6,147 research outputs found
Test Beam Results of Geometry Optimized Hybrid Pixel Detectors
The Multi-Chip-Module-Deposited (MCM-D) technique has been used to build
hybrid pixel detector assemblies. This paper summarises the results of an
analysis of data obtained in a test beam campaign at CERN. Here, single chip
hybrids made of ATLAS pixel prototype read-out electronics and special sensor
tiles were used. They were prepared by the Fraunhofer Institut fuer
Zuverlaessigkeit und Mikrointegration, IZM, Berlin, Germany. The sensors
feature an optimized sensor geometry called equal sized bricked. This design
enhances the spatial resolution for double hits in the long direction of the
sensor cells.Comment: Contribution to Proceedings of Pixel2005 Workshop, Bonn Germany 200
Equilibrium structure and shape of Ag and Pt nanoparticles grown on silica surfaces: from experimental investigations to the determination of a metal-silica potential
A combination of experimental and numerical calculations on metallic silver
and platinum nanoparticles deposited on silica substrates is presented, with a
focus on the metal-substrate interactions. Experimentally, the nanoparticles
are elaborated under ultra-high vacuum and characterized by Grazing-Incidence
Small-Angle X-ray Scattering (GISAXS) and High Resolution Transmission
Electronic Microscopy (HRTEM) to determine their structure and morphology, and
in particular their aspect ratio (height/diameter) which quantifies the
metal-substrate interaction. Numerically, the interactions between the metal
and the silica species are modeled with the Lennard-Jones (12, 6) potential,
with two parameters for each metal and silica species. The geometric parameters
were found in the literature, while the energetic parameters were determined
from our experimental measurements of the aspect ratio. The parameters are:
{\sigma}_{Ag-O} = 0.278 nm, {\sigma}_{Ag-Si} = 0.329 nm, {\epsilon}_{Ag-O} = 75
meV, and {\epsilon}_{Ag-Si} = 13 meV for Ag-silica and {\sigma}_{Pt-O} = 0.273
nm, {\sigma}_{Pt-Si} = 0.324 nm, {\epsilon}_{Pt-O} = 110 meV, and
{\epsilon}_{Pt-Si} = 18 meV for Pt-silica. The proposed Ag-silica potential
reproduces quantitatively the unexpected experimental observation of the
variation of the aspect ratio for Ag nanoparticles larger than 5 nm, which has
been interpreted as a consequence of the silica roughness. The nanoparticle
orientation, structure and disorder are also considered. This metal-silica
potential for Ag and Pt should be helpful for further studies on pure metals as
well as their alloys.Comment: 35p, 12 figure
Determination of the b quark mass at the M_Z scale with the DELPHI detector at LEP
An experimental study of the normalized three-jet rate of b quark events with
respect to light quarks events (light= \ell \equiv u,d,s) has been performed
using the CAMBRIDGE and DURHAM jet algorithms. The data used were collected by
the DELPHI experiment at LEP on the Z peak from 1994 to 2000. The results are
found to agree with theoretical predictions treating mass corrections at
next-to-leading order. Measurements of the b quark mass have also been
performed for both the b pole mass: M_b and the b running mass: m_b(M_Z). Data
are found to be better described when using the running mass. The measurement
yields: m_b(M_Z) = 2.85 +/- 0.18 (stat) +/- 0.13 (exp) +/- 0.19 (had) +/- 0.12
(theo) GeV/c^2 for the CAMBRIDGE algorithm. This result is the most precise
measurement of the b mass derived from a high energy process. When compared to
other b mass determinations by experiments at lower energy scales, this value
agrees with the prediction of Quantum Chromodynamics for the energy evolution
of the running mass. The mass measurement is equivalent to a test of the
flavour independence of the strong coupling constant with an accuracy of 7
permil.Comment: 24 pages, 10 figures, Accepted by Eur. Phys. J.
Measurement and Interpretation of Fermion-Pair Production at LEP energies above the Z Resonance
This paper presents DELPHI measurements and interpretations of
cross-sections, forward-backward asymmetries, and angular distributions, for
the e+e- -> ffbar process for centre-of-mass energies above the Z resonance,
from sqrt(s) ~ 130 - 207 GeV at the LEP collider. The measurements are
consistent with the predictions of the Standard Model and are used to study a
variety of models including the S-Matrix ansatz for e+e- -> ffbar scattering
and several models which include physics beyond the Standard Model: the
exchange of Z' bosons, contact interactions between fermions, the exchange of
gravitons in large extra dimensions and the exchange of sneutrino in R-parity
violating supersymmetry.Comment: 79 pages, 16 figures, Accepted by Eur. Phys. J.
