37 research outputs found
Production of Lambda and Sigma^0 hyperons in proton-proton collisions
This paper reports results on simultaneous measurements of the reaction
channels pp -> pK+\Lambda and pp -> pK+\Sigma^0 at excess energies of 204, 239,
and 284 MeV (\Lambda) and 127, 162, and 207 MeV (\Sigma^0). Total and
differential cross sections are given for both reactions. It is concluded from
the measured total cross sections that the high energy limit of the cross
section ratio is almost reached at an excess energy of only about 200 MeV. From
the differential distributions observed in the overall CMS as well as in the
Jackson and helicity frames, a significant contribution of interfering nucleon
resonances to the \Lambda production mechanism is concluded while resonant
\Sigma^0-production seems to be of lesser importance and takes place only
through specific partial waves of the entrance channel. The data also indicate
that kaon exchange plays a minor role in the case of \Lambda- but an important
role for \Sigma^0-production. Thus the peculiar energy dependence of the
\Lambda-to-\Sigma^0 cross section ratio appears in a new light as its
explanation requires more than mere differences between the p\Lambda and the
p\Sigma^0 final state interaction. The data provide a benchmark for theoretical
models already available or yet to come.Comment: 18 pages, 10 figures; accepted by The European Physical Journal A
(EPJ A
Search for Higgs Bosons in e+e- Collisions at 183 GeV
The data collected by the OPAL experiment at sqrts=183 GeV were used to
search for Higgs bosons which are predicted by the Standard Model and various
extensions, such as general models with two Higgs field doublets and the
Minimal Supersymmetric Standard Model (MSSM). The data correspond to an
integrated luminosity of approximately 54pb-1. None of the searches for neutral
and charged Higgs bosons have revealed an excess of events beyond the expected
background. This negative outcome, in combination with similar results from
searches at lower energies, leads to new limits for the Higgs boson masses and
other model parameters. In particular, the 95% confidence level lower limit for
the mass of the Standard Model Higgs boson is 88.3 GeV. Charged Higgs bosons
can be excluded for masses up to 59.5 GeV. In the MSSM, mh > 70.5 GeV and mA >
72.0 GeV are obtained for tan{beta}>1, no and maximal scalar top mixing and
soft SUSY-breaking masses of 1 TeV. The range 0.8 < tanb < 1.9 is excluded for
minimal scalar top mixing and m{top} < 175 GeV. More general scans of the MSSM
parameter space are also considered.Comment: 49 pages. LaTeX, including 33 eps figures, submitted to European
Physical Journal
A Measurement of the Product Branching Ratio f(b->Lambda_b).BR(Lambda_b->Lambda X) in Z0 Decays
The product branching ratio, f(b->Lambda_b).BR(Lambda_b->Lambda X), where
Lambda_b denotes any weakly-decaying b-baryon, has been measured using the OPAL
detector at LEP. Lambda_b are selected by the presence of energetic Lambda
particles in bottom events tagged by the presence of displaced secondary
vertices. A fit to the momenta of the Lambda particles separates signal from B
meson and fragmentation backgrounds. The measured product branching ratio is
f(b->Lambda_b).BR(Lambda_b->Lambda X) = (2.67+-0.38(stat)+0.67-0.60(sys))%
Combined with a previous OPAL measurement, one obtains
f(b->Lambda_b).BR(Lambda_b->Lambda X) = (3.50+-0.32(stat)+-0.35(sys))%.Comment: 16 pages, LaTeX, 3 eps figs included, submitted to the European
Physical Journal
Measurement of the Michel Parameters in Leptonic Tau Decays
The Michel parameters of the leptonic tau decays are measured using the OPAL
detector at LEP. The Michel parameters are extracted from the energy spectra of
the charged decay leptons and from their energy-energy correlations. A new
method involving a global likelihood fit of Monte Carlo generated events with
complete detector simulation and background treatment has been applied to the
data recorded at center-of-mass energies close to sqrt(s) = M(Z) corresponding
to an integrated luminosity of 155 pb-1 during the years 1990 to 1995. If e-mu
universality is assumed and inferring the tau polarization from neutral current
data, the measured Michel parameters are extracted. Limits on non-standard
coupling constants and on the masses of new gauge bosons are obtained. The
results are in agreement with the V-A prediction of the Standard Model.Comment: 32 pages, LaTeX, 9 eps figures included, submitted to the European
Physical Journal
The pK0\Sigma+ final state in proton-proton collisions
This paper reports results from a study of the reaction pp->pK0\Sigma+ at
beam momenta of p_{beam} = 2950, 3059, and 3200 MeV/c (excess energies of
\epsilon= 126, 161, and 206 MeV). Total cross sections were determined for all
energies; a set of differential cross sections (Dalitz plots; invariant mass
spectra of all two-body subsystems; angular distributions of all final state
particles; distributions in helicity and Jackson frames) are presented for
\epsilon= 161 MeV. The total cross sections are proportional to the volume of
available three-body phase-space indicating that the transition matrix element
