628 research outputs found
Plasma Wakefield Acceleration for Ultrahigh Energy Cosmic Rays
A cosmic acceleration mechanism is introduced which is based on the
wakefields excited by the Alfven shocks in a relativistically flowing plasma,
where the energy gain per distance of a test particle is Lorentz invariant. We
show that there exists a threshold condition for transparency below which the
accelerating particle is collision-free and suffers little energy loss in the
plasma medium. The stochastic encounters of the random
accelerating-decelerating phases results in a power-law energy spectrum: f(e)
1/e^2. The environment suitable for such plasma wakefield acceleration can be
cosmically abundant. As an example, we discuss the possible production of
super-GZK ultra high energy cosmic rays (UHECR) through this mechanism in the
atmosphere of gamma ray bursts. We show that the acceleration gradient can be
as high as G ~ 10^16 eV/cm. The estimated event rate in our model agrees with
that from UHECR observations.Comment: 11 pages, 1 figure, submitted to Phys. Rev. Let
Small Scale Anisotropy Predictions for the Auger Observatory
We study the small scale anisotropy signal expected at the Pierre Auger
Observatory in the next 1, 5, 10, and 15 years of operation, from sources of
ultra-high energy (UHE) protons. We numerically propagate UHE protons over
cosmological distances using an injection spectrum and normalization that fits
current data up to \sim 10^{20}\eV. We characterize possible sources of
ultra-high energy cosmic rays (UHECRs) by their mean density in the local
Universe, Mpc, with between 3 and 6.
These densities span a wide range of extragalactic sites for UHECR sources,
from common to rare galaxies or even clusters of galaxies. We simulate 100
realizations for each model and calculate the two point correlation function
for events with energies above 4 \times 10^{19}\eV and above 10^{20}\eV, as
specialized to the case of the Auger telescope. We find that for r\ga 4,
Auger should be able to detect small scale anisotropies in the near future.
Distinguishing between different source densities based on cosmic ray data
alone will be more challenging than detecting a departure from isotropy and is
likely to require larger statistics of events. Combining the angular
distribution studies with the spectral shape around the GZK feature will also
help distinguish between different source scenarios.Comment: 15 pages, 6 figures, 6 tables, submitted to JCA
Equation of State for Helium-4 from Microphysics
We compute the free energy of helium-4 near the lambda transition based on an
exact renormalization-group equation. An approximate solution permits the
determination of universal and nonuniversal thermodynamic properties starting
from the microphysics of the two-particle interactions. The method does not
suffer from infrared divergences. The critical chemical potential agrees with
experiment. This supports a specific formulation of the functional integral
that we have proposed recently. Our results for the equation of state reproduce
the observed qualitative behavior. Despite certain quantitative shortcomings of
our approximation, this demonstrates that ab initio calculations for collective
phenomena become possible by modern renormalization-group methods.Comment: 9 pages, 6 figures, revtex updated version, journal referenc
Planning the Future of U.S. Particle Physics (Snowmass 2013): Chapter 4: Cosmic Frontier
These reports present the results of the 2013 Community Summer Study of the
APS Division of Particles and Fields ("Snowmass 2013") on the future program of
particle physics in the U.S. Chapter 4, on the Cosmic Frontier, discusses the
program of research relevant to cosmology and the early universe. This area
includes the study of dark matter and the search for its particle nature, the
study of dark energy and inflation, and cosmic probes of fundamental
symmetries.Comment: 61 page
Unidentified EGRET Sources and the Extragalactic Gamma-Ray Background
The large majority of EGRET point sources remain to this day without an
identified low-energy counterpart. Whatever the nature of the EGRET
unidentified sources, faint unresolved objects of the same class must have a
contribution to the diffuse gamma-ray background: if most unidentified objects
are extragalactic, faint unresolved sources of the same class contribute to the
background, as a distinct extragalactic population; on the other hand, if most
unidentified sources are Galactic, their counterparts in external galaxies will
contribute to the unresolved emission from these systems. Understanding this
component of the gamma-ray background, along with other guaranteed
contributions from known sources, is essential in any attempt to use gamma-ray
observations to constrain exotic high-energy physics. Here, we follow an
empirical approach to estimate whether a potential contribution of unidentified
sources to the extragalactic gamma-ray background is likely to be important,
and we find that it is. Additionally, we comment on how the anticipated GLAST
measurement of the diffuse gamma-ray background will change, depending on the
nature of the majority of these sources.Comment: 6 pages, 3 figures, to appear in proceedings of "The Multi-Messenger
Approach to High Energy Gamma-Ray Sources", Barcelona, 4-7 July 2006;
comments welcom
On The Origin of Very High Energy Cosmic Rays
We discuss the most recent developments in our understanding of the
acceleration and propagation of cosmic rays up to the highest energies. In
particular we specialize our discussion to three issues: 1) developments in the
theory of particle acceleration at shock waves; 2) the transition from galactic
to extragalactic cosmic rays; 3) implications of up-to-date observations for
the origin of ultra high energy cosmic rays (UHECRs).Comment: Invited Review Article to appear in Modern Physics Letters A, Review
Sectio
A graviton propagator for inflation
We construct the scalar and graviton propagator in quasi de Sitter space up
to first order in the slow roll parameter . After
a rescaling, the propagators are similar to those in de Sitter space with an
correction to the effective mass. The limit
corresponds to the E(3) vacuum that breaks de Sitter symmetry, but does not
break spatial isotropy and homogeneity. The new propagators allow for a
self-consistent, dynamical study of quantum back-reaction effects during
inflation.Comment: 23 page
A Magnetized Local Supercluster and the Origin of the Highest Energy Cosmic Rays
A sufficiently magnetized Local Supercluster can explain the spectrum and
angular distribution of ultra-high energy cosmic rays. We show that the
spectrum of extragalactic cosmic rays with energies below eV may
be due to the diffusive propagation in the Local Supercluster with fields of
Gauss. Above eV, cosmic rays propagate
in an almost rectilinear way which is evidenced by the change in shape of the
spectrum at the highest energies. The fit to the spectrum requires that at
least one source be located relatively nearby at Mpc away from the
Milky Way. We discuss the origin of magnetic fields in the Local Supercluster
and the observable predictions of this model.Comment: 11 pages, 2 figures, submitted to PR
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