arXiv subjects

Feng-Kun Guo

Publications and source records attributed to Feng-Kun Guo.

At least 181 records · Page 10Linked to original sources

arXiv2016

Hindered magnetic dipole transitions between P-wave bottomonia and coupled-channel effects

In the hindered magnetic dipole transitions of heavy quarkonia, the coupled-channel effects originating from the coupling of quarkonia to a pair of heavy and anti-heavy mesons can play a dominant role. Here, we study the hindered magnetic dipole transitions between two $P$-wave bottomonia, $χ_b(n P)$ and $h_b(n^\prime P)$, with $n\neq n^\prime$. In these processes the coupled-channel effects are expected to lead to partial widths much larger than the quark model predictions. We estimate these partial widths which, however, are very sensitive to unknown coupling constants related to the vertices $χ_{b0}(nP)B\bar B$. A measurement of the hindered M1 transitions can shed light on the coupled-channel dynamics in these transitions and hence on the size of the coupling constants. We also suggest to check the coupled-channel effects by comparing results from quenched and fully dynamical lattice QCD calculations.

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arXiv2016

On the constituent counting rules for hard exclusive processes involving multiquark states

At high energies, the cross section of a hard exclusive process at finite scattering angle falls off as a negative power of the center-of-mass energy $\sqrt{s}$. If all involved quark-gluon compositions undergo hard momentum transfers, the scaling of the fall-off is determined by the underlying valence structures of the initial and final states, known as the constituent counting rules. It was argued in the literature that the counting rules are a powerful tool to determine the valence degrees of freedom inside multiquark states when applied to exclusive production processes. However, we demonstrate that for hadrons with hidden flavors the naive application of the constituent counting rules is problematic, since it is not mandatory for all components to participate the hard scattering at the scale $\sqrt{s}$. The correct scaling rules can be obtained easily by using effective field theory. A few examples involving the $Z_c(3900)^\pm$ and $X(3872)$ are discussed.

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arXiv2016

One-loop renormalization of the chiral Lagrangian for spinless matter fields in the SU(N) fundamental representation

We perform the leading one-loop renormalization of the chiral Lagrangian for spinless matter fields living in the fundamental representation of SU(N). The Lagrangian can also be applied to any theory with a spontaneous symmetry breaking of $SU(N)_L\times SU(N)_R$ to $SU(N)_V$ and spinless matter fields in the fundamental representation. For QCD, the matter fields can be kaons or pseudoscalar heavy mesons. Using the background field method and heat kernel expansion techniques, the divergences of the one-loop effective generating functional for correlation functions of single matter fields are calculated up to $\mathcal{O}(p^3)$. They are absorbed by counterterms not only from the third order but also from the second order chiral Lagrangian.

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arXiv2016

Disentangling the hadronic molecule nature of the $P_c(4380)$ pentaquark-like structure

We demonstrate that the relative ratio of the decays of hidden charm pentaquark-like structure $P^+_c(4380)$ to $\bar{D}^* Λ_c^+$ and $J/ψp$ are very different for $P^+_c$ being $\bar{D} Σ_c^*(2520)$ or $\bar{D}^* Σ_c(2455)$ molecule states. While the partial width of the $\bar{D} Σ_c^*(2520)$ molecule to the $\bar{D}^* Λ_c^+$ is much larger, by one order of magnitude, than that to the $J/ψp$, the $\bar{D}^* Σ_c(2455)$ molecule shows a different pattern. Our analysis shows that the $\bar{D} Σ_c^*$ bound state ansatz is more reasonable than the $\bar{D}^* Σ_c$ one to explain the broad $P_c(4380)$ structure. We suggest to search for the $P_c(4380)$ in the $\bar{D}^* Λ_c^+$ system, which can be used to disentangle the nature of the $P^+_c(4380)$ structure.

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arXiv2016

How the X(5568) challenges our understanding of QCD

We discuss the X(5568) particle recently announced by the D0 Collaboration. Several types of models were proposed to explain this structure in the literature. As pointed out by Burns and Swanson (arXiv:1603.04366), none of them provides a satisfactory description of the observation. We provide additional arguments using general properties of QCD, and conclude that the observation of the X(5568), if confirmed, poses serious challenges to our understanding of nonperturbative QCD.

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arXiv2016

Effect of $Z_b$ states on $Υ(3S)\toΥ(1S)ππ$ decays

Within the framework of dispersion theory, we analyze the dipion transitions between the lightest $Υ$ states, $Υ(nS) \rightarrow Υ(mS) ππ$ with $m < n \leq 3$. In particular, we consider the possible effects of two intermediate bottomoniumlike exotic states $Z_b(10610)$ and $Z_b(10650)$. The $ππ$ rescattering effects are taken into account in a model-independent way using dispersion theory. We confirm that matching the dispersive representation to the leading chiral amplitude alone cannot reproduce the peculiar two-peak $ππ$ mass spectrum of the decay $Υ(3S) \rightarrow Υ(1S) ππ$. The existence of the bottomoniumlike $Z_b$ states can naturally explain this anomaly. We also point out the necessity of a proper extraction of the coupling strengths for the $Z_b$ states to $Υ(nS)π$, which is only possible if a Flatté-like parametrization is used in the data analysis for the $Z_b$ states.

