Multi-photon production in ee collisions at \sqrt{s} = 181-209 GeV

The process is studied using data collected by the OPAL detector at LEP between the years 1997 and 2000. The data set corresponds to an integrated luminosity of 672.3 pb^-1 at centre-of-mass energies lying between 181 GeV and 209 GeV. Total and differential cross-sections are determined and found to be in good agreement with the predictions of QED. Fits to the observed angular distributions are used to set limits on parameters from several models of physics beyond the Standard Model such as cut-off parameters, contact interactions of the type , gravity in extra spatial dimensions and excited electrons. In events with three photons in the final state the mass spectrum of photon pairs is investigated. No narrow resonance is found and limits are placed on the product of the production cross-section and branching ratio. Received: 22 July 2002 / Published online: 8 November 2002     

Comparison of deep-inelastic electron-photon scattering data with the HERWIG and PHOJET Monte Carlo Models

Deep-inelastic electron-photon scattering is studied in the range from 1.2 to 30 GeV using the LEP1 data taken with the ALEPH, L3 and OPAL detectors at centre-of-mass energies close to the mass of the Z boson. Distributions of the measured hadronic final state are corrected to the hadron level and compared to the predictions of the HERWIG and PHOJET Monte Carlo models. For large regions in most of the distributions studied the results of the different experiments agree with one another. However, significant differences are found between the data and the models. Therefore the combined LEP data serve as an important input to improve on the Monte Carlo models. Received: 5 July 2000 / Published online: 15 February 2002     

Tests of the standard model and constraints on new physics from measurements of fermion-pair production at 189 GeV at LEP

Cross-sections and angular distributions for hadronic and lepton pair final states in collisions at a centre-of-mass energy near 189 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. The results are used to measure the energy dependence of the electromagnetic coupling constant , and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a sneutrino in supersymmetric theories with R-parity violation. A search for the indirect effects of the gravitational interaction in extra dimensions on the and final states is also presented. Received: 9 July 1999 / Published online: 17 February 2000     

Higher-order QED corrections to single-W production in electron–positron collisions

Four-fermion processes with a particle lost in the beam pipe are studied at LEP to perform precision tests of the electroweak theory. Leading higher-order QED corrections to such processes are analyzed within the framework of the structure function (SF) approach. The energy scale entering the QED SF is determined by inspection of the soft and collinear limit of the radiative corrections to the four-fermion final states, paying particular attention to the process of single-W production. Numerical predictions are shown in realistic situations for LEP experiments and compared with existing results. A Monte Carlo event generator, including exact tree-level matrix elements, vacuum polarization, higher-order leading QED corrections and anomalous trilinear gauge couplings, is presented. Received: 5 October 2000 / Published online: 11 May 2001     

Tests of the standard model and constraints on new physics from measurements of fermion-pair production at 189-209 GeV at LEP

Cross-sections and angular distributions for hadronic and lepton-pair final states in e ⁺ e^- collisions at centre-of-mass energies between 189 GeV and 209 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. The measurements are used to determine the electromagnetic coupling constant at LEP 2 energies. In addition, the results are used together with OPAL measurements at 91-183 GeV within the S-matrix formalism to determine the -Z interference term and to make an almost model-independent measurement of the Z mass. Limits on extensions to the Standard Model described by effective four-fermion contact interactions or the addition of a heavy Z boson are also presented.Received: 28 August 2003, Published online: 18 February 2004     

Measurement of the low-x behaviour of the photon structure function F_2^\gamma

The photon structure function has been measured using data taken by the OPAL detector at centre-of-mass energies of 91 GeV, 183 GeVand 189 \GeV, in ranges of 1.5–30.0 GeV (LEP1), and 7.0–30.0 GeV (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement. The results do not conclusively prove, but are completely consistent with, the presence of a rise in at low-x as expected from QCD. Received: 9 June 2000 / Published online: 13 November 2000     

Measurements of the QED structure of the photon

The structure of both quasi-real and highly virtual photons is investigated using the reaction , proceeding via the exchange of two photons. The results are based on the complete OPAL dataset taken at centre-of-mass energies close to the mass of the Z boson. The QED structure function and the differential cross-section for quasi-real photons are obtained as functions of the fractional momentum x from the muon momentum which is carried by the struck muon in the quasi-real photon for values of ranging from 1.5 to 400 GeV. The differential cross-section for highly virtual photons is measured for GeV and GeV, where and are the negative values of the four-momentum squared of the two photons such that . Based on azimuthal correlations the QED structure functions and for quasi-real photons are determined for an average of 5.4 GeV. Received: 8 February 1999 / Published online: 28 September 1999     

