arXiv subjects

Gang Li

Publications and source records attributed to Gang Li.

At least 685 records · Page 38Linked to original sources

arXiv2015

Emergence of Quantum Nonmagnetic Insulating Phase in Spin-Orbit Coupled Square Lattices

We investigate the metal-insulator transition (MIT) and phase diagram of the half-filled Fermi Hubbard model with Rashba-type spin-orbit coupling (SOC) on a square optical lattice. The interplay between the atomic interactions and SOC results in distinctive features of the MIT. Significantly, in addition to the diverse spin ordered phases, a nonmagnetic insulating phase emerges in a considerably large regime of parameters near the Mott transition. This phase has a finite single-particle gap but vanishing magnetization and spin correlation exhibits a power-law scaling, suggesting a potential algebraic spin-liquid ground state. These results are confirmed by the non-perturbative cluster dynamical mean-field theory.

cond-mat.str-el↗

arXiv2015

First Taste of Hot Channel in Interplanetary Space

Hot channel (HC) is a high temperature ($\sim$10 MK) structure in the inner corona revealed first by Atmospheric Imaging Assembly (AIA) on board \textit{Solar Dynamics Observatory}. Eruption of HC is often associated with flare and coronal mass ejection. Previous studies suggest that HC is a good proxy of magnetic flux rope (MFR) in the inner corona, in addition to another well-known MFR candidate, the prominence-cavity structure that is with a normal coronal temperature ($\sim$1-2 MK). In this paper, we report a high temperature structure (HTS, $\sim$1.5 MK) contained in an interplanetary coronal mass ejection induced by an HC eruption. According to the observations of bidirectional electrons, high temperature and density, strong magnetic field, and its association with the shock, sheath, and plasma pile-up region, we suggest that the HTS is the interplanetary counterpart of the HC. The scale of the measured HTS is around 14 R$_\odot$, and it maintained a much higher temperature than the background solar wind even at 1 AU. It is significantly different from the typical magnetic clouds (MCs), which usually have a much lower temperature. Our study suggests that the existence of a corotating interaction region ahead of the HC formed a magnetic container to inhibit the HC expansion and cooling down to a low temperature.

astro-ph.SR↗

arXiv2015

Hunting for the $X_b$ via hidden bottomonium decays

In this work, we study the isospin conserved hidden bottomonium decay of $X_b\to Υ(1S)ω$, where $X_b$ is taken to be the counterpart of the famous $X(3872)$ in the bottomonium sector as a candidate for the meson-meson molecule. Since it is likely that the $X_b$ is below the $B\bar B^*$ threshold and the mass difference between the neutral and charged bottom meson is small compared to the binding energy of the $X_b$, the isospin violating decay mode $X_b\to Υ(nS)π^+π^-$ would be greatly suppressed. We use the effective Lagrangian based on the heavy quark symmetry to explore the rescattering mechanism of $X_b\to Υ(1S)ω$ and calculate the partial widths. Our results show that the partial width for the $X_b\to Υ(1S)ω$ is about tens of keVs. Taking into account the fact that the total width of $X_b$ may be smaller than a few MeV like $X(3872)$, the calculated branching ratios may reach to orders of $10^{-2}$. These hidden bottomonium decay modes are of great importance in the experimental search for the $X_b$ particularly at the hadron collider. Also, the associated studies of hidden bottomonium decays $X_b \to Υ(nS) γ$, $Υ(nS)ω$, and $B\bar B γ$ may help us investigate the structure of $X_b$ deeply. The experimental observation of $X_b$ will provide us with further insight into the spectroscopy of exotic states and is helpful to probe the structure of the states connected by the heavy quark symmetry.

hep-ph↗

arXiv2015

Triangular Spin-Orbit-Coupled Lattice with Strong Coulomb Correlations: Sn Atoms on a SiC(0001) Substrate

