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W. Zhao

Publications and source records attributed to W. Zhao.

At least 73 records · Page 4Linked to original sources
arXiv2014

Limits on light WIMPs with a germanium detector at 177 eVee threshold at the China Jinping Underground Laboratory

The China Dark Matter Experiment reports results on light WIMP dark matter searches at the China Jinping Underground Laboratory with a germanium detector array with a total mass of 20 g. The physics threshold achieved is 177 eVee ("ee" represents electron equivalent energy) at 50% signal efficiency. With 0.784 kg-days of data, exclusion region on spin-independent coupling with the nucleon is derived, improving over our earlier bounds at WIMP mass less than 4.6 GeV.

hep-ex↗
arXiv2013

First results on low-mass WIMP from the CDEX-1 experiment at the China Jinping underground Laboratory

The China Dark matter Experiment collaboration reports the first experimental limit on WIMP dark matter from 14.6 kg-day of data taken with a 994 g p-type point-contact germanium detector at the China Jinping underground Laboratory where the rock overburden is more than 2400 m. The energy threshold achieved was 400 eVee. According to the 14.6 kg-day live data, we placed the limit of N= 1.75 * 10^{-40} cm^{2} at 90% confidence level on the spin-independent cross-section at WIMP mass of 7 GeV before differentiating bulk signals from the surface backgrounds.

hep-ex↗
arXiv2013

Anisotropy of Cosmic Acceleration

In this paper, we study the anisotropy of cosmic acceleration by dividing the Union2 Type Ia supernova dataset into 12 subsets according to their positions in Galactic coordinate system. In each region, we derive the deceleration parameter $q_0$ as the diagnostic to quantify the anisotropy level in the corresponding direction, and construct $q_0$ anisotropic maps by combining these $q_0$ values. In addition to the monopole component, we find the significant dipole effect in the $q_0$-maps with the amplitude $A_1=0.466^{+0.255}_{-0.205}$, which deviates from zero at more than 2-$σ$ level. The direction of the best-fit dipole is ($θ=108.8^{\circ}$, $ϕ=187.0^{\circ}$) in Galactic system. Interesting enough, we find the direction of this dipole is nearly perpendicular to the CMB kinematic dipole, and the angle between them is $95.7^{\circ}$. The perpendicular relation is anomalous at the 1-in-10 level.

astro-ph.CO↗
arXiv2013

Gravitational-Wave by Binary OJ287 in 3.5 PN Approximation

We compute the gravitational radiation of the supermassive binary black hole OJ287. By fitting the data of its recent seven outbursts, we obtain the orbital motion up to 3.5PN (Post-Newtonian) order, the energy and angular momentum fluxes and the waveform up to the 2PN. It is found that the 1PN term of the energy flux, has an opposite sign to the Newtonian flux, and the 2PN term has an opposite sign to the 1PN one. The same pattern is also found for the angular momentum fluxes. The total flux is reduced by 30.8% on average from the Newtonian flux for OJ287, resulting in a smaller orbital decay rate than that of 2.5PN calculation. Consisting with this, it is checked that, in the sequence of non-dissipative PN forces, each term of $i^{th}$ order (dissipative or non-dissipative) has a sign opposite to the $(i-1)^{th}$ order. The origin of this characteristic is traced to the appearance of the non-diagonal metric component $g_{0i}$ in PN approximation. This feature will have profound impact on estimation of gravitational waves from binary systems.

gr-qc↗
arXiv2013

Pavlov's dog associative learning demonstrated on synaptic-like organic transistors

In this letter, we present an original demonstration of an associative learning neural network inspired by the famous Pavlov's dogs experiment. A single nanoparticle organic memory field effect transistor (NOMFET) is used to implement each synapse. We show how the physical properties of this dynamic memristive device can be used to perform low power write operations for the learning and implement short-term association using temporal coding and spike timing dependent plasticity based learning. An electronic circuit was built to validate the proposed learning scheme with packaged devices, with good reproducibility despite the complex synaptic-like dynamic of the NOMFET in pulse regime.

