arXiv subjects

Z. Li

Publications and source records attributed to Z. Li.

At least 199 records · Page 11Linked to original sources

arXiv2017

Can the temperature of Ellerman Bombs be more than 10000K?

Ellerman bombs (EBs) are small brightening events in the solar lower atmosphere. By original definition, the main EB's characteristic is the two emission bumps in both wings of chromospheric lines, such as H$α$ and Ca II 8542 Å lines. Up to now, most authors found that the temperature increase of EBs around the temperature minimum region is in the range of 600K-3000K. However, with recent IRIS observations, some authors proposed that the temperature increase of EBs could be more than 10000K. Using non-LTE semi-empirical modeling, we investigate the line profiles, continuum emission and the radiative losses for the EB models with different temperature increases, and compare them with observations. Our result indicates that if the EB maximum temperature attains more than 10000K around the temperature minimum region, then the resulted H$α$ and Ca II 8542 Å line profiles and the continuum emission would be much stronger than that of EB observations. Moreover, due to the high radiative losses, the high temperature EB would have a very short lifetime, which is not comparable with the observations. Thus, our study does not support the proposal that the EB temperatures are higher than 10000K.

astro-ph.SR↗

arXiv2017

Design and expected performance of the MICE demonstration of ionization cooling

Muon beams of low emittance provide the basis for the intense, well-characterised neutrino beams necessary to elucidate the physics of flavour at a neutrino factory and to provide lepton-antilepton collisions at energies of up to several TeV at a muon collider. The international Muon Ionization Cooling Experiment (MICE) aims to demonstrate ionization cooling, the technique by which it is proposed to reduce the phase-space volume occupied by the muon beam at such facilities. In an ionization-cooling channel, the muon beam passes through a material in which it loses energy. The energy lost is then replaced using RF cavities. The combined effect of energy loss and re-acceleration is to reduce the transverse emittance of the beam (transverse cooling). A major revision of the scope of the project was carried out over the summer of 2014. The revised experiment can deliver a demonstration of ionization cooling. The design of the cooling demonstration experiment will be described together with its predicted cooling performance.

physics.acc-ph↗

arXiv2016

Robust topological insulator surface state in MBE grown (Bi_{1-x}Sb_x)_2Se_3

(Bi1-xSbx)2Se3 thin films have been prepared using molecular beam epitaxy (MBE). We demonstrate the angle-resolved photoemission spectroscopy (ARPES) and transport evidence for the existence of strong and robust topological surface states in this ternary system. Large tunability in transport properties by varying the Sb doping level has also been observed, where insulating phase could be achieved at x=0.5. Our results reveal the potential of this system for the study of tunable topological insulator and metal-insulator transition based device physics.

cond-mat.mes-hall↗

arXiv2016

Charge fluctuations and nodeless superconductivity in quasi-one-dimensional Ta$_{4}$Pd$_{3}$Te$_{16}$ revealed by $^{125}$Te-NMR and $^{181}$Ta-NQR

We report $^{125}$Te nuclear magnetic resonance and $^{181}$Ta nuclear quadrupole resonance studies on single-crystal Ta$_{4}$Pd$_{3}$Te$_{16}$, which has a quasi-one-dimensional structure and superconducts below $T_{\rm c}=4.3$ K. $^{181}$Ta with spin $I=7/2$ is sensitive to quadrupole interactions, while $^{125}$Te with spin $I=1/2$ can only relax by magnetic interactions. By comparing the spin-lattice relaxation rate ( $1/T_{1}$) of $^{181}$Ta and $^{125}$Te, we found that electric-field-gradient (EFG) fluctuations develop below $80$ K. The EFG fluctuations are enhanced with decreasing temperature due to the fluctuations of a charge density wave that sets in at $T_{\rm CDW}=20$ K, below which the spectra are broadened and $1/T_{1}T$ drops sharply. In the superconducting state, $1/T_{1}$ shows a Hebel-Slichter coherence peak just below $T_{\rm c}$ for $^{125}$Te, indicating that Ta$_{4}$Pd$_{3}$Te$_{16}$ is a full-gap superconductor without nodes in the gap function. The coherence peak is absent in the $1/T_{1}$ of $^{181}$Ta due to the strong EFG fluctuations.

