arXiv subjects

Yue Zhao

Publications and source records attributed to Yue Zhao.

At least 487 records · Page 27Linked to original sources

arXiv2016

Distribution System Outage Detection using Consumer Load and Line Flow Measurements

An outage detection framework for power distribution networks is proposed. Given the tree structure of the distribution system, a method is developed combining the use of real-time power flow measurements on edges of the tree with load forecasts at the nodes of the tree. A maximum a posteriori detector {\color{black} (MAP)} is formulated for arbitrary number and location of outages on trees which is shown to have an efficient detector. A framework relying on the maximum missed detection probability is used for optimal sensor placement and is solved for tree networks. Finally, a set of case studies is considered using feeder data from the Pacific Northwest National Laboratories. We show that a 10\% loss in mean detection reliability network wide reduces the required sensor density by 60 \% for a typical feeder if efficient use of measurements is performed.

eess.SY↗

arXiv2016

Minimum Sparsity of Unobservable Power Network Attacks

Physical security of power networks under power injection attacks that alter generation and loads is studied. The system operator employs Phasor Measurement Units (PMUs) for detecting such attacks, while attackers devise attacks that are unobservable by such PMU networks. It is shown that, given the PMU locations, the solution to finding the sparsest unobservable attacks has a simple form with probability one, namely, $κ(G^M) + 1$, where $κ(G^M)$ is defined as the vulnerable vertex connectivity of an augmented graph. The constructive proof allows one to find the entire set of the sparsest unobservable attacks in polynomial time. Furthermore, a notion of the potential impact of unobservable attacks is introduced. With optimized PMU deployment, the sparsest unobservable attacks and their potential impact as functions of the number of PMUs are evaluated numerically for the IEEE 30, 57, 118 and 300-bus systems and the Polish 2383, 2737 and 3012-bus systems. It is observed that, as more PMUs are added, the maximum potential impact among all the sparsest unobservable attacks drops quickly until it reaches the minimum sparsity.

math.OC↗

arXiv2016

Detecting Superlight Dark Matter with Fermi-Degenerate Materials

We examine in greater detail the recent proposal of using superconductors for detecting dark matter as light as the warm dark matter limit of O(keV). Detection of such light dark matter is possible if the entire kinetic energy of the dark matter is extracted in the scattering, and if the experiment is sensitive to O(meV) energy depositions. This is the case for Fermi-degenerate materials in which the Fermi velocity exceeds the dark matter velocity dispersion in the Milky Way of ~10^-3. We focus on a concrete experimental proposal using a superconducting target with a transition edge sensor in order to detect the small energy deposits from the dark matter scatterings. Considering a wide variety of constraints, from dark matter self-interactions to the cosmic microwave background, we show that models consistent with cosmological/astrophysical and terrestrial constraints are observable with such detectors. A wider range of viable models with dark matter mass below an MeV is available if dark matter or mediator properties (such as couplings or masses) differ at BBN epoch or in stellar interiors from those in superconductors. We also show that metal targets pay a strong in-medium suppression for kinetically mixed mediators; this suppression is alleviated with insulating targets.

hep-ph↗

arXiv2016

Explicitly Broken Supersymmetry with Exactly Massless Moduli

There is an avatar of the little hierarchy problem of the MSSM in 3-dimensional supersymmetry. We propose a solution to this problem in AdS$_3$ based on the AdS/CFT correspondence. The bulk theory is a supergravity theory in which U(1) $\times$ U(1) R-symmetry is gauged by Chern-Simons fields. The bulk theory is deformed by a boundary term quadratic in the gauge fields. It breaks SUSY completely and sources an exactly marginal operator in the dual CFT. SUSY breaking is communicated by gauge interactions to bulk scalar fields and their spinor superpartners. Since the R-charges of scalar and spinor differ, this generates a SUSY breaking shift of their masses. The Ward identity facilitates the calculation of these mass shifts to any desired order in the strength of the deformation. Moduli fields are massless $R$-neutral bulk scalars with vanishing potential in the undeformed theory. These properties are maintained to all orders in the deformation despite the fact that moduli couple in the bulk to loops of R-charged fields.

hep-th↗

arXiv2016

Suppressing supersymmetric flavor violations through quenched gaugino-flavor interactions