q-bio.NC↗
arXiv2012

Growth and Electronic Structure of Boron-Doped Graphene

The doping of graphene to tune its electronic structure is essential for its further use in carbon based electronics. Adapting strategies from classical silicon based semiconductor technology, we use the incorporation of heteroatoms in the 2D graphene network as a straightforward way to achieve this goal. Here, we report on the synthesis of boron-doped graphene on Ni(111) in a chemical vapor deposition process of triethylborane on the one hand and by segregation of boron from the bulk on the other hand. The chemical environment of boron was determined by x-ray photoelectron spectroscopy and angle resolved photoelectron spectroscopy was used to analyze the impact on the band structure. Doping with boron leads to a shift of the graphene bands to lower binding energies. The shift depends on the doping concentration and for a doping level of 0.3 ML a shift of up to 1.2 eV is observed. The experimental results are in agreement with density-functional calculations. Furthermore, our calculations suggest that doping with boron leads to graphene preferentially adsorbed in the top-fcc geometry, since the boron atoms in the graphene lattice are then adsorbed at substrate fcc-hollow sites. The smaller adsorption distance of boron compared to carbon leads to a bending of the graphene sheet in the vicinity of the boron atoms. By comparing calculations of doped and undoped graphene on Ni(111), as well as the respective free-standing cases, we are able to distinguish between the effects that doping and adsorption have on the band structure of graphene. Both, doping and bonding to the surface, result in opposing shifts on the graphene bands.

cond-mat.mtrl-sci↗
arXiv2012

Can residuals of the Solar system foreground explain low multipole anomalies of the CMB ?

The low multipole anomalies of the Cosmic Microwave Background has received much attention during the last few years. It is still not ascertained whether these anomalies are indeed primordial or the result of systematics or foregrounds. An example of a foreground, which could generate some non-Gaussian and statistically anisotropic features at low multipole range, is the very symmetric Kuiper Belt in the outer solar system. In this paper, expanding upon the methods presented by Maris et al. (2011), we investigate the contributions from the Kuiper Belt objects (KBO) to the WMAP ILC 7 map, whereby we can minimize the contrast in power between even and odd multipoles in the CMB, discussed discussed by Kim & Naselsky (2010). We submit our KBO de-correlated CMB signal to several tests, to analyze its validity, and find that incorporation of the KBO emission can decrease the quadrupole-octupole alignment and parity asymmetry problems, provided that the KBO signals has a non-cosmological dipole modulation, associated with the statistical anisotropy of the ILC 7 map. Additionally, we show that the amplitude of the dipole modulation, within a 2 sigma interval, is in agreement with the corresponding amplitudes, discussed by Lew (2008).

astro-ph.CO↗
arXiv2012

Faraday Rotation as a diagnostic of Galactic foreground contamination of CMB maps

The contribution from the residuals of the foreground can have a significant impact on the temperature maps of the Cosmic Microwave Background (CMB). Mostly, the focus has been on the galactic plane, when foreground cleaning has taken place. However, in this paper, we will investigate the possible foreground contamination, from sources outside the galactic plane in the CMB maps. We will analyze the correlation between the Faraday rotation map and the CMB temperature map. The Faraday rotation map is dependent on the galactic magnetic field, as well as the thermal electron density, and both may contribute to the CMB temperature. We find that the standard deviation for the mean cross correlation deviate from that of simulations at the 99.9% level. Additionally, a comparison between the CMB temperature extrema and the extremum points of the Faraday rotation is also performed, showing a general overlap between the two. Also we find that the CMB Cold Spot is located at an area of strong negative cross correlation, meaning that it may be explained by a galactic origin. Further, we investigate nearby supernova remnants in the galaxy, traced by the galactic radio loops. These super nova remnants are located at high and low galactic latitude, and thus well outside the galactic plane. We find some correlation between the Faraday Rotation and the CMB temperature, at select radio loops. This indicate, that the galactic foregrounds may affect the CMB, at high galactic latitudes

astro-ph.CO↗
arXiv2012

Is the CMB asymmetry due to the kinematic dipole?

Parity violation found in the Cosmic Microwave Background (CMB) radiation is a crucial clue for the non-standard cosmological model or the possible contamination of various foreground residuals and/or calibration of the CMB data sets. In this paper, we study the directional properties of the CMB parity asymmetry by excluding the $m=0$ modes in the definition of parity parameters. We find that the preferred directions of the parity parameters coincide with the CMB kinematic dipole, which implies that the CMB parity asymmetry may be connected with the possible contamination of the residual dipole component. We also find that such tendency is not only localized at $l=2,3$, but in the extended multipole ranges up to $l\sim 22$.

astro-ph.CO↗
arXiv2011

Testing inflationary consistency relations by the potential CMB observations

Testing the so-called consistency relations plays an important role for distinguishing the different classes of inflation models. In this paper, we investigate the possible testing based on the potential observations of the cosmic microwave background (CMB) radiation, including the planned CMBPol mission and the ideal CMB experiment where only the reduced cosmic weak lensing contamination for the B-mode polarization is considered. We find that for the canonical single-field inflation and the phantom inflation, the consistency relations are quite hard to be tested: the testing is possible only if $r>0.14$ for CMBPol mission, and $r>0.06$ for the ideal experiment. However, the situation could become much better for the general Lorentz invariant single-field inflation with large non-gaussian signal and the two-field inflation with strong correlation between the adiabatic and the isocurvature perturbations. We find that for these two classes of inflation the testing is possible if $r\gtrsim 10^{-2}$ or even smaller for both CMB experiments.

