Scale of mass generation for Majorana neutrinos
Scales of mass generation for Majorana neutrinos (as well as quarks and leptons) can be probed from high energy 2 → n inelastic scattering involving a multiple longitudinal gauge boson final state. We demonstrate that the unitarity of 2 → n scattering puts the strongest new upper limit on the scale of fermion mass generation, independent of the electroweak symmetry breaking scale v = ( √ 2GF) 1 2 . Strikingly, for Majorana neutrinos (quarks and leptons), we find that the strongest 2 → n limits fall in a narrow range, 136 − 170 TeV (3 − 107 TeV) with n = 20 − 24 (n = 2 − 12), depending on the observed fermion masses. Physical implications are discussed.
Scale of fermion mass generation
Unitarity of longitudinal weak vector boson scattering implies an upper bound on the scale of electroweak symmetry breaking, ${\ensuremath{\Lambda}}_{\mathrm{EWSB}}\ensuremath{\equiv}\sqrt{8\ensuremath{\pi}}v\ensuremath{\approx}1\mathrm{TeV}.$ Appelquist and Chanowitz [Phys. Rev. Lett. 59, 2405 (1987)] have derived an analogous upper bound on the scale of fermion mass generation, proportional to ${v}^{2}{/m}_{f},$ by considering the scattering of same-helicity fermions into pairs of longitudinal weak vector bosons in a theory without a standard Higgs boson. We show that there is no upper bound, beyond that on the scale of electroweak symmetry breaking, in such a theory. This result is obtained by considering the same process, but with a large number of longitudinal weak vector bosons in the final state. We further argue that there is no scale of (Dirac) fermion mass generation in the standard model. In contrast, there is an upper bound on the scale of Majorana-neutrino mass generation, given by ${\ensuremath{\Lambda}}_{\mathrm{Maj}}\ensuremath{\equiv}4\ensuremath{\pi}{v}^{2}{/m}_{\ensuremath{\nu}}.$ In general, the upper bound on the scale of fermion mass generation depends on the dimensionality of the interaction responsible for generating the fermion mass. We explore the scale of fermion mass generation in a variety of excursions from the standard model: models with fermions in nonstandard representations, a theory with higher-dimension interactions, a two-Higgs-doublet model, and models without a Higgs boson.
The effect of collimation on image quality and radiation safety in digital radiography of small animals
Collimating the primary x-ray beam to the specific area of anatomic interest has traditionally been strongly recommended to produce high quality radiographic images, even though some small animal patients can fit their entire torso on a single radiographic casette or detector. However, modern digital radiography may allow for whole-body projections for these patients at similar quality to specific collimated projections. Lateral and ventrodorsal projections of 31 small dogs were obtained to evaluate the effect of collimation on the quality of the resulting digital radiograph. Each projection was performed using both a collimated and whole-body technique. Scatter radiation was detected during each exposure as well to determine the effect of collimation on radiation safety for veterinary personnel. The resulting images were cropped to portray only the thorax, and were evaluated by five radiologists blinded to the procedure used for each image. Visual grading characteristics analysis was performed. An overall trend towards higher image quality was found in the collimated approach compared to the whole-body, but in most cases this change was not significant. Overall image quality for the ventrodorsal images was found to be significantly better in the collimated approach compared to the whole-body. Evaluation of image histograms suggests that the application of digital processing and the look-up table differs between them, and this may account for a large portion of any quality difference. Over twice as much scatter radiation was detected during the whole-body images as during the collimated images. Overall image quality was rated as adequate to high for almost all images with both approaches. Adequate collimation remains recommended to provide the highest quality images with the lowest exposure to veterinary personnel.