542 research outputs found
Pion-Muon Asymmetry Revisited
Long ago an unexpected and unexplainable phenomena was observed. The
distribution of muons from positive pion decay at rest was anisotropic with an
excess in the backward direction relative to the direction of the proton beam
from which the pions were created. Although this effect was observed by several
different groups with pions produced by different means, the result was not
accepted by the physics community, because it is in direct conflict with a
large set of other experiments indicating that the pion is a pseudoscalar
particle. It is possible to satisfy both sets of experiments if helicity-zero
vector particles exist and the pion is such a particle. Helicity-zero vector
particles have direction but no net spin. For the neutral pion to be a vector
particle requires an additional modification to conventional theory as
discussed herein. An experiment is proposed which can prove that the asymmetry
in the distribution of muons from pion decay is a genuine physical effect
because the asymmetry can be modified in a controllable manner. A positive
result will also prove that the pion is NOT a pseudoscalar particle.Comment: 9 pages, 3 figure
Mass Suppression in Octet Baryon Production
There is a striking suppression of the cross section for production of octet
baryons in annihilation, as the mass of the produced hadron
increases. We present a simple parametrization for the fragmentation functions
into octet baryons guided by two input models: the SU(3) flavor symmetry part
is given by a quark-diquark model, and the baryon mass suppression part is
inspired by the string model. We need only eight free parameters to describe
the fragmentation functions for all octet baryons. These free parameters are
determined by a fit to the experimental data of octet baryon production in annihilation. Then we apply the obtained fragmentation functions to
predict the cross section of the octet baryon production in charged lepton DIS
and find consistency with the available experimental data. Furthermore, baryon
production in collisions is suggested to be an ideal domain to check the
predicted mass suppression.Comment: 20 pages, 5 figure
Measurement of event shape distributions and moments in e+e- -> hadrons at 91-209 GeV and a determination of alpha_s
We have studied hadronic events from e+e- annihilation data at centre-of-mass
energies from 91 to 209 GeV. We present distributions of event shape
observables and their moments at each energy and compare with QCD Monte Carlo
models. From the event shape distributions we extract the strong coupling
alpha_s and test its evolution with energy scale. The results are consistent
with the running of alpha_s expected from QCD. Combining all data, the value of
alpha_s(M_Z) is determined to be alpha_s(M_Z) = 0.1191 +- 0.0005 (stat.) +-
0.0010 (expt.) +- 0.0011 (hadr.) +- 0.0044 (theo.). The energy evolution of the
moments is also used to determine a value of alpha_s with slightly larger
errors: alpha_s(M_Z) = 0.1223 +- 0.0005 (stat.) +- 0.0014 (expt.) +- 0.0016
(hadr.) +0.0054 -0.0036 (theo.).Comment: 63 pages 26 fi
Searches for Gauge-Mediated Supersymmetry Breaking Topologies in e+e- collisions at LEP2
In gauge-mediated supersymmetry (SUSY) breaking (GMSB) models the lightest
supersymmetric particle (LSP) is the gravitino and the phenomenology is driven
by the nature of the next-to-lightest SUSY particle (NLSP) which is either the
lightest neutralino, the stau or mass degenerate sleptons. Since the NLSP decay
length is effectively unconstrained, searches for all possible lifetime and
NLSP topologies predicted by GMSB models in e+e- collisions are performed on
the data sample collected by OPAL at centre-of-mass energies up to 209 GeV at
LEP. Results independent of the NLSP lifetime are presented for all relevant
final states including direct NLSP pair-production and, for the first time,
also NLSP production via cascade decays of heavier SUSY particles. None of the
searches shows evidence for SUSY particle production. Cross-section limits are
presented at the 95% confidence level both for direct NLSP production and for
cascade decays, providing the most general, almost model independent results.
These results are then interpreted in the framework of the minimal GMSB (mGMSB)
model, where large areas of the accessible parameter space are excluded. In the
mGMSB model, the NLSP masses are constrained to be larger than 53.5 GeV/c^2,
87.4 GeV/c^2 and 91.9 GeV/c^2 in the neutralino, stau and slepton co-NLSP
scenarios, respectively. A complete scan on the parameters of the mGMSB model
is performed, constraining the universal SUSY mass scale Lambda from the direct
SUSY particle searches: Lambda > 40, 27, 21, 17, 15 TeV/c^2 for messenger
indices N=1, 2, 3, 4, 5 respectively, for all NLSP lifetimes.Comment: 4 pages, 2 figures. To appear in Proceedings of SUSY06, the 14th
International Conference on Supersymmetry and the Unification of Fundamental
Interactions, UC Irvine, California, 12-17 June 200
Flavour Independent hA Search and Two Higgs Doublet Model Interpretation of Neutral Higgs Boson Searches at LEP
Upper limits on the cross-section of the pair-production process e+e- -> h0A0
assuming 100% decays into hadrons, are derived from a new search for the h0A0
-> hadrons topology, independent of the hadronic flavour of the decay products.
