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Kai Wang

Publications and source records attributed to Kai Wang.

At least 847 records · Page 47Linked to original sources
arXiv2015

Configurational space discretization and free energy calculation in complex molecular systems

Trajectories provide dynamical information that is discarded in free energy calculations, for which we sought to design a scheme with the hope of saving cost for generating dynamical information. We first demonstrated that snapshots in a converged trajectory set are associated with implicit conformers that have invariant statistical weight distribution (ISWD). Based on the thought that infinite number of sets of implicit conformers with ISWD may be created through independent converged trajectory sets, we hypothesized that explicit conformers with ISWD may be constructed for complex molecular systems through systematic increase of conformer fineness, and tested the hypothesis in lipid molecule palmitoyloleoylphosphatidylcholine (POPC). Furthermore, when explicit conformers with ISWD were utilized as basic states to define conformational entropy, change of which between two given macrostates was found to be equivalent to change of free energy except a mere difference of a negative temperature factor, and change of enthalpy essentially cancels corresponding change of average intra-conformer entropy. These findings suggest that entropy enthalpy compensation is inherently a local phenomenon in configurational space. By implicitly taking advantage of entropy enthalpy compensation and forgoing all dynamical information, constructing explicit conformers with ISWD and counting thermally accessible number of which for interested end macrostates is likely to be an efficient and reliable alternative end point free energy calculation strategy.

physics.chem-ph↗
arXiv2015

Smoothing methods comparison for CMB E- and B-mode separation

The anisotropies of the B-mode polarization in the cosmic microwave background radiation play a crucial role for the study of the very early Universe. However, in the real observation, the mixture of the E-mode and B-mode can be caused by the partial sky surveys, which must be separated before applied to the cosmological explanation. The separation method developed by Smith (\citealt{PhysRevD.74.083002}) has been widely adopted, where the edge of the top-hat mask should be smoothed to avoid the numerical errors. In this paper, we compare three different smoothing methods, and investigate the leakage residuals of the E-B mixture. We find that, if the less information loss is needed and the smaller region is smoothed in the analysis, the \textit{sin}- and \textit{cos}-smoothing methods are better. However, if we need a clean constructed B-mode map, the larger region around the mask edge should be smoothed. In this case, the \textit{Gaussian}-smoothing method becomes much better. In addition, we find that the leakage caused by the numerical errors in the \textit{Gaussian}-smoothing method mostly concentrates on two bands, which is quite easy to be reduced for the further E-B separations.

astro-ph.CO↗
arXiv2015

Measuring the Aharonov-Anandan phase in photonics

The Aharonov-Anandan phase is a description of the geometric nature in non-adiabatic cyclic evolutions of quantum states. Here we report on a measurement of the Aharonov-Anandan phase in photonics. We consider a time-independent quantum driven harmonic oscillator that is initially prepared at the vacuum state. We utilize evanescently coupled waveguides to realize this physical model and achieve a measurement of the Aharonov-Anandan phase via integrated interferometry.

physics.optics↗
arXiv2015

Pressure-induced reversible phase transition and amorphization of CH$_3$NH$_3$PbI$_3$

Recent advances of highly efficient solar cells based on organic-inorganic halide perovskites have triggered intense research efforts to establish the fundamental properties of these materials. In this work, we utilized diamond anvil cell to investigate the pressure-induced structural and electronic transformations in methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$) up to 7 GPa at room temperature. The synchrotron X-ray diffraction experiment show that the sample transformed from tetragonal to orthorhombic phase at 0.3 GPa and amorphized above 4 GPa. Further high pressure IR spectroscopy experiments illustrated the high pressure behavior of organic (CH$_3$NH$_3$)$^+$ cations. We also analyzed the pressure dependence of the band gap energy based on the optical absorption and photoluminescence (PL) results. Moreover, all the observed changes were fully reversible when the pressure was completely released. Our in situ high pressure studies provide essential information for the intrinsic properties and stability of organic-inorganic halide perovskites, which significantly affect the performance of perovskite solar cells.

