arXiv subjects

Xing-Bo Yuan

Publications and source records attributed to Xing-Bo Yuan.

29 records · Page 2Linked to original sources

arXiv2017

Tests for CPT sum rule and U-spin violation in Time-dependent CP violation of $B^0_s \to K^+ K^-$ and $B^0_d \to π^+ π^-$

Recent LHCb data for time-dependent CP violation in $B_d^0 \to π^+π^-$ and $B^0_s\to K^+K^-$ show deviations from theoretical predictions. Besides their central values for $\mathcal C_{K^+K^-}$, $\mathcal S_{K^+K^-}$ and $\mathcal A^{ΔΓ}_{K^+K^-}$ violate quantum mechanic CPT invariant sum rule (CPT sum rule) prediction of $|\mathcal C_{K^+K^-}|^2 + |\mathcal S_{K^+K^-}| ^2 + |\mathcal A^{ΔΓ}_{K^+K^-}|^2 = 1$ (LHCb data imply the sum to be $0.67\pm 0.20$.), their values for $\mathcal C_{K^+ K^-}= 0.24\pm 0.06\pm {0.02}$ and $\mathcal C_{π^+ π^-} = - 0.24\pm 0.07\pm 0.01$ also show large violation of SU(3) or its U-spin sub-group symmetry (SU(3)/U) relation $\mathcal C_{K^+ K^-} /\mathcal C_{π^- π^+} = - \mathcal B(B_d^0 \to π^- π^+)τ_{B^0_s}/\mathcal B(B^0_s \to K^+ K^-)τ_{B_d^0}$ (LHCb data imply the ratio of left-side to right-side to be $4.67\pm 1.88$.) . The LHCb results need to be further confirmed to be taken seriously. We suggest to use time-dependent CP violation in $B_s\to K^0\bar K^0, π^+π^-, π^0π^0$ to further test the CPT sum rule. Assuming that the sum rule holds, we propose that violation of the SU(3)/U relation may indicate a large FSI phase difference in the $π^+π^-$ and $K^+K^-$ re-scattering. We suggest several other U-spin pairs of $B\to PP$ decays to further test SU(3)/U relations.

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arXiv2017

Is there new particle running in the loop-induced $Hγγ$ and $Hgg$ vertex?

Loop-induced $Hgg$ and $Hγγ$ coupling play an important role in the Higgs production and decay at the LHC. These vertices can be affected by various New Physics contributions, including new particles, anomalous Yukawa couplings, and so on. We point out some ratios of the Higgs signal strengths could help disentangle the contributions of new particles from other sources of New Physics.

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arXiv2016

Revisiting $B_s \to μ^+μ^-$ in the two-Higgs doublet models with $Z_2$ symmetry

We revisit the rare leptonic decay $B_s \to μ^+ μ^-$ in the two-Higgs doublet models with a softly broken $Z_2$ symmetry, namely type-I, type-II, type-X and type-Y 2HDMs. We have derived the relevant full one-loop Wilson coefficients of the four 2HDMs from the recent calculation in the aligned two-Higgs doublet model by Li, Lu and Pich, which could be mapped to all the four 2HDMs for both large and small $\tanβ$. It is found that, a new term associated with the soft $Z_2$ symmetry breaking parameter $M$ can be enhanced by $\tan^2β$ in the type-II 2HDM, which has not been considered in the literature. Imposing both theoretical and experimental constraints, we have renewed the bounds on the parameter spaces of the four 2HDMs. Different from our previous paper, however, we find that all the four 2HDMs give sizable and similar contributions to $\overline{\mathcal B}(B_s \to μ^+ μ^-)$ within the stringently restricted parameter spaces, but very tiny as regards the mass-eigenstate rate asymmetry $\mathcal A_{ΔΓ}$; this makes it unfeasible to discriminate the four types of 2HDM with the correlations between the observables in $B_s \to μ^+ μ^-$ decay.

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arXiv2016

Constraints on $Z^\prime$ Boson within Minimal Flavor Violation

We explore a $Z^\prime$ boson coupled only with the Standard Model (SM) fermions ($Z^\prime f\bar{f}$) in the framework of Minimal Flavor Violation. We study its effects on the processes with lepton flavor violation $\ell_j\to \ell_i\ell_k\bar{\ell}_l$, $\ell_j\to\ell_iγ$, $μ^{-}N\to e^- N$, quark flavor changing neutral currents $b\to s \ell^+\ell^-$, neutral $B$ and $K$ meson mixing, and $e^+e^-\to f\bar{f}$ at the LEP experiment to constrain the parameter space of $Z^\prime$ mass and couplings. We find that among those relevant processes, $μ\to eγ$, $μ\to 3e$, $μ$-$e$ conversion and $e^+e^-\to f\bar{f}$ can put more stringent bounds on $Z^\prime f\bar{f}$ couplings normalized by $Z^\prime$ mass. Its implications on various processes are obtained, such as $B$ and $K$ mixing and $B/K\to M \ell_1 \bar\ell_2$ decays. In addition, we also make analysis on $Z^\prime$ signatures at the LHC with $\sqrt{s}=8$ and 13 TeV.

