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Chao-Qiang Geng

Publications and source records attributed to Chao-Qiang Geng.

At least 127 records · Page 7Linked to original sources

Semi-dileptonic decays of the light vector mesons in Light Front Quark Model

We study the transition form factors of the light vector to pseudoscalar mesons as functions of the momentum transfer $q^2$ within the light-front quark model. With these form factors, we calculate the decay branching ratios of all possible modes for $V\to P\ell^+\ell^-$ ($V=ω$ and $ϕ$, $P=π^0$, $η$ and $η^{\prime}$ and $\ell=e$ and $μ$). We find that our numerical results fit with the data, such as those of $ω\to π^0 \ell^+\ell^-$ and $ϕ\to π^0 e^+e^-$ by NA60 and $ϕ\toηe^+e^-$ by SND. We also predict that the branching ratios of $ϕ\to π^0 μ^+μ^-$, $ω\to ηe^+e^-$, $ω\to ημ^+μ^-$, $ϕ\to ημ^+μ^-$ and $ϕ\to η^{\prime} e^+e^-$ to be aroud $3.48\times 10^{-6}$, $3.22\times 10^{-6}$, $1.81\times 10^{-9}$, $6.86\times 10^{-6}$ $2.97\times 10^{-7}$, respectively.

hep-ph↗

Cosmic Ray Excesses from Multi-component Dark Matter Decays

We use multi-component decaying dark matter (DM) scenario to explain the possible cosmic ray excesses in the positron fraction recently confirmed by AMS-02 and the total $e^+ +e^-$ flux observed by Fermi-LAT. In the two-component DM models, we find an interesting variation of the flavor structure along with the cutoff of the heavy DM. For the three-component DM case, we focus on a particular parameter range in which the best fits prefer to open only 2 DM decay channels with a third DM contributing nothing to the electron and positron spectra. We show that all models give the reasonable fits to both the AMS-02 positron fraction and the Fermi-LAT total $e^++e^-$ flux, which are also consistent with the measured diffuse $γ$-ray flux by Fermi-LAT.

hep-ph↗

Dark radiation from a unified dark fluid model

We present a unified dark fluid model to describe the possible evolutionary behavior of $ΔN_\mathrm{eff}$ in dark radiation. This model can be viewed as an interacting model for the dark sectors, in which dark matter interacts with dark radiation. We show that the evolution of $ΔN_\mathrm{eff}$ can be nicely explained without some drawbacks, such as the blowup of $ΔN_\mathrm{eff}$ and the non-vanishing interaction at the late time.

astro-ph.CO↗

Imprint of Multi-component Dark Matter on AMS-02

The multi-component decaying dark matter (DM) scenario is investigated to explain the possible excesses in the positron fraction by PAMELA and recently confirmed by AMS-02, and in the total $e^+ +e^-$ flux observed by Fermi-LAT. By performing the $χ^2$ fits, we find that two DM components are already enough to give a reasonable fit of both AMS-02 and Fermi-LAT data. The best-fitted results show that the heavier DM component with its mass 1.5 TeV dominantly decays through the $μ$-channel, while the lighter one of 100 GeV mainly through the $τ$-channel. As a byproduct, the fine structure around 100 GeV observed by AMS-02 and Fermi-LAT can be naturally explained by the dropping due to the lighter DM component. With the obtained model parameters by the fitting, we calculate the diffuse $γ$-ray emission spectrum in this two-component DM scenario, and find that it is consistent with the data measured by Fermi-LAT. We also construct a microscopic particle DM model to naturally realize the two-component DM scenario, and point out an interesting neutrino signal which is possible to be measured in the near future by IceCube.

hep-ph↗

Parametrized Post-Newtonian Limit of Teleparallel Dark Energy Model

We study the post-Newtonian limit in the teleparallel equivalent of General Relativity with a scalar field which non-minimally couples to gravity. The metric perturbation is obtained from the vierbein field expansion with respect to the Minkowski background. Due to the structure of the teleparallel gravity Lagrangian, the potential of the scalar field shows no effect to the parametrized post-Newtonian parameters, and compatible results with Solar System observations are found.

gr-qc↗

Large-scale magnetic fields from inflation in teleparallel gravity

Generation of large-scale magnetic fields in inflationary cosmology is studied in teleparallelism, where instead of the scalar curvature in general relativity, the torsion scalar describes the gravity theory. In particular, we investigate a coupling of the electromagnetic field to the torsion scalar during inflation, which leads to the breaking of conformal invariance of the electromagnetic field. We demonstrate that for a power-law type coupling, the current magnetic field strength of $\sim 10^{-9}$ G on 1 Mpc scale can be generated, if the backreaction effects and strong coupling problem are not taken into consideration.

