48 research outputs found
Pion Interactions in Chiral Field Theories
We study in various chiral models the pion charge radius, form
factor ratio, amplitude, charge pion
polarizabilities, amplitude at low
energies and the s-wave I = 0 scattering length. We find that a
quark-level linear sigma-model approach (also being consistent with tree-level
vector meson dominance) is quite compatible with all of the above data.Comment: 12 pages, 9 eps figure
Roy-Steiner equations for pion-nucleon scattering
Starting from hyperbolic dispersion relations, we derive a closed system of
Roy-Steiner equations for pion-nucleon scattering that respects analyticity,
unitarity, and crossing symmetry. We work out analytically all kernel functions
and unitarity relations required for the lowest partial waves. In order to
suppress the dependence on the high-energy regime we also consider once- and
twice-subtracted versions of the equations, where we identify the subtraction
constants with subthreshold parameters. Assuming Mandelstam analyticity we
determine the maximal range of validity of these equations. As a first step
towards the solution of the full system we cast the equations for the
partial waves into the form of a Muskhelishvili-Omn\`es
problem with finite matching point, which we solve numerically in the
single-channel approximation. We investigate in detail the role of individual
contributions to our solutions and discuss some consequences for the spectral
functions of the nucleon electromagnetic form factors.Comment: 106 pages, 18 figures; version published in JHE
Selective methylation of histone H3 variant H3.1 regulates heterochromatin replication
Histone variants have been proposed to act as determinants for posttranslational modifications with widespread regulatory functions. We identify a histone-modifying enzyme that selectively methylates the replication-dependent histone H3 variant H3.1. The crystal structure of the SET domain of the histone H3 lysine-27 (H3K27) methyltransferase ARABIDOPSIS TRITHORAX-RELATED PROTEIN 5 (ATXR5) in complex with a H3.1 peptide shows that ATXR5 contains a bipartite catalytic domain that specifically "reads" alanine-31 of H3.1. Variation at position 31 between H3.1 and replication-independent H3.3 is conserved in plants and animals, and threonine-31 in H3.3 is responsible for inhibiting the activity of ATXR5 and its paralog, ATXR6. Our results suggest a simple model for the mitotic inheritance of the heterochromatic mark H3K27me1 and the protection of H3.3-enriched genes against heterochromatization during DNA replication
MO analysis of the high statistics Belle results on with chiral constraints
We reconsider Muskhelishvili-Omn\`es (MO) dispersive representations of
photon-photon scattering to two pions, motivated by the very high statistics
results recently released by the Belle collaboration for charged as well as
neutral pion pairs and also by recent progress in the determination of the
low-energy scattering amplitude. Applicability of this formalism is
extended beyond 1 GeV by taking into account inelasticity due to . A
modified MO representation is derived which has the advantage that all
polynomial ambiguities are collected into the subtraction constants and have
simple relations to pion polarizabilities. It is obtained by treating
differently the exactly known QED Born term and the other components of the
left-hand cut. These components are approximated by a sum over resonances. All
resonances up to spin two and masses up to GeV are included. The
tensor contributions to the left-hand cut are found to be numerically
important. We perform fits to the data imposing chiral constraints, in
particular, using a model independent sum rule result on the chiral
coupling . Such theoretical constraints are necessary because the
experimental errors are dominantly systematic. Results on further
couplings and pion dipole and quadrupole polarizabilities are then derived from
the fit. The relevance of the new data for distinguishing between two possible
scenarios of isospin breaking in the region is discussed.Comment: 44 pages, 12 figure
Single-neuron dynamics in human focal epilepsy
Epileptic seizures are traditionally characterized as the ultimate expression of monolithic, hypersynchronous neuronal activity arising from unbalanced runaway excitation. Here we report the first examination of spike train patterns in large ensembles of single neurons during seizures in persons with epilepsy. Contrary to the traditional view, neuronal spiking activity during seizure initiation and spread was highly heterogeneous, not hypersynchronous, suggesting complex interactions among different neuronal groups even at the spatial scale of small cortical patches. In contrast to earlier stages, seizure termination is a nearly homogenous phenomenon followed by an almost complete cessation of spiking across recorded neuronal ensembles. Notably, even neurons outside the region of seizure onset showed significant changes in activity minutes before the seizure. These findings suggest a revision of current thinking about seizure mechanisms and point to the possibility of seizure prevention based on spiking activity in neocortical neurons
