arXiv subjects

Xu Wang

Publications and source records attributed to Xu Wang.

At least 397 records · Page 22Linked to original sources

arXiv2020

Quality Assurance Test of Silicon Photomultipliers and Electronic Boards for STAR Event Plane Detector

The event plane detector (EPD), installed in the Solenoid Tracker at the Relativistic Heavy-Ion Collider located at the Brookhaven National Laboratory is a plastic scintillator-based device that measures the reaction centrality and event plane in the forward region of the relativistic heavy-ion collisions. We used silicon photomultiplier (SiPM) arrays to detect the photons produced in the scintillator via the fiber connection. Signals from the SiPM arrays were amplified by the front-end electronic (FEE) board, and sent to the analog-to-digital converter (ADC) boards for further processing via the receiver(RX) board. The full EPD system consisted of 24 super-sectors (SSs); each SS was equipped with two SiPM boards, two FEE boards and two RX boards, and they corresponded to 744 readout channels. All these boards were mass produced at the University of Science and Technology of China, with a dedicated quality assurance (QA) procedures applied to identify any problems before deployment. This article describes the details of the QA method and the related test system. The QA test results are presented along with the discussions.

physics.ins-det↗

arXiv2020

Ultra-broadband acoustic ventilation barriers via hybrid-functional metasurfaces

Ventilation barriers allowing simultaneous sound blocking and free airflow passage are of great challenge but necessary for particular scenarios calling for sound-proofing ventilation. Previous works based on local resonance or Fano-like interference serve a narrow working range around the resonant or destructive-interference frequency. Efforts made on broadband designs show a limited bandwidth typically smaller than half an octave. Here, we theoretically design an ultra-broadband ventilation barrier via hybridizing dissipation and interference. Confirmed by experiments, the synergistic effect from our hybrid-functional metasurface significantly expand the scope of its working frequencies, leading to an effective blocking of more than 90% of incident energy in the range of 650-2000 Hz, while its structural thickness is only 53 mm $(\sim λ/ 10)$. Our design shows great flexibility in customizing the broadband and is capable of handling sound coming from various directions, which has potential in air-permeable yet sound-proofing applications.

physics.app-ph↗

arXiv2020

Inverse Solidification Induced by Active Janus Particles

Crystals melt when thermal excitations or the concentration of defects in the lattice is sufficiently high. Upon melting, the crystalline long-range order vanishes, turning the solid to a fluid. In contrast to this classical scenario of solid melting, here we demonstrate a counter-intuitive behavior of the occurrence of crystalline long-range order in an initially disordered matrix. This unusual solidification is demonstrated in a system of passive colloidal particles accommodating chemically active defects -- photocatalytic Janus particles. The observed crystallization occurs when the amount of active-defect-induced fluctuations (which is the measure of the effective temperature) reaches critical value. The driving mechanism behind this unusual behavior is purely internal and resembles a blast-induced solidification. Here the role of "internal micro-blasts" is played by the photochemical activity of defects residing in the colloidal matrix. The defect-induced solidification occurs under non-equilibrium conditions: the resulting solid exists as long as a constant supply of energy in the form of ion flow is provided by the catalytic photochemical reaction at the surface of active Janus particle defects. Our findings could be useful for understanding of the phase transitions of matter under extreme conditions far from thermodynamic equilibrium.

cond-mat.soft↗

arXiv2020

An inverse random source problem for the one-dimensional Helmholtz equation with attenuation

This paper is concerned with an inverse random source problem for the one-dimensional stochastic Helmholtz equation with attenuation. The source is assumed to be a microlocally isotropic Gaussian random field with its covariance operator being a classical pseudo-differential operator. The random sources under consideration are equivalent to the generalized fractional Gaussian random fields which include rough fields and can be even rougher than the white noise, and hence should be interpreted as distributions. The well-posedness of the direct source scattering problem is established in the distribution sense. The micro-correlation strength of the random source, which appears to be the strength in the principal symbol of the covariance operator, is proved to be uniquely determined by the wave field in an open measurement set. Numerical experiments are presented for the white noise model to demonstrate the validity and effectiveness of the proposed method.

math.NA↗

arXiv2020

Active Gate Drive with Gate-Drain Discharge Compensation for Voltage Balancing in Series-Connected SiC MOSFETs

