arXiv subjects

Chao-Qiang Geng

Publications and source records attributed to Chao-Qiang Geng.

At least 91 records · Page 5Linked to original sources

arXiv2018

LHC Searches for Top-philic Kaluza-Klein Graviton

We study the phenomenology of a massive graviton $G$ with non-universal couplings to the Standard Model (SM) particles. Such a particle can arise as a warped Kaluza-Klein graviton from a framework of the Randall-Sundrum extra-dimension model. In particular, we consider a case in which $G$ is top-philic, i.e., $G$ interacts strongly with the right-handed top quark, resulting in the large top-loop contributions to its production via the gluon fusion and its decays to the SM gauge bosons. We take into account the constraints from the current 13 TeV LHC data on the channels of $t\bar{t}$, $γγ$, $jj (gg)$, $γZ$, and $ZZ$. Consequently, it is found that the strongest limit for this spin-2 resonance $G$ comes from the $t\bar{t}$ pair search, which constrains the cutoff scale to be of ${\cal O}$(100 GeV) for the right-top coupling of ${\cal O}(1)$ and the massive graviton mass in the range $m_G$=2-5 TeV, significantly relaxed compared with the universal $G$ coupling case.

hep-ph↗

arXiv2018

A model with flavor-dependent gauged $U(1)_{B-L_1}\times U(1)_{B-L_2-L_3}$ symmetry

We propose a new model with flavor-dependent gauged $U(1)_{B-L_1}\times U(1)_{B-L_{2}-L_{3}}$ symmetry in addition to the flavor-blind one in the standard model. The model contains three right-handed neutrinos to cancel gauge anomalies and several Higgses to construct the measured fermion masses. We show the generic feature of the model and explore its phenomenology. In particular, we discuss the current bounds on the extra gauge bosons from the K and B meson mixings as well as the LEP and LHC data and focus on their contributions to the lepton flavor violating processes of $\ell_{i+1}\to \ell_iγ$ (i=1,2).

hep-ph↗

arXiv2018

Cosmological constraints on $Λ(α)$CDM models with time-varying fine structure constant

We study the $Λ(α)$CDM models with $Λ(α)$ being a function of the time-varying fine structure constant $α$. We give a close look at the constraints on two specific $Λ(α)$CDM models with one and two model parameters, respectively, based on the cosmological observational measurements along with 313 data points for the time-varying $α$. We find that the model parameters are constrained to be around $10^{-4}$, which are similar to the results discussed previously but more accurately.

astro-ph.CO↗

arXiv2018

Neutrino masses, dark matter and leptogenesis with $U(1)_{B-L}$ gauge symmetry

We propose a model with an $U(1)_{B-L}$ gauge symmetry, in which small neutrino masses, dark matter and the matter-antimatter asymmetry in the Universe can be simultaneously explained. In particular, the neutrino masses are generated radiatively, while the matter-antimatter asymmetry is led by the leptogenesis mechanism, at TeV scale. We also explore allowed regions of the model parameters and discuss some phenomenological effects including lepton flavor violating processes.

hep-ph↗

arXiv2018

Cosmological perturbation and matter power spectrum in bimetric massive gravity

We discuss the linear perturbation equations with the synchronous gauge in a minimal scenario of the bimetric massive gravity theory. We find that the matter density perturbation and matter power spectrum are suppressed. We also examine the ghost and stability problems and show that the allowed deviation of this gravitational theory from the cosmological constant is constrained to be smaller than $\mathcal{O}(10^{-2})$ by the large scale structure observational data.

gr-qc↗

arXiv2017

Exothermic Dark Matter with Light Mediator after LUX and PandaX-II in 2016

Dark matter (DM) direct detections are investigated for models with the following properties: isospin-violating couplings, exothermic scatterings, and/or a light mediator, with the aim to reduce the tension between the CDMS-Si positive signals and other negative searches. In particular, we focus on the non-standard effective operators which could lead to the spin-independent DM-nucleus scatterings with non-trivial dependences on the transfer momentum or DM velocity. As a result, such effective operator choices have the very mild effects on the final fittings. Furthermore, by including the latest constraints from LUX, PandaX-II, XENON1T and PICO-60, we find that, for almost all the considered models, the predicted CDMS-Si signal regions are either severely constrained or completely excluded by the LUX, PandaX-II, XENON1T and PICO-60 data, including the most promising Xe-phobic exothermic DM models with/without a light mediator. Therefore, we conclude that it is very difficult for the present DM framework to explain the CDMS-Si excess.

