Observations of occultations of bright gamma-ray sources by the Sun may reveal predicted pair halos around blazars and/or new physics, such as, e.g., hypothetical light dark matter particles-axions. ...We use Fermi Gamma-Ray Space Telescope (Fermi) data to analyze four occultations of blazar 3C 279 by the Sun on October 8 each year from 2008 to 2011. A combined analysis of the observations of these occultations allows a point-like source at the position of 3C 279 to be detected with significance of approximately 3sigma, but does not reveal any significant excess over the flux expected from the quiescent Sun. The likelihood ratio test rules out complete transparency of the Sun to the blazar gamma-ray emission at a 3sigma confidence level.
The dwarf spheroidal satellite galaxies of the Milky Way are some of the most dark-matter-dominated objects known. Due to their proximity, high dark matter content, and lack of astrophysical ...backgrounds, dwarf spheroidal galaxies are widely considered to be among the most promising targets for the indirect detection of dark matter via gamma rays. Here we report on gamma -ray observations of 25 Milky Way dwarf spheroidal satellite galaxies based on 4 years of Fermi Large Area Telescope (LAT) data. None of the dwarf galaxies are significantly detected in gamma rays, and we present gamma -ray flux upper limits between 500 MeV and 500 GeV. We determine the dark matter content of 18 dwarf spheroidal galaxies from stellar kinematic data and combine LAT observations of 15 dwarf galaxies to constrain the dark matter annihilation cross section. We set some of the tightest constraints to date on the annihilation of dark matter particles with masses between 2 GeV and 10 TeV into prototypical standard model channels. We find these results to be robust against systematic uncertainties in the LAT instrument performance, diffuse gamma -ray background modeling, and assumed dark matter density profile.
We present a measurement of the left-right cross-section asymmetry ( A(LR)) for Z boson production by e(+)e(-) collisions. The measurement includes the final data taken with the SLD detector at the ...SLAC Linear Collider during the period 1996-1998. Using a sample of 383 487 Z decays collected during the 1996-1998 runs we measure the pole value of the asymmetry, A(0)(LR), to be 0.150 56+/-0.002 39 which is equivalent to an effective weak mixing angle of sin (2)straight theta(eff)(W) = 0.231 07+/-0.000 30. Our result for the complete 1992-1998 data set comprising approximately 537 000 Z decays is sin (2)straight theta(eff)(W) = 0.230 97+/-0.000 27.
It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our ...line of sight. The size of the emitting zone and the location of this region relative to the central supermassive black hole are, however, poorly known, with estimates ranging from light-hours to a light-year or more. Here we report the coincidence of a gamma (g)-ray flare with a dramatic change of optical polarization angle. This provides evidence for co-spatiality of optical and g-ray emission regions and indicates a highly ordered jet magnetic field. The results also require a non-axisymmetric structure of the emission zone, implying a curved trajectory for the emitting material within the jet, with the dissipation region located at a considerable distance from the black hole, at about 10 super(5) gravitational radii.
We report the detection of Delta *g-ray pulsations (>=0.1 GeV) from PSR J2229+6114 and PSR J1048-5832, the latter having been detected as a low-significance pulsar by EGRET. Data in the Delta *g-ray ...band were acquired by the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope, while the radio rotational ephemerides used to fold the Delta *g-ray light curves were obtained using the Green Bank Telescope, the Lovell telescope at Jodrell Bank, and the Parkes Telescope. The two young radio pulsars, located within the error circles of the previously unidentified EGRET sources 3EG J1048-5840 and 3EG J2227+6122, present spin-down characteristics similar to the Vela pulsar. PSR J1048-5832 shows two sharp peaks at phases 0.15 +/- 0.01 and 0.57 +/- 0.01 relative to the radio pulse confirming the EGRET light curve, while PSR J2229+6114 presents a very broad peak at phase 0.49 +/- 0.01. The Delta *g-ray spectra above 0.1 GeV of both pulsars are fit with power laws having exponential cutoffs near 3 GeV, leading to integral photon fluxes of (2.19 +/- 0.22 +/- 0.32) X 10-7 cm-2 s-1 for PSR J1048-5832 and (3.77 +/- 0.22 +/- 0.44) X 10-7 cm-2 s-1 for PSR J2229+6114. The first uncertainty is statistical and the second is systematic. PSR J1048-5832 is one of the two LAT sources which were entangled together as 3EG J1048-5840. These detections add to the growing number of young Delta *g-ray pulsars that make up the dominant population of GeV Delta *g-ray sources in the Galactic plane.
We have developed a new technique for inclusive reconstruction of the energy of B hadrons. The excellent efficiency and resolution of this technique allow us to make the most precise determination of ...the b -quark fragmentation function, using e{sup +}e{sup -}{yields}Z{sup 0} decays recorded in the SLAC Large Detector experiment. We compared our measurement with the predictions of a number of fragmentation models. We excluded several of these models and measured the average scaled energy of weakly decaying B hadrons to be <x{sub B}>=0.714{+-}0.005(stat ){+-}0.007(syst){+-}0.00 2 (model dependence). (c) 2000 The American Physical Society.
We have studied both the molecular-beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) grown AlGaAsAlAsGaAs double barrier multiple quantum well structures for 3–5 μm infrared ...(IR) photodetector application. The intersubband absorptions of these samples were measured at the Brewster angle as well as through a multipass 45° wedge waveguide. In the former case, we have also studied the polarisation dependence of the IR transmission. The MBE grown sample has an absorption at about 3.1 μm, while the absorption peak for the MOCVD sample is shifted slightly, depending on the wafer location. The low temperature photoluminescence peaks show a significant shift when the probe position is moved across the MOCVD wafer along the gas flow direction, owing to thickness non-uniformity. This variation was also confirmed by double crystal X-ray diffraction data. Theoretically, we have carried out a bound and quasibound state energy calculation for these structures as a function of well width and related them to the experimental results. In addition, the IR photoresponse for the MBE grown sample at about 80 K has been measured for IR photodetector application.
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