A Determination of the Centre-of-Mass Energy at LEP2 using Radiative 2-fermion Events
Using e+e- -> mu+mu-(gamma) and e+e- -> qqbar(gamma) events radiative to the
Z pole, DELPHI has determined the centre-of-mass energy, sqrt{s}, using energy
and momentum constraint methods. The results are expressed as deviations from
the nominal LEP centre-of-mass energy, measured using other techniques. The
results are found to be compatible with the LEP Energy Working Group estimates
for a combination of the 1997 to 2000 data sets.Comment: 20 pages, 6 figures, Accepted by Eur. Phys. J.
Study of Inclusive J/psi Production in Two-Photon Collisions at LEP II with the DELPHI Detector
Inclusive J/psi production in photon-photon collisions has been observed at
LEP II beam energies. A clear signal from the reaction gamma gamma -> J/psi+X
is seen. The number of observed N(J/psi -> mu+mu-) events is 36 +/- 7 for an
integrated luminosity of 617 pb^{-1}, yielding a cross-section of
sigma(J/psi+X) = 45 +/- 9 (stat) +/- 17 (syst) pb. Based on a study of the
event shapes of different types of gamma gamma processes in the PYTHIA program,
we conclude that (74 +/- 22)% of the observed J/psi events are due to
`resolved' photons, the dominant contribution of which is most probably due to
the gluon content of the photon.Comment: 13 pages, 8 figures, Accepted by Phys. Lett.
From Dusty Filaments to Cores to Stars: An Infrared Extinction Study of Lupus 3
We present deep NIR observations of a dense region of Lupus 3 obtained with
ESO's NTT and VLT. Using the NICE method we construct a dust extinction map of
the cloud, which reveals embedded globules, a dense filament, and a dense ring
structure. We derive dust column densities and masses for the entire cloud and
for the individual structures therein. We construct radial extinction profiles
for the embedded globules and find a range of profile shapes from relatively
shallow profiles for cores with low peak extinctions, to relatively steep
profiles for cores with high extinction. Overall the profiles are similar to
those of pressure truncated isothermal spheres of varying center-to-edge
density contrast. We apply Bonnor-Ebert analysis to compare the density
profiles of the embedded cores in a quantitative manner and derive physical
parameters such as temperatures, central densities, and external pressures. We
examine the stability of the cores and find that two cores are likely stable
and two are likely unstable. One of these latter cores is known to harbor an
active protostar. Finally, we discuss the relation between an emerging cluster
in Lupus 3 and the ring structure identified in our extinction map. Assuming
that the ring is the remnant of the core within which the cluster originally
formed we estimate that a star formation efficiency of ~ 30% characterized the
formation of the small cluster. Our observations of Lupus 3 suggest an intimate
link between the structure of a dense core and its state of star forming
activity. The dense cores are found to span the entire range of evolution from
a stable, starless core of modest central concentration, to an unstable,
star-forming core which is highly centrally concentrated, to a significantly
disrupted core from which a cluster of young stars is emerging.Comment: Accepted for publication in the Astrophysical Journal. Go to
http://cfa-www.harvard.edu/~clada/ or http://cfa-www.harvard.edu/~pteixeir/ a
version with higher resolution figure
Evidence for an Excess of Soft Photons in Hadronic Decays of Z^0
Soft photons inside hadronic jets converted in front of the DELPHI main
tracker (TPC) in events of qqbar disintegrations of the Z^0 were studied in the
kinematic range 0.2 < E_gamma < 1 GeV and transverse momentum with respect to
the closest jet direction p_T < 80 MeV/c. A clear excess of photons in the
experimental data as compared to the Monte Carlo predictions is observed. This
excess (uncorrected for the photon detection efficiency) is (1.17 +/- 0.06 +/-
0.27) x 10^{-3} gamma/jet in the specified kinematic region, while the expected
level of the inner hadronic bremsstrahlung (which is not included in the Monte
Carlo) is (0.340 +/- 0.001 +/- 0.038) x 10^{-3} gamma/jet. The ratio of the
excess to the predicted bremsstrahlung rate is then (3.4 +/- 0.2 +/- 0.8),
which is similar in strength to the anomalous soft photon signal observed in
fixed target experiments with hadronic beams.Comment: 37 pages, 9 figures, Accepted by Eur. Phys. J.
A Measurement of the Tau Hadronic Branching Ratios
The exclusive and semi-exclusive branching ratios of the tau lepton hadronic
decay modes (h- v_t, h- pi0 v_t, h- pi0 pi0 v_t, h- \geq 2pi0 v_t, h- \geq 3pi0
v_t, 2h- h+ v_t, 2h- h+ pi0 v_t, 2h- h+ \geq 2pi0 v_t, 3h- 2h+ v_t and 3h- 2h+
\geq 1pi0 v_t) were measured with data from the DELPHI detector at LEP.Comment: 53 pages, 18 figures, Accepted by Eur. Phys. J.
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