cond-mat.supr-con↗

arXiv2016

Thermodynamic Anomaly Above the Superconducting Critical Temperature in the Quasi One-Dimensional Superconductor Ta$_4$Pd$_3$Te$_{16}$

We study the intrinsic transport anisotropy and fermiology of the quasi one-dimensional superconductor Ta$_4$Pd$_3$Te$_{16}$. Below $T^*=20$ K we detect a thermodynamic phase transition that predominantly affects the conductivity perpendicular to the quasi one-dimensional chains, consistent with a thermodynamic transition related to the presence of charge order that precedes superconductivity. Remarkably the Fermi surface pockets detected by de Haas-van Alphen (dHvA) oscillations are unaffected by this transition, suggesting that the ordered state does not break any translational symmetries but rather alters the scattering of the quasiparticles themselves.

cond-mat.supr-con↗

arXiv2016

The Deep and Transient Universe in the SVOM Era: New Challenges and Opportunities - Scientific prospects of the SVOM mission

To take advantage of the astrophysical potential of Gamma-Ray Bursts (GRBs), Chinese and French astrophysicists have engaged the SVOM mission (Space-based multi-band astronomical Variable Objects Monitor). Major advances in GRB studies resulting from the synergy between space and ground observations, the SVOM mission implements space and ground instrumentation. The scientific objectives of the mission put a special emphasis on two categories of GRBs: very distant GRBs at z$>$5 which constitute exceptional cosmological probes, and faint/soft nearby GRBs which allow probing the nature of the progenitors and the physics at work in the explosion. These goals have a major impact on the design of the mission: the on-board hard X-ray imager is sensitive down to 4 keV and computes on line image and rate triggers, and the follow-up telescopes on the ground are sensitive in the NIR. At the beginning of the next decade, SVOM will be the main provider of GRB positions and spectral parameters on very short time scale. The SVOM instruments will operate simultaneously with a wide range of powerful astronomical devices. This rare instrumental conjunction, combined with the relevance of the scientific topics connected with GRB studies, warrants a remarkable scientific return for SVOM. In addition, the SVOM instrumentation, primarily designed for GRB studies, composes a unique multi-wavelength observatory with rapid slew capability that will find multiple applications for the whole astronomy community beyond the specific objectives linked to GRBs. This report lists the scientific themes that will benefit from observations made with SVOM, whether they are specific GRB topics, or more generally all the issues that can take advantage of the multi-wavelength capabilities of SVOM.

astro-ph.IM↗

arXiv2016

A Database for Storing the Results of Material Radiopurity Measurements

Searches for rare nuclear processes, such as neutrinoless double beta-decay and the interactions of WIMP dark matter, are motivating experiments with ever-decreasing levels of radioactive backgrounds. These background reductions are achieved using various techniques, but amongst the most important is minimizing radioactive contamination in the materials from which the experiment is constructed. To this end there have been decades of advances in material sourcing, manufacture and certification, during which researchers have accumulated many thousands of measurements of material radiopurity. Some of these assays are described in publications, others are in databases, but many are still communicated informally. Until this work, there has been no standard format for encoding assay results and no effective, central location for storing them. The aim of this work is to address these long-standing problems by creating a concise and flexible material assay data format and powerful software application to manipulate it. A public installation of this software, available at http://www.radiopurity.org, is the largest database of assay results ever compiled and is intended as a long-term repository for the community's data.

physics.ins-det↗

arXiv2016

Measurements of the atmospheric neutrino flux by Super-Kamiokande: energy spectra, geomagnetic effects, and solar modulation

A comprehensive study on the atmospheric neutrino flux in the energy region from sub-GeV up to several TeV using the Super-Kamiokande water Cherenkov detector is presented in this paper. The energy and azimuthal spectra of the atmospheric $ν_e+{\barν}_e$ and $ν_μ+{\barν}_μ$ fluxes are measured. The energy spectra are obtained using an iterative unfolding method by combining various event topologies with differing energy responses. The azimuthal spectra depending on energy and zenith angle, and their modulation by geomagnetic effects, are also studied. A predicted east-west asymmetry is observed in both the $ν_e$ and $ν_μ$ samples at 8.0 σ and 6.0 σ significance, respectively, and an indication that the asymmetry dipole angle changes depending on the zenith angle was seen at the 2.2 σ level. The measured energy and azimuthal spectra are consistent with the current flux models within the estimated systematic uncertainties. A study of the long-term correlation between the atmospheric neutrino flux and the solar magnetic activity cycle is also performed, and a weak indication of a correlation was seen at the 1.1 σ level, using SK I-IV data spanning a 20 year period. For particularly strong solar activity periods known as Forbush decreases, no theoretical prediction is available, but a deviation below the typical neutrino event rate is seen at the 2.4 σ level.

