arXiv subjects

H. Li

Publications and source records attributed to H. Li.

At least 145 records · Page 8Linked to original sources

arXiv2018

Statistical Description of Transport in Multimode Fibers with Mode-Dependent Loss

We analyze coherent wave transport in a new physical setting associated with multimode wave systems where reflection is completely suppressed and mode-dependent losses together with mode-mixing are dictating the wave propagation. An additional physical constraint is the fact that in realistic circumstances the access to the scattering (or transmission) matrix is incomplete. We have addressed all these challenges by providing a statistical description of wave transport which fuses together a free probability theory approach with a Filtered Random Matrix ensemble. Our theoretical predictions have been tested successfully against experimental data of light transport in multimode fibers.

physics.optics↗

arXiv2018

Limits on light WIMPs with a 1 kg-scale germanium detector at 160 eVee physics threshold at the China Jinping Underground Laboratory

We report results of a search for light weakly interacting massive particle (WIMP) dark matter from the CDEX-1 experiment at the China Jinping Underground Laboratory (CJPL). Constraints on WIMP-nucleon spin-independent (SI) and spin-dependent (SD) couplings are derived with a physics threshold of 160 eVee, from an exposure of 737.1 kg-days. The SI and SD limits extend the lower reach of light WIMPs to 2 GeV and improve over our earlier bounds at WIMP mass less than 6 GeV.

hep-ex↗

arXiv2018

Limits on Light Weakly Interacting Massive Particles from the First 102.8 kg ${\times}$ day Data of the CDEX-10 Experiment

We report the first results of a light weakly interacting massive particles (WIMPs) search from the CDEX-10 experiment with a 10 kg germanium detector array immersed in liquid nitrogen at the China Jinping Underground Laboratory with a physics data size of 102.8 kg day. At an analysis threshold of 160 eVee, improved limits of 8 $\times 10^{-42}$ and 3 $\times 10^{-36}$ cm$^{2}$ at a 90\% confidence level on spin-independent and spin-dependent WIMP-nucleon cross sections, respectively, at a WIMP mass ($m_χ$) of 5 GeV/${c}^2$ are achieved. The lower reach of $m_χ$ is extended to 2 GeV/${c}^2$.

hep-ex↗

arXiv2018

An Accurate Data Cleaning Procedure for Electron Cyclotron Emission Imaging on EAST Tokamak Based on Methodology of Machine Learning

A new data cleaning procedure for electron cyclotron emission imaging (ECEI) of EAST tokamak is constructed. Machine learning techniques, including SVM and Decision tree, are applied to identifying saturated, zero, and weak signals of ECEI raw data, which not only reduces the effort of researchers for data analysis, but also improves the accuracy of data preprocessing. To enhance the reliability of the procedure, proper training sets are sampled based on massive raw data from the experiments of ECEI on EAST tokamak. Window size of temporal signal, kernel function, and other model parameters are obtained after model training. Consequently, the recognition rates of saturated, zero, and weak signals in raw data are 99.4%, 99.86%, and 99.9%, respectively, which proves the accuracy of this procedure.

physics.data-an↗

arXiv2018

Extreme magnetoresistance in the topologically trivial lanthanum monopnictide LaAs

The family of binary Lanthanum monopnictides, LaBi and LaSb, have attracted a great deal of attention as they display an unusual extreme magnetoresistance (XMR) that is not well understood. Two classes of explanations have been raised for this: the presence of non-trivial topology, and the compensation between electron and hole densities. Here, by synthesizing a new member of the family, LaAs, and performing transport measurements, Angle Resolved Photoemission Spectroscopy (ARPES), and Density Functional Theory (DFT) calculations, we show that (a) LaAs retains all qualitative features characteristic of the XMR effect but with a siginificant reduction in magnitude compared to LaSb and LaBi, (b) the absence of a band inversion or a Dirac cone in LaAs indicates that topology is insignificant to XMR, (c) the equal number of electron and hole carriers indicates that compensation is necessary for XMR but does not explain its magnitude, and (d) the ratio of electron and hole mobilities is much different in LaAs compared to LaSb and LaBi. We argue that the compensation is responsible for the XMR profile and the mobility mismatch constrains the magnitude of XMR.

