166 research outputs found
Can Cosmic Structure form without Dark Matter?
One of the prime pieces of evidence for dark matter is the observation of
large overdense regions in the universe. Since we know from the cosmic
microwave background that the regions that contained the most baryons when the
universe was ~400,000 years old were overdense by only one part in ten
thousand, perturbations had to have grown since then by a factor greater than
where is the epoch of recombination. This enhanced
growth does not happen in general relativity, so dark matter is needed in the
standard theory. We show here that enhanced growth can occur in alternatives to
general relativity, in particular in Bekenstein's relativistic version of
MOdified Newtonian Dynamics (MOND). The vector field introduced in that theory
for a completely different reason plays a key role in generating the
instability that produces large cosmic structures today.Comment: 5 pages, 3 figure
On the influence of the cosmological constant on gravitational lensing in small systems
The cosmological constant Lambda affects gravitational lensing phenomena. The
contribution of Lambda to the observable angular positions of multiple images
and to their amplification and time delay is here computed through a study in
the weak deflection limit of the equations of motion in the Schwarzschild-de
Sitter metric. Due to Lambda the unresolved images are slightly demagnified,
the radius of the Einstein ring decreases and the time delay increases. The
effect is however negligible for near lenses. In the case of null cosmological
constant, we provide some updated results on lensing by a Schwarzschild black
hole.Comment: 8 pages, 1 figure; v2: extended discussion on the lens equation,
references added, results unchanged, in press on PR
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Design and Initial Tests of the Tracker-Converter ofthe Gamma-ray Large Area Space Telescope
The Tracker subsystem of the Large Area Telescope (LAT) science instrument of the Gamma-ray Large Area Space Telescope (GLAST) mission has been completed and tested. It is the central detector subsystem of the LAT and serves both to convert an incident gamma-ray into an electron-positron pair and to track the pair in order to measure the gamma-ray direction. It also provides the principal trigger for the LAT. The Tracker uses silicon strip detectors, read out by custom electronics, to detect charged particles. The detectors and electronics are packaged, along with tungsten converter foils, in 16 modular, high-precision carbon-composite structures. It is the largest silicon-strip detector system ever built for launch into space, and its aggressive design emphasizes very low power consumption, passive cooling, low noise, high efficiency, minimal dead area, and a structure that is highly transparent to charged particles. The test program has demonstrated that the system meets or surpasses all of its performance specifications as well as environmental requirements. It is now installed in the completed LAT, which is being prepared for launch in early 2008
The GINGER Project and status of the ring-laser of LNGS
A ring-laser attached to the Earth measures the absolute angular velocity of the Earth summed
to the relativistic precessions, de Sitter and Lense-Thirring. GINGER (Gyroscopes IN GEneral
Relativity) is a project aiming at measuring the LenseThirring effect with a ground based detector;
it is based on an array of ring-lasers. Comparing the Earth angular velocity measured
by IERS and the measurement done with the GINGER array, the Lense-Thirring effect can be
evaluated. Compared to the existing space experiments, GINGER provides a local measurement,
not the averaged value and it is unnecessary to model the gravitational field. It is a proposal,
but it is not far from being a reality. In fact the GrossRing G of the Geodesy Observatory of
Wettzell has a sensitivity very close to the necessary one. G ofWettzell is part of the IERS system
which provides the measure of the Length Of the DAY (LOD); G provides information on the fast
component of LOD. In the last few years, a roadmap toward GINGER has been outlined. The
experiment G-GranSasso, financed by the INFN Commission II, is developing instrumentations
and tests along the roadmap of GINGER. In this short paper the main activities of G-GranSasso
and some results will be presented. The first results of GINGERino will be reported, GINGERino
is the large ring-laser installed inside LNGS and now in the commissioning phase. Ring-lasers
provide as well important informations for geophysics, in particular the rotational seismology,
which is an emerging field of science. GINGERino is one of the three experiments of common
interest between INFN and INGV
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Search for Doubly Charged Higgs Bosons with Lepton-Flavor-Violating Decays involving Tau Leptons
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Exclusion of an Exotic Top Quark with -4/3 Electric Charge Using Soft Lepton Tagging
We present a measurement of the electric charge of the top quark using p{bar p} collisions corresponding to an integrated luminosity of 2.7 fb{sup -1} at the CDF II detector. We reconstruct t{bar t} events in the lepton+jets final state and use kinematic information to determine which b-jet is associated with the leptonically- or hadronically-decaying t-quark. Soft lepton taggers are used to determine the b-jet flavor. Along with the charge of the W boson decay lepton, this information permits the reconstruction of the top quark's electric charge. Out of 45 reconstructed events with 2.4 {+-} 0.8 expected background events, 29 are reconstructed as tt with the standard model +2/3 charge, whereas 16 are reconstructed as t{bar t} with an exotic -4/3 charge. This is consistent with the standard model and excludes the exotic scenario at 95% confidence level. This is the strongest exclusion of the exotic charge scenario and the first to use soft leptons for this purpose
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Search for Standard Model Higgs Boson Production in Association with a W Boson using a Neural Network
We present a search for standard model Higgs boson production in association with a W boson in proton-antiproton collisions (p{bar p} {yields} W{sup {+-}}H {yields} {ell}{nu}b{bar b}) at a center of mass energy of 1.96 TeV. The search employs data collected with the CDF II detector that correspond to an integrated luminosity of approximately 1.9 fb{sup -1}. We select events consistent with a signature of a single charged lepton (e{sup {+-}}/{mu}{sup {+-}}), missing transverse energy, and two jets. Jets corresponding to bottom quarks are identified with a secondary vertex tagging method, a jet probability tagging method, and a neural network filter. We use kinematic information in an artificial neural network to improve discrimination between signal and background compared to previous analyses. The observed number of events and the neural network output distributions are consistent with the standard model background expectations, and we set 95% confidence level upper limits on the production cross section times branching fraction ranging from 1.2 to 1.1 pb or 7.5 to 102 times the standard model expectation for Higgs boson masses from 110 to 150 GeV/c{sup 2}, respectively
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