arXiv subjects

Xin Zhang

Publications and source records attributed to Xin Zhang.

At least 1,099 records · Page 61Linked to original sources

arXiv2017

Intrapixel effects of CCD and CMOS detectors

Intrapixel nonuniformity is known to exist in CCD and CMOS image sensors, though the effects in backside illuminated (BSI) CCDs are too small to be a concern for most astronomical observations. However, projects like the Large Synoptic Survey Telescope require precise knowledge of the detector characteristics, and intrapixel effects may need more attention. By scanning CCD and CMOS cameras with a small light spot (unresolved by the optics), we find in the images that the spot's flux, centroid displacement, and ellipticity vary periodically on the pixel scale in most cases. The amplitude of variation depends on not only the detector but also how well the spot is sampled by the pixels. With a spot radius of 2 pixels (encircling 80% energy) as measured, the flux and the ellipticity extracted from the BSI CCD camera vary by 0.2-0.3% (rms) and 0.005 (rms), respectively, while the deviation of the centroid position (rms ~ 0.01 pixel) is not correlated with the pixels. The effects are more pronounced for the BSI CMOS camera and even worse for the frontside illuminated CMOS camera. The results suggest that a closer examination of the intrapixel effects is needed for precision astronomy.

astro-ph.IM↗

arXiv2017

A search for sterile neutrinos with the latest cosmological observations

We report the result of a search for sterile neutrinos with the latest cosmological observations. Both cases of massless and massive sterile neutrinos are considered in the $Λ$CDM cosmology. The cosmological observations used in this work include the Planck 2015 temperature and polarization data, the baryon acoustic oscillation data, the Hubble constant direct measurement data, the Planck Sunyaev-Zeldovich cluster counts data, the Planck lensing data, and the cosmic shear data. We find that the current observational data give a hint of the existence of massless sterile neutrino (as dark radiation) at the 1.44$σ$ level, and the consideration of an extra massless sterile neutrino can indeed relieve the tension between observations and improve the cosmological fit. For the case of massive sterile neutrino, the observations give a rather tight upper limit on the mass, which implies that actually a massless sterile neutrino is more favored. Our result is consistent with the recent result of neutrino oscillation experiment done by the Daya Bay and MINOS collaborations, as well as the recent result of cosmic ray experiment done by the IceCube collaboration.

astro-ph.CO↗

arXiv2017

Electronic structure and direct observation of ferrimagnetism in multiferroic hexagonal YbFeO3

The magnetic interaction between rare-earth and Fe ions in hexagonal rare-earth ferrites (h-REFeO3), may amplify the weak ferromagnetic moment on Fe, making these materials more appealing as multiferroics. To elucidate the interaction strength between the rare-earth and Fe ions as well as the magnetic moment of the rare-earth ions, element specific magnetic characterization is needed. Using X-ray magnetic circular dichroism, we have studied the ferrimagnetism in h-YbFeO3 by measuring the magnetization of Fe and Yb separately. The results directly show anti-alignment of magnetization of Yb and Fe ions in h-YbFeO3 at low temperature, with an exchange field on Yb of about 17 kOe. The magnetic moment of Yb is about 1.6 \muB at low-temperature, significantly reduced compared with the 4.5 \muB moment of a free Yb3+. In addition, the saturation magnetization of Fe in h-YbFeO3 has a sizable enhancement compared with that in h-LuFeO3. These findings directly demonstrate that ferrimagnetic order exists in h-YbFeO3; they also account for the enhancement of magnetization and the reduction of coercivity in h-YbFeO3 compared with those in h-LuFeO3 at low temperature, suggesting an important role for the rare-earth ions in tuning the multiferroic properties of h-REFeO3.

cond-mat.mtrl-sci↗

arXiv2017

Evaluating the System Compliance in Testing Carbon Nanotube Fibers

System compliances of two testing machines (Keysight T150 and Instron 3365) were studied for the tensile testing of carbon nanotube (CNT) fibers. Due to the different gripping methods, the T150's system compliance was nearly zero, while that of the Instron was nonnegligible. By analyzing the data dispersion with the Weibull theory, a safe gauge length of >10 mm was suggested for CNT fibers. Therefore, a standard test procedure should involve the compliance evaluation, choice of gauge length, and Weibull statistical analysis.

cond-mat.mtrl-sci↗

arXiv2017

Spatial Statistics of Apollonian Gaskets

Apollonian gaskets are formed by repeatedly filling the interstices between four mutually tangent circles with further tangent circles. We experimentally study the pair correlation, electrostatic energy, and nearest neighbor spacing of centers of circles from Apollonian gaskets. Even though the centers of these circles are not uniformly distributed in any `ambient' space, after proper normalization, all these statistics seem to exhibit some interesting limiting behaviors.

