arXiv subjects

X. Chen

Publications and source records attributed to X. Chen.

At least 469 records · Page 26Linked to original sources

arXiv2014

Measurement of the branching fraction for $ψ(3686)\toωK^+ K^-$

With $1.06\times 10^8$ $ψ(3686)$ events collected with the BESIII detector, the branching fraction of $ψ(3686) \to ωK^+ K^-$ is measured to be $(1.54 \pm 0.04 \pm 0.11) \times 10^{-4}$. This is the most precise result to date, due to the largest $ψ(3686)$ sample, improved signal reconstruction efficiency, good simulation of the detector performance, and a more accurate knowledge of the continuum contribution. Using the branching fraction of $J/ψ\to ωK^+ K^-$, the ratio $\mathcal{B}(ψ(3868) \to K^+K^-) / \mathcal{B}(J/ψ\to K^+K^-)$ is determined to be $(18.4 \pm 3.7)\,\%$. This constitutes a significantly improved test of the $12\,\%$ rule, with the uncertainty now dominated by the $J/ψ$ branching fraction.

hep-ex↗

arXiv2014

Observation of Electromagnetic Dalitz decays J/ψ\to P e^+e^-

Based on a sample of (225.3\pm2.8)\times 10^{6} J/ψevents collected with the BESIII detector, the electromagnetic Dalitz decays of J/ψ\to P e^+e^-(P=η'/η/π^0) are studied. By reconstructing the pseudoscalar mesons in various decay modes, the decays J/ψ\to η' e^+e^-, J/ψ\to ηe^+e^- and J/ψ\to π^0 e^+e^- are observed for the first time. The branching fractions are determined to be \mathcal{B}(J/ψ\to η' e^+e^-) = (5.81\pm0.16\pm0.31)\times10^{-5}, \mathcal{B}(J/ψ\to ηe^+e^-) = (1.16\pm0.07\pm0.06)\times10^{-5}, and \mathcal{B}(J/ψ\to π^0 e^+e^-)=(7.56\pm1.32\pm0.50)\times10^{-7}, where the first errors are statistical and the second ones systematic.

hep-ex↗

arXiv2014

Planck intermediate results. XV. A study of anomalous microwave emission in Galactic clouds

Anomalous microwave emission (AME) is believed to be due to electric dipole radiation from small spinning dust grains. The aim of this paper is a statistical study of the basic properties of AME regions and the environment in which they emit. We used WMAP and Planck maps, combined with ancillary radio and IR data, to construct a sample of 98 candidate AME sources, assembling SEDs for each source using aperture photometry on 1deg-smoothed maps from 0.408 GHz up to 3000 GHz. Each spectrum is fitted with a simple model of free-free, synchrotron (where necessary), cosmic microwave background (CMB), thermal dust, and spinning dust components. We find that 42 of the 98 sources have significant (>5sigma) excess emission at frequencies between 20 and 60 GHz. An analysis of the potential contribution of optically thick free-free emission from ultra-compact HII regions, using IR colour criteria, reduces the significant AME sample to 27 regions. The spectrum of the AME is consistent with model spectra of spinning dust. The AME regions tend to be more spatially extended than regions with little or no AME. The AME intensity is strongly correlated with the submillimetre/IR flux densities and comparable to previous AME detections in the literature. AME emissivity, defined as the ratio of AME to dust optical depth, varies by an order of magnitude for the AME regions. The AME regions tend to be associated with cooler dust in the range 14-20 K and an average emissivity index of +1.8, while the non-AME regions are typically warmer, at 20-27 K. In agreement with previous studies, the AME emissivity appears to decrease with increasing column density. The emerging picture is that the bulk of the AME is coming from the polycyclic aromatic hydrocarbons and small dust grains from the colder neutral interstellar medium phase (Abridged).

astro-ph.GA↗

arXiv2014

PandaX: A Liquid Xenon Dark Matter Experiment at CJPL

PandaX is a large upgradable liquid-xenon detector system that can be used for both direct dark-matter detection and $^{136}$Xe double-beta decay search. It is located in the Jinping Deep-Underground Laboratory in Sichuan, China. The detector operates in dual-phase mode, allowing detection of both prompt scintillation, and ionization charge through proportional scintillation. The central time projection chamber will be staged, with the first stage accommodating a target mass of about 120\,kg. In stage II, the target mass will be increased to about 0.5\,ton. In the final stage, the detector can be upgraded to a multi-ton target mass. In this paper a detailed description of the stage-I detector design and performance results established during the commissioning phase is presented.

