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Gang Li

Publications and source records attributed to Gang Li.

At least 721 records · Page 40Linked to original sources
arXiv2012

Probing scalar meson structures in $χ_{c1}$ decays into pseudoscalar and scalar

We evaluate the decay branching ratios of $χ_{c1}\to PS$, in a quark model parametrization scheme, where $P$ and $S$ stand for pseudoscalar and scalar meson, respectively. An interesting feature of this decay process is that the $c\bar{c}$ annihilate via the pQCD hair-pin diagram is supposed to be dominant. Hence, this decay process should be sensitive to the quark components of the final-state light mesons, and would provide a great opportunity for testing the mixing relations among the scalar mesons, i.e. $f_0(1370)$, $f_0(1500)$ and $f_0(1710)$, by tagging the final state pseudoscalar mesons.

hep-ph↗
arXiv2012

The nature of X(3900) and recognition of open charm effects

We identify open charm effects in a direct production process $e^+e^-\to J/ψπ^0$. A unique feature of this process is that the $D\bar{D}^*+c.c.$ threshold is located at a relatively isolated energy region, i.e. $\sim 3.876 \ \mathrm{GeV}$, which is far away from the well-established charmonia $ψ(3770)$ and $ψ(4040)$. Therefore, the cross section line-shape of this reaction provides an opportunity for singling out the open charm effects. A model-independent narrow enhancement between the thresholds of $D^0\bar{D}^{*0}+c.c.$ and $D^+D^{*-}+c.c.$ is predicted. This study can also help understand the X(3900) enhancement recently observed by the Belle and BaBar Collaboration in $e^+e^-\to D\bar{D}+c.c.$ We also show that the open charm effects play a crucial role for our understanding of the long-standing "$ρπ$ puzzle".

hep-ph↗
arXiv2012

Fast Algorithms for Sparse Recovery with Perturbed Dictionary

In this paper, we account for approaches of sparse recovery from large underdetermined linear models with perturbation present in both the measurements and the dictionary matrix. Existing methods have high computation and low efficiency. The total least-squares (TLS) criterion has well-documented merits in solving linear regression problems while FOCal Underdetermined System Solver (FOCUSS) has low-computation complexity in sparse recovery. Based on TLS and FOCUSS methods, the present paper develops more fast and robust algorithms, TLS-FOCUSS and SD-FOCUSS. TLS-FOCUSS algorithm is not only near-optimum but also fast in solving TLS optimization problems under sparsity constraints, and thus fit for large scale computation. In order to reduce the complexity of algorithm further, another suboptimal algorithm named D-FOCUSS is devised. SD-FOCUSS can be applied in MMV (multiple-measurement-vectors) TLS model, which fills the gap of solving linear regression problems under sparsity constraints. The convergence of TLS-FOCUSS algorithm and SD-FOCUSS algorithm is established with mathematical proof. The simulations illustrate the advantage of TLS-FOCUSS and SD-FOCUSS in accuracy and stability, compared with other algorithms.

cs.IT↗
arXiv2012

Open charm effects in the explanation of the long-standing "$ρπ$ puzzle"

A detailed analysis of the open charm effects on the decays of $J/ψ(ψ^\prime)\to VP$ is presented, where $V$ stands for light vector meson and $P$ for light pseudoscalar meson. These are the channels that the so-called "12% rule" of perturbative QCD (pQCD) is obviously violated. Nevertheless, they are also the channels that violate the pQCD helicity selection rule (HSR) at leading order. In this work, we put constraints on the electromagnetic (EM) contribution, short-distance contribution from the $c\bar{c}$ annihilation at the wavefunction origin, and long-distance contribution from the open charm threshold effects on these two decays. We show that interferences among these amplitudes, in particular, the destructive interferences between the short-distance and long-distance strong amplitudes play a key role to evade the HSR and cause the significant deviations from the pQCD expected "12% rule".

hep-ph↗
arXiv2012

A broken solar type II radio burst induced by a coronal shock propagating across the streamer boundary

