1,575 research outputs found
Global analysis of muon decay measurements
We have performed a global analysis of muon decay measurements to establish
model-independent limits on the space-time structure of the muon decay matrix
element. We find limits on the scalar, vector and tensor coupling of right- and
left-handed muons to right- and left-handed electrons. The limits on those
terms that involve the decay of right-handed muons to left-handed electrons are
more restrictive than in previous global analyses, while the limits on the
other non-standard model interactions are comparable. The value of the Michel
parameter eta found in the global analysis is -0.0036 \pm 0.0069, slightly more
precise than the value found in a more restrictive analysis of a recent
measurement. This has implications for the Fermi coupling constant G_F.Comment: 5 pages, 3 table
Sulfur reduction in sediments of marine and evaporite environments
Transformations of sulfur in sediments of ponds ranging in salinities from that of normal seawater to those of brines saturated with sodium chloride were examined. The chemistry of the sediment and pore waters were focused on with emphasis on the fate of sulfate reduction. The effects of increasing salinity on both forms of sulfur and microbial activity were determined. A unique set of chemical profiles and sulfate-reducing activity was found for the sediments of each of the sites examined. The quantity of organic matter in the salt pond sediments was significantly greater than that occurring in the adjacent intertidal site. The total quantitative and qualitative distribution of volatile fatty acids was also greater in the salt ponds. Volatile fatty acids increased with salinity
Refractive effects in the scattering of loosely bound nuclei
A study of the interaction of loosely bound nuclei 6,7Li at 9 and 19 AMeV
with light targets has been undertaken. With the determination of unambiguous
optical potentials in mind, elastic data for four projectile-target
combinations and one neutron transfer reaction 13C(7Li,8Li)12C have been
measured on a large angular range. The kinematical regime encompasses a region
where the mean field (optical potential) has a marked variation with mass and
energy, but turns out to be sufficiently surface transparent to allow strong
refractive effects to be manifested in elastic scattering data at intermediate
angles. The identified exotic feature, a "plateau" in the angular distributions
at intermediate angles, is fully confirmed in four reaction channels and
interpreted as a pre-rainbow oscillation resulting from the interference of the
barrier and internal barrier farside scattering subamplitudes.Comment: 19 pages, 14 figures, 3 tables to submit to Phys. Rev.
Gamma-ray probe of cosmic-ray pressure in galaxy clusters and cosmological implications
Cosmic rays produced in cluster accretion and merger shocks provide pressure
to the intracluster medium (ICM) and affect the mass estimates of galaxy
clusters. Although direct evidence for cosmic-ray ions in the ICM is still
lacking, they produce gamma-ray emission through the decay of neutral pions
produced in their collisions with ICM nucleons. We investigate the capability
of the Gamma-ray Large Area Space Telescope (GLAST) and imaging atmospheric
Cerenkov telescopes (IACTs) for constraining the cosmic-ray pressure
contribution to the ICM. We show that GLAST can be used to place stringent
upper limits, a few per cent for individual nearby rich clusters, on the ratio
of pressures of the cosmic rays and thermal gas. We further show that it is
possible to place tight (<~10%) constraints for distant (z <~ 0.25) clusters in
the case of hard spectrum, by stacking signals from samples of known clusters.
The GLAST limits could be made more precise with the constraint on the
cosmic-ray spectrum potentially provided by IACTs. Future gamma-ray
observations of clusters can constrain the evolution of cosmic-ray energy
density, which would have important implications for cosmological tests with
upcoming X-ray and Sunyaev-Zel'dovich effect cluster surveys.Comment: 12 pages, 5 figures; extended discussions; accepted by MNRA
The Energy Spectrum of Primary Cosmic Ray Electrons in Clusters of Galaxies and Inverse Compton Emission
Models for the evolution of the integrated energy spectrum of primary cosmic
ray electrons in clusters of galaxies have been calculated, including the
effects of losses due to inverse Compton (IC), synchrotron, and bremsstrahlung
emission, and Coulomb losses to the intracluster medium (ICM). The combined
time scale for these losses reaches a maximum of ~3e9 yr for electrons with a
Lorentz factor ~300. Only clusters in which there has been a substantial
injection of relativistic electrons since z <~ 1 will have any significant
population of primary cosmic ray electrons at present. In typical models, there
is a broad peak in the electron energy distribution extending to gamma~300, and
a steep drop in the electron population beyond this. In clusters with current
particle injection, there is a power-law tail of higher energy electrons with
an abundance determined by the current rate of injection. A significant
population of electrons with gamma~300, associated with the peak in the
particle loss time, is a generic feature of the models. The IC and synchrotron
emission from these models was calculated. In the models, EUV and soft X-ray
emission are nearly ubiquitous. This emission is produced by electrons with
gamma~300. The spectra are predicted to drop rapidly in going from the EUV to
the X-ray band. The IC emission also extends down the UV, optical, and IR bands
with a fairly flat spectrum. Hard X-ray (HXR) and diffuse radio emission due to
high energy electrons (gamma~10e4) is present only in clusters which have
current particle acceleration. Assuming that the electrons are accelerated in
ICM shocks, one would only expect diffuse HXR/radio emission in clusters which
are currently undergoing a large merger.Comment: Accepted for publication in the Astrophysical Journal, with minor
revisons to wording for clarity and one additional reference. 19 pages with
16 embedded Postscript figures in emulateapj.sty. Abbreviated abstract belo
Magnetic Field Evolution in Merging Clusters of Galaxies
We present initial results from the first 3-dimensional numerical
magnetohydrodynamical (MHD) simulations of magnetic field evolution in merging
clusters of galaxies. Within the framework of idealized initial conditions
similar to our previous work, we look at the gasdynamics and the magnetic field
evolution during a major merger event in order to examine the suggestion that
shocks and turbulence generated during a cluster/subcluster merger can produce
magnetic field amplification and relativistic particle acceleration and, as
such, may play a role in the formation and evolution of cluster-wide radio
halos. The ICM, as represented by the equations of ideal MHD, is evolved
self-consistently within a changing gravitational potential defined largely by
the collisionless dark matter component represented by an N-body particle
distribution. The MHD equations are solved by the Eulerian, finite-difference
code, ZEUS. The particles are evolved by a standard particle-mesh (PM) code. We
find significant evolution of the magnetic field structure and strength during
two distinct epochs of the merger evolution.Comment: 21 pages, 7 figures, Figure 2 is color postscript. Accepted for
publication in Ap
The Evolution of Diffuse Radio Sources in Galaxy Clusters
We investigate the evolution and number distribution of radio halos in galaxy
clusters. Without re-acceleration or regeneration, the relativistic electrons
responsible for the diffuse radio emission will lose their energy via
inverse-Compton and synchrotron losses in a rather short time, and radio halos
will have lifetimes 0.1 Gyr. Radio halos could last for Gyr if a
significant level of re-acceleration is involved. The lifetimes of radio halos
would be comparable with the cosmological time if the radio-emitting electrons
are mainly the secondary electrons generated by pion decay following
proton-proton collisions between cosmic-ray protons and the thermal
intra-cluster medium within the galaxy clusters. Adopting both observational
and theoretical constraints for the formation of radio halos, we calculate the
formation rates and the comoving number density of radio halos in the
hierarchical clustering scheme. Comparing with observations, we find that the
lifetimes of radio halos are Gyr. Our results indicate that a
significant level of re-acceleration is necessary for the observed radio halos
and the secondary electrons may not be a dominant origin for radio halos.Comment: 22 pages, 6 figures, ApJ, in press (v2:Corrected typos.
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