Two-dimensional (2D) atom lattices provide model setups for Coulomb correlations inducing competing ground states, partly with topological character. Hexagonal SiC(0001) is an intriguing wide-gap substrate, spectroscopically separated from the overlayer and hence reduced screening. We report the first study of an artificial high-Z atom lattice on SiC(0001) by Sn adatoms, based on combined experimental realization and theoretical modeling. Density-functional theory of our $\sqrt{3}$-structure model closely reproduces the scanning tunneling microscopy. Instead of metallic behavior, photoemission data show a deeply gapped state (~2 eV gap). Based on our calculations including dynamic mean-field theory, we argue that this reflects a pronounced Mott insulating scenario. We also find indications that the system is susceptible to antiferromagnetic superstructures. Such spin-orbit-coupled correlated heavy atom lattices on SiC(0001) thus form a novel testbed for peculiar quantum states of matter, with potential bearing for spin liquids and topological Mott insulators.

cond-mat.str-el↗

arXiv2014

Investigations on the charmless decays of $Y(4260)$

Apart from the charmful decay channels of $Y(4260)$, the charmless decay channels of $Y(4260)$ also provide us a good platform to study the nature and the decay mechanism of $Y(4260)$. In this paper, we propose to probe the structure of $Y(4260)$ through the charmless decays $Y(4260)\to VP$ via intermediate $D_1\bar D+c.c.$ meson loops, where $V$ and $P$ stand for light vector and pseudoscalar mesons, respectively. Under the molecule ansatz of $Y(4260)$, the predicted total branching ratio $BR_{VP}$ for all $Y(4260)\to VP$ processes are about $(0.34^{+0.32}_{-0.23})%$ to $(0.75^{+0.72}_{-0.52})%$ with the cutoff parameter $α=2\sim 3$. Numerical results show that the intermediate $D_1 \bar D+c.c.$ meson loops may be a possible transition mechanism in the $Y(4260)\to VP$ decays. These predicted branching ratios are the same order to that of $Y(4260) \to Z_c^+(3900) π^-$, which may be an evidence of $D_1D$ molecule and can be examined by the forthcoming BESIII data in the near future.

hep-ph↗

arXiv2014

Status of CSNS H- ion source

A new H^- ion source has been installed successfully and will be used to serve the China Spallation Neutron Source (CSNS). In this paper, we report various components of the ion source, including discharge chamber, temperature, cooling system, extraction electrodes, analyzing magnet, remote control system and so on. Compared to the previous experimental ion source, some improvements have been made to make the ion source more compact and convenient. In the present arrangement, the Penning field is generated by a pair of pole tip extensions on the analyzing magnet instead of by a separate circuit. For the remote control system, F3RP61-2L is applied to the accelerator online control system for the first time. In the running of the ion source, a stable pulse H- beam with a current of 50 mA at an energy of 50 keV is produced. The extraction frequency and pulse width is 25 Hz and 500microsecond, respectively. Furthermore, an emittance scanner has been installed and measurements are in progress.

physics.acc-ph↗

arXiv2014

Imaging and spectroscopic observations of magnetic reconnection and chromospheric evaporation in a solar flare

Magnetic reconnection is believed to be the dominant energy release mechanism in solar flares. The standard flare model predicts both downward and upward outflow plasmas with speeds close to the coronal Alfvén speed. Yet, spectroscopic observations of such outflows, especially the downflows, are extremely rare. With observations of the newly launched Interface Region Imaging Spectrograph (IRIS), we report the detection of greatly redshifted ($\sim$125 km s$^{-1}$ along line of sight) Fe {\sc{xxi}} 1354.08Å emission line with a $\sim$100 km s$^{-1}$ nonthermal width at the reconnection site of a flare. The redshifted Fe {\sc{xxi}} feature coincides spatially with the loop-top X-Ray source observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We interpret this large redshift as the signature of downward-moving reconnection outflow/hot retracting loops. Imaging observations from both IRIS and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) also reveal the eruption and reconnection processes. Fast downward-propagating blobs along these loops are also found from cool emission lines (e.g., Si {\sc{iv}}, O {\sc{iv}}, C {\sc{ii}}, Mg {\sc{ii}}) and images of AIA and IRIS. Furthermore, the entire Fe {\sc{xxi}} line is blueshifted by $\sim$260 km s$^{-1}$ at the loop footpoints, where the cool lines mentioned above all exhibit obvious redshift, a result that is consistent with the scenario of chromospheric evaporation induced by downward-propagating nonthermal electrons from the reconnection site.

