arXiv subjects

Qiang Li

Publications and source records attributed to Qiang Li.

At least 505 records · Page 28Linked to original sources

arXiv2018

Probing the Dark Sector through Mono-Z Boson Leptonic Decays

Collider search for dark matter production has been performed over the years based on high pT standard model signatures balanced by large missing transverse energy. The mono-Z boson production with leptonic decay has a clean signature with the advantage that the decaying electrons and muons can be precisely measured. This signature not only enables reconstruction of the Z boson rest frame, but also makes possible recovery of the underlying production dynamics through the decaying lepton angular distribution. In this work, we exploit full information carried by the leptonic Z boson decays to set limits on coupling strength parameters of the dark sector. We study simplified dark sector models with scalar, vector, and tensor mediators and observe among them different signatures in the distribution of angular coefficients.Specifically, we show that angular coefficients can be used to distinguish different scenarios of the spin-0 and spin-1 models, including the ones with parity-odd and charge conjugation parity-odd operators. To maximize the statistical power, we perform a matrix element method study with a dynamic construction of event likelihood function. We parametrize the test statistic such that sensitivity from the matrix element is quantified through a term measuring the shape difference. Our results show that the shape differences provide significant improvements in the limits, especially for the scalar mediator models. We also present an example application of a matrix-element-kinematic-discriminator, an easier approach that is applicable for experimental data.

hep-ph↗

arXiv2018

Tradeoff between Delay and Physical Layer Security in Wireless Networks

Exchange of crucial and confidential information leads to the unprecedented attention on the security problem in wireless networks. Though the security has been studied in a number of works, the joint optimization of the physical layer security and the end-to-end delay management, which requires a meticulous cross-layer design, has seldom been evaluated. In this work, by combining the tools from stochastic geometry and queueing theory, we analyze the tradeoff between the delay and the security performance in large wireless networks. We further propose a simple transmission mechanism which splits a message into two packets and evaluate its effect on the mean delay and the secrecy outage probability. Our numerical results reveal that the security performance is better for larger path loss exponent when the density of legitimate nodes is large, and it is reverse when the density is small. Moreover, it is observed that by introducing the simple mechanism of message split, the security performance is greatly improved in the backlogged scenario and slightly improved in the dynamic scenario when the density of legitimate transmitters is large. In summary, this work provides an understanding and a rule-of-thumb for the practical design of wireless networks where both the delay and the security are key concerns.

cs.IT↗

arXiv2018

Cell-less Communications in 5G Vehicular Networks Based on Vehicle-Installed Access Points

The development of intelligent transportation systems raises many requirements to the current vehicular networks. For instance, to ensure secure communications between vehicles, low latency, high connectivity and high data rate are required for vehicular networks. To meet such requirements, the 5G communication systems for vehicular networks should be improved accordingly. This article proposes a communication scheme for 5G vehicular networks, in which moving access points are deployed on vehicles to facilitate the access of vehicle users. Moreover, the adjacent vehicle-installed moving access points may cooperatively communicate with the vehicle users by joint transmissions and receptions. In this way, the vehicle users communicate with one or more unspecified cooperative access points in a cell-less manner instead of being associated to a single access point. To manage such cell-less networks, local moving software-defined cloudlets are adopted to perform transmission and scheduling management. Simulation results show that the proposed scheme significantly reduces the latency, while improving the connectivity of the vehicular networks, and can be considered as a research direction for the solution to 5G vehicular networks.

cs.NI↗

arXiv2018

A Normalization Model for Analyzing Multi-Tier Millimeter Wave Cellular Networks

Based on the distinguishing features of multi-tier millimeter wave (mmWave) networks such as different transmit powers, different directivity gains from directional beamforming alignment and path loss laws for line-of-sight (LOS) and non-line-of-sight (NLOS) links, we introduce a normalization model to simplify the analysis of multi-tier mmWave cellular networks. The highlight of the model is that we convert a multi-tier mmWave cellular network into a single-tier mmWave network, where all the base stations (BSs) have the same normalized transmit power 1 and the densities of BSs scaled by LOS or NLOS scaling factors respectively follow piecewise constant function which has multiple demarcation points. On this basis, expressions for computing the coverage probability are obtained in general case with beamforming alignment errors and the special case with perfect beamforming alignment in the communication. According to corresponding numerical exploration, we conclude that the normalization model for multi-tier mmWave cellular networks fully meets requirements of network performance analysis, and it is simpler and clearer than the untransformed model. Besides, an unexpected but sensible finding is that there is an optimal beam width that maximizes coverage probability in the case with beamforming alignment errors.

