Recent Physics Results
A selection of papers that have been published recently or will be published
soon by OPAL. Included are links to the abstracts, interesting plots and the
PostScript versions of the papers stored on the CERN preprint server.
The OPAL collaboration pursues research in a wide variety of topics:
Z properties
One of the primary purposes for building LEP was to measure the
properties of the of boson. These
properties include the lineshape
parameters (the mass and the total and partial widths) as well as
asymmetry measurements and rare decay modes.
- Gamma(b b-bar) / Gamma(hadrons)
OPAL has measured the ratio of the width of
decaying to b-quarks
to the width of decaying to hadrons; this measurement is not
sensitive to the Higgs boson mass nor to the strong coupling
constant, so it can therefore provide a good constraint on Standard
Model parameters. This analysis uses a double tagging
method. The OPAL measurement is
Gamma( -> b b-bar) /
Gamma( -> hadrons)
= 0.2171 +- 0.0021 +- 0.0021.
A paper on this topic has been submitted for publication in Zeitschrift für
Physik C.
You can look at:
- Tau polarization and forward-backward asymmetry
Due to parity non-conservation in weak interactions, one expects
different couplings of 's to left- and
right-handed fermions, and
therefore one expects polarization of the
decay products. OPAL
has measured the polarization of tau leptons to be (-14.9 +- 1.9 +-
1.3)% and the tau polarization forward-backward asymmetry to be
(-8.9 +- 2.2 +- 0.9)%. These results are consistent with lepton
universality.
A paper on this topic has been submitted for publication in
Zeitschrift für Physik C.
You can look at:
- Single photon production
Counting single photons is an elegant way to investigate the Z
lineshape and put limits on new physics, pioneered at LEP by OPAL
in Z.Phys.C50:373-384, 1991. In the Standard Model single
photons come from Z to two neutrinos decays accompanied by initial
state radiation; using this interpretation the OPAL data directly
measures the invisible width and number of light (M < 33 GeV)
neutrino generations. Using the indirect but more powerful
standard lineshape analysis, the single photon cross-section can
be used to constrain the production rates for a variety of new
physics sources.
A paper on this topic has been submitted for publication in
Zeitschrift für Physik C.
You can look at:
Tau physics
- Tau lifetime
OPAL has a new measurement of the tau lifetime of
288.8 +- 2.2 (statistical) +- 1.4 (systematic) fs.
This result uses impact parameters of tracks from one-prong tau
decays and decay lengths from three-prong tau decays.
This result uses all the data taken from 1990 to 1993, and so is a
significant improvement over the previous result of
291.9 +- 5.1 (statistical) +- 3.1 (systematic) fs, which was based
only on the 1990 and 1991 datasets.
A paper on this topic has been published in
Physics Letters B339 497-506.
You can look at:
- Branching ratios of tau to K
A paper on this topic has been published in
Physics Letters B339 278-292.
You can look at:
- Limit on the mass of the tau neutrino
By looking at the decay of tau leptons to 5 pions and a tau
neutrino, OPAL has determined an upper limit of the mass of the tau
neutrino. We use a new method using the
invariant mass and the total energy of the charged hadrons.
OPAL's upper limit is 74 MeV at the 95% confidence level.
A paper on this topic has been submitted for publication in Zeitschrift
für Physik C.
You can look at:
B physics
Resonant B-pi and B-K production
- Resonant B-pi and B-K production
OPAL has clear evidence for resonant B-pi and B-K production in a
large sample of partially reconstructed B mesons. The production
rate of these resonances is substantial, and the observed distributions
are consistent with the expectations based on narrow P-wave resonances
of b antiquarks and light quarks. In addition, we show that the
production flavor of B mesons can be tagged with the charge of
appropriately chosen pions, and that this tag yields superior
performance to lepton-based b flavor tags. This is of substantial
interest for the study of CP violation in B decays in future experiments.
A paper on this topic has been submitted for publication in Zeitschrift
für Physik C.
You can look at:
B decays
- Rare hadronic B decays
OPAL has searched for indications of a number of decays of
and
mesons which would imply the existence of flavor-changing
neutral currents. No evidence for such decays was found.
A paper on this topic was published in
Physics Letters B337 393-404.
You can look at:
B oscillations
In analogy to the kaon system, one expects oscillations between
neutral mesons that contain the b quark. This occurs primarily through
second-order weak transitions.
OPAL has measured the oscillation frequency between
and anti- with two techniques: using
jet charge and using leptons and D* mesons.
- Oscillation measurements using a jet charge technique
A paper on this topic was published in
Physics Letters B327 411-424.
You can look at:
- Oscillation measurements using leptons and D* mesons
A paper on this topic was published in Physics Letters B336
585-598.
You can look at:
Particle searches
We also search for new particles. In the standard model, the Higgs
boson and the tau neutrino are the only particles remaining to be
discovered. Other models (such as supersymmetry) predict that there
are more particles to be discovered (such as the supersymmetric Higgs
bosons and scalar top quarks).
Standard model
- Search for the standard model Higgs boson
OPAL has set a lower limit on the mass of a minimal standard model
Higgs boson of 56.9 GeV at the 95% confidence level.
A paper on this topic was published in
Physics Letters B327 397-410.
You can look at:
Supersymmetry
- Search for a supersymmetric Higgs boson
The minimal supersymmetric extension to the Standard Model predicts
five physical Higgs bosons: two charged,
and ; two CP-even neutrals,
and where
< ;
and one CP-odd neutral, .
OPAL sees no evidence for supersymmetric Higgs bosons. The actual
limits depend on the assumptions of the model, but for a typical
assumption (tan(beta) >= 1), OPAL's results
imply > 44.5 GeV and
> 24.3 GeV.
A paper on this topic was published in
Zeitschrift für Physik C64 1-13.
You can look at:
- Search for a scalar top quark
In many supersymmetric models, the partner of the top quark can be
the lightest supersymmetric particle. OPAL has searched for the
scalar top quark and found no evidence for it; for a certain set of
parameters, this excludes a scalar top quark mass of less than 45.1
GeV at a 95% confidence level.
A paper on this topic was published in
Physics Letters B337 207-218.
You can look at:
QCD
- QCD colour factor ratios
Ratios of the QCD color factors
and
, which are related to the
group structure of the theory, can be measured by investigating
angular correlations in 4-jet events. A three-dimensional fit of
group-independent kinematic functions to angular variables sensitive
to the color factors is described in this paper. New features of
this analysis include a calculation of jet energies with angular
information, an investigation of finite quark masses on the matrix
element calculation, and several other new systematic studies.
A paper on this topic has been submitted for publication in
Physics Letters B. You can look at:
Tests of the Standard Model
- Test of CP Invariance
A paper on this topic has been submitted for publication in Zeitschrift
für Physik C.
You can look at:
d.wagner,
p.hart
9-Jan-94