10,803 research outputs found
Probing the Atmospheres of Planets Orbiting Microlensed Stars via Polarization Variability
We present a new method to identify and probe planetary companions of stars
in the Galactic Bulge and Magellanic Clouds using gravitational microlensing.
While spectroscopic studies of these planets is well beyond current
observational techniques, monitoring polarization fluctuations during high
magnification events induced by binary microlensing events will probe the
composition of the planetary atmospheres, an observation which otherwise is
currently unattainable even for nearby planetary systems.Comment: 7 pages, 2 figures. To appear in Astrophysical Journal Letter
Technipion Limits from LHC Higgs Searches
LHC searches for the standard model Higgs Boson in di-photon or di-tau decay
modes place strong constraints on the light top-pion state predicted in
technicolor models that include colored technifermions. Compared with the
standard Higgs Boson, the top-pions have an enhanced production rate (largely
because the technipion decay constant is smaller than the weak scale) and also
enhanced branching ratios into di-photon and di-tau final states (largely due
to the suppression of WW decays of the technipions). These factors combine to
make the technipions more visible in both channels than a standard model Higgs
would be. Hence, the recent ATLAS and CMS searches for Higgs bosons exclude the
presence of technipions with masses from 110 GeV to nearly twice the top-quark
mass in technicolor models that (a) include colored technifermions (b) feature
topcolor dynamics and (c) have technicolor groups with three or more
technicolors. For certain models, the limits also apply out to higher
technipion masses or down to the minimum number of technicolors. The limits may
be softened somewhat in models where extended technicolor plays a significant
role in producing the top quark's mass. Additional LHC data on di-tau and
di-photon final states will be extremely valuable in further exploring
technicolor parameter space.Comment: 15 pages, 5 figures, PDF-LaTe
Making Meaning of a Life in Teaching: A MemoirâWriting Project for Seasoned Faculty
The University of Minnesotaâs faculty development project, âMaking Meaning of a Life in Teaching,â promotes collegiality and enhances self-reflection for those who are cxperienced classroom instructors. Started in October. 2003, this project provides a forum that invites participants to examine specific memories from their teaching lives and to transform those recollections into a written memoir. This chapter explores the use of memoir as an effective tool for faculty development, describes the projectâs structure and components, and presents both co-facilitator and participant perspectives on the process and the memoir product
Review of Top Quark Physics
We present an overview of Top Quark Physics - from what has been learned so
far at the Tevatron, to the searches that lie ahead at present and future
colliders. We summarize the richness of the measurements and discuss their
possible impact on our understanding of the Standard Model by pointing out
their key elements and limitations. When possible, we discuss how the top quark
may provide a connection to new or unexpected physics.Comment: 84 pp. With permission from the Annual Review of Nuclear & Particle
Science. Final version of this material is scheduled to appear in the Annual
Review of Nuclear & Particle Science Vol. 53, to be published in December
2003 by Annual Reviews (http://www.annualreviews.org
Conicoid Mirrors
The first order equation relating object and image location for a mirror of
arbitrary conic-sectional shape is derived. It is also shown that the parabolic
reflecting surface is the only one free of aberration and only in the limiting
case of distant sources.Comment: 9 page
Polynomial sequences for bond percolation critical thresholds
In this paper, I compute the inhomogeneous (multi-probability) bond critical
surfaces for the (4,6,12) and (3^4,6) lattices using the linearity
approximation described in (Scullard and Ziff, J. Stat. Mech. P03021),
implemented as a branching process of lattices. I find the estimates for the
bond percolation thresholds, p_c(4,6,12)=0.69377849... and
p_c(3^4,6)=0.43437077..., compared with Parviainen's numerical results of p_c
\approx 0.69373383 and p_c \approx 0.43430621 . These deviations are of the
order 10^{-5}, as is standard for this method, although they are outside
Parviainen's typical standard error of 10^{-7}. Deriving thresholds in this way
for a given lattice leads to a polynomial with integer coefficients, the root
in [0,1] of which gives the estimate for the bond threshold. I show how the
method can be refined, leading to a sequence of higher order polynomials making
predictions that likely converge to the exact answer. Finally, I discuss how
this fact hints that for certain graphs, such as the kagome lattice, the exact
bond threshold may not be the root of any polynomial with integer coefficients.Comment: submitted to Journal of Statistical Mechanic
Discovering Strong Top Dynamics at the LHC
We analyze the phenomenology of the top-pion and top-Higgs states in models
with strong top dynamics, and translate the present LHC searches for the
Standard Model Higgs into bounds on these scalar states. We explore the
possibility that the new state at a mass of approximately 125 GeV observed at
the LHC is consistent with a neutral pseudoscalar top-pion state. We
demonstrate that a neutral pseudoscalar top-pion can generate the diphoton
signal at the observed rate. However, the region of model parameter space where
this is the case does not correspond to classic topcolor-assisted technicolor
scenarios with degenerate charged and neutral top-pions and a top-Higgs mass of
order twice the top mass; rather, additional isospin violation would need to be
present and the top dynamics would be more akin to that in top seesaw models.
