Narrow bandwidth, low-emittance positron beams from a laser-wakefield accelerator.

The rapid progress that plasma wakefield accelerators are experiencing is now posing the question as to whether they could be included in the design of the next generation of high-energy electron-positron colliders. However, the typical structure of the accelerating wakefields presents challenging complications for positron acceleration. Despite seminal proof-of-principle experiments and theoretical proposals, experimental research in plasma-based acceleration of positrons is currently limited by the scarcity of positron beams suitable to seed a plasma accelerator. Here, we report on the first experimental demonstration of a laser-driven source of ultra-relativistic positrons with sufficient spectral and spatial quality to be injected in a plasma accelerator. Our results indicate, in agreement with numerical simulations, selection and transport of positron beamlets containing N e + ≥ 10 5 positrons in a 5% bandwidth around 600 MeV, with femtosecond-scale duration and micron-scale normalised emittance. Particle-in-cell simulations show that positron beams of this kind can be guided and accelerated in a laser-driven plasma accelerator, with favourable scalings to further increase overall charge and energy using PW-scale lasers. The results presented here demonstrate the possibility of performing experimental studies of positron acceleration in a laser-driven wakefield accelerator.

Paradental cysts: a role for food impaction in the pathogenesis? A review of cases from Northern Ireland.

We present the clinical, radiographic and histological findings of 15 paradental cysts. These were the second most common type of cyst associated with mandibular third molars, comprising 25% of all lesions at this site. Ten patients were aged 30 years or younger and 13 gave a history of pericoronitis. There was a close relationship between the site of the cyst and the angle of the impacted tooth; the cyst lay mesially to two mesioangularly impacted teeth, buccal to two vertically impacted teeth and distal/distobuccal to the remainder, all but one of which were distoangular in impaction. The pericoronal follicular space was preserved in nine cases, a useful diagnostic sign. Most cysts were lined with epithelium that resembled that seen in radicular cysts, although the lining of three cysts resembled that of unicystic ameloblastoma, a potential diagnostic pitfall. We propose that the relation between location of cyst and angle of impaction points to a role for food impaction in the pathogenesis of paradental cysts.

An overview of economics.

Economics influences health care at every level. Part of that influence is the result of the interaction of buyers and sellers in markets. Another part is the interaction among aggregates of different markets, the government, and international actors. This article provides a short, non-technical introduction to basic theories of both micro and macro economics as a necessary precursor to the understanding of more complex theories and debates about the economics of health care.