523 research outputs found
Influence of humidity on granular packings with moving walls
A significant dependence on the relative humidity H for the apparent mass
(Mapp) measured at the bottom of a granular packing inside a vertical tube in
relative motion is demonstrated experimentally. While the predictions of
Janssen's model are verified for all values of H investigated (25%< H <80%),
Mapp increases with time towards a limiting value at high relative humidities
(H>60%) but remains constant at lower ones (H=25%). The corresponding Janssen
length is nearly independent of the tube velocity for H>60% but decreases
markedly for H=25%. Other differences are observed on the motion of individual
beads in the packing. For H=25%, they are almost motionless while the mean
particle fraction of the packing remains constant; for H>60% the bead motion is
much more significant and the mean particle fraction decreases. The dependence
of these results on the bead diameter and their interpretation in terms of the
influence of capillary forces are discussed.Comment: 6 pages, 6 figure
The VLT-FLAMES Tarantula Survey. VII. A low velocity dispersion for the young massive cluster R136
Detailed studies of resolved young massive star clusters are necessary to
determine their dynamical state and evaluate the importance of gas expulsion
and early cluster evolution. In an effort to gain insight into the dynamical
state of the young massive cluster R136 and obtain the first measurement of its
velocity dispersion, we analyse multi-epoch spectroscopic data of the inner
regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of
the VLT-FLAMES Tarantula Survey. Following a quantitative assessment of the
variability, we use the radial velocities of non-variable sources to place an
upper limit of 6 km/s on the line-of-sight velocity dispersion of stars within
a projected distance of 5 pc from the centre of the cluster. After accounting
for the contributions of undetected binaries and measurement errors through
Monte Carlo simulations, we conclude that the true velocity dispersion is
likely between 4 and 5 km/s given a range of standard assumptions about the
binary distribution. This result is consistent with what is expected if the
cluster is in virial equilibrium, suggesting that gas expulsion has not altered
its dynamics. We find that the velocity dispersion would be ~25 km/s if
binaries were not identified and rejected, confirming the importance of the
multi-epoch strategy and the risk of interpreting velocity dispersion
measurements of unresolved extragalactic young massive clusters.Comment: 18 pages, 7 figures, accepted by A&
The periodic Anderson model from the atomic limit and FeSi
The exact Green's functions of the periodic Anderson model for
are formally expressed within the cumulant expansion in terms of an effective
cumulant. Here we resort to a calculation in which this quantity is
approximated by the value it takes for the exactly soluble atomic limit of the
same model. In the Kondo region a spectral density is obtained that shows near
the Fermi surface a structure with the properties of the Kondo peak.
Approximate expressions are obtained for the static conductivity
and magnetic susceptibility of the PAM, and they are employed to fit
the experimental values of FeSi, a compound that behaves like a Kondo insulator
with both quantities vanishing rapidly for . Assuming that the system
is in the intermediate valence region, it was possible to find good agreement
between theory and experiment for these two properties by employing the same
set of parameters. It is shown that in the present model the hybridization is
responsible for the relaxation mechanism of the conduction electrons.Comment: 26 pages and 8 figure
Superconductivity in diamond
We report the discovery of superconductivity in boron-doped diamond
synthesized at high pressure (8-9 GPa) and temperature (2,500-2,800 K).
Electrical resistivity, magnetic susceptibility, specific heat, and
field-dependent resistance measurements show that boron-doped diamond is a
bulk, type-II superconductor below the superconducting transition temperature
Tc=4 K; superconductivity survives in a magnetic field up to Hc2(0)=3.5 T. The
discovery of superconductivity in diamond-structured carbon suggests that Si
and Ge, which also form in the diamond structure, may similarly exhibit
superconductivity under the appropriate conditions.Comment: 13 pages, 4 figure
Dust in the Small Magellanic Cloud: Interstellar Polarization and Extinction
To elucidate the dust properties in the SMC we have for the first time
measured linear polarization in five colors in the optical region of the
spectrum for a sample of reddened stars. For two of these stars, for which
there were no existing UV spectrophotometric measurements, but for which we
measured a relatively large polarization, we have also obtained data from the
International Ultraviolet Explorer (IUE) in order to study the extinction. The
main results are: (1) the wavelength of maximum polarization, ,
in the SMC is typically smaller than that in the Galaxy; (2) however, AZV 456,
which shows the UV extinction bump, has a typical of that in
the Galaxy, its polarization curve is narrower, its bump is shifted to shorter
wavelengths as compared to the Galaxy and its UV extinction does not conform to
the Galactic analytical interpolation curve based on the ratio of total to
