Study of dimuon production in photon-photon collisions and measurement of QED photon structure functions at LEP

Muon pair production in the process is studied using the data taken at LEP1 () with the DELPHI detector during the years 1992-1995. The corresponding integrated luminosity is 138.5 pb. The QED predictions have been tested over the whole range accessible at LEP1 (from several GeV to several hundred GeV) by comparing experimental distributions with distributions resulting from Monte Carlo simulations using various generators. Selected events are used to extract the leptonic photon structure function . Azimuthal correlations are used to obtain information on additional structure functions, and , which originate from interference terms of the scattering amplitudes. The measured ratios and are significantly different from zero and consistent with QED predictions. Received: 23 February 2000 / Revised version: 7 December 2000 / Published online: 23 February 2001     

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     

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     

Photon and light meson production in hadronic Z^0decays

The inclusive production rates and differential cross-sections of photons and mesons with a final state containing photons have been measured with the OPAL detector at LEP. The light mesons covered by the measurements are the , , , , and a. The particle multiplicities per hadronic Zdecay, extrapolated to the full energy range, are: where the first errors are statistical and the second systematic. In general, the results are in agreement with the predictions of the JETSET and HERWIG Monte Carlo models. Received: 9 April 1998 / Published online: 12 August 1998     

Spin correlations in e^+e^-\to 4 jets

Existence of discrepancies between experimental data and Monte Carlo predictions for angular distributions in four-jet production via annihilation has been known for some time at LEP1. As such QCD processes constitute significant backgrounds to -jet production at LEP2, we consider the possibility that an erroneous modelling of the helicity structure of the final-state partons could affect the accuracy of experimental measurements of the boson parameters. Received: 21 September 1998 / Revised version: 20 January 1999 / Published online: 22 March 1999     

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     

Optimized second order QCD versus parton shower models ine + e − annihilation

We compare Monte Carlo models for hadronic final states, in which the perturbative phase is either based on the exact second order QCD matrix element (ME) or on parton shower evolution using the Leading Log Approximation (LLA). We find that up to present LEP energies both models are in agreement with each other and the data. It is important, thatO( _s ² ) ME Monte Carlos can be brought into agreement with the popular and successful parton shower models, since in contrast to the LLA the ME gives a correct treatment of hard gluon radiation, which allows quantitative studies like the determination of the strong coupling constant.     

QCD studies with e+e− annihilation data at 161 GeV

We have studied hadronic events produced at LEP at a centre-of-mass energy of 161 GeV. We present distributions of event shape variables, jet rates, charged particle momentum spectra and multiplicities. We determine the strong coupling strength to be α_s(161 GeV) = 0.101±0.005(stat.)±0.007(syst.), the mean charged particle multiplicity to be 〈n _ch〉(161 GeV) = 24.46 ± 0.45(stat.) ± 0.44(syst.) and the position of the peak in the ξ_p = ln(1/x _p) distribution to be ξ₀(161 GeV) = 4.00 ±0.03(stat.)±0.04(syst.). These results are compared to data taken at lower centre-of-mass energies and to analytic QCD or Monte Carlo predictions. Our measured value of α_s(161 GeV) is consistent with other measurements of α_s. Within the current statistical and systematic uncertainties, the PYTHIA, HERWIG and ARIADNE QCD Monte Carlo models and analytic calculations are in overall agreement with our measurements. The COJETS QCD Monte Carlo is in general agreement with the data for momentum weighted distributions like Thrust, but predicts a significantly larger charged particle multiplicity than is observed experimentally.     

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     

Uncertainties on the measurements of the top mass at a future e+e- collider

The uncertainties due to limited knowledge of the multi-hadron final state on the measurements of the top mass at future linear colliders are discussed. This study is performed for annihilation events at the center-of-mass energy of s ^1/2 =500 GeV using Monte Carlo models tuned to LEP experiments. The uncertainties are determined for the all-hadronic top-decay mode as well as for the lepton-plus-jets channel.     

String effects on Fermi–Dirac correlation measurements

We investigate some recent measurements of Fermi–Dirac correlations by the LEP collaborations indicating surprisingly small source radii for the production of baryons in e⁺e^- annihilation at the Z⁰ peak. In hadronization models there is besides the Fermi–Dirac correlation effect also a strong dynamical (anti-) correlation. We demonstrate that the extraction of the pure FD effect is highly dependent on a realistic Monte Carlo event generator, both for separation of those dynamical correlations that are not related to Fermi–Dirac statistics, and for corrections of the data and background subtractions. Although the model can be tuned to well reproduce single particle distributions, there are large model uncertainties when it comes to correlations between identical baryons. We therefore, unfortunately, have to conclude that it is at present not possible to draw any firm conclusion about the source radii relevant for baryon production at LEP. PACS 13.60.Rj; 13.66.Bc     

Radiative four-fermion processes at LEP2

The production of four fermions plus a visible photon in electron–positron collisions is analyzed, with particular emphasis on the LEP2 energy range. The study is based on the calculation of exact matrix elements, including the effect of fermion masses. In the light of the present measurements performed at LEP, triple and quartic anomalous gauge couplings are taken into account. Due to the presence of a visible photon in the final state, particular attention is paid to the treatment of higher-order QED corrections. Explicit results for integrated cross sections and differential distributions are shown and commented on. The features of the Monte Carlo program WRAP, used to perform the calculation and available for experimental analysis, are described. Received: 14 March 2001 / Revised version: 26 June 2001 / Published online: 3 August 2001     