Bosonic quartic couplings at LEP2

We list the set of and conserving anomalous quartic vector bosons self-couplings which can be tested at LEP2 through triple vector boson production. We show how this set can be embedded in manifestly gauge invariant operators exhibiting an global symmetry. We derive bounds on these various couplings and show the most relevant distributions that can enhance their contribution. We also find that an collider running at 500 GeV can improve the LEP2 limits by as much as three-orders of magnitude. Received: 9 September 1999 / Revised version: 19 October 1999 / Published online: 17 February 2000     

Studies of QCD at ${\rm e}^ + {\rm e}^-$ centre-of-mass energies between 91 and 209 GeV

The hadronic final states observed with the ALEPH detector at LEP in annihilation are analysed using 730 pb^-1 of data collected between 91 and 209 GeV in the framework of QCD. In particular event-shape variables and inclusive charged particle spectra are measured. The energy evolution of quantities derived from these measurements is compared to analytic QCD predictions. The mean charged particle multiplicity, the charged particle momentum spectrum and its peak position are compared to predictions of the modified-leading-logarithmic approximation. The strong coupling constant is determined from a fit of the QCD prediction to distributions of six event-shape variables at eight centre-of-mass energies. A study of non-perturbative power law corrections is presented.Received: 18 May 2004, Published online: 23 June 2004     

Di-Jet production in photon-photon collisions at \sqrt{s}_{\rm ee}=161 and 172 GeV

Di-jet production is studied in collisions of quasi-real photons radiated by the LEP beams at ee centre-of-mass energies 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 di-jet cross-section is measured as a function of and and compared to next-to-leading order perturbative QCD calculations. The inclusive di-jet cross-section as a function of 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. Received: 15 July 1999 / Published online: 12 August 1999     

Photoproduction of positive pions from hydrogen at large angles in the energy range 0.3–2.0 GeV

The differential cross-section for the reaction γp↦π ⁺ n was measured using the bremsstrahlung beam of the Bonn 2.5 GeV electron synchrotron. The pions were detected and momentum analysed in a multichannel magnet spectrometer. Data reduction resulted in 1278 measured cross-sections which are presented as energy distributions at six laboratory angles between 180° and 95°. The range of laboratory photon energies extended from 0.3 to 2.1 GeV. The statistical accuracy is better than 3 percent, the systematic error is about 5 percent. The data are compared with other experimental results and predictions of a phenomenological analysis. These cross-sections are the result of a measurement program which was started in the seventies. Further results will be reported in forthcoming publications. Received: 2 February 2000 / Accepted: 14 August 2001     

Study of tau-pair production in photon-photon collisions at LEP and limits on the anomalous electromagnetic moments of the tau lepton

Tau-pair production in the process was studied using data collected by the DELPHI experiment at LEP2 during the years 1997 - 2000. The corresponding integrated luminosity is 650 pb^-1. The values of the cross-section obtained are found to be in agreement with QED predictions. Limits on the anomalous magnetic and electric dipole moments of the tau lepton are deduced.Received: 18 September 2003, Revised: 26 March 2004, Published online: 11 May 2004     

Precision luminosity for Z^0 lineshape measurements with a silicon-tungsten calorimeter

The measurement of small–angle Bhabha scattering is used to determine the luminosity at the OPAL interaction point for the LEP I data recorded between 1993 and 1995. The measurement is based on the OPAL Silicon-Tungsten Luminometer which is composed of two calorimeters encircling the LEP beam pipe, on opposite sides of the interaction point. The luminometer detects electrons from small–angle Bhabha scattering at angles between 25 and 58 mrad. At LEP center-of-mass energies around the Z, about half of all Bhabha electrons entering the detector fall within a 79 nb fiducial acceptance region. The electromagnetic showers generated in the stack of 1 radiation length tungsten absorber plates are sampled by 608 silicon detectors with 38,912 radial pads of 2.5 mm width. The fine segmentation of the detector, combined with the precise knowledge of its physical dimensions, allows the trajectories of incoming 45 GeV electrons or photons to be determined with a total systematic error of less than 7 microns. We have quantified all significant sources of systematic experimental error in the luminosity determination by direct physical measurement. All measured properites of the luminosity event sample are found to be in agreement with current theoretical expectations. The total systematic measurement uncertainty is , significantly below the theoretical error of currently assigned to the QED calculation of the Bhabha acceptance, and contributes negligibly to the total uncertainty in the OPAL measurement of , a quantity of basic physical interest which depends crucially on the luminosity measurement. Received: 28 September 1999 / Published online: 14 April 2000     