astro-ph.CO↗

Form factors of $η_c$ in light front quark model

We study the form factors of the $η_c$ meson in the light-front quark model. We explicitly show that the transition form factor of $η_c \to γ^* γ$ as a function of the momentum transfer is consistent with the experimental data by the BaBar collaboration, while the decay constant of $η_c$ is found to be $f_{η_{c}}=230.5^{+52.2}_{-61.0}$ and $303.6^{+115.2}_{-116.4}$ MeV for $η_c\sim c\bar{c}$ by using two $η_c \to γγ$ decay widths of $5.3\pm 0.5$ and $7.2\pm2.1$ keV, given by Particle Data Group and Lattice QCD calculation, respectively.

hep-ph↗

Singularity Problem in Teleparallel Dark Energy Models

We study future singularity in teleparallel dark energy models, particularly its behavior and its (non)occurrence in the observationally viable models. For the models with a general self-potential of the scalar field, we point out that both at early times and in the future near the singularity the behavior of dark energy can be described by the analytic solutions of the scalar field we obtained for the model with no self-potential. As to the (non)occurrence in the viable models, we consider a natural binding-type self-potential, the quadratic potential, when fitting observational data, and illustrate the constraining region up to the $3σ$ confidence level as well as the region where a singularity will occur. As a result, the singularity region is outside the $3σ$ constraint. Thus, although the future singularity problem potentially exists in teleparallel dark energy models, the observationally viable models may not suffer this problem.

astro-ph.CO↗

Correlation of $h\rightarrowγγ$ and $Zγ$ in Type-II seesaw neutrino model

We study the charged scalar contributions to the Higgs decay channels of $h\rightarrow γγ$ and $h\rightarrow Zγ$ in the Type-II seesaw neutrino model. In most of the allowed parameter space in the model, the new contribution to $h\rightarrow Zγ$ is positively correlated with that to $h\rightarrowγγ$. If the current excess of the $h\rightarrow γγ$ rate measured by the ATLAS Collaboration persists, the $h\rightarrow Zγ$ rate should be also larger than the corresponding standard model prediction. We demonstrate that the anti-correlation between $h\rightarrowγγ$ and $h\rightarrow Zγ$ only exists in some special region.

hep-ph↗

Density Perturbation Growth in Teleparallel Cosmology

We study the cosmological perturbations in teleparallel dark energy models in which there is a dynamical scalar field with a non-minimal coupling to gravity. We find that the propagating degrees of freedom are the same as in quintessence cosmology despite that variables of the perturbed vierbein field are greater than those in metric theories. We numerically show some evident discrepancy from general relativity in the evolutions of the perturbations on all scales of the universe. We also demonstrate that the gravitational interactions are enhanced during the unique tracker evolutions in these models.

astro-ph.CO↗

New Scalar Contributions to $h\to Zγ$

We calculate the Higgs decay rate of $h\to Zγ$ by including the contributions from new scalars with arbitrary quantum numbers of the weak isospin ($T$) and hypercharge ($Y$) in the standard model. We find that our general formula for the decay rate of $h\to Zγ$ matches with that for $h\to γγ$ in the limit of $m_Z=0$, but it is different from those in the literature. To illustrate our result, by taking the current $2σ$ excess of the $h\to γγ$ rate measured by the LHC, we examine the corresponding shift for the $Zγ$ decay channel due to the new scalar. We show that the enhancement or reduction of the $h\to Zγ$ rate only depends on the relative size of $T$ and the absolute value of $Y$. Explicitly, we predict $0.76<R_{Zγ}\equiv Γ(h\to Zγ)/Γ_{SM}(h\to Zγ)<2.05$ by imposing the observed range of $1.5<R_{γγ}\equiv Γ(h\to Zγγ)/Γ_{SM}(h\toγγ)<2$, which is independent of the number of multiplets and the couplings to the Higgs particle as long as the scalars are heavier than 200 GeV. This result provides a clear signature for the future LHC measurements to test physics beyond the standard model.

hep-ph↗

Multi high charged scalars in the LHC searches and Majorana neutrino mass generations