does not change significantly in this range of excess energies. It is concluded
from the differential data that the reaction proceeds dominantly via the
N(1710)P_{11} and/or N(1720)P_{13} resonance(s); N(1650)S_{11} and
\Delta(1600)P_{33} could also contribute.Comment: 15 pages, 10 figure
Inclusive production of charged hadrons and mesons in photon-photon collisions
The production of charged hadrons and K_s mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at e+e- centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and K_s mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range 10-125 GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in gamma-gamma interactions with gamma-proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions.The production of charged hadrons and K_s mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at e+e- centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and K_s mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range 10-125 GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in gamma-gamma interactions with gamma-proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions
Measurement of the and lifetimes and search for CP(T) violation using reconstructed secondary vertices
The lifetimes of the B+ and B0 mesons, and their ratio, have been measured in the OPAL experiment using 2.4 million hadronic Z0 decays recorded at LEP. Z0 -> b bbar decays were tagged using displaced secondary vertices and high momentum electrons and muons. The lifetimes were then measured using well-reconstructed charged and neutral secondary vertices selected in this tagged data sample. The results are tau(B+) = 1.643 +- 0.037 +- 0.025 ps tau(B0) = 1.523 +- 0.057 +- 0.053 ps ratio tau(B+)/tau(B0) = 1.079 +- 0.064 +- 0.041 where in each case the first error is statistical and the second systematic. A larger data sample of 3.1 million hadronic Z0 decays has been used to search for CP and CPT violating effects by comparison of inclusive b and bbar hadron decays. No evidence for such effects is seen. The CP violation parameter Re(epsilon_B) is measured to be Re(epsilon_B) = 0.001 +- 0.014 +- 0.003 and the fractional difference between b and bbar hadron lifetimes is measured to be -0.001 +- 0.012 +- 0.00
A search for neutral Higgs bosons in the MSSM and models with two scalar field doublets
A search is described for the neutral Higgs bosons h^0 and A^0 predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the Z^0 h^0 and h^0 A^0 production channels is based on data corresponding to an integrated luminosity of 25 pb^{-1} from e^+e^- collisions at centre-of-mass energies between 130 and 172GeV collected with the OPAL detector at LEP. The observation of a number of candidates consistent with Standard Model background expectations is used in combination with earlier results from data collected at the Z^0 resonance to set limits on m_h and m_A in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for tan(beta) > 1, minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits m_h > 59.0 GeV and m_A > 59.5 GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan.A search is described for the neutral Higgs bosons h^0 and A^0 predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the Z^0 h^0 and h^0 A^0 production channels is based on data corresponding to an integrated luminosity of 25 pb^{-1} from e^+e^- collisions at centre-of-mass energies between 130 and 172 GeV collected with the OPAL detector at LEP. The observation of a number of candidates consistent with Standard Model background expectations is used in combination with earlier results from data collected at the Z^0 resonance to set limits on m_h and m_A in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for tan(beta) > 1, minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits m_h > 59.0 GeV and m_A > 59.5 GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan
Di-Jet Production in Photon-Photon Collisions at e+e- center-of-mass energies = 161 and 172 GeV
Di-jet production is studied in collisions of quasi-real photons radiated by
the LEP beams at e+e- centre-of-mass energies 161 and 172 GeV. The jets are
reconstructed using a cone jet finding algorithm. The angular distributions of
direct and double-resolved processes are measured and compared to the
predictions of leading order and next-to-leading order perturbative QCD. The
jet energy profiles are also studied. The inclusive two-jet cross-section is
measured as a function of transverse energy and rapidity and compared to
next-to-leading order perturbative QCD calculations. The inclusive two-jet
cross-section as a function of rapidity is compared to the prediction of the
leading order Monte Carlo generators PYTHIA and PHOJET. The Monte Carlo
predictions are calculated with different parametrisations of the parton
distributions of the photon. The influence of the `underlying event' has been
studied to reduce the model dependence of the predicted jet cross-sections from
the Monte Carlo generators.Comment: 31 pages, LaTeX, 15 eps figures included (1 in colour), submitted to
European Physical Journal