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Xin-Qiang Li

Publications and source records attributed to Xin-Qiang Li.

87 records · Page 5Linked to original sources
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

Determination of the strong coupling $g_{B^* Bπ}$ from semi-leptonic $B\to π\ell ν$ decay

According to heavy-meson chiral perturbation theory, the vector form factor $f_+(q^2)$ of exclusive semi-leptonic decay $B\to π\ell ν$ is closely related, at least in the soft-pion region (i.e., $q^{2} \sim (m_B-m_π)^2$), to the strong coupling $g_{B^* Bπ}$ or the normalized coupling $\hat g$. Combining the precisely measured $q^2$ spectrum of $B\to π\ell ν$ decay by the BaBar and Belle collaborations with several parametrizations of the form factor $f_+(q^2)$, we can extract these couplings from the residue of the form factor at the $B^*$ pole, which relies on an extrapolation of the form factor from the semi-leptonic region to the unphysical point $q^2=m_{B^*}^2$. Comparing the extracted values with the other experimental and theoretical estimates, we can test these various form-factor parametrizations, which differ from each other by the amount of physical information embedded in. It is found that the extracted values based on the BK, BZ and BCL parametrizations are consistent with each other and roughly in agreement with the other theoretical and lattice estimates, while the BGL ansatz, featured by a spurious, unwanted pole at the threshold of the cut, gives a neatly larger value.

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arXiv2011

The effects of a family non-universal Z-prime boson on B--->pipi decays

Motivated by the measured large branching ratio of $\bar{B}^{0}\toπ^0π^0$ (the so-called "$ππ$" puzzle), we investigate the effects of a family non-universal $Z^{\prime}$ model on the tree-dominated $B\toππ$ decays. We find that the $Z^{\prime}$ coupling parameter $ζ_{d}^{LR}\sim0.05$ with a nontrivial new weak phase $ϕ_d^L\sim-50^{\circ}$, which is relevant to the $Z^{\prime}$ contributions to the QCD penguin sector $\triangle C_5$, is needed to reconcile the observed discrepancy. Combined with the recent fitting results from $B\toπK$, $πK^{\ast}$ and $ρK$ decays, the $Z^{\prime}$ parameter spaces are severely reduced but still not excluded entirely, implying that both the "$ππ$" and "$πK$" puzzles could be accommodated simultaneously within such a family non-universal $Z^{\prime}$ model.

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arXiv2010

Heavy-to-light currents at NNLO in SCET and semi-inclusive anti-B ---> X(s) l+ l- decay

We perform the two-loop matching calculation for heavy-to-light currents from QCD onto soft-collinear effective theory for the complete set of Dirac structures. The newly obtained matching coefficients enter several phenomenological applications, of which we discuss heavy-to-light form factor ratios and exclusive radiative decays, as well as the semi-inclusive decay anti-B ---> X(s) l+ l-. For this decay, we observe a significant shift of the forward-backward asymmetry zero and find q_0^2 = (3.34 +0.22 -0.25) GeV^2 for an invariant mass cut m_X^cut=2.0 GeV.

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arXiv2010

$B \to K^{\ast} l^+ l^-$, $K l^+ l^-$ decays in a family non-universal $Z^{\prime}$ model

Motivated by the observed forward-backward asymmetry in $B\to K^{\ast} l^+ l^-$ decay, we perform a detailed analysis of this decay mode within a family non-universal $Z^{\prime}$ model. With the related coupling $Z^{\prime}-\bar{s}b$ constrained by $\bar{B}_s-B_s$ mixing, $B\to πK$, and $B\to X_s μ^{+}μ^{-}$ decays, we look for further constraint on the couplings $Z^{\prime}-μ^{+}μ^{-}$ from $A_{FB}(B\to K^{\ast} μ^{+}μ^{-})_{0 {\rm GeV}^2\leqslant q^2\leqslant 2 {\rm GeV}^2}$ and get numerically $B_{μμ}^{L,R}\sim{\cal O}(10^{-2})$. Moreover, we find that the relations, $B_{μμ}^{L}<B_{μμ}^{R}$ and $B_{μμ}^{L}+B_{μμ}^{R}<0$, with a small negative $ϕ_{s}^{L}$, are crucial to moderate the discrepancy for $A_{FB}(B\to K^{\ast} μ^{+}μ^{-})$ between the SM prediction and the experimental data. Numerically, comparing with the SM prediction, we find that $A_{FB}(B\to K^{\ast} μ^{+}μ^{-})_{0 {\rm GeV}^2\leqslant q^2\leqslant 2 {\rm GeV}^2}$ could be enhanced about 80% and 50% by $Z^{\prime}$ contribution at most in scenarios S1 and S2, corresponding to the two fitted results of $ϕ_{s}$ by UTfit collaboration, respectively. However, the results are still about $1.5σ$ lower than the experimental measurement.