physics.ins-det↗

arXiv2014

Long-term optical and radio variability of BL Lacertae

Well-sampled optical and radio light curves of BL Lacertae in B, V, R, I bands and 4.8, 8.0, 14.5 GHz from 1968 to 2014 were presented in this paper. A possible $1.26 \pm 0.05$ yr period in optical bands and a $7.50 \pm 0.15$ yr period in radio bands were detected based on discrete correlation function, structure function as well as Jurkevich method. Correlations among different bands were also analyzed and no reliable time delay was found between optical bands. Very weak correlations were detected between V band and radio bands. However, in radio bands the variation at low frequency lagged that at high frequency obviously. The spectrum of BL Lacertae turned mildly bluer when the object turned brighter, and stronger bluer-when-brighter trends were found for short flares. A scenario including a precessing helical jet and periodic shocks was put forward to interpret the variation characteristics of BL Lacertae.

astro-ph.HE↗

arXiv2014

Observation of $η^{\prime}\toπ^{+}π^{-}π^{+}π^{-}$ and $η^{\prime}\toπ^{+}π^{-}π^{0}π^{0}$

Using a sample of $1.3\times 10^9$ $J/ψ$ events collected with the BESIII detector, we report the first observation of $η^{\prime}\toπ^{+}π^{-}π^{+}π^{-}$ and $η^{\prime}\toπ^{+}π^{-}π^{0}π^{0}$. The measured branching fractions are $\mathcal{B}$($η^{\prime}\toπ^{+}π^{-}π^{+}π^{-}$) = $(8.53\pm0.69({\rm stat.})\pm0.64({\rm syst.}))\times10^{-5}$ and $\mathcal{B}$($η^{\prime}\toπ^{+}π^{-}π^{0}π^{0}$) = $(1.82\pm0.35({\rm stat.})\pm0.18({\rm syst.}))\times10^{-4}$, which are consistent with theoretical predictions based on a combination of chiral perturbation theory and vector-meson dominance.

hep-ex↗

arXiv2014

Investigating the TeV Morphology of MGRO J1908+06 with VERITAS

We report on deep observations of the extended TeV gamma-ray source MGRO J1908+06 made with the VERITAS very high energy (VHE) gamma-ray observatory. Previously, the TeV emission has been attributed to the pulsar wind nebula (PWN) of the Fermi-LAT pulsar PSR J1907+0602. We detect MGRO J1908+06 at a significance level of 14 standard deviations (14 sigma) and measure a photon index of 2.20 +/- 0.10_stat +/- 0.20_sys. The TeV emission is extended, covering the region near PSR J1907+0602 and also extending towards SNR G40.5--0.5. When fitted with a 2-dimensional Gaussian, the intrinsic extension has a standard deviation of sigma_src = 0.44 +/- 0.02 degrees. In contrast to other TeV PWNe of similar age in which the TeV spectrum softens with distance from the pulsar, the TeV spectrum measured near the pulsar location is consistent with that measured at a position near the rim of G40.5--0.5, 0.33 degrees away.

astro-ph.HE↗

arXiv2014

Planck 2013 results. XIV. Zodiacal emission

The Planck satellite provides a set of all-sky maps at nine frequencies from 30 GHz to 857 GHz. Planets, minor bodies, and diffuse interplanetary dust emission (IPD) are all observed. The IPD can be separated from Galactic and other emissions because Planck views a given point on the celestial sphere multiple times, through different columns of IPD. We use the Planck data to investigate the behaviour of zodiacal emission over the whole sky at sub-millimetre and millimetre wavelengths. We fit the Planck data to find the emissivities of the various components of the COBE zodiacal model -- a diffuse cloud, three asteroidal dust bands, a circumsolar ring, and an Earth-trailing feature. The emissivity of the diffuse cloud decreases with increasing wavelength, as expected from earlier analyses. The emissivities of the dust bands, however, decrease less rapidly, indicating that the properties of the grains in the bands are different from those in the diffuse cloud. We fit the small amount of Galactic emission seen through the telescope's far sidelobes, and place limits on possible contamination of the CMB results from both zodiacal and far-sidelobe emission. When necessary, the results are used in the Planck pipeline to make maps with zodiacal emission and far sidelobes removed. We show that the zodiacal correction to the CMB maps is small compared to the Planck CMB temperature power spectrum and give a list of flux densities for small Solar System bodies.

