arXiv subjects

Fu-Sheng Yu

Publications and source records attributed to Fu-Sheng Yu.

72 records · Page 4Linked to original sources

arXiv2018

Charged lepton flavor violating Higgs decays at future $e^+e^-$ colliders

After the discovery of the Higgs boson, several future experiments have been proposed to study the Higgs boson properties, including two circular lepton colliders, the CEPC and the FCC-ee, and one linear lepton collider, the ILC. We evaluate the precision reach of these colliders in measuring the branching ratios of the charged lepton flavor violating Higgs decays $H\to e^\pmμ^\mp$, $e^\pmτ^\mp$ and $μ^\pmτ^\mp$. The expected upper bounds on the branching ratios given by the circular (linear) colliders are found to be $\mathcal{B}(H\to e^\pmμ^\mp) < 1.2\ (2.1) \times 10^{-5}$, $\mathcal{B}(H\to e^\pmτ^\mp) < 1.6\ (2.4) \times 10^{-4}$ and $\mathcal{B}(H\to μ^\pmτ^\mp) < 1.4\ (2.3) \times 10^{-4}$ at 95\% CL, which are improved by one to two orders compared to the current experimental bounds. We also discuss the constraints that these upper bounds set on certain theory parameters, including the charged lepton flavor violating Higgs couplings, the corresponding parameters in the type-III 2HDM, and the new physics cut-off scales in the SMEFT, in RS models and in models with heavy neutrinos.

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arXiv2018

$D^0$-$\overline{D}^0$ mixing parameter $y$ in the factorization-assisted topological-amplitude approach

We calculate the $D^0$-$\overline{D}^0$ mixing parameter $y$ in the factorization-assisted topological-amplitude (FAT) approach, considering contributions from $D^{0}\to PP$, $PV$, and $VV$ modes, where $P$ ($V$) stands for a pseudoscalar (vector) meson. The $D^{0}\to PP$ and $PV$ decay amplitudes are extracted in the FAT approach, and the $D^{0}\to VV$ decay amplitudes with final states in the longitudinal polarization are estimated via the parameter set for $D^{0}\to PV$. It is found that the $VV$ contribution to $y$, being of order of $10^{-4}$, is negligible, and that the $PP$ and $PV$ contributions amount only up to $y_{PP+PV}=(0.21\pm0.07)\%$, a prediction more precise than those previously obtained in the literature, and much lower than the experimental data $y_{\rm exp}=(0.61\pm0.08)\%$. We conclude that $D^{0}$ meson decays into other two-body and multi-particle final states are relevant to the evaluation of $y$, so it is difficult to understand it fully in an exclusive approach.

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arXiv2018

$K_S^0-K_L^0$ Asymmetries and $CP$ Violation in Charmed Baryon Decays into Neutral Kaons

We study the $K^0_S-K^0_L$ asymmetries and $CP$ violations in charm-baryon decays with neutral kaons in the final state. The $K^0_S-K^0_L$ asymmetry can be used to search for two-body doubly Cabibbo-suppressed amplitudes of charm-baryon decays, with the one in $Λ^+_c\to pK^0_{S,L}$ as a promising observable. Besides, it is studied for a new $CP$-violation effect in these processes, induced by the interference between the Cabibbo-favored and doubly Cabibbo-suppressed amplitudes with the neutral kaon mixing. Once the new CP-violation effect is determined by experiments, the direct $CP$ asymmetry in neutral kaon modes can then be extracted and used to search for new physics. The numerical results based on $SU(3)$ symmetry will be tested by the experiments in the future.

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arXiv2018

Discovery Potentials of Doubly Charmed Baryons

The existence of doubly heavy flavor baryons has not been well established experimentally so far. In this Letter we systematically investigate the weak decays of the doubly charmed baryons, $Ξ_{cc}^{++}$ and $Ξ_{cc}^{+}$, which would be helpful for experimental searches for these particles. The branching fractions are studied under the factorization hypothesis for the short-distance contributions and considering the rescattering effect for the long-distance contributions which are significantly enhanced. Comparing all the decay modes, we recommend the processes of $Ξ_{cc}^{++}\to Λ_c^+K^-π^+π^+$ and $Ξ_c^+π^+$ as the first priority for experiments to search for the doubly heavy baryons.

