Two-loop corrections to the decay rate of parapositronium

Order $\alpha^2$ corrections to the decay rate of parapositronium are calculated. A QED scattering calculation of the amplitude for electron-positron annihilation into two photons at threshold is combined with the technique of effective field theory to determine an NRQED Hamiltonian, which is then used in a bound state calculation to determine the decay rate. Our result for the two-loop correction is $5.1243(33)$ in units of $(\alpha/\pi)^2$ times the lowest order rate. This is consistent with but more precise than the result $5.1(3)$ of a previous calculation.Comment: 26 pages, 7 figure

Radiative Corrections to One-Photon Decays of Hydrogenic Ions

Radiative corrections to the decay rate of n=2 states of hydrogenic ions are calculated. The transitions considered are the M1 decay of the 2s state to the ground state and the E1(M2) decays of the $2p_{1/2}$ and $2p_{3/2}$ states to the ground state. The radiative corrections start in order $\alpha (Z \alpha)^2$, but the method used sums all orders of $Z\alpha$. The leading $\alpha (Z\alpha)^2$ correction for the E1 decays is calculated and compared with the exact result. The extension of the calculational method to parity nonconserving transitions in neutral atoms is discussed.Comment: 22 pages, 2 figure

Quantum lump dynamics on the two-sphere

It is well known that the low-energy classical dynamics of solitons of Bogomol'nyi type is well approximated by geodesic motion in M_n, the moduli space of static n-solitons. There is an obvious quantization of this dynamics wherein the wavefunction evolves according to the Hamiltonian H_0 equal to (half) the Laplacian on M_n. Born-Oppenheimer reduction of analogous mechanical systems suggests, however, that this simple Hamiltonian should receive corrections including k, the scalar curvature of M_n, and C, the n-soliton Casimir energy, which are usually difficult to compute, and whose effect on the energy spectrum is unknown. This paper analyzes the spectra of H_0 and two corrections to it suggested by work of Moss and Shiiki, namely H_1=H_0+k/4 and H_2=H_1+C, in the simple but nontrivial case of a single CP^1 lump moving on the two-sphere. Here M_1=TSO(3), a noncompact kaehler 6-manifold invariant under an SO(3)xSO(3) action, whose geometry is well understood. The symmetry gives rise to two conserved angular momenta, spin and isospin. A hidden isometry of M_1 is found which implies that all three energy spectra are symmetric under spin-isospin interchange. The Casimir energy is found exactly on the zero section of TSO(3), and approximated numerically on the rest of M_1. The lowest 19 eigenvalues of H_i are found for i=0,1,2, and their spin-isospin and parity compared. The curvature corrections in H_1 lead to a qualitatively unchanged low-level spectrum while the Casimir energy in H_2 leads to significant changes. The scaling behaviour of the spectra under changes in the radii of the domain and target spheres is analyzed, and it is found that the disparity between the spectra of H_1 and H_2 is reduced when the target sphere is made smaller.Comment: 35 pages, 3 figure

Many-body-QED perturbation theory: Connection to the Bethe-Salpeter equation

The connection between many-body theory (MBPT)--in perturbative and non-perturbative form--and quantum-electrodynamics (QED) is reviewed for systems of two fermions in an external field. The treatment is mainly based upon the recently developed covariant-evolution-operator method for QED calculations [Lindgren et al. Phys. Rep. 389, 161 (2004)], which has a structure quite akin to that of many-body perturbation theory. At the same time this procedure is closely connected to the S-matrix and the Green's-function formalisms and can therefore serve as a bridge between various approaches. It is demonstrated that the MBPT-QED scheme, when carried to all orders, leads to a Schroedinger-like equation, equivalent to the Bethe-Salpeter (BS) equation. A Bloch equation in commutator form that can be used for an "extended" or quasi-degenerate model space is derived. It has the same relation to the BS equation as has the standard Bloch equation to the ordinary Schroedinger equation and can be used to generate a perturbation expansion compatible with the BS equation also for a quasi-degenerate model space.Comment: Submitted to Canadian J of Physic

The natural capital accounting opportunity: Let s really do the numbers

This work was conducted as a part of the “Accounting for U.S. Ecosystem Services at National and Subnational Scales” working group supported by the National Socio-Environmental Synthesis Center under funding received from the National Science Foundation (grant no. DBI-1052875) and the US Geological Survey John Wesley Powell Center for Analysis and Synthesis (grant no. GX16EW00ECSV00)

The theta^+ baryon in soliton models: large Nc QCD and the validity of rigid-rotor quantization

