Search for anomalous production of di-lepton events with missing transverse momentum in e(+)e(-) collisions at root s = 161 and 172 GeV

Events containing a pair of charged leptons and significant missing transverse momentum are selected from a data sample corresponding to a total integrated luminosity of 20.6 pb^-1 at centre-of-mass energies of 161 GeV and 172 GeV. The observed number of events, four at 161 GeV and nine at 172 GeV, is consistent with the number expected from Standard Model processes, predominantly arising 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. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is observed and limits on the production of scalar charged lepton pairs and other new particles are presented

Tests of the standard model and constraints on new physics from measurements of fermion-pair production at 130-172GeV at LEP

Production of events with hadronic and leptonic final states has been measured in e(+)e(-) collisions at centre-of-mass energies of 130-172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward asymmetries are presented, both including and excluding the dominant production of radiative Z gamma events, and compared to Standard Model expectations. The ratio R-b of the cross-section for production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have been used to obtain an improved precision on the measurement of gamma-Z interference. The energy dependence of alpha(em) has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described by effective four-fermion contact interactions, to search for t-channel contributions from new massive particles and to place limits on gaugino pair production with subsequent decay of the gaugino into a light gluino and a quark pair

Search for unstable heavy and excited leptons in e(+)e(-) collisions at root s=170-172 GeV

We have searched for unstable neutral and charged heavy leptons, N and L+/-, and for excited states of neutral and charged leptons, v*, e*, mu* and tau*, in e(+)e(-) collisions at centre-of-mass energies of 170 and 172 GeV using the OPAL detector at LEP. No evidence for their existence was found. Front the analysis of charged-current decays of pair-produced unstable heavy leptons, and of charged-current and photonic decays of pair-produced excited leptons, lower limits on their masses are derived. From the analysis of charged-current and photonic decays of singly-produced excited leptons, upper Limits on the ratio of the coupling to the compositeness scale, f/A, are determined for masses up to the kinematic limit

Search for chargino and neutralino production at √s=170 and 172 GeV at LEP

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 10.310.3 pb −1−1 at centre-of-mass energies of s√=s= 170 and 172 GeV with the OPAL detector at LEP. No evidence for these particles has been found. The results are combined with those from previous OPAL chargino and neutralino searches at lower energies to obtain limits. Exclusion regions at 95% C.L. of parameters of the Minimal Supersymmetric Standard Model are determined. Within this framework, for tanβ≥1.0tan⁡β≥1.0 , lower mass limits are placed on the lightest chargino and the three lightest neutralinos. The 95% C.L. lower mass limit on the lightest chargino, assuming that it is heavier than the lightest neutralino by more than 10 GeV, is 84.5 GeV for the case of a large universal scalar mass ( m0>m0> 1 TeV) and 65.7 GeV for the smallest m0m0 compatible with current limits on the sneutrino mass and slepton cross-sections. The lower limit on the lightest neutralino mass at 95% C.L. for tanβ≥1.0tan⁡β≥1.0 is 24.7 GeV for m0=1m0=1 TeV and 13.3 GeV for the minimum m0m0 scenario. These mass limits are higher for increasing values of tanβtan⁡β . The interpretation of the limits in terms of gluino and scalar quark mass limits is also given

Search for CP violation in Z0 ---> tau+ tau- and an upper limit on the weak dipole moment of the tau lepton

Contains fulltext : 124918.pdf (preprint version ) (Open Access

Search for a massive Di-photon resonance at root s=91-172 GeV

A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a total data sample of 25.7 pb(-1) taken a! centre-of-mass energies between 130 GeV and 172 GeV with the OPAL detector at LEP. The observed number of events is consistent with the expected number from Standard Model processes. The observed candidates are combined with search results from root s approximate to M-z to place limits on B(H-0 --> gamma gamma) within the Standard Model for Higgs boson masses up to 77 GeV, and on the production cross section of any scaler resonance decaying into di-photons. Upper limits on B(H-0 --> gamma gamma) x sigma(e(+)e(-)-->H2O) of 290 - 830 fb are obtained over 40 < M-H < 160 GeV. Higgs scalars which couple only to gauge bosons with Standard Model strength are ruled out lip to a mass of 76.5 GeV at the 95% confidence level

Multi-photon final states in e(+)e(-) collisions at root s=130-172 GeV

The process e(+)e(-)-->gamma gamma(gamma) is studied using data recorded with the OPAL detector at LEP. The data sample corresponds to a total integrated luminosity of 25.38 pb(-1) taken at centre-of-mass energies of 130-272 GeV. The measured cross-sections agree well with the expectation from QED, In a combined fit using data from all centre-of-mass energies. the angular distribution is used to obtain improved limits on the cut-off parameters: Lambda(+) > 195GeV and Lambda(-) > 210GeV (95% CL), In addition, limits on nonstandard e(+)e(-)gamma couplings and contact interactions, as well as a 95% CL mass limit for an excited electron. M-e*, > 194 GeV for an e(+)e(-)gamma coupling kappa = 1, are determined