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arXiv2018

Fragmentation-fraction ratio $f_{Ξ_b}/f_{Λ_b}$ in $b$- and $c$-baryon decays

We study the ratio of fragmentation fractions, $f_{Ξ_b}/f_{Λ_b}$, from the measurement of $Ξ_b^0\to Ξ_c^+π^-$ and $Λ_b^0\to Λ_c^+π^-$ with $Ξ_{c}^{+}/Λ_{c}^{+}\to p K^-π^+$. With the branching fraction $\mathcal{B}(Ξ_c^+\to pK^-π^+)=(2.2\pm0.8)\%$ obtained under the U-spin symmetry, the fragmentation ratio is determined as $f_{Ξ_b}/f_{Λ_b}$=$0.054\pm0.020$. To reduce the above uncertainties, we suggest to measure the branching fractions of $Ξ_c^+\to p \overline K^{*0}$ and $Λ_c^+\to Σ^+ K^{*0}$ at BESIII, Belle(II) and LHCb.

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arXiv2017

Weak Decays of Doubly Heavy Baryons: the $1/2\to 1/2$ case

Very recently, the LHCb collaboration has observed in the final state $Λ_c^+ K^-π^+π^+$ a resonant structure that is identified as the doubly-charmed baryon $Ξ_{cc}^{++}$. Inspired by this observation, we investigate the weak decays of doubly heavy baryons $Ξ_{cc}^{++}$, $Ξ_{cc}^{+}$, $Ω_{cc}^{+}$, $Ξ_{bc}^{(\prime)+}$, $Ξ_{bc}^{(\prime)0}$, $Ω_{bc}^{(\prime)0}$, $Ξ_{bb}^{0}$, $Ξ_{bb}^{-}$ and $Ω_{bb}^{-}$ and focus on the decays into spin $1/2$ baryons in this paper. At the quark level these decay processes are induced by the $c\to d/s$ or $b\to u/c$ transitions, and the two spectator quarks can be viewed as a scalar or axial vector diquark. We first derive the hadronic form factors for these transitions in the light-front approach and then apply them to predict the partial widths for the semi-leptonic and non-leptonic decays of doubly heavy baryons. We find that a number of decay channels are sizable and can be examined in future measurements at experimental facilities like LHC, Belle II and CEPC.

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arXiv2017

$CP$ asymmetries in charm decays into neutral kaons

We find a new $CP$-violation effect in charm decays into neutral kaons, which results from the interference between two tree (Cabibbo-favored and doubly Cabibbo-suppressed) amplitudes with the mixing of final-state mesons. This effect, estimated to be of an order of $10^{-3}$, is much larger than the direct $CP$ asymmetries in these decays, but missed in the literature. It can be revealed by measuring the difference of the time-dependent $CP$ asymmetries in the $D^{+}\toπ^{+}K_S^0$ and $D_{s}^{+}\to K^{+} K_S^0$ modes, which are accessible at the LHCb and Belle II experiments. If confirmed, the new effect has to be taken into account, as the above direct $CP$ asymmetries are used to search for new physics.

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arXiv2017

Masses of Scalar and Axial-Vector B Mesons Revisited

The SU(3) quark model encounters a great challenge in describing even-parity mesons. Specifically, the $q\bar q$ quark model has difficulties in understanding the light scalar mesons below 1 GeV, scalar and axial-vector charmed mesons and $1^+$ charmonium-like state $X(3872)$. A common wisdom for the resolution of these difficulties lies on the coupled channel effects which will distort the quark model calculations. In this work, we focus on the near mass degeneracy of scalar charmed mesons, $D_{s0}^*$ and $D_0^{*0}$, and its implications. Within the framework of heavy meson chiral perturbation theory, we show that near degeneracy can be qualitatively understood as a consequence of self-energy effects due to strong coupled channels. Quantitatively, the closeness of $D_{s0}^*$ and $D_0^{*0}$ masses can be implemented by adjusting two relevant strong couplings and the renormalization scale appearing in the loop diagram. Then this in turn implies the mass similarity of $B_{s0}^*$ and $B_0^{*0}$ mesons. The $P_0^* P'_1$ interaction with the Goldstone boson is crucial for understanding the phenomenon of near degeneracy. Based on heavy quark symmetry in conjunction with corrections from QCD and $1/m_Q$ effects, we obtain the masses of $B^*_{(s)0}$ and $B'_{(s)1}$ mesons, for example, $M_{B_{s0}^*}= (5715\pm1)\,{\rm MeV}+δΔ_S$, $M_{B'_{s1}}=(5763\pm1)\,{\rm MeV}+δΔ_S$ with $δΔ_S$ being $1/m_Q$ corrections. We find that the predicted mass difference of 48 MeV between $B'_{s1}$ and $B_{s0}^*$ is larger than that of $20\sim 30$ MeV inferred from the relativistic quark models, whereas the difference of 15 MeV between the central values of $M_{B'_{s1}}$ and $M_{B'_1}$ is much smaller than the quark model expectation of $60-100$ MeV.