astro-ph.CO↗

arXiv2014

Planck 2013 results. XXIX. Planck catalogue of Sunyaev-Zeldovich sources

We describe the all-sky Planck catalogue of clusters and cluster candidates derived from Sunyaev--Zeldovich (SZ) effect detections using the first 15.5 months of Planck satellite observations. The catalogue contains 1227 entries, making it over six times the size of the Planck Early SZ (ESZ) sample and the largest SZ-selected catalogue to date. It contains 861 confirmed clusters, of which 178 have been confirmed as clusters, mostly through follow-up observations, and a further 683 are previously-known clusters. The remaining 366 have the status of cluster candidates, and we divide them into three classes according to the quality of evidence that they are likely to be true clusters. The Planck SZ catalogue is the deepest all-sky cluster catalogue, with redshifts up to about one, and spans the broadest cluster mass range from (0.1 to 1.6) 10^{15}Msun. Confirmation of cluster candidates through comparison with existing surveys or cluster catalogues is extensively described, as is the statistical characterization of the catalogue in terms of completeness and statistical reliability. The outputs of the validation process are provided as additional information. This gives, in particular, an ensemble of 813 cluster redshifts, and for all these Planck clusters we also include a mass estimated from a newly-proposed SZ-mass proxy. A refined measure of the SZ Compton parameter for the clusters with X-ray counter-parts is provided, as is an X-ray flux for all the Planck clusters not previously detected in X-ray surveys.

astro-ph.CO↗

arXiv2014

Planck 2013 results. XVI. Cosmological parameters

We present the first results based on Planck measurements of the CMB temperature and lensing-potential power spectra. The Planck spectra at high multipoles are extremely well described by the standard spatially-flat six-parameter LCDM cosmology. In this model Planck data determine the cosmological parameters to high precision. We find a low value of the Hubble constant, H0=67.3+/-1.2 km/s/Mpc and a high value of the matter density parameter, Omega_m=0.315+/-0.017 (+/-1 sigma errors) in excellent agreement with constraints from baryon acoustic oscillation (BAO) surveys. Including curvature, we find that the Universe is consistent with spatial flatness to percent-level precision using Planck CMB data alone. We present results from an analysis of extensions to the standard cosmology, using astrophysical data sets in addition to Planck and high-resolution CMB data. None of these models are favoured significantly over standard LCDM. The deviation of the scalar spectral index from unity is insensitive to the addition of tensor modes and to changes in the matter content of the Universe. We find a 95% upper limit of r<0.11 on the tensor-to-scalar ratio. There is no evidence for additional neutrino-like relativistic particles. Using BAO and CMB data, we find N_eff=3.30+/-0.27 for the effective number of relativistic degrees of freedom, and an upper limit of 0.23 eV for the summed neutrino mass. Our results are in excellent agreement with big bang nucleosynthesis and the standard value of N_eff=3.046. We find no evidence for dynamical dark energy. Despite the success of the standard LCDM model, this cosmology does not provide a good fit to the CMB power spectrum at low multipoles, as noted previously by the WMAP team. While not of decisive significance, this is an anomaly in an otherwise self-consistent analysis of the Planck temperature data.

astro-ph.CO↗

arXiv2014

Radial Quasiballistic Transport in Time-Domain Thermoreflectance Studied Using Monte Carlo Simulations

Recently, a pump beam size dependence of thermal conductivity was observed in Si at cryogenic temperatures using time-domain thermal reflectance (TDTR). These observations were attributed to quasiballistic phonon transport, but the interpretation of the measurements has been semi-empirical. Here we present a numerical study of the heat conduction that occurs in the full 3D geometry of a TDTR experiment, including an interface, using the Boltzmann Transport Equation. We identify the radial suppression function that describes the suppression in heat flux, compared to Fourier's law, that occurs due to quasiballistic transport and demonstrate good agreement with experimental data. We also discuss unresolved discrepancies that are important topics for future study.

