314 research outputs found
The energy production rate & the generation spectrum of UHECRs
We derive simple analytic expressions for the flux and spectrum of ultra-high
energy cosmic-rays (UHECRs) predicted in models where the CRs are protons
produced by extra-Galactic sources. For a power-law scaling of the CR
production rate with redshift and energy, d\dot{n} /dE\propto E^-\alpha
(1+z)^m, our results are accurate at high energy, E>10^18.7 eV, to better than
15%, providing a simple and straightforward method for inferring d\dot{n}/dE
from the observed flux at E. We show that current measurements of the UHECR
spectrum, including the latest Auger data, imply
E^2d\dot{n}/dE(z=0)=(0.45\pm0.15)(\alpha-1) 10^44 erg Mpc^-3 yr^-1 at E<10^19.5
eV with \alpha roughly confined to 2\lesseq\alpha<2.7. The uncertainty is
dominated by the systematic and statistic errors in the experimental
determination of individual CR event energy, (\Delta E/E)_{sys} (\Delta
E/E)_{stat} ~20%. At lower energy, d\dot{n}/dE is uncertain due to the unknown
Galactic contribution. Simple models in which \alpha\simeq 2 and the transition
from Galactic to extra-Galactic sources takes place at the "ankle", E ~10^19
eV, are consistent with the data. Models in which the transition occurs at
lower energies require a high degree of fine tuning and a steep spectrum,
\alpha\simeq 2.7, which is disfavored by the data. We point out that in the
absence of accurate composition measurements, the (all particle) energy
spectrum alone cannot be used to infer the detailed spectral shapes of the
Galactic and extra-Galactic contributions.Comment: 9 pages, 11 figures, minor revision
Affine Gravity, Palatini Formalism and Charges
Affine gravity and the Palatini formalism contribute both to produce a simple
and unique formula for calculating charges at spatial and null infinity for
Lovelock type Lagrangians whose variational derivatives do not depend on
second-order derivatives of the field components. The method is based on the
covariant generalization due to Julia and Silva of the Regge-Teitelboim
procedure that was used to define properly the mass in the classical
formulation of Einstein's theory of gravity. Numerous applications reproduce
standard results obtained by other secure but mostly specialized methods. As a
novel application we calculate the Bondi energy loss in five dimensional
gravity, based on the asymptotic solution given by Tanabe, Tanahashi and
Shiromizu, and obtain, as expected, the same result. We also give the
superpotential for Einstein-Gauss-Bonnet gravity and find the superpotential
for Lovelock theories of gravity when the number of dimensions tends to
infinity with maximally symmetrical boundaries. The paper is written in
standard component formalism.Comment: The work is dedicated to Joshua Goldberg from whom I learned and got
interested in conservation laws in General Relativity (J.K
A Search for Correlation of Ultra-High Energy Cosmic Rays with IRAS-PSCz and 2MASS-6dF Galaxies
We study the arrival directions of 69 ultra-high energy cosmic rays (UHECRs)
observed at the Pierre Auger Observatory (PAO) with energies exceeding 55 EeV.
We investigate whether the UHECRs exhibit the anisotropy signal expected if the
primary particles are protons that originate in galaxies in the local universe,
or in sources correlated with these galaxies. We cross-correlate the UHECR
arrival directions with the positions of IRAS-PSCz and 2MASS-6dF galaxies
taking into account particle energy losses during propagation. This is the
first time that the 6dF survey is used in a search for the sources of UHECRs
and the first time that the PSCz survey is used with the full 69 PAO events.
The observed cross-correlation signal is larger for the PAO UHECRs than for 94%
(98%) of realisations from an isotropic distribution when cross-correlated with
the PSCz (6dF). On the other hand the observed cross-correlation signal is
lower than that expected from 85% of realisations, had the UHECRs originated in
galaxies in either survey. The observed cross-correlation signal does exceed
that expected by 50% of the realisations if the UHECRs are randomly deflected
by intervening magnetic fields by 5 degrees or more. We propose a new method of
analysing the expected anisotropy signal, by dividing the predicted UHECR
source distribution into equal predicted flux radial shells, which can help
localise and constrain the properties of UHECR sources. We find that the 69 PAO
events are consistent with isotropy in the nearest of three shells we define,
whereas there is weak evidence for correlation with the predicted source
distribution in the two more distant shells in which the galaxy distribution is
less anisotropic.Comment: 23 pages, version published in JCA
Exploring nu signals in dark matter detectors
We investigate standard and non-standard solar neutrino signals in direct
dark matter detection experiments. It is well known that even without new
physics, scattering of solar neutrinos on nuclei or electrons is an irreducible
background for direct dark matter searches, once these experiments each the ton
scale. Here, we entertain the possibility that neutrino interactions are
enhanced by new physics, such as new light force carriers (for instance a "dark
photon") or neutrino magnetic moments. We consider models with only the three
standard neutrino flavors, as well as scenarios with extra sterile neutrinos.
