arXiv subjects

Chao-Qiang Geng

Publications and source records attributed to Chao-Qiang Geng.

At least 145 records · Page 8Linked to original sources

arXiv2012

Scalar-Torsion Mode in a Cosmological Model of the Poincaré Gauge Theory of Gravity

We investigate the equation of state (EoS) of the scalar-torsion mode in Poincaré gauge theory of gravity. We concentrate on two cases with the constant curvature solution and positive kinetic energy, respectively. In the former, we find that the torsion EoS has different values in the various stages of the universe. In particular, it behaves like the radiation (matter) EoS of $w_r=1/3$ ($w_m=0$) in the radiation (matter) dominant epoch, while in the late time the torsion density is supportive for the accelerating universe. In the latter, our numerical analysis shows that in general the EoS has an asymptotic behavior in the high redshift regime, while it could cross the phantom divide line in the low redshift regime.

gr-qc↗

arXiv2012

Generation of large-scale magnetic fields from inflation in teleparallelism

We explore the generation of large-scale magnetic fields from inflation in teleparallelism, in which the gravitational theory is described by the torsion scalar instead of the scalar curvature in general relativity. In particular, we examine the case that the conformal invariance of the electromagnetic field during inflation is broken by a non-minimal gravitational coupling between the torsion scalar and the electromagnetic field. It is shown that for a power-law type coupling, the magnetic field on 1 Mpc scale with its strength of $\sim 10^{-9}$ G at the present time can be generated.

astro-ph.CO↗

arXiv2012

Study of pesudoscalar transition form factors within light front quark model

We study the transition form factors of the pesudoscalar mesons ($π,η$ and $η^{\prime}$) as functions of the momentum transfer $Q^2$ within the light-front quark model. We compare our results with the recent experimental data by CELLO, CLEO, BaBar and Belle. By considering the possible uncertainties from the quark masses, we illustrate that our predicted form factors can fit with all the data, including those at the large $Q^2$ regions.

hep-ph↗

arXiv2012

Direct CP Violation in Charm Decays due to Left-Right Mixing

Motivated by the $3.8σ$ deviation from no CP violation hypothesis for the CP asymmetry (CPA) difference between $D^0\to K^+ K^-$ and $D^0\to π^+ π^-$, reported recently by LHCb and CDF, we investigate the CP violating effect due to the left-right (LR) mixing in the general LR symmetric model. In particular, in the non-manifest LR model we show that the large CPA difference could be explained, while the constraints from $(ε'/ε)_K$ and $D^0$-$\bar D^0$ are satisfied.

hep-ph↗

arXiv2012

Singularity phenomena in viable f(R) gravity

The curvature singularity in viable f(R) gravity models is examined when the background density is dense. This singularity could be eliminated by adding the $R^{2}$ term in the Lagrangian. Some of cosmological consequences, in particular the source for the scalar mode of gravitational waves, are discussed.

astro-ph.CO↗

arXiv2012

Magnetic dipole moment and keV neutrino dark matter

We study magnetic dipole moments of right-handed neutrinos in a keV neutrino dark matter model. This model is a simple extension of the standard model with only right-handed neutrinos and a pair of charged particles added. One of the right-handed neutrinos is the candidate of dark matter with a keV mass. Some bounds on the dark matter magnetic dipole moment and model parameters are obtained from cosmological observations.

hep-ph↗

arXiv2012

CP violation in $D^0 \to (K^- K^+, π^- π^+)$ from diquarks

The explanation of the large CP asymmetries in $D^0\to (π^+π^-, K^+K^-)$ decays observed by the LHCb collaboration is likely to call for new physics beyond the CKM paradigm. We explore new contributions caused by the color-sextet scalar diquark, and demonstrate that the diquark with the mass of order 1 TeV and nominal couplings with quarks can generate the CP asymmetries at the percent level. Using the experimental data on branching ratios and CP asymmetries of $D^0\to (π^+π^-, K^+K^-)$, we derive the constraints on the diquark mass and couplings, which can be further examined on hadron colliders in the dijet final states.

hep-ph↗

arXiv2012

$π^0\toγ^*γ$ transition form factor within Light Front Quark Model

We study the transition form factor of $π^0\toγ^* γ$ as a function of the momentum transfer $Q^2$ within the light-front quark model (LFQM). We compare our result with the experimental data by BaBar as well as other calculations based on the LFQM in the literature. We show that our predicted form factor fits well with the experimental data, particularly those at the large $Q^2$ region.

hep-ph↗

arXiv2011

Observational Constraints on Teleparallel Dark Energy

We use data from Type Ia Supernovae (SNIa), Baryon Acoustic Oscillations (BAO), and Cosmic Microwave Background (CMB) observations to constrain the recently proposed teleparallel dark energy scenario based on the teleparallel equivalent of General Relativity, in which one adds a canonical scalar field, allowing also for a nonminimal coupling with gravity. Using the power-law, the exponential and the inverse hyperbolic cosine potential ansatzes, we show that the scenario is compatible with observations. In particular, the data favor a nonminimal coupling, and although the scalar field is canonical the model can describe both the quintessence and phantom regimes.