Dirac and Majorana heavy neutrinos at LEP II

The possibility of detecting single heavy Dirac and/or Majorana neutrinos at LEP II is investigated for heavy neutrino masses in the range . We study the process as a clear signature for heavy neutrinos. Numerical estimates for cross sections and distributions for the signal and the background are calculated and a Monte Carlo reconstruction of final state particles after hadronization is presented. Received: 20 September 2001 / Published online: 12 November 2001     

Monte Carlo simulation of quantum statistical lattice models

In this article we review recent developments in computational methods for quantum statistical lattice problems. We begin by giving the necessary mathematical basis, the generalized Trotter formula, and discuss the computational tools, exact summations and Monte Carlo simulation, that will be used to examine explicit examples. To illustrate the general strategy, the method is applied to an analytically solvable, non-trivial, model: the one-dimensional Ising model in a transverse field. Next it is shown how to generalized Trotter formula most naturally leads to different path-integral representations of the partition function by considering one-dimensional fermion lattice models. We show how to analyze the different representations and discuss Monte Carlo simulation results for one-dimensional fermions. Then Monte Carlo work on one- and two-dimensional spin-$\text{1}\text{2}$ models based upon the Trotter formula approach is reviewed and the more dedicated Handscomb Monte Carlo method is discussed. We consider electron-phonon models and discuss Monte Carlo simulation data on the Molecular Crystal Model in one, two and three dimensions and related one-dimensional polaron models. Exact numerical results are presented for free fermions and free bosons in the canonical ensemble. We address the main problem of Monte Carlo simulations of fermions in more than one dimension: the cancellation of large contributions. Free bosons on a lattice are compared with bosons in a box and the effects of finite size on Bose-Einstein condensation are discussed.     

Measurement of the mass of the W boson in <Emphasis Type="Italic">ee</Emphasis> collisions using the fully leptonic channel

A novel method of determining the mass of the W boson in the channel is presented and applied to 667 pb^-1 of data recorded at center-of-mass energies in the range 183-207 GeV with the OPAL detector at LEP. The measured energies of charged leptons and the results of a new procedure based on an approximate kinematic reconstruction of the events are combined to give: where the first error is statistical and the second is systematic. The systematic error is dominated by the uncertainty on the lepton energy, which is calibrated using data, and the parameterization of the variables used in the fitting, which is obtained using Monte Carlo events. Both of these are limited by statistics. Received: 13 March 2002 / Published online: 31 October 2002     

Intermittency and correlations in hadronic Z^0 decays

A multidimensional study of local multiplicity fluctuations and multiparticle correlations of hadrons produced in Z decays is performed. The study is based on the data sample of more than events recorded with the OPAL detector at LEP. The fluctuations and correlations are analysed in terms of the normalized scaled factorial moments and cumulants up to the fifth order. The moments are observed to have intermittency-like behaviour, which is found to be more pronounced with increasing dimension. The large data sample allows for the first time a study of the factorial cumulants in annihilation. The analysis of the cumulants shows the existence of genuine multiparticle correlations with a strong intermittency rise up to higher orders. These correlations are found to be stronger in higher dimensions. The decomposition of the factorial moments into lower-order correlations shows that the dynamical fluctuations have important contributions from genuine many-particle correlations. The Monte Carlo models Jetset 7.4 and Herwig 5.9 are found to reproduce the trend of the measured moments and cumulants but they underestimate the magnitudes. The results are found to be consistent with QCD jet formation dynamics, although additional contributions from other mechanisms cannot be excluded. Received: 8 February 1999 / Published online: 8 September 1999     

Measurement of the Michel parameters in leptonic tau decays

The Michel parameters of the leptonic decays are measured using the OPAL detector at LEP. The parameters , , (with ) and are extracted from the energy spectra of the charged decay leptons and from their energy–energy correlations. A new method involving a global likelihood fit of Monte Carlo generated events with complete detector simulation and background treatment has been applied to the data recorded at center-of-mass energies close to corresponding to an integrated luminosity of 155 pb. If e- universality is assumed and inferring the polarization from neutral current data, the measured Michel parameters are: where the value of has been constrained using the published OPAL measurements of the leptonic branching ratios and the lifetime. Limits on non-standard coupling constants and on the masses of new gauge bosons are obtained. The results are in agreement with the V–A prediction of the Standard Model. Received: 25 June 1998 / Published online: 1 March 1999     

Investigations of quark fragmentation universality

We propose event cuts in deep inelastic scattering, suitable for an examination of quark fragmentation universality. We compare the current hemisphere of the Breit frame with the hemispheres in annihilation events, using Monte Carlo simulations. The agreement between the two processes is improved after the suggested event cuts are introduced. A method to study the scale evolution in the quark hemispheres using data from fixed-energy experiments is presented. This makes it possible to use the high statistics from LEP1 also at scales below the mass. We also discuss observables which are sensitive to the dynamics of regions closer to the remnant. The observables probe the relatively clean region on the current side of the hardest emission in the event, and can be used to distinguish between different assumptions about remnant effects and mechanisms for the parton evolution. Received: 3 November 1998 / Revised version: 25 January 1999 / Published online: 30 June 1999     

Systematic event generator tuning for the LHC

In this article we describe Professor, a new program for tuning model parameters of Monte Carlo event generators to experimental data by parameterising the per-bin generator response to parameter variations and numerically optimising the parameterised behaviour. Simulated experimental analysis data is obtained using the Rivet analysis toolkit. This paper presents the Professor procedure and implementation, illustrated with the application of the method to tunes of the Pythia 6 event generator to data from the LEP/SLD and Tevatron experiments. These tunes are substantial improvements on existing standard choices, and are recommended as base tunes for LHC experiments, to be themselves systematically improved upon when early LHC data is available.