Moreover, the interpretation of the new state as a top-pion can be sustained
only if the ZZ (four-lepton) and WW (two-lepton plus missing energy) signatures
initially observed at the 3? level decline in significance as additional data
is accrued.Comment: 25 pages, pdf embedded figures. Submission extensively revised to
reflect discovery of a 125 GeV boso
A New Channel for the Detection of Planetary Systems Through Microlensing: II. Repeating Events
In the companion paper we began the task of systematically studying the
detection of planets in wide orbits () via microlensing surveys.
In this paper we continue, focusing on repeating events. We find that, if all
planetary systems are similar to our own Solar System, reasonable extensions of
the present observing strategies would allow us to detect 3-6 repeating events
per year along the direction to the Bulge. Indeed, if planetary systems with
multiple planets are common, then future monitoring programs which lead to the
discovery of thousands of stellar-lens events will likely discover events in
which several different planets within a single system serve as lenses, with
light curves exhibiting multiple repetitions. In this paper we discuss
observing strategies to maximize the discovery of all wide-orbit planet-lens
events. We also compare the likely detection rates of planets in wide orbits to
those of planets located in the zone for resonant lensing. We find that,
depending on the values of the planet masses and stellar radii of the lensed
sources (which determine whether or not finite source size is important), and
also on the sensitivity of the photometry used by observers, the detection of
planets in wide orbits may be the primary route to the discovery of planets via
microlensing. We also discuss how the combination of resonant and wide-orbit
events can help us to learn about the distribution of planetary system
properties (S 6.1). In addition, by determining the fraction of short-duration
events due to planets, we indirectly derive information about the fraction of
all short-duration events that may be due to low-mass MACHOs (S 6.2).Comment: 51 pages, 7 figures. To be published in the Astrophysical Journal, 20
February 1999. This completes the introduction to the discovery of planets in
wide orbits begun in astro-ph/9808075, also to appear in ApJ on 20 February
199
Reactive direction control for a mobile robot: A locust-like control of escape direction emerges when a bilateral pair of model locust visual neurons are integrated
Locusts possess a bilateral pair of uniquely identifiable visual neurons that respond vigorously to
the image of an approaching object. These neurons are called the lobula giant movement
detectors (LGMDs). The locust LGMDs have been extensively studied and this has lead to the
development of an LGMD model for use as an artificial collision detector in robotic applications.
To date, robots have been equipped with only a single, central artificial LGMD sensor, and this
triggers a non-directional stop or rotation when a potentially colliding object is detected. Clearly,
for a robot to behave autonomously, it must react differently to stimuli approaching from
different directions. In this study, we implement a bilateral pair of LGMD models in Khepera
robots equipped with normal and panoramic cameras. We integrate the responses of these LGMD
models using methodologies inspired by research on escape direction control in cockroaches.
Using ârandomised winner-take-allâ or âsteering wheelâ algorithms for LGMD model integration,
the khepera robots could escape an approaching threat in real time and with a similar
distribution of escape directions as real locusts. We also found that by optimising these
algorithms, we could use them to integrate the left and right DCMD responses of real jumping
locusts offline and reproduce the actual escape directions that the locusts took in a particular
trial. Our results significantly advance the development of an artificial collision detection and
evasion system based on the locust LGMD by allowing it reactive control over robot behaviour.
The success of this approach may also indicate some important areas to be pursued in future
biological research
Microlensing of Circumstellar Disks
We investigate the microlensing effects on a source star surrounded by a
circumstellar disk, as a function of wavelength. The microlensing light curve
of the system encodes the geometry and surface brightness profile of the disk.
In the mid- and far-infrared, the emission of the system is dominated by the
thermal emission from the cold dusty disk. For a system located at the Galactic
center, we find typical magnifications to be of order 10-20% or higher,
depending on the disk surface brightness profile, and the event lasts over one
year. At around 20 microns, where the emission for the star and the disk are
comparable, the difference in the emission areas results in a chromatic
microlensing event. Finally, in the near-infrared and visible, where the
emission of the star dominates, the fraction of star light directly reflected
by the disk slightly modifies the light curve of the system which is no longer
that of a point source. In each case, the corresponding light curve can be used
to probe some of the disk properties. A fraction of 0.1% to 1% optical
microlensing events are expected to be associated with circumstellar disk
systems. We show that the lensing signal of the disk can be detected with
sparse follow-up observations of the next generation space telescopes. While
direct imaging studies of circumstellar disks are limited to the solar
neighborhood, this microlensing technique can probe very distant disk systems
living in various environments and has the potential to reveal a larger
diversity of circumstellar disks.Comment: 9 pages, 7 figures. Accepted for publication in Ap
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