15 research outputs found
Mulit-Photon Events with Large Missing energy in e+e- collisions at sqrt(s)=192-209 GeV
Events with a final state consisting of two or more photons and large missing
transverse energy have been observed in e+e- collisions at centre-of-mass
energies in the range 192 - 209 GeV using the OPAL detector at LEP.
Cross-section measurements are performed within the kinematic acceptance of the
selection and compared with the expectation from the Standard Model process
e+e- -> nu nu gamma gamma(gamma). No evidence for new physics contributions to
this final state is observed. Upper limits on sigma (e+e- -> XX) BR^2(X ->
Ygamma) are derived for the case of stable and invisible Y. In the case of
massive Y the combined limits obtained from all the data range from 10 fb to 60
fb, while for the special case of massless Y the range is 20 fb to 40 fb. The
limits apply to pair production of excited neutrions (X=nu^*,Y=nu), to
neutralino production (X=~chi^0_2, Y=~chi^0_1) and to supersymmetric models in
which X=~chi^0_1 and Y=~G is a light gravitino.Comment: 18 pages, 4 figures, Submitted to Phys. Letts
Search for Associated Production of Massive States Decaying into Two Photonsin e+e- Annihilations at sqrt(s) = 88-209 GeV
A search is performed for production of short-lived particles in e+e- -> XY,
with X -> gamma gamma and Y -> ffbar, for scalar X and scalar or vector Y.
Model-independent limits in the range of 25-60 femtobarns are presented on
sigma (e+e- -> XY) x B(X -> ffbar) for centre-of-mass energies in the range
205-207 GeV. The data from all LEP centre-of-mass energies 88-209 GeV are also
interpreted in the context of fermiophobic Higgs boson models, for which a
lower mass limit of 105.5 GeV is obtained for a "benchmark" fermiophobic Higgs
boson.Comment: 17 pages, 4 figure
Improved extraction of humic acids from vermicomposted organic waste by a column-based continuous elution method
Fine root mass, distribution and regeneration in disturbed primary forests and secondary forests of the moist tropics
Bose-Einstein study of position-momentum correlations of charged pions in hadronic Z0 decays
A study of Bose-Einstein correlations in pairs of identically charged pions
produced in e+e- annihilations at the Z0 peak has been performed for the first
time assuming a non-static emitting source. The results are based on the high
statistics data obtained with the OPAL detector at LEP. The correlation
functions have been analyzed in intervals of the average pair transverse
momentum and of the pair rapidity, in order to study possible correlations
between the pion production points and their momenta (position-momentum
correlations). The Yano-Koonin and the Bertsch-Pratt parameterizations have
been fitted to the measured correlation functions to estimate the geometrical
parameters of the source as well as the velocity of the source elements with
respect to the overall centre-of-mass frame. The source rapidity is found to
scale approximately with the pair rapidity, and both the longitudinal and
transverse source dimensions are found to decrease for increasing average pair
transverse momenta.Comment: 25 pages, 10 figures, Submitted to Eur Phys J.
Inclusive production of charged hadrons in photon-photon collisions
The inclusive production of charged hadrons in the collisions of quasi-real
photons e+e- -> e+e- +X has been measured using the OPAL detector at LEP. The
data were taken at e+e- centre-of-mass energies from 183 to 209 GeV. The
differential cross-sections as a function of the transverse momentum and the
pseudorapidity of the hadrons are compared to theoretical calculations of up to
next-to-leading order (NLO) in the strong coupling constant alpha{s}. The data
are also compared to a measurement by the L3 Collaboration, in which a large
deviation from the NLO predictions is observed.Comment: 20 pages, 4 figures, Submitted to Phys. Letts.
A study of W+W-gamma events at LEP
A study of W+W- events accompanied by hard photon radiation, Egamma > 2.5 GeV, produced in e(+)e(-) collisions at LEP is presented. Events consistent with being two on-shell W-bosons and an isolated photon are selected from 681 pb(-1) of data recorded at 180 GeV < roots < 209 GeV. From the sample of 187 selected W(+)W(-)gamma candidates with photon energies greater than 2.5 GeV, the W(+)W(-)gamma cross-section is determined at five values of roots. The results are consistent with the Standard Model expectation. Averaging over all energies, the ratio of the observed cross-section to the Standard Model expectation is R(data/SM) = 0.99 +/- 0.09 +/- 0.04, where the errors represent the statistical and systematic uncertainties respectively. These data provide constraints on the related O(alpha) systematic uncertainties on the measurement of the W-boson mass at LEP. Finally, the data are used to derive 95% confidence level upper limits on possible anomalous contributions to the W(+)W(-)gammagamma and W(+)W(-)Z(0)y vertices: -0.020 GeV-2 < a(0)/Lambda(2) < 0.020 GeV-2, -0.053 GeV-2 < a(c)/Lambda(2) < 0.037 GeV-2, -0.16 GeV-2 < a(n)/Lambda(2) < 0.15 GeV-2, where Lambda represents the energy scale for new physics and a(0), a(c) and a(n) are dimensionless coupling constants. (C) 2003 Elsevier B.V. All rights reserved
Determination of the LEP Beam Energy using Radiative Fermion-pair Events, 2004
We present a determination of the LEP beam energy using "radiative return" fermion-pair events recorded at centre-of-mass energies from 183 GeV to 209 GeV. We find no evidence of a disagreement between the OPAL data and the LEP Energy Workings Group's standard calibration. Including the energy- averaged 11 MeV uncertainty in the standard determination, the beam energy we obtain from the OPAL data is higher than that obtained from the LEP calibration by 0+-34(stat.)+-27(syst.)Me
Measurement of alpha(s) with radiative hadronic events.
Hadronic final states with a hard isolated photon are studied using data taken at centre-of-mass energies around the mass of the Z boson with the OPAL detector at LEP. The strong coupling alpha(s) is extracted by comparing data and QCD predictions for event shape observables at average reduced centre-of-mass energies ranging from 24 GeV to 78 GeV, and the energy dependence of alpha(s) is studied. Our results are consistent with the running of alpha(s) as predicted by QCD and show that within the uncertainties of our analysis event shapes in hadronic Z decays with hard and isolated photon radiation can be described by QCD at reduced centre-of-mass energies. Combining all values from different event shape observables and energies gives alpha(s)(M-Z)=0.1182 +/- 0.0015(stat.)+/- 0.0101(syst.)