hep-ph↗

arXiv2017

Equation of State of Neutron Stars with Junction Conditions in the Starobinsky Model

We study the Starobinsky or $R^2$ model of $f(R)=R+αR^2$ for neutron stars with the structure equations represented by the coupled differential equations and the \emph{polytropic} type of the matter equation of state. The junction conditions of $f(R)$ gravity are used as the boundary conditions to match the Schwarschild solution at the surface of the star. Based on these the conditions, we demonstrate that the coupled differential equations can be solved \emph{directly}. In particular, from the dimensionless equation of state $\barρ = \bar{k}\, \bar{p}^{\,γ}$ with $\bar{k}\sim5.0$ and $γ\sim0.75$ and the constraint of $α\lesssim {1.47722}\times 10^{7}\, \text{m}^2$, we obtain the \emph{minimal} mass of the NS to be around 1.44 $M_{\odot}$. In addition, if $\bar{k}$ is larger than 5.0, the mass and radius of the NS would be smaller.

gr-qc↗

arXiv2017

Constraints on running vacuum model with $H(z)$ and $f σ_8$

We examine the running vacuum model with $Λ(H) = 3 νH^2 + Λ_0$, where $ν$ is the model parameter and $Λ_0$ is the cosmological constant. From the data of the cosmic microwave background radiation, weak lensing and baryon acoustic oscillation along with the time dependent Hubble parameter $H(z)$ and weighted linear growth $f (z)σ_8(z)$ measurements, we find that $ν=(1.37^{+0.72}_{-0.95})\times 10^{-4}$ with the best fitted $χ^2$ value slightly smaller than that in the $Λ$CDM model.

astro-ph.CO↗

arXiv2017

Observational constraints on successful model of quintessential Inflation

We study quintessential inflation using a generalized exponential potential $V(ϕ)\propto exp(-λϕ^n/Mpl^n), n>1$, the model admits slow-roll inflation at early times and leads to close-to-scaling behaviour in the post inflationary era with an exit to dark energy at late times. We present detailed investigations of the inflationary stage in the light of the Planck 2015 results, study post-inflationary dynamics and analytically confirm the existence of an approximately scaling solution. Additionally, assuming that standard massive neutrinos are non-minimally coupled, makes the field $ϕ$ dominant once again at late times giving rise to present accelerated expansion of the Universe. We derive observational constraints on the field and time-dependent neutrino masses. In particular, for $n=6 (8)$, the parameter $λ$ is constrained to be,$\log λ> -7.29 (-11.7)$; the model produces the spectral index of the power spectrum of primordial scalar (matter density) perturbations as $ n_s = 0.959 \pm 0.001 (0.961 \pm 0.001)$ and tiny tensor-to-scalar ratio, $r<1.72 \times 10^{-2} (2.32 \times 10^{-2})$ respectively. Consequently, the upper bound on possible values of the sum of neutrino masses $Σm_ν \lesssim 2.5$ eV significantly enhances compared to that in the standard $Λ$CDM model.

gr-qc↗

arXiv2017

Teleparallel Conformal Invariant Models induced by Kaluza-Klein Reduction

We study the extensions of teleparallism in the Kaluza-Klein (KK) scenario by writing the analogous form to the torsion scalar $T_{\text{NGR}}$ in terms of the corresponding antisymmetric tensors, given by $T_{\text{NGR}} = a\,T_{ijk} \, T^{ijk} + b\,T_{ijk} \,T^{kji} + c\,T^{j}{}_{ji} \, T^{k}{}_{k}{}^{i}$, in the four-dimensional New General Relativity (NGR) with arbitrary coefficients $a$, $b$ and $c$. After the KK dimensional reduction, the Lagrangian in the Einstein-frame can be realized by taking $2a+b+c=0$ with the ghost-free condition $c\leq0$ for the one-parameter family of teleparallelism. We demonstrate that the pure conformal invariant gravity models can be constructed by the requirements of $2a+b=0$ and $c=0$. In particular, the torsion vector can be identified as the conformal gauge field, while the conformal gauge theory can be obtained by $2a+b+4c=0$ or $2a+b=0$, which is described on the Weyl-Cartan geometry $Y_4$ with the ghost-free conditions $2a+b+c>0$ and $c\neq0$. We also consider the weak field approximation and discuss the non-minimal coupled term of the scalar current and torsion vector. For the conformal invariant models with $2a+b=0$, we find that only the anti-symmetric tensor field is allowed rather than the symmetric one.

