22,146 research outputs found
An efficient message passing algorithm for multi-target tracking
We propose a new approach for multi-sensor multi-target tracking by constructing statistical models on graphs with continuous-valued nodes for target states and discrete-valued nodes for data association hypotheses. These graphical representations lead to message-passing algorithms for the fusion of data across time, sensor, and target that are radically different than algorithms such as those found in state-of-the-art multiple hypothesis tracking (MHT) algorithms. Important differences include: (a) our message-passing algorithms explicitly compute different probabilities and estimates than MHT algorithms; (b) our algorithms propagate information from future data about past hypotheses via messages backward in time (rather than doing this via extending track hypothesis trees forward in time); and (c) the combinatorial complexity of the problem is manifested in a different way, one in which particle-like, approximated, messages are propagated forward and backward in time (rather than hypotheses being enumerated and truncated over time). A side benefit of this structure is that it automatically provides smoothed target trajectories using future data. A major advantage is the potential for low-order polynomial (and linear in some cases) dependency on the length of the tracking interval N, in contrast with the exponential complexity in N for so-called N-scan algorithms. We provide experimental results that support this potential. As a result, we can afford to use longer tracking intervals, allowing us to incorporate out-of-sequence data seamlessly and to conduct track-stitching when future data provide evidence that disambiguates tracks well into the past
Graphene Spin Transistor
Graphitic nanostructures, e.g. carbon nanotubes (CNT) and graphene, have been
proposed as ideal materials for spin conduction[1-7]; they have long electronic
mean free paths[8] and small spin-orbit coupling[9], hence are expected to have
very long spin-scattering times. In addition, spin injection and detection in
graphene opens new opportunities to study exotic electronic states such as the
quantum Hall[10,11] and quantum spin Hall[9] states, and spin-polarized edge
states[12] in graphene ribbons. Here we perform the first non-local four-probe
experiments[13] on graphene contacted by ferromagnetic Permalloy electrodes. We
observe sharp switching and often sign-reversal of the non-local resistance at
the coercive field of the electrodes, indicating definitively the presence of a
spin current between injector and detector. The non-local resistance changes
magnitude and sign quasi-periodically with back-gate voltage, and
Fabry-Perot-like oscillations[6,14,15] are observed, consistent with
quantum-coherent transport. The non-local resistance signal can be observed up
to at least T = 300 K
Majorana spin liquids and projective realization of SU(2) spin symmetry
We revisit the fermionic parton approach to S = 1/2 quantum spin liquids with
SU(2) spin rotation symmetry, and the associated projective symmetry group
(PSG) classification. We point out that the existing PSG classification is
incomplete; upon completing it, we find spin liquid states with S=1 and S=0
Majorana fermion excitations coupled to a deconfined Z2 gauge field. The
crucial observation leading us to this result is that, like space group and
time reversal symmetries, spin rotations can act projectively on the fermionic
partons; that is, a spin rotation may be realized by simultaneous SU(2) spin
and gauge rotations. We show that there are only two realizations of spin
rotations acting on fermionic partons: the familiar naive realization where
spin rotation is not accompanied by any gauge transformation, and a single type
of projective realization. We discuss the PSG classification for states with
projective spin rotations. To illustrate these results, we show that there are
four such PSGs on the two-dimensional square lattice. We study the properties
of the corresponding states, finding that one -- with gapless Fermi points --
is a stable phase beyond mean-field theory. In this phase, depending on
parameters, a small Zeeman magnetic field can open a partial gap for the
Majorana fermion excitations. Moreover, there are nearby gapped phases
supporting Z2 vortex excitations obeying non-Abelian statistics. We conclude
with a discussion of various open issues, including the challenging question of
where such S=1 Majorana spin liquids may occur in models and in real systems.Comment: 19 pages, 8 figures. Typos corrected, references adde
HST Detection of Extended Neutral Hydrogen in a Massive Elliptical at z = 0.4