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arXiv2010

Family Non-universal Z^\prime effects on \bar{B}_q-B_q$ mixing, B\to X_s μ^+μ^- and B_s\to μ^+μ^- Decays

Motivated by the large discrepancy of CP-violating phase in $\bar{B}_s-B_s$ mixing between the experimental data and the Standard Model prediction, we pursue possible solutions within a family non-universal $Z^{\prime}$ model. Within such a specific model, we find that both the $\bar{B}_s-B_s$ mixing anomaly and the well-known "$πK$ puzzle" could be moderated simultaneously with a nontrivial new weak phase, $ϕ^L_s\sim-72^{\circ}$ (S1) or $-82^{\circ}$ (S2). With the stringently constrained $Z^{\prime}$ coupling $B_{sb}^{L}$, we then study the $Z^{\prime}$ effects on the rare $B\to X_s μ^+μ^-$ and $B_s\to μ^+μ^-$ decays, which are also induced by the same $b\to s$ transition. The observables of $B\to X_s μ^+μ^-$, at both high and low $q^2$ regions, are found to be able to put strong constraints on the $μ-μ-Z^{\prime}$ coupling, $B_{μμ}^{L,R}\sim10^{-2}$. It is also shown that the combined constraints from $\bar{B}_s-B_s$ mixing, $B\to πK$ and $B\to X_s μ^+μ^-$ do not allow a large $Z^{\prime}$ contribution to the pure leptonic $B_s\toμ^+μ^-$ decay.

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arXiv2009

NNLO vertex corrections to non-leptonic B decays: Tree amplitudes

The colour-suppressed tree amplitude in non-leptonic B decays is particularly sensitive to perturbative and non-perturbative corrections. We calculate the two-loop (NNLO) vertex corrections to the colour-suppressed and colour-allowed tree amplitudes in QCD factorization. Our results are given completely analytically, including the full dependence on the charm quark mass. We then update theoretical predictions for a range of interesting observables derived from pi pi, pi rho, and rho rho final states that do not depend (significantly) on penguin contributions, and hence are now available with NNLO accuracy. We observe good agreement with experimental data within experimental and theoretical errors, except for observables involving the pi^0 pi^0 branching fraction.

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arXiv2009

Constraints on the nonuniversal Z^\prime couplings from B\toπK, πK^{\ast} and ρK Decays

Motivated by the large difference between the direct CP asymmetries $A_{CP}(B^-\to π^0 K^-)$ and $A_{CP}(\bar{B}^{0}\to π^{+} K^{-})$, we combine the up-to-date experimental information on $B\toπK$, $πK^{\ast}$ and $ρK$ decays to pursue possible solutions with the nonuniversal $Z^{\prime}$ model. Detailed analyses of the relative impacts of different types of couplings are presented in four specific cases. Numerically, we find that the new coupling parameters, $ξ^{LL}$ and $ξ^{LR}$ with a common nontrivial new weak phase $ϕ_L\sim-86^{\circ}$, which are relevant to the $Z^{\prime}$ contributions to the electroweak penguin sector $\triangle C_9$ and $\triangle C_7$, are crucial to the observed "$πK$ puzzle". Furthermore, they are found to be definitely unequal and opposite in sign. We also find that $A_{CP}(B^-\to ρ^0 K^-)$ can put a strong constraint on the new $Z^{\prime}$ couplings, which implies the $Z^{\prime}$ contributions to the coefficient of QCD penguins operator $O_3$ involving the parameter $ζ^{LL}$ required.

hep-ph↗
arXiv2009

Hadronic B decays in the MSSM with large tan(beta)

We present an analysis of non-leptonic B decays in the minimally flavour-violating MSSM with large tan(beta). We relate the Wilson coefficients of the relevant hadronic scalar operators to leptonic observables, showing that the present limits on the Bs->mu+ mu- and B+->tau nu_tau branching fractions exclude any visible effect in hadronic decays. We study the transverse helicity amplitudes of B->VV decays, which exhibit an enhanced sensitivity to the scalar operators, showing that even though an order one modification relative to the SM is not excluded in some of these amplitudes, they are too small to be detected at B factories.

