5,533 research outputs found
Proposed New Antiproton Experiments at Fermilab
Fermilab operates the world's most intense source of antiprotons. Recently
various experiments have been proposed that can use those antiprotons either
parasitically during Tevatron Collider running or after the Tevatron Collider
finishes in about 2010. We discuss the physics goals and prospects of the
proposed experiments.Comment: 6 pages, 2 figures, to appear in Proceedings of IXth International
Conference on Low Energy Antiproton Physics (LEAP'08), Vienna, Austria,
September 16 to 19, 200
Nambu-Goldstone Modes in Gravitational Theories with Spontaneous Lorentz Breaking
Spontaneous breaking of Lorentz symmetry has been suggested as a possible
mechanism that might occur in the context of a fundamental Planck-scale theory,
such as string theory or a quantum theory of gravity. However, if Lorentz
symmetry is spontaneously broken, two sets of questions immediately arise: what
is the fate of the Nambu-Goldstone modes, and can a Higgs mechanism occur? A
brief summary of some recent work looking at these questions is presented here.Comment: 6 pages. Presented at the meeting "From Quantum to Cosmos,"
Washington, D.C., May 2006; published in Int. J. Mod. Phys. D16:2357-2363,
200
The optical depth of the Universe to ultrahigh energy cosmic ray scattering in the magnetized large scale structure
This paper provides an analytical description of the transport of ultrahigh
energy cosmic rays in an inhomogeneously magnetized intergalactic medium. This
latter is modeled as a collection of magnetized scattering centers such as
radio cocoons, magnetized galactic winds, clusters or magnetized filaments of
large scale structure, with negligible magnetic fields in between. Magnetic
deflection is no longer a continuous process, it is rather dominated by
scattering events. We study the interaction between high energy cosmic rays and
the scattering agents. We then compute the optical depth of the Universe to
cosmic ray scattering and discuss the phenomological consequences for various
source scenarios. For typical parameters of the scattering centers, the optical
depth is greater than unity at 5x10^{19}eV, but the total angular deflection is
smaller than unity. One important consequence of this scenario is the
possibility that the last scattering center encountered by a cosmic ray be
mistaken with the source of this cosmic ray. In particular, we suggest that
part of the correlation recently reported by the Pierre Auger Observatory may
be affected by such delusion: this experiment may be observing in part the last
scattering surface of ultrahigh energy cosmic rays rather than their source
population. Since the optical depth falls rapidly with increasing energy, one
should probe the arrival directions of the highest energy events beyond
10^{20}eV on an event by event basis to circumvent this effect.Comment: version to appear in PRD; substantial improvements: extended
introduction, sections added on angular images and on direction dependent
effects with sky maps of optical depth, enlarged discussion of Auger results
(conclusions unchanged); 27 pages, 9 figure
First-order thermal correction to the quadratic response tensor and rate for second harmonic plasma emission
Three-wave interactions in plasmas are described, in the framework of kinetic
theory, by the quadratic response tensor (QRT). The cold-plasma QRT is a common
approximation for interactions between three fast waves. Here, the first-order
thermal correction (FOTC) to the cold-plasma QRT is derived for interactions
between three fast waves in a warm unmagnetized collisionless plasma, whose
particles have an arbitrary isotropic distribution function. The FOTC to the
cold-plasma QRT is shown to depend on the second moment of the distribution
function, the phase speeds of the waves, and the interaction geometry. Previous
calculations of the rate for second harmonic plasma emission (via Langmuir-wave
coalescence) assume the cold-plasma QRT. The FOTC to the cold-plasma QRT is
used here to calculate the FOTC to the second harmonic emission rate, and its
importance is assessed in various physical situations. The FOTC significantly
increases the rate when the ratio of the Langmuir phase speed to the electron
thermal speed is less than about 3.Comment: 11 pages, 2 figures, submitted to Physics of Plasma
FIRe glider: Mapping in situ chlorophyll variable fluorescence with autonomous underwater gliders
Nutrient and light availability regulate phytoplankton physiology and photosynthesis in the ocean. These physiological processes are difficult to sample in time and space over physiologically and ecologically relevant scales using traditional shipboard techniques. Gliders are changing the nature of data collection, by allowing a sustained presence at sea over regional scales, collecting data at resolution not possible using traditional techniques. The integration of a fluorescence induction and relaxation (FIRe) sensor in a Slocum glider allows autonomous high‐resolution and vertically‐resolved measurements of photosynthetic physiological variables together with oceanographic data. In situ measurements of variable fluorescence under ambient light allows a better understanding of the physical controls of primary production (PP). We demonstrate this capability in a laboratory setting and with several glider deployments in the Southern Ocean. Development of these approaches will allow for the in situ evaluation of phytoplankton light stress and photoacclimation mechanisms, as well as the role of vertical mixing in phytoplankton dynamics and the underlying physiology, especially in remote locations and for prolonged duration
The Lorentz Integral Transform (LIT) method and its applications to perturbation induced reactions
The LIT method has allowed ab initio calculations of electroweak cross
sections in light nuclear systems. This review presents a description of the
method from both a general and a more technical point of view, as well as a
summary of the results obtained by its application. The remarkable features of
the LIT approach, which make it particularly efficient in dealing with a
general reaction involving continuum states, are underlined. Emphasis is given
on the results obtained for electroweak cross sections of few--nucleon systems.