hep-ex↗

arXiv2016

Search for Neutrinos in Super-Kamiokande associated with Gravitational Wave Events GW150914 and GW151226

We report the results from a search in Super-Kamiokande for neutrino signals coincident with the first detected gravitational wave events, GW150914 and GW151226, using a neutrino energy range from 3.5 MeV to 100 PeV. We searched for coincident neutrino events within a time window of $\pm$500 seconds around the gravitational wave detection time. Four neutrino candidates are found for GW150914 and no candidates are found for GW151226. The remaining neutrino candidates are consistent with the expected background events. We calculated the 90\% confidence level upper limits on the combined neutrino fluence for both gravitational wave events, which depends on event energy and topologies. Considering the upward going muon data set (1.6 GeV - 100 PeV) the neutrino fluence limit for each gravitational wave event is 14 - 37 (19 - 50) cm$^{-2}$ for muon neutrinos (muon antineutrinos), depending on the zenith angle of the event. In the other data sets, the combined fluence limits for both gravitational wave events range from 2.4$\times 10^{4}$ to 7.0$\times 10^{9}$ cm$^{-2}$.

astro-ph.HE↗

arXiv2016

Solar Neutrino Measurements in Super-Kamiokande-IV

Upgraded electronics, improved water system dynamics, better calibration and analysis techniques allowed Super-Kamiokande-IV to clearly observe very low-energy 8B solar neutrino interactions, with recoil electron kinetic energies as low as 3.49 MeV. Super-Kamiokande-IV data-taking began in September of 2008; this paper includes data until February 2014, a total livetime of 1664 days. The measured solar neutrino flux is (2.308+-0.020(stat.) + 0.039-0.040(syst.)) x 106/(cm2sec) assuming no oscillations. The observed recoil electron energy spectrum is consistent with no distortions due to neutrino oscillations. An extended maximum likelihood fit to the amplitude of the expected solar zenith angle variation of the neutrino-electron elastic scattering rate in SK-IV results in a day/night asymmetry of (-3.6+-1.6(stat.)+-0.6(syst.))%. The SK-IV solar neutrino data determine the solar mixing angle as sin2 theta_12 = 0.327+0.026-0.031, all SK solar data (SK-I, SK-II, SK III and SKIV) measures this angle to be sin2 theta_12 = 0.334+0.027-0.023, the determined mass-squared splitting is Delta m2_21 = 4.8+1.5-0.8 x10-5 eV2.

hep-ex↗

arXiv2016

First simultaneous microlensing observations by two space telescopes: $Spitzer$ & $Swift$ reveal a brown dwarf in event OGLE-2015-BLG-1319

Simultaneous observations of microlensing events from multiple locations allow for the breaking of degeneracies between the physical properties of the lensing system, specifically by exploring different regions of the lens plane and by directly measuring the "microlens parallax". We report the discovery of a 30-55$M_J$ brown dwarf orbiting a K dwarf in microlensing event OGLE-2015-BLG-1319. The system is located at a distance of $\sim$5 kpc toward the Galactic bulge. The event was observed by several ground-based groups as well as by $Spitzer$ and $Swift$, allowing the measurement of the physical properties. However, the event is still subject to an 8-fold degeneracy, in particular the well-known close-wide degeneracy, and thus the projected separation between the two lens components is either $\sim$0.25 AU or $\sim$45 AU. This is the first microlensing event observed by $Swift$, with the UVOT camera. We study the region of microlensing parameter space to which $Swift$ is sensitive, finding that while for this event $Swift$ could not measure the microlens parallax with respect to ground-based observations, it can be important for other events. Specifically, for detecting nearby brown dwarfs and free-floating planets in high magnification events.

