1,203 research outputs found
Loop-Generated Bounds on Changes to the Graviton Dispersion Relation
We identify the effective theory appropriate to the propagation of massless
bulk fields in brane-world scenarios, to show that the dominant low-energy
effect of asymmetric warping in the bulk is to modify the dispersion relation
of the effective 4-dimensional modes. We show how such changes to the graviton
dispersion relation may be bounded through the effects they imply, through
loops, for the propagation of standard model particles. We compute these bounds
and show that they provide, in some cases, the strongest constraints on
nonstandard gravitational dispersions. The bounds obtained in this way are the
strongest for the fewest extra dimensions and when the extra-dimensional Planck
mass is the smallest. Although the best bounds come for warped 5-D scenarios,
for which the 5D Planck Mass is O(TeV), even in 4 dimensions the graviton loop
can lead to a bound on the graviton speed which is comparable with other
constraints.Comment: 18 pages, LaTeX, 4 figures, uses revte
Phi meson production in relativistic heavy ion collisions
Within a multiphase transport model we study phi meson production in
relativistic heavy ion collisions from both superposition of initial multiple
proton-proton interactions and the secondary collisions in the produced
hadronic matter. The yield of phi mesons is then reconstructed from their
decaying product of either the kaon-antikaon pairs or the dimuon pairs. Since
the kaon-antikaon pairs at midrapidity with low transverse momenta are
predominantly rescattered or absorbed in the hadronic medium, they can not be
used to reconstruct the phi meson and lead thus to a smaller reconstructed phi
meson yield than that reconstructed from the dimuon channel. With in-medium
mass modifications of kaons and phi mesons, the phi yield from dimuons is
further enhanced compared to that from the kaon-antikaon pairs. The model
result is compared with the experimental data at the CERN/SPS and RHIC energies
and its implications to quark-gluon plasma formation are discussed.Comment: Revised version, to appear in Nucl. Phys.
The trans-activation domain of the sporulation response regulator Spo0A revealed by X-ray crystallography
Sporulation in Bacillus involves the induction of scores of genes in a temporally and spatially co-ordinated programme of cell development. Its initiation is under the control of an expanded two-component signal transduction system termed a phosphorelay. The master control element in the decision to sporulate is the response regulator, Spo0A, which comprises a receiver or phosphoacceptor domain and an effector or transcription activation domain. The receiver domain of Spo0A shares sequence similarity with numerous response regulators, and its structure has been determined in phosphorylated and unphosphorylated forms. However, the effector domain (C-Spo0A) has no detectable sequence similarity to any other protein, and this lack of structural information is an obstacle to understanding how DNA binding and transcription activation are controlled by phosphorylation in Spo0A. Here, we report the crystal structure of C-Spo0A from Bacillus stearothermophilus revealing a single alpha -helical domain comprising six alpha -helices in an unprecedented fold. The structure contains a helix-turn-helix as part of a three alpha -helical bundle reminiscent of the catabolite gene activator protein (CAP), suggesting a mechanism for DNA binding. The residues implicated in forming the sigma (A)-activating region clearly cluster in a flexible segment of the polypeptide on the opposite side of the structure from that predicted to interact with DNA. The structural results are discussed in the context of the rich array of existing mutational data
Pentaquark baryon production from photon-neuteron reactions
Extending the hadronic Lagrangians that we recently introduced for studying
pentaquark baryon production from meson-proton, proton-proton, and
photon-proton reactions near threshold to include the anomalous interaction
between and , we evaluate the cross section for
production from photon-neutron reactions, in which the was first
detected in the SPring-8 experiment in Japan and the CLAS experiment at Thomas
Jefferson National Laboratory. With empirical coupling constants and form
factors, and assuming that the decay width of is 20 MeV, the
predicted cross section is found to have a peak value of about 280 nb, which is
substantially larger than that for production from photon-proton
reactions.Comment: 13 pages, 6 figure
Using Nonlinear Response to Estimate the Strength of an Elastic Network
Disordered networks of fragile elastic elements have been proposed as a model
