9,676 research outputs found
Measuring WWZ and WWgamma coupling constants with Z-pole data
Triple gauge boson couplings between Z, gamma and the W boson are determined
by exploiting their impact on radiative corrections to fermion-pair production
in e+e- interactions at centre-of-mass energies near the Z-pole. Recent values
of observables in the electroweak part of the Standard model are used to
determine the four parameters epsilon_1, epsilon_2, epsilon_3 and epsilon_b. In
a second step the results on the four epsilon parameters are used to determine
the couplings Delta-g^1_Z and Delta-kappa_gamma. For a wide range of scales,
these indirect coupling measurements are more precise than recent direct
measurements at LEP 2 and at the TEVATRON. The Standard model predictions agree
well with these measurements.Comment: 6 pages, 2 tables, 4 figure
Dissipative effects from transport and viscous hydrodynamics
We compare 2->2 covariant transport theory and causal Israel-Stewart
hydrodynamics in 2+1D longitudinally boost invariant geometry with RHIC-like
initial conditions and a conformal e = 3p equation of state. The pressure
evolution in the center of the collision zone and the final differential
elliptic flow v2(pT) from the two theories agree remarkably well for a small
shear viscosity to entropy density ratio eta/s ~ 1/(4 pi), and also for a large
cross section sigma ~ 50 mb. A key to this agreement is keeping ALL terms in
the Israel-Stewart equations of motion. Our results indicate promising
prospects for the applicability of Israel-Stewart dissipative hydrodynamics at
RHIC, provided the shear viscosity of hot and dense quark-gluon matter is
indeed very small for the relevant temperatures T ~ 200-500 MeV.Comment: Presentation at Quark Matter 2008. 4 pages, 3 figure
The applicability of causal dissipative hydrodynamics to relativistic heavy ion collisions
We utilize nonequilibrium covariant transport theory to determine the region
of validity of causal Israel-Stewart dissipative hydrodynamics (IS) and
Navier-Stokes theory (NS) for relativistic heavy ion physics applications. A
massless ideal gas with 2->2 interactions is considered in a 0+1D Bjorken
scenario, appropriate for the early longitudinal expansion stage of the
collision. In the scale invariant case of a constant shear viscosity to entropy
density ratio eta/s ~ const, we find that Israel-Stewart theory is 10% accurate
in calculating dissipative effects if initially the expansion timescale exceeds
half the transport mean free path tau0/lambda0 > ~2. The same accuracy with
Navier-Stokes requires three times larger tau0/lambda0 > ~6. For dynamics
driven by a constant cross section, on the other hand, about 50% larger
tau0/lambda0 > ~3 (IS) and ~9 (NS) are needed. For typical applications at RHIC
energies s_{NN}**(1/2) ~ 100-200 GeV, these limits imply that even the
Israel-Stewart approach becomes marginal when eta/s > ~0.15. In addition, we
find that the 'naive' approximation to Israel-Stewart theory, which neglects
products of gradients and dissipative quantities, has an even smaller range of
applicability than Navier-Stokes. We also obtain analytic Israel-Stewart and
Navier-Stokes solutions in 0+1D, and present further tests for numerical
dissipative hydrodynamics codes in 1+1, 2+1, and 3+1D based on generalized
conservation laws.Comment: 30 pages, 26 EPS figures, revtex stylefil
Electrothermal feedback in superconducting nanowire single-photon detectors
We investigate the role of electrothermal feedback in the operation of
superconducting nanowire single-photon detectors (SNSPDs). It is found that the
desired mode of operation for SNSPDs is only achieved if this feedback is
unstable, which happens naturally through the slow electrical response
associated with their relatively large kinetic inductance. If this response is
sped up in an effort to increase the device count rate, the electrothermal
feedback becomes stable and results in an effect known as latching, where the
device is locked in a resistive state and can no longer detect photons. We
present a set of experiments which elucidate this effect, and a simple model
which quantitatively explains the results
Has the time come for an older driver vehicle?
