132 research outputs found
On neutral pion electroproduction off deuterium
Threshold neutral pion electroproduction on the deuteron is studied in the
framework of baryon chiral perturbation theory at next-to-leading order in the
chiral expansion. To this order in small momenta, the amplitude is finite and a
sum of two- and three-body interactions with no undetermined parameters. We
calculate the S-wave multipoles for threshold production and the deuteron
S-wave cross section as a function of the photon virtuality. We also discuss
the sensitivity to the elementary neutron amplitudes.Comment: 6 pp, revtex, 3 figs, corrected version, to appear in Phys. Rev.
New developments in threshold pion photo- and electroproduction
Photoproduction of neutral and charged pions off nucleons and deuterium has
been precisely calculated in baryon chiral perturbation theory. I review the
predictions with the accurate data that have become available over the last few
years. Some progress in the description of neutral pion electroproduction off
protons is also discussed.Comment: 17 pp, LaTeX, uses epsf and lamuphys.sty, invited plenary talk,
Workshop on ``Chiral Dynamics: Theory and Experiment'', Mainz, September
1997, to appear in the proceeding
Modification to the power spectrum in the brane world inflation driven by the bulk inflaton
We compute the cosmological perturbations generated in the brane world
inflation driven by the bulk inflaton. Different from the model that the
inflation is a brane effect, we exhibit the modification of the power spectrum
of scalar perturbations due to the existence of the fifth dimension. With the
change of the initial vacuum, we investigate the dependence of the correction
of the power spectrum on the choice of the vacuum.Comment: replaced with the revised version, accepted for publication in PR
Squeezed States in the de Sitter Vacuum
We discuss the treatment of squeezed states as excitations in the Euclidean
vacuum of de Sitter space. A comparison with the treatment of these states as
candidate no-particle states, or alpha-vacua, shows important differences
already in the free theory. At the interacting level alpha-vacua are
inconsistent, but squeezed state excitations seem perfectly acceptable. Indeed,
matrix elements can be renormalized in the excited states using precisely the
standard local counterterms of the Euclidean vacuum. Implications for
inflationary scenarios in cosmology are discussed.Comment: 15 pages, no figures. One new citation in version 3; no other change
Threshold neutral pion photoproduction off the tri-nucleon to O(q^4)
We calculate electromagnetic neutral pion production off tri-nucleon bound
states (3H, 3He) at threshold in chiral nuclear effective field theory to
fourth order in the standard heavy baryon counting. We show that the fourth
order two-nucleon corrections to the S-wave multipoles at threshold are very
small. This implies that a precise measurement of the S-wave cross section for
neutral pion production off 3He allows for a stringent test of the chiral
perturbation theory prediction for the S-wave electric multipole E_{0+}^{pi0
n}.Comment: 17 pages, 5 figures, title changed, final version to appear in EPJA.
arXiv admin note: substantial text overlap with arXiv:1103.340
Astrophysical Uncertainties in the Cosmic Ray Electron and Positron Spectrum From Annihilating Dark Matter
In recent years, a number of experiments have been conducted with the goal of
studying cosmic rays at GeV to TeV energies. This is a particularly interesting
regime from the perspective of indirect dark matter detection. To draw reliable
conclusions regarding dark matter from cosmic ray measurements, however, it is
important to first understand the propagation of cosmic rays through the
magnetic and radiation fields of the Milky Way. In this paper, we constrain the
characteristics of the cosmic ray propagation model through comparison with
observational inputs, including recent data from the CREAM experiment, and use
these constraints to estimate the corresponding uncertainties in the spectrum
of cosmic ray electrons and positrons from dark matter particles annihilating
in the halo of the Milky Way.Comment: 21 pages, 9 figure
Primeval Corrections to the CMB Anisotropies
We show that deviations of the quantum state of the inflaton from the thermal
vacuum of inflation may leave an imprint in the CMB anisotropies. The quantum
dynamics of the inflaton in such a state produces corrections to the
inflationary fluctuations, which may be observable. Because these effects
originate from IR physics below the Planck scale, they will dominate over any
trans-Planckian imprints in any theory which obeys decoupling. Inflation sweeps
away these initial deviations and forces its quantum state closer to the
thermal vacuum. We view this as the quantum version of the cosmic no-hair
theorem. Such imprints in the CMB may be a useful, independent test of the
duration of inflation, or of significant features in the inflaton potential
about 60 e-folds before inflation ended, instead of an unlikely discovery of
the signatures of quantum gravity. The absence of any such substructure would
suggest that inflation lasted uninterrupted much longer than
e-folds.Comment: 17 pages, latex, no figures; v3: added references and comments, final
version to appear in Phys. Rev.
Indirect search for dark matter: prospects for GLAST
Possible indirect detection of neutralino, through its gamma-ray annihilation
product, by the forthcoming GLAST satellite from our galactic halo, M31, M87
and the dwarf galaxies Draco and Sagittarius is studied. Gamma-ray fluxes are
evaluated for the two representative energy thresholds, 0.1 GeV and 1.0 GeV, at
which the spatial resolution of GLAST varies considerably. Apart from dwarfs
which are described either by a modified Plummer profile or by a
tidally-truncated King profiles, fluxes are compared for halos with central
cusps and cores. It is demonstrated that substructures, irrespective of their
profiles, enhance the gamma-ray emission only marginally. The expected
gamma-ray intensity above 1 GeV at high galactic latitudes is consistent with
the residual emission derived from EGRET data if the density profile has a
central core and the neutralino mass is less than 50 GeV, whereas for a central
cusp only a substantial enhancement would explain the observations. From M31,
the flux can be detected above 0.1 GeV and 1.0 GeV by GLAST only if the
neutralino mass is below 300 GeV and if the density profile has a central cusp,
case in which a significant boost in the gamma-ray emission is produced by the
central black hole. For Sagittarius, the flux above 0.1 GeV is detectable by
GLAST provided the neutralino mass is below 50 GeV. From M87 and Draco the
fluxes are always below the sensitivity limit of GLAST.Comment: 14 Pages, 7 Figures, 3 Tables, version to appear on Physical Review
Coherent Compton scattering on light nuclei in the delta resonance region
Coherent Compton scattering on light nuclei in the delta resonance region is
studied in the impulse approximation and is shown to be a sensitive probe of
the in-medium properties of the delta resonance. The elementary amplitude on a
single nucleon is calculated from the unitary K-matrix approach developed
previously. Modifications of the properties of the delta resonance due to the
nuclear medium are accounted for through the self-energy operator of the delta,
calculated from the one-pion loop. The dominant medium effects such as the
Pauli blocking, mean-field modification of the nucleon and delta masses, and
particle-hole excitations in the pion propagator are consistently included in
nuclear matter.Comment: 30 pages, 11 figures, accepted for publication in Phys. Rev.
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