of inner porous regions of large bones [Gunaratne et.al., cond-mat/0009221,
http://xyz.lanl.gov]. It is shown that the ratio of responses of such
a network to static and periodic strain can be used to estimate its ultimate
(or breaking) stress. Since bone fracture in older adults results from the
weakening of porous bone, we discuss the possibility of using as a
non-invasive diagnostic of osteoporotic bone.Comment: 4 pages, 4 figure
A note on spin-s duality
Duality is investigated for higher spin (), free, massless, bosonic
gauge fields. We show how the dual formulations can be derived from a common
"parent", first-order action. This goes beyond most of the previous treatments
where higher-spin duality was investigated at the level of the equations of
motion only. In D=4 spacetime dimensions, the dual theories turn out to be
described by the same Pauli-Fierz (s=2) or Fronsdal () action (as it
is the case for spin 1). In the particular s=2 D=5 case, the Pauli-Fierz action
and the Curtright action are shown to be related through duality. A crucial
ingredient of the analysis is given by the first-order, gauge-like,
reformulation of higher spin theories due to Vasiliev.Comment: Minor corrections, reference adde
Seeing phi meson through the dilepton spectra in heavy-ion collisions
Dilepton spectra from the decay of phi mesons produced in heavy-ion
collisions at SIS/GSI energies ( GeV/nucleon) are studied in the
relativistic transport model. We include phi mesons produced from
baryon-baryon, pion-baryon, and kaon-antikaon collisions. The cross sections
for the first two processes are obtained from an one-boson-exchange model,
while that for the last process is taken to be the Breit-Wigner form through
the phi meson resonance. For dileptons with invariant mass near the phi meson
peak, we also include contributions from neutron-proton bremsstrahlung,
pion-pion annihilation, and the decay of rho and omega mesons produced in
baryon-baryon and meson-baryon collisions. Effects due to medium modifications
of the kaon and vector (rho, omega and phi) meson properties are investigated.
We find that the kaon medium effects lead to a broadening of the dilepton
spectrum as a result of the increase of phi meson decay width. Furthermore, the
dropping of phi meson mass in nuclear medium leads to a shoulder structure in
the dilepton spectrum besides the main peak at the bare phi meson mass. The
experimental measurement of the dilepton spectra from heavy-ion collisions is
expected to provide useful information about the phi meson properties in dense
matter.Comment: RevTeX, 18 pages, including 13 postscript figures, submitted to
Nuclear Physics
Axion-like particles as ultra high energy cosmic rays?
If Ultra High Energy Cosmic Rays (UHECRs) with E>4 10^{19} eV originate from
BL Lacertae at cosmological distances as suggested by recent studies, the
absence of the GZK cutoff can not be reconciled with Standard-Model particle
properties. Axions would escape the GZK cutoff, but even the coherent
conversion and back-conversion between photons and axions in large-scale
magnetic fields is not enough to produce the required flux. However, one may
construct models of other novel (pseudo)scalar neutral particles with
properties that would allow for sufficient rates of particle production in the
source and shower production in the atmosphere to explain the observations. As
an explicit example for such particles we consider SUSY models with light
sgoldstinos.Comment: 5 pages, 2 postscript figures, ref. adde
Phi Meson Production in Heavy-Ion Collisions at SIS Energies
Phi meson production in heavy-ion collisions at SIS/GSI energies (
GeV/nucleon) is studied in the relativistic transport model. We include
contributions from baryon-baryon, pion-baryon, and kaon-antikaon collisions.
The cross sections for the first two processes are obtained in an
one-boson-exchange model, while that for the last process is taken to be of
Breit-Wigner form through the phi meson resonance. The dominant contribution to
phi meson production in heavy ion collisions at these energies is found to come
from secondary pion-nucleon collisions. Effects due to medium modifications of
kaon masses are also studied and are found to reduce the phi meson yield by
about a factor of two, mainly because of increased phi decay width as a result
of dropping kaon-antikaon masses. In this case, the ratio is about
4%, which is a factor of 2-3 below preliminary experimental data from the FOPI
collaboration at GSI. Including also the reduction of phi meson mass in medium
increases this ratio to about 8%, which is then in reasonable agreement with
the data.Comment: 46 pages, including 21 postscript figure
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