arXiv subjects

Fu-Guang Cao

Publications and source records attributed to Fu-Guang Cao.

At least 19 recordsLinked to original sources

arXiv2021

Radial excitation of light mesons and the $γγ^* \rightarrow π^0$ transition form factor

We investigate radial excitation of the quark-antiquark pair in the $π^0$ meson and its effects on the $γγ^* \rightarrow π^0$ transition form factor in the framework of light-cone perturbative QCD. The existing constraints on the light-cone wave function of the lowest Fock state $|q { \bar q} \rangle$ in the $π^0$ meson allow a sizeable radial excitation of the quark-antiquark pair. We construct the light-cone wave function for the quark-antiquark pair in the first radially excited state (the 2S state) using a simple harmonic oscillator potential. The distribution amplitude obtained for the 2S state has two nodes in $x$ at low scale of $Q$ and thereby has a much strong scale dependence than the 1S state. Contributions from this radial excitation to the $γγ^* \rightarrow π^0$ transition form factor exhibit different $Q^2$-dependence behavior from the ground state and thus can modify the prediction for the transition form factor in the medium-large region of $Q^2$.

hep-ph↗

arXiv2019

Meaningful Details: The value of adding baseline dependence to the Neutrino-Dark Matter Effect

The possible effect on the flavour spectra of astronomical neutrinos from a neutrino-dark matter interaction has been investigated for decoherent neutrinos. In this work, we report results calculated for coherent neutrinos. This was done with two different models for the neutrino dark-matter interactions: a flavour state interaction,as for the weak interaction in the Standard Model, and a mass state interaction, which is predicted by certain non-Standard Models (specifically Scotogenic models). It was found that using a coherent analysis dramatically increased the explorable parameter space for the neutrino-dark matter interaction. However, the detection of coherent astronomical neutrinos presents a significant challenge to experimentalists, because such a detection would require an improvement in energy resolution by at least six orders of magnitude, with similar improvements in astronomical distance determinations.

astro-ph.HE↗

arXiv2018

Extraction of neutrino mixing parameters from experiments with multiple identical detectors

The statistical method used in the analyses of measurements of neutrino oscillation mixing angle $θ_{13}$ by the Daya Bay Collaboration is based on variational minimization of a $χ^2$ function defined in terms of quantities of interest and pull factors which are introduced to deal with effects of systematic uncertainties. For both experiments, the number of parameters that need to be determined is great than the number of available data points (20 vs 6 for the Daya Bay and 12 vs 2 for the RENO). While the results for the mixing angle and the normalization factor were reported, results for the other parameters (pull factors) were omitted in their publications. There exist multiple sets of parameters from the minimization of the $χ^2$ function. We investigate the sensitivity of the extracted mixing angle on this non-uniqueness of minimization results for the Daya Bay data using two methods of minimization. We report results for all parameters, including those of physics interest and pull factors. The obtained results for the mixing angle and the normalization factor are in agreement with those reported by the Daya Bay Collaboration. Furthermore, we present plots of confidence level contours in the space of the mixing angle and normalization factor. We also present results from fittings using a reduced $χ^2$ function with fewer parameters than the one employed by the Daya Bay Collaboration.

hep-ph↗

arXiv2012

Perturbative and nonperturbative contributions to the strange quark asymmetry in the nucleon

There are two mechanisms for the generation of an asymmetry between the strange and anti-strange quark distributions in the nucleon: nonperturbative contributions originating from nucleons fluctuating into virtual baryon-meson pairs such as $ΛK$ and $ΣK$, and perturbative contributions arising from gluons splitting into strange and anti-strange quark pairs. While the nonperturbative contributions are dominant in the large-$x$ region, the perturbative contributions are more significant in the small-$x$ region. We calculate this asymmetry taking into account both nonperturbative and perturbative contributions, thus giving a more accurate evaluation of this asymmetry over the whole domain of $x$. We find that the perturbative contributions are generally a few times larger in magnitude than the nonperturbative contributions, which suggests that the best region to detect this asymmetry experimentally is in the region $0.02 < x < 0.03$. We find that the asymmetry may have more than one node, which is an effect that should be taken into account, e.g. for parameterizations of the strange and anti-strange quark distributions used in global analysis of parton distributions.