The population of the world is growing older. As people grow older they are more likely to
experience declines that can make operating a personal automobile more difficult. Once driving
abilities begin to decline, older adults are often faced with decreased mobility. Due to the preference
for and pervasiveness of the personal automobile for satisfying mobility needs, there is a global
necessity to keep older adults driving for as long as they can safely do so. In this report we explore the
question: Has the time come for an older driver vehicle? Great gains in safe mobility could be made by
designing automobiles that take into account, and help overcome, some of the deficits in abilities
common in older people. The report begins by providing a background and rationale for an older driver
vehicle, including discussions of relevant trends, age-related declines in functional abilities, and the
adverse consequences of decreased mobility. The next section discusses research and issues related to
vehicle design and advanced technology with respect to older drivers. The next section explores
crashworthiness issues and the unique requirements for older adults. The following section discusses
the many issues related to marketing a vehicle that has been designed for older drivers. The report
concludes that there is a clear global opportunity to improve the safety, mobility, and quality of life of
older adults by designing vehicles and vehicle technologies that help overcome common age-related
deficits. The marketing of these vehicles to older consumers, however, will be challenging and will
likely require further market research. The development of vehicle design features, new automotive
technologies, and crashworthiness systems in the future should be guided by both knowledge of the
effects of frailty/fragility of the elderly on crash outcomes, as well as knowledge of common drivingrelated
declines in psychomotor, visual, and cognitive abilities. Design strategies that allow for some
degree of customization may be particularly beneficial. It is clear that training and education efforts for
using new vehicle features will need to be improved.The University of Michigan Sustainable Worldwide Transportationhttp://deepblue.lib.umich.edu/bitstream/2027.42/89960/1/102821.pd
Meson and baryon elliptic flow at high pT from parton coalescence
The large and saturating differential elliptic flow v2(pT) observed in Au+Au
reactions at RHIC so far could only be explained assuming an order of magnitude
denser initial parton system than estimated from perturbative QCD.
Hadronization via parton coalescence can resolve this ``opacity puzzle''
because it enhances hadron elliptic flow at large pT relative to that of
partons at the same transverse momentum. An experimentally testable consequence
of the coalescence scenario is that v2(pT) saturates at about 50% higher values
for baryons than for mesons. In addition, if strange quarks have weaker flow
than light quarks, hadron v2 at high pT decreases with relative strangeness
content.Comment: Talk at SQM2003 [7th Int. Conf. on Strangeness in Quark Matter
(Atlantic Beach, NC, USA, Mar 12-17, 2003)] - 6 pages, 5 eps figs, IOP style
file
Free-form lens model and mass estimation of the high redshift galaxy cluster ACT-CL J0102-4915, "El Gordo"
We examine the massive colliding cluster El Gordo, one of the most massive
clusters at high redshift. We use a free-form lensing reconstruction method
that avoids making assumptions about the mass distribution. We use data from
the RELICS program and identify new multiply lensed system candidates. The new
set of constraints and free-form method provides a new independent mass
estimate of this intriguing colliding cluster. Our results are found to be
consistent with earlier parametric models, indirectly confirming the
assumptions made in earlier work. By fitting a double gNFW profile to the lens
model, and extrapolating to the virial radius, we infer a total mass for the
cluster of M. We
estimate the uncertainty in the mass due to errors in the photometric
redshifts, and discuss the uncertainty in the inferred virial mass due to the
extrapolation from the lens model. We also find in our lens map a mass
overdensity corresponding to the large cometary tail of hot gas, reinforcing
its interpretation as a large tidal feature predicted by hydrodynamical
simulations that mimic El Gordo. Finally, we discuss the observed relation
between the plasma and the mass map, finding that the peak in the projected
mass map may be associated with a large concentration of colder gas, exhibiting
possible star formation. El Gordo is one of the first clusters that will be
observed with JWST, which is expected to unveil new high redshift lensed
galaxies around this interesting cluster, and provide a more accurate
estimation of its mass.Comment: 19 pages, 10 figures. Updated figure
Acoustic phonon scattering in a low density, high mobility AlGaN/GaN field effect transistor
We report on the temperature dependence of the mobility, , of the
two-dimensional electron gas in a variable density AlGaN/GaN field effect
transistor, with carrier densities ranging from 0.4 cm to
3.0 cm and a peak mobility of 80,000 cm/Vs. Between
20 K and 50 K we observe a linear dependence T
indicating that acoustic phonon scattering dominates the temperature dependence
of the mobility, with being a monotonically increasing function of
decreasing 2D electron density. This behavior is contrary to predictions of
scattering in a degenerate electron gas, but consistent with calculations which
account for thermal broadening and the temperature dependence of the electron
screening. Our data imply a deformation potential D = 12-15 eV.Comment: 3 pages, 2 figures, RevTeX. Submitted to Appl Phys Let
Characterization of SU(1,1) coherent states in terms of affine group wavelets
The Perelomov coherent states of SU(1,1) are labeled by elements of the
quotient of SU(1,1) by the compact subgroup. Taking advantage of the fact that
this quotient is isomorphic to the affine group of the real line, we are able
to parameterize the coherent states by elements of that group or equivalently
by points in the half-plane. Such a formulation permits to find new properties
of the SU(1,1) coherent states and to relate them to affine wavelets.Comment: 11 pages, latex, to be published in J. Phys. A : Math. Ge
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