431 research outputs found
Evidence of "crossed" transitions in dots-in-a-well structures through waveguide absorption measurements
In-plane absorption measurements were performed at room temperature by means of a waveguide transmission setup on a Stranski-Krastanov InAs dots-in-a-well system emitting at 1.3 mu m embedded in a p-i-n structure. The polarization dependence of quantum dot (QD) absorption was exploited to resolve its discrete and continuous spectral components and study them separately under reverse bias application. The quantum confined Stark effect observed in the discrete spectral component gave evidence of an upward built-in QD dipole of about 9.5x10(-29) C m. The continuous component was found to originate from electronic transitions involving a QD state and a quantum well state. (C) 2008 American Institute of Physics. (DOI: 10.1063/1.3000381
An EBIS for charge state breeding in the SPES project
The 'charge state breeder', BRIC (breeding ion charge) is in construction at the INFN section of Bari (Italy). It is based on EBIS scheme and it is designed to accept radioactive ion beam (RIB) with charge state +1 in a slow injection mode. This experiment can be considered as a first step towards the design and construction of a charge breeder for the SPES project. The new feature of BRIC, with respect to the classical EBIS, is given by the insertion, in the ion chamber, of a rf-quadrupole aiming at filtering the unwanted masses and then making a more efficient containment of the wanted ions. In this paper, the breeder design, the simulation results of the electron and ion beam propagation and the construction problems of the device will be reported
Search for Exotic Strange Quark Matter in High Energy Nuclear Reactions
We report on a search for metastable positively and negatively charged states
of strange quark matter in Au+Pb reactions at 11.6 A GeV/c in experiment E864.
We have sampled approximately six billion 10% most central Au+Pb interactions
and have observed no strangelet states (baryon number A < 100 droplets of
strange quark matter). We thus set upper limits on the production of these
exotic states at the level of 1-6 x 10^{-8} per central collision. These limits
are the best and most model independent for this colliding system. We discuss
the implications of our results on strangelet production mechanisms, and also
on the stability question of strange quark matter.Comment: 21 pages, 9 figures, to be published in Nuclear Physics A (Carl Dover
memorial edition
Antiproton Production in 11.5 A GeV/c Au+Pb Nucleus-Nucleus Collisions
We present the first results from the E864 collaboration on the production of
antiprotons in 10% central 11.5 A GeV/c Au+Pb nucleus collisions at the
Brookhaven AGS. We report invariant multiplicities for antiproton production in
the kinematic region 1.4<y<2.2 and 50<p_T<300 MeV/c, and compare our data with
a first collision scaling model and previously published results from the E878
collaboration. The differences between the E864 and E878 antiproton
measurements and the implications for antihyperon production are discussed.Comment: 4 pages, 4 figures; accepted for publication in Physical Review
Letter
Simultaneous Planck, Swift, and Fermi Observations of X-ray and Gamma-ray Selected Blazars
We present simultaneous Planck, Swift, Fermi, and ground-based data for 105 blazars belonging to three samples with flux limits in the soft X-ray, hard X-ray, and gamma-ray bands, with additional 5 GHz flux-density limits to ensure a good probability of a Planck detection. We compare our results to those of a companion paper presenting simultaneous Planck and multi-frequency observations of 104 radio-loud northern active galactic nuclei selected at radio frequencies. While we confirm several previous results, our unique data set allows us to demonstrate that the selection method strongly influences the results, producing biases that cannot be ignored. Almost all the BL Lac objects have been detected by the Fermi Large Area Telescope (LAT), whereas 30% to 40% of the flat-spectrum radio quasars (FSRQs) in the radio, soft X-ray, and hard X-ray selected samples are still below the gamma-ray detection limit even after integrating 27 months of Fermi-LAT data. The radio to sub-millimetre spectral slope of blazars is quite flat, with (alpha) approx 0 up to about 70GHz, above which it steepens to (alpha) approx -0.65. The BL Lacs have significantly flatter spectra than FSRQs at higher frequencies. The distribution of the rest-frame synchrotron peak frequency (nu(sup s)(sub peak)) in the spectral energy distribution (SED) of FSRQs is the same in all the blazar samples with (nu(sup s)(sub peak)) = 10(exp 13.1 +/- 0.1) Hz, while the mean inverse Compton peak frequency, (nu(sup IC)(sub peak)), ranges from 10(exp 21) to 10(exp 22) Hz. The distributions of nu(sup s)(sub peak) and nu(sup IC)(sub peak) of BL Lacs are much broader and are shifted to higher energies than those of FSRQs; their shapes strongly depend on the selection method. The Compton dominance of blazars. defined as the ratio of the inverse Compton to synchrotron peak luminosities, ranges from less than 0.2 to nearly 100, with only FSRQs reaching values larger than about 3. Its distribution is broad and depends strongly on the selection method, with gamma-ray selected blazars peaking at approx 7 or more, and radio-selected blazars at values close to 1, thus implying that the common assumption that the blazar power budget is largely dominated by high-energy emission is a selection effect. A comparison of our multi-frequency data with theoretical predictions shows that simple homogeneous SSC models cannot explain the simultaneous SEDs of most of the gamma-ray detected blazars in all samples. The SED of the blazars that were not detected by Fermi~LAT may instead be consistent with SSC emission. Our data challenge the correlation between bolometric luminosity and nu(sup s)(sub peak) predicted by the blazar sequence