cond-mat.mtrl-sci↗

arXiv2018

Innovative design and construction technique for the Cylindrical GEM detector for the BESIII experiment

Gas detector are very light instrument used in high energy physics to measure the particle properties: position and momentum. Through high electric field is possible to use the Gas Electron Multiplier (GEM) technology to detect the particles and to exploit the its properties to construct a large area detector, such as the new IT for BESIII. The state of the art in the GEM production allow to create very large area GEM foils (up to 50x100 cm2) and thanks to the small thickness of these foil is it possible to shape it to the desired form: a Cylindrical Gas Electron Multiplier (CGEM) is then proposed. The innovative construction technique based on Rohacell, a PMI foam, will give solidity to cathode and anode with a very low impact on material budget. The entire detector is sustained by permaglass rings glued at the edges. These rings are use to assembly the CGEM together with a dedicated Vertical Insertion System and moreover there is placed the On-Detector electronic. The anode has been improved w.r.t. the state of the art through a jagged readout that minimize the inter-strip capacitance. The mechanical challenge of this detector requires a precision of the entire geometry within few hundreds of microns in the whole area. In this presentation will be presented an overview of the construction technique and the validation of this technique through the realization of a CGEM and its first tests. These activities are performed within the framework of the BESIIICGEM Project (645664), funded by the European Commission in the action H2020-RISE-MSCA-2014.

physics.ins-det↗

arXiv2018

Performance of the micro-TPC Reconstruction for GEM Detectors at High Rate

Gas detectors are one of the pillars of the research in fundamental physics. Since many years, a new concept of detectors, the Micro Pattern Gas Detectors, allows to overcome many of the problems of other types of commonly used detectors, as drift chambers and microstrips, reducing the discharge rate and increasing the radiation tolerance. Among these, one of the most commonly used is the Gas Electron Multiplier. Commonly deployed as fast timing detectors and triggers, due to their fast response, high rate capability and high radiation hardness, they can also be used as trackers. The center of gravity readout technique allows to overcome the limit of the digital pads, whose spatial resolution is constrained by the pitch size. The presence of a high external magnetic field can distort the electronic cloud and affect the spatial resolution. The micro-TPC reconstruction method allows to reconstruct the three dimensional particle position as in a traditional Time Projection Chamber, but within a drift gap of a few millimeters. This method brings these detectors into a new perspective for what concerns the spatial resolution in strong magnetic field. In this report, the basis of this new technique will be shown and it will be compared to the traditional center of gravity. The results of a series of test beam performed with 10 x 10 cm2 planar prototypes in magnetic field will also be presented. This is one of the first implementations of this technique for GEM detectors in magnetic field and allows to reach unprecedented performance for gas detectors, up to a limit of 120 micron at 1T, one of the world's best results for MPGDs in strong magnetic field. The micro-TPC reconstruction has been recently tested at very high rates in a test beam at the MAMI facility; preliminary results of the test will be presented.

physics.ins-det↗

arXiv2018

Test beam results of a Cylindrical GEM detector for the BESIII experiment

Gas detector are very light instrument used in high energy physics to measure the particle properties: position and momentum. Through high electric field is possible to use the Gas Electron Multiplier (GEM) technology to detect the charged particles and to exploit their properties to construct a large area detector, such as the new IT for BESIII. The state of the art in the GEM production allows to create very large area GEM foils (up to 50x100 $\mathrm{cm}^2$) and thanks to the small thickness of these foils is it possible to shape it to the desired form: a Cylindrical Gas Electron Multiplier (CGEM) is then proposed. The innovative construction technique based on Rohacell, a PMI foam, will give solidity to cathode and anode with a very low impact on material budget. The entire detector is sustained by Permaglass rings glued at the edges. These rings are used to assembly the CGEM, together with a dedicated Vertical Insertion System and moreover they host the On-Detector electronic. The anode has been improved w.r.t. the state of the art through a jagged readout that minimize the inter-strip capacitance. The mechanical challenge of this detector requires a precision of the entire geometry within few hundreds of microns in the whole area. In this contribution an overview of the construction technique, the validation of this technique through the realization of a CGEM, and its first tests will be presented. These activities are performed within the framework of the BESIIICGEM Project (645664), funded by the European Commission in the action H2020-RISE-MSCA-2014.