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arXiv2017

$CP$ violation in charmed hadron decays into neutral kaons

We find a new $CP$ violating effect in charmed hadron decays into neutral kaons, which is induced by the interference between the Cabibbo-favored and doubly Cabibbo-suppressed amplitudes with the $K^{0}-\overline K^{0}$ mixing. It is estimated to be of order of $\mathcal{O}(10^{-3})$, much larger than the direct $CP$ asymmetry, but missed in the literature. To reveal this new $CP$ violation effect, we propose a new observable, the difference of the $CP$ asymmetries in the $D^{+}\to π^{+}K_S^0$ and $D_{s}^{+}\to K^{+} K_S^0$ modes. Once the new effect is determined by experiments, the direct $CP$ asymmetry then can be extracted and used to search for new physics.

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arXiv2017

$K_{S}^{0}-K_{L}^{0}$ asymmetries in $D$-meson decays

The $K_{S}^{0}-K_{L}^{0}$ asymmetries in the $D$ meson decays, induced by the interference between the Cabibbo-favored and the doubly Cabibbo-suppressed amplitudes, can help to understand the dynamics of charm decays. All possible processes of two-body non-leptonic $D$ decays into one neutral kaon and another pseudoscalar or vector meson are considered. We study the $K_{S}^{0}-K_{L}^{0}$ asymmetries and the branching fractions of corresponding processes in the factorization-assisted topological-amplitude approach in which significant flavor $SU(3)$ symmetry breaking effects are included. The branching fractions of $K_{L}^{0}$ modes are predicted. It is first found that the $K_{S}^{0}-K_{L}^{0}$ asymmetries in the $D^0$-meson decays are shifted by the $D^0-\overline D^0$ mixing parameter $y_{D}\simeq0.006$, to be $0.113\pm0.001$ for all the relevant $D^{0}$ decay modes. Our results on $K_{S}^{0}-K_{L}^{0}$ asymmetries are consistent with the current data and could be tested by experiments in the future.

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arXiv2017

Doubly-Heavy Baryon Weak Decays: $Ξ_{bc}^{0}\to pK^{-}$ and $Ξ_{cc}^{+}\to Σ_{c}^{++}(2520)K^{-}$

Doubly-heavy baryons, with two heavy and one light quarks, are expected to exist in QCD and their masses have been predicted in the quark model. However their existence is not well established so far in experiment. In this work, we explore the possibility of searching for $Ξ_{bc}$ and $Ξ_{cc}^{+}$ in the $W$-exchange processes, $Ξ_{bc}^{0}\to pK^{-}$ and $Ξ_{cc}^{+}\to Σ_{c}^{++}(2520)K^{-}$. On the basis of perturbative calculations, we estimate the branching ratio of the first decay as ${\cal BR}(Ξ_{bc}^0\to p^+ K^-)\approx3.21\times {\cal R}^2_f \times {\cal R}_τ\times 10^{-7}$, where ${\cal R}_f$ (${\cal R}_τ$) are the ratios of the decay constants (lifetimes) of $Ξ_{bc}^{0}$ and $Λ_b^{0}$. The branching ratio of $Ξ_{cc}^{+}\to Σ_{c}^{++}(2520)K^{-}$ is related to that of $Λ_c^+\to Δ^{++} K^-$, and thereby a conjectured topology analysis leads to the range for the branching ratio as: ${\cal BR}(Ξ_{cc}^+\to Σ_{c}^{++}(2520) K^-)\in \left[0.36\%,1.80\%\right]$. The decay $Ξ_{cc}^+\to Σ_{c}^{++}(2520) K^-$ would be reconstructed in the $ Λ_{c}^{+}K^{-}π^{+}$ final state which is easy to access even at a hadron collider. Based on the two facts that abundant heavy quarks can be produced at a hadron collider like LHC, and the branching ratios of $Ξ_{bc}^{0}\to pK^{-}$ and $Ξ_{cc}^{+}\to Σ_{c}^{++}(2520)K^{-}$ are sizable, we urge our experimental colleagues to perform a search at LHCb. This will presumably lead to the discovery of the $Ξ_{bc}$ and $Ξ_{cc}^{+}$ , and precision measurements of the branching ratios in the future are helpful to investigate their decay mechanism.