astro-ph.SR↗

arXiv2014

Possible role of coronal streamer as magnetically-closed structure in shock-induced energetic electrons and metric type II radio bursts

Two solar type II radio bursts, separated by ~24 hours in time, are examined together. Both events are associated with coronal mass ejections (CMEs) erupting from the same active region (NOAA 11176) beneath a well-observed helmet streamer. We find that the type II emissions in both events ended once the CME/shock fronts passed the white-light streamer tip, which is presumably the magnetic cusp of the streamer. This leads us to conjecture that the closed magnetic arcades of the streamer may play a role in electron acceleration and type II excitation at coronal shocks. To examine such a conjecture, we conduct a test-particle simulation for electron dynamics within a large-scale partially-closed streamer magnetic configuration swept by a coronal shock. We find that the closed field lines play the role of an electron trap, via which the electrons are sent back to the shock front for multiple times, and therefore accelerated to high energies by the shock. Electrons with an initial energy of 300eV can be accelerated to tens of keV concentrating at the loop apex close to the shock front with a counter-streaming distribution at most locations. These electrons are energetic enough to excite Langmuir waves and radio bursts. Considering the fact that most solar eruptions originate from closed field regions, we suggest that the scenario may be important to the generation of more metric type IIs. This study also provides an explanation to the general ending frequencies of metric type IIs at or above 20-30 MHz and the disconnection issue between metric and interplanetary type IIs.

astro-ph.SR↗

arXiv2014

Gap phenomena and curvature estimates for Conformally Compact Einstein Manifolds

In this paper we first use the result in $[12]$ to remove the assumption of the $L^2$ boundedness of Weyl curvature in the gap theorem in $[9]$ and then obtain a gap theorem for a class of conformally compact Einstein manifolds with very large renormalized volume. We also uses the blow-up method to derive curvature estimates for conformally compact Einstein manifolds with large renormalized volume. The second part of this paper is on conformally compact Einstein manifolds with conformal infinities of large Yamabe constants. Based on the idea in $[15]$ we manage to give the complete proof of the relative volume inequality $(1.9)$ on conformally compact Einstein manifolds. Therefore we obtain the complete proof of the rigidity theorem for conformally compact Einstein manifolds in general dimensions with no spin structure assumption (cf. $[29, 15]$) as well as the new curvature pinch estimates for conformally compact Einstein manifolds with conformal infinities of very large Yamabe constant. We also derive the curvature estimates for conformally compact Einstein manifolds with conformal infinities of large Yamabe constant.

math.DG↗

arXiv2014

More hidden heavy quarkonium molecules and their discovery decay modes

To validate the molecular description of the observed $Z_b(10610)/Z_b(10650)$ and $Z_c(3900)/Z_c(4025)$, it is valuable to investigate their counterparts, denoted as $Z_{QV}^{(\prime)}$ in this work, and the corresponding decay modes. In this work, we present an analysis of the $Z_{QV}^{(\prime)}$ using flavor symmetry. We also use the effective Lagrangian based on the heavy quark symmetry to explore the rescattering mechanism and calculate the partial widths for the isospin conserved channels $Z_{QV}^{(\prime)} \to η_Q V$. The predicted partial widths are of an order of MeV for $Z_{QV} \to η_Q V$, which correspond to branching ratios of the order of $10^{-2}\sim 10^{-1}$. For $Z_{QV}^\prime \to η_Q V$, the partial widths are a few hundreds of keV and the branching ratios are about $10^{-3}$. Future experimental measurements can test our predictions on the partial widths and thus examine the molecule description of heavy quarkoniumlike exotic states.

