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Feng-Kun Guo

Publications and source records attributed to Feng-Kun Guo.

At least 145 records · Page 8Linked to original sources
arXiv2020

New parametrization of the form factors in $\bar{B}\to D\ell\barν_\ell$ decays

A new model-independent parametrization is proposed for the hadronic form factors in the semileptonic $\bar{B}\to D\ell\barν_\ell$ decay. By a combined consideration of the recent experimental and lattice QCD data, we determine precisely the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cb}|=41.01(75)\times 10^{-3}$ and the ratio $\mathcal{R}_D=\frac{\mathcal{BR}(\bar{B}\to D τ\barν_τ)}{\mathcal{BR}(\bar{B}\to D \ell \barν_\ell)}=0.301(5)$. The coefficients in this parametrization, related to phase shifts by sumrulelike dispersion relations and hence called phase moments, encode important scattering information of the $\bar{B}\bar{D}$ interactions which are poorly known so far. Thus, we give strong hints about the existence of at least one bound and one virtual $\bar B \bar D$ $S$-wave $0^+$ states, subject to uncertainties produced by potentially sizable inelastic effects. This formalism is also applicable for any other semileptonic processes induced by the weak $b\to c$ transition.

hep-ph↗
arXiv2020

Threshold cusps and triangle singularities in hadronic reactions

The spectrum of hadrons is the manifestation of color confinement of quantum chromodynamics. Hadronic resonances correspond to poles of the S-matrix. Since 2003, lots of new hadron resonant structures were discovered in the mass regions from light mesons to hadrons containing a pair of a heavy quark and an antiquark. Many of them are candidates of exotic hadrons, and they are usually observed as peaks in invariant mass distributions. However, the S-matrix also has kinematical singularities due to the on-shellness of intermediate particles for a process, such as two-body thresholds and triangle singularities, and they can produce peaks as well. On the one hand, such singularities may be misidentified as resonances; on the other hand, they can be used as tools for precision measurements. In this paper, we review the threshold cusps and various triangle singularities in hadronic reactions, paying attention to their manifestations in phenomena related to exotic hadron candidates.

hep-ph↗
arXiv2019

Revisiting $X(3872)\to D^0 \bar{D}^0 π^0$ in XEFT

The calculation of the decay $X(3872)\to D^0 \bar{D}^0 π^0$ in effective field theory is revisited to include final state $π^0 D^0$, $π^0 \bar{D}^0$and $D^0\bar{D}^0$ rescattering diagrams. These introduce significant uncertainty into the prediction for the partial width as a function of the binding energy. The differential distribution in the pion energy is also studied for the first time. The normalization of the distribution is again quite uncertain due to higher order effects but the shape of the distribution is unaffected by higher order corrections. Furthermore the shape of the distribution and the location of the peak are sensitive to the binding energy of $X(3872)$. The shape is strongly impacted by the presence of virtual $D^{*0}$ graphs which highlights the molecular nature of the $X(3872)$. Measurement of the pion energy distribution in the decay $X(3872)\to D^0 \bar{D}^0 π^0$ can reveal interesting information about the binding nature of the $X(3872)$.

hep-ph↗
arXiv2019

Light-quark components analysis and the nature of the $Y(4260)$

We study the processes $e^+ e^- \to Y(4260) \to J/ψππ(K\bar{K})$. The strong final-state interactions, especially the coupled-channel ($ππ$ and $K\bar{K}$) final-state interaction in the $S$-wave are taken into account in a model-independent way using dispersion theory. It is found that the light-quark SU(3) octet state plays a significant role in these transitions, implying that the $Y(4260)$ contains a large light-quark component. Our findings suggest that the $Y(4260)$ is neither a hybrid nor a conventional charmonium state. Furthermore, through an analysis of the ratio of the light-quark SU(3) octet and singlet components, we show that the $Y(4260)$ does not behave like a pure $\bar D D_1$ hadronic molecule as well.