astro-ph.EP↗

arXiv2016

ASCR/HEP Exascale Requirements Review Report

This draft report summarizes and details the findings, results, and recommendations derived from the ASCR/HEP Exascale Requirements Review meeting held in June, 2015. The main conclusions are as follows. 1) Larger, more capable computing and data facilities are needed to support HEP science goals in all three frontiers: Energy, Intensity, and Cosmic. The expected scale of the demand at the 2025 timescale is at least two orders of magnitude -- and in some cases greater -- than that available currently. 2) The growth rate of data produced by simulations is overwhelming the current ability, of both facilities and researchers, to store and analyze it. Additional resources and new techniques for data analysis are urgently needed. 3) Data rates and volumes from HEP experimental facilities are also straining the ability to store and analyze large and complex data volumes. Appropriately configured leadership-class facilities can play a transformational role in enabling scientific discovery from these datasets. 4) A close integration of HPC simulation and data analysis will aid greatly in interpreting results from HEP experiments. Such an integration will minimize data movement and facilitate interdependent workflows. 5) Long-range planning between HEP and ASCR will be required to meet HEP's research needs. To best use ASCR HPC resources the experimental HEP program needs a) an established long-term plan for access to ASCR computational and data resources, b) an ability to map workflows onto HPC resources, c) the ability for ASCR facilities to accommodate workflows run by collaborations that can have thousands of individual members, d) to transition codes to the next-generation HPC platforms that will be available at ASCR facilities, e) to build up and train a workforce capable of developing and using simulations and analysis to support HEP scientific research on next-generation systems.

physics.comp-ph↗

arXiv2015

Two successive magneto-structural transformations and their relation to enhanced magnetocaloric effect for Ni55.3Mn19.7Ga25 Heusler alloy

In the present work, two successive magneto-structural transformations (MSTs) consisting of martensitic and intermartensitic transitions have been reported in polycrystalline Ni55.8Mn18.1Ga26.1 Heusler alloy. Benefiting from the additional latent heat contributed from intermediate phase, this alloy exhibits a large transition entropy change ΔStr with the value of ~28 J/kg K. Moreover, the magnetocaloric effect (MCE) has been also evaluated in terms of Maxwell relation. For the magnetic field change of 3 T, it is found that the calculated value of refrigeration capacity for Ni55.8Mn18.1Ga26.1 attains to ~72 J/kg around room temperature, which significantly surpasses those obtained in many Ni-Mn based Heusler alloys. The mechanism underlying the enhanced MCE is believed to be responsible for these multiple transformations, which can sustain the pronounced isothermal entropy change ΔST over a relatively wide temperature interval.

cond-mat.mtrl-sci↗

arXiv2015

Search for Nucleon and Dinucleon Decays with an Invisible Particle and a Charged Lepton in the Final State at the Super-Kamiokande Experiment

Search results for nucleon decays $p \rightarrow e^+X$, $p \rightarrow μ^+X$, $n \rightarrow νγ$ (where $X$ is an invisible, massless particle) as well as dinucleon decays $np \rightarrow e^+ν$, $np \rightarrow μ^+ν$ and $np \rightarrow τ^+ν$ in the Super-Kamiokande experiment are presented. Using single-ring data from an exposure of 273.4 kton $\cdot$ years, a search for these decays yields a result consistent with no signal. Accordingly, lower limits on the partial lifetimes of $τ_{p \rightarrow e^+X} > 7.9 \times 10^{32}$ years, $τ_{p \rightarrow μ^+X} > 4.1 \times 10^{32}$ years, $τ_{n \rightarrow νγ} > 5.5 \times 10^{32}$ years, $τ_{np \rightarrow e^+ν} > 2.6 \times 10^{32}$ years, $τ_{np \rightarrow μ^+ν} > 2.2 \times 10^{32}$ years and $τ_{np \rightarrow τ^+ν} > 2.9 \times 10^{31}$ years at a $90 \% $ confidence level are obtained. Some of these searches are novel.