One natural way to understand the excess of the measured $H\toγγ$ rate over the standard model (SM) expectation at the Large Hadron Collider (LHC) is to have charged scalar bosons, existing in most of the SM extensions. Motivated by this LHC result, we explore if it also sheds light on solving the small neutrino mass generation problem. We concentrate on a class of models with high dimensional representations of scalars to realize Majorana neutrino masses at two-loop level without imposing any new symmetry. In these models, multi scalars with the electric charges higher than two are naturally expected, which not only enhance the $H\toγγ$ rate, but provide more searching grounds at the LHC. In particular, the rate of $H\to Zγ$ also changes similar to that of the diphoton channel.

hep-ph↗

Einstein Static Universe in Exponential $f(T)$ Gravity

We analyze the stability of the Einstein static closed and open universe in two types of exponential $f(T)$ gravity theories. We show that the stable solutions exist in these two models. In particular, we find that large regions of parameter space in equation of state $w=p/ρ$ for the stable universe are allowed in the $f(T)$ theories.

gr-qc↗

Note on 125 GeV Spin-2 particle

A new boson around 125 GeV without specific spin has been observed by both ATLAS and CMS at the LHC. Since its decay into a diphoton excludes the spin-1 case by the Landau-Yang theorem,it leaves 0 or 2 as the possible lowest spin for the new boson. Instead of the well-established spin-0 Higgs-like boson, we take this new boson to be a spin-2 massive Graviton-like particle denoted as $G$, which exists copiously in extra-dimension theories, and concentrate on its phenomenology. In particular, we calculate the three-body decays of $G\rightarrow V f\bar{f}'$ with $V$ and $f^{(\prime)}$ the gauge boson and fermions in the standard model (SM) and compare our results with those of the SM Higgs boson. The couplings between $G$ and $V$s are also estimated by fitting the data. A new observable that can distinguish $G$ from the Higgs is proposed.

hep-ph↗

Asymptotic Cosmological Behavior of Scalar-Torsion Mode in Poincare Gauge Theory

We study the cosmological effect of the simple scalar-torsion ($0^+$) mode in Poincaré gauge theory of gravity. We find that for the non-constant (affine) curvature case, the early evolution of the torsion density $ρ_T$ has a radiation-like asymptotic behavior of $a^{-4}$ with $a$ representing the scale factor, along with the stable point of the torsion pressure ($P_T$) and density ratio $P_T/ρ_T\rightarrow 1/3$ in the high redshift regime $(z \gg 0)$, which is different from the previous result in the literature. We use the Laurent expansion to resolve the solution. We also illustrate our result by the execution of numerical computations.

astro-ph.CO↗

Teleparallel Dark Energy with Purely Non-minimal Coupling to Gravity

We propose the simplest model of teleparallel dark energy with purely a non-minimal coupling to gravity but no self-potential, a single model possessing various interesting features: simplicity, self-potential-free, the guaranteed late-time cosmic acceleration driven by the non-minimal coupling to gravity, tracker behavior of the dark energy equation of state at earlier times, a crossing of the phantom divide at a late time, and the existence of a finite-time future singularity. We find the analytic solutions of the dark-energy scalar field respectively in the radiation, matter, and dark energy dominated eras, thereby revealing the above features. We further illustrate possible cosmic evolution patterns and present the observational constraint of this model obtained by numerical analysis and data fitting.

astro-ph.CO↗

Primordial Fluctuations within Teleparallelism

We study the cosmological perturbations for the possible inflation scenario in the teleparallel equivalence of general relativity specified with parallelizable topological conditions. By acquiring the identical physical observables to general relativity under the teleparallel formalism, we perform a 3+1 decomposition of the vierbein field, which can be interpreted as the time gauge fixing between coordinate and tangent frames. We also extend our discussion to the higher-order action, $f(T)$ gravity.

gr-qc↗

Matter Density Perturbations in Modified Teleparallel Theories

We study the matter density perturbations in modified teleparallel gravity theories, where extra degrees of freedom arise from the local Lorentz violation in the tangent space. We formulate a vierbein perturbation with variables addressing all the 16 components of the vierbein field. By assuming the perfect fluid matter source, we examine the cosmological implication of the 6 unfamiliar new degrees of freedom in modified $f(T)$ gravity theories. We find that despite the new modes in the vierbein scenario provide no explicit significant effect in the small-scale regime, they exhibit some deviation from the standard general relativity results in super-horizon scales.

gr-qc↗