Imbalanced voltage sharing during the turn-off transient is a challenge for series-connected silicon carbide (SiC) MOSFET application. This article first discusses the influence of the gate-drain discharge deviation on the voltage imbalance ratio, and its primary causes are also presented and verified by LTspice simulation. Accordingly, a novel active gate drive, which aims to compensate the discharge difference between devices connected in series, is proposed and analyzed. By only using the original output of the driving IC, the proposed gate drive is realized by implementing an auxiliary circuit on the existing commercial gate drive. Therefore, unlike other active gate drives for balancing control, no extra isolations for power/signal are needed, and the number of the devices in series is unlimited. The auxiliary circuit includes three sub-circuits as a high-bandwidth current sink for regulating switching performance, a relative low-frequency but reliable sampling and control circuit for closed-loop control, and a trigger combining the former and the latter. The operational principle and the design guideline for each part are presented in detail. Experimental results validate the performance of the proposed gate drive and its voltage balancing control algorithm.

eess.SY↗

arXiv2020

Nonlinear optical effects in a nucleus

Intense laser technologies generate light with unprecedented and growing intensities. The possibility emerges that a nucleus responds nonlinearly to an intense light field, pointing to a yet little explored research area of nuclear nonlinear optics. We consider two-photon and three-photon absorption (with subsequent disintegration) processes of the deuteron, the simplest and the most fundamental nontrivial nucleus, as prototypes of nuclear nonlinear optical effects. Quantitative calculations are performed on these processes and novel observable effects are predicted.

physics.atom-ph↗

arXiv2020

Nuclear fission in intense laser fields

Rapid-advancing intense laser technologies enable the possibility of a direct laser-nucleus coupling. In this paper the effect of intense laser fields on a series of nuclear fission processes, including proton decay, alpha decay, and cluster decay, is theoretically studied with the help of nuclear double folding potentials. The results show that the half-lives of these decay processes can be modified by non-negligible amounts, for example on the order of 0.01 or 0.1 percents in intense laser fields available in the forthcoming years. In addition to numerical results, an approximate analytical formula is derived to connect the laser-induced modification to the decay half-life and the decay energy.

physics.atom-ph↗

arXiv2020

Statistical Inference for Networks of High-Dimensional Point Processes

Fueled in part by recent applications in neuroscience, the multivariate Hawkes process has become a popular tool for modeling the network of interactions among high-dimensional point process data. While evaluating the uncertainty of the network estimates is critical in scientific applications, existing methodological and theoretical work has primarily addressed estimation. To bridge this gap, this paper develops a new statistical inference procedure for high-dimensional Hawkes processes. The key ingredient for this inference procedure is a new concentration inequality on the first- and second-order statistics for integrated stochastic processes, which summarize the entire history of the process. Combining recent results on martingale central limit theory with the new concentration inequality, we then characterize the convergence rate of the test statistics. We illustrate finite sample validity of our inferential tools via extensive simulations and demonstrate their utility by applying them to a neuron spike train data set.

stat.ML↗

arXiv2020

Substantially enhanced deuteron-triton fusion probabilities in intense low-frequency laser fields

Deuteron-triton (DT) fusion is the primary fusion reaction used in controlled fusion research, mainly for its relatively high reaction cross sections compared to other fusion options. Even so, to attain appreciable reaction probabilities very high temperatures (on the order of 10-100 million kelvins) are required, which are extremely challenging to achieve and maintain. We show that intense low-frequency laser fields, such as those in the near-infrared regime for the majority of intense laser facilities around the world, are highly effective in transferring energy to the DT system and enhancing the DT fusion probabilities. The fusion probabilities are shown to be enhanced by at least an order of magnitude in 800-nm laser fields with intensities on the order of 10$^{21}$ W/cm$^2$. The demanding temperature requirement of controlled nuclear fusion may be relaxed if intense low-frequency lasers are exploited.

physics.atom-ph↗

arXiv2020

Analysis of the strong decays of the $P_c(4312)$ as a pentaquark molecular state with QCD sum rules

