14,654 research outputs found
The Mars ancient cratered terrain, smooth plains boundary: Implications of Viking color data for evolution of the Amenthes Region
The global color set compiled by the Mars Consortium was investigated. The problem of application of the martian surface color data to geologic interpretation are atmospheric contributions which increase with latitude, and the high correlation among the three color bands. In southern Amenthes the classified units show areas of possible mixing between cratered tarrain and smooth plains. It is suggested that some geologically meaningful correlation exists between surface units and the transformed color data in the Amenthes region. The knobby terrain protruding through the plains units appears to be remnants of ancient cratered terrain extending northward beneath the more youthful smooth plains
The Martian crustal dichotomy: Product of accretion and not a specific event?
Attempts to explain the fundamental crustal dichotomy on Mars range from purely endogenic to extreme exogenic processes, but to date no satisfactory theory has evolved. What is accepted is: (1) the dichotomy is an ancient feature of the Martian crust, and (2) the boundary between the cratered highlands and northern plains which marks the dichotomy in parts of Mars has undergone significant and variable modification during the observable parts of Martian history. Some ascribe it to a single mega-impact event, essentially an instantaneous rearrangement of the crustal structures (topography and lithospheric thickness). Others prefer an internal mechanism: a period of vigorous convection subcrustally erodes the northern one third of Mars, causing foundering and isostatic lowering of that part of Mars. The evidence for each theory is reviewed, with the conclusion that there is little to recommend either. An alternative is suggested: the formation of the crustal dichotomy on Mars was not a specific tectonic event but a byproduct of the accretionary process and therefore a primordial characteristic of the Martian crust, predating the oldest recognizable landforms
Progress in three-particle scattering from LQCD
We present the status of our formalism for extracting three-particle
scattering observables from lattice QCD (LQCD). The method relies on relating
the discrete finite-volume spectrum of a quantum field theory with its
scattering amplitudes. As the finite-volume spectrum can be directly determined
in LQCD, this provides a method for determining scattering observables, and
associated resonance properties, from the underlying theory. In a pair of
papers published over the last two years, two of us have extended this approach
to apply to relativistic three-particle scattering states. In this talk we
summarize recent progress in checking and further extending this result. We
describe an extension of the formalism to include systems in which two-to-three
transitions can occur. We then present a check of the previously published
formalism, in which we reproduce the known finite-volume energy shift of a
three-particle bound state.Comment: 9 pages, 3 figures, proceedings for XIIth Quark Confinement and the
Hadron Spectrum (CONF12
Three-particle systems with resonant subprocesses in a finite volume
In previous work, we have developed a relativistic, model-independent
three-particle quantization condition, but only under the assumption that no
poles are present in the two-particle K matrices that appear as scattering
subprocesses. Here we lift this restriction, by deriving the quantization
condition for identical scalar particles with a G-parity symmetry, in the case
that the two-particle K matrix has a pole in the kinematic regime of interest.
As in earlier work, our result involves intermediate infinite-volume quantities
with no direct physical interpretation, and we show how these are related to
the physical three-to-three scattering amplitude by integral equations. This
work opens the door to study processes such as , in which the is rigorously treated as a resonance state.Comment: 46 pages, 9 figures, JLAB-THY-18-2819, CERN-TH-2018-21
Numerical study of the relativistic three-body quantization condition in the isotropic approximation
We present numerical results showing how our recently proposed relativistic
three-particle quantization condition can be used in practice. Using the
isotropic (generalized -wave) approximation, and keeping only the leading
terms in the effective range expansion, we show how the quantization condition
can be solved numerically in a straightforward manner. In addition, we show how
the integral equations that relate the intermediate three-particle
infinite-volume scattering quantity, , to the
physical scattering amplitude can be solved at and below threshold. We test our
methods by reproducing known analytic results for the expansion of the
threshold state, the volume dependence of three-particle bound-state energies,
and the Bethe-Salpeter wavefunctions for these bound states. We also find that
certain values of lead to unphysical finite-volume
energies, and give a preliminary analysis of these artifacts.Comment: 32 pages, 21 figures, JLAB-THY-18-2657, CERN-TH-2018-046; version 2:
corrected typos, updated references, minor stylistic changes---consistent
with published versio
Unitarity of the infinite-volume three-particle scattering amplitude arising from a finite-volume formalism
In a previous publication, two of us derived a relation between the
scattering amplitude of three identical bosons, , and a real
function referred to as the {divergence-free} K matrix and denoted . The result arose in the context of a relation between
finite-volume energies and , derived to all orders in
the perturbative expansion of a generic low-energy effective field theory. In
this work we set aside the role of the finite volume and focus on the
infinite-volume relation between and .
