Observation of the π0 → e+e− decay

We have observed for the first time the very rare π⁰ → e⁺e^? decay. The tagged π⁰'s come from K⁺ → π⁺π⁰ decays in flight. We find the branching ratio to be (2.23_?1.1^+2.4) × 10^?7 (90% C.L.). This value is well above the QED unitary limit and comes in rough agreement with a second-order electromagnetic process mediated by vector mesons.     

The use of QCD in hadronic cascades between heavy quark-antiquark bound states

We argue that a perturbative calculation of the spectra and rates of hadronic transitions between heavy quark-antiquark bound states makes sense within QCD provided suitable spin-parity projections are performed. Applications are made to various kinds of cascades. We obtain the correct ψ′ → J/ψππ mass spectrum. The cascades involving the emission of at least three gluons appear very strongly suppressed as compared to those allowing two-gluon emission. Application to the ? family is made.     

A comparative photophysical and photochemical study of nitropyrene isomers occurring in the environment

Ground state absorption, first excited-singlet state, and properties of reactive intermediates of mononitropyrene isomers encountered in the atmospheric aerosol have been studied under different conditions that could mimic the environment. The nitro group can present different orientations relative to the pyrene ring depending on its geometric location and could induce differences in the photochemistry of the isomers. The 2-NO(2)Py isomer has the largest red shift and lowest oscillator strength in the UV-visible band associated with the nitro group. The isomers show very low fluorescence yields (10(-3)-10(-4)). Only 1-NO(2)Py and 4-NO(2)Py have phosphorescence emission (Φ(p) ≈ 10(-4)), indicating that the lowest triplet state decays mainly through effective radiationless channels. Laser photolysis produces a low-lying triplet state (τ(T) = 10(-5)-10(-6) s), a long-lived pyrenoxy radical, and a PyNO(2)H radical in solvents in which the triplet can abstract a hydrogen atom. Similar triplet yields were calculated (0.1-0.6) for the isomers, while significant differences in the relative yield of the long-lived species were determined. Differences in the quenching rate constants of the triplet by water and phenols suggest a strong hydrogen-bond interaction with the nitro group in the C-2 position, which provides for radiationless deactivation routes.     

Stability analysis of implicit time discretizations for the Compton-scattering Fokker–Planck equation

The Fokker–Planck equation is a widely used approximation for modeling the Compton scattering of photons in high energy density applications. In this paper, we perform a stability analysis of three implicit time discretizations for the Compton-Scattering Fokker–Planck equation. Specifically, we examine (i) a Semi-Implicit (SI) scheme that employs backward-Euler differencing but evaluates temperature-dependent coefficients at their beginning-of-time-step values, (ii) a Fully Implicit (FI) discretization that instead evaluates temperature-dependent coefficients at their end-of-time-step values, and (iii) a Linearized Implicit (LI) scheme, which is developed by linearizing the temperature dependence of the FI discretization within each time step. Our stability analysis shows that the FI and LI schemes are unconditionally stable and cannot generate oscillatory solutions regardless of time-step size, whereas the SI discretization can suffer from instabilities and nonphysical oscillations for sufficiently large time steps. With the results of this analysis, we present time-step limits for the SI scheme that prevent undesirable behavior. We test the validity of our stability analysis and time-step limits with a set of numerical examples.     

Three-dimensional coronary artery MR imaging using prospective real-time respiratory navigator and linear phase shift processing: comparison with conventional coronary angiography.

Respiratory gating with navigator echo is a recent technique to detect diaphragm position in 3D magnetic resonance (MR) coronary angiography. The purpose of our study was to image proximal coronary arteries and to detect significant stenoses in patients with coronary artery diseases and to compare with contrast enhanced angiography results. Twenty patients with coronary artery diseases who were referred for conventional angiography underwent magnetic resonance angiography (MRA). Three-dimensional gradient echo volumes were acquired using cardiac and respiratory gating and fat suppression. Using reformatted oblique planes and maximum intensity projection technique, visualization coronary segments and detection of significant coronary stenoses were made. Eighty-three coronary segments were analyzed. The sensitivity and specificity were 65% and 93%, respectively. The corresponding positive and negative predictive values were 69% and 91%. This study shows the ability to image correctly coronary arteries and to identify proximal stenoses, but image quality need to be improved for an efficiency detection of coronary artery stenoses in clinical practice.     

Electric quadrupole moment of the neutron-rich 33Al

The nuclear electric quadrupole moment (Q moment) of the neutron-rich nucleus ³³Al (I = 5/2 , T _1/2 = 41 ms) has been measured by the $ \beta$ -ray detected nuclear quadrupole resonance ( $ \beta$ -NQR) method for the first time. The ³³Al nucleus is considered to be on the border of the island of inversion within which a significant intrusion of the pf orbits occurs across the N = 20 shell gap and the intrusion causes an anomalous enhancement of the Q moment. Polarized ³³Al nuclei were produced from ³⁶S (77.5MeV/u beams through the fragmentation process and separated by LISE fragment separator at GANIL. The ³³Al nuclei were implanted into a Al₂O₃ single-crystal plate and the $ \beta$ -NQR spectrum was successfully obtained.     

Nonlinear variants of the TR/BDF2 method for thermal radiative diffusion

We apply the Trapezoidal/BDF2 (TR/BDF2) temporal discretization scheme to nonlinear grey radiative diffusion. This is a scheme that is not well-known within the radiation transport community, but we show that it offers many desirable characteristics relative to other second-order schemes. Several nonlinear variants of the TR/BDF2 scheme are defined and computationally compared with the Crank–Nicholson scheme. It is found for our test problems that the most accurate TR/BDF2 schemes are those that are fully iterated to nonlinear convergence, but the most efficient TR/BDF2 scheme is one based upon a single Newton iteration. It is also shown that neglecting the contributions to the Jacobian matrix from the cross-sections, which is often done due to a lack of smooth interpolations for tabular cross-section data, has a significant impact upon efficiency.     

Synthesis of ortho- and meta-Re(I)-metallocarboranes in water

A series of metallocarboranes of the types rac-[M(CO)3(eta(5)-7-R-7,8-C2B9H11)]-, rac-[M(CO)3(eta(5)-7-R-8-R'-7,8-C2B9H11)]-, and rac-[M(CO)3(eta(5)-7-R-7,9-C2B9H11)]- (M=Re) were prepared by reacting [NEt4]2[Re(CO)3Br3] or [Re(CO)3(OH2)3]Br with the corresponding carboranes in the presence of aqueous solutions of either alkali metal or tetraalkylammonium fluoride salts. Carborane derivatives that were investigated included those containing pyridine, amino, carboxylic acid, carbohydrate, and aryl substituents. During the course of the research, it was discovered that Re metallocarboranes can be prepared directly from the respective closo-clusters under similar reaction conditions used with nido-carboranes. Reaction yields ranged from modest to excellent depending on the carborane isomer and the nature of the cage substituent(s). A crystal structure of an amine-substituted Re metallocarborane was obtained where the complex crystallized in the orthorhombic space group P2(1)2(1)2(1) with a=8.982(2) A, b=11.563(3) A, c=16.811(4) A, alpha=beta=gamma=90 degrees, V=1746.1(7) A3, Z=4, and R1=0.0684.     

Quark jet as a trigger for gluon jet in deep inelastic muon scattering

We calculate, to first order in QCD, the probability for a fast hadron observed at laboratory angle θ _h from the virtual photon directionq, to originate from the quark of a quark-gluon final state. Given this probability, the average gluon energy and angle with respect toq are then calculated. This approach provides a method for gluon jet direction in forthcoming experiments.