Radiative pion capture by a nucleon

The differential cross sections for $\pi^- p \to \gamma n$ and $\pi^+ n \to \gamma p$ are computed up to $O(p^3)$ in heavy baryon chiral perturbation theory (HBChPT). The expressions at $O(p)$ and $O(p^2)$ have no free parameters. There are three unknown parameters at $O(p^3)$, low energy constants of the HBChPT Lagrangian, which are determined by fitting to experimental data. Two acceptable fits are obtained, which can be separated by comparing with earlier dispersion relation calculations of the inverse process. Expressions for the multipoles, with emphasis on the p-wave multipoles, are obtained and evaluated at threshold. Generally the results obtained from the best of the two fits are in good agreement with the dispersion relation predictions.Comment: 24 pages, Latex, using RevTe

The anomalous chiral perturbation theory meson Lagrangian to order p6p^6 revisited

We present a revised and extended construction of the mesonic Lagrangian density in chiral perturbation theory (ChPT) at order $p^6$ in the anomalous (or epsilon) sector, ${\cal{L}}_{6,\epsilon}$. After improving several aspects of the strategy we used originally, i.e., a more efficient application of partial integration, the implementation of so-called Bianchi identities, and additional trace relations, we find the new monomial sets to include 24 $SU(N_f)$, 23 SU(3), and 5 SU(2) elements. Furthermore, we introduce 8 supplementary terms due to the extension of the chiral group to $SU(N_f)_L \times SU(N_f)_R \times U(1)_V$.Comment: 21 pages, Latex, using RevTe

Polarized photons in radiative muon capture

We discuss the measurement of polarized photons arising from radiative muon capture. The spectrum of left circularly polarized photons or equivalently the circular polarization of the photons emitted in radiative muon capture on hydrogen is quite sensitive to the strength of the induced pseudoscalar coupling constant $g_P$. A measurement of either of these quantities, although very difficult, might be sufficient to resolve the present puzzle resulting from the disagreement between the theoretical prediction for $g_P$ and the results of a recent experiment. This sensitivity results from the absence of left-handed radiation from the muon line and from the fact that the leading parts of the radiation from the hadronic lines, as determined from the chiral power counting rules of heavy-baryon chiral perturbation theory, all contain pion poles.Comment: 10 pages, 6 figure

biomimetic sensor suite for flight control of a micromechanical flying insect: design and experimental results, in

Four types of biomimetic sensors have been designed and simulated for flight control of a robotic flying insect. The ocelli use four photodiodes to detect changes in light intensity in the surrounding. The halteres use piezo-actuated vibrating structures to sense the Coriolis forces to detect angular velocities. The optic flow sensors consist of linear arrays of elementary motion detectors to register optic flows. The MEMS compass uses three metal loops to detect changes in the magnetic field. Despite simplicity and novelty, the preliminary tests on these devices showed promising performances for using such biomimetic sensors on a robotic flying insect.

Baryon chiral perturbation theory with virtual photons and leptons

We construct the general pion-nucleon SU(2) Lagrangian including both virtual photons and leptons for relativistic baryon chiral perturbation theory up to fourth order. We include the light leptons as explicit dynamical degrees of freedom by introducing new building blocks which represent these leptons.Comment: 11 page

Field transformations and simple models illustrating the impossibility of measuring off-shell effects

In the context of simple models illustrating field transformations in Lagrangian field theories we discuss the impossibility of measuring off-shell effects in nucleon-nucleon bremsstrahlung, Compton scattering, and related processes. To that end we introduce a simple phenomenological Lagrangian describing nucleon-nucleon bremsstrahlung and perform an appropriate change of variables leading to different off-shell behavior in the nucleon-nucleon amplitude as well as the photon-nucleon vertex. As a result we obtain a class of equivalent Lagrangians, generating identical S-matrix elements, of which the original Lagrangian is but one representative. We make use of this property in order to show that what appears as an off-shell effect in an S-matrix element for one Lagrangian may originate in a contact term from an equivalent Lagrangian. By explicit calculation we demonstrate for the case of nucleon-nucleon bremsstrahlung as well as nucleon Compton scattering the equivalence of observables from which we conclude that off-shell effects cannot in any unambiguous way be extracted from an S-matrix element. Finally, we also discuss some implications of introducing off-shell effects on a phenomenological basis, resulting from the requirement that the description of one process be consistent with that of other processes described by the same Lagrangian.Comment: 19 pages, Latex, using RevTe

The Off-Shell Nucleon-Nucleon Amplitude: Why it is Unmeasurable in Nucleon-Nucleon Bremsstrahlung

Nucleon-nucleon bremsstrahlung has long been considered a way of getting information about the off-shell nucleon-nucleon amplitude which would allow one to distinguish among nucleon-nucleon potentials based on their off-shell properties. There have been many calculations and many experiments devoted to this aim. We show here, in contrast to this standard view, that such off-shell amplitudes are not measurable as a matter of principle. This follows formally from the invariance of the S-matrix under transformations of the fields. This result is discussed here and illustrated via two simple models, one applying to spin zero, and one to spin one half, processes. The latter model is very closely related to phenomenological models which have been used to study off-shell effects at electromagnetic vertices.Comment: 6 pages, Latex, uses FBSsuppl.cls - Invited plenary talk at the Asia Pacific Conference on Few Body Problems in Physics, Noda/Kashiwa, Japan, August, 1999 - To be published in Few Body Systems Supp