We show that, for any real choice of ,
satisfies the three-particle unitarity constraint to all orders. Given that
is also free of a class of kinematic divergences,
the function may provide a useful tool for parametrizing three-body scattering
data. Applications include the phenomenological analysis of experimental data
(where the connection to the finite volume is irrelevant) as well as
calculations in lattice quantum chromodynamics (where the volume plays a key
role).Comment: 19 pages, 4 figures, JLAB-THY-19-2945, CERN-TH-2019-07
Oxygen Cost of Recreational Horse-Riding in Females
Version: as accepted for publication.BACKGROUND: The purpose of this study was to characterize the physiological demands of a riding session comprising different types of recreational horse riding in females. METHODS: Sixteen female recreational riders (aged 17 to 54 years) completed an incremental cycle ergometer exercise test to determine peak oxygen consumption (VO₂peak) and a 45-minute riding session based upon a British Horse Society Stage 2 riding lesson (including walking, trotting, cantering and work without stirrups). Oxygen consumption (VO₂), from which metabolic equivalent (MET) and energy expenditure values were derived, was measured throughout. RESULTS: The mean VO₂ requirement for trotting/cantering (18.4 ± 5.1 ml·kg⁻¹·min⁻¹; 52 ± 12% VO₂peak; 5.3 ± 1.1 METs) was similar to walking/trotting (17.4 ± 5.1 ml·kg⁻¹·min⁻¹; 48 ± 13% VO₂peak; 5.0 ± 1.5 METs) and significantly higher than for work without stirrups (14.2 ± 2.9 ml·kg⁻¹·min⁻¹; 41 ± 12% VO₂peak; 4.2 ± 0.8 METs) (P = .001). CONCLUSIONS: The oxygen cost of different activities typically performed in a recreational horse riding session meets the criteria for moderate intensity exercise (3-6 METs) in females, and trotting combined with cantering imposes the highest metabolic demand. Regular riding could contribute to the achievement of the public health recommendations for physical activity in this population
A Crystal Structure of the Bifunctional Antibiotic Simocyclinone D8, Bound to DNA Gyrase
Simocyclinones are bifunctional antibiotics that inhibit bacterial DNA gyrase by preventing DNA binding to the enzyme. We report the crystal structure of the complex formed between the N-terminal domain of the Escherichia coli gyrase A subunit and simocyclinone D8, revealing two binding pockets that separately accommodate the aminocoumarin and polyketide moieties of the antibiotic. These are close to, but distinct from, the quinolone-binding site, consistent with our observations that several mutations in this region confer resistance to both agents. Biochemical studies show that the individual moieties of simocyclinone D8 are comparatively weak inhibitors of gyrase relative to the parent compound, but their combination generates a more potent inhibitor. Our results should facilitate the design of drug molecules that target these unexploited binding pockets
Non-equilibrium and information: the role of cross-correlations
We discuss the relevance of information contained in cross-correlations among
different degrees of freedom, which is crucial in non-equilibrium systems. In
particular we consider a stochastic system where two degrees of freedom
and - in contact with two different thermostats - are coupled together.
The production of entropy and the violation of equilibrium
fluctuation-dissipation theorem (FDT) are both related to the cross-correlation
between and . Information about such cross-correlation may be lost
when single-variable reduced models, for , are considered. Two different
procedures are typically applied: (a) one totally ignores the coupling with
; (b) one models the effect of as an average memory effect,
obtaining a generalized Langevin equation. In case (a) discrepancies between
the system and the model appear both in entropy production and linear response;
the latter can be exploited to define effective temperatures, but those are
meaningful only when time-scales are well separated. In case (b) linear
response of the model well reproduces that of the system; however the loss of
information is reflected in a loss of entropy production. When only linear
forces are present, such a reduction is dramatic and makes the average entropy
production vanish, posing problems in interpreting FDT violations.Comment: 30 pages, 4 figures, 4 appendixe
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