5,739 research outputs found
Isoelectronic series of oxygen deficient centers in silica: experimental estimation of homogeneous and inhomogeneous spectral widths
We report nanosecond time-resolved photoluminescence measurements on the
isoelectronic series of oxygen deficient centers in amorphous silica,
Si-ODC(II), Ge-ODC(II) and Sn-ODC(II) which are responsible of fluorescence
activities at 4 eV under excitation at 5 eV. The dependence of the
first moment of their emission band on time, and that of the radiative decay
lifetime on emission energy are analyzed within a theoretical model able to
describe the effects introduced by disorder on the optical properties of the
defects. We obtain separate estimates of the homogeneous and inhomogeneous
contributions to the measured emission linewidth and we derive homogeneous
spectroscopic features of the investigated point defects (Huangh-Rhys factor,
homogeneous width, oscillator strength, vibrational frequency). The results
point to a picture in which an oxygen deficient center localized on a heavier
atom features a higher degree of inhomogeneity due to stronger local distortion
of the surrounding matrix. For Si, Ge, Sn related defects the parameter
, able to quantify inhomogeneity, results to be 65, 78 and 90$%,
respectively
Studies of the Response of the Prototype CMS Hadron Calorimeter, Including Magnetic Field Effects, to Pion, Electron, and Muon Beams
We report on the response of a prototype CMS hadron calorimeter module to
charged particle beams of pions, muons, and electrons with momenta up to 375
GeV/c. The data were taken at the H2 and H4 beamlines at CERN in 1995 and 1996.
The prototype sampling calorimeter used copper absorber plates and scintillator
tiles with wavelength shifting fibers for readout. The effects of a magnetic
field of up to 3 Tesla on the response of the calorimeter to muons, electrons,
and pions are presented, and the effects of an upstream lead tungstate crystal
electromagnetic calorimeter on the linearity and energy resolution of the
combined calorimetric system to hadrons are evaluated. The results are compared
with Monte Carlo simulations and are used to optimize the choice of total
absorber depth, sampling frequency, and longitudinal readout segmentation.Comment: 89 pages, 41 figures, to be published in NIM, corresponding author: P
de Barbaro, [email protected]
Structural relaxation of E' gamma centers in amorphous silica
We report experimental evidence of the existence of two variants of the E'
gamma centers induced in silica by gamma rays at room temperature. The two
variants are distinguishable by the fine features of their line shapes in
paramagnetic resonance spectra. These features suggest that the two E' gamma
differ for their topology. We find a thermally induced interconversion between
the centers with an activation energy of about 34 meV. Hints are also found for
the existence of a structural configuration of minimum energy and of a
metastable state.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Determination of alpha_s using Jet Rates at LEP with the OPAL detector
Hadronic events produced in e+e- collisions by the LEP collider and recorded
by the OPAL detector were used to form distributions based on the number of
reconstructed jets. The data were collected between 1995 and 2000 and
correspond to energies of 91 GeV, 130-136 GeV and 161-209 GeV. The jet rates
were determined using four different jet-finding algorithms (Cone, JADE, Durham
and Cambridge). The differential two-jet rate and the average jet rate with the
Durham and Cambridge algorithms were used to measure alpha(s) in the LEP energy
range by fitting an expression in which order alpah_2s calculations were
matched to a NLLA prediction and fitted to the data. Combining the measurements
at different centre-of-mass energies, the value of alpha_s (Mz) was determined
to be
alpha(s)(Mz)=0.1177+-0.0006(stat.)+-0.0012$(expt.)+-0.0010(had.)+-0.0032(theo.)
\.Comment: 40 pages, 17 figures, Submitted to Euro. Phys. J.
Colour reconnection in e+e- -> W+W- at sqrt(s) = 189 - 209 GeV
The effects of the final state interaction phenomenon known as colour
reconnection are investigated at centre-of-mass energies in the range sqrt(s) ~
189-209 GeV using the OPAL detector at LEP. Colour reconnection is expected to
affect observables based on charged particles in hadronic decays of W+W-.
Measurements of inclusive charged particle multiplicities, and of their angular
distribution with respect to the four jet axes of the events, are used to test
models of colour reconnection. The data are found to exclude extreme scenarios
of the Sjostrand-Khoze Type I (SK-I) model and are compatible with other
models, both with and without colour reconnection effects. In the context of
the SK-I model, the best agreement with data is obtained for a reconnection
probability of 37%. Assuming no colour reconnection, the charged particle
multiplicity in hadronically decaying W bosons is measured to be (nqqch) =
19.38+-0.05(stat.)+-0.08 (syst.).Comment: 30 pages, 9 figures, Submitted to Euro. Phys. J.
Scaling violations of quark and gluon jet fragmentation functions in e+e- annihilations at sqrt(s) = 91.2 and 183-209 GeV
Flavour inclusive, udsc and b fragmentation functions in unbiased jets, and
flavour inclusive, udsc, b and gluon fragmentation functions in biased jets are
measured in e+e- annihilations from data collected at centre-of-mass energies
of 91.2, and 183-209 GeV with the OPAL detector at LEP. The unbiased jets are
defined by hemispheres of inclusive hadronic events, while the biased jet
measurements are based on three-jet events selected with jet algorithms.