Muon Energy Estimate Through Multiple Scattering with the Macro Detector
Muon energy measurement represents an important issue for any experiment
addressing neutrino induced upgoing muon studies. Since the neutrino
oscillation probability depends on the neutrino energy, a measurement of the
muon energy adds an important piece of information concerning the neutrino
system. We show in this paper how the MACRO limited streamer tube system can be
operated in drift mode by using the TDC's included in the QTPs, an electronics
designed for magnetic monopole search. An improvement of the space resolution
is obtained, through an analysis of the multiple scattering of muon tracks as
they pass through our detector. This information can be used further to obtain
an estimate of the energy of muons crossing the detector. Here we present the
results of two dedicated tests, performed at CERN PS-T9 and SPS-X7 beam lines,
to provide a full check of the electronics and to exploit the feasibility of
such a multiple scattering analysis. We show that by using a neural network
approach, we are able to reconstruct the muon energy for 40 GeV. The
test beam data provide an absolute energy calibration, which allows us to apply
this method to MACRO data.Comment: 25 pages, 11 figures, Submitted to Nucl. Instr. & Meth.
Multiwavelength Evidence for Quasi-periodic Modulation in the Gamma-ray Blazar PG 1553+113
We report for the first time a gamma-ray and multi-wavelength nearly-periodic
oscillation in an active galactic nucleus. Using the Fermi Large Area Telescope
(LAT) we have discovered an apparent quasi-periodicity in the gamma-ray flux (E
>100 MeV) from the GeV/TeV BL Lac object PG 1553+113. The marginal significance
of the 2.18 +/-0.08 year-period gamma-ray cycle is strengthened by correlated
oscillations observed in radio and optical fluxes, through data collected in
the OVRO, Tuorla, KAIT, and CSS monitoring programs and Swift UVOT. The optical
cycle appearing in ~10 years of data has a similar period, while the 15 GHz
oscillation is less regular than seen in the other bands. Further long-term
multi-wavelength monitoring of this blazar may discriminate among the possible
explanations for this quasi-periodicity.Comment: 8 pages, 5 figures. Accepted to The Astrophysical Journal Letters.
Corresponding authors: S. Ciprini (ASDC/INFN), S. Cutini (ASDC/INFN), S.
Larsson (Stockholm Univ/KTH), A. Stamerra (INAF/SNS), D. J. Thompson (NASA
GSFC
The Observation of Up-going Charged Particles Produced by High Energy Muons in Underground Detectors
An experimental study of the production of up-going charged particles in
inelastic interactions of down-going underground muons is reported, using data
obtained from the MACRO detector at the Gran Sasso Laboratory. In a sample of
12.2 10^6 single muons, corresponding to a detector livetime of 1.55 y, 243
events are observed having an up-going particle associated with a down-going
muon. These events are analysed to determine the range and emission angle
distributions of the up-going particle, corrected for detection and
reconstruction efficiency. Measurements of the muon neutrino flux by
underground detectors are often based on the observation of through-going and
stopping muons produced in interactions in the rock below the
detector. Up-going particles produced by an undetected down-going muon are a
potential background source in these measurements. The implications of this
background for neutrino studies using MACRO are discussed.Comment: 18 pages, 9 figures. Accepted by Astrop. Physic
Gamma-ray flares from the Crab Nebula
A young and energetic pulsar powers the well-known Crab Nebula. Here we
describe two separate gamma-ray (photon energy >100 MeV) flares from this
source detected by the Large Area Telescope on board the Fermi Gamma-ray Space
Telescope. The first flare occurred in February 2009 and lasted approximately
16 days. The second flare was detected in September 2010 and lasted
approximately 4 days. During these outbursts the gamma-ray flux from the nebula
increased by factors of four and six, respectively. The brevity of the flares
implies that the gamma rays were emitted via synchrotron radiation from PeV
(10^15 eV) electrons in a region smaller than 1.4 10^-2 pc. These are the
highest energy particles that can be associated with a discrete astronomical
source, and they pose challenges to particle acceleration theory.Comment: Contact authors: Rolf Buehler,[email protected]; Stefan
Funk,[email protected]; Roger Blandford,rdb3@stanford ; 16 pages,2
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