cs.IT↗

arXiv2018

Periodic solutions for neutral evolution equations with delays

The aim is to study the periodic solution problem for neutral evolution equation $$(u(t)-G(t,u(t-ξ)))'+Au(t)=F(t,u(t),u(t-τ)),\ \ \ \ t\in\R$$in Banach space $X$, where $A:D(A)\subset X\rightarrow X$ is a closed linear operator, and $-A$ generates a compact analytic operator semigroup $T(t)(t\geq0)$. With the aid of the analytic operator semigroup theories and some fixed point theorems, we obtain the existence and uniqueness of periodic mild solution for neutral evolution equations. The regularity of periodic mild solution for evolution equation with delay is studied, and some the existence results of the classical and strong solutions are obtained. In the end, we give an example to illustrate the applicability of abstract results. Our works greatly improve and generalize the relevant results of existing literatures.

math.FA↗

arXiv2018

Positive periodic solutions for abstract evolution equations with delay

In this paper, we discuss the existence and asymptotic stability of the positive periodic mild solutions for the abstract evolution equation with delay in an ordered Banach space $E$, $$u'(t)+Au(t)=F(t,u(t),u(t-τ)),\ \ \ \ t\in\R,$$ where $A:D(A)\subset E\rightarrow E$ is a closed linear operator and $-A$ generates a positive $C_{0}$-semigroup $T(t)(t\geq0)$, $F:\R\times E\times E\rightarrow E$ is a continuous mapping which is $ω$-periodic in $t$. Under order conditions on the nonlinearity $F$ concerning the growth exponent of the semigroup $T(t)(t\geq0)$ or the first eigenvalue of the operator $A$, we obtain the existence and asymptotic stability results of the positive $ω$-periodic mild solutions by applying operator semigroup theory. In the end, an example is given to illustrate the applicability of our abstract results.

math.FA↗

arXiv2018

Monotone iterative technique for delayed evolution equation periodic problems in Banach spaces

In this paper, we deal with the existence of $ω$-periodic mild solutions for the abstract evolution equation with delay in an ordered Banach space $E$ $$u'(t)+Au(t)=F(t,u(t),u(t-τ)),\ \ \ \ t\in\R,$$ where $A:D(A)\subset E\rightarrow E$ is a closed linear operator and $-A$ generates a positive $C_{0}$-semigroup $T(t)(t\geq0)$, $F:\R\times E\times E\rightarrow E$ is a continuous mapping which is $ω$-periodic in $t$, and $τ\geq0$ is a constant. Under some weaker assumptions, we construct monotone iterative method for the delayed evolution equation periodic problems, and obtain the existence of maximal and minimal periodic mild solutions. The results obtained generalize the recent conclusions on this topic. Finally, we present two applications to illustrate the feasibility of our abstract results.

math.FA↗

arXiv2018

Existence and Asymptotic Stability of Periodic Solutions for Impulsive Delay Evolution Equations

In this paper, we are devoted to consider the periodic problem for the impulsive evolution equations with delay in Banach space. By using operator semigroups theory and fixed point theorem, we establish some new existence theorems of periodic mild solutions for the equations. In addition, with the aid of an integral inequality with impulsive and delay, we present essential conditions on the nonlinear and impulse functions to guarantee that the equations have an asymptotically stable $ω$-periodic mild solution.

math.FA↗

arXiv2017

Fast simulation of the CEPC detector with Delphes

Fast simulation tools are highly appreciated in particle physics phenomenology studies, especially in the exploration of the physics potential of future experimental facilities. The Circular Electron Positron Collider is a proposed Higgs and Z factory that can precisely measure the Higgs boson properties and the electroweak precision observables. A fast-simulation toolkit dedicated to the CEPC detector has been developed using Delphes. The comparison shows that this fast simulation tool is highly consistent with the full simulation, on a set of benchmark distributions. Therefore, we recommend this fast simulation toolkit for CEPC phenomenological investigations.