In this article, we tentatively assign the $P_c(4312)$ to be the $\bar{D}Σ_c$ pentaquark molecular state with the spin-parity $J^P={\frac{1}{2}}^-$, and discuss the factorizable and non-factorizable contributions in the two-point QCD sum rules for the $\bar{D}Σ_c$ molecular state in details to prove the reliability of the single pole approximation in the hadronic spectral density. We study its two-body strong decays with the QCD sum rules, special attentions are paid to match the hadron side with the QCD side of the correlation functions to obtain solid duality. We obtain the partial decay widths $Γ\left(P_c(4312)\to η_c p\right)=0.255\,{\rm{MeV}}$ and $Γ\left(P_c(4312)\to J/ψp\right)=9.296^{+19.542}_{-9.296}\,\,{\rm{MeV}}$, which are compatible with the experimental value of the total width, and support assigning the $P_c(4312)$ to be the $\bar{D}Σ_c$ pentaquark molecular state.

hep-ph↗

arXiv2020

Automate Obstructive Sleep Apnea Diagnosis Using Convolutional Neural Networks

Identifying sleep problem severity from overnight polysomnography (PSG) recordings plays an important role in diagnosing and treating sleep disorders such as the Obstructive Sleep Apnea (OSA). This analysis traditionally is done by specialists manually through visual inspections, which can be tedious, time-consuming, and is prone to subjective errors. One of the solutions is to use Convolutional Neural Networks (CNN) where the convolutional and pooling layers behave as feature extractors and some fully-connected (FCN) layers are used for making final predictions for the OSA severity. In this paper, a CNN architecture with 1D convolutional and FCN layers for classification is presented. The PSG data for this project are from the Cleveland Children's Sleep and Health Study database and classification results confirm the effectiveness of the proposed CNN method. The proposed 1D CNN model achieves excellent classification results without manually preprocesssing PSG signals such as feature extraction and feature reduction.

cs.LG↗

arXiv2020

Ergodicity breaking of an inorganic glass in aging near ${T_g}$ probed by elasticity relaxation

We performed a series of aging experiments of an inorganic glass (As${_2}$Se${_3}$) at a temperature ${T_2}$ near the glass transition point ${T_g}$ by first relaxing it at ${T_1}$. The relaxation of Young's modulus was monitored, which was(almost if not ideally) exponential with a ${T_1}$-dependent relaxation time ${τ(T_1)}$. We demostrate the Kovacs' paradox for the first time in an inorganic glasses. Associated with the divergence of $τ$, the quasi-equilibrated Young's modulus ${E_\infty}$ does not converge either. An elastic model of relaxation time and a Mori-Tanaka analysis of ${E_\infty}$ lead to a similar estimate of the persistent memory of the history, ergodicity breaking within the accessible experimental time. Experiments with different ${T_2}$ exhibits a critical temperature ${T_p \sim T_g}$, i.e., when ${T_2 > T_p}$, both $τ$ and ${E_\infty}$ converge.

cond-mat.dis-nn↗

arXiv2020

Characterization of the photomultiplier tubes for the scintillation detectors of GRANDProto35 experiment

GRANDProto35 is the first stage of the GRAND project. It will be composed of an array of 35 radio antennas and 24 scintillation detectors, the radio and scintillating subarrays being triggered independently. The scintillation detector array allow to cross check,through an offline treatment, if the selected radio candidates are indeed air shower events and thus quantitatively determine the detection efficiency of the radio array. The Hamamatsu R7725 is candidate for the scintillation detector photomultiplier. The characteristics of the PMT will directly affects the time and energy resolution, dynamic detection range of a scintillation detector.In order to cover the large dynamic range, voltage divider circuit featured with dual-readoutwas designed for the PMT. In this paper,details about the system setup, measurement method and results will be described. Some characteristics of PMT were calibrated and researched: the absolution gain, single photoelectron (SPE) energy resolution, transit time spread (TTS), linear dynamic range and temperature dependence of PMT gain.

physics.ins-det↗

arXiv2020

Hybrid Attention for Automatic Segmentation of Whole Fetal Head in Prenatal Ultrasound Volumes