We find that low-energy neutrino--electron and neutrino--nucleus scattering
rates can be enhanced by several orders of magnitude, potentially enough to
explain the event excesses observed in CoGeNT and CRESST. We also investigate
temporal modulation in these neutrino signals, which can arise from geometric
effects, oscillation physics, non-standard neutrino energy loss, and
direction-dependent detection efficiencies. We emphasize that, in addition to
providing potential explanations for existing signals, models featuring new
physics in the neutrino sector can also be very relevant to future dark matter
searches, where, on the one hand, they can be probed and constrained, but on
the other hand, their signatures could also be confused with dark matter
signals.Comment: 38 pages, 8 figures, 1 table; v3: eq 3 and nuclear recoil plots
corrected, footnote added, conclusions unchange
Placebo-controlled study in neuromyelitis optica : ethical and design considerations
BACKGROUND: To date, no treatment for neuromyelitis optica (NMO) has been granted regulatory approval, and no controlled clinical studies have been reported. OBJECTIVE: To design a placebo-controlled study in NMO that appropriately balances patient safety and clinical-scientific integrity. METHODS: We assessed the "standard of care" for NMO to establish the ethical framework for a placebo-controlled trial. We implemented measures that balance the need for scientific robustness while mitigating the risks associated with a placebo-controlled study. The medical or scientific community, patient organizations, and regulatory authorities were engaged early in discussions on this placebo-controlled study, and their input contributed to the final study design. RESULTS: The N-MOmentum study (NCT02200770) is a clinical trial that randomizes NMO patients to receive MEDI-551, a monoclonal antibody that depletes CD19+ B-cells, or placebo. The study design has received regulatory, ethical, clinical, and patient approval in over 100 clinical sites in more than 20 countries worldwide. CONCLUSION: The approach we took in the design of the N-MOmentum trial might serve as a roadmap for other rare severe diseases when there is no proven therapy and no established clinical development path
Gamma-Ray Bursts: The Underlying Model
A pedagogical derivation is presented of the ``fireball'' model of gamma-ray
bursts, according to which the observable effects are due to the dissipation of
the kinetic energy of a relativistically expanding wind, a ``fireball.'' The
main open questions are emphasized, and key afterglow observations, that
provide support for this model, are briefly discussed. The relativistic outflow
is, most likely, driven by the accretion of a fraction of a solar mass onto a
newly born (few) solar mass black hole. The observed radiation is produced once
the plasma has expanded to a scale much larger than that of the underlying
``engine,'' and is therefore largely independent of the details of the
progenitor, whose gravitational collapse leads to fireball formation. Several
progenitor scenarios, and the prospects for discrimination among them using
future observations, are discussed. The production in gamma- ray burst
fireballs of high energy protons and neutrinos, and the implications of burst
neutrino detection by kilometer-scale telescopes under construction, are
briefly discussed.Comment: In "Supernovae and Gamma Ray Bursters", ed. K. W. Weiler, Lecture
Notes in Physics, Springer-Verlag (in press); 26 pages, 2 figure
Phenomenology of flavor-mediated supersymmetry breaking
The phenomenology of a new economical SUSY model that utilizes dynamical SUSY
breaking and gauge-mediation (GM) for the generation of the sparticle spectrum
and the hierarchy of fermion masses is discussed. Similarities between the
communication of SUSY breaking through a messenger sector, and the generation
of flavor using the Froggatt-Nielsen (FN) mechanism are exploited, leading to
the identification of vector-like messenger fields with FN fields, and the
messenger U(1) as a flavor symmetry. An immediate consequence is that the first
and second generation scalars acquire flavor-dependent masses, but do not
violate FCNC bounds since their mass scale, consistent with effective SUSY, is
of order 10 TeV. We define and advocate a minimal flavor-mediated model (MFMM),
recently introduced in the literature, that successfully accommodates the small
flavor-breaking parameters of the standard model using order one couplings and
ratios of flavon field vevs. The mediation of SUSY breaking occurs via two-loop
log-enhanced GM contributions, as well as several one-loop and two-loop
Yukawa-mediated contributions for which we provide analytical expressions. The
MFMM is parameterized by a small set of masses and couplings, with values
restricted by several model constraints and experimental data. The
next-to-lightest sparticle (NLSP) always has a decay length that is larger than
the scale of a detector, and is either the lightest stau or the lightest
neutralino. Similar to ordinary GM models, the best collider search strategies
are, respectively, inclusive production of at least one highly ionizing track,
or events with many taus plus missing energy. In addition, D^0 - \bar{D}^0
mixing is also a generic low energy signal. Finally, the dynamical generation
of the neutrino masses is briefly discussed.Comment: 54 pages, LaTeX, 8 figure
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