hep-ph↗
arXiv2019

Triangle singularities in ${J/ψ\rightarrowηπ^0ϕ}$ and ${π^0π^0ϕ}$

The BESIII Collaboration recently reported the observation of the $a_0(980)^0-f_0(980)$ mixing in the isospin breaking decay $J/ψ\to ηπ^0ϕ$. In the Dalitz plot for that decay with the $η$ reconstructed from two photons, there is a band around $1.4$~GeV on the $π^0 ϕ$ distribution. In general, this peak can be due to a resonance or a kinematic effect. In this paper, we study the effects of a set of $K^*K\bar K$ triangle diagrams, and show that due to triangle singularities such diagrams can lead to a peak around 1.4~GeV in the $π^0ϕ$ invariant mass distribution. The Dalitz plot induced by such a mechanism has a feature consistent with the BESIII observation, namely events along the band accumulate at both ends close to the Dalitz plot boundary. The effect of the same mechanism on the $J/ψ\to π^0π^0ϕ$ and $J/ψ\to ηπ^0 K^+K^-$ decays are also investigated. We suggest to take more data for the $J/ψ\to ηπ^0ϕ\to ηπ^0 K^+ K^-$ and check whether the structure around $1.4$~GeV persists for the $K^+K^-$ invariant mass away from the $ϕ$ mass region. This is crucial for understanding whether the band is due to triangle singularities or due to a resonance. Were it the latter, the band should remain while it would not if it is due to the former.

hep-ph↗
arXiv2019

Decays of $P_c$ into $J/ψN$ and $η_cN$ with heavy quark spin symmetry

We investigate the consequences of heavy quark spin symmetry (HQSS) on hidden-charm pentaquark $P_c$ states. As has been proposed before, assuming the $P_c(4440)$ and the $P_c(4457)$ as $S$-wave $\bar{D}^*Σ_c$ molecules, seven hadronic molecular states composed of $\bar{D}Σ_c$, $\bar{D}Σ_c^*$, $\bar{D}^*Σ_c$, and $\bar{D}^*Σ_c^*$ can be obtained, with the $\bar{D}Σ_c$ molecule corresponding to the $P_c(4312)$. These seven states can decay into $J/ψN$ and $η_c N$, and we use HQSS to predict ratios of partial widths of the $S$-wave decays. For the decays into $J/ψN$, it is found that among all six $P_c$ molecules with spin $1/2$ or $3/2$, at least four states decay much more easily into the $J/ψN$ than the $P_c(4312)$, and two of them couple dominantly to the $\bar D^*Σ_c^*$. While no significant peak around the $\bar{D}^*Σ_c^*$ threshold is found in the $J/ψp$ distribution, these higher $P_c$ states either are produced with lower rates or some special production mechanism for the observed $P_c$ states might play an important role, such as an intricate interplay between the production of pentaquarks and triangle singularities.

hep-ph↗
arXiv2019

Chromopolarizabilities of bottomonia from the $Υ(2S,3S,4S) \to Υ(1S,2S)ππ$ transitions

The dipion transitions $Υ(2S,3S,4S) \to Υ(1S,2S)ππ$ are systematically studied by considering the mechanisms of the hadronization of soft gluons, exchanging the bottomoniumlike $Z_b$ states, and the bottom-meson loops. The strong pion-pion final-state interaction, especially including the channel coupling to $K\bar{K}$ in the $S$-wave, is taken into account in a model-independent way using the dispersion theory. Through fitting to the available experimental data, we extract values of the transition chromopolarizabilities $|α_{Υ(mS)Υ(nS)}|$, which measure the chromoelectric couplings of the bottomonia with soft gluons. It is found that the $Z_b$ exchange has a slight impact on the extracted chromopolarizablity values, and the obtained $|α_{Υ(2S)Υ(1S)}|$ considering the $Z_b$ exchange is $(0.29\pm 0.20)~\text{GeV}^{-3}$. Our results could be useful in studying the interactions of bottomonium with light hadrons.

hep-ph↗
arXiv2019

Aspects of the QCD $θ$-vacuum

This paper addresses two aspects concerning the $θ$-vacuum of Quantum Chromodynamics. First, large-$N_c$ chiral perturbation theory is used to calculate the first two non-trivial cumulants of the distribution of the winding number, i.\,e. the topological susceptibility, $χ_\mathrm{top}$, and the fourth cumulant, $c_4$, up to next-to-leading order. Their large-$N_c$ scaling is discussed, and compared to lattice results. It is found that $χ_\mathrm{top}=\mathcal{O}(N_c^0)$, as known before, and $c_4=\mathcal{O}(N_c^{-3})$, correcting the assumption of $\mathcal{O}(N_c^{-2})$ in the literature. Second, we discuss the properties of QCD at $θ\simπ$ using chiral perturbation theory for the case of $2+1$ light flavors, i.\,e. by taking the strange quark mass heavier than the degenerate up and down quark masses. It is shown that --- in accordance with previous findings for $N_f=2$ and $N_f=3$ mass-degenerate flavors --- in the region $θ\simπ$ two vacuum states coexist, which become degenerate at $θ=π$. The wall tension of the energy barrier between these degenerate vacua is determined as well as the decay rate of a false vacuum.