selective extinction; (3) the 'typical', monotonic SMC extinction curve can be
best fit with amorphous carbon and silicate grains; (4) the extinction towards
AZV456 may only be explained by assuming a larger gas-to-dust ratio than the
observed N(HI)/A(V) value, with a small amount of the available carbon in
graphite form; (5) from an analysis of both the extinction and polarization
data and our model fits it appears that the SMC has typically smaller grains
than those in the Galaxy.Comment: To appear in the ApJ, 50 pages, latex fil
An excess of massive stars in the local 30 Doradus starburst
The 30 Doradus star-forming region in the Large Magellanic Cloud is a nearby analog of large star-formation events in the distant universe. We determined the recent formation history and the initial mass function (IMF) of massive stars in 30 Doradus on the basis of spectroscopic observations of 247 stars more massive than 15 solar masses ([Formula: see text]). The main episode of massive star formation began about 8 million years (My) ago, and the star-formation rate seems to have declined in the last 1 My. The IMF is densely sampled up to 200 [Formula: see text] and contains 32 ± 12% more stars above 30 [Formula: see text] than predicted by a standard Salpeter IMF. In the mass range of 15 to 200 [Formula: see text], the IMF power-law exponent is [Formula: see text], shallower than the Salpeter value of 2.35
Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector
The inclusive and dijet production cross-sections have been measured for jets
containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass
energy of sqrt(s) = 7 TeV, using the ATLAS detector at the LHC. The
measurements use data corresponding to an integrated luminosity of 34 pb^-1.
The b-jets are identified using either a lifetime-based method, where secondary
decay vertices of b-hadrons in jets are reconstructed using information from
the tracking detectors, or a muon-based method where the presence of a muon is
used to identify semileptonic decays of b-hadrons inside jets. The inclusive
b-jet cross-section is measured as a function of transverse momentum in the
range 20 < pT < 400 GeV and rapidity in the range |y| < 2.1. The bbbar-dijet
cross-section is measured as a function of the dijet invariant mass in the
range 110 < m_jj < 760 GeV, the azimuthal angle difference between the two jets
and the angular variable chi in two dijet mass regions. The results are
compared with next-to-leading-order QCD predictions. Good agreement is observed
between the measured cross-sections and the predictions obtained using POWHEG +
Pythia. MC@NLO + Herwig shows good agreement with the measured bbbar-dijet
cross-section. However, it does not reproduce the measured inclusive
cross-section well, particularly for central b-jets with large transverse
momenta.Comment: 10 pages plus author list (21 pages total), 8 figures, 1 table, final
version published in European Physical Journal
Measurement of χ c1 and χ c2 production with s√ = 7 TeV pp collisions at ATLAS
The prompt and non-prompt production cross-sections for the χ c1 and χ c2 charmonium states are measured in pp collisions at s√ = 7 TeV with the ATLAS detector at the LHC using 4.5 fb−1 of integrated luminosity. The χ c states are reconstructed through the radiative decay χ c → J/ψγ (with J/ψ → μ + μ −) where photons are reconstructed from γ → e + e − conversions. The production rate of the χ c2 state relative to the χ c1 state is measured for prompt and non-prompt χ c as a function of J/ψ transverse momentum. The prompt χ c cross-sections are combined with existing measurements of prompt J/ψ production to derive the fraction of prompt J/ψ produced in feed-down from χ c decays. The fractions of χ c1 and χ c2 produced in b-hadron decays are also measured
Measurements of fiducial and differential cross sections for Higgs boson production in the diphoton decay channel at s√=8 TeV with ATLAS
Measurements of fiducial and differential cross sections are presented for Higgs boson production in proton-proton collisions at a centre-of-mass energy of s√=8 TeV. The analysis is performed in the H → γγ decay channel using 20.3 fb−1 of data recorded by the ATLAS experiment at the CERN Large Hadron Collider. The signal is extracted using a fit to the diphoton invariant mass spectrum assuming that the width of the resonance is much smaller than the experimental resolution. The signal yields are corrected for the effects of detector inefficiency and resolution. The pp → H → γγ fiducial cross section is measured to be 43.2 ±9.4(stat.) − 2.9 + 3.2 (syst.) ±1.2(lumi)fb for a Higgs boson of mass 125.4GeV decaying to two isolated photons that have transverse momentum greater than 35% and 25% of the diphoton invariant mass and each with absolute pseudorapidity less than 2.37. Four additional fiducial cross sections and two cross-section limits are presented in phase space regions that test the theoretical modelling of different Higgs boson production mechanisms, or are sensitive to physics beyond the Standard Model. Differential cross sections are also presented, as a function of variables related to the diphoton kinematics and the jet activity produced in the Higgs boson events. The observed spectra are statistically limited but broadly in line with the theoretical expectations
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