math.MG↗

arXiv2017

Equivalence Between the Gauge $n\cdot\partial n\cdot A=0$ and the Axial Gauge

Discontinuity of gauge theory in the gauge condition $n\cdot\partial n\cdot A=0$, which emerges at $n\cdot k=0$, is studied here. Such discontinuity is different from that one confronts in axial gauge and can not be regularized by conventional analytical continuation method. The Faddeev-Popov determinate of the gauge $n\cdot\partial n\cdot A=0$, which is solved explicitly in the manuscript, behaves like a $δ$-functional of gauge potentials once singularities in the functional integral is neglected and the length along $n^μ$ direction of the space tends to infinity. As a sequence, perturbation series in the gauge $n\cdot\partial n\cdot A=0$ returns to that in axial gauge for short-range correlated objects that are free from singularities in path integral. However, the equivalence between the gauge $n\cdot\partial n\cdot A=0$ and axil gauge is nontrivial for long-range correlated objects and quantities that are affected by singularities in path integral. Continuity of gauge links one encounter in perturbation theory and lattice calculation is affected by such discontinuity.

hep-th↗

arXiv2017

No evidence for the evolution of mass density power-law index $γ$ from strong gravitational lensing observation

In this paper, we consider the singular isothermal sphere lensing model that has a spherically symmetric power-law mass distribution $ρ_{tot}(r)\sim r^{-γ}$. We investigate whether the mass density power-law index $γ$ is cosmologically evolutionary by using the strong gravitational lensing (SGL) observation, in combination with other cosmological observations. We also check whether the constraint result of $γ$ is affected by the cosmological model, by considering several simple dynamical dark energy models. We find that the constraint on $γ$ is mainly decided by the SGL observation and independent of the cosmological model, and we find no evidence for the evolution of $γ$ from the SGL observation.

astro-ph.CO↗

arXiv2017

Constraining neutrino mass and extra relativistic degrees of freedom in dynamical dark energy models using Planck 2015 data in combination with low-redshift cosmological probes: basic extensions to $Λ$CDM cosmology

We investigate how the properties of dark energy affect the cosmological measurements of neutrino mass and extra relativistic degrees of freedom. We limit ourselves to the most basic extensions of $Λ$ cold dark matter (CDM) model, i.e. the $w$CDM model with one additional parameter $w$, and the $w_{0}w_{a}$CDM model with two additional parameters, $w_{0}$ and $w_{a}$. In the cosmological fits, we employ the 2015 cosmic microwave background temperature and polarization data from the Planck mission, in combination with low-redshift measurements such as the baryon acoustic oscillations, Type Ia supernovae and the Hubble constant ($H_{0}$). Given effects of massive neutrinos on large-scale structure, we further include weak lensing, redshift space distortion, Sunyaev--Zeldovich cluster counts and Planck lensing data. We show that, though the cosmological constant $Λ$ is still consistent with the current data, a phantom dark energy ($w<-1$) or an early phantom dark energy (i.e. quintom evolving from $w<-1$ to $w>-1$) is slightly more favoured by current observations, which leads to the fact that in both $w$CDM and $w_0w_a$CDM models we obtain a larger upper limit of $\sum m_ν$. We also show that in the three dark energy models, the constraints on $N_{\rm eff}$ are in good accordance with each other, all in favour of the standard value 3.046, which indicates that the dark energy parameters almost have no impact on constraining $N_{\rm eff}$. Therefore, we conclude that the dark energy parameters can exert a significant influence on the cosmological weighing of neutrinos, but almost cannot affect the constraint on dark radiation.

astro-ph.CO↗

arXiv2017

Impact of the latest measurement of Hubble constant on constraining inflation models

We investigate how the constraint results of inflation models are affected by considering the latest local measurement of $H_0$ in the global fit. We use the observational data, including the Planck CMB full data, the BICEP2 and Keck Array CMB B-mode data, the BAO data, and the latest measurement of Hubble constant, to constrain the $Λ$CDM+$r$+$N_{\rm eff}$ model, and the obtained 1$σ$ and 2$σ$ contours of $(n_s, r)$ are compared to the theoretical predictions of selected inflationary models. We find that, in this fit, the scale invariance is only excluded at the 3.3$σ$ level, and $ΔN_{\rm eff}>0$ is favored at the 1.6$σ$ level. The natural inflation model is now excluded at more than 2$σ$ level; the Starobinsky $R^2$ model becomes only favored at around 2$σ$ level; the most favored model becomes the spontaneously broken SUSY inflation model; and, the brane inflation model is also well consistent with the current data, in this case.