hep-th↗
arXiv2019

Novel Method for Precisely Measuring the $X(3872)$ Mass

The $X(3872)$ is the first and the most interesting one amongst the abundant $XYZ$ states. Its mass coincides exactly with the $D^0\bar D^{*0}$ threshold with an uncertainty of 180 keV. Precise knowledge of its mass is crucial to understand the $X(3872)$. However, whether it is above or below the $D^0\bar D^{*0}$ threshold is still unknown. We propose a completely new method to measure the $X(3872)$ mass precisely by measuring the $X(3872)γ$ line shape between 4010 and 4020 MeV, which is strongly sensitive to the $X(3872)$ mass relative to the $D^0\bar D^{*0}$ threshold due to a triangle singularity. This method can be applied to experiments which produce copious $D^{*0}\bar D^{*0}$ pairs, such as electron-positron, proton-antiproton and other experiments, and may lead to much more precise knowledge about the $X(3872)$ mass.

hep-ph↗
arXiv2019

Isospin breaking decays as a diagnosis of the hadronic molecular structure of the $P_c(4457)$

The LHCb Collaboration announced the observation of three narrow structures consistent with hidden-charm pentaquark states. They are candidates of hadronic molecules formed of a pair of a charmed baryon and an anticharmed meson. Among them, the $P_c(4457)$ mass is consistent with earlier predictions of a $Σ_c\bar D^*$ molecule with $I=1/2$. We point out that if such a picture were true, one would have $\mathcal{B}(P_c(4457)\to J/ψΔ^+)/\mathcal{B}(P_c(4457)\to J/ψp)$ at the level ranging from a few percent to about 30%. Such a large isospin breaking decay ratio is two to three orders of magnitude larger than that for normal hadron resonances. It is a unique feature of the $Σ_c\bar D^*$ molecular model, and can be checked by LHCb.

hep-ph↗
arXiv2019

Constraints on Disconnected Contributions in $ππ$ Scattering

The accuracy of the lattice QCD computation of hadron-hadron scattering at low isospin depends critically on the ability to compute correlation functions with fermionic disconnected Wick contractions. This happens, for instance, in isospin $I=0$ $ππ$ scattering, which receives contributions from rectangular and vacuum types of contractions among other easier calculable ones. Combining Lüscher's formula and partially-quenched chiral perturbation theory, we provide precise theory predictions of the discrete energy levels extracted from specific linear combinations of lattice correlation functions corresponding to various types of contractions. Expressions are provided for extracting the unphysical low-energy constants in the partially-quenched chiral perturbation theory from the energy levels for these contractions. The predictions for the rectangular and vacuum contractions may serve as solid tests of the accuracy for existing and future lattice studies of $ππ$ scattering.

hep-lat↗
arXiv2019

Nature of the $Y(4260)$: A light-quark perspective

The $Y(4260)$ has been one of the most puzzling pieces among the so-called $XYZ$ states. In this paper, we try to gain insights into the structure of the $Y(4260)$ from the light-quark perspective. We study the dipion invariant mass spectrum of the $e^+ e^- \to Y(4260) \to J/ψπ^+π^-$ process and the ratio of the cross sections ${σ(e^+e^- \to J/ψK^+ K^-)}/{σ(e^+e^- \to J/ψπ^+π^-)}$. In particular, we consider the effects of different light-quark SU(3) eigenstates inside the $Y(4260)$. The strong pion-pion final-state interactions as well as the $K\bar{K}$ coupled channel in the $S$-wave are taken into account in a model-independent way using dispersion theory. We find that the SU(3) octet state plays a significant role in these transitions, implying that the $Y(4260)$ contains a large light-quark component. Our findings suggest that the $Y(4260)$ is neither a hybrid nor a conventional charmonium state, and they are consistent with the $Y(4260)$ having a sizeable $\bar D D_1$ component which, however, is not completely dominant.

hep-ph↗
arXiv2019

Implications of chiral symmetry on $S$-wave pionic resonances and the scalar charmed mesons

The chiral symmetry of QCD requires energy-dependent pionic strong interactions at low energies. This constraint, however, is not fulfilled by the usual Breit--Wigner parameterization of pionic resonances, leading to masses larger than the real ones. We derive relations between nonleptonic three-body decays of the $B$-meson into a $D$-meson and a pair of light pseudoscalar mesons based on SU(3) chiral symmetry. Employing effective field theory methods, we demonstrate that taking into account the final-state interactions, the experimental data of the decays $B^-\to D^+π^-π^-$, $B_s^0\to \bar{D}^0K^-π^+$, $B^0\to\bar{D}^0π^-π^+$, $B^-\to D^+π^-K^-$ and $B^0\to\bar{D}^0π^-K^+$ can all be described by the nonperturbative $π/η/K$-$D/D_s$ scattering amplitudes previously obtained from a combination of chiral effective field theory and lattice QCD calculations. The results provide a strong support of the scenario that the broad scalar charmed meson $D^\ast_0(2400)$ should be replaced by two states, the lower one of which has a mass of around 2.1 GeV, much smaller than that extracted from experimental data using a Breit--Wigner parameterization.

