arXiv subjects

Cai-Dian Lü

Publications and source records attributed to Cai-Dian Lü.

At least 55 records · Page 3Linked to original sources

arXiv2018

Perturbative QCD Analysis of Exclusive Processes $e^+e^-\rightarrow VP$ and $e^+e^-\rightarrow TP$

We study the $e^+e^-\to VP$ and $e^+e^-\to TP$ processes in the perturbative QCD approach based on $k_T$ factorization, where the $P,V$ and $T$ denotes a light pseudo-scalar, vector and tensor meson, respectively. We point out in the case of $e^+e^-\to TP$ transition due to charge conjugation invariance, only three channels are allowed: $e^+e^-\to a_2^{\pm} π^\mp$, $e^+e^-\to K_2^{*\pm} K^\mp$ and the V-spin suppressed $e^+e^-\to K_2^{*0} \bar K^0+\overline K_2^{*0} K^0 $. Cross sections of $e^+e^-\to VP$ and $e^+e^-\to TP$ at $\sqrt{s}=3.67$ GeV and $\sqrt{s}=10.58$ GeV are calculated and the invariant mass dependence is found to favor the $1/s^4$ power law. Most of our theoretical results are consistent with the available experimental data and other predictions can be tested at the ongoing BESIII and forthcoming Belle-II experiments.

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arXiv2017

Three-body decays $B \to ϕ(ρ) K γ$ in perturbative QCD approach

We study the three-body radiative decays $B\to ϕ(ρ) Kγ$ induced by a flavor-changing neutral current in the perturbative QCD approach. Pseudoscalar-vector ($PV$) distribution amplitudes (DAs) are introduced for the final-state $ϕK$ ($ρK$) pair to capture important infrared dynamics in the region with a small $PV$-pair invariant mass. The dependence of these $PV$ DAs on the parton momentum fraction is parametrized in terms of the Gegenbauer polynomials, and the dependence on the meson momentum fraction is derived through their normalizations to time-like $PV$ form factors. In addition to the dominant electromagnetic penguin, the subleading chromomagnetic penguin, quark-loop and annihilation diagrams are also calculated. After determining the $PV$ DAs from relevant branching-ratio data, the direct $CP$ asymmetries and decay spectra in the $PV$-pair invariant mass are predicted for each $B\to ϕ(ρ) Kγ$ mode.

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arXiv2017

Perturbative corrections to $B \to D$ form factors in QCD

We compute perturbative QCD corrections to $B \to D$ form factors at leading power in $Λ/m_b$, at large hadronic recoil, from the light-cone sum rules (LCSR) with $B$-meson distribution amplitudes in HQET. QCD factorization for the vacuum-to-$B$-meson correlation function with an interpolating current for the $D$-meson is demonstrated explicitly at one loop with the power counting scheme $m_c \sim {\cal O} \left (\sqrt{Λ\, m_b} \right ) $. The jet functions encoding information of the hard-collinear dynamics in the above-mentioned correlation function are complicated by the appearance of an additional hard-collinear scale $m_c$, compared to the counterparts entering the factorization formula of the vacuum-to-$B$-meson correction function for the construction of $B \to π$ from factors. Inspecting the next-to-leading-logarithmic sum rules for the form factors of $B \to D \ell ν$ indicates that perturbative corrections to the hard-collinear functions are more profound than that for the hard functions, with the default theory inputs, in the physical kinematic region. We further compute the subleading power correction induced by the three-particle quark-gluon distribution amplitudes of the $B$-meson at tree level employing the background gluon field approach. The LCSR predictions for the semileptonic $B \to D \ell ν$ form factors are then extrapolated to the entire kinematic region with the $z$-series parametrization. Phenomenological implications of our determinations for the form factors $f_{BD}^{+, 0}(q^2)$ are explored by investigating the (differential) branching fractions and the $R(D)$ ratio of $B \to D \ell ν$ and by determining the CKM matrix element $|V_{cb}|$ from the total decay rate of $B \to D μν_μ$.

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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

$b\to ss{\bar d} $ decay in Randall-Sundrum models

The extremely small branching ratio of $b\to ss{\bar d}$ decay in the Standard Model makes it a suitable channel to explore new-physics signals. We study this $ΔS=2$ process in Randall-Sundrum models, including the custodially protected and the bulk-Higgs Randall-Sundrum models. Exploring the experimentally favored parameter spaces of these models, it suggests a possible enhancement of the decay rate, compared to the Standard Model result, by at most two orders of magnitude.