gr-qc↗

arXiv2017

Probing gravitational non-minimal coupling with dark energy surveys

We investigate observational constraints on a specific one-parameter extension to the minimal quintessence model, where the quintessence field acquires a quadratic coupling to the scalar curvature through a coupling constant $ξ$. The value of $ξ$ is highly suppressed in typical tracker models if the late-time cosmic acceleration is driven at some field values near the Planck scale. We test $ξ$ in a second class of models in which the field value today becomes a free model parameter. We use the combined data from type-Ia supernovae, cosmic microwave background, baryon acoustic oscillations and matter power spectrum, to weak lensing measurements and find a best-fit value $ξ> 0.289$ where $ξ= 0$ is excluded outside the 95 per cent confidence region. The effective gravitational constant $G_{\rm eff}$ subject to the hint of a non-zero $ξ$ is constrained to $-0.003 < 1- G_{\rm eff}/G < 0.033$ at the same confidence level on cosmological scales, and can be narrowed down to $1- G_{\rm eff}/G < 2.2 \times 10^{-5}$ when combining with Solar System tests.

astro-ph.CO↗

arXiv2017

Large $ν$-$\barν$ Oscillations from High-Dimensional Lepton Number Violating Operator

It is usually believed that the observation of the neutrino-antineutrino ($ν$-$\barν$) oscillations is almost impossible since the oscillation probabilities are expected to be greatly suppressed by the square of tiny ratio of neutrino masses to energies. Such an argument is applicable to most models for neutrino mass generation based on the Weinberg operator, including the seesaw models. However, in the present paper, we shall give a counterexample to this argument, and show that large $ν$-$\barν$ oscillation probabilities can be obtained in a class of models in which both neutrino masses and neutrinoless double beta ($0νββ$) decays are induced by the high-dimensional lepton number violating operator ${\cal O}_7 = \bar{u}_R l^c_R \bar{L}_L H^*d_R + {\rm H.c.}$ with $u$ and $d$ representing the first two generations of quarks. In particular, we find that the predicted $0νββ$ decay rates have already placed interesting constraints on the $ν_e \leftrightarrow \barν_e$ oscillation. Moreover, we provide an UV-complete model to realize this scenario, in which a dark matter candidate naturally appears due to the new $U(1)_d$ symmetry.

hep-ph↗

arXiv2016

Matter density perturbation and power spectrum in running vacuum model

We investigate the matter density perturbation $δ_m$ and power spectrum $P(k)$ in the running vacuum model (RVM) with the cosmological constant being a function of the Hubble parameter, given by $Λ= Λ_0 + 6 σH H_0+ 3νH^2$, in which the linear and quadratic terms of $H$ would originate from the QCD vacuum condensation and cosmological renormalization group, respectively. Taking the dark energy perturbation into consideration, we derive the evolution equation for $δ_m$ and find a specific scale $d_{cr}=2 π/k_{cr}$, which divides the evolution of the universe into the sub and super-interaction regimes, corresponding to $k \ll k_{cr}$ and $k \gg k_{cr}$, respectively. For the former, the evolution of $δ_m$ has the same behavior as that in the $Λ$CDM model, while for the latter, the growth of $δ_m$ is frozen (greatly enhanced) when $ν+ σ>(<)0$ due to the couplings between radiation, matter and dark energy. It is clear that the observational data rule out the cases with $ν<0$ and $ν+ σ<0$, while the allowed window for the model parameters is extremely narrow with $ν, |σ| \lesssim \mathcal{O}(10^{-7})$.

