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Chao-Qiang Geng

Publications and source records attributed to Chao-Qiang Geng.

At least 163 records · Page 9Linked to original sources
arXiv2010

Z-mediated charge and CP asymmetries and FCNCs in $B_{d,s}$ processes

We show model-independently that the negative like-sign charge asymmetry $(-A^b_{s\ell})$ is less than $ 3.16\times 10^{-3}$ when the constraints from the $B_q-\bar B_q$ mixings and the time-dependent CP asymmetries (CPAs) for $B_q\to J/ΨM_q$ with $M_q=K,ϕ$ and $q=d,s$ are taken into account. Although the result is smaller than the measured value by the DØ Collaboration at Fermilab, there is still plenty of room to have new physics, which is sensitive to new CP violating effects, as the standard model (SM) prediction is $(2.3_{-0.5}^{+0.6})\times 10^{-4}$. To illustrate the potential large $|A^{b}_{s\ell}|$, we show the influence of new $SU(2)_L$ singlet exotic quarks in the vector-like quark model, where the $Z$-mediated flavor changing neutral currents (FCNCs) are generated at tree level. In particular, we demonstrate that (a) the like-sign charge asymmetry could be enhanced by a factor of two in magnitude; (b) the CPA of $\sin2β^{J/Ψϕ}_s$ could reach to $-15%$; (c) the CPA of $\sin2β_{ϕK_S}$ could be higher than $\sin2β_{J/ΨK_S}$ when $|A^b_{s\ell}|$ is larger than the SM prediction; and (d) the branching ratio for $B_s\to μ^+ μ^-$ could be as large as $0.6\times 10^{-8}$.

hep-ph↗
arXiv2010

Generic feature of future crossing of phantom divide in viable $f(R)$ gravity models

We study the equation of state for dark energy and explicitly demonstrate that the future crossings of the phantom divide line $w_{\mathrm{DE}}=-1$ are the generic feature in the existing viable $f(R)$ gravity models. We also explore the future evolution of the cosmological horizon entropy and illustrate that the cosmological horizon entropy oscillates with time due to the oscillatory behavior of the Hubble parameter. The important cosmological consequence is that in the future, the sign of the time derivative of the Hubble parameter changes from negative to positive in these viable $f(R)$ gravity models.

astro-ph.CO↗
arXiv2010

Observational Constraints on Exponential Gravity

We study the observational constraints on the exponential gravity model of f(R)=-beta*Rs(1-e^(-R/Rs)). We use the latest observational data including Supernova Cosmology Project (SCP) Union2 compilation, Two-Degree Field Galaxy Redshift Survey (2dFGRS), Sloan Digital Sky Survey Data Release 7 (SDSS DR7) and Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP7) in our analysis. From these observations, we obtain a lower bound on the model parameter beta at 1.27 (95% CL) but no appreciable upper bound. The constraint on the present matter density parameter is 0.245< Omega_m^0<0.311 (95% CL). We also find out the best-fit value of model parameters on several cases.

astro-ph.CO↗
arXiv2010

Cosmological evolution in exponential gravity

We explore the cosmological evolution in the exponential gravity $f(R)=R +c_1 (1-e^{- c_2 R})$ ($c_{1, 2} = \mathrm{constant}$). We summarize various viability conditions and explicitly demonstrate that the late-time cosmic acceleration following the matter-dominated stage can be realized. We also study the equation of state for dark energy and confirm that the crossing of the phantom divide from the phantom phase to the non-phantom (quintessence) one can occur. Furthermore, we illustrate that the cosmological horizon entropy globally increases with time.

astro-ph.CO↗
arXiv2010

Thermodynamics in $f(R)$ gravity in the Palatini formalism

We investigate thermodynamics of the apparent horizon in $f(R)$ gravity in the Palatini formalism with non-equilibrium and equilibrium descriptions. We demonstrate that it is more transparent to understand the horizon entropy in the equilibrium framework than that in the non-equilibrium one. Furthermore, we show that the second law of thermodynamics can be explicitly verified in both phantom and non-phantom phases for the same temperature of the universe outside and inside the apparent horizon.

