arXiv subjects

G. Abbiendi

Publications and source records attributed to G. Abbiendi.

123 records · Page 7Linked to original sources

arXiv2000

Bose-Einstein correlations in charged kaon pairs from Z0 decays into two hadronic jets

Bose-Einstein correlations in pairs of charged kaons produced in a sample of 3.9 million hadronic Z0 decays have been measured with the OPAL experiment at LEP. Charged kaons were identified in the central tracking detector using their specific energy loss in the drift chamber gas. The correlation function was studied in two-jet events using a double ratio, formed by the number of like-sign pairs normalised by a reference sample in the data, divided by the same ratio in a Monte Carlo simulation. The enhancement at small values of the four-momentum difference of the pair was parametrised using a Gaussian function. The parameters of the Bose-Einstein correlations were measured to be lambda = 0.82+-0.22(+0.17,-0.12) for the strength and R0 = 0.56+-0.08(+0.08,-0.06) fm for the kaon source radius, where the first errors are statistical and the second systematic. Corrections for final-state interactions are discussed.

hep-ex↗

arXiv1999

Precision Luminosity for Z0 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 58mrad. At LEP center-of-mass energies around the Z0, about half of all Bhabha electrons entering the detector fall within a 79nb 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 2.5mm width.The fine segmentation of the detector, combined with the precise knowledge of its physical dimensions, allows the trajectories of incoming 45GeV electrons or photons to be determined with a total systematic error of less than 7microns. 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 3.4x10^-4, significantly below the theoretical error of 5.4x10^-4 currently assigned to the QED calculation of the Bhabha acceptance, and contributes negligibly to the total uncertainty in the OPAL measurement of Gamma{invisible}/Gamma{l+l-}, a quantity of basic physical interest which depends crucially on the luminosity measurement.

hep-ex↗

arXiv1999

Tau decays with neutral kaons

The branching ratio of the tau lepton to a neutral K meson is measured from a sample of approximately 200,000 tau decays recorded by the OPAL detector at centre-of-mass energies near the Z0 resonance. The measurement is based on two samples which identify one-prong tau decays with KL and KS mesons. The combined branching ratios are measured to be B(tau- -->pi- K0bar nutau) = (9.33+-0.68+-0.49)x10^-3 B(tau- -->pi- K0bar [>=1pi0] nutau) = (3.24+-0.74+-0.66)x10^-3 B(tau- -->K- K0bar [>=0pi0] nutau) = (3.30+-0.55+-0.39)x10^-3 where the first error is statistical and the second systematic.

hep-ex↗

arXiv1999

Inclusive production of D*+- mesons in photon-photon collisions at sqrt(s) = 183 and 189 GeV and a first measurement of the the charm structure function of the photon

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 ηare presented in the kinematic region 2<p_t<12GeV and η<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.

hep-ex↗

arXiv1999

Total Hadronic Cross-Section of Photon-Photon Interactions at LEP

The total hadronic cross-section sigma_gg(W) for the interaction of real photons, gg->hadrons, is measured for gg centre-of-mass energies 10 e+e-g*g* -> e+e- hardrons, using a luminosity function for the photon flux together with form factors for extrapolating to real photons (Q^2=0 GeV^2). The data were taken with the OPAL detector at LEP at e+e- centre-of-mass energies 161, 172 and 183 GeV. The cross-section sigma_gg(W) is compared with Regge factorisation and with the energy dependence observed in gp and pp interactions. The data are also compared to models which predict a faster rise of sigma_gg(W) compared to gp and pp interactions due to additional hard gg interactions not present in hadronic collisions.

hep-ex↗

arXiv1999

Measurement of the W+W-gamma Cross-section and First direct Limits on Anomalous Electroweak Quartic Gauge Couplings