physics.ins-det↗

arXiv2018

The new cylindrical GEM inner tracker of BESIII

The Cylindrical GEM-Inner Tracker (CGEM-IT) is the upgrade of the internal tracking system of the BESIII experiment. It consists of three layers of cylindrically-shaped triple GEMs, with important innovations with respect to the existing GEM detectors, in order to achieve the best performance with the lowest material budget. It will be the first cylindrical GEM running with analog readout inside a 1T magnetic field. The simultaneous measurement of both the deposited charge and the signal time will permit to use a combination of two algorithms to evaluate the spatial position of the charged tracks inside the CGEM-IT: the charge centroid and the micro time projection chamber modes. They are complementary and can cope with the asymmetry of the electron avalanche when running in magnetic field and with non-orthogonal incident tracks. To evaluate the behavior under different working settings, both planar chambers and the first cylindrical prototype have been tested during various test beams at CERN with 150 GeV/c muons and pions. This paper reports the results obtained with the two reconstruction methods and a comparison between the planar and cylindrical chambers.

physics.ins-det↗

arXiv2018

Test beam results with prototypes for the new Cylindrical GEM Inner Tracker of the BESIII experiment

A cylindrical GEM tracker is under construction in order to replace and improve the inner tracking system of the BESIII experiment. Tests with planar chamber prototypes were carried out on the H4 beam line of SPS (CERN) with muons of 150 GeV/c momentum, to evaluate the efficiency and resolution under different working conditions. The obtained efficiency was in the 96 - 98% range. Two complementary algorithms for the position determination were developed: the charge centroid and the micro-TPC methods. With the former, resolutions <100 micron and <200 micron were achieved without and with magnetic field, respectively. The micro-TPC improved these results. By the end of 2016, the first cylindrical prototype was tested on the same beam line. It showed optimal stability under different settings. The comparison of its performance with respect to the planar chambers is ongoing. Here, the results of the planar prototype tests will be addressed.

physics.ins-det↗

arXiv2018

Spectroscopic Observations of a Current Sheet in a Solar Flare

Current sheet is believed to be the region of energy dissipation via magnetic reconnection in solar flares. However, its properties, for example, the dynamic process, have not been fully understood. Here we report a current sheet in a solar flare (SOL2017-09-10T16:06) that was clearly observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory as well as the EUV Imaging Spectrometer on Hinode. The high-resolution imaging and spectroscopic observations show that the current sheet is mainly visible in high temperature (>10 MK) passbands, particularly in the Fe XXIV 192.03 line with a formation temperature of ~18 MK. The hot Fe XXIV 192.03 line exhibits very large nonthermal velocities up to 200 km/s in the current sheet, suggesting that turbulent motions exist there. The largest turbulent velocity occurs at the edge of the current sheet, with some offset with the strongest line intensity. At the central part of the current sheet, the turbulent velocity is negatively correlated with the line intensity. From the line emission and turbulent features we obtain a thickness in the range of 7--11 Mm for the current sheet. These results suggest that the current sheet has internal fine and dynamic structures that may help the magnetic reconnection within it proceeds efficiently.