gr-qc↗
arXiv2010

Equilibrium thermodynamics in modified gravitational theories

We show that it is possible to obtain a picture of equilibrium 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 that respects to a local energy conservation in the Jordan frame. In this framework the horizon entropy $S$ corresponding to equilibrium thermodynamics is equal to a quarter of the horizon area $A$ in units of gravitational constant $G$, as in Einstein gravity. For a flat cosmological background with a decreasing Hubble parameter, $S$ globally increases with time, as it happens for viable $f(R)$ inflation and dark energy models. We also show that the equilibrium description in terms of the horizon entropy $S$ is convenient because it takes into account the contribution of both the horizon entropy $\hat{S}$ in non-equilibrium thermodynamics and an entropy production term.

gr-qc↗
arXiv2010

Crossing of Phantom Divide in $F(R)$ Gravity

An explicit model of $F(R)$ gravity with realizing a crossing of the phantom divide is reconstructed. In particular, it is shown that the Big Rip singularity may appear in the reconstructed model of $F(R)$ gravity. Such a Big Rip singularity could be avoided by adding $R^2$ term or non-singular viable $F(R)$ theory to the model because phantom behavior becomes transient.

hep-th↗
arXiv2009

Equivalence of modified gravity equation to the Clausius relation

We explicitly show that the equations of motion for modified gravity theories of $F(R)$-gravity, the scalar-Gauss-Bonnet gravity, $F(\mathcal{G})$-gravity and the non-local gravity are equivalent to the Clausius relation in thermodynamics. In addition, we discuss the relation between the expression of the entropy and the contribution from the modified gravity as well as the matter to the definition of the energy flux (heat).

hep-th↗
arXiv2009

Oscillating phantom in $F(R)$ gravity

We investigate the oscillating effective equation of state (EoS) of the universe around the phantom divide in the framework of $F(R)$ gravity. We illustrate the behavior of $F(R)$ with realizing multiple crossings of the phantom divide.

astro-ph.CO↗
arXiv2009

$B_{s,d}-\bar B_{s,d}$ mixings and $b\to q (\ga, \ell \bar \ell)$ decays in private Higgs model

We study the low energy phenomena induced by the lightest charged Higgs in the private Higgs (PH) model, in which each quark flavor is associated with a Higgs doublet. We show that the couplings of the charged Higgs scalars to fermions are fixed and the unknown parameters are only the masses and mixing elements of the charged Higgs scalars. As the charged Higgs masses satisfy with $M_b<M_{c}\ll M_{s} \ll M_{d,u}$, processes involving $B$-meson are expected to be the ideal places to test the PH model. In particular, we explore the constraints on the model from experimental data in $B$ physics, such as the branching ratio (BR) and CP asymmetry (CPA) of $B\to X_s \ga$, $B_{d,s}-\bar B_{d,s}$ mixings and the BR for $B\to K^* \ell^{+} \ell^{-}$. We illustrate that the sign of the Wilson coefficient for $B\to X_s \ga$ can be different from that in the standard model, while this flipped sign can be displayed by the forward-backward asymmetry of $B\to V \ell^{+} \ell^{-}$ with $V$ a vector meson. We also demonstrate that $B_{d,s}-\bar B_{d,s}$ mixings and their time-dependent CPAs are negligible small and the BR of $B\to K^* \ell^{+} \ell^{-}$ can have a more strict bound than that of $B\to X_s \ga$.

hep-ph↗
arXiv2009

Thermodynamics in $F(R)$ gravity with phantom crossing

We study thermodynamics of the apparent horizon in $F(R)$ gravity. In particular, we demonstrate that a $F(R)$ gravity model with realizing a crossing of the phantom divide can satisfy the second law of thermodynamics in the effective phantom phase as well as non-phantom one.