We discuss an intriguing type II radio burst that occurred on 2011 March 27. The dynamic spectrum was featured by a sudden break at about 43 MHz on the well-observed harmonic branch. Before the break, the spectrum drifted gradually with a mean rate of about -0.05 MHz/s. Following the break, the spectrum jumped to lower frequencies. The post-break emission lasted for about three minutes. It consisted of an overall slow drift which appeared to have a few fast drift sub-bands. Simultaneous observations from the Solar TErrestrial RElations Observatory (STEREO) and the Solar Dynamics Observatory (SDO) were also available and are examined for this event. We suggest that the slow-drift period before the break was generated inside a streamer by a coronal eruption driven shock, and the spectral break as well as the relatively wide spectrum after the break is a consequence of the shock crossing the streamer boundary where density drops abruptly. It is suggested that this type of radio bursts can be taken as a unique diagnostic tool for inferring the coronal density structure, as well as the radio emitting source region.

astro-ph.SR↗
arXiv2012

Efficient treatment of the high-frequency tail of the self-energy function and its relevance for multi-orbital models

In this paper, we present an efficient and stable method to determine the one-particle Green's function in the hybridization-expansion continuous-time (CT-HYB) quantum Monte Carlo method, within the framework of the dynamical mean-field theory. The high-frequency tail of the impurity self-energy is replaced with a noise-free function determined by a dual-expansion around the atomic limit. This method does not depend on the explicit form of the interaction term. More advantageous, it does not introduce any additional numerical cost to the CT-HYB simulation. We discuss the symmetries of the two-particle vertex, which can be used to optimize the simulation of the four-point correlation functions in the CT-HYB. Here, we adopt it to accelerate the dual-expansion calculation, which turns out to be especially suitable for the study of material systems with complicated band structures. As an application, a two-orbital Anderson impurity model with a general on-site interaction form is studied. The phase diagram is extracted as a function of the Coulomb interactions for two different Hund's coupling strengths. In the presence of the hybridization between different orbitals, for smaller interaction strengths, this model shows a transition from metal to band-insulator. Increasing the interaction strengths, this transition is replaced by a crossover from Mott insulator to band-insulator behavior.

cond-mat.str-el↗
arXiv2011

Magnetic-Order Induced Spectral-Weight Redistribution in a Triangular Surface System

The Sn-induced $\sqrt3 \times \sqrt3$ surface reconstruction on Si(111) has been investigated by material-specific many-body calculations and by angle-resolved photoelectron spectroscopy (ARPES). This triangular surface system in the low adatom coverage regime is governed by rather localized dangling bond orbitals with enhanced electronic correlations and it is prone to exhibit magnetic frustration. We find a rather good overall agreement of the spectral function and its temperature-dependence between theory and experiment. Changes in the ARPES band topology in comparison to the density functional calculations can be explained as a spectral weight redistribution with respect to an additional symmetry which is not due to any geometrical change. This new symmetry corresponds to a magnetic order, which is found to be more complex than the canonical $120^{\circ}$ anti-ferromagnetic order on a triangular lattice with nearest-neighbor coupling only.

cond-mat.str-el↗
arXiv2011

Revisit the radiative decays of $J/ψ$ and $ψ'\to γη_c (γη_c^\prime)$

With the new measurements of $J/ψ$ and $ψ^\prime\to γη_c \ (γη_c^\prime)$ from CLEO and BES-III Collaboration, we re-investigate the intermediate meson loop (IML) contributions to these radiative decays in association with the quark model M1 transitions in an effective Lagrangian approach. It shows that the "unquenched" effects due to the intermediate hadron loops can be better quantified by the new data for $J/ψ\to γη_c$. Although the IML contributions are relatively small in $J/ψ\to γη_c$, they play a crucial role in $ψ^\prime\to γη_c \ (γη_c^\prime)$. A prediction for the IML contributions to $ψ(3770)\to γη_c \ (γη_c^\prime)$ is made. Such "unquenched" effects allow us to reach a coherent description of those three radiative transitions, and gain some insights into the underlying dynamics.