Their implications for the present understanding of microscopic nuclear
dynamics are discussed.Comment: 83 pages, 31 figures. Topical review. Corrected typo
Brane Universes with Gauss-Bonnet-Induced-Gravity
The DGP brane world model allows us to get the observed late time
acceleration via modified gravity, without the need for a ``dark energy''
field. This can then be generalised by the inclusion of high energy terms, in
the form of a Gauss-Bonnet bulk. This is the basis of the
Gauss-Bonnet-Induced-Gravity (GBIG) model explored here with both early and
late time modifications to the cosmological evolution. Recently the simplest
GBIG models (Minkowski bulk and no brane tension) have been analysed. Two of
the three possible branches in these models start with a finite density
``Big-Bang'' and with late time acceleration. Here we present a comprehensive
analysis of more general models where we include a bulk cosmological constant
and brane tension. We show that by including these factors it is possible to
have late time phantom behaviour.Comment: 12 pages, 19 figures. Minor modifications to text, comments on
phantom behaviour added. References added. As submitted to JCA
The obscured gamma-ray and UHECR universe
Auger results on clustering of > 60 EeV ultra-high energy cosmic ray (UHECR)
ions and the interpretation of the gamma-ray spectra of TeV blazars are
connected by effects from the extragalactic background light (EBL). The EBL
acts as an obscuring medium for gamma rays and a reprocessing medium for UHECR
ions and protons, causing the GZK cutoff. The study of the physics underlying
the coincidence between the GZK energy and the clustering energy of UHECR ions
favors a composition of > 60 EeV UHECRs in CNO group nucleons. This has
interesting implications for the sources of UHECRs. We also comment on the
Auger analysis.Comment: 11 pages, 10 figures, in the International Conference on Topics in
Astroparticle and Underground Physics (TAUP) 2007, Sendai, Japan, September
11-15, 200
Restrictions on the lifetime of sterile neutrinos from primordial nucleosynthesis
We analyze the influence of decaying sterile neutrinos with the masses in the
range 1-140 MeV on the primordial Helium-4 abundance, explicitly solving the
Boltzmann equations for all particle species, taking into account neutrino
flavour oscillations, and paying special attention to systematic uncertainties.
We show that the Helium abundance depends only on the sterile neutrino lifetime
and not on the way the active-sterile mixing is distributed between flavours,
and derive an upper bound on the lifetime. We also demonstrate that the recent
results of Izotov & Thuan [arXiv:1001.4440], who find 2sigma higher than
predicted by the standard primordial nucleosynthesis value of Helium-4
abundance, are consistent with the presence in the plasma of sterile neutrinos
with the lifetime 0.01-2 seconds. The decay of these particles perturbs the
spectra of (decoupled) neutrinos and heats photons, changing the ratio of
neutrino to photon energy density, that can be interpreted as extra neutrino
species at the recombination epoch.Comment: 17 pp. + Appendices. Analysis of deuterium bounds and more accurate
account of CMB bounds on Helium-4 is added. Final version to appear in JCA
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