{+} \nu\bar\nu$. We show that both decays could provide strong constraints on the free parameters in the NSIs. We point out that the branching ratio of $D^{+}\to \pi^{+} \nu\bar\nu$ could be at the level of $10^{-8}$., Comment: 7 pages and 2 figures, reference added, version to appear in Phys. Rev. D

We study the contributions of the non-standard neutrino interactions (NSIs) to the rare meson decays of $K^{+}\to ��^{+} ��\bar��$ and $D^{+}\to ��^{+} ��\bar��$. We show that both decays could provide strong constraints on the free parameters in the NSIs. We point out that the branching ratio of $D^{+}\to ��^{+} ��\bar��$ could be at the level of $10^{-8}$., 7 pages and 2 figures, reference added, version to appear in Phys. Rev. D

We study the $D-\bar D$ mixing and rare D decays in the Littlest Higgs model. As the new weak singlet quark with the electric charge of 2/3 is introduced to cancel the quadratic divergence induced by the top-quark, the standard unitary $3\times 3$ Cabibbo-Kobayashi-Maskawa matrix is extended to a non-unitary $4\times 3$ matrix in the quark charged currents and Z-mediated flavor changing neutral currents are generated at tree level. In this model, we show that the $D-\bar D$ mixing parameter can be as large as the current experimental value and the decay branching ratio (BR) of $D\to X_u \ga$ is small but its direct CP asymmetry could be $O(10%)$. In addition, we find that the BRs of $D\to X_u \ell^{+} \ell^{-}$, $D\to X_u\nu \bar \nu$ and $D\to \mu^{+} \mu^{-}$ could be enhanced to be $O(10^{-9})$, $O(10^{-8})$ and $O(10^{-9})$, respectively., Comment: 18 pages, 6 figures, references added and updated, version to appear in Phys. Lett. B

In the quark-flavor mixing scheme, $��$ and $��'$ are linear combinations of flavor states $��_{q}=(u\bar{u}+d\bar{d})/\sqrt{2}$ and $��_{s}=s\bar{s}$ with the masses of $m_{qq}$ and $m_{ss}$, respectively. Phenomenologically, $m_{ss}$ is strictly fixed to be around 0.69, which is close to $\sqrt{2m^{2}_{K}-m^{2}_��}$ by the approximate flavor symmetry, while $m_{qq}$ is found to be $0.18\pm 0.08$ GeV. For a large allowed value of $m_{qq}$, we show that the BRs for $B\to ��^{(\prime)} X$ decays with $X=(\ell^{-} \bar��_{\ell}, \ell^{+} \ell^{-})$ are enhanced. We also illustrate that $BR(B\to��X)> BR(B\to ��^{\prime} X)$ in the mechanism without the flavor-singlet contribution. Moreover, we demonstrate that the decay branching ratios (BRs) for $B\to ��^{(\prime)}K^{[*]}$ are consistent with the data. In particular, the puzzle of the large $BR(B\to ��^{\prime} K)$ can be solved. In addition, we find that the CP asymmetry for $B^{\pm}\to ��K^{\pm}$ can be as large as -30%, which agrees well with the data. However, we cannot accommodate the CP asymmetries of $B\to ��K^*$ in our analysis, which could indicate the existence of some new CP violating sources., 18 pages, 4 figures, typos corrected and version revised, version to appear in Phys. Rev. D

In the quark-flavor mixing scheme, $\eta$ and $\eta'$ are linear combinations of flavor states $\eta_{q}=(u\bar{u}+d\bar{d})/\sqrt{2}$ and $\eta_{s}=s\bar{s}$ with the masses of $m_{qq}$ and $m_{ss}$, respectively. Phenomenologically, $m_{ss}$ is strictly fixed to be around 0.69, which is close to $\sqrt{2m^{2}_{K}-m^{2}_{\pi}}$ by the approximate flavor symmetry, while $m_{qq}$ is found to be $0.18\pm 0.08$ GeV. For a large allowed value of $m_{qq}$, we show that the BRs for $B\to \eta^{(\prime)} X$ decays with $X=(\ell^{-} \bar\nu_{\ell}, \ell^{+} \ell^{-})$ are enhanced. We also illustrate that $BR(B\to\eta X)> BR(B\to \eta^{\prime} X)$ in the mechanism without the flavor-singlet contribution. Moreover, we demonstrate that the decay branching ratios (BRs) for $B\to \eta^{(\prime)}K^{[*]}$ are consistent with the data. In particular, the puzzle of the large $BR(B\to \eta^{\prime} K)$ can be solved. In addition, we find that the CP asymmetry for $B^{\pm}\to \eta K^{\pm}$ can be as large as -30%, which agrees well with the data. However, we cannot accommodate the CP asymmetries of $B\to \eta K^*$ in our analysis, which could indicate the existence of some new CP violating sources., Comment: 18 pages, 4 figures, typos corrected and version revised, version to appear in Phys. Rev. D