hep-ph↗
arXiv2011

A Statistical Study on the Morphology of Rays and Dynamics of Blobs in the Wake of Coronal Mass Ejections

In this paper, with a survey through the Large Angle and Spectrometric Coronagraph (LASCO) data from 1996 to 2009, we present 11 events with plasma blobs flowing outwards sequentially along a bright coronal ray in the wake of a coronal mass ejection. The ray is believed to be associated with the current sheet structure that formed as a result of solar eruption, and the blobs are products of magnetic reconnection occurring along the current sheet. The ray morphology and blob dynamics are investigated statistically. It is found that the apparent angular widths of the rays at a fixed time vary in a range of 2.1-6.6 (2.0-4.4) degrees with an average of 3.5 (2.9) degrees at 3 (4) Rs, respectively, and the observed durations of the events vary from 12 h to a few days with an average of 27 h. It is also found, based on the analysis of blob motions, that 58% (26) of the blobs were accelerated, 20% (9) were decelerated, and 22% (10) moved with a nearly-constant speed. Comparing the dynamics of our blobs and those that are observed above the tip of a helmet streamer, we find that the speeds and accelerations of the blobs in these two cases differ significantly. It is suggested that these differences of the blob dynamics stem from the associated magnetic reconnection involving different magnetic field configurations and triggering processes.

astro-ph.SR↗
arXiv2011

Analysis of Block OMP using Block RIP

Orthogonal matching pursuit (OMP) is a canonical greedy algorithm for sparse signal reconstruction. When the signal of interest is block sparse, i.e., it has nonzero coefficients occurring in clusters, the block version of OMP algorithm (i.e., Block OMP) outperforms the conventional OMP. In this paper, we demonstrate that a new notion of block restricted isometry property (Block RIP), which is less stringent than standard restricted isometry property (RIP), can be used for a very straightforward analysis of Block OMP. It is demonstrated that Block OMP can exactly recover any block K-sparse signal in no more than K steps if the Block RIP of order K+1 with a sufficiently small isometry constant is satisfied. Using this result it can be proved that Block OMP can yield better reconstruction properties than the conventional OMP when the signal is block sparse.

cs.IT↗
arXiv2011

SAR Imaging of Moving Targets via Compressive Sensing

An algorithm based on compressive sensing (CS) is proposed for synthetic aperture radar (SAR) imaging of moving targets. The received SAR echo is decomposed into the sum of basis sub-signals, which are generated by discretizing the target spatial domain and velocity domain and synthesizing the SAR received data for every discretized spatial position and velocity candidate. In this way, the SAR imaging problem is converted into sub-signal selection problem. In the case that moving targets are sparsely distributed in the observed scene, their reflectivities, positions and velocities can be obtained by using the CS technique. It is shown that, compared with traditional algorithms, the target image obtained by the proposed algorithm has higher resolution and lower side-lobe while the required number of measurements can be an order of magnitude less than that by sampling at Nyquist sampling rate. Moreover, multiple targets with different speeds can be imaged simultaneously, so the proposed algorithm has higher efficiency.

cs.IT↗
arXiv2011

Effect of charmed meson loops on charmonium transitions

The effects of intermediate charmed mesons on charmonium transitions with the emission of one pion or eta are studied systematically. Based on a non-relativistic effective field theory we show that charmed meson loops are enhanced compared to the corresponding tree-level contributions for transitions between two S-wave charmonia as well as for transitions between two P-wave charmonia. On the contrary, for the transitions between one S-wave and one P-wave charmonium state, the loops need to be analyzed case by case and often appear to be suppressed. The relation to and possible implications for an effective Lagrangian approach are also discussed. This study at the same time provides a cross check for the numerical evaluations.

hep-ph↗
arXiv2010

Elimination of degenerate trajectory of single atom strongly coupled to the tilted cavity TEM10 mode