hep-ph↗

arXiv2012

Neutrino masses from lepton and quark mass relations and neutrino oscillations

Determining the absolute masses of neutrinos is of fundamental importance in particle physics, nuclear physics, and astrophysics. We conjecture that intrinsic mass relations exist between leptons and quarks. Using these relations and neutrino oscillation data, we show that the inverted neutrino mass hierarchy is strongly disfavored and estimate the absolute neutrino masses to be m_{1}=0.21^{+1.70}_{-0.21} \times 10^{-4} {eV}, m_{2}=(8.7 \pm 0.1) \times 10^{-3} {eV}, and m_{3}=(4.9 \pm 0.1) \times 10^{-2} {eV}.

hep-ph↗

arXiv2012

Determination of the $η$-$η^\prime$ mixing angle

We extract $η$-$η^\prime$ mixing angle and the ratios of decay constants of light pseudoscalar mesons $π^0$, $η$ and $η^\prime$ using recently available BaBar measurements on $η$-photon and $η^\prime$-photon transition form factors and more accurate experimental data for the masses and two-photon decay widths of the light pseduoscalar mesons.

hep-ph↗

arXiv2011

Applications of Symmetry Breaking in Determining PDFs of the Nucleon

Studying the possible breaking of various parton model symmetries by the parton distribution functions of the nucleon can provide important information for the non-perturbative structure of hadrons and the strong interaction. We review theoretical calculations for the breaking of flavor symmetry, quark-antiquark symmetry and charge symmetry in the unpolarized and polarized nucleons using the meson cloud model. We report an estimation for the total distribution of strange and antistrange quarks in the nucleon by combining theoretical calculations of SU(3) flavor symmetry breaking with light antiquark distributions obtained from global analysis of available experimental data.

hep-ph↗

arXiv2011

Meson Transition Form Factors in Light-Front Holographic QCD

We study the photon-to-meson transition form factors (TFFs) F_{M γ}(Q^2) for gamma gamma^* \to M using light-front holographic methods. The Chern-Simons action, which is a natural form in 5-dimensional anti-de Sitter (AdS) space, leads directly to an expression for the photon-to-pion TFF for a class of confining models. Remarkably, the predicted pion TFF is identical to the leading order QCD result where the distribution amplitude has asymptotic form. The Chern-Simons form is local in AdS space and is thus somewhat limited in its predictability. It only retains the q \bar q component of the pion wavefunction, and further, it projects out only the asymptotic form of the meson distribution amplitude. It is found that in order to describe simultaneously the decay process π^0 \rightarrow gamma gamma and the pion TFF at the asymptotic limit, a probability for the q \bar q component of the pion wavefunction P_{q \bar q}=0.5 is required; thus giving indication that the contributions from higher Fock states in the pion light-front wavefunction need to be included in the analysis. The probability for the Fock state containing four quarks (anti-quarks) which follows from analyzing the hadron matrix elements, P_{q \bar q q \bar q} \sim 10 %, agrees with the analysis of the pion elastic form factor using light-front holography including higher Fock components in the pion wavefunction. The results for the TFFs for the eta and eta^\prime mesons are also presented. The rapid growth of the pion TFF exhibited by the BaBar data at high Q^2 is not compatible with the models discussed in this article, whereas the theoretical calculations are in agreement with the experimental data for the eta and eta^\prime TFFs.

hep-ph↗

arXiv2011

AdS/QCD and Applications of Light-Front Holography

Light-Front Holography leads to a rigorous connection between hadronic amplitudes in a higher dimensional anti-de Sitter (AdS) space and frame-independent light-front wavefunctions of hadrons in 3+1 physical space-time, thus providing a compelling physical interpretation of the AdS/CFT correspondence principle and AdS/QCD, a useful framework which describes the correspondence between theories in a modified AdS$_5$ background and confining field theories in physical space-time. To a first semiclassical approximation, where quantum loops and quark masses are not included, this approach leads to a single-variable light-front Schrödinger equation which determines the eigenspectrum and the light-front wavefunctions of hadrons for general spin and orbital angular momentum. The coordinate $z$ in AdS space is uniquely identified with a Lorentz-invariant coordinate $ζ$ which measures the separation of the constituents within a hadron at equal light-front time. The internal structure of hadrons is explicitly introduced and the angular momentum of the constituents plays a key role. We give an overview of the light-front holographic approach to strongly coupled QCD. In particular, we study the photon-to-meson transition form factors (TFFs) $F_{M γ}(Q^2)$ for $γγ^* \to M$ using light-front holographic methods. The results for the TFFs for the $η$ and $η^\prime$ mesons are also presented. Some novel features of QCD are discussed, including the consequences of confinement for quark and gluon condensates. A method for computing the hadronization of quark and gluon jets at the amplitude level is outlined.