hep-ph↗

arXiv2014

First- and Second-order Fermi Acceleration at Parallel Shocks

We report on a new Monte Carlo method for simulating diffusive shock acceleration (DSA) of solar energetic particles at upstream and downstream regions of quasi-parallel collisionless shock waves under the influence of self-generated turbulence. By way of example, we apply the model to a fast 1500 km \mathrm{s}^{-1} coronal mass ejection at ten solar radii. Results indicate that the maximum energies at outer corona are likely to be limited to few MeV, due to lack of suprathermal protons for appreciable wave growth, and insufficient time required acceleration. We find that the second-order Fermi acceleration, although being a too slow process to have a notable effect at the highest energies, significantly flattens energy spectra at low energy end. Simulations indicate that protons continue to damp waves efficiently several solar radii from the shock in the downstream region, which may be an important mechanism for heating suprathermals. Our simulations also suggest that models assuming a simple isotropic scattering are likely to predict too efficient acceleration.

physics.space-ph↗

arXiv2014

Efficient Regularized Regression for Variable Selection with L0 Penalty

Variable (feature, gene, model, which we use interchangeably) selections for regression with high-dimensional BIGDATA have found many applications in bioinformatics, computational biology, image processing, and engineering. One appealing approach is the L0 regularized regression which penalizes the number of nonzero features in the model directly. L0 is known as the most essential sparsity measure and has nice theoretical properties, while the popular L1 regularization is only a best convex relaxation of L0. Therefore, it is natural to expect that L0 regularized regression performs better than LASSO. However, it is well-known that L0 optimization is NP-hard and computationally challenging. Instead of solving the L0 problems directly, most publications so far have tried to solve an approximation problem that closely resembles L0 regularization. In this paper, we propose an efficient EM algorithm (L0EM) that directly solves the L0 optimization problem. $L_0$EM is efficient with high dimensional data. It also provides a natural solution to all Lp p in [0,2] problems. The regularized parameter can be either determined through cross-validation or AIC and BIC. Theoretical properties of the L0-regularized estimator are given under mild conditions that permit the number of variables to be much larger than the sample size. We demonstrate our methods through simulation and high-dimensional genomic data. The results indicate that L0 has better performance than LASSO and L0 with AIC or BIC has similar performance as computationally intensive cross-validation. The proposed algorithms are efficient in identifying the non-zero variables with less-bias and selecting biologically important genes and pathways with high dimensional BIGDATA.

cs.LG↗

arXiv2014

A solar type II radio burst from CME-coronal ray interaction: simultaneous radio and EUV imaging

Simultaneous radio and extreme ultraviolet (EUV)/white-light imaging data are examined for a solar type II radio burst occurring on 2010 March 18 to deduce its source location. Using a bow-shock model, we reconstruct the 3-dimensional EUV wave front (presumably the type-II emitting shock) based on the imaging data of the two STEREO spacecraft. It is then combined with the Nançay radio imaging data to infer the 3-dimensional position of the type II source. It is found that the type II source coincides with the interface between the CME EUV wave front and a nearby coronal ray structure, providing evidence that the type II emission is physically related to the CME-ray interaction. This result, consistent with those of previous studies, is based on simultaneous radio and EUV imaging data for the first time.

physics.space-ph↗

arXiv2014

Decentralized Subspace Pursuit for Joint Sparsity Pattern Recovery

To solve the problem of joint sparsity pattern recovery in a decen-tralized network, we propose an algorithm named decentralized and collaborative subspace pursuit (DCSP). The basic idea of DCSP is to embed collaboration among nodes and fusion strategy into each iteration of the standard subspace pursuit (SP) algorithm. In DCSP, each node collaborates with several of its neighbors by sharing high-dimensional coefficient estimates and communicates with other remote nodes by exchanging low-dimensional support set estimates. Experimental evaluations show that, compared with several existing algorithms for sparsity pattern recovery, DCSP produces satisfactory results in terms of accuracy of sparsity pattern recovery with much less communication cost.