Realizing that couplings related by supersymmetry (SUSY) can be disentangled when SUSY is broken, it is suggested that unwanted flavor and CP violating SUSY couplings may be suppressed via quenched gaugino-flavor interactions, which may be accomplished by power-law running of sfermion anomalous dimensions. A simple theoretical framework to accomplish this is exemplified and the defeated constraints are tallied. One key implication of the scenario is the expectation of enhanced top, bottom and tau production at the LHC, accompanied by large missing energy. Also, direct detection signals of dark matter may be more challenging to find than in conventional SUSY scenarios.

hep-ph↗

arXiv2016

Adaptive estimation of the copula correlation matrix for semiparametric elliptical copulas

We study the adaptive estimation of copula correlation matrix $Σ$ for the semi-parametric elliptical copula model. In this context, the correlations are connected to Kendall's tau through a sine function transformation. Hence, a natural estimate for $Σ$ is the plug-in estimator $\hatΣ$ with Kendall's tau statistic. We first obtain a sharp bound on the operator norm of $\hatΣ-Σ$. Then we study a factor model of $Σ$, for which we propose a refined estimator $\widetildeΣ$ by fitting a low-rank matrix plus a diagonal matrix to $\hatΣ$ using least squares with a nuclear norm penalty on the low-rank matrix. The bound on the operator norm of $\hatΣ-Σ$ serves to scale the penalty term, and we obtain finite sample oracle inequalities for $\widetildeΣ$. We also consider an elementary factor copula model of $Σ$, for which we propose closed-form estimators. All of our estimation procedures are entirely data-driven.

stat.ML↗

arXiv2015

Dark Matter Annihilation Decay at The LHC

Collider experiments provide an opportunity to shed light on dark matter (DM) self-interactions. In this work, we study the possibility of generating DM bound states -- the Darkonium -- at the LHC and discuss how the annihilation decay of the Darkonium produces force carriers. We focus on two popular scenarios that contain large DM self-couplings: the Higgsinos in the $λ$-SUSY model, and self-interacting DM (SIDM) framework. After forming bound states, the DM particles annihilate into force mediators, which decay into the standard model particles either through a prompt or displaced process. This generates interesting signals for the heavy resonance search. We calculate the production rate of bound states and study the projected future constraints from the existing heavy resonance searches.

hep-ph↗

arXiv2015

Superconducting Detectors for Super Light Dark Matter

We propose and study a new class of superconducting detectors which are sensitive to O(meV) electron recoils from dark matter-electron scattering. Such devices could detect dark matter as light as the warm dark matter limit, mX > keV. We compute the rate of dark matter scattering off of free electrons in a (superconducting) metal, including the relevant Pauli blocking factors. We demonstrate that classes of dark matter consistent with terrestrial and cosmological/astrophysical constraints could be detected by such detectors with a moderate size exposure.

hep-ph↗

arXiv2015

A Radio for Hidden-Photon Dark Matter Detection

We propose a resonant electromagnetic detector to search for hidden-photon dark matter over an extensive range of masses. Hidden-photon dark matter can be described as a weakly coupled "hidden electric field," oscillating at a frequency fixed by the mass, and able to penetrate any shielding. At low frequencies (compared to the inverse size of the shielding), we find that observable effect of the hidden photon inside any shielding is a real, oscillating magnetic field. We outline experimental setups designed to search for hidden-photon dark matter, using a tunable, resonant LC circuit designed to couple to this magnetic field. Our "straw man" setups take into consideration resonator design, readout architecture and noise estimates. At high frequencies,there is an upper limit to the useful size of a single resonator set by $1/ν$. However, many resonators may be multiplexed within a hidden-photon coherence length to increase the sensitivity in this regime. Hidden-photon dark matter has an enormous range of possible frequencies, but current experiments search only over a few narrow pieces of that range. We find the potential sensitivity of our proposal is many orders of magnitude beyond current limits over an extensive range of frequencies, from 100 Hz up to 700 GHz and potentially higher.

hep-ph↗

arXiv2015

AdS/CFT and the Little Hierarchy Problem

The AdS/CFT correspondence is applied to a close analogue of the little hierarchy problem in $AdS_{d+1}$, $d \geq 3$. The new mechanism requires a Maxwell field that gauges a $U(1)_R$ symmetry in a bulk supergravity theory with a negative cosmological constant. Supersymmetry is explicitly broken by a non-local boundary term with dimensionless coupling $h$. Non-locality appears to cause no pathology, and the SUSY breaking deformation engendered is exactly marginal. SUSY breaking effects in the bulk are computed using the $U(1)$ Ward identity. Conformal dimensions and thus masses of scalar and spinor partners are split simply because they carry different R-charges. However SUSY breaking effects cancel to all orders for R-neutral fields, even in diagrams with internal R-charged loops. SUSY breaking corrections can be summed to all orders in $h$. Diagrams involving graviton loops do not introduce any further SUSY breaking corrections. A possible scenario for a flat spacetime limit is outlined.