cond-mat.mtrl-sci↗
arXiv2015

Dixmier Trace for Toeplitz Operators on Symmetric Domains

For Toeplitz operators on bounded symmetric domains of arbitrary rank, we define a Hilbert quotient module corresponding to partitions of length $1$ and prove that it belongs to the Macaev class ${\mathcal{L}}^{n,\infty}$. We next obtain an explicit formula for the Dixmier trace of Toeplitz commutators in terms of the underlying boundary geometry.

math.FA↗
arXiv2015

Ideal gas behavior of rotamerically defined conformers in native globular proteins

Protein conformational transitions, which are essential for function, may be driven either by entropy or enthalpy when molecular systems comprising solute and solvent molecules are the focus. Revealing thermodynamic origin of a given molecular process is an important but difficult task, and general principles governing protein conformational distributions remain elusive. Here we demonstrate that when protein molecules are taken as thermodynamic systems and solvents being treated as the environment, conformational entropy is an excellent proxy for free energy and is sufficient to explain protein conformational distributions. Specifically, by defining each unique combination of side chain torsional state as a conformer, the population distribution (or free energy) on an arbitrarily given order parameter is approximately a linear function of conformational entropy. Additionally, span of various microscopic potential energy terms is observed to be highly correlated with both conformational entropy and free energy. Presently widely utilized free energy proxies, including minimum potential energy, average potential energy terms by themselves or in combination with vibrational entropy\cite, are found to correlate with free energy rather poorly. Therefore, our findings provide a fundamentally new theoretical base for development of significantly more reliable and efficient next generation computational tools, where the number of available conformers,rather than poential energy of microscopic configurations, is the central focus. We anticipate that many related research fields, including structure based drug design and discovery, protein design, docking and prediction of general intermolecular interactions involving proteins, are expected to benefit greatly.

q-bio.BM↗
arXiv2014

Pure leptonic proposal to $W^{+}W^{-}$ excess and neutrino mass generation

We investigate the TeV models for neutrino mass generation as candidate models to explain the recent 2$σ$ excess of leptonic $W^{+}W^{-}$ pair production at LHC. Several models with singly charged exotic states that may explain the excess require light masses completely excluded by LEP experiments. One possible model with new lepton doublets can fit the observation and evade all direct search bounds but with tuned Yukawa structure to satisfy lepton universality. The new exotic leptons $L^{\pm}$ decay into $L^{\pm}\to \ell^{\pm} ϕ$ where $ϕ$ is a light singlet scalar of $\cal O$(MeV) that decays into neutrinos. Drell-Yan production of $L^{+}L^{-}\to \ell^{+}\ell^{-}+\cancel{E}_{T}$ fits the excess and $L^{\pm}L^{0}\to \ell^{\pm}+\cancel{E}_{T}$ is completely buried in SM background.

hep-ph↗
arXiv2014

Characterizing the Impact of the Workload on the Value of Dynamic Resizing in Data Centers

Energy consumption imposes a significant cost for data centers; yet much of that energy is used to maintain excess service capacity during periods of predictably low load. Resultantly, there has recently been interest in developing designs that allow the service capacity to be dynamically resized to match the current workload. However, there is still much debate about the value of such approaches in real settings. In this paper, we show that the value of dynamic resizing is highly dependent on statistics of the workload process. In particular, both slow time-scale non-stationarities of the workload (e.g., the peak-to-mean ratio) and the fast time-scale stochasticity (e.g., the burstiness of arrivals) play key roles. To illustrate the impact of these factors, we combine optimization-based modeling of the slow time-scale with stochastic modeling of the fast time scale. Within this framework, we provide both analytic and numerical results characterizing when dynamic resizing does (and does not) provide benefits.

cs.PF↗
arXiv2014

Polarizations of $χ_{c1}$ and $χ_{c2}$ in prompt production at the LHC

Prompt $χ_c$ production at hadron colliders may provide a unique test for the color-octet mechanism in nonrelativistic QCD. We present an analysis for the polarization observables of $χ_{c1}$ and $χ_{c2}$ at next-to-leading order in $α_S$, and propose to measure them at the LHC, which is expected to be important for testing the validity of NRQCD.