astro-ph.CO↗
arXiv2011

Radio structure of the blazar 1156+295 with sub-pc resolution

1156+295 is a flat-spectrum quasar which is loud at radio and gamma-ray. Previous observations of the source revealed a radio morphology on pc to kpc scales consistent with a helical jet model. In our present research, this source was observed with the VLBA at 86, 43 and 15 GHz on four epochs from 10 May 2003 to 13 March 2005 aiming at studying the structure of the innermost jet in order to understand the relation between the helical structure and the astrophysical processes in the central engine. A core-jet structure with six jet components is identified. The apparent transverse velocities of the six jet components derived from proper motion measurements are in the range between 3.6 c and 11.6 c. The overall jet shape shows oscillatory morphology with multiple curvatures on pc scales which might be indicative of a helical pattern. Models of helical jet are discussed on the basis of both Kelvin-Helmholtz (K-H) instability and jet precession. The K-H instability model shows better agreement with the observed data. The overall radio structure on the scale from sub-pc to kpc appears to be fitted with a hydrodynamic model with the fundamental helical mode in Kelvin-Helmholtz (K-H) instability. This helical mode with an initial characteristic wavelength of 0.2 pc is excited at the base of the jet on the scale of 0.005 pc (or 1000R_s, the typical size of the broad line region for a super massive black hole of $4.3\times10^8M_{\odot}$). A presessing jet model can also fit the observed jet structure on the scale between 10 pc and 300 pc. However, additional astrophysical processes may be required for the presessing jet model in order to explain the bendings on the inner jet structure (1 to 10 pc) and re-collimation of the large scale jet outflow (>300 pc).

astro-ph.CO↗
arXiv2011

Determination of Dark Energy by the Einstein Telescope: Comparing with CMB, BAO and SNIa Observations

A design study is currently in progress for a third generation gravitational-wave (GW) detector called Einstein Telescope (ET). An important kind of source for ET will be the inspiral and merger of binary neutron stars (BNS) up to $z \sim 2$. If BNS mergers are the progenitors of short-hard $γ$-ray bursts, then some fraction of them will be seen both electromagnetically and through GW, so that the luminosity distance and the redshift of the source can be determined separately. An important property of these `standard sirens' is that they are \emph{self-calibrating}: the luminosity distance can be inferred directly from the GW signal, with no need for a cosmic distance ladder. Thus, standard sirens will provide a powerful independent check of the $Λ$CDM model. In previous work, estimates were made of how well ET would be able to measure a subset of the cosmological parameters (such as the dark energy parameter $w_0$) it will have access to, assuming that the others had been determined to great accuracy by alternative means. Here we perform a more careful analysis by explicitly using the potential Planck CMB data as prior information for these other parameters. We find that ET will be able to constrain $w_0$ and $w_a$ with accuracies $Δw_0 = 0.099$ and $Δw_a = 0.302$, respectively. These results are compared with projected accuracies for the JDEM Baryon Acoustic Oscillations project and the SNAP Type Ia supernovae observations.

astro-ph.CO↗
arXiv2010

Relic gravitational waves in the light of 7-year Wilkinson Microwave Anisotropy Probe data and improved prospects for the Planck mission

The new release of data from Wilkinson Microwave Anisotropy Probe improves the observational status of relic gravitational waves. The 7-year results enhance the indications of relic gravitational waves in the existing data and change to the better the prospects of confident detection of relic gravitational waves by the currently operating Planck satellite. We apply to WMAP7 data the same methods of analysis that we used earlier [W. Zhao, D. Baskaran, and L.P. Grishchuk, Phys. Rev. D 80, 083005 (2009)] with WMAP5 data. We also revised by the same methods our previous analysis of WMAP3 data. It follows from the examination of consecutive WMAP data releases that the maximum likelihood value of the quadrupole ratio $R$, which characterizes the amount of relic gravitational waves, increases up to $R=0.264$, and the interval separating this value from the point $R=0$ (the hypothesis of no gravitational waves) increases up to a $2σ$ level. The primordial spectra of density perturbations and gravitational waves remain blue in the relevant interval of wavelengths, but the spectral indices increase up to $n_s =1.111$ and $n_t=0.111$. Assuming that the maximum likelihood estimates of the perturbation parameters that we found from WMAP7 data are the true values of the parameters, we find that the signal-to-noise ratio $S/N$ for the detection of relic gravitational waves by the Planck experiment increases up to $S/N=4.04$, even under pessimistic assumptions with regard to residual foreground contamination and instrumental noises. We comment on theoretical frameworks that, in the case of success, will be accepted or decisively rejected by the Planck observations.