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arXiv2015

$Z_c(3900)$: what has been really seen?

The $Z^\pm_c(3900)/Z^\pm_c(3885)$ resonant structure has been experimentally observed in the $Y(4260) \to J/ψππ$ and $Y(4260) \to \bar{D}^\ast D π$ decays. This structure is intriguing since it is a prominent candidate of an exotic hadron. Yet, its nature is unclear so far. In this work, we simultaneously describe the $\bar{D}^\ast D$ and $J/ψπ$ invariant mass distributions in which the $Z_c$ peak is seen using amplitudes with exact unitarity. Two different scenarios are statistically acceptable, where the origin of the $Z_c$ state is different. They correspond to using energy dependent or independent $\bar D^* D$ $S$-wave interaction. In the first one, the $Z_c$ peak is due to a resonance with a mass around the $D\bar D^*$ threshold. In the second one, the $Z_c$ peak is produced by a virtual state which must have a hadronic molecular nature. In both cases the two observations, $Z^\pm_c(3900)$ and $Z^\pm_c(3885)$, are shown to have the same common origin, and a $\bar D^* D$ bound state solution is not allowed. Precise measurements of the line shapes around the $D\bar D^*$ threshold are called for in order to understand the nature of this state.

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arXiv2015

One-loop analysis of the interactions between charmed mesons and Goldstone bosons

We derive the scattering amplitude for Goldstone bosons of chiral symmetry off the pseudoscalar charmed mesons up to leading one-loop order in a covariant chiral effective field theory, using the so-called extended-on-mass-shell renormalization scheme. Then we use unitarized chiral perturbation theory to fit to the available lattice data of the S-wave scattering lengths. The lattice data are well described. However, most of the low-energy constants determined from the fit bear large uncertainties. Lattice simulations in more channels are necessary to pin down these values which can then be used to make predictions in other processes related by chiral and heavy quark symmetries.

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arXiv2015

One-loop corrections to the Higgs electroweak chiral Lagrangian

In this talk we study beyond Standard Model scenarios where the Higgs is non-linearly realized. The one-loop ultraviolet divergences of the low-energy effective theory at next-to-leading order, O(p^4), are computed by means of the background-field method and the heat-kernel expansion. The power counting in non-linear theories shows that these divergences are determined by the leading-order effective Lagrangian L_2. We focus our attention on the most important O(p^4) divergences, which are provided by the loops of Higgs and electroweak Goldstones, as these particle are the only ones that couple through derivatives in L_2. The one-loop divergences are renormalized by O(p^4) effective operators, and set their running. This implies the presence of chiral logarithms in the amplitudes along with the O(p^4) low-energy couplings, which are of a similar importance and should not be neglected in next-to-leading order effective theory calculations, e.g. in composite scenarios.

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arXiv2015

How to reveal the exotic nature of the P_c(4450)

The LHCb Collaboration announced two pentaquark-like structures in the $J/ψp$ invariant mass distribution. We show that the current information on the narrow structure at 4.45 GeV is compatible with kinematical effects of the rescattering from $χ_{c1} p$ to $J/ψp$: First, it is located exactly at the $χ_{c1} p$ threshold. Second, the mass of the four-star well-established $Λ(1890)$ is such that a leading Landau singularity from a triangle diagram can coincidentally appear at the $χ_{c1} p$ threshold, and third, there is a narrow structure at the $χ_{c1}\,p$ threshold but not at the $χ_{c0} p$ and $χ_{c2} p$ thresholds. In order to check whether that structure corresponds to a real exotic resonance, one has to measure the process $Λ_b^0\to K^-χ_{c1} p$. If the $P_c(4450)$ structure exists in the $χ_{c1} p$ invariant mass distribution as well, then the structure cannot be just a kinematical effect but is a real resonance, otherwise, one cannot conclude the $P_c(4450)$ to be another exotic hadron. In addition, it is also worthwhile to measure the decay $Υ(1S)\to J/ψp \bar p$: a narrow structure at 4.45 GeV but not at the $χ_{c0} p$ and $χ_{c2} p$ thresholds would exclude the possibility of a pure kinematical effect.

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arXiv2015

The nature of near-threshold XYZ states

We demonstrate that the recently observed $X$, $Y$, $Z$ states cannot be purely from kinematic effect. Especially the narrow near-threshold structures in elastic channels call for nearby poles of the $S$-matrix which are qualified as states. We propose a way to distinguish cusp effects from genuine states and demonstrate that (not all of) the recently observed $X$, $Y$, $Z$ states cannot be purely from kinematic effects. Especially, we show that the narrow near-threshold structures in elastic channels call for nearby poles of the $S$-matrix, since the normal kinematic cusp effect cannot produce that narrow structures in the elastic channels in contrast to genuine $S$-matrix poles. In addition, it is also discussed how spectra can be used to distinguish different scenarios proposed for the structure of those poles, such as hadro-quarkonia, tetraquarks and hadronic molecules. The basic tool employed is heavy quark spin symmetry.