A study of W+W- events accompanied by hard photon radiation produced in e+e- collisions at LEP is presented. Events consistent with two on-shell W-bosons and an isolated photon are selected from 183pb^-1 of data recorded at root{s}=189GeV. From these data, 17 W+W-gamma candidates are selected with photon energy greater than 10GeV, consistent with the Standard Model expectation. These events are used to measure the e+e- to W+W-gamma cross-section within a set of geometric and kinematic cuts; sigma{W+W-gamma} = 136+-37+-8 fb, where the first error is statistical and the second systematic. The photon energy spectrum is used to set the first direct, albeit weak, limits on possible anomalous contributions to the {W+ W- gamma gamma} and {W+ W- gamma Z0} vertices: -0.070GeV^{-2} < a_0/Lambda^2 < 0.070GeV^{-2}, -0.13GeV^{-2} < a_c/Lambda^2 < 0.19GeV^{-2}, -0.61GeV^{-2} < a_n/Lambda^2 < 0.57GeV^{-2}, where Lambda represents the energy scale for new physics.

hep-ex↗

arXiv1999

Search for Chargino and Neutralino Production at sqrt{s} = 189 GeV at LEP

A search for charginos and neutralinos, predicted by supersymmetric theories, is performed using a data sample of 182.1 pb-1 taken at a centre-of-mass energy of 189 GeV with the OPAL detector at LEP. No evidence for chargino or neutralino production is found. Upper limits on chargino and neutralino pair production cross-sections are obtained as a function of the chargino mass, the lightest neutralino mass and the second lightest neutralino mass. Within the Constrained Minimal Supersymmetric Standard Model framework, and for a chargino - neutralino mass difference of more than 5 GeV, the 95% confidence level lower limits on the chargino mass are 93.6 GeV for tan{beta} = 1.5 and 94.1 GeV for tan{beta} = 35. These limits are obtained assuming a universal scalar mass m_0 > 500 GeV. The corresponding limits for all m_0 are 78.0 and 71.7 GeV. The 95% confidence level lower limits on the lightest neutralino mass, valid for any value of tan{beta} are 32.8 GeV for m_0 > 500 GeV and 31.6 GeV for all m_0.

hep-ex↗

arXiv1999

Search for Anomalous Production of Acoplanar Di-lepton Events in e+e- Collisions at sqrt{s} = 183 and 189 GeV

A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 237.4 pb-1 at e+e- centre-of-mass energies of 183 GeV and 189 GeV. The observed numbers of events - 78 at 183 GeV and 301 at 189 GeV - are consistent with the numbers expected from Standard Model processes, which arise predominantly from W+W- production with each W decaying leptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Discrimination techniques are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is apparent and model independent limits are presented on the production cross-section times branching ratio squared for sleptons and for leptonically decaying charginos and charged Higgs. Assuming a 100 % branching ratio for the decay of a slepton to a lepton and the lightest neutralino, we exclude at 95 % CL: right-handed smuons with masses below 82.3 GeV for a smuon-neutralino mass difference > 3 Gev and right-handed staus with masses below 81.0 GeV for a stau-neutralino mass difference > 8 GeV. Right-handed selectrons are excluded at 95% CL for masses below 87.1 GeV for a selectron-neutralino mass diffeerence > 5GeV, within the framework of the Minimal Supersymmetric Standard Model assuming mu < -100 GeV and tan{beta} = 1.5. Charged Higgs bosons are excluded at 95% CL for masses below 82.8GeV, assuming a 100% branching ratio for their decay to tau+neutrino.