gr-qc↗
arXiv2010

Search for neutral Higgs bosons decaying into four taus at LEP2

A search for the production and non-standard decay of a Higgs boson, h, into four taus through intermediate pseudoscalars, a, is conducted on 683 pb-1 of data collected by the ALEPH experiment at centre-of-mass energies from 183 to 209 GeV. No excess of events above background is observed, and exclusion limits are placed on the combined production cross section times branching ratio, ξ^2 = σ(e+e- --> Zh)/σ_{SM}(e+e- --> Zh) x B(h --> aa)x B(a --> τ^+τ^-)^2. For mh < 107 GeV/c2 and 4 < ma < 10 GeV/c2, ξ^2 > 1 is excluded at the 95% confidence level.

hep-ex↗
arXiv2010

Primordial Gravitational Waves and Cosmic Microwave Background Radiation

This is a summary of presentations delivered at the OC1 parallel session "Primordial Gravitational Waves and the CMB" of the 12th Marcel Grossmann meeting in Paris, July 2009. The reports and discussions demonstrated significant progress that was achieved in theory and observations. It appears that the existing data provide some indications of the presence of gravitational wave contribution to the CMB anisotropies, while ongoing and planned observational efforts are likely to convert these indications into more confident statements about the actual detection.

gr-qc↗
arXiv2009

Stable indications of relic gravitational waves in Wilkinson Microwave Anisotropy Probe data and forecasts for the Planck mission

The relic gravitational waves (gw) are the cleanest probe of the violent times in the very early history of the Universe. They are expected to leave signatures in the observed cosmic microwave background anisotropies. We significantly improved our previous analysis [1] of the 5-year WMAP $TT$ and $TE$ data at lower multipoles $\ell$. This more general analysis returned essentially the same maximum likelihood (ML) result (unfortunately, surrounded by large remaining uncertainties): the relic gw are present and they are responsible for approximately 20% of the temperature quadrupole. We identify and discuss the reasons by which the contribution of gw can be overlooked in a data analysis. One of the reasons is a misleading reliance on data from very high multipoles $\ell$, another - a too narrow understanding of the problem as the search for $B$-modes of polarization, rather than the detection of relic gw with the help of all correlation functions. Our analysis of WMAP5 data has led to the identification of a whole family of models characterized by relatively high values of the likelihood function. Using the Fisher matrix formalism we formulated forecasts for {\it Planck} mission in the context of this family of models. We explore in details various `optimistic', `pessimistic' and `dream case' scenarios. We show that in some circumstances the $B$-mode detection may be very inconclusive, at the level of signal-to-noise ratio $S/N =1.75$, whereas a smarter data analysis can reveal the same gw signal at $S/N= 6.48$. The final result is encouraging. Even under unfavourable conditions in terms of instrumental noises and foregrounds, the relic gw, if they are characterized by the ML parameters that we found from WMAP5 data, will be detected by {\it Planck} at the level $S/N = 3.65$.

gr-qc↗
arXiv2009

Kinematics of the parsec-scale radio jet in 3C48

We present results on the compact steep-spectrum quasar 3C 48 from observations with the VLBA, MERLIN and EVN at multiple radio frequencies. In the 1.5-GHz VLBI images, the radio jet is characterized by a series of bright knots. The active nucleus is embedded in the southernmost VLBI component A, which is further resolved into two sub-components A1 and A2 at 4.8 and 8.3 GHz. A1 shows a flat spectrum and A2 shows a steep spectrum. The most strongly polarized VLBI components are located at component C $\sim$0.25 arcsec north of the core. The polarization angles at C show gradual changes across the jet width at all observed frequencies, indicative of a gradient in the emission-weighted intrinsic polarization angle across the jet and possibly a systematic gradient in the rotation measure; moreover, the percentage of polarization increases near the curvature at C, likely consistent with the presence of a local jet-ISM interaction and/or changing magnetic-field directions. The hot spot B shows a higher rotation measure, and has no detected proper motion. These facts provide some evidence for a stationary shock in the vicinity of B. Comparison of the present VLBI observations with those made 8.43 years ago suggests a proper motion of $β_{app}=3.7\pm0.4 c$ for A2 to the north. The apparent superluminal motion suggests that the relativistic jet plasma moves at a velocity of $\gtrsim0.96 c$ if the jet is viewed at an inclination angle less than $20\degr$. A simple precessing jet model and a hydrodynamical isothermal jet model with helical-mode Kelvin-Helmholtz instabilities are used to fit the oscillatory jet trajectory of 3C 48 defined by the bright knots.

astro-ph.CO↗