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 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 GeV, the 95% confidence level lower limits on are 93.6 GeV for and 94.1 GeV for . These limits are obtained assuming a universal scalar mass 500 GeV. The corresponding limits for all are 78.0 and 71.7 GeV. The 95% confidence level lower limits on the lightest neutralino mass, valid for any value of are 32.8 GeV for GeV and 31.6 GeV for all . Received: 14 October 1999 / Published online: 6 March 2000     

Search for chargino and neutralino production at \sqrt{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 pb at centre-of-mass energies of 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 , 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 ( 1 TeV) and 65.7 GeV for the smallest 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 is 24.7 GeV for TeV and 13.3 GeV for the minimum scenario. These mass limits are higher for increasing values of . The interpretation of the limits in terms of gluino and scalar quark mass limits is also given. Received: 11 July 1997 / Online publication: February 26, 1998     

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     

Search for neutralino pair production at \sqrt{s} = 189 GeV

A search for pair-production of neutralinos at a LEP centre-of-mass energy of 189 GeV gave no evidence for a signal. This limits the neutralino production cross-section and excludes regions in the parameter space of the Minimal Supersymmetric Standard Model (MSSM). Received: 22 November 2000 / Revised version: 11 January 2001 / Published online: 15 March 2001     

Search for resonant $\tilde\nu$ production at $\sqrt{s} = $ 183 to 208 GeV

Searches for resonant production in e ⁺ e ^- collisions under the assumption that R-parity is not conserved and that the dominant R-parity violating coupling is or used data recorded by DELPHI in 1997 to 2000 at centre-of-mass energies of 183 to 208 GeV. No deviation from the Standard Model was observed. Upper limits are given for the and couplings as a function of the sneutrino mass and total width. The limits are especially stringent for sneutrino masses equal to the centre-of-mass energies with the highest integrated luminosities recorded. Received: 14 January 2003 / Published online: 14 March 2003     

Search for pair-produced leptoquarks in e + e- interactions at $\sqrt{s} \simeq 189\text{--}209$ GeV

A search for pair-produced leptoquarks is performed using collision events collected by the OPAL detector at LEP at centre-of-mass energies between 189 and 209 GeV. The data sample corresponds to a total integrated luminosity of 596 pb^-1. The leptoquarks are assumed to be produced via couplings to the photon and the Z⁰. For a given search channel only leptoquark decays involving a single lepton generation are considered. No evidence for leptoquark pair production is observed. Lower limits on masses for scalar and vector leptoquarks are calculated. The results improve most of the LEP limits derived from previous searches for the pair production process by 10-25 GeV, depending on the leptoquark quantum numbers.Received: 6 May 2003, Published online: 24 October 2003     

Search for R-parity violating production of single sneutrinos in ee collisions at $\sqrt{s}$ = 189-209 GeV

A search for single sneutrino production under the assumption that R-parity is violated via a single dominant coupling is presented. This search considers the process and is performed using the data collected by the ALEPH detector at centre-of-mass energies from 189 GeV up to 209 GeV corresponding to an integrated luminosity of 637.1 pb^-1. The numbers of observed candidate events are in agreement with Standard Model expectations and 95% confidence level upper limits on five of the couplings are given as a function of the assumed sneutrino mass. Received: 19 December 2001 / Published online: 26 July 2002     

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     

Single- and multi-photon production in ee collisions at $\sqrt{s}$ up to 209 GeV

Events containing only energetic photons are analysed in a sample of of data recorded from collisions at centre-of-mass energies between 189 and 209 GeV by the ALEPH detector at LEP. The and cross sections are measured and found to be in agreement with the standard model predictions. The number of light neutrino generations is determined to be . Upper limits are derived on the cross sections for photon production in the context of several supersymmetric models. Limits are also set on the parameters of models with extra spatial dimensions, with contact interactions and with excited electrons. Received: 27 May 2002 / Published online: 14 March 2003     

Search for stable hadronizing squarks and gluinosin e + e- collisions up to \sqrt{s} = 209 GeV

