The relationship between the matrix element of a local operator and the Fourier transform of the associated form factor fails for systems such as the nucleon where its intrinsic size is of order its ...Compton wavelength. Although one can conceive of an intrinsic charge density distribution in the proton, there does not seem to be an unambiguous way to define, compute, or measure it precisely.
The first evidence for quantum chromodynamics (QCD), the theory of the strong interactions, came from the systematics of baryon and meson spectroscopy. An important early observation was the apparent ...absence of exotics, baryons requiring more than three quarks or mesons requiring more than
q
q
¯
. Years later, QCD is well established, hadron spectroscopy has been relatively inactive, but the absence of exotics remains poorly understood. The recent observation of narrow, prominent exotic baryons has stirred up new interest in hadron spectroscopy. At present the experimental situation is confused; so is theory. The recent discoveries are striking. So too is the complete
absence of exotic mesons, and, except for the recent discoveries, of exotic baryons as well. Whether or not the new states are confirmed, the way we look at complicated states of confined quarks and gluons has changed. Perhaps the most lasting result, and the one emphasized in these notes, is a new appreciation for the role of diquark correlations in QCD.
This work introduces a novel methodology for the quantification of uncertainties associated with potential energy surfaces (PESs) computed from first-principles quantum mechanical calculations. The ...methodology relies on Bayesian inference and machine learning techniques to construct a stochastic PES and to express the inadequacies associated with the ab initio data points and their fit. By combining high fidelity calculations and reduced-order modeling, the resulting stochastic surface is efficiently forward propagated via quasi-classical trajectory and master equation calculations. In this way, the PES contribution to the uncertainty on predefined quantities of interest (QoIs) is explicitly determined. This study is done at both microscopic (e.g., rovibrational-specific rate coefficients) and macroscopic (e.g., thermal and chemical relaxation properties) levels. A correlation analysis is finally applied to identify the PES regions that require further refinement, based on their effects on the QoI reliability. The methodology is applied to the study of singlet (11A′) and quintet (25A′) PESs describing the interaction between O2 molecules and O atoms in their ground electronic state. The investigation of the singlet surface reveals a negligible uncertainty on the kinetic properties and relaxation times, which are found to be in excellent agreement with the ones previously published in the literature. On the other hand, the methodology demonstrated significant uncertainty on the quintet surface, due to inaccuracies in the description of the exchange barrier and the repulsive wall. When forward propagated, this uncertainty is responsible for the variability of 1 order of magnitude in the vibrational relaxation time and of factor four in the exchange reaction rate coefficient, both at 2500 K.
We collected mercury observations as part of the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign over the southeastern US between 1 June and ...15 July 2013. We use the GEOS-Chem chemical transport model to interpret these observations and place new constraints on bromine radical initiated mercury oxidation chemistry in the free troposphere. We find that the model reproduces the observed mean concentration of total atmospheric mercury (THg) (observations: 1.49 ± 0.16 ng m−3, model: 1.51 ± 0.08 ng m−3), as well as the vertical profile of THg. The majority (65 %) of observations of oxidized mercury (Hg(II)) were below the instrument's detection limit (detection limit per flight: 58–228 pg m−3), consistent with model-calculated Hg(II) concentrations of 0–196 pg m−3. However, for observations above the detection limit we find that modeled Hg(II) concentrations are a factor of 3 too low (observations: 212 ± 112 pg m−3, model: 67 ± 44 pg m−3). The highest Hg(II) concentrations, 300–680 pg m−3, were observed in dry (RH < 35 %) and clean air masses during two flights over Texas at 5–7 km altitude and off the North Carolina coast at 1–3 km. The GEOS-Chem model, back trajectories and observed chemical tracers for these air masses indicate subsidence and transport from the upper and middle troposphere of the subtropical anticyclones, where fast oxidation of elemental mercury (Hg(0)) to Hg(II) and lack of Hg(II) removal lead to efficient accumulation of Hg(II). We hypothesize that the most likely explanation for the model bias is a systematic underestimate of the Hg(0) + Br reaction rate. We find that sensitivity simulations with tripled bromine radical concentrations or a faster oxidation rate constant for Hg(0) + Br, result in 1.5–2 times higher modeled Hg(II) concentrations and improved agreement with the observations. The modeled tropospheric lifetime of Hg(0) against oxidation to Hg(II) decreases from 5 months in the base simulation to 2.8–1.2 months in our sensitivity simulations. In order to maintain the modeled global burden of THg, we need to increase the in-cloud reduction of Hg(II), thus leading to faster chemical cycling between Hg(0) and Hg(II). Observations and model results for the NOMADSS campaign suggest that the subtropical anticyclones are significant global sources of Hg(II).
A systematic molecular dynamics study shows that the contact angle of a water droplet on graphite changes significantly as a function of the water−carbon interaction energy. Together with the ...observation that a linear relationship can be established between the contact angle and the water monomer binding energy on graphite, a new route to calibrate interaction potential parameters is presented. Through a variation of the droplet size in the range from 1000 to 17 500 water molecules, we determine the line tension to be positive and on the order of 2 × 10-10 J/m. To recover a macroscopic contact angle of 86°, a water monomer binding energy of −6.33 kJ mol-1 is required, which is obtained by applying a carbon−oxygen Lennard-Jones potential with the parameters εCO = 0.392 kJ mol-1 and σCO = 3.19 Å. For this new water−carbon interaction potential, we present density profiles and hydrogen bond distributions for a water droplet on graphite.
We study the Casimir force acting on a conducting piston with arbitrary cross section. We find the exact solution for a rectangular cross section and the first three terms in the asymptotic expansion ...for small height to width ratio when the cross section is arbitrary. Though weakened by the presence of the walls, the Casimir force turns out to be always attractive. Claims of repulsive Casimir forces for related configurations, like the cube, are invalidated by cutoff dependence.
We develop an exact method for computing the Casimir energy between arbitrary compact objects, either dielectrics or perfect conductors. The energy is obtained as an interaction between multipoles, ...generated by quantum current fluctuations. The objects' shape and composition enter only through their scattering matrices. The result is exact when all multipoles are included, and converges rapidly. A low frequency expansion yields the energy as a series in the ratio of the objects' size to their separation. As an example, we obtain this series for two dielectric spheres and the full interaction at all separations for perfectly conducting spheres.
We find the exact Casimir force between a plate and a cylinder, a geometry intermediate between parallel plates, where the force is known exactly, and the plate sphere, where it is known at large ...separations. The force has an unexpectedly weak decay approximately L/H3 ln(H/R) at large plate-cylinder separations H (L and R are the cylinder length and radius), due to transverse magnetic modes. Path integral quantization with a partial wave expansion additionally gives a qualitative difference for the density of states of electric and magnetic modes, and corrections at finite temperatures.
In the majority of these studies, cTnT or cTnI cutoff concentrations were higher than either the 99th percentile or the 10% CV value. These and other studies have consistently shown that ...postprocedural increases in cTn and/or CK-MB are associated with major adverse clinical events.
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