astro-ph.SR↗

arXiv2018

Dynamic magnetic-transformation-induced exchange bias in (Fe2O3)0.1-(FeTiO3)0.9

Up to now, for the conventional exchange bias (EB) systems there has been one pinning phase and one pinned phase, and the pinning and pinned phases are inherent to the material and do not mutually transform into each other. Interestingly, we show here that EB is observed in a special system (Fe2O3)0.1(FeTiO3)0.9 (HI9) different from the conventional EB system. Neutron powder diffraction and magnetic measurement confirm that for HI9: i) two types of short range antiferromagnetic ordering coexist, ii) there are two pinning phases and one pinned phase, iii) the pinned phase is not intrinsic to the structure but can be dynamically produced from the pinning phase with the help of an external magnetic field. Consequently, two anomalous EB behaviors are observed: i) both the coercivity (HC) and the exchange bias field (HE) simultaneously decrease to zero at 30 K, ii) for a high cooling field (Hcool) HE decreases logarithmically with increasing Hcool. Using Arrott plots it is confirmed that the first order magnetic phase transformation (FOMPT) from the AFM Fe2+ to ferromagnetic (FM) Fe2+ and the second order magnetic phase transformation (SOMPT) for the process whereby the FM Fe2+ aligns with the external field direction coexist in HI-9. The Morin transition and FOMPT cause the anomalous EB behaviors. This work may provide fresh ideas for research into EB behavior.

cond-mat.mtrl-sci↗

arXiv2017

Steering proton migration in hydrocarbons using intense few-cycle laser fields

Proton migration is a ubiquitous process in chemical reactions related to biology, combustion, and catalysis. Thus, the ability to control the movement of nuclei with tailored light, within a hydrocarbon molecule holds promise for far-reaching applications. Here, we demonstrate the steering of hydrogen migration in simple hydrocarbons, namely acetylene and allene, using waveform-controlled, few-cycle laser pulses. The rearrangement dynamics are monitored using coincident 3D momentum imaging spectroscopy, and described with a quantum-dynamical model. Our observations reveal that the underlying control mechanism is due to the manipulation of the phases in a vibrational wavepacket by the intense off-resonant laser field.

physics.chem-ph↗

arXiv2017

Adiabatic Decoherence-Free Subspaces and its Shortcuts

The adiabatic theorem and "shortcuts to adiabaticity" for the adiabatic dynamics of time-dependent decoherence-free subspaces are explored in this paper. Starting from the definition of the dynamical stable decoherence-free subspaces, we show that, under a compact adiabatic condition, the quantum state follows time-dependent decoherence-free subspaces (the adiabatic decoherence free subspaces) into the target subspace with extremely high purity, even though the dynamics of the quantum system may be non-adiabatic. The adiabatic condition mentioned in the adiabatic theorem is very similar with the adiabatic condition for closed quantum systems, except that the operators required to be "slowness" is on the Lindblad operators. We also show that the adiabatic decoherence-free subspaces program depends on the existence of instantaneous decoherence-free subspaces, which requires that the Hamiltonian of open quantum systems has to be engineered according to the incoherent control program. Besides, "the shortcuts to adiabaticity" for the adiabatic decoherence-free subspaces program is also presented based on the transitionless quantum driving method. Finally, we provide an example of physical systems that support our programs. Our approach employs Markovian master equations and applies primarily to finite-dimensional quantum systems.

quant-ph↗

arXiv2017

Unbounded norm topology beyond normed lattices

In this paper, we generalize the concept of unbounded norm (un) convergence: let $X$ be a normed lattice and $Y$ a vector lattice such that $X$ is an order dense ideal in $Y$; we say that a net $(y_α)$ un-converges to $y$ in $Y$ with respect to $X$ if $\Bigl\lVert\lvert y_α-y\rvert \wedge x\Bigr\rVert\to 0$ for every $x\in X_+$. We extend several known results about un-convergence and un-topology to this new setting. We consider the special case when $Y$ is the universal completion of $X$. If $Y=L_0(μ)$, the space of all $μ$-measurable functions, and $X$ is an order continuous Banach function space in $Y$, then the un-convergence on $Y$ agrees with the convergence in measure. If $X$ is atomic and order complete and $Y=\mathbb R^A$ then the un-convergence on $Y$ agrees with the coordinate-wise convergence.

math.FA↗

arXiv2017

Development and Test of a uTPC Cluster Reconstruction for a Triple GEM Detector in Strong Magnetic Field

