Neutrons, X Rays, and Gamma Rays: Imaging Detectors, Material Characterization Techniques, and Applications, 1993
Scintillating fibers are of growing interest in high energy physics for applications in calorimet... more Scintillating fibers are of growing interest in high energy physics for applications in calorimetry and in tracking detectors At present plastic scintillating fibers are mainly used in these applications because of their high light yield and their fast decay rates; however, in thin fibers, required for high spatial resolution, these suffer from low attenuation lengths. Moreover, cross-talk is still a severe problem. As alternatives we will discuss the following two concepts: (1) using Ce- and Tb-doped multicomponent glasses as active core material of glass fibers and (2) using liquid scintillator filled glass capillary arrays. The optical properties of the rare earth doped glasses are described and the scintillation efficiency of the fibers and fiber bundles utilizing these glasses as core material are presented. Broader applications appear to the possible with liquid scintillator filled capillary arrays. Suitable liquid scintillators with high refractive index solvents and locally emitting solutes with high yields, short decay times and large Stokes-shifts are available. Arrays can be produced with and without extra mural absorber in various sizes and shapes. Theoretical estimates show that reflection losses at the liquid/glass interface do not effect the overall transmission up to length/diameter ratios of 105. In addition recent results have shown that the system resists radiation doses in the 100 kGy range. Further experimental results obtained at CERN with these arrays will be discussed.
This paper reports a study of B meson oscillations using hadronic Z 0 decays with two identified ... more This paper reports a study of B meson oscillations using hadronic Z 0 decays with two identified leptons, and updates a previous publication by including data collected in 1994. Decay times are reconstructed for each of the semileptonic B decays by forming vertices which include the lepton and by estimating the B meson momentum. The mass difference, ∆m d , between the two mass eigenstates in the B 0 d system is measured to be 0.430 ± 0.043 +0.028 −0.030 ps −1 , where the first error is statistical and the second error is systematic. For the B 0 s system, a lower limit of ∆m s > 2.2 ps −1 is obtained at 95% C.L.
Photonic events with large missing energy have been observed with the OPAL detector at LEP using ... more Photonic events with large missing energy have been observed with the OPAL detector at LEP using two search topologies. The rst topology looks for events with signicant missing energy and at least one photon in j cos j < 0:7 with large scaled energy, x > 0:2. The second topology looks for events with an acoplanar photon pair where both photons are in j cos j < 0:7 and have energy exceeding 1.75 GeV. In the data sample corresponding to an integrated luminosity of 10.0 pb 1 , 1 1 e v ents are selected for the rst topology and a cross-section of 1.6 0.5 pb is measured within the kinematic acceptance, consistent with that expected for the Standard Model process e + e ! (). No events are selected for the second topology giving a 95% CL cross-section upper limit of 0.41 pb within the kinematic acceptance. Upper limits on the cross-section times branching ratio for new particle production are also derived.
A study of b quark fragmentation at LEP is presented using a sample of semileptonic B decays cont... more A study of b quark fragmentation at LEP is presented using a sample of semileptonic B decays containing a fully reconstructed charm meson. The data are compared to several theoretical models for heavy quark fragmentation; the free parameters in these models are tted and the sensitivity of the model parameters to the rate of P-wave B meson production is studied. The mean scaled energy fraction of B 0 and B + mesons has been determined to be hx E i = 0 : 695 0:006 0:003 0:007, where the errors are statistical, systematic and model dependence respectively. This result is consistent with previous, less direct measurements from inclusive leptonic B decays. Also presented is a model independent t to the shape of the energy distribution of weakly decaying B mesons at LEP.
A new search for decays of heavy isosinglet neutrinos produced by neutral-current neutrino intera... more A new search for decays of heavy isosinglet neutrinos produced by neutral-current neutrino interactions in the CERN wide-band neutrino beam has been conducted by the CHARM-II Collaboration. We searched for heavy neutrinos created by scattering muon neutrinos on nucleons and decaying into + . Production and decay of heavy neutrinos would appear as double events in the detector. Each double event candidate was tested wheather the measured quantities are compatible with the kinematics of this process. No event passed this test. The analysis is based on 2 10 7 neutral-current neutrino events collected from 1987-1991. The experiment is sensitive to heavy neutrinos in the mass range 0.3-2.4 GeV=c 2 . The best limit for the mixing parameter is jUij 2 < 3 10 5 for a mass around 2 GeV=c 2 . The analysis improved by an order of magnitude previous results derived from wide-band neutrino beam exposures of the CHARM detector.
