To be updated
Physics analyses by the OPAL Bologna group.
The Bologna group participates in the OPAL Z0 Lineshape working group and in the
Luminosity working group.
In particular, the luminosity measurement was performed at the 0.5 per cent accuracy
level using the
Forward Detector
and at the 0.07 per cent level with the small angle
Silicon Tungsten
Calorimeter (the so called Luminometer).
This new subdetector is fundamental for accurate measurements of
the Z0 parameters.
In a high precision scan with centre-of-mass energies within 2 GeV from the Z
mass, the hadronic and leptonic cross sections and the leptonic for/back
asymmetries in electron-positron annichilation were measured. The mass and
width of the Z were determined with an uncertainty of a few MeV. The ratio of
the invisible to leptonic widths - a critical test of the EW couplings in the
Standard Model - was measured to be in excellent agreement with the SM
expectations.
The Bologna group partecipates in the OPAL Charm working group.
Studies are beeing performed about J/PSI mesons production, both from B
hadron decays and from charm fragmentation.
The separation of the two components
is done statistically and it is based on the long lifetime of B mesons.
J/Psi's are reconstructed throught their decay into a lepton pair.
Others charmonium states, like psi' and chi_c, have been reconstructed from
their decay respectively into J/Psi pi+ pi- and J/psi gamma.
A measure has been performed of the ratio Br(Z0--->psi'+X)/Br(Z0--->j/psi+X).
The results are consistent with the ones obtained by other
experiments both at LEP and at the Y(4S) energy.
A measurement of the B0-B0bar mixing was obtained from a sample of
hadronic decays of the Z.
Events were required to contain two lepton candidates,
either electrons or muons, in opposite hemispheres with respect to the
thrust direction.
The signal for B0_B0bar mixing was observed using the sign of the lepton
charge to tag the charge of the b quark in decaying b-flavoured hadrons.
A variable which discriminated the process b--lepton from the background
was construced from the lepton momentum and its component perpendicular
to the jet axis. By fitting the fraction of selected events for which the
two lepton charges were of the same sign, as a function of this variable,
the average mixing parameter was measured.
The Forward-Backward
asymmety in the Z0 --> b bbar decay is measured.
The relative long lifetime of hadrons containing a b-quark yelds significantly disp
displaced secondary vertices, with which one may enrich the sample of b-bbar
events.
A "Jet Charge" method, based on the charge distribution of the final
state particles, is used to determine the direction of the primary quark
produced in the Z0 decay.
This technique allows to measure the b-quark Forward-Backward asymmetry
with an uncertainty of about 8%.
The corresponding Effective Weinberg Angle is determined at the 0.6% level.
Light quark forward-backward asymmetries at the Z0 peak
are measured using high momentum charged tracks, identified as pions,
kaons or protons via dE/dx. Reliability of primary quark charge
determination is measured from data, using events with two opposite
charged tracks. Contamination in the sample by b and c decays of the Z0
is determined from experimental data using respecively secondary vertices
and high momentum D*.
A Study has been performed on Bhabha events in which both initial-state
electrons radiate a large fraction of their energy, using the OPAL
Forward Detector.
These doubly radiative events e+e- --> e+e- gamma gamma
represent a reference process to investigate the size of second order QED
contributions to the Bhabha cross section. The measured yield
of (0.31 +- 0.02) % is compared to the predictions of the Monte
Carlos used for the luminosity measurement at LEP.
The process e+e- --> e+e- mu+mu- has been studied.
Bose-Einstein correlations (BECs) manifest as enhancements in the production
of identical bosons which are close to one another in phase space and allow
to probe the space-time structure of the source emitting particles.
Several analyses of BECs in pairs of pions with the same charge have
been done, based on the high statistics sample of Z0 hadronic decays.
The dimension of the region of homogeneity ("radius" of the emitting source)
has been measured to extend to about 1 fm [1] and to depend slightly on the
charged multiplicity of the hadronic final state [2].
BECs have also been studied as a function of the three components of the
momentum difference, transverse ("out" and "side") and longitudinal with
respect to the thrust direction of the event.
A significant difference between the transverse and longitudinal dimensions
has been observed, indicating that the emitting source of identical pions is
not spherical but is elongated in shape [3].
In addition, three dimensional Bose-Einstein correlation functions have
been analysed in intervals of the average pair transverse momentum and of
the pair rapidity.
The Yano-Koonin and the Bertsch-Pratt parameterizations have
been fit to the measured correlation functions. The rapidity of each source
element scales approximately with the rapidity of the pion pairs emitted by
the source element. Both the longitudinal and transverse source dimensions
decrease for increasing average pair transverse momenta. These findings
point to a pion source which is not static but rather expands during the particle
emission process [4].
References:
1. "A study of Bose-Einstein correlations in e+e- annihilations at LEP".
P.D. Acton et al., Phys. Lett. B267 (1991) 143-153 (OPAL pub. PR042)
2. "Multiplicity dependence of Bose-Einstein correlations in hadronic Z0 decays".
G.Alexander et al., Zeit. fur Physik C72 (1996) 389-398 (OPAL pub. PR164)
3. "Transverse and longitudinal Bose-Einstein correlations in hadronic Z0 decays".
G. Abbiendi et al., Eur. Phys. J. C16 (2000) 423-433 (OPAL pub. PR302)
4. "Bose-Einstein study of position-momentum correlations of charged pions in hadronic Z0 decays".
G. Abbiendi et al., Submitted to Eur. Phys J.C. (OPAL pub. PR423)
The Bologna group is involved in several aspects of jet production in
hadronic Z0 decays, measurements of the strong coupling costant,
alpha_s, on charge multiplicity and
transverse momentum correlations.
The strong coupling constant, alpha_s(M_Z0), was determined with
a relative uncertainty of about 5% from a study of several observables in
hadronic Z0 decays: event shape variables, lineshape and decay asymmetries,
hadronic branching fractions of tau leptons.
The good agreement among the
values obtained from different variables is an important consistency
check of perturbative QCD. The relatively small uncertainty
of these measurements, together with previous results at lower
energies, provided a first clear evidence of the running character of alpha_s.
The average number of charged hadrons produced in positron-electron collisions
at 91 GeV centre of mass energy is 21.4+-0.04.
The shape of the multiplicity distribution is well
described by QCD based parton shower models.
Long range "forward-backward" correlations between the number of charged
hadrons produced in two opposite event hemispheres and the dependence
of the average transverse momentum of the produced charged hadrons on the
charged multiplicity have been studied. In both cases exist positive
correlations, which are understood in terms of known phenomena, like
hard gluon radiation, decay of resonances and higher
mean multiplicities for events initiated by heavy quarks.
Searches for free gluons have been performed as well as for particles
with anomalous ionization.
A search for free gluons was performed by looking at interactions of
electrally neutral particles in the
hadron calorimeter.
The negative result was interpretedin terms of the parameters of a
phenomenological model.
Limits were obtained for the production of fractionally-charged particles with
Q/e=2/3 and 4/3 as well as for particles with Q/e=2. The limits, at the
level of 10**-4 of the muon production cross section, are relative to masses
up to mZ/2. Limits on charge one particles are used to obtain limits on
charged gluino-quark-quark composites of low mass.
Last modified: Tue Jan 5 12:20:59 MET
Z0 Lineshape and Luminosity measurement.