361 research outputs found
A new photon recoil experiment: towards a determination of the fine structure constant
We report on progress towards a measurement of the fine structure constant to
an accuracy of or better by measuring the ratio of the
Planck constant to the mass of the cesium atom. Compared to similar
experiments, ours is improved in three significant ways: (i) simultaneous
conjugate interferometers, (ii) multi-photon Bragg diffraction between same
internal states, and (iii) an about 1000 fold reduction of laser phase noise to
-138 dBc/Hz. Combining that with a new method to simultaneously stabilize the
phases of four frequencies, we achieve 0.2 mrad effective phase noise at the
location of the atoms. In addition, we use active stabilization to suppress
systematic effects due to beam misalignment.Comment: 12 pages, 9 figure
Probing generalized parton distributions in pi N -> l+ l- N
We study the exclusive reactions pi- p -> l+ l- n and pi+ n -> l+ l- p$ in
view of possible future experiments with high-intensity pion beams. For large
invariant mass of the lepton pair l+ l- and small squared momentum transfer to
the nucleon these are hard-scattering processes providing access to generalized
parton distributions. We estimate the cross section for these reactions,
explore their connection with the pion form factor, and discuss the role they
can play in improving our understanding of the relevant reaction mechanisms.Comment: 13 pages, 5 figure
Persistent Currents in 1D Disordered Rings of Interacting Electrons
We calculate the persistent current of 1D rings of spinless fermions with
short-range interactions on a lattice with up to 20 sites, and in the presence
of disorder, for various band fillings. We find that {\it both} disorder and
interactions always decrease the persistent current by localizing the
electrons. Away from half-filling, the interaction has a much stronger
influence in the presence of disorder than in the pure case.Comment: Latex file, 11 pages, 5 figures available on request, Report
LPQTH-93/1
Femto-Photography of Protons to Nuclei with Deeply Virtual Compton Scattering
Developments in deeply virtual Compton scattering allow the direct
measurements of scattering amplitudes for exchange of a highly virtual photon
with fine spatial resolution. Real-space images of the target can be obtained
from this information. Spatial resolution is determined by the momentum
transfer rather than the wavelength of the detected photon. Quantum photographs
of the proton, nuclei, and other elementary particles with resolution on the
scale of a fraction of a femtometer is feasible with existing experimental
technology.Comment: To be published in Physical Review D. Replaces previous version with
minor changes in presentatio
COMUNIDADE DE PRÁTICAS EMANCIPATÓRIAS: PRÁTICAS TERRITORIAIS VOLTADAS PARA A INFÂNCIA-JUVENTUDE NO MUNICÍPIO DE PELOTAS, RIO GRANDE DO SUL.
Introdução: levando-se em conta a vulnerabilidade e a condição infanto-juvenil no
território Dunas, docentes do núcleo de Terapia Ocupacional Social da UFPel propuseram
a criação de uma “comunidade de práticas emancipatórias” CoPE, a fim de fomentar
alternativas de pesquisa, formação e extensão que perseguissem a democratização do bem
público universitário por meio de práticas coletivas. Objetivo: relatar as experiências
territoriais desenvolvidas junto às escolas e UBS, destinadas à infância/juventude.
Metodologia: Elegeu-se a pesquisa-ação emancipatória (PAE), que tem como finalidade a
produção de saberes implicados com a transformação social, por meio de processo
educativo facilitador, resguardando a participação radical das pessoas envolvidas e o
desenvolvimento de consciências críticas. Processos: foram empregadas diferentes
técnicas, como as caminhadas, reuniões com equipamentos públicos e lideranças sociais,
oficinas de atividades, dinâmicas e projetos, rodas formativas. Conclusões: As ações
realizadas até o presente momento, proporcionaram, simultaneamente, a produção de
saberes sobre o território, bem como (re)conhecimentos entre atores, com vistas à
consolidação de uma comunidade de práticas. O estímulo à participação social de todos os
atores aconteceu mediante as estratégias de deflagração da realidade e convite ao
engajamento nos problemas coletivos
Optical absorption spectra in SrCu_2O_3 two-leg spin ladder
We calculate the phonon-assisted optical-absorption spectra in SrCu_2O_3
two-leg spin-ladder systems. The results for two models proposed for SrCu_2O_3
are compared. In the model including the effects of a cyclic four-spin
interaction, the shoulder structure appears at 978 cm^{-1} and the peak appears
at 1975 cm^{-1} in the spectrum for polarization of the electric field parallel
to the legs. In the other model which describes a pure two-leg ladder, the peak
appears around the lower edge of the spectrum at 1344 cm^{-1}. The feature can
be effective in determining the proper model for SrCu_2O_3.Comment: 5 pages, 5 figures, to appear in PRB vol. 67 (2003
Correlation effects in ionic crystals: I. The cohesive energy of MgO
High-level quantum-chemical calculations, using the coupled-cluster approach
and extended one-particle basis sets, have been performed for (Mg2+)n (O2-)m
clusters embedded in a Madelung potential. The results of these calculations
are used for setting up an incremental expansion for the correlation energy of
bulk MgO. This way, 96% of the experimental cohesive energy of the MgO crystal
is recovered. It is shown that only 60% of the correlation contribution to the
cohesive energy is of intra-ionic origin, the remaining part being caused by
van der Waals-like inter-ionic excitations.Comment: LaTeX, 20 pages, no figure
The immunopeptidome landscape associated with T cell infiltration, inflammation and immune editing in lung cancer.
One key barrier to improving efficacy of personalized cancer immunotherapies that are dependent on the tumor antigenic landscape remains patient stratification. Although patients with CD3 <sup>+</sup> CD8 <sup>+</sup> T cell-inflamed tumors typically show better response to immune checkpoint inhibitors, it is still unknown whether the immunopeptidome repertoire presented in highly inflamed and noninflamed tumors is substantially different. We surveyed 61 tumor regions and adjacent nonmalignant lung tissues from 8 patients with lung cancer and performed deep antigen discovery combining immunopeptidomics, genomics, bulk and spatial transcriptomics, and explored the heterogeneous expression and presentation of tumor (neo)antigens. In the present study, we associated diverse immune cell populations with the immunopeptidome and found a relatively higher frequency of predicted neoantigens located within HLA-I presentation hotspots in CD3 <sup>+</sup> CD8 <sup>+</sup> T cell-excluded tumors. We associated such neoantigens with immune recognition, supporting their involvement in immune editing. This could have implications for the choice of combination therapies tailored to the patient's mutanome and immune microenvironment
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
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