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arXiv2016

Analysis of Charmless Two-body B decays in Factorization Assisted Topological Amplitude Approach

We analyze charmless two-body non-leptonic B decays $B \to PP, PV$ under the framework of factorization assisted topological amplitude approach, where $P(V)$ denotes a light pseudoscalar (vector) meson. Compared with the conventional flavor diagram approach, we consider flavor $SU(3)$ breaking effect assisted by factorization hypothesis for topological diagram amplitudes of different decay modes, factorizing out the corresponding decay constants and form factors. The non-perturbative parameters of topology diagram magnitudes $χ$ and strong phase $ϕ$ are universal that can be extracted by $χ^2$ fit from current abundant experimental data of charmless B decays. The number of free parameters and the $χ^2$ per degree of freedom are both reduced comparing with previous analysis. With these best fitted parameters, we predict branching fractions and $CP$ asymmetry parameters of nearly 100 $B_{u,d}$ and $B_s$ decay modes. The long-standing $ππ$ and $πK$-$CP$ puzzles are resolved simultaneoulsy.

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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

Constrain the UT angle $γ$ by CP violation parameters in $B^0 \to π^+ π^-$

We calculate the tree and penguin amplitudes in the $B^0\toπ^+π^-$ decay channel employing the perturbative QCD factorization approach. Using the amplitudes as input with the theoretical uncertainties sufficiently considered, we constrain the UT angle $γ$ to $53^\circ\leqγ\leq70^\circ$, from the measurements of the CP violation parameters $C_{π^+π^-}$ and $S_{π^+π^-}$ in $B^0\toπ^+π^-$. The U-spin breaking effect between $B^0\toπ^+π^-$ and $B_s^0\to K^+K^-$ is estimated to be around 30\%.

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arXiv2015

$S$-wave resonance contributions to the $B^0_{(s)}\to J/ψπ^+π^-$ and $B_s\toπ^+π^-μ^+μ^-$ decays

We study $S$-wave resonance contributions to the $B^0_{(s)}\to J/ψπ^+π^-$ and $B_s\toπ^+π^-\ell^+\ell^-$ decays in the perturbative QCD (PQCD) framework by introducing two-hadron distribution amplitudes for final states. The Breit-Wigner formula for the $f_0(500)$, $f_0(1500)$ and $f_0(1790)$ resonances and the Flatté model for the $f_0(980)$ resonance are adopted to parameterize the time-like scalar form factors in the two-pion distribution amplitudes, which include both resonant and nonresonant contributions. The resultant branching fraction and differential branching fraction in the pion-pair invariant mass for each resonance channel are consistent with experimental data. The determined $S$-wave two-pion distribution amplitudes, containing the information of both resonant and nonresonant rescattering phases, can be employed to predict direct CP asymmetries of other three-body hadronic $B$ meson decays in various localized regions of two-pion phase space.

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arXiv2014

Semileptonic decays $B_c^+\to D^{(*)}_{(s)}(l^+ν,l^+l^-,ν\barν)$ in the perturbative QCD approach

In this paper we study the semileptonic decays of $B_c^+\to D^{(*)}_{(s)}(l^+ν_l,l^+l^-,ν\barν)$ (here $l$ stands for $e$, $μ$, or $τ$). After evaluating the $B_c^+ \to (D_{(s)},D^*_{(s)})$ transition form factors $F_{0,+,T}(q^2)$ and $V(q^2), A_{0,1,2}(q^2), T_{1,2,3}(q^2)$ by employing the perturbative QCD factorization approach, we calculate the branching ratios for all these semileptonic decays. Our predictions for the values of the $B_c^+ \to D_{(s)}$ and $B_c^+ \to D^*_{(s)}$ transition form factors are consistent with those obtained by using other methods. The branching ratios of the decay modes with $\barνν$ are almost an order of magnitude larger than the corresponding decays with $l^+l^-$ after the summation over the three neutrino generations. The branching ratios for the decays with $b\to d$ transitions are much smaller than those decays with the $b\to s$ transitions, due to the Cabibbo-Kobayashi-Maskawa suppression. We define ratios $R_D$ and $R_{D^*}$ for the branching ratios with the $τ$ lepton versus $μ$, $e$ lepton final states to cancel the uncertainties of the form factors, which could possibly be tested in the near future.

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arXiv2014

Direct CP asymmetries of three-body $B$ decays in perturbative QCD

We propose a theoretical framework for analyzing three-body hadronic $B$ meson decays based on the perturbative QCD approach. The crucial nonperturbative input is a two-hadron distribution amplitude for final states, whose involved time-like form factor and rescattering phase are fit to relevant experimental data. Together with the short-distance strong phase from the $b$-quark decay kernel, we are able to make predictions for direct CP asymmetries in, for example, the $B^\pm \to π^+ π^- π^\pm$ and $π^+π^-K^\pm$ modes, which are consistent with the LHCb data in various localized regions of phase space. Applications of our formalism to other three-body hadronic and radiative $B$ meson decays are mentioned.