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arXiv2016

Test Flavor SU(3) symmetry in Exclusive $Λ_c$ decays

Flavor SU(3) symmetry is a powerful tool to analyze charmed baryon decays, however its applicability remains to be experimentally validated. Since there is not much data on $Ξ_c$ decays, various exclusive $Λ_c$ decays especially the ones into a neutron state are essential for the test of flavor symmetry. These decay modes are also helpful to investigate final state interactions in charmed baryon decays. In this work, we discuss the explicit roles of $Λ_c$ decays into a neutron in testing the flavor symmetry and exploring final state interactions. The involved decay modes include semileptonic decays, two-body and three-body non-leptonic decays, but all of them have not been experimentally observed to date.

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arXiv2015

Analysis of Two-body Charmed $B$ Meson Decays in Factorization-Assisted Topological-Amplitude Approach

Within the factorization-assisted topological-amplitude approach, we study the two-body charmed $B$ meson decays $B_{u,d,s} \to D^{(*)}M$, with $M$ denoting a light pseudoscalar (or vector) meson. The meson decay constants and transition form factors are factorized out from the hadronic matrix element of topological diagrams. Therefore the effect of SU(3) symmetry breaking is retained, which is different from the conventional topological diagram approach. The number of free nonperturbative parameters to be fitted from experimental data is also much less. Only four universal nonperturbative parameters $χ^C$, $ϕ^C$, $χ^E$ and $ϕ^E$ are introduced to describe the contribution of the color suppressed tree and $W$-exchanged diagrams for all the decay channels. With the fitted parameters from 31 decay modes induced by $b\to c$ transition, we then predict the branching fractions of 120 decay modes induced by both $b\to c$ and $b\to u$ transitions. Our results are well consistent with the measured data or to be tested in the LHCb and Belle-II experiments in the future. Besides, the SU(3) symmetry breaking, isospin violation and $CP$ asymmetry are also investigated.

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arXiv2014

Near Mass Degeneracy in the Scalar Meson Sector: Implications for $B^*_{(s)0}$ and $B'_{(s)1}$ Mesons

The empirical observation of near degeneracy of scalar mesons above 1 GeV, namely, the mass of the strange-flavor scalar meson is similar to that of the non-strange one, is at variance with the naive expectation of the quark model. Qualitatively, the approximate mass degeneracy can be understood as a consequence of self-energy effects due to strong coupled channels which will push down the mass of the heavy scalar meson in the strange sector more than that in the non-strange partner. However, it works in the conventional model without heavy quark expansion, but not in the approach of heavy meson chiral perturbation theory as mass degeneracy and the physical masses of $D_{s0}^*$ and $D_0^*$ cannot be accounted for simultaneously. In the heavy quark limit, near mass degeneracy observed in the scalar charm sector will imply the same phenomenon in the $B$ system. We have the prediction $M_{B_0^*}\approx M_{B_{s0}^*}\approx 5715\,{\rm MeV}+δΔ_S$ based on heavy quark symmetry and the leading-order QCD correction, where $δΔ_S$ arises from $1/m_Q$ corrections. A crude estimate indicates that $δΔ_S$ is of order $-35$ MeV or less. We stress that the closeness of $B_{s0}^*$ and $B_0^*$ masses implied by heavy quark symmetry is not spoiled by $1/m_Q$ or QCD corrections. The mass-shift effect on $K_0^*(1430)$ is discussed.

