arXiv subjects

Feng-Kun Guo

Publications and source records attributed to Feng-Kun Guo.

At least 163 records · Page 9Linked to original sources

arXiv2018

Towards a new paradigm for heavy-light meson spectroscopy

Since 2003 many new hadrons, including the lowest-lying positive-parity charm-strange mesons ${D_{s0}^*(2317)}$ and ${D_{s1}(2460)}$, were observed that do not conform with quark model expectations. It was recently demonstrated that various puzzles in the charm meson spectrum find a natural resolution, if the SU(3) multiplets for the lightest scalar and axial-vector states, amongst them the ${D_{s0}^*(2317)}$ and the ${D_{s1}(2460)}$, owe their existence to the nonperturbative dynamics of Goldstone-Boson scattering off $D_{(s)}$ and $D^*_{(s)}$ mesons. Most importantly the ordering of the lightest strange and nonstrange scalars becomes natural. In this work we demonstrate for the first time that this mechanism is strongly supported by the recent high quality data on the ${B^-\to D^+π^-π^- }$ provided by the LHCb experiment. This implies that the lowest quark-model positive-parity charm mesons, together with their bottom counterparts, if realized in nature, do not form the ground-state multiplet. This is similar to the pattern that has been established for the scalar mesons made from light up, down and strange quarks, where the lowest multiplet is considered to be made of states not described by the quark model. In a broader view, the hadron spectrum must be viewed as more than a collection of quark model states.

hep-ph↗

arXiv2018

Interactions of Charmed Mesons with Light Pseudoscalar Mesons from Lattice QCD and Implications on the Nature of the D_{s0}^*(2317)

We study the scattering of light pseudoscalar mesons ($π$, $K$) off charmed mesons ($D$, $D_s$) in full lattice QCD. The S-wave scattering lengths are calculated using Lüscher's finite volume technique. We use a relativistic formulation for the charm quark. For the light quark, we use domain-wall fermions in the valence sector and improved Kogut-Susskind sea quarks. We calculate the scattering lengths of isospin-3/2 $Dπ$, $D_sπ$, $D_sK$, isospin-0 $D\bar{K}$ and isospin-1 $D\bar{K}$ channels on the lattice. For the chiral extrapolation, we use a chiral unitary approach to next-to-leading order, which at the same time allows us to give predictions for other channels. It turns out that our results support the interpretation of the $D_{s0}^*(2317)$ as a $DK$ molecule. At the same time, we also update a prediction for the isospin breaking hadronic decay width $Γ(D_{s0}^*(2317)\to D_sπ)$ to $(133\pm22)$ keV.

hep-lat↗

arXiv2018

Spectroscopy and decays of the fully-heavy tetraquarks

We discuss the possible existence of the fully-heavy tetraquarks. We calculate the ground-state energy of the $bb \bar b \bar b$ bound state, where $b$ stands for the bottom quark, in a nonrelativistic effective field theory framework with one-gluon-exchange (OGE) color Coulomb interaction, and in a relativized diquark model characterized by OGE plus a confining potential. Our analysis advocates the existence of uni-flavor heavy four-quark bound states. The ground state $bb\bar b\bar b$ tetraquark mass is predicted to be $(18.72\pm0.02)$~GeV. Mass inequality relations among the lowest $QQ\bar{Q}\bar{Q}$ state, where $Q\in \{c, b\}$, and the corresponding heavy quarkonia are presented, which give the upper limit on the mass of ground state $QQ\bar{Q}\bar{Q}$. The possible decays of the lowest $bb\bar{b}\bar{b}$ are highlighted, which might provide useful references in the search for them in ongoing LHC experiments, and its width is estimated to be a few tens of MeV.

hep-ph↗

arXiv2018

Novel Soft-Pion Theorem for Long-Range Nuclear Parity Violation

The parity-odd effect in the Standard Model weak neutral current reveals itself in the long-range parity-violating nuclear potential generated by the pion exchanges in the $ΔI=1$ channel with the parity-odd pion-nucleon coupling constant $h_π^1$. Despite decades of experimental and theoretical efforts, the size of this coupling constant is still not well-understood. In this Letter we derive a soft-pion theorem relating $h_π^1$ and the neutron-proton mass-splitting induced by an artificial parity-even counterpart of the $ΔI=1$ weak Lagrangian, and demonstrate that the theorem still holds exact at the next-to-leading order in chiral perturbation theory. A considerable amount of simplification is expected in the study of $h_π^1$ by using either lattice or other QCD models following its reduction from a parity-odd proton-neutron-pion matrix element to a simpler spectroscopic quantity. The theorem paves the way to much more precise calculations of $h_π^1$, and thus a quantitative test of the strangeness-conserving neutral current interaction of the Standard Model is foreseen.