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arXiv2008

Revisiting B\toπK, πK^{\ast} and ρK decays: CP violations and implication for New Physics

Combining the up-to-date experimental information on $B\toπK, πK^{\ast}$ and $ρK$ decays, we revisit the decay rates and CP asymmetries of these decays within the framework of QCD factorization. Using an infrared finite gluon propagator of Cornwall prescription, we find that the time-like annihilation amplitude could contribute a large strong phase, while the space-like hard spectator scattering amplitude is real. Numerically, we find that all the branching ratios and most of the direct CP violations, except $A_{CP}(B^{\pm}\to K^{\pm}π^{0})$, agree with the current experimental data with an effective gluon mass $m_g\simeq0.5 {\rm GeV}$. Taking the unmatched difference in direct CP violations between $B\toπ^{0} K^{\pm}$ and $π^{\mp}K^{\pm}$ decays as a hint of new physics, we perform a model-independent analysis of new physics contributions with a set of $\bar{s}(1+γ_{5})b\otimes\bar{q}(1+γ_{5})q$ (q=u,d) operators. Detail analyses of the relative impacts of the operators are presented in five cases. Fitting the twelve decay modes, parameter spaces are found generally with nontrivial weak phases. Our results may indicate that both strong phase from annihilation amplitude and new weak phase from new physics are needed to resolve the $πK$ puzzle. To further test the new physics hypothesis, the mixing-induced CP violations in $B\toπ^{0}K_{S}$ and $ρ^{0}K_{S}$ are discussed and good agreements with the recent experimental data are found.

hep-ph↗
arXiv2008

Contribution of $DK$ Continuum in the QCD Sum Rule for $D_{sJ}(2317)$

Using the soft-pion theorem and the assumption on the final-state interactions, we include the contribution of $DK$ continuum into the QCD sum rules for $D_{sJ}(2317)$ meson. We find that this contribution can significantly lower the mass and the decay constant of $D_s(0^+)$ state. For the value of the current quark mass $m_c(m_c)=1.286 {\rm GeV}$, we obtain the mass of $D_s(0^+)$ $M=2.33 \pm 0.02 {\rm GeV}$ in the interval $s_0=7.5-8.0 {\rm GeV}^2$, being in agreement with the experimental data, and the vector current decay constant of $D_s(0^+)$ $f_0=0.128 \pm 0.013 {\rm GeV}$, much lower than those obtained in previous literature.

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arXiv2007

Constraints on the anomalous tensor operators from B\toϕK^{\ast}, ηK^{\ast}$ and ηK decays

We investigate whether the anomalous tensor operators with the Lorentz structure $σ_{μν} (1+γ_{5})\otimes σ^{μν}(1+γ_{5})$, which could provide a simple resolution to the polarization anomaly observed in $B\to ϕK^{\ast}$ decays, could also provide a coherent resolution to the large ${\cal B}(B\to ηK^{\ast})$ and survive bounds from $B\toηK$ decays. Parameter spaces satisfying all these experimental data are obtained, and found to be dominated by the color-octet tensor operator contribution. Constraints for the equivalent solution with $(1+γ_{5})\otimes(1+γ_{5})$ operators are also derived and found to be dominated by the color-singlet one. With the constrained parameter spaces, we finally give predictions for $B_{s}\to ϕϕ$ decay, which could be tested at the Fermilab Tevatron and the LHC-b experiments.

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arXiv2005

Reexamining $ B\to ππ,πK$ Decays in QCD Factorization Approach

Motivated by the recent experimental data, we have revisited the $B\to πK,ππ$ decays in the framework of QCD factorization, with inclusion of the important strong penguin corrections of order $α_s^2$ induced by $b\to D g^\ast g^\ast$ ($D=d$ or $s$ and $g^\ast$ denotes an off-shell gluon) transitions. We find that these higher order strong penguin contributions can provide $\sim 30%$ enhancement to the penguin-dominated $B\to πK$ decay rates, and such an enhancement can improve the consistency between the theoretical predictions and the experimental data significantly, while for the tree-dominated $B\to ππ$ decays, these higher order contributions play only a minor role. When these strong penguin contributions are summed, only a small strong phase remains and the direct CP asymmetries get small corrections. We also find patterns of the ratios between the CP-averaged branching fractions remain nearly unaffected even after including these higher order corrections and the $πK$ puzzle still persists. Our results may indicate that to resolve the puzzle one would have to resort to new physics contributions in the electroweak penguin sector as found by Buras {\it et al}.

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