The fraction of Z to bbbar events in hadronic Z decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The Z to bbbar decays were tagged using displaced secondary vertices, and high momentum electrons and muons. Systematic uncertainties were reduced by measuring the b-tagging efficiency using a double tagging technique. Efficiency correlations between opposite hemispheres of an event are small, and are well understood through comparisons between real and simulated data samples. A value of Rb = 0.2178 +- 0.0011 +- 0.0013 was obtained, where the first error is statistical and the second systematic. The uncertainty on Rc, the fraction of Z to ccbar events in hadronic Z decays, is not included in the errors. The dependence on Rc is Delta(Rb)/Rb = -0.056*Delta(Rc)/Rc where Delta(Rc) is the deviation of Rc from the value 0.172 predicted by the Standard Model. The result for Rb agrees with the value of 0.2155 +- 0.0003 predicted by the Standard Model.

The correlated production of Lambda and Lambdabar baryons has been studied using 4.3 million multihadronic Zo decays recorded with the OPAL detector at LEP. Di-lambda pairs were investigated in the full data sample and for the first time also in 2-jet and 3-jet events selected with the k_t algorithm. The distributions of rapidity differences from correlated Lambda-Lambdabar pairs exhibit short-range, local correlations and prove to be a sensitive tool to test models, particularly for 2-jet events. The JETSET model describes the data best but some extra parameter tuning is needed to improve agreement with the experimental results in the rates and the rapidity spectra simultaneously. The recently developed modification of JETSET, the MOdified Popcorn Scenarium (MOPS), and also HERWIG do not give satisfactory results. This study of di-lambda production in 2- and 3-jet events supports the short-range compensation of quantum numbers.

The helicity density matrix elements rho[00] of rho(770)+- and omega(782) mesons produced in Z decays have been measured using the OPAL detector at LEP. Over the measured meson energy range, the values are compatible with 1/3, corresponding to a statistical mix of helicity -1, 0 and +1 states. For the highest accessible scaled energy range 0.3 < x_E < 0.6, the measured rho[00] values of the rho(770)+- and the omega are 0.373 +- 0.052 and 0.142 +- 0.114, respectively. These results are compared to measurements of other vector mesons.

About 4.4 million hadronic decays of Z bosons, recorded by the OPAL detector at LEP at a centre-of-mass energy of around sqrt(s) = 91.2 GeV, are used to determine the mean charged particle multiplicities for the three light quark flavours. Events from primary u, d, and s quarks are tagged by selecting characteristic particles which carry a large fraction of the beam energy. The charged particle multiplicities are measured in the hemispheres opposite to these particles. An unfolding procedure is applied to obtain these multiplicities for each primary light quark flavour. This yields <n_u> = 17.77 +- 0.51 +0.86 -1.20, <n_d> = 21.44 +- 0.63 +1.46 -1.17, <n_s> = 20.02 +- 0.13 +0.39 -0.37, where statistical and systematic errors are given. The results for <n_u> and <n_d> are almost fully statistically anti-correlated. Within the errors the result is consistent with the flavour independence of the strong interaction for the particle multiplicities in events from the light up, down, and strange quarks.

The predicted effects of final state interactions such as colour reconnection are investigated by measuring properties of hadronic decays of W bosons, recorded at a centre-of-mass energy of sqrt(s)=182.7 GeV in the OPAL detector at LEP. Dependence on the modelling of hadronic W decays is avoided by comparing W+W- -> qqqq events with the non-leptonic component of W+W- -> qqlnu events. The scaled momentum distribution, its mean value, x_p, and that of the charged particle multiplicity, n_ch, are measured and found to be consistent in the two channels. The measured differences are: Diff(x_p) = +0.7 +- 0.8 +- 0.6 and Diff(n_ch) = (-0.09 +- 0.09 +-0.05)*10**-2. In addition, measurements of rapidity and thrust are performed for W+W- -> qqqq events. The data are described well by standard QCD models and disfavour one model of colour reconnection within the ARIADNE program. The current implementation of the ELLIS-GEIGER model of colour reconnection is excluded. At the current level of statistical precision no evidence for colour reconnection effects was found in the observables studied. The predicted effect of colour reconnection on OPAL measurements of M_W is also quantified in the context of models studied.

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.

Gluon jets are identified in hadronic Z0 decays as all the particles in a hemisphere opposite to a hemisphere containing two tagged quark jets. Gluon jets defined in this manner are equivalent to gluon jets produced from a color singlet point source and thus correspond to the definition employed for most theoretical calculations. In a separate stage of the analysis, we select quark jets in a manner to correspond to calculations, as the particles in hemispheres of flavor tagged light quark (uds) events. We present the distributions of rapidity, scaled energy, the logarithm of the momentum, and transverse momentum with respect to the jet axes, for charged particles in these gluon and quark jets. We also examine the charged particle multiplicity distributions of the jets in restricted intervals of rapidity. For soft particles at large transverse momentum, we observe the charged particle multiplicity ratio of gluon to quark jets to be 2.29 +- 0.09 +- 0.15 in agreement with the prediction that this ratio should approximately equal the ratio of QCD color factors, CA/CF = 2.25. The intervals used to define soft particles and large transverse momentum for this result, p<4 GeV/c and 0.8<p_t<3.0 GeV/c, are motivated by the predictions of the Herwig Monte Carlo multihadronic event generator. Additionally, our gluon jet data allow a sensitive test of the phenomenon of non-leading QCD terms known as color reconnection. We test the model of color reconnection implemented in the Ariadne Monte Carlo multihadronic event generator and find it to be disfavored by our data.

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.

The inclusive production of D*+- mesons in photon-photon collisions has been measured using the OPAL detector at LEP at e+e- centre-of-mass energies of 183 and 189GeV. The D* mesons are reconstructed in their decay to D0pi+ with the D0 observed in the two decay modes Kpi+ and Kpi+pi-pi+. After background subtraction, 100.4+-12.6(stat) D*+- mesons have been selected in events without observed scattered beam electron ("anti-tagged") and 29.8+-5.9 (stat) D*+- mesons in events where one beam electron is scattered into the detector ("single-tagged"). Direct and single-resolved events are studied separately. Differential cross-sections as functions of the D* transverse momentum p_t and pseudorapidity \eta are presented in the kinematic region 2<p_t<12GeV and \eta<1.5. They are compared to next-to-leading order (NLO) perturbative QCD calculations. The total cross-section for the process (e+e- to e+e-ccbar), where the charm quarks are produced in the collision of two quasi-real photons, is measured to be 842+-97(stat)+-75(syst)+-196(extrapolation)pb. A first measurement of the charm structure function F2 of the photon is performed in the kinematic range 0.0014<x<0.87 and 5<Q^2<100 GeV^2, and the result is compared to a NLO perturbative QCD calculation.

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.