We demonstrate the trajectory measurement of the single neutral atoms deterministically using a high-finesse optical micro-cavity. Single atom strongly couples to the high-order transverse vacuum TEM_{10} mode, instead of the usual TEM_{00} mode, and the parameter of the system is (g_{10},κ,γ)=2π\times (20.5,2.6,2.6)MHz. The atoms simply fall down freely from the magneto-optic trap into the cavity modes and the trajectories of the single atoms are linear. The transmission spectrums of atoms passing through the TEM10 mode are detected by a single photon counting modules and well fitted. Thanks to the tilted cavity transverse TEM10 mode, which is inclined to the vertical direction about 45 degrees and it helps us, for the first time, to eliminate the degenerate trajectory of the single atom falling through the cavity and get the unique atom trajectory. Atom position with high precision of 0.1μm in the off-axis direction (axis y) is obtained, and the spatial resolution of 5.6μm is achieved in time of 10μs along the vertical direction (axis x). The average velocity of the atoms is also measured from the atom transits, which determines the temperature of the atoms in magneto-optic trap, 186μK {\pm} 19μK.

quant-ph↗
arXiv2010

$B_s\to D_s(3040)$ form factors and $B_s$ decays into $D_s(3040)$

Under the assignment of $D_s(3040)$ as a radially excited p-wave $\bar cs$ state with $J^P=1^+$, we compute the $B_s\to D_s (3040)$ form factors within the covariant light-front quark model. Two classification schemes for the p-wave $\bar cs$ meson are adopted. We also use our results to predict the branching ratios (BRs), polarization fractions and angular asymmetries in semileptonic $B_s\to D_s(3040)\ell\barν(\ell=e,μ,τ)$. The BRs are found to have the order of $10^{-3}$ for $\ell=e,μ$ and $10^{-5}$ for $\ell=τ$. We find that the polarization fractions and the angular asymmetries could be useful to pin down the ambiguities of the quantum numbers for $D_s(3040)$. In addition, we investigate the nonleptonic $B_s\to D_s(3040)M$ decays under the factorization method, where $M$ denotes a charged pseudoscalar or a vector meson. The BRs of $B_s\to D_s(3040)ρ$ and $B_s\to D_s(3040)D_s^*$ reach the order of $10^{-3}$, while the other channels are typically smaller by 1-2 orders.

hep-ph↗
arXiv2010

Airborne Radar STAP using Sparse Recovery of Clutter Spectrum

Space-time adaptive processing (STAP) is an effective tool for detecting a moving target in spaceborne or airborne radar systems. Statistical-based STAP methods generally need sufficient statistically independent and identically distributed (IID) training data to estimate the clutter characteristics. However, most actual clutter scenarios appear only locally stationary and lack sufficient IID training data. In this paper, by exploiting the intrinsic sparsity of the clutter distribution in the angle-Doppler domain, a new STAP algorithm called SR-STAP is proposed, which uses the technique of sparse recovery to estimate the clutter space-time spectrum. Joint sparse recovery with several training samples is also used to improve the estimation performance. Finally, an effective clutter covariance matrix (CCM) estimate and the corresponding STAP filter are designed based on the estimated clutter spectrum. Both the Mountaintop data and simulated experiments have illustrated the fast convergence rate of this approach. Moreover, SR-STAP is less dependent on prior knowledge, so it is more robust to the mismatch in the prior knowledge than knowledge-based STAP methods. Due to these advantages, SR-STAP has great potential for application in actual clutter scenarios.

cs.IT↗
arXiv2010

Novel analysis of the decays psi' -> h_c pi^0 and eta_c'-> chi_{c0} pi^0

We show that in the transitions psi' -> h_c π^0 and eta_c' -> chi_{c0} pi^0 the contributions from charmed meson loops are highly suppressed, in contrast to various other charmonium decays. We calculate the width of the psi' -> h_c pi^0, which agrees with the recent BES-III data, and predict the width of the eta_c' -> chi_{c0} pi^0, Γ(eta_c' -> chi_{c0} pi^0) = 1.5\pm0.4 keV. A confirmation of this prediction would also provide additional support for a recent analysis of psi' -> J/psi pi^0 (eta), where loops are claimed to play a prominent role.

hep-ph↗