On-Shell Gauge Invariant Three-Point Amplitudes

Assuming locality, Lorentz invariance and parity conservation we obtain a set of differential equations governing the 3-point interactions of massless bosons, which in turn determines the polynomial ring of these amplitudes. We derive all possible 3-point interactions for tensor fields with polarisations that have total symmetry and mixed symmetry under permutations of Lorentz indices. Constraints on the existence of gauge-invariant cubic vertices for totally symmetric fields are obtained in general spacetime dimensions and are compared with existing results obtained in the covariant and light-cone approaches. Expressing our results in spinor helicity formalism we reproduce the perhaps mysterious mismatch between the covariant approach and the light cone approach in 4 dimensions. Our analysis also shows that there exists a mismatch, in the 3-point gauge invariant amplitudes corresponding to cubic self-interactions, between a scalar field $\phi$ and an antisymmetric rank-2 tensor field $A_{\mu\nu}$. Despite the well-known fact that in 4 dimensions rank-2 anti-symmetric fields are dual to scalar fields in free theories, such duality does not extend to interacting theories.Comment: significantly revised, final version published in JHE

Electron Energy Distributions at Relativistic Shock Sites: Observational Constraints from the Cygnus A Hotspots

We report new detections of the hotspots in Cygnus A at 4.5 and 8.0 microns with the Spitzer Space Telescope. Together with detailed published radio observations and synchrotron self-Compton modeling of previous X-ray detections, we reconstruct the underlying electron energy spectra of the two brightest hotspots (A and D). The low-energy portion of the electron distributions have flat power-law slopes (s~1.5) up to the break energy which corresponds almost exactly to the mass ratio between protons and electrons; we argue that these features are most likely intrinsic rather than due to absorption effects. Beyond the break, the electron spectra continue to higher energies with very steep slopes s>3. Thus, there is no evidence for the `canonical' s=2 slope expected in 1st order Fermi-type shocks within the whole observable electron energy range. We discuss the significance of these observations and the insight offered into high-energy particle acceleration processes in mildly relativistic shocks.Comment: 5 pages, 3 figures, in Extragalactic Jets: Theory and Observation from Radio to Gamma Ray, Eds. T. A. Rector and D. S. De Youn

Strange-Beauty Meson Production at ppˉp\bar p Colliders

The production rates and transverse momentum distributions of the strange-beauty mesons $B_s$ and $B_s^*$ at $p\bar p$ colliders are calculated assuming fragmentation is the dominant process. Results are given for the Tevatron in the large transverse momentum region, where fragmentation is expected to be most important.Comment: Minor changes in the discussion section. Also available at http://www.ph.utexas.edu/~cheung/paper.htm

Evolution of optical gain properties through three generations of electroluminescent fluorene-based polymers

Conjugated polymer semiconductors combine the processing and mechanical characteristics of plastics with the desirable optical and electronic properties of semiconductors. The aim of the research reported in this thesis was to investigate the evolution of the optical gain properties through three generations of electroluminescent fluorene-based polymers. Detailed optical, optoelectrical and gain characterisations were carried out on a range of different electroluminescent polyfluorene-based polymers. It was discovered that not all of the polymers were gain media as some were unable to give ASE. SC006 was found to be the most intriguing material among the rest of the tested polymers; this third generation polymer was found to be a non ASE material while achieving a high PLQE of 96% with 1.3ns-long excited state lifetime. Therefore it was evident that optimised highly efficient light emitting conjugate polymers for PLEDs are not necessarily effective optical gain media, and high steady state PLQE and long excited state lifetime are insufficient for good optical gain properties. Furthermore, in order to investigate the ASE quenching mechanism in SC006, a series of solvatochromism studies were carried out on this polymer. The time-resolved PL characteristics were compared between polymers of second and third generations. The combination of intermolecular and intramolecular energy transfer process was found to be responsible for the ASE quenching. Moreover, the effects of the differences in Yamamoto and Suzuki synthesis routes on optical gain properties of the first generation statistical and alternating copolymers were investigated and were found to be insignificant. Finally, the application of the gain quenching mechanism was demonstrated by an optical switching process performed on a polymer DFB laser. This enabled complete control over the laser emission from the polymer laser, thus achieving a minimum of a thirty fold reduction in the visible light output in the presence of a control pulse