astro-ph.CO↗

arXiv2016

Running cosmological constant with observational tests

We investigate the running cosmological constant model with dark energy linearly proportional to the Hubble parameter, $Λ= σH + Λ_0$, in which the $Λ$CDM limit is recovered by taking $σ=0$. We derive the linear perturbation equations of gravity under the Friedmann-Lemaïtre-Robertson-Walker cosmology, and show the power spectra of the CMB temperature and matter density distribution. By using the Markov chain Monte Carlo method, we fit the model to the current observational data and find that $σH_0/ Λ_0 \lesssim 2.63 \times 10^{-2}$ and $6.74 \times 10^{-2}$ for $Λ(t)$ coupled to matter and radiation-matter, respectively, along with constraints on other cosmological parameters.

astro-ph.CO↗

arXiv2016

Note on Spin-2 Particle Interpretation of the 750~GeV Diphoton Excess

We explore the possibility to explain the 750 GeV diphoton excess recently measured by ATLAS and CMS collaborations in terms of a massive spin-2 particle. In particular, we consider the case in which the top-quark loops can give the similar amplitudes as the tree-level contributions to the diphoton and digluon channels. Such a scenario can be naturally realized in the generalized warped extra dimension models. As a result, the parameter space favored by the diphoton data implies that the top-quark loop contribution can compete to and even dominate over the tree-level amplitude in the spin-2 particle production via the gluon-gluon fusion process. Similar results are obtained for the case in which the spin-2 particle induces large invisible decay branching ratios, as indicated by the ATLAS data.

hep-ph↗

arXiv2016

Deflation of the cosmological constant associated with inflation and dark energy

In order to solve the fine-tuning problem of the cosmological constant, we propose a simple model with the vacuum energy non-minimally coupled to the inflaton field. In this model, the vacuum energy decays to the inflaton during pre-inflation and inflation eras, so that the cosmological constant effectively deflates from the Planck mass scale to a much smaller one after inflation and plays the role of dark energy in the late-time of the universe. We show that our deflationary scenario is applicable to arbitrary slow-roll inflation models. We also take two specific inflation potentials to illustrate our results.

gr-qc↗

arXiv2016

Some heavy vector and tensor meson decay constants in light-front quark model

We study the decay constants ($f_M$) of the heavy vector ($D^{*}$, $D^{*}_{s}$, $B^{*}$, $B^{*}_{s}$, $B^{*}_{c}$) and tensor ($D_{2}^{*}$, $D_{s2}^{*}$, $B^{*}_{2}$, $B^{*}_{s2}$) mesons in the light front quark model. With the known pseudoscalar meson decay constants of $f_D$, $f_{D_s}$, $f_B$, $f_{B_s}$, and $f_{B_c}$ as the input parameters to determine the light-front meson wave functions, we obtain that $f_{D^{*}, D^{*}_{s}, B^{*},B^{*}_s,B^{*}_c} = (252.0^{+13.8}_{-11.6}$, $318.3^{+15.3}_{-12.6}$ , $201.9^{+43.2}_{-41.4}$, $244.2\pm7.0$, $473.4\pm18.2$) and $(264.9^{+10.2}_{-9.5}$, $330.9^{+9.9}_{-9.0}$, $220.2^{+49.1}_{-46.2}$, $265.7\pm8.0$, $487.6\pm19.2$) MeV with Gaussian and power-law wave functions, respectively, while $f_{D_{2}^{*},D_{s2}^{*},B^{*}_{2},B^{*}_{s2}}$= ($143.6^{+24.9}_{-21.8}$, $209.5^{+29.1}_{-24.2}$, $80.9^{+33.8}_{-27.7}$, $109.7^{+15.7}_{-15.0}$) MeV with only Gaussian wave functions.

hep-ph↗

arXiv2016

Direct Detection of Exothermic Dark Matter with Light Mediator

We study the dark matter (DM) direct detection for the models with the effects of the isospin-violating couplings, exothermic scatterings, and/or the lightness of the mediator, proposed to relax the tension between the CDMS-Si signals and null experiments. In the light of the new updates of the LUX and CDMSlite data, we find that many of the previous proposals are now ruled out, including the Ge-phobic exothermic DM model and the Xe-phobic DM one with a light mediator. We also examine the exothermic DM models with a light mediator but without the isospin violation, and we are unable to identify any available parameter space that could simultaneously satisfy all the experiments. The only models that can partially relax the inconsistencies are the Xe-phobic exothermic DM models with or without a light mediator. But even in this case, a large portion of the CDMS-Si regions of interest has been constrained by the LUX and SuperCDMS data.

hep-ph↗