274 research outputs found
High energy cosmic rays, gamma rays and neutrinos from AGN
The author reviews a model for the emission of high energy cosmic rays,
gamma-rays and neutrinos from AGN (Active Galactic Nuclei) that he has proposed
since 1985. Further discussion of the knee energy phenomenon of the cosmic ray
energy spectrum requires the existence of a heavy particle with mass in the
knee energy range. A possible method of detecting such a particle in the Pierre
Auger Project is suggested. Also presented is a relation between the spectra of
neutrinos and gamma-rays emitted from AGN. This relation can be tested by high
energy neutrino detectors such as ICECUBE, the Mediterranean Sea Detector and
possibly by the Pierre Auger Project.Comment: 4 pages, no figure
Two components of dark matter in the DAMA data
It is shown that the DAMA data indicate two dark matter components, one that
circulates around the galactic center (GC) and another that is emitted from the
GC. From the location of the maximum yearly variation, one can compute the
ratio of the two components.Comment: 4 pages, no figur
Interplay of the Chiral and Large N_c Limits in pi N Scattering
Light-quark hadronic physics admits two useful systematic expansions, the
chiral and 1/N_c expansions. Their respective limits do not commute, making
such cases where both expansions may be considered to be especially
interesting. We first study pi N scattering lengths, showing that (as expected
for such soft-pion quantities) the chiral expansion converges more rapidly than
the 1/N_c expansion, although the latter nevertheless continues to hold. We
also study the Adler-Weisberger and Goldberger-Miyazawa-Oehme sum rules of pi N
scattering, finding that both fail if the large N_c limit is taken prior to the
chiral limit.Comment: 10 pages, ReVTe
On relativistic approaches to the pion self-energy in nuclear matter
We argue that, in contrast to the non-relativistic approach, a relativistic
evaluation of the nucleon--hole and delta-isobar--nucleon hole contributions to
the pion self-energy incorporates the s-wave scattering, which requires a more
accurate evaluation. Therefore relativistic approach containing only these
diagrams does not describe appropriately the pion self-energy in isospin
symmetric nuclear matter. We conclude that, a correct relativistic approach to
the pion self-energy should involve a more sophisticated calculation in order
to satisfy the known experimental results on the near-threshold behaviour of
the pion-nucleon (forward) scattering amplitude.Comment: 7 pages,1 figur
Characterization of the glass transition in vitreous silica by temperature scanning small-angle X-ray scattering
The temperature dependence of the x-ray scattering in the region below the
first sharp diffraction peak was measured for silica glasses with low and high
OH content (GE-124 and Corning 7980). Data were obtained upon scanning the
temperature at 10, 40 and 80 K/min between 400 K and 1820 K. The measurements
resolve, for the first time, the hysteresis between heating and cooling through
the glass transition for silica glass, and the data have a better signal to
noise ratio than previous light scattering and differential thermal analysis
data. For the glass with the higher hydroxyl concentration the glass transition
is broader and at a lower temperature. Fits of the data to the
Adam-Gibbs-Fulcher equation provide updated kinetic parameters for this very
strong glass. The temperature derivative of the observed X-ray scattering
matches that of light scattering to within 14%.Comment: EurophysicsLetters, in pres
Relativistic two-pion exchange nucleon-nucleon potential: configuration space
We have recently performed a relativistic chiral expansion of the
two-pion exchange potential, and here we explore its configuration space
content. Interactions are determined by three families of diagrams, two of
which involve just and , whereas the third one depends on
empirical coefficients fixed by subthreshold data. In this sense, the
calculation has no adjusted parameters and gives rise to predictions, which are
tested against phenomenological potentials. The dynamical structure of the
eight leading non-relativistic components of the interaction is investigated
and, in most cases, found to be clearly dominated by a well defined class of
diagrams. In particular, the central isovector and spin-orbit, spin-spin, and
tensor isoscalar terms are almost completely fixed by just and .
The convergence of the chiral series in powers of the ratio (pion mass/nucleon
mass) is studied as a function of the internucleon distance and, for 1 fm,
found to be adequate for most components of the potential. An important
exception is the dominant central isoscalar term, where the convergence is
evident only for 2.5 fm. Finally, we compare the spatial behavior of the
functions that enter the relativistic and heavy baryon formulations of the
interaction and find that, in the region of physical interest, they differ by
about 5%.Comment: 27 pages, 33 figure
Quasinormal distributions and expansion at the mode
The Gram-Charlier series of type A is discussed in terms of deviants which are related to moments in a way similar to the way Hermite polynomials are related to the powers. Distribution functions are also expressed in terms of the mode and moments (cumulants or deviants), which are useful expansions when the distributions are approximately normal. It is shown that such expansions as well as the Gram-Charlier series are valid asymptotically for discrete distributions defined on the semiinfinite interval [0, â].Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45129/1/10955_2005_Article_BF01010217.pd
The in-medium isovector pi N amplitude from low energy pion scattering
Differential cross sections for elastic scattering of 21.5 MeV positive and
negative pions by Si, Ca, Ni and Zr have been measured as part of a study of
the pion-nucleus potential across threshold. The `anomalous' repulsion in the
s-wave term was observed, as is the case with pionic atoms. The extra repulsion
can be accounted for by a chiral-motivated model where the pion decay constant
is modified in the medium. Unlike in pionic atoms, the anomaly cannot be
removed by merely introducing an empirical on-shell energy dependence.Comment: 9 pages, 2 figures. Minor changes, to appear in PR
Elastic scattering of low energy pions by nuclei and the in-medium isovector pi N amplitude
Measurements of elastic scattering of 21.5 MeV pi+ and pi- by Si, Ca, Ni and
Zr were made using a single arm magnetic spectrometer. Absolute calibration was
made by parallel measurements of Coulomb scattering of muons. Parameters of a
pion-nucleus optical potential were obtained from fits to all eight angular
distributions put together. The `anomalous' s-wave repulsion known from pionic
atoms is clearly observed and could be removed by introducing a
chiral-motivated density dependence of the isovector scattering amplitude,
which also greatly improved the fits to the data. The empirical energy
dependence of the isoscalar amplitude also improves the fits to the data but,
contrary to what is found with pionic atoms, on its own is incapable of
removing the anomaly.Comment: 20 pages, 5 figures, 5 tables. V2 added details on
uncertainties,extended discussion. To appear in PR
Quantitative AFM analysis of phase separated borosilicate glass surfaces
Phase separated borosilicate glass samples were prepared by applying various
heat treatments. Using selective chemical etching we performed AFM measurement
on the phase separated glass surfaces. A quantitative roughness analysis
allowed us to measure precisely the dependence of the characteristic size of
the phase domains on heating time and temperature. The experimental
measurements are very well described by the theoretically expected scaling
laws. Interdiffusion coefficients and activation energy are estimated from this
analysis and are consistent with literature data
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