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arXiv2016

Exploring top quark FCNC within 2HDM type III in association with flavor physics

The top quark flavor changing neutral current (FCNC) process is an excellent probe to search for new physics in top sector since the Standard Model expectation is extremely suppressed. We explore Higgs-mediated top quark FCNC, focusing on $H$-$t$-$c$ Yukawa coupling $λ_{ct}$ within the general two Higgs doublet model. After electroweak symmetry breaking the top quark FCNC couplings are included in the charged Higgs Yukawa sector so that they contribute to various processes in flavor physics. To probe $λ_{ct}$, we study anomalous single top production and the same sign top pair production at the LHC in association with flavor physics from the tree-level processes $B\to D^{(*)}τν$, $B\to τν$ as well as from the loop-level processes $B_d \to X_s γ$, $B_{d,s}-{\overline B}_{d,s}$ mixing. We perform combined analysis of all the constraints regarding the fine-tuning argument to fit the data and discuss future prospect. The recently updated measurements on $B\to D^{(*)}τν$ still prefer large $λ_{ct}$, but we show that the current bound on the same sign top pair production at the LHC gives the most significant upper bound on $λ_{ct}$ to be less than $10\sim30$ depending on neutral heavy Higgs masses. We also find that for the given upper bound on $λ_{ct}$, $B\to D^{(*)}τν$ put significant lower bound on $H$-$τ$-$τ$ Yukawa coupling, and the bound is proportional to the charged Higgs mass.

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arXiv2014

Phenomenological discriminations of the Yukawa interactions in two-Higgs doublet models with $Z_2$ symmetry

There are four types of two-Higgs doublet models under a discrete $Z_2$ symmetry imposed to avoid tree-level flavour-changing neutral current, i.e. type-I, type-II, type-X and type-Y models. We investigate the possibility to discriminate the four models in the light of the flavour physics data, including $B_s-\bar B_s$ mixing, $B_{s,d} \to μ^+ μ^-$, $B\to τν$ and $\bar B \to X_s γ$ decays, the recent LHC Higgs data, the direct search for charged Higgs at LEP, and the constraints from perturbative unitarity and vacuum stability. After deriving the combined constraints on the Yukawa interaction parameters, we have shown that the correlation between the mass eigenstate rate asymmetry $A_{ΔΓ}$ of $B_{s} \to μ^+ μ^-$ and the ratio $R={\cal B}(B_{s} \to μ^+ μ^-)_{exp}/ {\cal B}(B_{s} \to μ^+ μ^-)_{SM}$ could be sensitive probe to discriminate the four models with future precise measurements of the observables in the $B_{s} \to μ^+ μ^-$ decay at LHCb.

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arXiv2014

Exclusive radiative B-meson decays within minimal flavour violating 2HDMs

In the "Higgs basis" for a generic 2HDM, only one doublet gets a nonzero vacuum expectation value and, under the criterion of minimal flavour violation, the other one is fixed to be either colour-singlet or colour-octet, referred respectively as the type-III and the type-C model. Both of them can naturally avoid large FCNC transitions and provide very interesting phenomena in some low-energy processes. In this paper, we study their effects on exclusive radiative B-meson decays due to the exchange of colourless or coloured charged Higgs. It is found that, while constraints from the branching ratios are slightly weaker than the ones from the inclusive $B\to X_{s} γ$ decay, the isospin asymmetries in exclusive decays provide very complementary bounds on the model parameters. As the two models predict similar corrections to the dipole coefficient $C_7^{\rm eff}$ whereas similar magnitudes but with opposite signs to $C_8^{\rm eff}$, the branching ratios cannot discriminate the two models, and we have to resort to the direct CP and isospin asymmetries of $b\to s$ processes, which are more sensitive to $C_8^{\rm eff}$. Due to the CKM factors $|λ_u^{(d)}|\sim|λ_t^{(d)}|$, the terms proportional to $λ_u^{(d)}$ make the observables of $b\to d$ processes exhibit a different dependence on the possible new physics phase. In addition, correlations between the various observables in the exclusive $B\to Vγ$ and the inclusive $B\to X_{s,d}γ$ decays are investigated, which could provide further insights into the models with more precise experimental measurements and theoretical predictions for these decays.

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arXiv2013

Constraints on anomalous $tcZ$ coupling from $\bar B \to \bar K^* μ^+ μ^-$ and $B_s \to μ^+ μ^-$ decays

In this paper, we analyze the possible anomalous $tcZ$ coupling effects in the $b \to s $ mediated decays $\bar B \to \bar K^* μ^+ μ^-$ and $B_s \to μ^+ μ^-$. After exploiting the available experimental data, the combined constraints on the anomalous coupling $X_{ct}^L$ are derived. It is found that, the bound on the magnitude $|X_{ct}^L|$ is dominated by the branching ratios of these two decays. Furthermore, one sign-flipped solution is excluded by the longitudinal fraction of $\bar B \to \bar K^* μ^+ μ^-$ at the low dilepton mass region. After considering the combined constraints, for general complex coupling $X_{ct}^L$, the predicted upper bound on $\mathcal B (t \to cZ)$ are compatible with that from the recent CMS direct search. In particular, for the case of real coupling $X_{ct}^L$, the upper bound reads $\mathcal B (t \to cZ)<1.0\times 10^{-4}$, which is much lower than the current CMS bound but still accessible at the LHC. With improved measurements at the LHC, the colser correlations between the $t \to cZ$ and $b \to s$ mediated (semi-) leptonic decays are expected in the near future.