cs.IT↗

arXiv2014

Hunting for the X_b via Radiative Decays

In this paper, we study radiative decays of X_b, the counterpart of the famous X(3872) in the bottomonium-sector as a candidate for meson-meson molecule, into the γΥ(nS) (n=1, 2, 3). Since it is likely that the X_b is below the B\bar B^* threshold and the mass difference between the neutral and charged bottom meson is small compared to the binding energy of the X_b, the isospin violating decay mode X_b\to Υ(nS)π^+π^- would be greatly suppressed. This will promote the importance of the radiative decays. We use the effective Lagrangian based on the heavy quark symmetry to explore the rescattering mechanism and calculate the partial widths. Our results show that the partial widths into γΥ(nS) are about 1 keV, and thus the branching fractions may be sizeable, considering the fact the total width may also be smaller than a few MeV like the X(3872). These radiative decay modes are of great importance in the experimental search for the X_b particularly at hadron collider. An observation of the X_b will provide a deeper insight into the exotic hadron spectroscopy and is helpful to unravel the nature of the states connected by the heavy quark symmetry.

hep-ph↗

arXiv2014

A solar eruption driven by rapid sunspot rotation

We present the observation of a major solar eruption that is associated with fast sunspot rotation. The event includes a sigmoidal filament eruption, a coronal mass ejection, and a GOES X2.1 flare from NOAA active region 11283. The filament and some overlying arcades were partially rooted in a sunspot. The sunspot rotated at $\sim$10$^\circ$ per hour rate during a period of 6 hours prior to the eruption. In this period, the filament was found to rise gradually along with the sunspot rotation. Based on the HMI observation, for an area along the polarity inversion line underneath the filament, we found gradual pre-eruption decreases of both the mean strength of the photospheric horizontal field ($B_h$) and the mean inclination angle between the vector magnetic field and the local radial (or vertical) direction. These observations are consistent with the pre-eruption gradual rising of the filament-associated magnetic structure. In addition, according to the Non-Linear Force-Free-Field reconstruction of the coronal magnetic field, a pre-eruption magnetic flux rope structure is found to be in alignment with the filament, and a considerable amount of magnetic energy was transported to the corona during the period of sunspot rotation. Our study provides evidences that in this event sunspot rotation plays an important role in twisting, energizing, and destabilizing the coronal filament-flux rope system, and led to the eruption. We also propose that the pre-event evolution of $B_h$ may be used to discern the driving mechanism of eruptions.

astro-ph.SR↗

arXiv2014

Stable Embedding of Grassmann Manifold via Gaussian Random matrices

In this paper, we explore a volume-based stable embedding of multi-dimensional signals based on Grassmann manifold, via Gaussian random measurement matrices. The Grassmann manifold is a topological space in which each point is a linear vector subspace, and is widely regarded as an ideal model for multi-dimensional signals. In this paper, we formulate the linear subspace spanned by multi-dimensional signal vectors as points on the Grassmann manifold, and use the volume and the product of sines of principal angles (also known as the product of principal sines) as the generalized norm and distance measure for the space of Grassmann manifold. We prove a volume-preserving embedding property for points on the Grassmann manifold via Gaussian random measurement matrices, i.e., the volumes of all parallelotopes from a finite set in Grassmann manifold are preserved upon compression. This volume-preserving embedding property is a multi-dimensional generalization of the conventional stable embedding properties, which only concern the approximate preservation of lengths of vectors in certain unions of subspaces. Additionally, we use the volume-preserving embedding property to explore the stable embedding effect on a generalized distance measure of Grassmann manifold induced from volume. It is proved that the generalized distance measure, i.e., the product of principal sines between different points on the Grassmann manifold, is well preserved in the compressed domain via Gaussian random measurement matrices.Numerical simulations are also provided for validation.

cs.IT↗

arXiv2014

Glueballs in charmonia radiative decays

Scalar \cite{scalar_paper} and tensor \cite{tensor_paper} glueballs created in $J/ψ$ radiative decays are studied in quenched lattice QCD. Using two anisotropic lattices to approach the continuum limit, we compute the relevant form factors responsible for the decay rates for $J/ψ\rightarrowγG_{0^{++}}$ and $J/ψ\rightarrowγG_{2^{++}}$. Comparing with the existing experimental data, it is argued that $f_0(1710)$ is a favorable candidate for scalar glueball. The decay rate for $J/ψ\rightarrowγG_{2^{++}}$ is found to be quite substantial. A comprehensive search in the tensor channel on BESIII is therefore suggested.

hep-lat↗