astro-ph.CO↗

arXiv2011

Thermodynamics of cosmological horizons in $f(T)$ gravity

We explore thermodynamics of the apparent horizon in $f(T)$ gravity with both equilibrium and non-equilibrium descriptions. We find the same dual equilibrium/non-equilibrium formulation for $f(T)$ as for $f(R)$ gravity. In particular, we show that the second law of thermodynamics can be satisfied for the universe with the same temperature of the outside and inside the apparent horizon.

gr-qc↗

arXiv2011

"Teleparallel" Dark Energy

Using the "teleparallel" equivalent of General Relativity as the gravitational sector, which is based on torsion instead of curvature, we add a canonical scalar field, allowing for a nonminimal coupling with gravity. Although the minimal case is completely equivalent to standard quintessence, the nonminimal scenario has a richer structure, exhibiting quintessence-like or phantom-like behavior, or experiencing the phantom-divide crossing. The richer structure is manifested in the absence of a conformal transformation to an equivalent minimally-coupled model.

hep-th↗

arXiv2011

Large θ_13 from finite quantum corrections in quasi-degenerate neutrino mass spectrum

We study finite quantum corrections for several well known neutrino mixing matrices and find that it is hard to account for the large value of θ_13 recently reported by T2K and MINOS. To nicely reproduce all experimentally favored neutrino mixing angles and masses, we propose a new neutrino mixing pattern. We also demonstrate a simple realization by slightly extending the standard model to illustrate the quantum corrections.

hep-ph↗

arXiv2011

Phantom crossing in viable $f(R)$ theories

We review the equation of state for dark energy in modified gravity theories. In particular, we summarize the generic feature of the phantom divide crossing in the past and future in viable $f(R)$ gravity models.

gr-qc↗

arXiv2011

Gravitational Waves in Viable f(R) Models

We study gravitational waves in viable $f(R)$ theories under a non-zero background curvature. In general, an $f(R)$ theory contains an extra scalar degree of freedom corresponding to a massive scalar mode of gravitational wave. For viable $f(R)$ models, since there always exits a de-Sitter point where the background curvature in vacuum is non-zero, the mass squared of the scalar mode of gravitational wave is about the de-Sitter point curvature $R_{d}\sim10^{-66}eV^{2}$. We illustrate our results in two types of viable $f(R)$ models: the exponential gravity and Starobinsky models. In both cases, the mass will be in the order of $10^{-33}eV$ when it propagates in vacuum. However, in the presence of matter density in galaxy, the scalar mode can be heavy. Explicitly, in the exponential gravity model, the mass becomes almost infinity, implying the disappearance of the scalar mode of gravitational wave, while the Starobinsky model gives the lowest mass around $10^{-24}eV$, corresponding to the lowest frequency of $10^{-9}$ Hz, which may be detected by the current and future gravitational wave probes, such as LISA and ASTROD-GW.

astro-ph.CO↗

arXiv2011

Finite quantum corrections to the tribimaximal neutrino mixing

We calculate finite quantum corrections to the tribimaximal neutrino mixing pattern V_TB in three generic classes of neutrino mass models. We show that three flavor mixing angles can all depart from their tree-level results described by V_TB, among which θ_12 is most sensitive to such quantum effects, and the Dirac CP-violating phase can radiatively arise from two Majorana CP-violating phases. This theoretical scheme offers a new way to understand why θ_13 is naturally small and how three CP-violating phases are presumably correlated.

hep-ph↗

arXiv2011

Study of the Radivative Pion Decay

We study the radiative pion decay of $π^{+} \to e^{+}ν_{e}γ$ in the chiral perturbation theory (ChPT) and light front quark model (LFQM). The vector and axial-vector hadronic form factors ($F_{V,A}$) for the $π\toγ$ transition are evaluated in the whole allowed momentum transfer. In terms of these momentum dependent form factors, we calculate the decay branching ratio and compare our results with the experimental data and other theoretical predictions in the literature. We also constrain the possible size of the tensor interaction in the LFQM.

hep-ph↗

arXiv2011

Thermodynamics in Modified Gravity Theories

We demonstrate that there does exist an equilibrium description of thermodynamics on the apparent horizon in the expanding cosmological background for a wide class of modified gravity theories with the Lagrangian density $f(R, ϕ, X)$, where $R$ is the Ricci scalar and $X$ is the kinetic energy of a scalar field $ϕ$. This comes from a suitable definition of an energy momentum tensor of the "dark" component obeying the local energy conservation law in the Jordan frame. It is shown that the equilibrium description in terms of the horizon entropy $S$ is convenient because it takes into account the contribution of the horizon entropy $\hat{S}$ in non-equilibrium thermodynamics as well as an entropy production term.

gr-qc↗

arXiv2011

Equation of state for dark energy in $f(T)$ gravity

We study the cosmological evolutions of the equation of state for dark energy $w_{\mathrm{DE}}$ in the exponential and logarithmic as well as their combination $f(T)$ theories. We show that the crossing of the phantom divide line of $w_{\mathrm{DE}} = -1$ can be realized in the combined $f(T)$ theory even though it cannot be in the exponential or logarithmic $f(T)$ theory. In particular, the crossing is from $w_{\mathrm{DE}} > -1$ to $w_{\mathrm{DE}} < -1$, in the opposite manner from $f(R)$ gravity models. We also demonstrate that this feature is favored by the recent observational data.

astro-ph.CO↗