Searches for the neutral Higgs bosons h0 and A0, are used to obtain constraints
on the Type II Two Higgs Doublet Model (2HDM(11)) with no CP violation in the
Higgs sector and no additional non Standard Model particles besides the five
Higgs bosons. The analysis combines LEP1 and LEP2 data collected with the OPAL
detctor up to the highest available centre-of-mass energies. The searches are
sensitive to the h0, A0 -> qq, gg,tau+tau- and h0 -> A0A0 decay modes of the
Higgs bosons. The 2HDM(II) parameter space is explored in a detailed scan.
Large regions of the 2HDM(II) parameter space are excluded at the 95% CL in the
(mh, mA), (mh, tanb) and (mA, tanb) planes, using both direct neutral Higgs
boson searches and indirect limits derived from Standard Model high precision
measurements. The region 1 lesssim mh lesssim 55 GeV and 3 lesssim mA lesssim
63 GeV is excluded at 95% CL independently of the choice of the 2HDM(II)
parameters.Comment: 37 pages, 11 figures, Submitted to Eur. Phys. J.
Genuine Correlations of Like-Sign Particles in Hadronic Z0 Decays
Correlations among hadrons with the same electric charge produced in Z0
decays are studied using the high statistics data collected from 1991 through
1995 with the OPAL detector at LEP. Normalized factorial cumulants up to fourth
order are used to measure genuine particle correlations as a function of the
size of phase space domains in rapidity, azimuthal angle and transverse
momentum. Both all-charge and like-sign particle combinations show strong
positive genuine correlations. One-dimensional cumulants initially increase
rapidly with decreasing size of the phase space cells but saturate quickly. In
contrast, cumulants in two- and three-dimensional domains continue to increase.
The strong rise of the cumulants for all-charge multiplets is increasingly
driven by that of like-sign multiplets. This points to the likely influence of
Bose-Einstein correlations. Some of the recently proposed algorithms to
simulate Bose-Einstein effects, implemented in the Monte Carlo model PYTHIA,
are found to reproduce reasonably well the measured second- and higher-order
correlations between particles with the same charge as well as those in
all-charge particle multiplets.Comment: 26 pages, 6 figures, Submitted to Phys. Lett.
A measurement of the tau mass and the first CPT test with tau leptons
We measure the mass of the tau lepton to be 1775.1+-1.6(stat)+-1.0(syst.) MeV
using tau pairs from Z0 decays. To test CPT invariance we compare the masses of
the positively and negatively charged tau leptons. The relative mass difference
is found to be smaller than 3.0 10^-3 at the 90% confidence level.Comment: 10 pages, 4 figures, Submitted to Phys. Letts.
First Measurement of Z/gamma* Production in Compton Scattering of Quasi-real Photons
We report the first observation of Z/gamma* production in Compton scattering
of quasi-real photons. This is a subprocess of the reaction e+e- to
e+e-Z/gamma*, where one of the final state electrons is undetected.
Approximately 55 pb-1 of data collected in the year 1997 at an e+e-
centre-of-mass energy of 183 GeV with the OPAL detector at LEP have been
analysed. The Z/gamma* from Compton scattering has been detected in the
hadronic decay channel. Within well defined kinematic bounds, we measure the
product of cross-section and Z/gamma* branching ratio to hadrons to be
(0.9+-0.3+-0.1) pb for events with a hadronic mass larger than 60 GeV,
dominated by (e)eZ production. In the hadronic mass region between 5 GeV and 60
GeV, dominated by (e)egamma* production, this product is found to be
(4.1+-1.6+-0.6) pb. Our results agree with the predictions of two Monte Carlo
event generators, grc4f and PYTHIA.Comment: 18 pages, LaTeX, 5 eps figures included, submitted to Physics Letters
Measurement of the B0 Lifetime and Oscillation Frequency using B0->D*+l-v decays
The lifetime and oscillation frequency of the B0 meson has been measured
using B0->D*+l-v decays recorded on the Z0 peak with the OPAL detector at LEP.
The D*+ -> D0pi+ decays were reconstructed using an inclusive technique and the
production flavour of the B0 mesons was determined using a combination of tags
from the rest of the event. The results t_B0 = 1.541 +- 0.028 +- 0.023 ps, Dm_d
= 0.497 +- 0.024 +- 0.025 ps-1 were obtained, where in each case the first
error is statistical and the second systematic.Comment: 17 pages, 4 figures, submitted to Phys. Lett.
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
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