astro-ph.CO↗

arXiv2017

Weighing neutrinos in dynamical dark energy models

We briefly review the recent results of constraining neutrino mass in dynamical dark energy models using cosmological observations and summarize the findings. (i) In dynamical dark energy models, compared to $Λ$CDM, the upper limit of $\sum m_ν$ can become larger and can also become smaller. In the cases of phantom and early phantom (i.e., the quintom evolving from $w<-1$ to $w>-1$), the constraint on $\sum m_ν$ becomes looser; but in the cases of quintessence and early quintessence (i.e., the quintom evolving from $w>-1$ to $w<-1$), the constraint on $\sum m_ν$ becomes tighter. (ii) In the holographic dark energy (HDE) model, the tightest constraint on $\sum m_ν$, i.e., $\sum m_ν<0.105$ eV, is obtained, which is almost equal to the lower limit of $\sum m_ν$ of IH case. Thus, it is of great interest to find that the future neutrino oscillation experiments would potentially offer a possible falsifying scheme for the HDE model. (iii) The mass splitting of neutrinos can influence the cosmological fits. We find that the NH case fits the current observations slightly better than the IH case, although the difference of $χ^2$ of the two cases is still not significant enough to definitely distinguish the neutrino mass hierarchy.

astro-ph.CO↗

arXiv2017

Exact solution of the relativistic quantum Toda chain

The relativistic quantum Toda chain model is studied with the generalized algebraic Bethe Ansatz method. By employing a set of local gauge transformations, proper local vacuum states can be obtained for this model. The exact spectrum and eigenstates of the model are thus constructed simultaneously.

math-ph↗

arXiv2017

Weighing neutrinos in the scenario of vacuum energy interacting with cold dark matter: application of the parameterized post-Friedmann approach

We constrain the neutrino mass in the scenario of vacuum energy interacting with cold dark matter by using current cosmological observations. To avoid the large-scale instability problem in interacting dark energy models, we employ the parameterized post-Friedmann (PPF) approach to do the calculation of perturbation evolution, for the $Q=βHρ_{\rm c}$ and $Q=βHρ_Λ$ models. The current observational data sets used in this work include Planck (cosmic microwave background), BSH (baryon acoustic oscillations, type Ia supernovae, and Hubble constant), and LSS (redshift space distortions and weak lensing). According to the constraint results, we find that $β>0$ at more than $1σ$ level for the $Q=βHρ_{\rm c}$ model, which indicates that cold dark matter decays into vacuum energy; while $β=0$ is consistent with the current data at $1σ$ level for the $Q=βHρ_Λ$ model. Taking the $Λ$CDM model as a baseline model, we find that a smaller upper limit, $\sum m_ν<0.11$ eV ($2σ$), is induced by the latest BAO BOSS DR12 data and the Hubble constant measurement $H_{0} = 73.00 \pm 1.75$ km~s$^{-1}$~Mpc$^{-1}$. For the $Q=βHρ_{\rm c}$ model, we obtain $\sum m_ν<0.20$ eV ($2σ$) from Planck+BSH. For the $Q=βHρ_Λ$ model, $\sum m_ν<0.10$ eV ($2σ$) and $\sum m_ν<0.14$ eV ($2σ$) are derived from Planck+BSH and Planck+BSH+LSS, respectively. We show that these smaller upper limits on $\sum m_ν$ are affected more or less by the tension between $H_{0}$ and other observational data.

astro-ph.CO↗

arXiv2017

Max-Margin based Discriminative Feature Learning

In this paper, we propose a new max-margin based discriminative feature learning method. Specifically, we aim at learning a low-dimensional feature representation, so as to maximize the global margin of the data and make the samples from the same class as close as possible. In order to enhance the robustness to noise, a $l_{2,1}$ norm constraint is introduced to make the transformation matrix in group sparsity. In addition, for multi-class classification tasks, we further intend to learn and leverage the correlation relationships among multiple class tasks for assisting in learning discriminative features. The experimental results demonstrate the power of the proposed method against the related state-of-the-art methods.

cs.LG↗

arXiv2017

Probing the interaction between dark energy and dark matter with the parametrized post-Friedmann approach

There possibly exists some direct, non-gravitational coupling between dark energy and dark matter. This possibility should be seriously tested by using observations, which requires us to understand such a scenario from the aspects of both expansion history and growth of structure. It is found that once calculating the perturbations in the interacting dark energy (IDE) scenario, for most cases the curvature perturbation on superhorizon scales is divergent, which is a catastrophe for the IDE cosmology. We found a solution to this issue, which is to establish an effective theory to treat the dark energy perturbations totally based on the basic facts of dark energy. This scheme generalizes the parametrized post-Friedmann framework of uncoupled dark energy and can be used to cure the instability of the IDE cosmology. The whole parameter space of IDE models can henceforward be explored by observational data. The IDE scenario can thus be tested or falsified with current and future observational data by using the PPF approach. We expect that the future highly accurate observational data would offer the certain answer to the question whether there is a direct coupling between dark energy and dark matter.