hep-ex↗

arXiv2017

Measurement of $\mathrm{H} \to μ^+ μ^-$ production in association with a Z boson at the CEPC

The Circular Electron-Positron Collider (CEPC) is a future Higgs factory proposed by the Chinese high energy physics community. It will operate at a center-of-mass energy of 240-250 GeV and is expected to accumulate an integrated luminosity of 5 ab$^{\rm{-1}}$ with ten years of operation. At CEPC, Higgs bosons are dominantly produced from $ZH$ associated process. Vast amount of Higgs events collected will enable precise studies of its properties including Yukawa couplings to massive particles. With {\sc GEANT4}-based simulation of detector effects, we study CEPC feasibility on measuring Higgs boson decaying into a pair of muons. The results with or without information from Z boson decay products are provided, which show a signal significance of over 10 standard deviations can be achieved and the H-$\mathrmμ$-$\mathrmμ$ coupling can be measured within $10\%$ accuracy.

hep-ex↗

arXiv2017

Wear Analysis of a Heterogeneous Annular Cylinder

The wear of a cylindrical punch composed by two different materials alternatively distributed in annular forms is studied with the Method of Dimensionality Reduction (MDR). The changes in surface topography and pressure distribution during the wear process is obtained and validated by the Boundary Element Method (BEM). The pressure in each annular ring approaches constant in a stationary state where the surface topography does not change any more. Furthermore, in an easier way, using the direct integration the limiting profile in steady wear state is theoretically calculated, as well as the root mean square (RMS) of its surface gradient which is closely related to the coefficient of friction between this kind of surface and an elastomer. The dependence on the wear coefficients and the width of the annular areas of two phases is obtained.

cond-mat.soft↗

arXiv2017

$H \rightarrow e^+ e^- $ at CEPC: ISR effect with MadGraph

The Circular Electron Positron Collider (CEPC) is a future Higgs factory proposed by the Chinese high energy physics community. It will operate at a center-of-mass energy of 240-250 GeV. The CEPC will accumulate an integrated luminosity of 5 ab$^{\rm{-1}}$ in ten years' operation. With GEANT4-based full simulation samples for CEPC, Higgs boson decaying into electron pair is studied at the CEPC. The upper limit of ${\cal B}(H \rightarrow e^+ e^-)$ could reach 0.024\% at 95\% confidence level. The signal process is generated by MadGraph, with Initial State Radiation (ISR) implemented, as a first step to adjust MadGraph for a electron positron Collider.

hep-ph↗

arXiv2017

Adversarial Network Embedding

Learning low-dimensional representations of networks has proved effective in a variety of tasks such as node classification, link prediction and network visualization. Existing methods can effectively encode different structural properties into the representations, such as neighborhood connectivity patterns, global structural role similarities and other high-order proximities. However, except for objectives to capture network structural properties, most of them suffer from lack of additional constraints for enhancing the robustness of representations. In this paper, we aim to exploit the strengths of generative adversarial networks in capturing latent features, and investigate its contribution in learning stable and robust graph representations. Specifically, we propose an Adversarial Network Embedding (ANE) framework, which leverages the adversarial learning principle to regularize the representation learning. It consists of two components, i.e., a structure preserving component and an adversarial learning component. The former component aims to capture network structural properties, while the latter contributes to learning robust representations by matching the posterior distribution of the latent representations to given priors. As shown by the empirical results, our method is competitive with or superior to state-of-the-art approaches on benchmark network embedding tasks.

cs.LG↗

arXiv2017

Multi-Path Cooperative Communications Networks for Augmented and Virtual Reality Transmission

Augmented and/or virtual reality (AR/VR) are emerging as one of the main applications in future fifth generation (5G) networks. To meet the requirements of lower latency and massive data transmission in AR/VR applications, a solution with software-defined networking (SDN) architecture is proposed for 5G small cell networks. On this basis, a multi-path cooperative route (MCR) scheme is proposed to facilitate the AR/VR wireless transmissions in 5G small cell networks, in which the delay of MCR scheme is analytically studied. Furthermore, a service effective energy optimal (SEEO) algorithm is developed for AR/VR wireless transmission in 5G small cell networks. Simulation results indicate that both the delay and service effective energy (SEE) of the proposed MCR scheme outperform the delay and SEE of the conventional single path route scheme in 5G small cell networks.