We report new constraints on the existence of additional Z bosons derived from neutrino{electron ... more We report new constraints on the existence of additional Z bosons derived from neutrino{electron scattering data. The analysis is based on data taken by the CHARM II Collaboration during ve y ears of exposure of the detector to the wide band neutrino beam at CERN. The limits on the mass of these new particles are evaluated in the framework of E 6 superstring-inspired theories and Left-Right symmetric models. Limits on the so-called Z 1 are also reported.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1997
We discuss the operation principle of a detector based on superheated droplets of Freon-12 and it... more We discuss the operation principle of a detector based on superheated droplets of Freon-12 and its feasibility for the search of weakly interacting cold dark matter particles. In particular we are interested in a neutralino search experiment in the mass range from 10 to 10 4 GeV/c 2 and with a sensitivity of better than 10 −2 events/kg/d. We show that our new proposed detector can be operated at ambient pressure and room temperature in a mode where it is exclusively sensitive to nuclear recoils like those following neutralino interactions, which allows a powerful background discrimination. An additional advantage of this technique is due to the fact that the detection material, Freon-12, is cheap and readily available in large quantities. Moreover we were able to show that piezoelectric transducers allow efficient event localization in large volumes.
Neutron response functions for superheated droplet detectors have been analyzed from threshold up... more Neutron response functions for superheated droplet detectors have been analyzed from threshold up to 2 MeV incident energy, taking into account a gradual threshold function, in terms of temperature and incident energy. A method for obtaining fitted parameters is described. Detection mechanisms up to 14 MeV are discussed.
The rate of secondary charm-quark-pair production has been measured in 4.4 million hadronic Z 0 d... more The rate of secondary charm-quark-pair production has been measured in 4.4 million hadronic Z 0 decays collected by OPAL. By selecting events with three jets and tagging charmed hadrons in the gluon jet candidate using leptons and D ± mesons, the average number of secondary charm-quark pairs per hadronic event is found to be (3.20±0.21±0.38)×10 −2 .
Using data from e + e − annihilation into hadrons, taken with the OPAL detector at LEP at the Z p... more Using data from e + e − annihilation into hadrons, taken with the OPAL detector at LEP at the Z pole between 1991 and 1995, we performed a simultaneous measurement of the colour factors of the underlying gauge group of the strong interaction, C F and C A , and the strong coupling, α s. The measurement was carried out by fitting next-to-leading order perturbative predictions to measured angular correlations of 4-jet events together with multi-jet related variables. Our results, C A = 3.02 ± 0.25(stat.) ± 0.49(syst.) , C F = 1.34 ± 0.13(stat.) ± 0.22(syst.) , α s (M Z) = 0.120 ± 0.011(stat.) ± 0.020(syst.) , provide a test of perturbative QCD in which the only assumptions are non-abelian gauge symmetry and standard hadronization models. The measurements are in agreement with SU(3) expectations for C F and C A and the world average of α s (M Z).
The European Physical Journal C - Particles and Fields, 2002
A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks (q) in Rp... more A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks (q) in Rparity violating SUSY models has been performed using e + e − collisions collected with the OPAL detector at LEP at an e + e − centre-of-mass energy √ s ee of 189 GeV. The data correspond to an integrated luminosity of about 160 pb −1. The dominant process for this search is eq → LQ/q → eq, νq, where a photon, which has been radiated by one of the beam electrons, serves as a source of quarks. The numbers of selected events found in the two decay channels are in agreement with the expectations from Standard Model processes. This result allows to set lower limits at the 95 % confidence level on the mass of first generation scalar and vector leptoquarks, and of squarks in R-parity violating SUSY models. For Yukawa couplings λ to fermions larger than √ 4πα em , the mass limits range from 121 GeV/c 2 to 175 GeV/c 2 (149 GeV/c 2 to 188 GeV/c 2) depending on the branching ratio β of the scalar (vector) leptoquark state. Furthermore, limits are set on the Yukawa couplings λ for leptoquarks and λ 1jk for squarks, and on β as a function of the scalar leptoquark/squark mass.
The energy distribution and type of the particle with the highest momentum in quark jets are dete... more The energy distribution and type of the particle with the highest momentum in quark jets are determined for each of the five quark flavours making only minimal model assumptions. The analysis is based on a large statistics sample of hadronic Z 0 decays collected with the OPAL detector at the LEP e + e − collider. These results provide a basis for future studies of light flavour production at other centre-of-mass energies. We use our results to study the hadronisation mechanism in light flavour jets and compare the data to the QCD models JETSET and HERWIG. Within the JETSET model we also directly determine the suppression of strange quarks to be γs = 0.422 ± 0.049(stat.) ± 0.059(syst.) by comparing the production of charged and neutral kaons in strange and non-strange light quark events. Finally we study the features of baryon production.