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arXiv2015

Cumulants of the QCD topological charge distribution

The distribution of the QCD topological charge can be described by cumulants, with the lowest one being the topological susceptibility. The vacuum energy density in a theta-vacuum is the generating function for these cumulants. In this paper, we derive the vacuum energy density in SU(2) chiral perturbation theory up to next-to-leading order keeping different up and down quark masses, which can be used to calculate any cumulant of the topological charge distribution. We also give the expression for the case of SU(N) with degenerate quark masses. In this case, all cumulants depend on the same linear combination of low-energy constants and chiral logarithm, and thus there are sum rules between the N-flavor quark condensate and the cumulants free of next-to-leading order corrections.

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arXiv2015

Production cross section estimates for strongly-interacting Electroweak Symmetry Breaking Sector resonances at particle colliders

We are exploring a generic strongly-interacting Electroweak Symmetry Breaking Sector (EWSBS) with the low-energy effectie field theory for the four experimentally known particles ($W_L^\pm$, $Z_L$, $h$) and its dispersion-relation based unitary extension. In this contribution we provide simple estimates for the production cross section of pairs of the EWSBS bosons and their resonances at proton-proton colliders as well as in a future $e^-e^+$ (or potentially a $μ^-μ^+$) collider with a typical few-TeV energy. We examine the simplest production mechanisms, tree-level production through a $W$ (dominant when quantum numbers allow) and the simple effective boson approximation (in which the electroweak bosons are considered as collinear partons of the colliding fermions). We exemplify with custodial isovector and isotensor resonances at 2 TeV, the energy currently being discussed because of a slight excess in the ATLAS 2-jet data. We find it hard, though not unthinkable, to ascribe this excess to one of these $W_LW_L$ rescattering resonances. An isovector resonance could be produced at a rate smaller than, but close to earlier CMS exclusion bounds, depending on the parameters of the effective theory. The $ZZ$ excess is then problematic and requires additional physics (such as an additional scalar resonance). The isotensor one (that would describe all charge combinations) has a smaller cross section.

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arXiv2015

$θ$-dependence of the lightest meson resonances in QCD

We derive the pion mass and the elastic pion-pion scattering amplitude in the QCD theta-vacuum up to next-to-leading order in chiral perturbation theory. Using the modified inverse amplitude method, we study the theta-dependence of the mass and width of the light scalar meson sigma(500) and the vector meson rho(770).

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arXiv2015

Employing spin symmetry to disentangle different models for the XYZ states

In order to test different models proposed for some states discovered recently in the charmonium mass range that do not fit into the pattern predicted by the conventional quark model, we derive predictions for the spectrum within the hadro-charmonium picture, the tetraquark picture as well as the hadronic molecular approach. We exploit heavy quark spin symmetry for the hadro-charmonium and hadronic molecule scenarios. The patterns that emerge from the different models turn out to be quite distinct. For example, only within the hadro-charmonium picture a pseudoscalar state emerges that is lighter than the Y(4260). Possible discovery channels of these additional states are discussed.

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arXiv2015

One-loop renormalization of the electroweak chiral Lagrangian with a light Higgs

We consider the general chiral effective action which parametrizes the nonlinear realization of the spontaneous breaking of the electroweak symmetry with a light Higgs, and compute the one-loop ultraviolet divergences coming from Higgs and electroweak Goldstone fluctuations using the background field method. The renormalization of the divergences is carried out through operators of next-to-leading order in the chiral counting, i.e. of O(p^4). Being of the same order in power counting, the logarithmic corrections linked to these divergences can be as important as the tree-level contributions from the O(p^4) operators, and must be accounted for in the phenomenological analysis of experimental data. Deviations in the O(p^2) (leading-order) couplings with respect to the Standard Model values, e.g., in the h->WW coupling, would generate contributions from the 1-loop chiral logarithms computed in this work to a vast variety of observables, which do not have a counterpart in the conventional electroweak effective theory with a linearly transforming Higgs complex doublet.

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arXiv2015

Could the near-threshold $XYZ$ states be simply kinematic effects?

We demonstrate that the spectacular structures discovered recently in various experiments and named as $X$, $Y$ and $Z$ states cannot be purely kinematic effects. Their existence necessarily calls for nearby poles in the $S$--matrix and they therefore qualify as states. We propose a way to distinguishing kinematic cusp effects from genuine $S$--matrix poles: the kinematic threshold cusp cannot produce a narrow peak in the invariant mass distribution in the elastic channel in contrast with a genuine $S$--matrix pole.

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arXiv2015

What can radiative decays of the X(3872) teach us about its nature?

Starting from the hypothesis that the X(3872) is a $D\bar D^*$ molecule, we discuss the radiative decays of the X(3872) into $γJ/ψ$ and $γψ'$ from an effective field theory point of view. We show that radiative decays are very weakly sensitive to the long-range structure of the X(3872). In particular, contrary to earlier claims, we argue that the experimentally determined ratio of the mentioned branching fractions is not in conflict with a wave function of the X(3872) that is dominated by the $D\bar D^*$ hadronic molecular component.

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