astro-ph.CO↗

arXiv2017

$Λ_b\toΛ_cτ\barν_τ$ decay in scalar and vector leptoquark scenarios

It has been shown that the anomalies observed in $\bar B\to D^{(\ast)}τ\barν_τ$ and $\bar B\to \bar K\ell^+\ell^-$ decays can be resolved by adding a single scalar or vector leptoquark to the Standard Model, while constraints from other precision measurements in the flavour sector can be satisfied without fine-tuning. To further explore these two interesting scenarios, in this paper, we study their effects in the semi-leptonic $Λ_b\toΛ_cτ\barν_τ$ decay. Using the best-fit solutions for the operator coefficients allowed by the current data of mesonic decays, we find that (i) the two scenarios give similar amounts of enhancements to the branching fraction $\mathcal B(Λ_b\toΛ_cτ\barν_τ)$ and the ratio $R_{Λ_c}=\mathcal B(Λ_b\toΛ_c τ\barν_τ)/\mathcal B(Λ_b\toΛ_c\ell\barν_\ell)$, (ii) the two best-fit solutions in each of these two scenarios are also indistinguishable from each other, (iii) both scenarios give nearly the same predictions as those of the Standard Model for the longitudinal polarizations of $Λ_c$ and $τ$ as well as the lepton-side forward-backward asymmetry. With future measurements of these observables in $Λ_b\toΛ_cτ\barν_τ$ decay at the LHCb, the two leptoquark scenarios could be further tested, and even differentiated from the other NP explanations for the $R_{D^{(\ast)}}$ anomalies. We also discuss the feasibility for the measurements of these observables at the LHC and the future $e^+e^-$ colliders.

hep-ph↗

arXiv2017

Extreme-Strike Asymptotics for General Gaussian Stochastic Volatility Models

We consider a stochastic volatility asset price model in which the volatility is the absolute value of a continuous Gaussian process with arbitrary prescribed mean and covariance. By exhibiting a Karhunen-Loève expansion for the integrated variance, and using sharp estimates of the density of a general second-chaos variable, we derive asymptotics for the asset price density for large or small values of the variable, and study the wing behavior of the implied volatility in these models. Our main result provides explicit expressions for the first five terms in the expansion of the implied volatility. The expressions for the leading three terms are simple, and based on three basic spectral-type statistics of the Gaussian process: the top eigenvalue of its covariance operator, the multiplicity of this eigenvalue, and the $L^{2}$ norm of the projection of the mean function on the top eigenspace. The fourth term requires knowledge of all eigen-elements. We present detailed numerics based on realistic liquidity assumptions in which classical and long-memory volatility models are calibrated based on our expansion.

q-fin.MF↗

arXiv2017

The Continuity of the Gauge Fixing Condition $n\cdot\partial n\cdot A=0$ for $SU(2)$ Gauge Theory

The continuity of the gauge fixing condition $n\cdot\partial n\cdot A=0$ for $SU(2)$ gauge theory on the manifold $R\bigotimes S^{1}\bigotimes S^{1}\bigotimes S^{1}$ is studied here, where $n^μ$ stands for directional vector along $x_{i}$-axis($i=1,2,3$). It is proved that the gauge fixing condition is continuous given that gauge potentials are differentiable with continuous derivatives on the manifold $R\bigotimes S^{1}\bigotimes S^{1}\bigotimes S^{1}$ which is compact.

physics.gen-ph↗

arXiv2017

Redshift drift constraints on holographic dark energy

The Sandage-Loeb (SL) test is a promising method for probing dark energy because it measures the redshift drift in the spectra of Lyman-$α$ forest of distant quasars, covering the "redshift desert" of $2\lesssim z\lesssim5$, which is not covered by existing cosmological observations. Therefore, it could provide an important supplement to current cosmological observations. In this paper, we explore the impact of SL test on the precision of cosmological constraints for two typical holographic dark energy models, i.e., the original holographic dark energy (HDE) model and the Ricci holographic dark energy (RDE) model. To avoid data inconsistency, we use the best-fit models based on current combined observational data as the fiducial models to simulate 30 mock SL test data. The results show that SL test can effectively break the existing strong degeneracy between the present-day matter density $Ω_{m0}$ and the Hubble constant $H_0$ in other cosmological observations. For the considered two typical dark energy models, not only can a 30-year observation of SL test improve the constraint precision of $Ω_{m0}$ and $h$ dramatically, but can also enhance the constraint precision of the model parameters $c$ and $α$ significantly.

astro-ph.CO↗