nucl-th↗

arXiv2018

Disentangling the role of the $Y(4260)$ in $e^+e^-\to D^\bar{D}^$ and $D_s^\bar{D}_s^$ via line shape studies

Whether the $Y(4260)$ can couple to open charm channels has been a crucial issue for understanding its nature. The available experimental data suggest that the cross section line shapes of exclusive processes in $e^+e^-$ annihilations have nontrivial structures around the mass region of the $Y(4260)$. As part of a series of studies of the $Y(4260)$ as mainly a $\bar{D}D_1(2420)+c.c.$ molecular state, we show that the partial widths of the $Y(4260)$ to the two-body open charm channels of $e^+e^-\to D^*\bar{D}^*$ and $D_s^*\bar{D}_s^*$ are much smaller than that to $\bar{D}D^*π+c.c.$. The line shapes measured by the Belle Collaboration for these two channels can be well described by the vector charmonium states $ψ(4040)$, $ψ(4160)$ and $ψ(4415)$ together with the $Y(4260)$. It turns out that the interference of the $Y(4260)$ with the other charmonia produces a dip around 4.22~GeV in the $e^+e^-\to D^*\bar{D}^*$ cross section line shape. The data also show an evidence for the strong coupling of the $Y(4260)$ to the $D\bar D_1(2420)$, in line with the expectation in the hadronic molecular scenario for the $Y(4260)$.

hep-ph↗

arXiv2018

Traps in hadron spectroscopy: Thresholds, triangle singularities, etc

The knowledge of hadron spectrum is based on experimental observations of hadronic resonances. The resonances are normally observed as peaks in certain invariant mass distributions. However, neither is a peak necessarily due to the presence of a resonance, nor does a resonance necessarily lead to a peak. Kinematic singularities can also produce peaks. Here, we discuss such possibilities and methods distinguishing genuine resonances from kinematic effects.

hep-ph↗

arXiv2017

Scalar form factors of semi-leptonic $D\toπ/ \bar{K}$ transitions with coupled-channel effects

Coupled-channel effects are taken into account for the study of scalar form factors in semi-leptonic $D\toπ\bar{\ell}ν_\ell$ and $D\to \bar{K}\bar{\ell}ν_\ell$ decays, by solving the Muskhelishvili-Omnès integral equations. As inputs, we employ the unitarized amplitudes taken from chiral effective theory for the region not far away from thresholds, while, at higher energies of the Goldstone bosons, proper asymptotic conditions are employed. Within Muskhelishvili-Omnès formalism, the scalar form factors are represented in terms of Omnès matrix multiplied by a vector of polynomials. We reduce the number of subtraction constants by matching to the scalar form factors derived in chiral perturbation theory up to next-to-leading order. The recent lattice QCD data by ETM collaboration for ${D\toπ}$ and ${D\to\bar{K}}$ scalar form factors are simultaneously well described. The scalar form factors for $D\toη$, $D_s\to \bar{K}$ and $D_s\to η$ transitions are predicted in their whole kinematical regions. Using our fitting parameters, we also extract the following Cabibbo-Kobayashi-Maskawa elements: $|V_{cd}|=0.243(_{-12}^{+11})_{\rm sta.}({3})_{\rm sys.}(3)_{\rm exp.}$ and $|V_{cs}|=0.950(_{-40}^{+39})_{\rm sta.}(1)_{\rm sys.}(7)_{\rm exp.}$. The approach used in this work can be straightforwardly extended to the semileptonic $B$ decays.

hep-ph↗

arXiv2017

Note on X(3872) production at hadron colliders and its molecular structure

The production of the X(3872) as a hadronic molecule in hadron colliders is clarified. We show that the conclusion of Bignamini et al., Phys. Rev. Lett. 103 (2009) 162001, that the production of the X(3872) at high $p_T$ implies a non-molecular structure, does not hold. In particular, using the well understood properties of the deuteron wave function as an example, we identify the relevant scales in the production process.

hep-ph↗

arXiv2017

Triangular singularity and a possible $ϕp$ resonance in the $Λ^+_c \to π^0 ϕp$ decay

We study the $Λ^+_c \to π^0 ϕp$ decay by considering a triangle singularity mechanism. In this mechanism, the $Λ^+_c$ decays into the $K^* Σ^*(1385)$, the $Σ^*(1385)$ decays into the $π^0 Σ$ (or $Λ$), and then the $K^* Σ$ (or $Λ$) interact to produce the $ϕp$ in the final state. This mechanism produces a peak structure around $2020$ MeV. In addition, the possibility that there is a hidden-strange pentaquark-like state is also considered by taking into account the final state interactions of $K^* Λ$, $K^* Σ$, and $ϕp$. We conclude that it is difficult to search for the hidden-strange analogue of the $P_c$ states in this decay. However, we do expect nontrivial behavior in the $ϕp$ invariant mass distribution. The predictions can be tested by experiments such as BESIII, LHCb and Belle-II.