hep-ph↗

arXiv2011

Evolved QCD predictions for the meson-photon transition form factors

The QCD evolution of the pion distribution amplitude (DA) $ϕ_π(x,Q^2)$ is computed for several commonly used models. Our analysis includes the nonperturbative form predicted by light-front holographic QCD, thus combining the nonperturbative bound state dynamics of the pion with the perturbative ERBL evolution of the pion distribution amplitude. We calculate the meson-photon transition form factors for the $π^0$, $η$ and $η^\prime$ using the hard-scattering formalism. We point out that a widely-used approximation of replacing $ϕ(x, (1-x) Q)$ with $ϕ(x,Q)$ in the calculations will unjustifiably reduce the predictions for the meson-photon transition form factors. It is found that the four models of the pion DA discussed give very different predictions for the $Q^2$ dependence of the meson-photon transition form factors in the region of $Q^2>30$ GeV$^2$. More accurate measurements of these transition form factors at the large $Q^2$ region will be able to distinguish the four models of the pion DA. The rapid growth of the large $Q^2$ data for the pion-photon transition form factor reported by the BaBar Collaboration is difficult to explain within the current framework of QCD. If the BaBar data for the meson-photon transition form factor for the $π^0$ is confirmed, it could indicate physics beyond-the-standard model, such as a weakly-coupled elementary $C=+$ axial vector or pseudoscalar $z^0$ in the few GeV domain, an elementary field which would provide the coupling $γ^*$ $γ$ to $z^0$ to $π^0$ at leading twist. Our analysis thus indicates the importance of additional measurements of the pion-photon transition form factor at large $Q^2$.

hep-ph↗

arXiv2011

Determining the strange and antistrange quark distributions of the nucleon

The difference between the strange and antistrange quark distributions, $δs(x)=s(x)-\sbar(x)$, and the combination of light quark sea and strange quark sea, $Δ(x)=\dbar(x)+\ubar(x)-s(x)-\sbar(x)$, are originated from non-perturbative processes, and can be calculated using non-perturbative models of the nucleon. We report calculations of $δs(x)$ and $Δ(x)$ using the meson cloud model. Combining our calculations of $Δ(x)$ with relatively well known light antiquark distributions obtained from global analysis of available experimental data, we estimate the total strange sea distributions of the nucleon.

hep-ph↗

arXiv2007

Phenomenological analysis of nucleon strangeness and meson-nucleon sigma terms

We calculate the nucleon strangeness $y_N$ in the chiral quark model and the meson cloud model. With the internal relation between the sigma term of $π$$N$ ($σ_{πN}$) and $y_N$, we present the results of $σ_{πN}$ in these two models. Our calculations show that $y_N$ from the chiral quark model is significant larger than that from the meson cloud model, whereas the difference of $σ_{πN}$ between the two models is relatively small. We also present the results of $σ_{K N}$ and $σ_{ηN}$, which could be determined by $σ_{πN}$ and $y_N$ from their definition in the current algebra, and find that these two physical parameters are quite sensitive to $y_N$. The results indicate the necessity to restrict the parameters of the two models from more precision measurements.

hep-ph↗

arXiv2003

Non-perturbative structure of the polarized nucleon sea

We investigate the flavour and quark-antiquark structure of the polarized nucleon by calculating the parton distribution functions of the nucleon sea using the meson cloud model. We find that the SU(2) flavor symmetry in the light antiquark sea and quark-antiquark symmetry in the strange quark sea are broken, {\it i.e.} $Δ\ubar < Δ\dbar$ and $Δs < Δ\sbar$. The polarization of the strange sea is found to be positive, which is in contradiction to previous analyses. We predict a much larger quark-antiquark asymmetry in the polarized strange quark sea than that in the unpolarized strange quark sea. Our results for both polarized light quark sea and polarized strange quark sea are consistent with the recent HERMES data.