cs.NI↗

arXiv2017

Energy Efficiency of Multi-user Multi-antenna Random Cellular Networks with Minimum Distance Constraints

Compared with conventional regular hexagonal cellular models, random cellular network models resemble real cellular networks much more closely. However, most studies of random cellular networks are based on the Poisson point process and do not take into account the fact that adjacent base stations (BSs) should be separated with a minimum distance to avoid strong interference among each other. In this paper, based on the hard core point process (HCPP), we propose a multi-user multi-antenna random cellular network model with the aforementioned minimum distance constraint for adjacent BSs. Taking into account the effects of small scale fading and shadowing, interference and capacity models are derived for the multi-user multi-antenna HCPP random cellular networks. Furthermore, a spectrum efficiency model as well as an energy efficiency model is presented, based on which, the maximum achievable energy efficiency of the considered multi-user multiantenna HCPP random cellular networks is investigated. Simulation results demonstrate that the energy efficiency of conventional Poison point process (PPP) cellular networks is underestimated when the minimum distance between adjacent BSs is ignored.

cs.NI↗

arXiv2017

Testing the nature of neutrinos from four-body $τ$ decays

This paper is designed to discuss four-body lepton number violating tau decay. We study the processes $τ^+ \to e^+ e^+ π^- ν_τ$ and $τ^+ \to e^+ e^+ π^- ν_e$ to determine the nature of neutrino. The first process violates lepton number by two units which can only happen through an internal Majorana. The second one conserves lepton number but violates lepton flavor which can take place with both Majorana neutrino and Dirac neutrino. We calculate their branching ratio $Br$ and differential branching ratio $dBr/dE_π$ to distinguish Majorana neutrino vs. Dirac neutrino. We also propose the possibility of experiment to perform this detection.

hep-ph↗

arXiv2017

Four-body decays of $B$ meson with lepton number violation

This paper is designed to calculate the branching ratio of four-body decays of $B$ meson with lepton number changed by 2. With the new experimental data limit to lepton-number violation processes, we update the upper limits of mixing parameters between heavy Majorana neutrino and charged leptons. Afterwards, we calculate the branching ratio of $B^0(P)\rightarrow D^{*-}(P_1)\ell^+_1(P_2)\ell^+_2(P_3)M_2^-(P_4)$ using the updated parameters. It is found that the most hopeful decay channel is $B^0(P)\rightarrow D^{*-}(P_1)e^+_1(P_2)e^+_2(P_3)ρ^-(P_4)$ or $B^0(P)\rightarrow D^{*-}(P_1)e(μ)^+_1(P_2)μ(e)^+_2(P_3)ρ^-(P_4)$, whose branching ratio can reach about $10^{-4}$ with heavy Majorana neutrino mass range around $2~\mathrm{GeV}$.

hep-ph↗

arXiv2017

Suppression of the antiferromagnetic order when approaching the superconducting state in a phase-separated crystal of K$_x$Fe$_{2-y}$Se$_2$

We have combined elastic and inelastic neutron scattering techniques, magnetic susceptibility and resistivity measurements to study single-crystal samples of K$_x$Fe$_{2-y}$Se$_2$, which contain the superconducting phase that has a transition temperature of $\sim$31 K. In the inelastic neutron scattering measurements, we observe both the spin-wave excitations resulting from the block antiferromagnetic ordered phase and the resonance that is associated with the superconductivity in the superconducting phase, demonstrating the coexistence of these two orders. From the temperature dependence of the intensity of the magnetic Bragg peaks, we find that well before entering the superconducting state, the development of the magnetic order is interrupted, at $\sim$42 K. We consider this result to be evidence for the physical separation of the antiferromagnetic and superconducting phases; the suppression is possibly due to the proximity effect of the superconducting fluctuations on the antiferromagnetic order.

cond-mat.supr-con↗