hep-ex↗

arXiv2015

Configuration Mixing of Quark States in Baryons

Mixing angles are used to describe the $SU(6)\otimes O(3)$ symmetry breaking in $\left[70,~1^-\right]$ multiplet in the sector of the lowest mass nucleon resonances, which are investigated extensively in constituent quark models for baryon spectroscopy. The transition amplitudes for the meson photoproduction off nucleon can also be expressed in terms of the mixing angles to take into account the configuration mixing. Those amplitudes are derived as a function of the mixing angles between $|N^2P_M{J}^-\rangle$ and $|N^4P_M{J}^-\rangle$ states, with $J$=1/2 and 3/2, for the processes $γp \to ηp,~K^+ Λ,~K^+ Σ^\circ,~K^\circ Σ^+$. The present status of our knowledge on the mixing angles between $S_{11}(1535)$ and $S_{11}(1650)$ ($θ_{S}$), as well as between $D_{13}(1520)$ and $D_{13} (1700)$ ($θ_{D}$) is reported. Since these resonances play very important role in the threshold region for both $η$ and kaon production mechanisms, they are expected to provide crucial tests of different quark models for the baryon spectroscopy.

hep-ph↗

arXiv2015

Diagnostics of Ellerman Bombs with High-resolution Spectral Data

Ellerman bombs (EBs) are tiny brightenings often observed near sunspots. The most impressive characteristic of the EB spectra is the two emission bumps in both wings of the H$α$ and \ion{Ca}{II} 8542 Å lines. High-resolution spectral data of three small EBs were obtained on 2013 June 6 with the largest solar telescope, the 1.6 meter New Solar Telescope (NST), at the Big Bear Solar Observatory. The characteristics of these EBs are analyzed. The sizes of the EBs are in the range of 0.3\arcsec\--0.8\arcsec\ and their durations are only 3--5 minutes. Our semi-empirical atmospheric models indicate that the heating occurs around the temperature minimum region with a temperature increase of 2700--3000 K, which is surprisingly higher than previously thought. The radiative and kinetic energies are estimated to be as high as 5$\times$10$^{25}$--3.0$\times$10$^{26}$ ergs despite the small size of these EBs. Observations of the magnetic field show that the EBs appeared just in a parasitic region with mixed polarities and accompanied by mass motions. Nonlinear force-free field extrapolation reveals that the three EBs are connected with a series of magnetic field lines associated with bald patches, which strongly implies that these EBs should be produced by magnetic reconnection in the solar lower atmosphere. According to the lightcurves and the estimated magnetic reconnection rate, we propose that there is a three phase process in EBs: pre-heating, flaring and cooling phases.

astro-ph.SR↗

arXiv2014

Mass predictions of superheavy nuclei from the systematics of $α$-Decay Energies

A recent proposed method for $α$-decay energies ($Q_α$) [J.M. Dong, W. Zuo, and W. Scheid, Phys. Rev. Lett. \textbf{107}, 012501 (2011)] can reproduce experimental data of superheavy nuclei (SHN) with an $rms$-value of less than 100 keV. However, a sinusoid-like periodic deviation from experiments, which limits the accuracy in predictions, is observed when using different reference nuclei. In this paper, we have further extended this hybrid method, i.e., to predict $Q_α$ of the as-yet-unobserved SHN with the help of known nuclei. It is found that the systematic deviation in previous study is rooted in the nuclear mass model employed. By further analyzing the source of errors, different nuclear mass models are evaluated based on the same procedure.

nucl-th↗

arXiv2014

Study of the $\vec{p}d \to n\{pp\}_{s}$ charge-exchange reaction using a polarised deuterium target

The vector and tensor analysing powers, $A_y$ and $A_{yy}$, of the $\vec{p}d \to n\{pp\}_{s}$ charge-exchange reaction have been measured at a beam energy of 600 MeV at the COSY-ANKE facility by using an unpolarised proton beam incident on an internal storage cell target filled with polarised deuterium gas. The low energy recoiling protons were measured in a pair of silicon tracking telescopes placed on either side of the target. Putting a cut of 3 MeV on the diproton excitation energy ensured that the two protons were dominantly in the $^{1}S_{0}$ state, here denoted by $\{pp\}_{s}$. The polarisation of the deuterium gas was established through measurements in parallel of proton-deuteron elastic scattering. By analysing events where both protons entered the same telescope, the charge-exchange reaction was measured for momentum transfers $q\geq 160$ MeV/$c$. These data provide a good continuation of the earlier results at $q\leq 140$ MeV/$c$ obtained with a polarised deuteron beam. They are also consistent with impulse approximation predictions with little sign evident for any modifications due to multiple scatterings.

nucl-ex↗