cond-mat.mtrl-sci↗

arXiv2014

Deep Broadband Observations of the Distant Gamma-ray Blazar PKS 1424+240

We present deep VERITAS observations of the blazar PKS 1424+240, along with contemporaneous Fermi Large Area Telescope, Swift X-ray Telescope and Swift UV Optical Telescope data between 2009 February 19 and 2013 June 8. This blazar resides at a redshift of $z\ge0.6035$, displaying a significantly attenuated gamma-ray flux above 100 GeV due to photon absorption via pair-production with the extragalactic background light. We present more than 100 hours of VERITAS observations from three years, a multiwavelength light curve and the contemporaneous spectral energy distributions. The source shows a higher flux of (2.1$\pm0.3$)$\times10^{-7}$ ph m$^{-2}$s$^{-1}$ above 120 GeV in 2009 and 2011 as compared to the flux measured in 2013, corresponding to (1.02$\pm0.08$)$\times10^{-7}$ ph m$^{-2}$s$^{-1}$ above 120 GeV. The measured differential very high energy (VHE; $E\ge100$ GeV) spectral indices are $Γ=$3.8$\pm$0.3, 4.3$\pm$0.6 and 4.5$\pm$0.2 in 2009, 2011 and 2013, respectively. No significant spectral change across the observation epochs is detected. We find no evidence for variability at gamma-ray opacities of greater than $τ=2$, where it is postulated that any variability would be small and occur on longer than year timescales if hadronic cosmic-ray interactions with extragalactic photon fields provide a secondary VHE photon flux. The data cannot rule out such variability due to low statistics.

astro-ph.HE↗

arXiv2014

Search for the rare decays $J/ψ\to D_s^-ρ^+$ and $J/ψ\to \overline{D}{}^0\overline{K}{}^{\ast0}$

A search for the rare decays of $J/ψ\to D_s^-ρ^+ + c.c.$ and $J/ψ\to \overline{D}{}^0\overline{K}{}^{\ast0} + c.c.$ is performed with a data sample of 225.3 million $J/ψ$ events collected with the BESIII detector. No evident signal is observed. Upper limits on the branching fractions are determined to be $\mathcal{B}(J/ψ\to D_s^-ρ^+ + c.c.) < 1.3 \times 10^{-5}$ and $\mathcal{B}(J/ψ\to \overline{D}{}^0\overline{K}{}^{\ast0} + c.c.) < 2.5 \times 10^{-6}$ at the $90\%$ confidence level.

hep-ex↗

arXiv2014

2DEGs at perovskite interfaces between KTaO3 or KNbO3 and stannates

We report density functional studies of electron rich interfaces between KTaO3 or KNbO3 and CaSnO3 or ZnSnO3 and in particular the nature of the interfacial electron gasses that can be formed. We find that depending on the details these may occur on either the transition metal or stannate sides of the interface and in the later case can be shifted away from the interface by ferroelectricity. We also present calculations for bulk KNbO3, KTaO3, CaSnO3, BaSnO3 and ZnSnO3, showing the different transport and optical properties that may be expected on the two sides of such interfaces. The results suggest that these interfaces may display a wide range of behaviors depending on conditions, and in particular the interplay with ferroelectricity suggests that electrical control of these properties may be possible.

cond-mat.mtrl-sci↗

arXiv2014

Planck 2013 results. II. Low Frequency Instrument data processing

We describe the data processing pipeline of the Planck Low Frequency Instrument (LFI) data processing centre (DPC) to create and characterize full-sky maps based on the first 15.5 months of operations at 30, 44 and 70 GHz. In particular, we discuss the various steps involved in reducing the data, starting from telemetry packets through to the production of cleaned, calibrated timelines and calibrated frequency maps. Data are continuously calibrated using the modulation induced on the mean temperature of the cosmic microwave background radiation by the proper motion of the spacecraft. Sky signals other than the dipole are removed by an iterative procedure based on simultaneous fitting of calibration parameters and sky maps. Noise properties are estimated from time-ordered data after the sky signal has been removed, using a generalized least square map-making algorithm. A destriping code (Madam) is employed to combine radiometric data and pointing information into sky maps, minimizing the variance of correlated noise. Noise covariance matrices, required to compute statistical uncertainties on LFI and Planck products, are also produced. Main beams are estimated down to the -20 dB level using Jupiter transits, which are also used for the geometrical calibration of the focal plane.