hep-ph↗

arXiv2017

Connected and Disconnected Contractions in Pion-Pion Scattering

We show that the interplay of chiral effective field theory and lattice QCD can be used in the evaluation of so-called disconnected diagrams, which appear in the study of the isoscalar and isovector channels of pion-pion scattering and have long been a major challenge for the lattice community. By means of partially-quenched chiral perturbation theory, we distinguish and analyze the effects from different types of contraction diagrams to the pion-pion scattering amplitude, including its scattering lengths and the energy-dependence of its imaginary part. Our results may be used to test the current degree of accuracy of lattice calculation in the handling of disconnected diagrams, as well as to set criteria for the future improvement of relevant lattice computational techniques that may play a critical role in the study of other interesting QCD matrix elements.

hep-lat↗

arXiv2017

Study of open-charm $0^+$ states in unitarized chiral effective theory with one-loop potentials

Chiral potentials are derived for the interactions between Goldstone bosons and pseudoscalar charmed mesons up to next-to-next-to-leading order in a covariant chiral effective field theory with explicit vector charmed-meson degrees of freedom. Using the extended-on-mass-shell scheme, we demonstrate that the ultraviolet divergences and the so-called power counting breaking terms can be properly absorbed by the low-energy constants of the chiral Lagrangians. We calculate the scattering lengths by unitarizing the one-loop potentials and fit them to the data extracted from lattice QCD. The obtained results are compared to the ones without an explicit contribution of vector charmed mesons given previously. It is found that the difference is negligible for $S$-wave scattering in the threshold region. This validates the use of $D^\ast$-less one-loop potentials in the study of the pertinent scattering lengths. We search for dynamically generated open-charm states with $J^P=0^+$ as poles of the $S$-matrix on various Riemann sheets. The trajectories of those poles for varying pion masses are presented as well.

hep-ph↗

arXiv2017

Effects of $Z_b$ states and bottom meson loops on $Υ(4S) \to Υ(1S,2S) π^+π^-$ transitions

We study the dipion transitions $Υ(4S) \rightarrow Υ(nS) π^+π^-$ $(n=1,2)$. In particular, we consider the effects of the two intermediate bottomoniumlike exotic states $Z_b(10610)$ and $Z_b(10650)$ as well as bottom meson loops. The strong pion-pion final-state interactions, especially including channel coupling to $K\bar{K}$ in the $S$-wave, are taken into account model-independently by using dispersion theory. Based on a nonrelativistic effective field theory we find that the contribution from the bottom meson loops is comparable to those from the chiral contact terms and the $Z_b$-exchange terms. For the $Υ(4S) \rightarrow Υ(2S) π^+π^-$ decay, the result shows that including the effects of the $Z_b$-exchange and the bottom meson loops can naturally reproduce the two-hump behavior of the $ππ$ mass spectra. Future angular distribution data are decisive for the identification of different production mechanisms. For the $Υ(4S) \rightarrow Υ(1S) π^+π^-$ decay, we show that there is a narrow dip around 1 GeV in the $ππ$ invariant mass distribution, caused by the final-state interactions. The distribution is clearly different from that in similar transitions from lower $Υ$ states, and needs to be verified by future data with high statistics. Also we predict the decay width and the dikaon mass distribution of the $Υ(4S) \rightarrow Υ(1S) K^+ K^-$ process.

hep-ph↗

arXiv2017

Role of a triangle singularity in the $γp\rightarrow K^+ Λ(1405)$ reaction

We show the effects of a triangle singularity mechanism for the $γp \to K^+Λ(1405)$ reaction. The mechanism has a $N^*$ resonance around 2030 MeV, which decays into $K^* Σ$. The $K^*$ decays to $K^+ π$, and the $πΣ$ merge to form the $Λ(1405)$. This mechanism produces a peak around $\sqrt{s} = 2110$ MeV, and has its largest contribution around cos$θ=0$. The addition of this mechanism to other conventional ones, leads to a good reproduction of ${\rm d}σ/{\rm dcos}θ$ and the integrated cross section around this energy, providing a solution to a problem encountered in previous theoretical models.

hep-ph↗

arXiv2016

Subtraction of power counting breaking terms in chiral perturbation theory: spinless matter fields

When matter fields are included in chiral perturbation theory, the nonvanishing mass in the chiral limit introduces a new energy scale so that the loop diagrams including such matter field propagators spoil the usual power counting. However, the power counting breaking terms can be absorbed into counterterms in the chiral Lagrangian. In this paper, we systematically derive these terms to leading one-loop order (next-to-next-to leading order in the chiral expansion) at once by calculating the generating functional using the path integral. They are then absorbed by counterterms in the next-to-leading order Lagrangian. The method can be extended to calculating power counting breaking terms for other matter fields.