A light collective theta+ baryon state (with strangeness +1) was predicted via rigid-rotor collective quantization of SU(3) chiral soliton models. This paper explores the validity of this treatment. A number of rather general analyses suggest that predictions of exotic baryon properties based on this approximation do not follow from large Nc QCD. These include an analysis of the baryon's width, a comparison of the predictions with general large Nc consistency conditions of the Gervais-Sakita-Dashen-Manohar type; an application of the technique to QCD in the limit where the quarks are heavy; a comparison of this method with the vibration approach of Callan and Klebanov; and the 1/Nc scaling of the excitation energy. It is suggested that the origin of the problem lies in an implicit assumption in the that the collective motion is orthogonal to vibrational motion. While true for non-exotic motion, the Wess-Zumino term induces mixing at leading order between collective and vibrational motion with exotic quantum numbers. This suggests that successful phenomenological predictions of theta+ properties based on rigid-rotor quantization were accidental.Comment: 19 pages; A shorter more readable versio

Estimation of cooling rates during close-coupled gas atomization using secondary dendrite arm spacing measurement

Al-4 wt pct Cu alloy has been gas atomized using a commercial close-coupled gas-atomization system. The resulting metal powders have been sieved into six size fractions, and the SDAS has been determined using electron microscopy. Cooling rates for the powders have been estimated using a range of published conversion factors for Al-Cu alloy, with reasonable agreement being found between sources. We find that cooling rates are very low relative to those often quoted for gas-atomized powders, of the order of 10 K s for sub-38 μm powders. We believe that a number of numerical studies of gas atomization have overestimated the cooling rate during solidification, probably as a consequence of overestimating the differential velocity between the gas and the particles. From the cooling rates measured in the current study, we estimate that such velocities are unlikely to exceed 20 m s

Ten simple rules for implementing open and reproducible research practices after attending a training course

Open, reproducible, and replicable research practices are a fundamental part of science. Training is often organized on a grassroots level, offered by early career researchers, for early career researchers. Buffet style courses that cover many topics can inspire participants to try new things; however, they can also be overwhelming. Participants who want to implement new practices may not know where to start once they return to their research team. We describe ten simple rules to guide participants of relevant training courses in implementing robust research practices in their own projects, once they return to their research group. This includes (1) prioritizing and planning which practices to implement, which involves obtaining support and convincing others involved in the research project of the added value of implementing new practices; (2) managing problems that arise during implementation; and (3) making reproducible research and open science practices an integral part of a future research career. We also outline strategies that course organizers can use to prepare participants for implementation and support them during this process

Overview of the Characteristic Features of the Magnetic Phase Transition with Regards to the Magnetocaloric Effect: the Hidden Relationship Between Hysteresis and Latent Heat

This article was published in the journal, Metallurgical and Materials Transactions E [Springer / © The Minerals, Metals & Materials Society and ASM International]. The final publication is available at Springer via http://dx.doi.org/10.1007/s40553-014-0015-8The magnetocaloric effect has seen a resurgence in interest over the last 20 years as a means towards an alternative energy efficient cooling method. This has resulted in a concerted effort to develop the so-called “giant” magnetocaloric materials with large entropy changes that often come at the expense of hysteretic behavior. But do the gains offset the disadvantages? In this paper, we review the relationship between the latent heat of several giant magnetocaloric systems and the associated magnetic field hysteresis. We quantify this relationship by the parameter Δμ 0 H/ΔS L, which describes the linear relationship between field hysteresis, Δμ 0 H, and entropy change due to latent heat, ΔS L. The general trends observed in these systems suggest that itinerant magnets appear to consistently show large ΔS L accompanied by small Δμ 0 H (Δμ 0 H/ΔS L = 0.02 ± 0.01 T/(J K−1 kg−1)), compared to local moment systems, which show significantly larger Δμ 0 H as ΔS L increases (Δμ 0 H/ΔS L = 0.14 ± 0.06 T/(J K−1 kg−1))

J/ψ polarization in p+p collisions at s=200 GeV in STAR

AbstractWe report on a polarization measurement of inclusive J/ψ mesons in the di-electron decay channel at mid-rapidity at 2<pT<6 GeV/c in p+p collisions at s=200 GeV. Data were taken with the STAR detector at RHIC. The J/ψ polarization measurement should help to distinguish between different models of the J/ψ production mechanism since they predict different pT dependences of the J/ψ polarization. In this analysis, J/ψ polarization is studied in the helicity frame. The polarization parameter λθ measured at RHIC becomes smaller towards high pT, indicating more longitudinal J/ψ polarization as pT increases. The result is compared with predictions of presently available models