Background and Objective: Biometric measurements of fetal head are important indicators for maternal and fetal health monitoring during pregnancy. 3D ultrasound (US) has unique advantages over 2D scan in covering the whole fetal head and may promote the diagnoses. However, automatically segmenting the whole fetal head in US volumes still pends as an emerging and unsolved problem. The challenges that automated solutions need to tackle include the poor image quality, boundary ambiguity, long-span occlusion, and the appearance variability across different fetal poses and gestational ages. In this paper, we propose the first fully-automated solution to segment the whole fetal head in US volumes. Methods: The segmentation task is firstly formulated as an end-to-end volumetric mapping under an encoder-decoder deep architecture. We then combine the segmentor with a proposed hybrid attention scheme (HAS) to select discriminative features and suppress the non-informative volumetric features in a composite and hierarchical way. With little computation overhead, HAS proves to be effective in addressing boundary ambiguity and deficiency. To enhance the spatial consistency in segmentation, we further organize multiple segmentors in a cascaded fashion to refine the results by revisiting context in the prediction of predecessors. Results: Validated on a large dataset collected from 100 healthy volunteers, our method presents superior segmentation performance (DSC (Dice Similarity Coefficient), 96.05%), remarkable agreements with experts. With another 156 volumes collected from 52 volunteers, we ahieve high reproducibilities (mean standard deviation 11.524 mL) against scan variations. Conclusion: This is the first investigation about whole fetal head segmentation in 3D US. Our method is promising to be a feasible solution in assisting the volumetric US-based prenatal studies.

eess.IV↗

arXiv2020

Cohomologies of complex manifolds with symplectic $(1,1)$-forms

Let $(X, J)$ be a complex manifold with a non-degenerated smooth $d$-closed $(1,1)$-form $ω$. Then we have a natural double complex $\overline{\partial}+\overline{\partial}^Λ$, where $\overline{\partial}^Λ$ denotes the symplectic adjoint of the $\overline{\partial}$-operator. We study the Hard Lefschetz Condition on the Dolbeault cohomology groups of $X$ with respect to the symplectic form $ω$. In \cite{TW}, we proved that such a condition is equivalent to a certain symplectic analogous of the $\partial\overline{\partial}$-Lemma, namely the $\overline{\partial}\, \overline{\partial}^Λ$-Lemma, which can be characterized in terms of Bott--Chern and Aeppli cohomologies associated to the above double complex. We obtain Nomizu type theorems for the Bott--Chern and Aeppli cohomologies and we show that the $\overline{\partial}\, \overline{\partial}^Λ$-Lemma is stable under small deformations of $ω$, but not stable under small deformations of the complex structure. However, if we further assume that $X$ satisfies the $\partial\overline{\partial}$-Lemma then the $\overline{\partial}\, \overline{\partial}^Λ$-Lemma is stable.

math.DG↗

arXiv2020

Extended virtual detector theory including quantum interferences

We extend our earlier "virtual detector" method [X. Wang, J. Tian, and J. H. Eberly, Phys. Rev. Lett. 110, 243001 (2013)], a hybrid quantum mechanical and classical trajectory method, to include phases in the classical trajectories. Effects of quantum interferences, lost in the earlier method, are restored. The obtained photoelectron momentum distributions agree well with the corresponding numerical solutions of the time-dependent Schrödinger equation.

physics.atom-ph↗

arXiv2020

An inverse random source problem for Maxwell's equations

This paper is concerned with an inverse random source problem for the three-dimensional time-harmonic Maxwell equations. The source is assumed to be a centered complex-valued Gaussian vector field with correlated components, and its covariance operator is a pseudo-differential operator. The well-posedness of the direct source scattering problem is established and the regularity of the electromagnetic field is given. For the inverse source scattering problem, the micro-correlation strength matrix of the covariance operator is shown to be uniquely determined by the high frequency limit of the expectation of the electric field measured in an open bounded domain disjoint with the support of the source. In particular, we show that the diagonal entries of the strength matrix can be uniquely determined by only using the amplitude of the electric field. Moreover, this result is extended to the almost surely sense by deducing an ergodic relation for the electric field over the frequencies.

math.AP↗

arXiv2020

Timing Performance of a Micro-Channel-Plate Photomultiplier Tube

The spatial dependence of the timing performance of the R3809U-50 Micro-Channel-Plate PMT (MCP-PMT) by Hamamatsu was studied in high energy muon beams. Particle position information is provided by a GEM tracker telescope, while timing is measured relative to a second MCP-PMT, identical in construction. In the inner part of the circular active area (radius r$<$5.5\,mm) the time resolution of the two MCP-PMTs combined is better than 10~ps. The signal amplitude decreases in the outer region due to less light reaching the photocathode, resulting in a worse time resolution. The observed radial dependence is in quantitative agreement with a dedicated simulation. With this characterization, the suitability of MCP-PMTs as $\text{t}_\text{0}$ reference detectors has been validated.

physics.ins-det↗