Inspired by the new measurements on $B^{-}\to \eta^{(\prime)}\ell \bar\nu_{\ell}$ from the BaBar Collaboration, we examine the constraint on the flavor-singlet mechanism, proposed to understand the large branching ratios for $B\to \eta^{\prime} K$ decays. Based on the mechanism, we study the decays of $\bar B_{d,s}\to \eta^{(\prime)} \ell^{+} \ell^{-}$ and find that they are sensitive to the flavor-singlet effects. In particular, we show that the decay branching ratios of $\bar B_{d,s}\to \eta^{\prime} \ell^{+} \ell^{-}$ can be as large as $O(10^{-8})$ and $O(10^{-6})$, respectively., Comment: 10 pages, 2 figures, final version to appear in Phys. Lett. B

We study the charged Higgs effects on the decays of $B^{-} \to \tau \bar\nu_{\tau}$ and $\bar B\to P(V) \ell \bar\nu_{\ell}$ with $P=\pi^{+},D^{+}$ and $V=\rho^{+},D^{*+}$. We concentrate on the minimal supersymmetric standard model with nonholomorphic terms at a large $\tan\beta$. To extract new physics contributions, we define several physical quantities related to the decay rate and angular distributions to reduce uncertainties from the QCD as well as the CKM elements. With the constraints from the recent measurement on the decay branching ratio of $B^{-} \to \tau \bar\nu_{\tau}$, we find that the charged Higgs effects could be large and measurable., Comment: 21 pages, 10 tables, typos corrected

We study the lepton flavor violation (LFV) in tau decays in the framework of the supersymmetric seesaw mechanism with nonholomorphic terms for the lepton sector at a large $\tan\beta$. In particular, we analyze two new decay modes $\tau\to \ell f_0(980)$ and $\tau\to \ell K^{+} K^{-}$ arising from the scalar boson exchanges contrast to $\tau\to \ell\eta^{(\prime)}$ from the pseudoscalar ones. We find that the decay branching ratios of the two new modes could be not only as large as the current upper limits of $O(10^{-7})$, but also larger than those of $\tau\to \ell \eta^{(\prime)}$. Experimental searches for the two modes are important for the LFV induced by the scalar-mediated mechanism. In addition, we show that the decay branching ratios of $\tau\to \ell \mu^{+} \mu^{-}$ are related to those of $\tau\to \ell \eta$ and $\tau\to \ell f_0(980)$., Comment: 11 pages, 3 figures, typos corrected, references added, version to fit journal

Inspired by the new measurements on $B^{-}\to ��^{(\prime)}\ell \bar��_{\ell}$ from the BaBar Collaboration, we examine the constraint on the flavor-singlet mechanism, proposed to understand the large branching ratios for $B\to ��^{\prime} K$ decays. Based on the mechanism, we study the decays of $\bar B_{d,s}\to ��^{(\prime)} \ell^{+} \ell^{-}$ and find that they are sensitive to the flavor-singlet effects. In particular, we show that the decay branching ratios of $\bar B_{d,s}\to ��^{\prime} \ell^{+} \ell^{-}$ can be as large as $O(10^{-8})$ and $O(10^{-6})$, respectively., 10 pages, 2 figures, final version to appear in Phys. Lett. B

We study the charged Higgs effects on the decays of $B^{-} \to ��\bar��_��$ and $\bar B\to P(V) \ell \bar��_{\ell}$ with $P=��^{+},D^{+}$ and $V=��^{+},D^{*+}$. We concentrate on the minimal supersymmetric standard model with nonholomorphic terms at a large $\tan��$. To extract new physics contributions, we define several physical quantities related to the decay rate and angular distributions to reduce uncertainties from the QCD as well as the CKM elements. With the constraints from the recent measurement on the decay branching ratio of $B^{-} \to ��\bar��_��$, we find that the charged Higgs effects could be large and measurable., 21 pages, 10 tables, typos corrected