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arXiv2014

The NLO twist-3 contributions to $B \to π$ form factors in $k_{T}$ factorization

In this paper, we calculate the next-to-leading-order (NLO) twist-3 contribution to the form factors of $B \to π$ transitions by employing the $k_{T}$ factorization theorem. All the infrared divergences regulated by the logarithms $\ln(k_{iT}^{2})$ cancel between those from the quark diagrams and from the effective diagrams for the initial $B$ meson wave function and the final pion meson wave function. An infrared finite NLO hard kernel is therefore obtained, which confirms the application of the $k_{T}$ factorization theorem to $B$ meson semileptonic decays at twist-3 level. From our analytical and numerical evaluations, we find that the NLO twist-3 contributions to the form factors $f^{+,0}(q^2)$ of $B \to π$ transition are similar in size, but have an opposite sign with the NLO twist-2 contribution, which leads to a large cancelation between these two NLO parts. For the case of $f^+(0)$, for example, the $24\%$ NLO twist-2 enhancement to the full LO prediction is largely canceled by the negative ( about $-17\%$ ) NLO twist-3 contribution, leaving a small and stable $7\%$ enhancement to the full LO prediction in the whole range of $0\leq q^2\leq 12$ GeV$^2$. At the full NLO level, the perturbative QCD prediction is $F^{B \to π}(0)=0.269^{+0.054}_{-0.050}$. We also studied the possible effects on the pQCD predictions when different sets of the B meson and pion distribution amplitudes are used in the numerical evaluation.

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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

Perturbative QCD study of $B_s$ decays to a pseudoscalar meson and a tensor meson

We study two-body hadronic $B_s\to PT$ decays, with $P (T)$ being a light pseudoscalar (tensor) meson, in the perturbative QCD approach. The CP-averaged branching ratios and the direct CP asymmetries of the $ΔS=0$ modes are predicted, where $ΔS$ is the difference between the strange numbers of final and initial states. We also define and calculate experimental observables for the $ΔS=1$ modes under the $B_s^0-\bar{B}_s^0$ mixing, including CP averaged branching ratios, time-integrated CP asymmetries, and the CP observables $C_{f}$, $D_{f}$ and $S_{f}$. Results are compared to the $B_s\to PV$ ones in the literature, and to the $B\to PT$ ones, which indicate considerable U-spin symmetry breaking. Our work provides theoretical predictions for the $B_s\to PT$ decays for the first time, some of which will be potentially measurable at future experiments.

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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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arXiv2013

Constraints on the R-parity violating couplings using the newest measurement of the decay $B_s^0 \rightarrow μ^+ μ^-$

Recently, the LHCb collaboration reported the first evidence for the decay $B_s^0 \rightarrow μ^+ μ^-$. A branching ratio of $\mathcal{B}(B_s^0 \rightarrow μ^+ μ^-) = (3.2^{+1.5}_{-1.2}) \times 10^{-9}$ is given. Using the newest data, and together with the most precise predictions of the Standard Model contributions to the decay, we derive the constraints on the combinations of the R-parity violating parameters. Our results are several orders of magnitudes stronger than the constraints in the previous literature. We also update the constraints on the relevant parameters using the upper limit of $\mathcal{B}(B_d^0 \rightarrow μ^+ μ^-)$.

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arXiv2013

Pure annihilation type $ D\to PP(V)$ decays in the perturbative QCD approach

The annihilation type diagrams are difficult to calculate in any kind of models or method. Encouraged by the the successful calculation of pure annihilation type B decays in the perturbative QCD factorization approach, we calculate the pure annihilation type $D\to PP(V)$ decays in the perturbative QCD approach based on the $k_T$ factorization. Although the expansion parameter $1/m_D$ is not very small, our leading order numerical results agree with the existing experiment data for most channels. We expect the more accurate observation from experiments, which can help us learn about the dynamics of $D$ meson weak decays.

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arXiv2013

Hunting for a scalar glueball in exclusive B decays

Using flavor SU(3) symmetry for the light quarks validated by the available experimental data, we propose an intuitive way to hunt for a scalar glueball in B decays. In the presence of mixing between the glueball and ordinary scalar mesons, we explore the extraction of the mixing parameters. In particular, we discuss the implication from the recently available experimental data and show the sensitivities of B decays as a probe to the scalar structures. The future Super KEKB factory would allow access to establishing the mixing pattern among the scalars, and possibly allow one to disentangle the long-standing puzzle concerning the existence and mixings of the scalar glueball predicted by QCD.

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