astro-ph.IM↗

arXiv2014

Planck 2013 results. VIII. HFI photometric calibration and mapmaking

This paper describes the processing applied to the HFI cleaned time-ordered data to produce photometrically calibrated maps. HFI observes the sky over a broad range of frequencies, from 100 to 857 GHz. To get the best accuracy on the calibration on such a large range, two different photometric calibration schemes have to be used. The 545 and 857 \GHz\ data are calibrated using Uranus and Neptune flux density measurements, compared with models of their atmospheric emissions to calibrate the data. The lower frequencies (below 353 GHz) are calibrated using the cosmological microwave background dipole.One of the components of this anisotropy results from the orbital motion of the satellite in the Solar System, and is therefore time-variable. Photometric calibration is thus tightly linked to mapmaking, which also addresses low frequency noise removal. The 2013 released HFI data show some evidence for apparent gain variations of the HFI bolometers' detection chain. These variations were identified by comparing observations taken more than one year apart in the same configuration. We developed an effective correction to limit its effect on calibration, and assess its accuracy. We present several methods used to estimate the precision of the photometric calibration. We distinguish relative (from one detector to another, or from one frequency to another) and absolute uncertainties. In both cases, we found that these uncertainties range from a few $10^{-3}$ to several per cents from 100 to 857 GHz. We describe the pipeline producing the maps from the HFI timelines, based on the photometric calibration parameters and we detail the scheme used to a posteriori set the zero level of the maps. We also briefly discuss the cross-calibration between HFI and the SPIRE instrument on board Herschel. We finally summarize the basic characteristics of the set of the HFI maps from the 2013 Planck data release.

astro-ph.CO↗

arXiv2014

Observations of the unidentified gamma-ray source TeV J2032+4130 by VERITAS

TeV J2032+4130 was the first unidentified source discovered at very high energies (VHE; E $>$ 100 GeV), with no obvious counterpart in any other wavelength. It is also the first extended source to be observed in VHE gamma rays. Following its discovery, intensive observational campaigns have been carried out in all wavelengths in order to understand the nature of the object, which have met with limited success. We report here on a deep observation of TeV J2032+4130, based on 48.2 hours of data taken from 2009 to 2012 by the VERITAS (Very Energetic Radiation Imaging Telescope Array System) experiment. The source is detected at 8.7 standard deviations ($σ$) and is found to be extended and asymmetric with a width of 9.5$^{\prime}$$\pm$1.2$^{\prime}$ along the major axis and 4.0$^{\prime}$$\pm$0.5$^{\prime}$ along the minor axis. The spectrum is well described by a differential power law with an index of 2.10 $\pm$ 0.14$_{stat}$ $\pm$ 0.21$_{sys}$ and a normalization of (9.5 $\pm$ 1.6$_{stat}$ $\pm$ 2.2$_{sys}$) $\times$ 10$^{-13}$TeV$^{-1}$ cm$^{-2}$ s$^{-1}$ at 1 TeV. We interpret these results in the context of multiwavelength scenarios which particularly favor the pulsar wind nebula (PWN) interpretation.

astro-ph.HE↗

arXiv2014

Amplitude Analysis of the $D^+ \to K^0_S π^+ π^0$ Dalitz Plot

We perform an analysis of the $D^+ \to K^0_S π^+ π^0$ Dalitz plot using a data set of 2.92 fb$^{-1}$ of $e^+e^-$ collisions at the $ψ(3770)$ mass accumulated by the BESIII Experiment, in which 166694 candidate events are selected with a background of 15.1%. The Dalitz plot is found to be well-represented by a combination of six quasi-two-body decay channels ($K^0_Sρ^+$, $K^0_Sρ(1450)^+$, $\overline{K}^{*0}π^+$, $\overline{K}_0(1430)^0π^+$, $\overline{K}(1680)^0π^+$, $\overlineκ^0π^+$) plus a small non-resonant component. Using the fit fractions from this analysis, partial branching ratios are updated with higher precision than previous measurements.

hep-ex↗