We study the lepton flavor violation (LFV) in tau decays in the framework of the supersymmetric seesaw mechanism with nonholomorphic terms for the lepton sector at a large $\tan��$. In particular, we analyze two new decay modes $��\to \ell f_0(980)$ and $��\to \ell K^{+} K^{-}$ arising from the scalar boson exchanges contrast to $��\to \ell��^{(\prime)}$ from the pseudoscalar ones. We find that the decay branching ratios of the two new modes could be not only as large as the current upper limits of $O(10^{-7})$, but also larger than those of $��\to \ell ��^{(\prime)}$. Experimental searches for the two modes are important for the LFV induced by the scalar-mediated mechanism. In addition, we show that the decay branching ratios of $��\to \ell ��^{+} ��^{-}$ are related to those of $��\to \ell ��$ and $��\to \ell f_0(980)$., 11 pages, 3 figures, typos corrected, references added, version to fit journal

We try to understand the polarization puzzle in $B\to \phi K^*$ decays with a simple Higgs model associated with flavor changing neutral current at tree level. The new interactions can effectively reduce the longitudinal polarization $|A_0|^2$. In particular, we find that if the couplings of $b$-quark in different chiralities to Higgs are the same, the transverse polarization $|A_\perp|^2$ can receive the largest contribution and its value can be as large as 30%. On the other hand, with opposite sign in the couplings, the other transverse polarization $|A_{\parallel}|^2$ is enhanced., Comment: 12 pages, 2 figures, Revtex4, relative phases added and typos corrected, version to appear in Phys. Rev. D

We study $B$ physics phenomenology in the scenario of split supersymmetry without R-parity. By assuming the constraints of bilinear (trilinear) R-parity violating couplings, which are introduced to solve the problem of the atmospheric (solar) neutrino mass, we show that the decay branching ratios of $B_{s}\to \ell^{+} \ell^{-}$ and the mixing of $B_s-\bar{B}_s$ can be large. Explicitly, we find that $B(B_s\to\mu^+\mu^-)=O(10^{-7})$ and $\Delta m_{B_s}=O(10^{-9}) GeV$, which should be observed at future hadron colliders., Comment: 10 pages, 4 figures, version to appear in Phys. Rev. D

We try to understand the polarization puzzle in $B\to ��K^*$ decays with a simple Higgs model associated with flavor changing neutral current at tree level. The new interactions can effectively reduce the longitudinal polarization $|A_0|^2$. In particular, we find that if the couplings of $b$-quark in different chiralities to Higgs are the same, the transverse polarization $|A_\perp|^2$ can receive the largest contribution and its value can be as large as 30%. On the other hand, with opposite sign in the couplings, the other transverse polarization $|A_{\parallel}|^2$ is enhanced., 12 pages, 2 figures, Revtex4, relative phases added and typos corrected, version to appear in Phys. Rev. D

The Circular Electron Positron Collider (CEPC) is a large international scientific facility proposed by the Chinese particle physics community to explore the Higgs boson and provide critical tests of the underlying fundamental physics principles of the Standard Model that might reveal new physics. The CEPC, to be hosted in China in a circular underground tunnel of approximately 100 km in circumference, is designed to operate as a Higgs factory producing electron-positron collisions with a center-of-mass energy of 240 GeV. The collider will also operate at around 91.2 GeV, as a Z factory, and at the WW production threshold (around 160 GeV). The CEPC will produce close to one trillion Z bosons, 100 million W bosons and over one million Higgs bosons. The vast amount of bottom quarks, charm quarks and tau-leptons produced in the decays of the Z bosons also makes the CEPC an effective B-factory and tau-charm factory. The CEPC will have two interaction points where two large detectors will be located. This document is the second volume of the CEPC Conceptual Design Report (CDR). It presents the physics case for the CEPC, describes conceptual designs of possible detectors and their technological options, highlights the expected detector and physics performance, and discusses future plans for detector R&D and physics investigations. The final CEPC detectors will be proposed and built by international collaborations but they are likely to be composed of the detector technologies included in the conceptual designs described in this document. A separate volume, Volume I, recently released, describes the design of the CEPC accelerator complex, its associated civil engineering, and strategic alternative scenarios., 424 pages