hep-ph↗
arXiv2014

Light Top Squark in Precision Top Quark Sample

The uncertainty of $t\bar{t}$ production cross section measurement at LHC is at a-few-percent level which still allows the stop pair production $\tilde{t}\tilde{t}^{*}$ with identical final states $2b+\ell+nj+{E}_{T}$. In this paper, we attempt to use the existing measurement of $W$-polarization in top quark decay to improve the distinction between stop and top quark states. We apply the ATLAS $χ^2$ method of $W$-polarization measurement in semi-leptonic $t\bar{t}$ final state to semi-leptonic stop pair samples and study its prediction. We find that the faked top events from stop mostly contribute to the left-handed polarized $W$ due to the reconstruction. The benchmark point with maximal contribution to top events only changes $F_{L}$ by 1%. After comparing with the current experiments, we conclude that the current measurement of $W$-polarization in $t$ decay cannot exclude the light stop scenario.

hep-ph↗
arXiv2013

Higgs Precision Measurements and Flavor Physics: A Supersymmetric Example

[Review for "Chinese Science Bulletin" and CEPC+SPPC Proposal] Rare decays in flavor physics often suffer from Helicity suppress and Loop suppress. Helicity flip is a direct consequence of chiral $U(3)$ symmetry breaking and electroweak symmetry breaking. The identical feature is also shared by the mass generation of SM fermions. In this review, we use MSSM as an example to illustrate an explicit connection between bottom Yukawa coupling and rare decay process of $b\to sγ$. We take a symmetry approach to study the common symmetry breaking in supersymmetric correction to bottom quark mass generation and $b\to sγ$. We show that Large Peccei-Quinn symmetry breaking effect and $R$-symmetry breaking effect required by $b\to sγ$ inevitably lead to significant reduction of bottom Yukawa ${y}_{b}$. To compromise the reduction in $b\bar{b}$, a new decay is also needed to keep the Higgs total width as the SM value.

hep-ph↗
arXiv2013

Is polarization effect visible in leptonic SUSY searches?

On-shell effective theory approach has been widely used in search of various supersymmetric signals, in particular, gluino/squark pairs with long cascade decay chains in which complete matrix element calculations may encounter over-20 dimensional integrations. On the other hand, leptons from polarized chargino decays may show significant boost or anti-boost effect in some scenarios and simulation without polarization information may underestimate or overestimate the lepton $p_{T}$ cut efficiencies in the first place. We study the polarization effects in supersymmetry searches of multi-jets plus leptons final states. We find it justifiable for first two generations to only use on-shell effective theories. While for measurements related to third generation squarks, for instance, polarization effect of charginos from stop may reduce the lepton $p_{T}$ cut efficiencies in cross section measurements by $25\%$ when slepton contributions dominate in chargino decay or $W$ are on-shell. The signal is then underestimated if only on-shell effective theory approach is taken in simulation of signal and the real bound on squark/gluino should be more stringent.

hep-ph↗
arXiv2013

Flavor dependence of annihilation parameters in QCD factorization

For $B_{d,s} \to π^\mp K^\pm$ and $K^{(\ast)} K^{(\ast)}$ decays, the flavor symmetry breaking effects may be particularly small since the final state interactions should be the same between the corresponding $B_d$ and $B_s$ decays due to the charge conjugation symmetry of the final states. This is consistent with the newly measured direct CP asymmetry of $B_s \to π^+ K^-$. These decays are thus supposed to be important in testing the Standard Model and in probing new physics effects. However, the observation of pure annihilation decay $B_s \to π^+ π^-$ appears to imply a large annihilation scenario with $ρ_A \sim 3$, in contrast to the case of $ρ_A \sim 1$ in $B_{u,d}$ decays in the framework of QCD factorization. This seems to indicate unexpectedly large flavor symmetry breaking effects between the annihilation amplitudes of $B_s$ and $B_{u,d}$ decays. This apparent contradiction could be resolved by noticing that there is a priori no reason to justify the common practice of assuming the universality of annihilation parameters for different Dirac structures of effective operators. We then argue that, for $B_{d,s} \to π^\mp K^\pm$ decays, the flavor symmetry breaking effects of annihilation amplitudes have all been included in the initial state decay constants and are thus small. But the flavor symmetry breaking effects in $B_{d,s} \to K^{(\ast)} K^{(\ast)}$ decays are likely to be much larger, as part of the annihilation topologies of $B_s \to K K$ decay could be related to $B_s \to π^+ π^-$ decay. Therefore when new physics effects are searched for in these decay channels, care must be taken to consider the potentially large flavor symmetry breaking effects in more details.

