7,294 research outputs found
The QCD pomeron at TESLA - motivation and exclusive J/psi production
We briefly present the motivation for studying the processes mediated by the
QCD pomeron at high energy e+e- colliders. We describe the behaviour of the
cross-section for the reaction gamma gamma to J/psi J/psi obtained from the
BFKL equation with dominant non-leading corrections. We give the predictions
for the rates of anti-tagged e+e- to e+e- J/psi J/psi events in TESLA and
conclude that such reactions may be excellent probes of the hard pomeron.Comment: 8 pages, 3 figures, to appear in the Proceedings of the 2nd Joint
ECFA/DESY Study on Physics and Detectors for a Linear Electron-Positron
Collider, Obernai, France, October 199
How a small quantum bath can thermalize long localized chains
We investigate the stability of the many-body localized (MBL) phase for a
system in contact with a single ergodic grain, modelling a Griffiths region
with low disorder. Our numerical analysis provides evidence that even a small
ergodic grain consisting of only 3 qubits can delocalize a localized chain, as
soon as the localization length exceeds a critical value separating localized
and extended regimes of the whole system. We present a simple theory,
consistent with the arguments in [Phys. Rev. B 95, 155129 (2017)], that assumes
a system to be locally ergodic unless the local relaxation time, determined by
Fermi's Golden Rule, is larger than the inverse level spacing. This theory
predicts a critical value for the localization length that is perfectly
consistent with our numerical calculations. We analyze in detail the behavior
of local operators inside and outside the ergodic grain, and find excellent
agreement of numerics and theory.Comment: 4 pages, 5 figure
Quantum Brownian Motion in a Simple Model System
We consider a quantum particle coupled (with strength λ) to a spatial array of independent non-interacting reservoirs in thermal states (heat baths). Under the assumption that the reservoir correlations decay exponentially in time, we prove that the motion of the particle is diffusive at large times for small, but finite λ. Our proof relies on an expansion around the kinetic scaling limit ( , while time and space scale as λ−2) in which the particle satisfies a Boltzmann equation. We also show an equipartition theorem: the distribution of the kinetic energy of the particle tends to a Maxwell-Boltzmann distribution, up to a correction of O(λ2
Approach to ground state and time-independent photon bound for massless spin-boson models
It is widely believed that an atom interacting with the electromagnetic field
(with total initial energy well-below the ionization threshold) relaxes to its
ground state while its excess energy is emitted as radiation. Hence, for large
times, the state of the atom+field system should consist of the atom in its
ground state, and a few free photons that travel off to spatial infinity.
Mathematically, this picture is captured by the notion of asymptotic
completeness. Despite some recent progress on the spectral theory of such
systems, a proof of relaxation to the ground state and asymptotic completeness
was/is still missing, except in some special cases (massive photons, small
perturbations of harmonic potentials). In this paper, we partially fill this
gap by proving relaxation to an invariant state in the case where the atom is
modelled by a finite-level system. If the coupling to the field is sufficiently
infrared-regular so that the coupled system admits a ground state, then this
invariant state necessarily corresponds to the ground state. Assuming slightly
more infrared regularity, we show that the number of emitted photons remains
bounded in time. We hope that these results bring a proof of asymptotic
completeness within reach.Comment: 45 pages, published in Annales Henri Poincare. This archived version
differs from the journal version because we corrected an inconsequential
mistake in Section 3.5.1: to do this, a new paragraph was added after Lemma
3.
Extended Weak Coupling Limit for Friedrichs Hamiltonians
We study a class of self-adjoint operators defined on the direct sum of two
Hilbert spaces: a finite dimensional one called sometimes a ``small subsystem''
and an infinite dimensional one -- a ``reservoir''. The operator, which we call
a ``Friedrichs Hamiltonian'', has a small coupling constant in front of its
off-diagonal term. It is well known that under some conditions in the weak
coupling limit the appropriately rescaled evolution in the interaction picture
converges to a contractive semigroup when restricted to the subsystem. We show
that in this model, the properly renormalized and rescaled evolution converges
on the whole space to a new unitary evolution, which is a dilation of the above
mentioned semigroup. Similar results have been studied before \cite{AFL} in
more complicated models and they are usually referred to as "stochastic Limit".Comment: changes in notation and title, minor correction
Statistical Modeling of Lower Limb Kinetics During Deep Squat and Forward Lunge.
PURPOSE: Modern statistics and higher computational power have opened novel possibilities to complex data analysis. While gait has been the utmost described motion in quantitative human motion analysis, descriptions of more challenging movements like the squat or lunge are currently lacking in the literature. The hip and knee joints are exposed to high forces and cause high morbidity and costs. Pre-surgical kinetic data acquisition on a patient-specific anatomy is also scarce in the literature. Studying the normal inter-patient kinetic variability may lead to other comparable studies to initiate more personalized therapies within the orthopedics. METHODS: Trials are performed by 50 healthy young males who were not overweight and approximately of the same age and activity level. Spatial marker trajectories and ground reaction force registrations are imported into the Anybody Modeling System based on subject-specific geometry and the state-of-the-art TLEM 2.0 dataset. Hip and knee joint reaction forces were obtained by a simulation with an inverse dynamics approach. With these forces, a statistical model that accounts for inter-subject variability was created. For this, we applied a principal component analysis in order to enable variance decomposition. This way, noise can be rejected and we still contemplate all waveform data, instead of using deduced spatiotemporal parameters like peak flexion or stride length as done in many gait analyses. In addition, this current paper is, to the authors' knowledge, the first to investigate the generalization of a kinetic model data toward the population. RESULTS: Average knee reaction forces range up to 7.16 times body weight for the forwarded leg during lunge. Conversely, during squat, the load is evenly distributed. For both motions, a reliable and compact statistical model was created. In the lunge model, the first 12 modes accounts for 95.26% of inter-individual population variance. For the maximal-depth squat, this was 95.69% for the first 14 modes. Model accuracies will increase when including more principal components. CONCLUSION: Our model design was proved to be compact, accurate, and reliable. For models aimed at populations covering descriptive studies, the sample size must be at least 50
Derivation of some translation-invariant Lindblad equations for a quantum Brownian particle
We study the dynamics of a Brownian quantum particle hopping on an infinite
lattice with a spin degree of freedom. This particle is coupled to free boson
gases via a translation-invariant Hamiltonian which is linear in the creation
and annihilation operators of the bosons. We derive the time evolution of the
reduced density matrix of the particle in the van Hove limit in which we also
rescale the hopping rate. This corresponds to a situation in which both the
system-bath interactions and the hopping between neighboring sites are small
and they are effective on the same time scale. The reduced evolution is given
by a translation-invariant Lindblad master equation which is derived
explicitly.Comment: 28 pages, 4 figures, minor revisio
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