2,253 research outputs found
The Acceleration of the Expansion of the Universe: A Brief Early History of the Supernova Cosmology Project (SCP)
It is now about 10 years since the evidence, based on Type Ia supernovae, for
the acceleration of the expansion of the Universe was discovered. I will
discuss some aspects of the work and events in the Supernova Cosmology Project
(SCP), during the period 1988 to 1998, which led to this discovery.Comment: 20 pages, 10 figures. To appear in the Proceedings of the 8th UCLA
Dark Matter Symposium, Marina del Rey, USA, 20-22 February 2008. Revision
with references corrected, new references added, and minor text update
Evidence of Klein tunneling in graphene p-n junctions
Transport through potential barriers in graphene is investigated using a set
of metallic gates capacitively coupled to graphene to modulate the potential
landscape. When a gate-induced potential step is steep enough, disorder becomes
less important and the resistance across the step is in quantitative agreement
with predictions of Klein tunneling of Dirac fermions up to a small correction.
We also perform magnetoresistance measurements at low magnetic fields and
compare them to recent predictions.Comment: Major changes made: 1) Taking into account properly the contribution
of the resistance of monopolar junctions to the odd part of the resistance.
To better present the results we use a fitting parameter for the amplitude of
screening in graphene. 2) Wrong data for the diffusive model in figures 3, 9
and 10 was plotted in former version. 3) Figure 5 moved to EPAP
Gauge Invariance and the Pauli-Villars Regulator in Lorentz- and CPT-Violating Electrodynamics
We examine the nonperturbative structure of the radiatively induced
Chern-Simons term in a Lorentz- and CPT-violating modification of QED. Although
the coefficient of the induced Chern-Simons term is in general undetermined,
the nonperturbative theory appears to generate a definite value. However, the
CPT-even radiative corrections in this same formulation of the theory generally
break gauge invariance. We show that gauge invariance may yet be preserved
through the use of a Pauli-Villars regulator, and, contrary to earlier
expectations, this regulator does not necessarily give rise to a vanishing
Chern-Simons term. Instead, two possible values of the Chern-Simons coefficient
are allowed, one zero and one nonzero. This formulation of the theory therefore
allows the coefficient to vanish naturally, in agreement with experimental
observations.Comment: 8 page
Heavy Meson Physics: What have we learned in Twenty Years?
I give a personal account of the development of the field of heavy quarks.
After reviewing the experimental discovery of charm and bottom quarks, I
describe how the field's focus shifted towards determination of CKM elements
and how this has matured into a precision science.Comment: This talk was presented during the ceremony awarding the Medalla 2003
of the Division of Particles and Fields of The Mexican Phsyical Society, at
the IX Mexican Workshop on Particles and Fields; submitted for proceedings; 9
pages, 9 figures; replacement: fix multiple typo
Contact resistance and shot noise in graphene transistors
Potential steps naturally develop in graphene near metallic contacts. We
investigate the influence of these steps on the transport in graphene Field
Effect Transistors. We give simple expressions to estimate the
voltage-dependent contribution of the contacts to the total resistance and
noise in the diffusive and ballistic regimes.Comment: 6 pages, 4 figures; Figs 3 and 4 completed and appendix adde
Should I Stay or Should I Go Now? An Analysis of Pension Structure and Retirement Timing
Over the last two decades, twenty-two states have moved away from traditional defined benefit (DB) pension systems and toward pension plan structures like the defined contribution (DC) plans now prevalent in the private sector. Others are considering such a reform as it is seen as a means of limiting future pension funding risk. It is important to understand the implications of such reforms for end-of-career exit patterns and workforce composition. Empirical evidence on the relationship between pension plan structure and retirement timing is currently limited, primarily because, most state pension reforms are so new that few employees enrolled in those alternative plans have reached retirement age. An exception, and the subject of our analysis, is the teacher retirement system in Washington State, which introduced a hybrid DB-DC plan in 1996 and allowed employees in its traditional DB plan to transfer into the new plan. Our analysis focuses on a years-of-service threshold, the crossing of which grants employees early retirement eligibility and, in many cases, a large upward shift in retirement wealth. The financial implications of crossing this threshold are far greater under the state’s traditional DB plan than under the hybrid plan. We find that employees are responsive to crossing the years-of-service threshold, but we fail to find significant evidence that the propensity to exit the workforce varies according to plan enrollment
The Rise Times of High and Low Redshift Type Ia Supernovae are Consistent
We present a self-consistent comparison of the rise times for low- and
high-redshift Type Ia supernovae. Following previous studies, the early light
curve is modeled using a t-squared law, which is then mated with a modified
Leibundgut template light curve. The best-fit t-squared law is determined for
ensemble samples of low- and high-redshift supernovae by fitting simultaneously
for all light curve parameters for all supernovae in each sample. Our method
fully accounts for the non-negligible covariance amongst the light curve
fitting parameters, which previous analyses have neglected. Contrary to Riess
et al. (1999), we find fair to good agreement between the rise times of the
low- and high-redshift Type Ia supernovae. The uncertainty in the rise time of
the high-redshift Type Ia supernovae is presently quite large (roughly +/- 1.2
days statistical), making any search for evidence of evolution based on a
comparison of rise times premature. Furthermore, systematic effects on rise
time determinations from the high-redshift observations, due to the form of the
late-time light curve and the manner in which the light curves of these
supernovae were sampled, can bias the high-redshift rise time determinations by
up to +3.6/-1.9 days under extreme situations. The peak brightnesses - used for
cosmology - do not suffer any significant bias, nor any significant increase in
uncertainty.Comment: 18 pages, 4 figures, Accepted for publication in the Astronomical
Journal. Also available at http://www.lbl.gov/~nugent/papers.html Typos were
corrected and a few sentences were added for improved clarit
Teaching and understanding of quantum interpretations in modern physics courses
Just as expert physicists vary in their personal stances on interpretation in
quantum mechanics, instructors vary on whether and how to teach interpretations
of quantum phenomena in introductory modern physics courses. In this paper, we
document variations in instructional approaches with respect to interpretation
in two similar modern physics courses recently taught at the University of
Colorado, and examine associated impacts on student perspectives regarding
quantum physics. We find students are more likely to prefer realist
interpretations of quantum-mechanical systems when instructors are less
explicit in addressing student ontologies. We also observe contextual
variations in student beliefs about quantum systems, indicating that
instructors who choose to address questions of ontology in quantum mechanics
should do so explicitly across a range of topics.Comment: 18 pages, references, plus 2 pages supplemental materials. 8 figures.
PACS: 01.40.Fk, 03.65.-
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