hep-th↗

arXiv2015

3D Automatic Segmentation Method for Retinal Optical Coherence Tomography Volume Data Using Boundary Surface Enhancement

With the introduction of spectral-domain optical coherence tomography (SDOCT), much larger image datasets are routinely acquired compared to what was possible using the previous generation of time-domain OCT. Thus, there is a critical need for the development of 3D segmentation methods for processing these data. We present here a novel 3D automatic segmentation method for retinal OCT volume data. Briefly, to segment a boundary surface, two OCT volume datasets are obtained by using a 3D smoothing filter and a 3D differential filter. Their linear combination is then calculated to generate new volume data with an enhanced boundary surface, where pixel intensity, boundary position information, and intensity changes on both sides of the boundary surface are used simultaneously. Next, preliminary discrete boundary points are detected from the A-Scans of the volume data. Finally, surface smoothness constraints and a dynamic threshold are applied to obtain a smoothed boundary surface by correcting a small number of error points. Our method can extract retinal layer boundary surfaces sequentially with a decreasing search region of volume data. We performed automatic segmentation on eight human OCT volume datasets acquired from a commercial Spectralis OCT system, where each volume of data consisted of 97 OCT images with a resolution of 496 512; experimental results show that this method can accurately segment seven layer boundary surfaces in normal as well as some abnormal eyes.

cs.CV↗

arXiv2015

Weak convergence of empirical copula processes indexed by functions

Weak convergence of the empirical copula process indexed by a class of functions is established. Two scenarios are considered in which either some smoothness of these functions or smoothness of the underlying copula function is required. A novel integration by parts formula for multivariate, right continuous functions of bounded variation, which is perhaps of independent interest, is proved. It is a key ingredient in proving weak convergence of a general empirical process indexed by functions of bounded variation.

math.ST↗

arXiv2015

Creating and Probing Electron Whispering Gallery Modes in Graphene

Designing high-finesse resonant cavities for electronic waves faces challenges due to short electron coherence lengths in solids. Previous approaches, e.g. the seminal nanometer-sized quantum corrals, depend on careful positioning of adatoms at clean surfaces. Here we demonstrate an entirely different approach, inspired by the peculiar acoustic phenomena in whispering galleries. Taking advantage of graphene's unique properties, namely gate-tunable light-like carriers, we create Whispering Gallery Mode (WGM) resonators defined by circular pn-junctions, induced by a scanning tunneling probe. We can tune the resonator size and the carrier concentration under the probe in a back-gated graphene device over a wide range, independently and in situ. The confined modes, revealed through characteristic resonances in the tunneling spectrum, originate from Klein scattering at pn junction boundaries. The WGM-type confinement and resonances are a new addition to the quantum electron-optics toolbox, paving the way to real-world electronic lenses and resonators.

cond-mat.mes-hall↗

arXiv2014

Semiparametric Gaussian copula classification

This paper studies the binary classification of two distributions with the same Gaussian copula in high dimensions. Under this semiparametric Gaussian copula setting, we derive an accurate semiparametric estimator of the log density ratio, which leads to our empirical decision rule and a bound on its associated excess risk. Our estimation procedure takes advantage of the potential sparsity as well as the low noise condition in the problem, which allows us to achieve faster convergence rate of the excess risk than is possible in the existing literature on semiparametric Gaussian copula classification. We demonstrate the efficiency of our empirical decision rule by showing that the bound on the excess risk nearly achieves a convergence rate of $n^{-1/2}$ in the simple setting of Gaussian distribution classification.