A search for pair produced scalar fermions with couplings that violate R-parity has been performe... more A search for pair produced scalar fermions with couplings that violate R-parity has been performed using a data sample corresponding to an integrated luminosity of 56 pb −1 at a centre-of-mass energy of √ s = 183 GeV collected with the OPAL detector at LEP. An important consequence of R-parity breaking interactions is that the lightest supersymmetric particle is expected to be unstable. Searches for R-parity violating decays of charged sleptons, sneutrinos and stop quarks have been performed under the assumptions that the lightest supersymmetric particle decays promptly and that only one of the R-parity violating couplings is dominant for each of the decay modes considered. Such processes would yield multi-leptons, jets plus leptons or multi-jets, with or without missing energy, in the final state. No significant excess of such events has been observed. Limits on the production cross-sections of scalar fermions in R-parity violating scenarios are obtained. Mass exclusion regions are also presented in the framework of the Constrained Minimal Supersymmetric Standard Model.
We have searched for unstable neutral and charged heavy leptons, N and L ± , and for excited stat... more We have searched for unstable neutral and charged heavy leptons, N and L ± , and for excited states of neutral and charged leptons, ν * , e * , µ * and τ * , 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. From the analysis of charged-current decays of pair-produced unstable heavy leptons, and of charged-current and photonic decays of pairproduced 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 /Λ, are determined for masses up to the kinematic limit.
From an analysis of the ionisation energy loss of charged particles selected from a sample of 147... more From an analysis of the ionisation energy loss of charged particles selected from a sample of 147926 e + e − → τ + τ − candidates recorded in the OPAL detector at e + e − centre-of-mass energies near the Z 0 resonance, we determine the branching ratios:
Searches for the neutral Higgs bosons h 0 and A 0 are used to obtain limits on the Type II Two Hi... more Searches for the neutral Higgs bosons h 0 and A 0 are used to obtain limits on the Type II Two Higgs Doublet Model (2HDM(II)) with no CP-violation in the Higgs sector and no additional particles besides the five Higgs bosons. The analysis combines approximately 170 pb −1 of data collected with the OPAL detector at √ s ≈ 189 GeV with previous runs at √ s ≈ m Z and √ s ≈ 183 GeV. The searches are sensitive to the h 0 , A 0 →qq, gg, τ + τ − and h 0 →A 0 A 0 decay modes of the Higgs bosons. For the first time, the 2HDM(II) parameter space is explored in a detailed scan, and new flavour independent analyses are applied to examine regions in which the neutral Higgs bosons decay predominantly into light quarks or gluons. Model-independent limits are also given.
Photonic events with large missing energy have been observed in e + e − collisions at centreof-ma... more Photonic events with large missing energy have been observed in e + e − collisions at centreof-mass energies of 130, 136 and 183 GeV collected in 1997 using the OPAL detector at LEP. Results are presented for event topologies with a single photon and missing transverse energy or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection. These results are compared with the expectations from the Standard Model process e + e − → νν + photon(s). No evidence is observed for new physics contributions to these final states. Using the data at √ s = 183 GeV, upper limits on σ(e + e − → XY) • BR(X → Yγ) and σ(e + e − → XX) • BR 2 (X → Yγ) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = ν * , Y = ν), to neutralino production (X =χ 0 2 , Y =χ 0 1) and to supersymmetric models in which X =χ 0 1 and Y =G is a light gravitino.
In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks... more In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks were selected using the long lifetime of b flavoured hadrons. Comparing the 3-jet rate in b events with that in d,u,s and c quark events, a significant difference was observed. Using O(α 2 S) calculations for massive quarks, this difference was used to determine the b quark mass in the MS renormalisation scheme at the scale of the Z boson mass. By combining the results from seven different jet finders the running b quark mass was determined to be m b (m Z) = (2.67 ± 0.03 (stat.) +0.29 −0.37 (syst.) ± 0.19 (theo.)) GeV. Evolving this value to the b quark mass scale itself yields m b (m b) = (3.95 +0.52 −0.62) GeV, consistent with results obtained at the b quark production threshold. This determination confirms the QCD expectation of a scale dependent quark mass. A constant mass is ruled out by 3.9 standard deviations.