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arXiv2012

Anomalous $tqZ$ coupling effects in rare B- and K-meson decays

As a top-factory, the LHC is performing a direct study of top-quark anomalous FCNC couplings, which are, however, correlated closely with the rare B- and K-meson decays. In this paper, we study the effects of anomalous $tqZ$ (with $q=u,c$) couplings in the rare decays $B_{s,d}\to μ^+μ^-$, $B\to X_s ν\barν$, $B\to K^{(*)}ν\barν$, $K^+\to π^+ ν\barν$, and $K_L\to π^0 ν\barν$. With the up-to-date experimental bounds on the branching ratios of these channels, constraints on the left-handed anomalous couplings $X_{ct}^L$ and $X_{ut}^L$ are derived, respectively. With these low-energy constraints taken into account, we find that, for real couplings $X_{ct}^L$ and $X_{ut}^L$, the indirect upper bounds on $\mathcal B(t\to qZ)$ are much lower than that from the D0 collaboration, but are still compatible with the $5σ$ discovery potential of ATLAS with an integrated luminosity of $10 {\rm fb}^{-1}$. With refined measurements to be available at the LHCb, the future super-B factories, the NA62 at CERN, and the KOTO at J-PARC, closer correlations between the $t\to qZ$ and the rare B- and K-meson decays are expected in the near future, which will be helpful for the searches of the top-quark FCNC decays at the LHC.

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arXiv2011

Anomalous $tqγ$ coupling effects in exclusive radiative B-meson decays

The top-quark FCNC processes will be searched for at the CERN LHC, which are correlated with the B-meson decays. In this paper, we study the effects of top-quark anomalous interactions $tqγ$ in the exclusive radiative $B\to K^*γ$ and $B\toργ$ decays. With the current experimental data of the branching ratios, the direct CP and the isospin asymmetries, bounds on the coupling $κ_{tcR}^γ$ from $B\to K^*γ$ and $κ_{tuR}^γ$ from $B\to ργ$ decays are derived, respectively. The bound on $|κ_{tcR}^γ|$ from ${\mathcal B}(B\to K^{*}γ)$ is generally compatible with that from ${\mathcal B}(B\to X_{s}γ)$. However, the isospin asymmetry $Δ(K^{*}γ)$ further restrict the phase of $κ_{tcR}^γ$, and the combined bound results in the upper limit, $\mathcal B(t\to cγ)<0.21%$, which is lower than the CDF result. For real $κ_{tcR}^γ$, the upper bound on $\mathcal B(t\to cγ)$ is about of the same order as the $5σ$ discovery potential of ATLAS with an integrated luminosity of $10 {\rm fb}^{-1}$. For $B\toργ$ decays, the NP contribution is enhanced by a large CKM factor $|V_{ud}/V_{td}|$, and the constraint on $tuγ$ coupling is rather restrictive, $\mathcal B(t\to uγ)<1.44\times 10^{-5}$. With refined measurements to be available at the LHCb and the future super-B factories, we can get close correlations between $B\to V γ$ and the rare $t\to qγ$ decays, which will be studied directly at the LHC ATLAS and CMS.

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arXiv2011

B -> Xsγconstraints on the top quark anomalous t-> cγcoupling

Observation of top quark flavor changing neutral process $t\to c +γ$ at the LHC would be the signal of physics beyond the Standard Model. If anomalous $t\to cγ$ coupling exists, it will affect the precisely measured $\mathcal{B}(\bar{B} \to X_s γ)$. In this paper, we study the effects of a dimension 5 anomalous $ tc γ$ operator in $\bar{B} \to X_s γ$ decay to derive constraints on its possible strength. It is found that, for real anomalous $t\to cγ$ coupling $κ_{\rm{tcR}}^γ$, the constraints correspond to the upper bounds $\mathcal B (t \to c + γ)<6.54 \times 10^{-5}$ (for $κ_{\rm{tcR}}^γ>0$) and $\mathcal B (t \to c + γ)<8.52 \times 10^{-5}$ (for $κ_{\rm{tcR}}^γ<0$), respectively, which are about the same order as the $5σ$ discovery potential of ATLAS ($9.4\times 10^{-5}$) and slightly lower than that of CMS ($4.1\times 10^{-4}$) with $10 \ \rm{fb}^{-1}$ integrated luminosity operating at $\sqrt{s} =14$ TeV.

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