4,256 research outputs found
Log it: How to fit an active Brownian particle's mean squared displacement with improved parameter estimation
The active Brownian particle (ABP) model is widely used to describe the
dynamics of active matter systems, such as Janus microswimmers. In particular,
the analytical expression for an ABP's mean-squared-displacement (MSD) is
useful as it provides a means to describe the essential physics of a
self-propelled, spherical Brownian particle. However, the truncated or
'short-time' form of the MSD equation is typically fitted, which can lead to
significant problems in parameter estimation. Furthermore, heteroscedasticity
and the often statistically dependent observations of an ABP's MSD lead to a
situation where standard ordinary least squares (OLS) regression will obtain
biased estimates and unreliable confidence intervals. Here, we propose to
revert to always fitting the full expression of an ABP's MSD at short
timescales, using bootstrapping to construct confidence intervals of the fitted
parameters. Additionally, after comparison between different fitting
strategies, we propose to extract the physical parameters of an ABP using its
mean logarithmic squared displacement (MLSD). These steps improve the
estimation of an ABP's physical properties, and provide more reliable
confidence intervals, which are critical in the context of a growing interest
in the interactions of microswimmers with confining boundaries and the
influence on their motion.Comment: 20 pages, 5 figure
The Microcanonical Fermionic Average method for Asymptotically Free Theories: a test in the Schwinger Modelâ
Abstract We have applied the Microcanonical Fermionic Average method to QED2, i.e. the Schwinger Model, to test its applicability to Asymptotically Free Theories. We present here the results of the simulations, compared to the continuum results. Since the M.F.A. method allows the study of the whole ÎČ, mf plane at very small computer cost, we are able to verify the scaling of the chiral condensate at a high degree, and obtain the continuum result within 3 decimal places. We present also results for the plaquette energy
Preferential adsorption to air-water interfaces: a novel cryoprotective mechanism for LEA proteins
Late embryogenesis abundant (LEA) proteins comprise a diverse family whose members play a key role in abiotic stress tolerance. As intrinsically disordered proteins, LEA proteins are highly hydrophilic and inherently stress tolerant. They have been shown to stabilize multiple client proteins under a variety of stresses, but current hypotheses do not fully explain how such broad range stabilization is achieved. Here, using neutron reflection and surface tension experiments, we examine in detail the mechanism by which model LEA proteins, AavLEA1 and ERD10, protect the enzyme citrate synthase from aggregation during freeze-thaw. We find that a major contributing factor to citrate synthase aggregation is the formation of air bubbles during the freeze-thaw process. This greatly increases the air-water interfacial area, which is known to be detrimental to folded protein stability. Both model LEA proteins preferentially adsorb to this interface and compete with citrate synthase, thereby reducing surface induced aggregation. This novel surface activity provides a general mechanism by which diverse members of the LEA protein family might function to provide aggregation protection that is not specific to the client protein.Canadian Research Council for PhD studentship
+ ERC gran
Understanding the passivation layer formed by tolyltriazole on copper, bronze, and brass surfaces
Funding: Lubrizol Ltd is acknowledged for funding and supplying TTAH. EPSRC is acknowledged (EP/L017008/1, EP/T019298/1, and EP/R023751/1) for electron microscopy.Tolyltriazole (TTAH) is used industrially as a corrosion inhibitor for copper alloys, particularly in organic media. In this study, the morphology and chemistry of the layer formed by TTAH on copper and copper alloys under realistic conditions is investigated, with focus on the effects due to the presence of tin or zinc in the substrates. A combination of X-ray photoelectron spectroscopy (XPS), medium energy ion scattering (MEIS), and scanning transmission electron microscopy (STEM) has been used. It was found that an inhomogeneous metalâorganic layer forms on the surface of copper specimens, likely in the form of copper nanoparticles surrounded by CuxTTAy complexes. This layer increases in thickness for at least 30 days. Chemically, the copper species in the layer are initially in the +2 oxidation state, but after longer exposure to TTAH, mostly Cu(I) is observed. In bronze samples, tin does not appear to segregate to the surface layer. In brass samples, zinc is depleted from the bulk and forms a thicker ZnxTTAy layer.Peer reviewe
The stellar masses and specific star-formation rates of submillimetre galaxies
Establishing the stellar masses (M*), and hence specific star-formation rates
(sSFRs) of submillimetre galaxies (SMGs) is crucial for determining their role
in the cosmic galaxy/star formation. However, there is as yet no consensus over
the typical M* of SMGs. Specifically, even for the same set of SMGs, the
reported average M* have ranged over an order of magnitude, from ~5x10^10 Mo to
~5x10^11 Mo. Here we study how different methods of analysis can lead to such
widely varying results. We find that, contrary to recent claims in the
literature, potential contamination of IRAC 3-8 um photometry from hot dust
associated with an active nucleus is not the origin of the published
discrepancies in derived M*. Instead, we expose in detail how inferred M*
depends on assumptions made in the photometric fitting, and quantify the
individual and cumulative effects of different choices of initial mass
function, different brands of evolutionary synthesis models, and different
forms of assumed star-formation history. We review current observational
evidence for and against these alternatives as well as clues from the
hydrodynamical simulations, and conclude that, for the most justifiable choices
of these model inputs, the average M* of SMGs is ~2x10^11 Mo. We also confirm
that this number is perfectly reasonable in the light of the latest
measurements of their dynamical masses, and the evolving M* function of the
overall galaxy population. M* of this order imply that the average sSFR of SMGs
is comparable to that of other star-forming galaxies at z>2, at 2-3 Gyr^-1.
