134 research outputs found
Intersection of two TASEP traffic lanes with frozen shuffle update
Motivated by interest in pedestrian traffic we study two lanes
(one-dimensional lattices) of length that intersect at a single site. Each
lane is modeled by a TASEP (Totally Asymmetric Exclusion Process). The
particles enter and leave lane (where ) with probabilities
and , respectively. We employ the `frozen
shuffle' update introduced in earlier work [C. Appert-Rolland et al, J. Stat.
Mech. (2011) P07009], in which the particle positions are updated in a fixed
random order. We find analytically that each lane may be in a `free flow' or in
a `jammed' state. Hence the phase diagram in the domain
consists of four regions with boundaries
depending on and . The regions meet in a single point on the
diagonal of the domain. Our analytical predictions for the phase boundaries as
well as for the currents and densities in each phase are confirmed by Monte
Carlo simulations.Comment: 7 figure
Measurement of pi^0 photoproduction on the proton at MAMI C
Differential cross sections for the gamma p -> pi^0 p reaction have been
measured with the A2 tagged-photon facilities at the Mainz Microtron, MAMI C,
up to the center-of-mass energy W=1.9 GeV. The new results, obtained with a
fine energy and angular binning, increase the existing quantity of pi^0
photoproduction data by ~47%. Owing to the unprecedented statistical accuracy
and the full angular coverage, the results are sensitive to high partial-wave
amplitudes. This is demonstrated by the decomposition of the differential cross
sections in terms of Legendre polynomials and by further comparison to model
predictions. A new solution of the SAID partial-wave analysis obtained after
adding the new data into the fit is presented.Comment: 13 pages, 12 figures, 1 tabl
Determination of the scalar polarizabilities of the proton using beam asymmetry in Compton scattering
The scalar dipole polarizabilities, and , are
fundamental properties related to the internal dynamics of the nucleon. The
currently accepted values of the proton polarizabilities were determined by
fitting to unpolarized proton Compton scattering cross section data. The
measurement of the beam asymmetry in a certain kinematical range
provides an alternative approach to the extraction of the scalar
polarizabilities. At the Mainz Microtron (MAMI) the beam asymmetry was measured
for Compton scattering below pion photoproduction threshold for the first time.
The results are compared with model calculations and the influence of the
experimental data on the extraction of the scalar polarizabilities is
determined.Comment: 6 pages, 5 figure
Helicity-dependent cross sections and double-polarization observable E in η photoproduction from quasifree protons and neutrons
Precise helicity-dependent cross sections and the double-polarization observable E were measured for η
photoproduction from quasifree protons and neutrons bound in the deuteron. The η → 2γ and η → 3π0 → 6γ
decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The
measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz.
A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon
beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with
the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98% of the full solid angle. The results
show that the narrow structure observed earlier in the unpolarized excitation function of η photoproduction off
the neutron appears only in reactions with antiparallel photon and nucleon spin (σ1/2). It is absent for reactions
with parallel spin orientation (σ3/2) and thus very probably related to partial waves with total spin 1/2. The
behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which
uses an interference of P11 and S11 partial waves to explain the narrow structure
Compton Scattering from the Deuteron Above Pion-Production Threshold
The electromagnetic polarizabilities of the nucleon are fundamental nucleon-structure observables that characterize its response to external electromagnetic fields. The neutron polarizabilities can be accessed from Compton-scattering data on light nuclear targets. Recent measurements of the differential cross section for Compton scattering on the deuteron below the pion-production threshold have decreased the uncertainties in the neutron polarizabilities, yet the proton polarizabilities remain known substantially more accurately. As the sensitivity of the cross section to the polarizabilities increases with incident photon energy, measurements above the pion threshold may offer a way for an improved determination of the neutron polarizabilities. In this Rapid Communiciation, the first measurement of the cross section for coherent Compton scattering on the deuteron above the pion-production threshold is presented
Evaluation of the E2/M1 ratio in the transition from the reaction
A new data set for the helicity-dependent differential cross section of the
single-meson photoproduction reaction was obtained for