hep-ph↗

arXiv2001

The flavour asymmetry of polarized anti-quarks in the nucleon

We present a study of the flavour asymmetry of polarized anti-quarks in the nucleon using the meson cloud model. We include contributions both from the vector mesons and the interference terms of pseudoscalar and vector mesons. Employing the bag model, we first give the polarized valence quark distribution of the $ρ$ meson and the interference distributions. Our calculations show that the interference effect mildly increases the prediction for $Δ\dbar(x)-Δ\ubar(x)$ at intermediate $x$ region. We also discuss the contribution of `Pauli blocking' to the asymmetry.

hep-ph↗

arXiv2000

Charge symmetry violation in the parton distributions of the nucleon

We point out that charge symmetry violation in both the valence and sea quark distributions of the nucleon has a non-perturbative source. We calculate this non-perturbative charge symmetry violation using the meson cloud model, which has earlier been successfully applied to both the study of SU(2) flavour asymmetry in the nucleon sea and quark-antiquark asymmetry in the nucleon. We find that the charge symmetry violation in the valence quark distribution is well below 1%, which is consistent with most low energy tests but significantly smaller than the quark model prediction about 5%-10%. Our prediction for the charge symmetry violation in the sea quark distribution is also much smaller than the quark model calculation.

hep-ph↗

arXiv2000

On the phenomenological analyses of $s$-${\bar s}$ asymmetry in the nucleon sea

Two phenomenological models which give opposite predictions for the $s$-${\bar s}$ asymmetry in the nucleon sea are re-analyzed carefully. It is pointed out that although the quantitative results in both models depend dramatically on the parameters, the predictions for the shape of $s(x)-{\bar s}(x)$ in the two models are parameter independent and opposite. Thereby the coming experiments are likely to be able to distinguish the two models. We find that the reason for the two models giving opposite predictions is that the fluctuation functions and parametrizations for the strange (anti-strange) quark distribution in the baryon (meson) in the two models are quite different. To further investigate these models, we use the same parametrizations for the strange (anti-strange) distributions of the baryon (meson) in the two models. We find that one of the models depends strongly on the parameter which controls the behavior of the meson-baryon fluctuation function. Also the two models agree on the shape and size of $s$-${\bar s}$ for some values of the model parameters, but can disagree strongly for others.

hep-ph↗

arXiv2000

Two analytical constraints on the $η$-$η^\prime$ mixing

We obtained two analytical constraints on the $η$-$η^\prime$ mixing parameters by considering two-photon decays of $η$ and $η^\prime$ [$η(η^\prime) \ra γγ$], and productions of $η$ and $η^\prime$ in the $e^+ e^-$ scattering at large momentum transfer ($Q^2 \ra \infty$). Using the data given in the PDG98 for the decay processes and recent CLEO measurements on the meson-photon transition form factors, we estimate for the $η_8$-$η_1$ mixing scheme the mixing angle to be $θ=-14.5^\circ \pm 2.0^\circ$ and the ratio of the decay constants of singlet to octet to be $f_1/f_8=1.17 \pm 0.08$. Applying our approach to the recently proposed $q \qbar$-$s \sbar$ mixing scheme, we obtain the mixing angle to be $ϕ=39.8^\circ \pm 1.8^\circ$ and the ratio of the decay constants of $s\sbar$ state to $q\qbar$ state to be $f_s/f_q=1.20 \pm 0.10$.

hep-ph↗

arXiv2000

The flavour asymmetry and quark-antiquark asymmetry in the $Σ^+$-sea

The sea quark content of the $Σ^+$ baryon is investigated using light-cone baryon-meson fluctuation model suggested by Brodsky and Ma. It is found that the $Σ^+$ sea is flavour asymmetric ($\dbar > \ubar > \sbar$) and quark-antiquark asymmetric ($q \not= \qbar$). Our prediction for the flavour asymmetry, $\dbar > \ubar > \sbar$, is significantly different from the SU(3) prediction ($\dbar < \ubar < \sbar$), while our prediction for the $d$-$\dbar$ asymmetry is consistent with the SU(3) prediction.

hep-ph↗