Searches for stable, hadronizing scalar quarks and gluinos are performed using the data collected with the ALEPH detector at LEP. Gluon splitting into a gluino or a squark pair is searched for at centre-of-mass energies around the Z resonance, in the e ⁺ e^- and processes. Stable squark pair production, and stop pair production with subsequent decays into a stable gluino, , are also directly searched for at centre-of-mass energies from 183 to 209 GeV. Altogether, stable hadronizing stop (sbottom) quarks are excluded up to masses of 95 (92) , and stable hadronizing gluinos are excluded up to 26.9 , at 95% confidence level. In the framework of R-parity-conserving supersymmetric models in which the gluino and the stop quark are the two lightest supersymmetric particles, a 95% C.L. lower limit of 80 is set on the stop quark mass.Received: 23 May 2003, Published online: 24 October 2003     

Photon events with missing energy in e + e- collisions at $\sqrt{s} = {}$130 to 209 GeV

The production of single- and multi-photon events has been studied in the reaction . The data collected with the DELPHI detector during the years 1999 and 2000 at centre-of-mass energies between 191 GeV and 209 GeV was combined with earlier data to search for phenomena beyond the Standard Model. The measured number of light neutrino families was consistent with three and the absence of an excess of events beyond that predicted by the Standard Model processes was used to set limits on new physics. Both model-independent searches and searches for new processes predicted by supersymmetric and extra-dimensional models have been made. Limits on new non-standard model interactions between neutrinos and electrons were also determined.Received: 31 March 2004, Revised: 14 October 2004, Published online: 7 December 2004     

Measurement of isolated prompt photon production in photon-photon collisions at $\sqrt{s_{\rm ee}} = 183$-209 GeV

For the first time at LEP the production of prompt photons is studied in the collisions of quasi-real photons using the OPAL data taken at centre-of-mass energies between 183 GeV and 209 GeV. The total inclusive production cross-section for isolated prompt photons in the kinematic range of transverse momentum GeV and pseudorapidity is determined to be . Differential cross-sections are compared to the predictions of a next-to-leading-order (NLO) calculation.Received: 6 May 2003, Published online: 4 November 2003     

Study of Z pair production and anomalous couplingsin e + e- collisions at $\sqrt{s}$ between 190 GeV and 209 GeV

A study of Z-boson pair production in e ⁺ e^- annihilation at center-of-mass energies between 190 GeV and 209 GeV is reported. Final states containing only leptons, ( and ), quark and lepton pairs, ( , ) and only hadrons ( ) are considered. In all states with at least one Z boson decaying hadronically, lifetime, lepton and event-shape tags are used to separate pairs from final states. Limits on anomalous ZZ and ZZZ couplings are derived from the measured cross sections and from event kinematics using an optimal observable method. Limits on low scale gravity with large extra dimensions are derived from the cross sections and their dependence on polar angle.Received: 14 July 2003, Published online: 18 December 2003     

Photon events with missing energy at \sqrt{s} =183 to 189 GeV

The production of single photons has been studied in the reaction at centre-of-mass energies of 183 GeV and 189 GeV. A previously published analysis of events with multi-photon final states accompanied by missing energy has been updated with 189 GeV data. The data were collected with the DELPHI detector and correspond to integrated luminosities of about 51 pb and 158 pb at the two energies. The number of light neutrino families is measured to be . The absence of an excess of events beyond that expected from Standard Model processes is used to set limits on new physics as described by supersymmetric and composite models. A limit on the gravitational scale is also determined. Received: 28 January 2000 / Revised version: 27 April 2000 / Published online: 26 July 2000     

Search for excited leptons in e+e- collisions at \sqrt{s}=189–209 GeV

A search for excited lepton production in e⁺e^- collisions was performed using the data collected by the DELPHI detector at LEP at centre-of-mass energies ranging from 189 GeV to 209 GeV, corresponding to an integrated luminosity of approximately 600 pb^-1. No evidence for excited lepton production was found. In searches for pair-produced excited leptons, lower mass limits were established in the range 94–103 GeV/c², depending on the channel and model assumptions. In searches for singly-produced excited leptons, upper limits on the parameter f/Λ were established as a function of the mass.