We report the first detection of extended neutral hydrogen (HI) gas in the
interstellar medium (ISM) of a massive elliptical galaxy beyond z~0. The
observations utilize the doubly lensed images of QSO HE 0047-1756 at z_QSO =
1.676 as absorption-line probes of the ISM in the massive (M_star ~ 10^11
M_sun) elliptical lens at z = 0.408, detecting gas at projected distances of d
= 3.3 and 4.6 kpc on opposite sides of the lens. Using the Space Telescope
Imaging Spectrograph (STIS), we obtain UV absorption spectra of the lensed QSO
and identify a prominent flux discontinuity and associated absorption features
matching the Lyman series transitions at z = 0.408 in both sightlines. The HI
column density is log N(HI) = 19.6-19.7 at both locations across the lens,
comparable to what is seen in 21 cm images of nearby ellipticals. The HI gas
kinematics are well-matched with the kinematics of the FeII absorption complex
revealed in ground-based echelle data, displaying a large velocity shear of 360
km/s across the galaxy. We estimate an ISM Fe abundance of 0.3-0.4 solar at
both locations. Including likely dust depletions increases the estimated Fe
abundances to solar or supersolar, similar to those of the hot ISM and stars of
nearby ellipticals. Assuming 100% covering fraction of this Fe-enriched gas,we
infer a total Fe mass of M_cool(Fe)~(5-8)x10^4 M_sun in the cool ISM of the
massive elliptical lens, which is no more than 5% of the total Fe mass observed
in the hot ISM.Comment: 6 pages, 2 figures; Accepted for publication in ApJ Letter
A piloted simulation of helicopter air combat to investigate effects of variations in selected performance and control response characteristics
A piloted simulation study investigating handling qualities and flight characteristics required for helicopter air to air combat is presented. The Helicopter Air Combat system was used to investigate this role for Army rotorcraft. Experimental variables were the maneuver envelope size (load factor and sideslip), directional axis handling qualities, and pitch and roll control-response type. Over 450 simulated, low altitude, one-on-one engagements were conducted. Results from the experiment indicate that a well damped directional response, low sideforce caused by sideslip, and some effective dihedral are all desirable for weapon system performance, good handling qualities, and low pilot workload. An angular rate command system was favored over the attitude type pitch and roll response for most applications, and an enhanced maneuver envelope size over that of current generation aircraft was found to be advantageous. Pilot technique, background, and experience are additional factors which had a significant effect on performance in the air combat tasks investigated. The implication of these results on design requirements for future helicopters is presented
Anomaly-safe discrete groups
We show that there is a class of finite groups, the so-called perfect groups,
which cannot exhibit anomalies. This implies that all non-Abelian finite simple
groups are anomaly-free. On the other hand, non-perfect groups generically
suffer from anomalies. We present two different ways that allow one to
understand these statements.Comment: 11 page
Applications of neuroimaging to disease-modification trials in Alzheimer's disease.
Critical to development of new therapies for Alzheimer's disease (AD) is the ability to detect clinical or pathological change over time. Clinical outcome measures typically used in therapeutic trials have unfortunately proven to be relatively variable and somewhat insensitive to change in this slowly progressive disease. For this reason, development of surrogate biomarkers that identify significant disease-associated brain changes are necessary to expedite treatment development in AD. Since AD pathology is present in the brain many years prior to clinical manifestation, ideally we want to develop biomarkers of disease that identify abnormal brain structure or function even prior to cognitive decline. Magnetic resonance imaging, fluorodeoxyglucose positron emission tomography, new amyloid imaging techniques, and spinal fluid markers of AD all have great potential to provide surrogate endpoint measures for AD pathology. The Alzheimer's disease neuroimaging initiative (ADNI) was developed for the distinct purpose of evaluating surrogate biomarkers for drug development in AD. Recent evidence from ADNI demonstrates that imaging may provide more sensitive, and earlier, measures of disease progression than traditional clinical measures for powering clinical drug trials in Alzheimer's disease. This review discusses recently presented data from the ADNI dataset, and the importance of imaging in the future of drug development in AD
- …