896 research outputs found
Modelling DNA Response to THz Radiation
Collective response of DNA to THz electric fields is studied in a simple pair
bond model. We confirm, with some caveats, a previous observation of
destabilising DNA breather modes and explore the parameter-dependence of these
modes. It is shown that breather modes are eliminated under reasonable physical
conditions and that thermal effects are significant.Comment: 6 pages, 3 figures. version to appear in Phys. Rev.
A Coulomb Gauge Model of Mesons
A model of mesons which is based on the QCD Hamiltonian in Coulomb gauge is
presented. The model relies on a novel quasiparticle basis to improve the
reliability of the Fock space expansion. It is also relativistic, yields chiral
pions, and is tightly constrained by QCD (quark masses are the only
parameters). Applications to hidden flavor mesons yield results which are
comparable to phenomenological constituent quark models while revealing the
limitations of such models.Comment: 13 pages, 1 eps figure, 5 table
Spatial tracking for optical heterodyne communication
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1984.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.Includes bibliographical reference.by Eric A. Swanson.M.S
A Phosphatidylinositol-3-Kinase-Dependent Signal Transition Regulates ARF1 and ARF6 during Fcγ Receptor-Mediated Phagocytosis
Fcγ receptor (FcγR)–mediated phagocytosis of IgG-coated particles is regulated by 3′-phosphoinositides (3′PIs) and several classes of small GTPases, including ARF6 from the ADP Ribosylation Factor subfamily. The insensitivity of phagocytosis to brefeldin A (BFA), an inhibitor of certain ARF guanine nucleotide exchange factors (GEFs), previously indicated that ARF1 did not participate in phagocytosis. In this study, we show that ARF1 was activated during FcγR-mediated phagocytosis and that blocking normal ARF1 cycling inhibited phagosome closure. We examined the distributions and activation patterns of ARF6 and ARF1 during FcγR-mediated phagocytosis using fluorescence resonance energy transfer (FRET) stoichiometric microscopy of macrophages expressing CFP- or YFP-chimeras of ARF1, ARF6, and a GTP-ARF-binding protein domain. Both GTPases were activated by BFA-insensitive factors at sites of phagocytosis. ARF6 activation was restricted to the leading edge of the phagocytic cup, while ARF1 activation was delayed and delocalized over the phagosome. Phagocytic cups formed after inhibition of PI 3-kinase (PI-3K) contained persistently activated ARF6 and minimally activated ARF1. This indicates that a PI-3K-dependent signal transition defines the sequence of ARF GTPase activation during phagocytosis and that ARF6 and ARF1 coordinate different functions at the forming phagosome
Dynamic Properties of Charmonium
Nonrelativistic quark models of charmonia are tested by comparison of
theoretical charmonium decay constants, form factors, and widths
with experiment and lattice gauge computations. The importance of relativistic
effects, a running coupling, and the correct implementation of bound state
effects are demonstrated. We describe how an improved model and computational
techniques resolve several outstanding issues in previous nonrelativistic quark
models such as the use of `correction' factors in quark model form factors,
artificial energy prescriptions in decay constant calculations, and ad hoc
phase space modifications. We comment on the small experimental value of
and the D-wave component of the . Decay constants and
widths for bottomonium are also presented.Comment: 22 pages, 22 ps figures (table entries corrected, text modified
Unquenching the Quark Model and Screened Potentials
The low-lying spectrum of the quark model is shown to be robust under the
effects of `unquenching'. In contrast, the use of screened potentials is shown
to be of limited use in models of hadrons. Applications to unquenching the
lattice Wilson loop potential and to glueball mixing in the adiabatic hybrid
spectrum are also presented.Comment: 6 pages, 3 ps figures, revtex. Version to appear in J. Phys.
Distinguishing Among Strong Decay Models
Two competing models for strong hadronic decays, the and
models, are currently in use.
Attempts to rule out one or the other have been hindered by a poor
understanding of final state interactions and by ambiguities in the treatment
of relativistic effects.
In this article we study meson decays in both models, focussing on certain
amplitude ratios for which the relativistic uncertainties largely cancel out
(notably the ratios in and
), and using a Quark Born Formalism to estimate the
final state interactions.
We find that the model is strongly favoured.
In addition, we predict a amplitude ratio of for the decay
.
We also study the parameter-dependence of some individual amplitudes (as
opposed to amplitude ratios), in an attempt to identify a ``best'' version of
the model.Comment: 20 pages, uuencoded postscript file with 7 figures, MIT-CTP-2295;
CMU-HEP94-1
Saponin TQL1055 adjuvant-containing vaccine confers protection upon Mycobacterium tuberculosis challenge in mice
Tuberculosis (TB), caused by the intracellular pathoge
Hybrid Decays
The heavy quark expansion of Quantum Chromodynamics and the strong coupling
flux tube picture of nonperturbative glue are employed to develop the
phenomenology of hybrid meson decays. The decay mechanism explicitly couples
gluonic degrees of freedom to the pair produced quarks and hence does not obey
the well known, but model-dependent, selection rule which states that hybrids
do not decay to pairs of L=0 mesons. However, the nonperturbative nature of
gluonic excitations in the flux tube picture leads to a new selection rule:
light hybrids do not decay to pairs of identical mesons. New features of the
model are highlighted and partial widths are presented for several low lying
hybrid states.Comment: 13 pages, 1 table, revte
From Current to Constituent Quarks: a Renormalization Group Improved Hamiltonian-based Description of Hadrons
A model which combines the perturbative behavior of QCD with low energy
phenomenology in a unified framework is developed. This is achieved by applying
a similarity transformation to the QCD Hamiltonian which removes interactions
between the ultraviolet cutoff and an arbitrary lower scale. Iteration then
yields a renormalization group improved effective Hamiltonian at the hadronic
energy scale. The procedure preserves the standard ultraviolet behavior of QCD.
Furthermore, the Hamiltonian evolves smoothly to a phenomenological low energy
behavior below the hadronic scale. This method has the benefit of allowing
radiative corrections to be directly incorporated into nonperturbative
many-body techniques. It is applied to Coulomb gauge QCD supplemented with a
low energy linear confinement interaction. A nontrivial vacuum is included in
the analysis via a Bogoliubov-Valatin transformation. Finally, the formalism is
applied to the vacuum gap equation, the quark condensate, and the dynamical
quark mass.Comment: 36 pages, RevTeX, 5 ps figures include
- …