204,029 research outputs found
New Constraints (and Motivations) for Abelian Gauge Bosons in the MeV-TeV Mass Range
We survey the phenomenological constraints on abelian gauge bosons having
masses in the MeV to multi-GeV mass range (using precision electroweak
measurements, neutrino-electron and neutrino-nucleon scattering, electron and
muon anomalous magnetic moments, upsilon decay, beam dump experiments, atomic
parity violation, low-energy neutron scattering and primordial
nucleosynthesis). We compute their implications for the three parameters that
in general describe the low-energy properties of such bosons: their mass and
their two possible types of dimensionless couplings (direct couplings to
ordinary fermions and kinetic mixing with Standard Model hypercharge). We argue
that gauge bosons with very small couplings to ordinary fermions in this mass
range are natural in string compactifications and are likely to be generic in
theories for which the gravity scale is systematically smaller than the Planck
mass - such as in extra-dimensional models - because of the necessity to
suppress proton decay. Furthermore, because its couplings are weak, in the
low-energy theory relevant to experiments at and below TeV scales the charge
gauged by the new boson can appear to be broken, both by classical effects and
by anomalies. In particular, if the new gauge charge appears to be anomalous,
anomaly cancellation does not also require the introduction of new light
fermions in the low-energy theory. Furthermore, the charge can appear to be
conserved in the low-energy theory, despite the corresponding gauge boson
having a mass. Our results reduce to those of other authors in the special
cases where there is no kinetic mixing or there is no direct coupling to
ordinary fermions, such as for recently proposed dark-matter scenarios.Comment: 49 pages + appendix, 21 figures. This is the final version which
appears in JHE
A cryogenic dc-dc power converter for a 100kW synchronous HTS generator at liquid nitrogen temperatures
A dc-dc converter has been developed for retrofitting inside the vacuum space of the HTS rotor of a synchronous generator. The heavy copper sections of the current leads used for energising the HTS field winding were replaced by cryogenic power electronics; consisting of the converter and a rotor control unit. The converter board was designed using an H-bridge configuration with two 5A rated wires connecting the cryogenic boards to the stator control board located on the outside of the generator and drawing power from a (5A, 50V) dc power source. The robustness of converter board was well demonstrated when it was powered up from a cold start at 82K. When charging the field winding with moderate currents (30A), the heat in-leak to the âcoldâ rotor core was only 2W. It continued to function down to 74K, surviving several quenches. However, the quench protection function failed when injecting 75A into the field winding, resulting in the burn out of one of the DC-link capacitors. The magnitudes of the critical currents measured with the original current leads were compared to the quench currents, which was defined as the current which triggered quench protection protocol. The difference between the two currents was rather large, (~20A). However, additional measurements using a single HTS coil in liquid nitrogen found that this reduction should not be so dramatic and in the region of 4A. Our conclusions identified the converterâs switching voltage and its operating frequency as two parameters, which could have contributed to lowering the quench current. Magnetic fields and eddy currents are expected to be more prominent the field winding and its impact on the converter also need further investigation
A Simple Model for Predicting Sprint Race Times Accounting for Energy Loss on the Curve
The mathematical model of J. Keller for predicting World Record race times,
based on a simple differential equation of motion, predicted quite well the
records of the day. One of its shortcoming is that it neglects to account for a
sprinter's energy loss around a curve, a most important consideration
particularly in the 200m--400m. An extension to Keller's work is considered,
modeling the aforementioned energy loss as a simple function of the centrifugal
force acting on the runner around the curve. Theoretical World Record
performances for indoor and outdoor 200m are discussed, and the use of the
model at 300m is investigated. Some predictions are made for possible 200m
outdoor and indoor times as run by Canadian 100m WR holder Donovan Bailey,
based on his 100m final performance at the 1996 Olympic Games in Atlanta.Comment: 20pp, latex; submitted to the "Canadian Journal of Physics
Hawthorn Lace Bug (Hemiptera: Tingidae), First Record of Injury to Roses, with a Review of Host Plants
Hawthorn lace bug, Coryrhucha cydoniae (Fitch), is reported for the first time as damag- ing roses. Injury to climbing and hybrid Tea roses is described, and a list of known host plants is provided based on observations in Pennsylvania and review of literature. Preferred hosts are native and cultivated species of Amelanchier and Craraegus and ornamental Cotoneasrer and Pyracanrha. Damage to crabapple. fruit trees, mountain ash. and other rosaceous plants may occur when they are grown near more favored hosts
The Carina Project IX: on Hydrogen and helium burning variables
We present new multi-band (UBVI) time-series data of helium burning variables
in the Carina dwarf spheroidal galaxy. The current sample includes 92 RR
Lyrae-six of them are new identifications-and 20 Anomalous Cepheids, one of
which is new identification. The analysis of the Bailey diagram shows that the
luminosity amplitude of the first overtone component in double-mode variables
is located along the long-period tail of regular first overtone variables,
while the fundamental component is located along the short-period tale of
regular fundamental variables. This evidence further supports the transitional
nature of these objects. Moreover, the distribution of Carina double-mode
variables in the Petersen diagram (P_1/P_0 vs P_0) is similar to metal-poor
globulars (M15, M68), to the dwarf spheroidal Draco and to the Galactic Halo.
This suggests that the Carina old stellar population is metal-poor and affected
by a small spread in metallicity. We use trigonometric parallaxes for five
field RR Lyrae stars to provide an independent estimate of the Carina distance
using the observed reddening free Period--Wesenheit [PW, (BV)] relation. Theory
and observations indicate that this diagnostic is independent of metallicity.
We found a true distance modulus of \mu=20.01\pm0.02 (standard error of the
mean) \pm0.05 (standard deviation) mag. We also provided independent estimates
of the Carina true distance modulus using four predicted PW relations (BV, BI,
VI, BVI) and we found: \mu=(20.08\pm0.007\pm0.07) mag,
\mu=(20.06\pm0.006\pm0.06) mag, \mu=(20.07\pm0.008\pm0.08) mag and
\mu=(20.06\pm0.006\pm0.06) mag. Finally, we identified more than 100 new SX
Phoenicis stars that together with those already known in the literature (340)
make Carina a fundamental laboratory to constrain the evolutionary and
pulsation properties of these transitional variables.Comment: 44 pages, 13 tables, 13 figures, Accepted for publication in Ap
The Impact of Dual Enrollment on College Degree Attainment: Do Low-SES Students Benefit?
Dual enrollment in high school is viewed by many as one mechanism for increasing college admission and completion of low-income students. However, little evidence demonstrates that these students discretely benefit from dual enrollment and whether these programs narrow attainment gaps vis-Ă -vis students from middle-class or affluent family backgrounds. Using the National Education Longitudinal Study (N = 8,800), this study finds significant benefits in boosting rates of college degree attainment for low-income students while holding weaker effects for peers from more affluent backgrounds. These results remain even with analyses from newer data of college freshman of 2004. This report conducts sensitivity analyses and found that these results are robust to relatively large unobserved confounders. However, expanding dual enrollment programs would modestly reduce gaps in degree attainment
- âŠ