This supports the view that, while rare outliers may be found at any M*, most
SMGs simply form the top end of the main-sequence of star-forming galaxies at
these redshifts. Conversely, this argues strongly against the viewpoint that
SMGs are extreme pathological objects, of little relevance in the cosmic
history of star-formation.Comment: Accepted to A&A. 13 pages, 5 figures, 3 tables. Main changes: 1)
investigation that the main-sequence does not change the location as much as
SMGs when changing SFHs; 2) a new table added with all stellar mass estimates
for individual SMGs (machine-readable version in the source file). V3:
missing references adde
A persistent excess of galaxy-galaxy strong lensing observed in galaxy clusters
Previous studies have revealed that the estimated probability of
galaxy-galaxy strong lensing in observed galaxy clusters exceeds the
expectations from the Cold Dark Matter cosmological model by one
order of magnitude. We aim to understand the origin of this excess by analyzing
a larger set of simulated galaxy clusters and investigating how the theoretical
expectations vary under different adopted prescriptions and numerical
implementations of star formation and feedback in simulations. We perform a
ray-tracing analysis of 324 galaxy clusters from the Three Hundred project,
comparing the Gadget-X and Gizmo-Simba runs. These simulations, which start
from the same initial conditions, are performed with different implementations
of hydrodynamics and galaxy formation models tailored to match different
observational properties of the Intra-Cluster-Medium and cluster galaxies. We
find that galaxies in the Gizmo-Simba simulations develop denser stellar cores
than their Gadget-X counterparts. Consequently, their probability for
galaxy-galaxy strong lensing is higher by a factor of . This increment
is still insufficient to fill the gap with observations, as a discrepancy by a
factor still persists. In addition, we find that several simulated
galaxies have Einstein radii that are too large compared to observations. We
conclude that a persistent excess of galaxy-galaxy strong lensing exists in
observed galaxy clusters. The origin of this discrepancy with theoretical
predictions is still unexplained in the framework of the cosmological
hydrodynamical simulations. This might signal a hitherto unknown issue with
either the simulation methods or our assumptions regarding the standard
cosmological model.Comment: 6 pages, 2 figures, accepted for publication on A&A Letters. See
companion paper Srivastava et al. (2023
Measurement of the cosmic ray spectrum above eV using inclined events detected with the Pierre Auger Observatory
A measurement of the cosmic-ray spectrum for energies exceeding
eV is presented, which is based on the analysis of showers
with zenith angles greater than detected with the Pierre Auger
Observatory between 1 January 2004 and 31 December 2013. The measured spectrum
confirms a flux suppression at the highest energies. Above
eV, the "ankle", the flux can be described by a power law with
index followed by
a smooth suppression region. For the energy () at which the
spectral flux has fallen to one-half of its extrapolated value in the absence
of suppression, we find
eV.Comment: Replaced with published version. Added journal reference and DO
Energy Estimation of Cosmic Rays with the Engineering Radio Array of the Pierre Auger Observatory
The Auger Engineering Radio Array (AERA) is part of the Pierre Auger
Observatory and is used to detect the radio emission of cosmic-ray air showers.
These observations are compared to the data of the surface detector stations of
the Observatory, which provide well-calibrated information on the cosmic-ray
energies and arrival directions. The response of the radio stations in the 30
to 80 MHz regime has been thoroughly calibrated to enable the reconstruction of
the incoming electric field. For the latter, the energy deposit per area is
determined from the radio pulses at each observer position and is interpolated
using a two-dimensional function that takes into account signal asymmetries due
to interference between the geomagnetic and charge-excess emission components.
The spatial integral over the signal distribution gives a direct measurement of
the energy transferred from the primary cosmic ray into radio emission in the
AERA frequency range. We measure 15.8 MeV of radiation energy for a 1 EeV air
shower arriving perpendicularly to the geomagnetic field. This radiation energy
-- corrected for geometrical effects -- is used as a cosmic-ray energy
estimator. Performing an absolute energy calibration against the
surface-detector information, we observe that this radio-energy estimator
scales quadratically with the cosmic-ray energy as expected for coherent
emission. We find an energy resolution of the radio reconstruction of 22% for
the data set and 17% for a high-quality subset containing only events with at
least five radio stations with signal.Comment: Replaced with published version. Added journal reference and DO
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