math.ST↗

arXiv2014

Detecting Boosted Dark Matter from the Sun with Large Volume Neutrino Detectors

We study novel scenarios where thermal dark matter (DM) can be efficiently captured in the Sun and annihilate into boosted dark matter. In models with semi-annihilating DM, where DM has a non-minimal stabilization symmetry, or in models with a multi-component DM sector, annihilations of DM can give rise to stable dark sector particles with moderate Lorentz boosts. We investigate both of these possibilities, presenting concrete models as proofs of concept. Both scenarios can yield viable thermal relic DM with masses O(1)-O(100) GeV. Taking advantage of the energetic proton recoils that arise when the boosted DM scatters off matter, we propose a detection strategy which uses large volume terrestrial detectors, such as those designed to detect neutrinos or proton decays. In particular, we propose a search for proton tracks pointing towards the Sun. We focus on signals at Cherenkov-radiation-based detectors such as Super-Kamiokande (SK) and its upgrade Hyper-Kamiokande (HK). We find that with spin-dependent scattering as the dominant DM-nucleus interaction at low energies, boosted DM can leave detectable signals at SK or HK, with sensitivity comparable to DM direct detection experiments while being consistent with current constraints. Our study provides a new search path for DM sectors with non-minimal structure.

hep-ph↗

arXiv2014

A Parametrically Enhanced Hidden Photon Search

Many theories beyond the Standard Model contain hidden photons. A light hidden photon will generically couple to the Standard Model through a kinetic mixing term, giving a powerful avenue for detection using "Light-Shining-Through-A-Wall"-type transmission experiments with resonant cavities. We demonstrate a parametric enhancement of the signal in such experiments, resulting from transmission of the longitudinal mode of the hidden photon. While previous literature has focused on the production and detection of transverse modes, the longitudinal mode allows a significant improvement in experimental sensitivity. Although optical experiments such as ALPS are unable to take useful advantage of this enhancement, the reach of existing microwave cavity experiments such as CROWS is significantly enhanced beyond their published results. Future microwave cavity experiments, designed with appropriate geometry to take full advantage of the longitudinal mode, will provide a powerful probe of hidden-photon parameter space extending many orders of magnitude beyond current limits, including significant regions where the hidden photon can be dark matter.

hep-ph↗

arXiv2014

Indirect Detection Signatures for the Origin of Asymmetric Dark Matter

We study the decay signatures of Asymmetric Dark Matter (ADM) via higher dimension operators which are responsible for generating the primordial dark matter (DM) asymmetry. Since the signatures are sensitive both to the nature of the higher dimension operator generating the DM asymmetry and to the sign of the baryon or lepton number that the DM carries, indirect detection may provide a window into the nature of the mechanism which generates the DM asymmetry. We consider in particular dimension-6 fermionic operators of the form ${\cal O}_{ADM} = X {\cal O}_{B-L}/M^2$, where ${\cal O}_{B-L} = u^c d^c d^c,~\ell \ell e^c,~q \ell d^c$ (or operators related through a Hermitian conjugate) with the scale $M$ around or just below the GUT scale. We derive constraints on ADM particles both in the natural mass range (around a few GeV), as well as in the range between 100 GeV to 10 TeV. For light ADM, we focus on constraints from both the low energy gamma ray data and proton/anti-proton fluxes. For heavy ADM, we consider $γ$-rays and proton/anti-proton fluxes, and we fit $e^+/e^-$ data from AMS-02 and H.E.S.S. (neglecting the Fermi charged particle fluxes which disagree with AMS-02 below 100 GeV). We show that, although the best fit regions from electron/positron measurement are still in tension with other channels on account of the H.E.S.S. measurement at high energies, compared to an ordinary symmetric dark matter scenario, the decay of DM with a primordial asymmetry reduces the tension. Better measurement of the flux at high energy will be necessary to draw a definite conclusion about the viability of decaying DM as source for the signals.

hep-ph↗

arXiv2014

Dark Matter Induced Nucleon Decay: Model and Signatures

If dark matter (DM) carries anti-baryon number, a DM particle may annihilate with a nucleon by flipping to anti-DM. Inspired by Hylogenesis models, we introduce a single component DM model where DM is asymmetric and carries B and L as -1/2. It can annihilate with a nucleon to an anti-lepton and an anti-DM at leading order or with an additional meson at sub-leading order. Such signals may be observed in proton decay experiments. If DM is captured in the Sun, the DM induced nucleon decay can generate a large flux of anti-neutrinos, which could be observed in neutrino experiments. Furthermore, the anti-DM particle in the final state obtains a relatively large momentum (few hundred MeV), and escapes the Sun. These fast-moving anti-DM particles could also induce interesting signals in various underground experiments.

hep-ph↗