Performance of triple GEM prototypes has been evaluated by means of a muon beam at the H4 line of the SPS test area at CERN. The data from two planar prototypes have been reconstructed and analyzed offline with two clusterization methods: the enter of gravity of the charge distribution and the micro Time Projection Chamber (\muTPC). Concerning the spatial resolution, the charge centroid cluster reconstruction performs extremely well with no magnetic field: the resolution is well below 100 \mum . Increasing the magnetic field intensity, the resolution degrades almost linearly as effect of the Lorentz force that displaces, broadens and asymmetrizes the electron avalanche. Tuning the electric fields of the GEM prototype we could achieve the unprecedented spatial resolution of 190 \mum at 1 Tesla. In order to boost the spatial resolution with strong magnetic field and inclined tracks a \muTPC cluster reconstruction has been investigated. Such a readout mode exploits the good time resolution of the GEM detector and electronics to reconstruct the trajectory of the particle inside the conversion gap. Beside the improvement of the spatial resolution, information on the track angle can be also extracted. The new clustering algorithm has been tested with diagonal tracks with no magnetic field showing a resolution between 100 um and 150 um for the incident angle ranging from 10° to 45° . Studies show similar performance with 1 Tesla magnetic field. This is the first use of a \muTPC readout with a triple GEM detector in magnetic field. This study has shown that a combined readout is capable to guarantee stable performance over a broad spectrum of particle momenta and incident angles, up to a 1 Tesla magnetic field.

physics.ins-det↗

arXiv2017

The Cylindrical GEM Inner Tracker of the BESIII experiment: prototype test beam results

A cylindrical GEM detector is under development, to serve as an upgraded inner tracker at the BESIII spectrometer. It will consist of three layers of cylindrically-shaped triple GEMs surrounding the interaction point. The experiment is taking data at the e+e- collider BEPCII in Beijing (China) and the GEM tracker will be installed in 2018. Tests on the performances of triple GEMs in strong magnetic field have been run by means of the muon beam available in the H4 line of SPS (CERN) with both planar chambers and the first cylindrical prototype. Efficiencies and resolutions have been evaluated using different gains, gas mixtures, with and without magnetic field. The obtained efficiency is 97-98% on single coordinate view, in many operational arrangements. The spatial resolution for planar GEMs has been evaluated with two different algorithms for the position determination: the charge centroid and the micro time projection chamber (mu-TPC) methods. The two modes are complementary and are able to cope with the asymmetry of the electron avalanche when running in magnetic field, and with non-orthogonal incident tracks. With the charge centroid, a resolution lower than 100 micron has been reached without magnetic field and lower than 200 micron with a magnetic field up to 1 T. The mu-TPC mode showed to be able to improve those results. In the first beam test with the cylindrical prototype, the detector had a very good stability under different voltage configurations and particle intensities. The resolution evaluation is in progress.

physics.ins-det↗

arXiv2017

A custom readout electronics for the BESIII CGEM detector

For the upgrade of the inner tracker of the BESIII spectrometer, planned for 2018, a lightweight tracker based on an innovative Cylindrical Gas Electron Multiplier (CGEM) detector is now under development. The analogue readout of the CGEM enables the use of a charge centroid algorithm to improve the spatial resolution to better than 130 um while loosening the pitch strip to 650 um, which allows to reduce the total number of channels to about 10 000. The channels are readout by 160 dedicated integrated 64-channel front-end ASICs, providing a time and charge measurement and featuring a fully-digital output. The energy measurement is extracted either from the time-over-threshold (ToT) or the 10-bit digitisation of the peak amplitude of the signal. The time of the event is generated by quad-buffered low-power TDCs, allowing for rates in excess of 60 kHz per channel. The TDCs are based on analogue interpolation techniques and produce a time stamp (or two, if working in ToT mode) of the event with a time resolution better than 50 ps. The front-end noise, based on a CSA and CR-RC2 shapers, dominate the channel intrinsic time jitter, which is less than 5 ns r.m.s.. The time information of the hit can be used to reconstruct the track path, operating the detector as a small TPC and hence improving the position resolution when the distribution of the cloud, due to large incident angle or magnetic field, is very broad. Event data is collected by an off-detector motherboard, where each GEM-ROC readout card handles 4 ASIC carrier PCBs (512 channels). Configuration upload and data readout between the off-detector electronics and the VME-based data collector cards are managed by bi-directional fibre optical links.

physics.ins-det↗