hep-ph↗
arXiv2013

Revisit to Non-decoupling MSSM

Dipole operator $\bar{s}σ_{μν}F^{μν}b$ requires the helicity flip in the involving quark states thus the breaking of chiral $U(3)_{Q}\times U(3)_{d}$. On the other hand, the $b$-quark mass generation is also a consequence of chiral $U(3)_{Q}\times U(3)_{d}$ symmetry breaking. Therefore, in many models, there might be strong correlation between the $b\to sγ$ and $b$ quark Yukawa coupling. We use non-decoupling MSSM model to illustrate this feature. The light Higgs boson may evade the direct search experiments at LEPII or Tevatron while the 125 GeV Higgs-like boson is identified as the heavy Higgs boson in the spectrum. A light charged Higgs is close to the heavy Higgs boson which is of 125 GeV and its contribution to $b\to s γ$ requires large supersymmetric correction with large PQ and $R$ symmetry breaking. The large supersymmetric contribution at the same time significantly modifies the $b$ quark Yukawa co upling. With combined flavor constraints $B\to X_{s}γ$ and $B_{s}\to μ^{+}μ^{-}$ and direct constraints on Higgs properties, we find best fit scenarios with light stop of $\cal O$(500 GeV), negative $A_{t}$ around -750 GeV and large $μ$-term of 2-3 TeV. In addition, reduction in $b\bar{b}$ partial width may also result in large enhancement of $ττ$ decay branching fraction. Large parameter region in the survival space under all bounds may be further constrained by $H\to ττ$ if no excess of $ττ$ is confirmed at LHC. We only identify a small parameter region with significant $H\to hh$ decay that is consistent with all bounds and reduced $ττ$ decay branching fraction.

hep-ph↗
arXiv2013

Laterally emitted surface second harmonic generation in a single ZnTe nanowire

We report a direct observation on the unique laterally emitted surface second harmonic generation (SHG) in a single ZnTe nanowire. The highly directional surface SHG signal that polarizes along the direction vertical to the nanowire growth axis, is significantly larger than the bulk SHG contribution, indicating a high efficiency of surface SHG. Two strong SHG peaks are observed on both sides of the nanowire surfaces in the far-field scanning images, which is further supported by FDTD simulations, demonstrating that the unique laterally emitted signal is ascribed to surface SHG in the ZnTe nanowire. The surface SHG in a single ZnTe nanowire with unique lateral emission and high conversion efficiency shows great potential applications in short-wavelength nanolasers, nonlinear microscopy and polarization dependent photonic integrating.

physics.optics↗
arXiv2013

Comprehensive Constraints on a Spin-3/2 Singlet Particle as a Dark Matter Candidate

We consider the proposal that dark matter (DM) is composed of a spin-3/2 particle that is a singlet of the standard model (SM). Its leading effective interactions with ordinary matter involve a pair of their fields and a pair of SM fermions, in the form of products of chiral currents. We make a comprehensive analysis on possible phenomenological effects of the interactions in various experiments and observations. These include collider searches for monojet plus missing transverse energy events, direct detections of DM scattering off nuclei, possible impacts on the gamma rays and antiproton-to-proton flux ratio in cosmic rays, and the observed relic density. The current data already set strong constraints on the effective interactions in a complementary manner. The constraint from collider searches is most effective at a relatively low mass of DM, and the antiproton-to-proton flux ratio offers the best bound for a heavy DM, while the spin-independent direct detection is the best in between. For DM mass of order 10 GeV to 1 TeV, the effective interaction scale is constrained to be typically above a few tens TeV.

hep-ph↗