Strongly Interacting Vector Bosons at TeV e+-e- Linear Colliders
In the absence of light Higgs bosons, the W and Z bosons become strongly
interacting particles at energies of about 1 TeV. If the longitudinal W,Z
components are generated by Goldstone modes associated with spontaneous
symmetry breaking in a new strong interaction theory, the quasi-elastic W,Z
scattering amplitudes can be predicted as a systematic chiral expansion in the
energy. We study the potential of TeV e+e- and e-e- linear colliders in
investigating these scattering processes. We estimate the accuracy with which
the coefficients of the chiral expansion can be measured in a multi-parameter
analysis. The measurements will provide us with a quantitative test of the
dynamics underlying the W,Z interactions.Comment: 40 pages, LaTeX2e, uses feynmp.sty [included]. Improvements in text,
minor typos correcte
Proton-Antiproton Annihilation and Meson Spectroscopy with the Crystal Barrel
This report reviews the achievements of the Crystal Barrel experiment at the
Low Energy Antiproton Ring (LEAR) at CERN. During seven years of operation
Crystal Barrel has collected very large statistical samples in pbarp
annihilation, especially at rest and with emphasis on final states with high
neutral multiplicity. The measured rates for annihilation into various two-body
channels and for electromagnetic processes have been used to test simple models
for the annihilation mechanism based on the quark internal structure of
hadrons. From three-body annihilations three scalar mesons, a0(1450), f0(1370)
and f0(1500) have been established in various decay modes. One of them,
f0(1500), may be identified with the expected ground state scalar glueball.Comment: 64 pages, LATEX file, 36 figures are available as ps files at
http://afuz01.cern.ch/claude/ Submitted to Reviews of Modern Physic
Predictors of Access to Rehabilitation in the Year Following Traumatic Brain Injury : A European Prospective and Multicenter Study
Background Although rehabilitation is beneficial for individuals with traumatic brain injury (TBI), a significant proportion of them do not receive adequate rehabilitation after acute care. Objective Therefore, the goal of this prospective and multicenter study was to investigate predictors of access to rehabilitation in the year following injury in patients with TBI. Methods Data from a large European study (CENTER-TBI), including TBIs of all severities between December 2014 and December 2017 were used (N = 4498 patients). Participants were dichotomized into those who had and those who did not have access to rehabilitation in the year following TBI. Potential predictors included sociodemographic factors, psychoactive substance use, preinjury medical history, injury-related factors, and factors related to medical care, complications, and discharge. Results In the year following traumatic injury, 31.4% of patients received rehabilitation services. Access to rehabilitation was positively and significantly predicted by female sex (odds ratio [OR] = 1.50), increased number of years of education completed (OR = 1.05), living in Northern (OR = 1.62; reference: Western Europe) or Southern Europe (OR = 1.74), lower prehospital Glasgow Coma Scale score (OR = 1.03), higher Injury Severity Score (OR = 1.01), intracranial (OR = 1.33) and extracranial (OR = 1.99) surgery, and extracranial complication (OR = 1.75). On contrast, significant negative predictors were lack of preinjury employment (OR = 0.80), living in Central and Eastern Europe (OR = 0.42), and admission to hospital ward (OR = 0.47; reference: admission to intensive care unit) or direct discharge from emergency room (OR = 0.24). Conclusions Based on these findings, there is an urgent need to implement national and international guidelines and strategies for access to rehabilitation after TBI.Peer reviewe
Quantum Spacetime Phenomenology
I review the current status of phenomenological programs inspired by
quantum-spacetime research. I stress in particular the significance of results
establishing that certain data analyses provide sensitivity to effects
introduced genuinely at the Planck scale. And my main focus is on
phenomenological programs that managed to affect the directions taken by
studies of quantum-spacetime theories.Comment: 125 pages, LaTex. This V2 is updated and more detailed than the V1,
particularly for quantum-spacetime phenomenology. The main text of this V2 is
about 25% more than the main text of the V1. Reference list roughly double