hep-ex↗

arXiv1999

A Study of Three-Prong Tau Decays with Charged Kaons

From an analysis of the ionisation energy loss of charged particles selected from a sample of 147926 e+e- -> tau+tau- candidates recorded in the OPAL detector at e+e- centre-of-mass energies near the Z0 resonance, we determine the branching ratios: BR(tau- -> nu_tau K- pi- pi+ (pi0)) = 0.343 +- 0.073 +- 0.031 % BR(tau- -> nu_tau K- pi- K+ (pi0)) = 0.159 +- 0.053 +- 0.020 % where the (pi0) notation refers to decay modes with or without an accompanying pizero. The tau- -> nu_tau K- pi- pi+ (pi0) final states occurring through tau- -> nu_tau K- K_s (pi0) are treated as background in this analysis. We also examine the resonant structure of tau- -> nu_tau K- pi- pi+ candidates. Under the assumption that the resonant structure is dominated by the K_1 resonances, we determine the ratio of the BR (tau- -> K_1(1270)) to the sum of the BRs (tau -> K_1(1270) and (tau -> K_1(1400)) to be 0.71 +- 0.16 +- 0.11. In all results, the first uncertainties are statistical and the second are systematic.

hep-ex↗

arXiv1999

Search for pair-produced leptoquarks in e+e- interactions at sqrt(s) = 183 GeV

A search for pair-produced leptoquarks has been performed using a sample of e+e- collision events collected by the OPAL detector at LEP at e+e- centre-of-mass energies of about 183 GeV. The data sample corresponds to an integrated luminosity of 55.9 pb-1. The leptoquarks were assumed to be produced via couplings to the photon and the Z0 and then to decay within a single fermion generation. No evidence for contributions from leptoquark pair production processes was observed. Lower limits on scalar and vector leptoquark masses are obtained. The existing limits are improved in the region of large decay branching ratio to quark-neutrino.

hep-ex↗

arXiv1999

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 e+e- 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 alpha_em, 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 mu+mu- and tau+tau- final states is also presented.

hep-ex↗

arXiv1999

Measurement of the Production Rate of Charm Quark Pairs from Gluons in Hadronic Z0 Decays

The rate of secondary charm-quark-pair production has been measured in 4.4 million hadronic Z0 decays collected by OPAL. By selecting events with three jets and tagging charmed hadrons in the gluon jet candidate using leptons and charged D* mesons, the average number of secondary charm-quark pairs per hadronic event is found to be (3.20+-0.21+-0.38)x10-2.

hep-ex↗

arXiv1999

Multi-photon production in e+e- collisions at sqrt(s)= 189 geV

The process e+e- to 2 (or 3) gammas is studied using data recorded with the OPAL detector at LEP. The data sample taken at a centre-of-mass energy of 189 GeV corresponds to a total integrated luminosity of 178 pb-1. The measured cross-section agrees well with the expectation from QED. A fit to the angular distribution is used to obtain improved limits at 95% CL on the QED cut-off parameters: Lambda+ > 304 GeV and Lambda- > 295 GeV as well as a mass limit for an excited electron, Me* > 306 GeV assuming equal e*egamma and eegamma couplings. Graviton exchange in the context of theories with higher dimensions is excluded for scales G+ < 660 GeV and G- < 634 GeV. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons and produced in association with another photon.

hep-ex↗

arXiv1996

High p_T Charm Photoproduction

The expected rates for charm-tagged jet photoproduction are evaluated for a number of tagging procedures, and some of the physics potential is discussed. Charm in jets is tagged using $D^*$'s, $μ$'s, or tracks from secondary vertices which might be identified in a microvertex detector. We find high expected event rates, leading to the possibility of placing strong constraints on the kinematics of charm production and on the gluon content of the proton and the charm content of the photon.

hep-ph↗

arXiv1996

HERWIG version 5.9

HERWIG (Hadron Emission Reactions With Interfering Gluons) is a multipurpose Monte Carlo event generator for the simulation of hard collisions between hadrons, leptons and photons. This note describes the new features of version 5.9. These include adding over 100 new particles, 1800 new decay modes and supporting new decay types. A complete space-time picture, including heavy meson mixing and secondary decays, is available for all event types and includes an optional colour rearrangement model. Initial state radition of photons in e^+e^- is is now included. Many other processes and features have been upgraded.

hep-ph↗