Several methods are employed to extract the fragmentation functions over a wide
range of scales. Possible biases are studied in the results are obtained. The
fragmentation functions are compared to results from lower energy e+e-
experiments and with earlier LEP measurements and are found to be consistent.
Scaling violations are observed and are found to be stronger for the
fragmentation functions of gluon jets than for those of quarks. The measured
fragmentation functions are compared to three recent theoretical
next-to-leading order calculations and to the predictions of three Monte Carlo
event generators. While the Monte Carlo models are in good agreement with the
data, the theoretical predictions fail to describe the full set of results, in
particular the b and gluon jet measurements.Comment: 46 pages, 17 figures, Submitted to Eur. Phys J.
Search for Yukawa Production of a Light Neutral Higgs Boson at LEP
Within a Two-Higgs-Doublet Model (2HDM) a search for a light Higgs boson in
the mass range of 4-12 GeV has been performed in the Yukawa process e+e- -> b
bbar A/h -> b bbar tau+tau-, using the data collected by the OPAL detector at
LEP between 1992 and 1995 in e+e- collisions at about 91 GeV centre-of-mass
energy. A likelihood selection is applied to separate background and signal.
The number of observed events is in good agreement with the expected
background. Within a CP-conserving 2HDM type II model the cross-section for
Yukawa production depends on xiAd = |tan beta| and xihd = |sin alpha/cos beta|
for the production of the CP-odd A and the CP-even h, respectively, where tan
beta is the ratio of the vacuum expectation values of the Higgs doublets and
alpha is the mixing angle between the neutral CP-even Higgs bosons. From our
data 95% C.L. upper limits are derived for xiAd within the range of 8.5 to 13.6
and for xihd between 8.2 to 13.7, depending on the mass of the Higgs boson,
assuming a branching fraction into tau+tau- of 100%. An interpretation of the
limits within a 2HDM type II model with Standard Model particle content is
given. These results impose constraints on several models that have been
proposed to explain the recent BNL measurement of the muon anomalous magnetic
moment.Comment: 24 pages, 9 figures, Submitted to Euro. Phys. J.
Measurement of triple gauge boson couplings from WW production at LEP energies up to 189 GeV
A measurement of triple gauge boson couplings is presented, based on W-pair
data recorded by the OPAL detector at LEP during 1998 at a centre-of-mass
energy of 189 GeV with an integrated luminosity of 183 pb^-1. After combining
with our previous measurements at centre-of-mass energies of 161-183 GeV we
obtain k_g=0.97 +0.20 -0.16, g_1^z=0.991 +0.060 -0.057 and lambda_g=-0.110
+0.058 -0.055, where the errors include both statistical and systematic
uncertainties and each coupling is determined by setting the other two
couplings to their SM values. These results are consistent with the Standard
Model expectations.Comment: 28 pages, 8 figures, submitted to Eur. Phys. J.
Tests of model of color reconnection and a search for glueballs using gluon jets with a rapidity gap
Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from
hadronic Z0 decay events produced in e+e- annihilations. A subsample of these
jets is identified which exhibits a large gap in the rapidity distribution of
particles within the jet. After imposing the requirement of a rapidity gap, the
gluon jet purity is 86%. These jets are observed to demonstrate a high degree
of sensitivity to the presence of color reconnection, i.e. higher order QCD
processes affecting the underlying color structure. We use our data to test
three QCD models which include a simulation of color reconnection: one in the
Ariadne Monte Carlo, one in the Herwig Monte Carlo, and the other by Rathsman
in the Pythia Monte Carlo. We find the Rathsman and Ariadne color reconnection
models can describe our gluon jet measurements only if very large values are
used for the cutoff parameters which serve to terminate the parton showers, and
that the description of inclusive Z0 data is significantly degraded in this
case. We conclude that color reconnection as implemented by these two models is
disfavored. The signal from the Herwig color reconnection model is less clear
and we do not obtain a definite conclusion concerning this model. In a separate
study, we follow recent theoretical suggestions and search for glueball-like
objects in the leading part of the gluon jets. No clear evidence is observed
for these objects.Comment: 42 pages, 18 figure
W Boson Polarisation at LEP2
Elements of the spin density matrix for W bosons in e+e- -> W+W- -> qqln
events are measured from data recorded by the OPAL detector at LEP. This
information is used calculate polarised differential cross-sections and to
search for CP-violating effects. Results are presented for W bosons produced in
e+e- collisions with centre-of-mass energies between 183 GeV and 209 GeV. The
average fraction of W bosons that are longitudinally polarised is found to be
(23.9 +- 2.1 +- 1.1)% compared to a Standard Model prediction of (23.9 +-
0.1)%. All results are consistent with CP conservation.Comment: 20 pages, 3 figures, Submitted to Phys. Letts.
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