Neutrons, X Rays, and Gamma Rays: Imaging Detectors, Material Characterization Techniques, and Applications, 1993
Scintillating fibers are of growing interest in high energy physics for applications in calorimet... more Scintillating fibers are of growing interest in high energy physics for applications in calorimetry and in tracking detectors At present plastic scintillating fibers are mainly used in these applications because of their high light yield and their fast decay rates; however, in thin fibers, required for high spatial resolution, these suffer from low attenuation lengths. Moreover, cross-talk is still a severe problem. As alternatives we will discuss the following two concepts: (1) using Ce- and Tb-doped multicomponent glasses as active core material of glass fibers and (2) using liquid scintillator filled glass capillary arrays. The optical properties of the rare earth doped glasses are described and the scintillation efficiency of the fibers and fiber bundles utilizing these glasses as core material are presented. Broader applications appear to the possible with liquid scintillator filled capillary arrays. Suitable liquid scintillators with high refractive index solvents and locally emitting solutes with high yields, short decay times and large Stokes-shifts are available. Arrays can be produced with and without extra mural absorber in various sizes and shapes. Theoretical estimates show that reflection losses at the liquid/glass interface do not effect the overall transmission up to length/diameter ratios of 105. In addition recent results have shown that the system resists radiation doses in the 100 kGy range. Further experimental results obtained at CERN with these arrays will be discussed.
This paper reports a study of B meson oscillations using hadronic Z 0 decays with two identified ... more This paper reports a study of B meson oscillations using hadronic Z 0 decays with two identified leptons, and updates a previous publication by including data collected in 1994. Decay times are reconstructed for each of the semileptonic B decays by forming vertices which include the lepton and by estimating the B meson momentum. The mass difference, ∆m d , between the two mass eigenstates in the B 0 d system is measured to be 0.430 ± 0.043 +0.028 −0.030 ps −1 , where the first error is statistical and the second error is systematic. For the B 0 s system, a lower limit of ∆m s > 2.2 ps −1 is obtained at 95% C.L.
Photonic events with large missing energy have been observed with the OPAL detector at LEP using ... more Photonic events with large missing energy have been observed with the OPAL detector at LEP using two search topologies. The rst topology looks for events with signicant missing energy and at least one photon in j cos j < 0:7 with large scaled energy, x > 0:2. The second topology looks for events with an acoplanar photon pair where both photons are in j cos j < 0:7 and have energy exceeding 1.75 GeV. In the data sample corresponding to an integrated luminosity of 10.0 pb 1 , 1 1 e v ents are selected for the rst topology and a cross-section of 1.6 0.5 pb is measured within the kinematic acceptance, consistent with that expected for the Standard Model process e + e ! (). No events are selected for the second topology giving a 95% CL cross-section upper limit of 0.41 pb within the kinematic acceptance. Upper limits on the cross-section times branching ratio for new particle production are also derived.
A study of b quark fragmentation at LEP is presented using a sample of semileptonic B decays cont... more A study of b quark fragmentation at LEP is presented using a sample of semileptonic B decays containing a fully reconstructed charm meson. The data are compared to several theoretical models for heavy quark fragmentation; the free parameters in these models are tted and the sensitivity of the model parameters to the rate of P-wave B meson production is studied. The mean scaled energy fraction of B 0 and B + mesons has been determined to be hx E i = 0 : 695 0:006 0:003 0:007, where the errors are statistical, systematic and model dependence respectively. This result is consistent with previous, less direct measurements from inclusive leptonic B decays. Also presented is a model independent t to the shape of the energy distribution of weakly decaying B mesons at LEP.
A new search for decays of heavy isosinglet neutrinos produced by neutral-current neutrino intera... more A new search for decays of heavy isosinglet neutrinos produced by neutral-current neutrino interactions in the CERN wide-band neutrino beam has been conducted by the CHARM-II Collaboration. We searched for heavy neutrinos created by scattering muon neutrinos on nucleons and decaying into + . Production and decay of heavy neutrinos would appear as double events in the detector. Each double event candidate was tested wheather the measured quantities are compatible with the kinematics of this process. No event passed this test. The analysis is based on 2 10 7 neutral-current neutrino events collected from 1987-1991. The experiment is sensitive to heavy neutrinos in the mass range 0.3-2.4 GeV=c 2 . The best limit for the mixing parameter is jUij 2 < 3 10 5 for a mass around 2 GeV=c 2 . The analysis improved by an order of magnitude previous results derived from wide-band neutrino beam exposures of the CHARM detector.