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arXiv2014

Branching ratios and direct CP asymmetries in $D\to PV$ decays

We study the two-body hadronic $D\to PV$ decays, where $P$ ($V$) denotes a pseudoscalar (vector) meson, in the factorization-assisted topological-amplitude approach proposed in our previous work. This approach is based on the factorization of short-distance and long-distance dynamics into Wilson coefficients and hadronic matrix elements of four-fermion operators, respectively, with the latter being parametrized in terms of several nonperturbative quantities. We further take into account the $ρ$-$ω$ mixing effect, which improves the global fit to the branching ratios involving the $ρ^0$ and $ω$ mesons. Combining short-distance dynamics associated with penguin operators and the hadronic parameters determined from the global fit to branching ratios, we predict direct $CP$ asymmetries. In particular, the direct $CP$ asymmetries in the $D^0\to K^0\overline{K}^{*0},~\overline{K}^0K^{*0}$, $D^+\toπ^+ρ^0$, and $D_s^+\to K^+ω,~K^+ϕ$ decays are found to be of ${\cal O}(10^{-3})$, which can be observed at the LHCb or future Belle II experiment. We also predict the $CP$ asymmetry observables of some neutral $D$ meson decays.

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arXiv2014

Photon Polarization in the b->s gamma processes in the Left-Right Symmetric Model

The circular-polarization of the photon in the radiative B decays is dominantly left-handed in the Standard Model (SM), but the right-handed polarization may be enhanced in some new physics models with right-handed currents, such as the Left-Right Symmetric Model (LRSM). In this article, we investigate how large this wrong polarization could be within the allowed parameter space of the LRSM. We show that in LRSM, the right-handed polarization of the photon in the $b\to sγ$ processes could be largely enhanced by the $W_L-W_R$ mixing contributions because of the helicity flip on the internal top quark line of the penguin diagrams and the enhancement by the CKM factor $V_{ts}^R/V_{ts}^L$. We discuss the sensitivity of the proposed methods to determine the $b\to sγ$ photon polarization to the LRSM as well as their complementary role compared to the direct search of right-handed charged gauge bosons at LHC.

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arXiv2012

Branching ratios and direct CP asymmetries in $D\to PP$ decays

We propose a theoretical framework for analyzing two-body nonleptonic $D$ meson decays, based on the factorization of short-distance (long-distance) dynamics into Wilson coefficients (hadronic matrix elements of four-fermion operators). The parametrization of hadronic matrix elements in terms of several nonperturbative quantities is demonstrated for the $D\to PP$ decays, $P$ denoting a pseudoscalar meson. We consider the evolution of Wilson coefficients with energy release in individual decay modes, and the Glauber strong phase associated with the pion in nonfactorizable annihilation amplitudes, that is attributed to the unique role of the pion as a Nambu-Goldstone boson and a quark-anti-quark bound state simultaneously. The above inputs improve the global fit to the branching ratios involving the $η'$ meson, and resolves the long-standing puzzle from the $D^0\toπ^+π^-$ and $D^0\to K^+K^-$ branching ratios, respectively. Combining short-distance dynamics associated with penguin operators and the hadronic parameters determined from the global fit to branching ratios, we predict direct CP asymmetries, to which the quark loops and the scalar penguin annihilation give dominant contributions. In particular, we predict $ΔA_{\rm CP}\equiv A_{\rm CP}(K^+K^-)-A_{\rm CP}(π^+π^-)=-1.00\times 10^{-3}$, lower than the LHCb and CDF data.

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arXiv2011

Nonleptonic two-body decays of charmed mesons

Nonleptonic decays of charmed mesons into two pseudoscalar mesons or one pseudoscalar meson and one vector meson are studied on the basis of a generalized factorization method considering the resonance effects in the pole model for the annihilation contributions. Large strong phases between different topological diagrams are considered in this work, simply taking the phase in the coefficients $a_i$. We find that the annihilation-type contributions calculated in the pole model are large in both of the $PP$ and $PV$ modes, which make our numerical results agree with the experimental data better than those previous calculations.

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