Measurement of Z/\gamma^* production in compton scattering of quasi-real photons

The process is studied with the OPAL detector at LEP at a centre of mass energy of = 189 GeV. The cross-section times the branching ratio of the Z/ decaying into hadrons is measured within Lorentz invariant kinematic limits to be pb for invariant masses of the hadronic system between 5 GeV and 60 GeV and pb for hadronic masses above 60 GeV. The differential cross-sections of the Mandelstam variables , and are measured and compared with the predictions from the Monte Carlo generators grc4f and PYTHIA. From this, based on a factorisation ansatz, the total and differential cross-sections for the subprocess are derived. Received: 16 July 2001 / Published online: 5 April 2002     

Measurement of event shape and inclusive distributions at ${sqrt s} = 130$ and 136 GeV

Inclusive charged particle and event shape distributions are measured using 321 hadronic events collected with the DELPHI experiment at LEP at effective centre of mass energies of 130 to 136 GeV. These distributions are presented and compared to data at lower energies, in particular to the precise Z data. Fragmentation models describe the observed changes of the distributions well. The energy dependence of the means of the event shape variables can also be described using second order QCD plus power terms. A method independent of fragmentation model corrections is used to determine α_s from the energy dependence of the mean thrust and heavy jet mass. It is measured to be: $$←pha _s(133 {? GeV})={0.116}pm {0.007}_{exp-0.004theo}^{+0.005}$$ from the high energy data.     

Measurement of the W mass and width in e^+e^- collisions at 189 GeV

The mass of the W boson is determined in ee collisions at LEP by the direct reconstruction of W decays in and events, supplemented by measurements using the kinematic properties of the leptons in the decay channel. The main sample of W pairs is selected from an integrated luminosity of 174 pb collected with the ALEPH detector in 1998 at a centre-of-mass energy of 188.63 GeV. The combined result from all channels is where FSI represents the possible effects of final state interactions in the channel. In a second two-parameter fit to the , and channels, where the W mass and width are decoupled, the average W width is found to be { GeV/}, consistent with the Standard Model prediction. The combination of the mass measurement presented in this paper together with those derived previously from the W pair cross section at 161 and 172 GeV and direct reconstruction at 172 and 183 GeV gives Received: 15 March 2000 / Published online: 8 September 2000     

Charged particle momentum spectra in ee annihilation at $\sqrt{s} = 192$-209 GeV

Charged particle momentum distributions are studied in the reaction hadrons, using data collected with the OPAL detector at centre-of-mass energies from 192 GeV to 209 GeV. The data correspond to an average centre-of-mass energy of 201.7 GeV and a total integrated luminosity of 433 pb^-1. The measured distributions and derived quantities, in combination with corresponding results obtained at lower centre-of-mass energies, are compared to QCD predictions in various theoretical approaches to study the energy dependence of the strong interaction and to test QCD as the theory describing it. In general, a good agreement is found between the measurements and the corresponding QCD predictions. Received: 17 July 2002 / Published online: 3 March 2003     

Total hadronic cross-section of photon-photon interactions at LEP

The total hadronic cross-section for the interaction of real photons, hadrons, is measured for centre-of-mass energies GeV. The cross-section is extracted from a measurement of the process hadrons, using a luminosity function for the photon flux together with form factors for extrapolating to real photons ( GeV). The data were taken with the OPAL detector at LEP at centre-of-mass energies , 172$ and 183 GeV. The cross-section is compared with Regge factorisation and with the energy dependence observed in p and pp interactions. The data are also compared to models which predict a faster rise of compared to p and pp interactions due to additional hard interactions not present in hadronic collisions. Received: 3 June 1999 / Published online: 6 April 2000     

\mathrm{W^+W^-} production and triple gauge boson couplings at LEP energies up to 183 GeV

A study of W-pair production in annihilations at LEP is presented, based on candidates corresponding to an integrated luminosity of 57 pb at GeV. Assuming that the angular distributions of the W-pair production and decay, as well as their branching fractions, are described by the Standard Model, the W-pair production cross-section is measured to be pb. Assuming lepton universality and combining with our results from lower centre-of-mass energies, the W branching fraction to hadrons is determined to be %. The number of W-pair candidates and the angular distributions for each final state (, are used to determine the triple gauge boson couplings. After combining these values with our results from lower centre-of-mass energies we obtain , and , where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to the Standard Model value. The fraction of W bosons produced with a longitudinal polarisation is measured to be . All these measurements are consistent with the Standard Model expectations. Received: 23 October 1998 / Published online: 1 March 1999     

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 and predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the and production channels is based on data corresponding to an integrated luminosity of 25 pb from 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 resonance to set limits on and in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for , minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits GeV and GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan. Received: 20 February 1998 / Published online: 13 July 1998