734 research outputs found
Selfish Dark Matter
We present a mechanism where a particle asymmetry in one sector is used to
generate an asymmetry in another sector. The two sectors are not coupled
through particle number violating interactions and are not required to be in
thermal contact with each other. When this mechanism is applied to baryogenesis
in asymmetric dark matter models, we find that the dark matter particles can be
extremely light, e.g. much lighter than an eV, and that in some cases there is
no need to annihilate away the symmetric component of dark matter. We discuss a
concrete realization of the mechanism with signals in direct detection, at the
LHC, at -factories or future beam dump experiments.Comment: 18+5 pages, 2 figures; Journal version: Added references, small
changes to the free-streaming length estimate
Bounds on Cross-sections and Lifetimes for Dark Matter Annihilation and Decay into Charged Leptons from Gamma-ray Observations of Dwarf Galaxies
We provide conservative bounds on the dark matter cross-section and lifetime
from final state radiation produced by annihilation or decay into charged
leptons, either directly or via an intermediate particle . Our analysis
utilizes the experimental gamma-ray flux upper limits from four Milky Way dwarf
satellites: HESS observations of Sagittarius and VERITAS observations of Draco,
Ursa Minor, and Willman 1. Using 90% confidence level lower limits on the
integrals over the dark matter distributions, we find that these constraints
are largely unable to rule out dark matter annihilations or decays as an
explanation of the PAMELA and ATIC/PPB-BETS excesses. However, if there is an
additional Sommerfeld enhancement in dwarfs, which have a velocity dispersion
~10 to 20 times lower than that of the local Galactic halo, then the
cross-sections for dark matter annihilating through 's required to
explain the excesses are very close to the cross-section upper bounds from
Willman 1. Dark matter annihilation directly into 's is also marginally
ruled out by Willman 1 as an explanation of the excesses, and the required
cross-section is only a factor of a few below the upper bound from Draco.
Finally, we make predictions for the gamma-ray flux expected from the dwarf
galaxy Segue 1 for the Fermi Gamma-ray Space Telescope. We find that for a
sizeable fraction of the parameter space in which dark matter annihilation into
charged leptons explains the PAMELA excess, Fermi has good prospects for
detecting a gamma-ray signal from Segue 1 after one year of observation.Comment: 11 pages, 4 figures. References added. Final published versio
Altitude Performance and Operational Characteristics of an XT38-A-2 Turboprop Engine
The overall engine performance and the starting and windmilling characteristics of an XT38-A-2 turboprop engine have been investigated in the NACA Lewis altitude wind tunnel. The simulated flight conditions ranged from altitudes of 5000 to 45,000 feet at a flight Mach number of 0.30 and from Mach numbers of 0.301 to 0.557 at an altitude of 35,000 feet. The engine, equipped with a standard-area exhaust nozzle, was operated with independent control of fuel flow and propeller pitch; operation was thereby allowed over a wide range of engine conditions. Windmilling characteristics were obtained at altitudes of 15,000 feet and 35,000 feet. Analysis of the performance maps obtained at each flight condition revealed that both altitude and flight Mach number had a major effect on corrected engine variables. The large reductions in corrected shaft horsepower occurring when the altitude was increased were the result of decreases in compressor and turbine efficiencies. Windmilling engine starts were made at altitudes as high as 35,000 feet at an engine speed of 2000 rpm
Constructing the Western Landscape: National Park Architecture
This thesis explores how National Park Architecture has helped shape this country's attitude toward the American West, nature, and tourism. In the 19th century, a specific image of the parks was constructed, which implied the ideal interaction between man and nature. Over the years, as this relationship has changed, so has the architecture. Each generation has reinterpreted the idea of what a national park represents and how it fits into American culture. The image of the parks has been carefully controlled in order to serve a particular purpose. This provides the opportunity to design a building that not only functions as a visitor center, but one that stands as a recognizable model for how to build and interact with the natural environment. This thesis addresses the existing site of the Old Faithful visitor center and the larger complex in which it is situated. While the site exists within the "wilderness" of Yellowstone National Park, it accommodates 25,000 daily visitors, and therefore, presents numerous urban challenges
An Electron Fixed Target Experiment to Search for a New Vector Boson A' Decaying to e+e-
We describe an experiment to search for a new vector boson A' with weak
coupling alpha' > 6 x 10^{-8} alpha to electrons (alpha=e^2/4pi) in the mass
range 65 MeV < m_A' < 550 MeV. New vector bosons with such small couplings
arise naturally from a small kinetic mixing of the "dark photon" A' with the
photon -- one of the very few ways in which new forces can couple to the
Standard Model -- and have received considerable attention as an explanation of
various dark matter related anomalies. A' bosons are produced by radiation off
an electron beam, and could appear as narrow resonances with small production
cross-section in the trident e+e- spectrum. We summarize the experimental
approach described in a proposal submitted to Jefferson Laboratory's PAC35,
PR-10-009. This experiment, the A' Experiment (APEX), uses the electron beam of
the Continuous Electron Beam Accelerator Facility at Jefferson Laboratory
(CEBAF) at energies of ~1-4 GeV incident on 0.5-10% radiation length Tungsten
wire mesh targets, and measures the resulting e+e- pairs to search for the A'
using the High Resolution Spectrometer and the septum magnet in Hall A. With a
~1 month run, APEX will achieve very good sensitivity because the statistics of
e+e- pairs will be ~10,000 times larger in the explored mass range than any
previous search for the A' boson. These statistics and the excellent mass
resolution of the spectrometers allow sensitivity to alpha'/alpha one to three
orders of magnitude below current limits, in a region of parameter space of
great theoretical and phenomenological interest. Similar experiments could also
be performed at other facilities, such as the Mainz Microtron.Comment: 19 pages, 12 figures, 2 table
Light dark forces at flavor factories
SuperB experiment could represent an ideal environment to test a new U (1)
symmetry related to light dark forces candidates. A promising discovery channel
is represented by the resonant production of a boson U, followed by its decay
into lepton pairs. Beyond approximations adopted in the literature, an exact
tree level calculation of the radiative processes and corresponding QED
backgrounds is performed, including also the most important higher-order
corrections. The calculation is implemented in a release of the generator
BabaYaga@NLO useful for data analysis and interpretation. The distinct features
of U boson production are shown and the statistical significance is analysed
Probing Dark Forces and Light Hidden Sectors at Low-Energy e+e- Colliders
A dark sector -- a new non-Abelian gauge group Higgsed or confined near the
GeV scale -- can be spectacularly probed in low-energy e+e- collisions. A
low-mass dark sector can explain the annual modulation signal reported by
DAMA/LIBRA and the PAMELA, ATIC, and INTEGRAL observations by generating small
mass splittings and new interactions for weak-scale dark matter. Some of these
observations may be the first signs of a low-mass dark sector that collider
searches can definitively confirm. Production and decay of O(GeV)-mass dark
states is mediated by a Higgsed Abelian gauge boson that mixes kinetically with
hypercharge. Existing data from BaBar, BELLE, CLEO-c, and KLOE may contain
thousands of striking dark-sector events with a high multiplicity of leptons
that reconstruct mass resonances and possibly displaced vertices. We discuss
the production and decay phenomenology of Higgsed and confined dark sectors and
propose e+e- collider search strategies. We also use the DAMA/LIBRA signal to
estimate the production cross-sections and decay lifetimes for dark-sector
states.Comment: 42 pages, 19 figures. References and minor clarifications added.
Final published versio
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