hep-ph↗
arXiv2019

Production of the $Z_b^{(\prime)}$ states from the $Υ(5S,6S)$ decays

In the present work, we investigate the production mechanism of the $Z_b(10610)$ and $Z_b(10650)$ states from the $Υ(5S,6S)$ decays. Two types of bottom-meson loops are discussed. We show that the loop contributions with all intermediate states being the $S$-wave ground state bottom mesons are negligible, while the loops with one bottom meson being the broad $B_0^\ast$ or $B_1^\prime$ resonance could provide the dominant contributions to the $Υ(5S) \to Z_b^{(\prime)} π$. It is found that such a mechanism is not suppressed by the large width of the $B_0/B_1'$ resonance. In addition, we also estimate the branching ratios for the $Υ(6S) \to Z_b^{(\prime)} π$ which could be tested by future precise measurements at Belle-II.

hep-ph↗
arXiv2019

Effective Field Theory in The Study of Long Range Nuclear Parity Violation on Lattice

A non-zero signal $A_γ^\mathrm{np}=(-3.0\pm1.4\pm0.2)\times 10^{-8}$ of the gamma-ray asymmetry in the neutron-proton capture was recently reported by the NPDGamma Collaboration which provides the first determination of the $ΔI=1$ parity-odd pion-nucleon coupling constant $h_π^1=(2.6\pm 1.2\pm 0.2)\times 10^{-7}$. The ability to reproduce this value from first principles serves as a direct test of our current understanding of the interplay between the strong and weak interaction at low energy. To motivate new lattice studies of $h_π^1$, we review the current status of the theoretical understanding of this coupling, which includes our recent work that relates it to a nucleon mass-splitting by a soft-pion theorem. We further investigate the possibility of calculating the mass-splitting on the lattice by providing effective field theory parameterizations of all the involved quark contraction diagrams. We show that the lattice calculations of the easier connected diagrams will provide information of the chiral logarithms in the much harder quark loop diagrams and thus help in the chiral extrapolation of the latter.

nucl-th↗
arXiv2018

Interactions between vector mesons and dynamically generated resonances

The $ρρ$ interaction and the corresponding dynamically generated bound states are revisited. We demonstrate that an improved unitarization method is necessary to study the pole structures of amplitudes outside the near-threshold region. In this work, we extend the study of the covariant $ρρ$ scattering in a unitarized chiral theory to the $S$-wave interactions for the whole vector-meson nonet. We demonstrate that there are unphysical left-hand cuts in the on-shell factorization approach of the Bethe-Salpeter equation. This is in conflict with the correct analytic behavior and makes the so-obtained poles, corresponding to possible bound states or resonances, unreliable. To avoid this difficulty, we employ the first iterated solution of the $N/D$ method and investigate the possible dynamically generated resonances from vector-vector interactions. A comparison with the results from the nonrelativistic calculation is provided as well.

hep-ph↗
arXiv2018

New spectrum of negative-parity doubly charmed baryons: Possibility of two quasistable states

The discovery of $Ξ_{cc}^{++}$ by the LHCb Collaboration triggers predictions of more doubly charmed baryons. By taking into account both the $P$-wave excitations between the two charm quarks and the scattering of light pseudoscalar mesons off the ground state doubly charmed baryons, a set of negative-parity spin-1/2 doubly charmed baryons are predicted already from a unitarized version of leading order chiral perturbation theory. Moreover, employing heavy antiquark-diquark symmetry the relevant low-energy constants in the next-to-leading order are connected with those describing light pseudoscalar mesons scattering off charmed mesons, which have been well determined from lattice calculations and experimental data. Our calculations result in a spectrum richer than that of heavy mesons. We find two very narrow $J^P=1/2^-$ $Ω_{cc}^P$, which very likely decay into $Ω_{cc}π^0$ breaking isospin symmetry. In the isospin-1/2 $Ξ_{cc}^P$ sector, three states are predicted to exist below 4.2~GeV with the lowest one being narrow and the other two rather broad. We suggest to search for the $Ξ_{cc}^{P}$ states in the $Ξ_{cc}^{++}π^-$ mode. Searching for them and their analogues are helpful to establish the hadron spectrum.

hep-ph↗