We report new constraints on the existence of additional Z bosons derived from neutrino{electron ... more We report new constraints on the existence of additional Z bosons derived from neutrino{electron scattering data. The analysis is based on data taken by the CHARM II Collaboration during ve y ears of exposure of the detector to the wide band neutrino beam at CERN. The limits on the mass of these new particles are evaluated in the framework of E 6 superstring-inspired theories and Left-Right symmetric models. Limits on the so-called Z 1 are also reported.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1997
We discuss the operation principle of a detector based on superheated droplets of Freon-12 and it... more We discuss the operation principle of a detector based on superheated droplets of Freon-12 and its feasibility for the search of weakly interacting cold dark matter particles. In particular we are interested in a neutralino search experiment in the mass range from 10 to 10 4 GeV/c 2 and with a sensitivity of better than 10 −2 events/kg/d. We show that our new proposed detector can be operated at ambient pressure and room temperature in a mode where it is exclusively sensitive to nuclear recoils like those following neutralino interactions, which allows a powerful background discrimination. An additional advantage of this technique is due to the fact that the detection material, Freon-12, is cheap and readily available in large quantities. Moreover we were able to show that piezoelectric transducers allow efficient event localization in large volumes.
Neutron response functions for superheated droplet detectors have been analyzed from threshold up... more Neutron response functions for superheated droplet detectors have been analyzed from threshold up to 2 MeV incident energy, taking into account a gradual threshold function, in terms of temperature and incident energy. A method for obtaining fitted parameters is described. Detection mechanisms up to 14 MeV are discussed.
The rate of secondary charm-quark-pair production has been measured in 4.4 million hadronic Z 0 d... more The rate of secondary charm-quark-pair production has been measured in 4.4 million hadronic Z 0 decays collected by OPAL. By selecting events with three jets and tagging charmed hadrons in the gluon jet candidate using leptons and D ± mesons, the average number of secondary charm-quark pairs per hadronic event is found to be (3.20±0.21±0.38)×10 −2 .
Using data from e + e − annihilation into hadrons, taken with the OPAL detector at LEP at the Z p... more Using data from e + e − annihilation into hadrons, taken with the OPAL detector at LEP at the Z pole between 1991 and 1995, we performed a simultaneous measurement of the colour factors of the underlying gauge group of the strong interaction, C F and C A , and the strong coupling, α s. The measurement was carried out by fitting next-to-leading order perturbative predictions to measured angular correlations of 4-jet events together with multi-jet related variables. Our results, C A = 3.02 ± 0.25(stat.) ± 0.49(syst.) , C F = 1.34 ± 0.13(stat.) ± 0.22(syst.) , α s (M Z) = 0.120 ± 0.011(stat.) ± 0.020(syst.) , provide a test of perturbative QCD in which the only assumptions are non-abelian gauge symmetry and standard hadronization models. The measurements are in agreement with SU(3) expectations for C F and C A and the world average of α s (M Z).
The European Physical Journal C - Particles and Fields, 2002
A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks (q) in Rp... more A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks (q) in Rparity violating SUSY models has been performed using e + e − collisions collected with the OPAL detector at LEP at an e + e − centre-of-mass energy √ s ee of 189 GeV. The data correspond to an integrated luminosity of about 160 pb −1. The dominant process for this search is eq → LQ/q → eq, νq, where a photon, which has been radiated by one of the beam electrons, serves as a source of quarks. The numbers of selected events found in the two decay channels are in agreement with the expectations from Standard Model processes. This result allows to set lower limits at the 95 % confidence level on the mass of first generation scalar and vector leptoquarks, and of squarks in R-parity violating SUSY models. For Yukawa couplings λ to fermions larger than √ 4πα em , the mass limits range from 121 GeV/c 2 to 175 GeV/c 2 (149 GeV/c 2 to 188 GeV/c 2) depending on the branching ratio β of the scalar (vector) leptoquark state. Furthermore, limits are set on the Yukawa couplings λ for leptoquarks and λ 1jk for squarks, and on β as a function of the scalar leptoquark/squark mass.
The energy distribution and type of the particle with the highest momentum in quark jets are dete... more The energy distribution and type of the particle with the highest momentum in quark jets are determined for each of the five quark flavours making only minimal model assumptions. The analysis is based on a large statistics sample of hadronic Z 0 decays collected with the OPAL detector at the LEP e + e − collider. These results provide a basis for future studies of light flavour production at other centre-of-mass energies. We use our results to study the hadronisation mechanism in light flavour jets and compare the data to the QCD models JETSET and HERWIG. Within the JETSET model we also directly determine the suppression of strange quarks to be γs = 0.422 ± 0.049(stat.) ± 0.059(syst.) by comparing the production of charged and neutral kaons in strange and non-strange light quark events. Finally we study the features of baryon production.