hep-th↗
arXiv2009

Neutrino Masses, Leptogenesis and Decaying Dark Matter

We study a simple extension of the standard model to simultaneously explain neutrino masses, dark matter and the matter-antimatter asymmetry of the Universe. In our model, the baryon asymmetry is achieved by the leptogenesis mechanism, while the decaying dark matter with the lifetime of O(10^26 s) provides a natural solution to the electron and positron excesses in Fermi and PAMELA satellite experiments. In particular, we emphasize that our model is sensitive to the structure at the endpoint around 1 TeV of the Fermi data. In addition, some of new particles proposed in the model are within the reach at the near future colliders, such as the Large Hadron Collider.

hep-ph↗
arXiv2009

Crossing of the phantom divide in modified gravity

We reconstruct an explicit model of modified gravity in which a crossing of the phantom divide can be realized. It is shown that the (finite-time) Big Rip singularity appears in the model of modified gravity (i.e., in the so-called Jordan frame), whereas that in the corresponding scalar field theory obtained through the conformal transformation (i.e., in the so-called Einstein frame) the singularity becomes the infinite-time one. Furthermore, we investigate the relations between the scalar field theories with realizing a crossing of the phantom divide and the corresponding modified gravitational theories by using the inverse conformal transformation. It is demonstrated that the scalar field theories describing the non-phantom phase (phantom one with the Big Rip) can be represented as the theories of real (complex) $F(R)$ gravity through the inverse (complex) conformal transformation. We also study a viable model of modified gravity in which the transition from the de Sitter universe to the phantom phase can occur. In addition, we explore the stability for the obtained solutions of the crossing of the phantom divide under a quantum correction coming from conformal anomaly.

hep-th↗
arXiv2009

ATIC/PAMELA anomaly from fermionic decaying Dark Matter

We demonstrate that an economical two Higgs doublet model can explain the electron and positron excesses in the recent ATIC and PAMELA experiments by the three body decays of the dark matter (DM) fermions without requiring the fine turning of the couplings and degeneracy of masses. We also show that the mass and lifetime of the decaying DM particle may not be fixed to be around 1 TeV and 10^{26} sec, respectively. Moreover, we note that this model includes a stable dark matter candidate as well.

hep-ph↗
arXiv2008

New CP phase of $B_s-\bar B_s$ mixing on ${\rm T}$ violation in $B_{d(s)}\to K^*(ϕ) \ell^{+} \ell^{-}$

A large CP-violating phase uncovered recently by CDF and D$Ø$ collaborations in the time-dependent CP asymmetry (CPA) of the $B_s\to J/Ψϕ$ decay clearly indicates that a non-Cabibbo-Kobayashi-Maskawa (CKM) phase has to be brought into the $b\to s$ transition. We find that the model with $SU(2)_L$ singlet exotic quarks can not only provide the new phase induced from the Z-mediated flavor changing neutral current (FCNC) at tree level, but also strongly relate the $B_s-\bar B_s$ mixing, $B_q\to V_q \ell^{+} \ell^{-}$ ($V_{d[s]}=K^*[ϕ])$ and $B_s\to μ^{+} μ^{-}$ together. In particular, we show that the new CP phase can be unambiguously exposed by the large statistical significances of $T$ violating observables in $B_q\to V_q \ell^{+} \ell^{-}$, while the branching ratio of $B_s\to μ^{+} μ^{-}$ can be enhanced to be $O(10^{-8})$.

hep-ph↗
arXiv2008

U-boson and the HyperCP exotic events

We show that the very light spin-1 gauge U-boson of the extra $U(1)'$ gauge model in the framework of the supersymmetric standard model extension can be a good candidate of the new light particle suggested by the HyperCP experiment. We demonstrate that the flavor changing neutral currents (FCNCs) for the HyperCP events in the decay of $Σ^{+}\to p μ^{+} μ^{-}$ can be generated at both tree and loop levels. In particular, we find that the loop induced $s\to d U$ transition due to the tensor-type interaction with the dimension-5 electric dipole operator plays a very important role on the FCNCs. Our explanation of the HyperCP data with the spin-1 U-boson is different from that based on a light pseudoscalar Higgs boson or sgoldstino in the literature. In particular, the U-boson involves a rich phenomenology in particle physics as well as cosmology.

hep-ph↗