the photon energy interval 150-400 MeV. The experiment was performed at the A2
tagged photon facility of the Mainz Microtron MAMI using a circularly polarized
photon beam and a longitudinally polarized proton target. The reaction products
were detected with the large acceptance Crystal Ball/TAPS calorimeter covering
97\% of the full solid angle. These new results, obtained with a fine energy
and polar angle binning, greatly increase both the existing quantity and
quality of the data available for this observable. A moment analysis, based on
a finite expansion in Legendre polynomials, was applied to these data by using
a bootstrap-based fitting method to correctly account for their systematic
uncertainties. From the resulting decomposition of the differential cross
sections, the ratio for the transition was
determined to be . Combining this value with previous results also allowed us to
evaluate the most precise available estimate of the ratio to be used
for all further reference and model comparisons.Comment: 16 pages, 14 figure
Brain-based classification of youth with anxiety disorders: transdiagnostic examinations within the ENIGMA-Anxiety database using machine learning
Neuroanatomical findings on youth anxiety disorders are notoriously difficult to replicate, small in effect size,
and have limited clinical relevance. These concerns have prompted a paradigm shift towards highly powered
(i.e., big data) individual-level inferences, which are data-driven, transdiagnostic, and neurobiologically
informed. Hence, we uniquely built/validated supervised neuroanatomical machine learning (ML) models for
individual-level inferences, using the largest up to date neuroimaging database on youth anxiety disorders:
ENIGMA Anxiety Consortium (N=3,343; Age: 10-25 years; Global Sites: 32). Modest, yet robust, brain-based
classifications were achieved for specific anxiety disorders (Panic Disorder), but also transdiagnostically for all
anxiety disorders when patients were subgrouped according to their sex, medication status, and symptom
severity (AUC’s 0.59-0.63). Classifications were driven by neuroanatomical features (cortical thickness/surface
area, subcortical volumes) in fronto-striato-limbic and temporo-parietal regions. This benchmark study provides
estimates on individual-level classification performances that can be realistically achieved with ML using
neuroanatomical data, within a large, heterogenous, and multi-site sample of youth with anxiety disorders
First measurement of polarisation transfer in deuteron photodisintegration
A first measurement of the polarisation transfer from a circularly-polarised
photon to the final state neutron () in deuterium photodisintegration
has been carried out. This quantity is determined over the photon energy range
370~--~700~MeV and for neutron centre-of-mass breakup angles
. The polarisation of the final state neutrons was
determined by an ancillary large-acceptance nucleon polarimeter, surrounding a
cryogenic liquid deuterium target within the Crystal Ball detector at MAMI. The
polarimeter characterised charge exchange of the ejected neutrons to
determine their polarisation. The new data are also compared to a
theoretical model based on nucleonic and nucleon resonance degrees of freedom
constrained by the current world-database of deuterium photodisintegration
measurements. Structures in observed in the region of the
could not be explained by conventional models of deuteron
photodisintegration
Signatures of the hexaquark in d(,)
We report a measurement of the spin polarisation of the recoiling neutron in deuterium photodisintegration, utilising a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300~--~700~MeV provides the first measurement of recoil neutron polarisation at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarisation in a narrow structure centred around ~570~MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behaviour could be related to the excitation of the hexaquark
Single Production Off Neutrons Bound in Deuteron with Linearly Polarized Photons
The quasifree photon beam asymmetry,
, has been measured at photon energies, , from 390 to 610
MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the
first time. The data were collected in the A2 hall of the MAMI electron beam
facility with the Crystal Ball and TAPS calorimeters covering pion
center-of-mass angles from 49 to 148. In this kinematic region,
polarization observables are sensitive to contributions from the and resonances. The extracted values of have been
compared to predictions based on partial-wave analyses (PWAs) of the existing
pion photoproduction database. Our comparison includes the SAID, MAID, and
Bonn-Gatchina analyses; while a revised SAID fit, including the new
measurements, has also been performed. In addition, isospin symmetry is
examined as a way to predict photoproduction observables, based on
fits to published data in the channels , , and .Comment: 11 pages, 9 figures, 5 tables; fixed 2 glitche
- …