A search for pair produced scalar fermions with couplings that violate R-parity has been performe... more A search for pair produced scalar fermions with couplings that violate R-parity has been performed using a data sample corresponding to an integrated luminosity of 56 pb −1 at a centre-of-mass energy of √ s = 183 GeV collected with the OPAL detector at LEP. An important consequence of R-parity breaking interactions is that the lightest supersymmetric particle is expected to be unstable. Searches for R-parity violating decays of charged sleptons, sneutrinos and stop quarks have been performed under the assumptions that the lightest supersymmetric particle decays promptly and that only one of the R-parity violating couplings is dominant for each of the decay modes considered. Such processes would yield multi-leptons, jets plus leptons or multi-jets, with or without missing energy, in the final state. No significant excess of such events has been observed. Limits on the production cross-sections of scalar fermions in R-parity violating scenarios are obtained. Mass exclusion regions are also presented in the framework of the Constrained Minimal Supersymmetric Standard Model.
We have searched for unstable neutral and charged heavy leptons, N and L ± , and for excited stat... more We have searched for unstable neutral and charged heavy leptons, N and L ± , and for excited states of neutral and charged leptons, ν * , e * , µ * and τ * , 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. From the analysis of charged-current decays of pair-produced unstable heavy leptons, and of charged-current and photonic decays of pairproduced 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 /Λ, are determined for masses up to the kinematic limit.
From an analysis of the ionisation energy loss of charged particles selected from a sample of 147... more From an analysis of the ionisation energy loss of charged particles selected from a sample of 147926 e + e − → τ + τ − candidates recorded in the OPAL detector at e + e − centre-of-mass energies near the Z 0 resonance, we determine the branching ratios:
Searches for the neutral Higgs bosons h 0 and A 0 are used to obtain limits on the Type II Two Hi... more Searches for the neutral Higgs bosons h 0 and A 0 are used to obtain limits on the Type II Two Higgs Doublet Model (2HDM(II)) with no CP-violation in the Higgs sector and no additional particles besides the five Higgs bosons. The analysis combines approximately 170 pb −1 of data collected with the OPAL detector at √ s ≈ 189 GeV with previous runs at √ s ≈ m Z and √ s ≈ 183 GeV. The searches are sensitive to the h 0 , A 0 →qq, gg, τ + τ − and h 0 →A 0 A 0 decay modes of the Higgs bosons. For the first time, the 2HDM(II) parameter space is explored in a detailed scan, and new flavour independent analyses are applied to examine regions in which the neutral Higgs bosons decay predominantly into light quarks or gluons. Model-independent limits are also given.
Photonic events with large missing energy have been observed in e + e − collisions at centreof-ma... more Photonic events with large missing energy have been observed in e + e − collisions at centreof-mass energies of 130, 136 and 183 GeV collected in 1997 using the OPAL detector at LEP. Results are presented for event topologies with a single photon and missing transverse energy or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection. These results are compared with the expectations from the Standard Model process e + e − → νν + photon(s). No evidence is observed for new physics contributions to these final states. Using the data at √ s = 183 GeV, upper limits on σ(e + e − → XY) • BR(X → Yγ) and σ(e + e − → XX) • BR 2 (X → Yγ) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = ν * , Y = ν), to neutralino production (X =χ 0 2 , Y =χ 0 1) and to supersymmetric models in which X =χ 0 1 and Y =G is a light gravitino.
In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks... more In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks were selected using the long lifetime of b flavoured hadrons. Comparing the 3-jet rate in b events with that in d,u,s and c quark events, a significant difference was observed. Using O(α 2 S) calculations for massive quarks, this difference was used to determine the b quark mass in the MS renormalisation scheme at the scale of the Z boson mass. By combining the results from seven different jet finders the running b quark mass was determined to be m b (m Z) = (2.67 ± 0.03 (stat.) +0.29 −0.37 (syst.) ± 0.19 (theo.)) GeV. Evolving this value to the b quark mass scale itself yields m b (m b) = (3.95 +0.52 −0.62) GeV, consistent with results obtained at the b quark production threshold. This determination confirms the QCD expectation of a scale dependent quark mass. A constant mass is ruled out by 3.9 standard deviations.
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Papers by Viktor Zacek