hep-ph↗

arXiv2016

Two-pole structure of the $D^\ast_0(2400)$

The so far only known charmed non-strange scalar meson is dubbed as $D_0^*(2400)$ in the Review of Particle Physics. We show, within the framework of unitarized chiral perturbation theory, that there are in fact two $(I=1/2, J^P=0^+)$ poles in the region of the $D_0^*(2400)$ in the coupled-channel $Dπ$, $Dη$ and $D_s\bar K$ scattering amplitudes. With all the parameters previously fixed, we predict the energy levels for the coupled-channel system in a finite volume, and find that they agree remarkably well with recent lattice QCD calculations. This successful description of the lattice data is regarded as a strong evidence for the two-pole structure of the $D_0^*(2400)$. With the physical quark masses, the poles are located at $\left(2105^{+6}_{-8}-i\,102^{+10}_{-12}\right)$~MeV and $\left(2451^{+36}_{-26}-i\,134^{+7}_{-8}\right)$~MeV, with the largest couplings to the $Dπ$ and $D_s\bar K$ channels, respectively. Since the higher pole is close to the $D_s\bar K$ threshold, we expect it to show up as a threshold enhancement in the $D_s\bar K$ invariant mass distribution. This could be checked by high-statistic data in future experiments. We also show that the lower pole belongs to the same SU(3) multiplet as the $D_{s0}^*(2317)$ state. Predictions for partners in the bottom sector are also given.

hep-ph↗

arXiv2016

Aspects of the low-energy constants in the chiral Lagrangian for charmed mesons

We investigate the numerical values of the low-energy constants in the chiral effective Lagrangian for the interactions between the charmed mesons and the lightest pseudoscalar mesons, the Goldstone bosons of the spontaneous breaking of chiral symmetry for QCD. This problem is tackled from two sides: estimates using the resonance exchange model, and positivity constraints from the general properties of the $S$-matrix including analyticity, crossing symmetry and unitarity. These estimates and constraints are compared with the values determined from fits to lattice data of the scattering lengths. Tensions are found, and possible reasons are discussed. We conclude that more data from lattice calculations and experiments are necessary to fix these constants better. As a by-product, we also estimate the coupling constant $g_{DDa_2}$, with $a_2$ the light tensor meson, via the QCD sum rule approach.

hep-ph↗

arXiv2016

Remarks on the $P_c$ structures and triangle singularities

It was proposed that the narrow $P_c(4450)$ structure observed by the LHCb Collaboration in the reaction $Λ_b\to J/ψp K$ might be due to a triangle singularity around the $χ_{c1}$--proton threshold at 4.45~GeV. We discuss the occurrence of a similar triangle singularity in the $J/ψp$ invariant mass distribution for the decay $Λ_b\to J/ψp π$, which could explain the bump around 4.45~GeV in the data. More precise measurements of this process would provide valuable information towards an understanding of the $P_c$ structures.

hep-ph↗

arXiv2016

A Discussion on Triangle Singularities in the $Λ_b \to J/ψK^{-} p$ Reaction

We have analyzed the singularities of a triangle loop integral in detail and derived a formula for an easy evaluation of the triangle singularity on the physical boundary. It is applied to the $Λ_b \rightarrow J/ψK^{-} p$ process via $Λ^*$-charmonium-proton intermediate states. Although the evaluation of absolute rates is not possible, we identify the $χ_{c1}$ and the $ψ(2S)$ as the relatively most relevant states among all possible charmonia up to the $ψ(2S)$. The $Λ(1890)\, χ_{c1}\, p$ loop is very special as its normal threshold and triangle singularities merge at about 4.45 GeV, generating a narrow and prominent peak in the amplitude in the case that the $χ_{c1}\, p$ is in an $S$-wave. We also see that loops with the same charmonium and other $Λ^*$ hyperons produce less dramatic peaks from the threshold singularity alone. For the case of $χ_{c1}\, p \rightarrow J/ψ\, p$ and quantum numbers $3/2^-$ or $5/2^+$ one needs $P$- and $D$-waves, respectively, in the $χ_{c1}\, p$, which drastically reduce the strength of the contribution and smooth the threshold peak. In this case we conclude that the singularities cannot account for the observed narrow peak. In the case of $1/2^+$, $3/2^+$ quantum numbers, where $χ_{c1}\, p \rightarrow J/ψ\, p$ can proceed in an $S$-wave, the $Λ(1890)\,χ_{c1}\,p$ triangle diagram could play an important role, though can neither assert their strength without further input from experiments and lattice QCD calculations.

hep-ph↗