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ALEPH Collaboration

Publications and source records attributed to ALEPH Collaboration.

At least 55 records · Page 3Linked to original sources
arXiv2001

Flavour Independent Search for Hadronically Decaying Neutral Higgs Bosons at LEP

Hadronic decays of Higgs bosons, not necessarily into b-quarks, have been searched for using data collected at LEP-2. Such searches are complementary to the usual Standard Model Higgs searches and lead to more model-independence. Preliminary results obtained by the four LEP collaborations are presented for the hZ production mechanism (using the q-qbarq-qbar, q-qbarnu-nubar and q-qbarl+l- channels) and combined for the first time.

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arXiv2001

Measurement of Ab_FB using inclusive b-hadron decays

Based on a sample of four million events collected by ALEPH from 1991 to 1995, a measurement of the forward-backward asymmetry in Z -> b-barb decays using inclusive final states is presented. High-performance tagging of b-barb events in a wide angular range is achieved using neural network techniques. An optimal hemisphere charge estimator is built by merging primary and secondary vertex information, leading kaon identification and jet charge in a neural network. The average charge asymmetry, the flavour tagging efficiencies and mean b-hemisphere charges are measured from data and used to extract the pole b asymmetry in the Standard Model A^{0,b}_{FB} = 0.1009 +- 0.0027 (stat) +- 0.0015 (syst), corresponding to a value of the effective weak mixing angle of sin^2(theta_W^{eff}) = 0.23193 +- 0.00056.

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arXiv2001

Study of the fragmentation of b quarks into B mesons at the Z peak

The fragmentation of b quarks into B mesons is studied with four million hadronic Z decays collected by the ALEPH experiment during the years 1991-1995. A semi-exclusive reconstruction of B->l nu D(*) decays is performed, by combining lepton candidates with fully reconstructed D(*) mesons while the neutrino energy is estimated from the missing energy of the event. The mean value of xewd, the energy of the weakly-decaying B meson normalised to the beam energy, is found to be mxewd = 0.716 +- 0.006 (stat) +- 0.006 (syst) using a model-independent method; the corresponding value for the energy of the leading B meson is mxel = 0.736 +- 0.006 (stat) +- 0.006 (syst). The reconstructed spectra are compared with different fragmentation models.

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arXiv2001

Measurement of the Tau Polarisation at LEP

The polarisation of $τ$'s produced in Z decay is measured using 160 pb$^{-1}$ of data accumulated at LEP by the ALEPH detector between 1990 and 1995. The variation of the polarisation with polar angle yields the two parameters ${\cal A}_e = 0.1504 \pm 0.0068 $ and ${\cal A}_τ = 0.1451 \pm 0.0059$ which are consistent with the hypothesis of $e$-$τ$ universality. Assuming universality, the value ${\cal A}_{e{-}τ} = 0.1474 \pm 0.0045$ is obtained from which the effective weak mixing angle $\sin^2 {θ_{\mathrm{W}}^{\mathrm{eff}}} =0.23147 \pm 0.00057 $ is derived.

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arXiv2001

Measurement of Triple Gauge-Boson Couplings at LEP up to 189GeV

The triple gauge-boson couplings involving the W are determined using data samples collected with the ALEPH detector at mean centre-of-mass energies of 183 GeV and 189 GeV, corresponding to integrated luminosities of 57 pb^-1 and 174 pb^-1, respectively. The couplings, g^Z_1, Kappa_gamma and lambda_gamma, are measured using W-pair events, single-W production and single-gamma production. Each coupling is measured individually with the other two coupling fixed at their Standard Model value. Including ALEPH results from lower energies, the 95% confidence level intervals for the deviation to the Standard Model are -0.087 < Dg^Z_1 < 0.141 -0.200 < DKappa_gamma < 0.258 -0.062 < Lambda_gamma < 0.147. Fits are also presented where two or all three couplings are allowed to vary. In addition, W-pair events are used to set limits on the C- or P-violating couplings g^V_4, g^V_5, Kappa_V, and Lambda_V, where V denotes either gamma or Z. No deviations from the Standard Model expectations are observed.

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arXiv2001

Combination procedure for the precise determination of Z boson parameters from results of the LEP experiments

The precise determination of the Z boson parameters from the measurements at the Z resonance by the four collaborations ALEPH, DELPHI, L3 and OPAL in e+e- collisions at the large electron positron collider LEP at CERN is a landmark for precision tests of the electroweak theory. The four experiments measured quantities which were used to extract the mass and width of the Z boson, the hadronic cross-section at the pole of the resonance, the ratio of hadronic and leptonic decay widths, and the leptonic forward-backward asymmetries at the pole. The combination procedure based on these parameters is presented in this paper.

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arXiv2000

Observation of an Excess in the Search for the Standard Model Higgs Boson at ALEPH

A search has been performed for the Standard Model Higgs boson in the data sample collected with the ALEPH detector at LEP, at centre-of-mass energies up to 209GeV. An excess of 3sigma beyond the background expectation is found, consistent with the production of the Higgs boson with a mass near 114GeV/c2. Much of this excess is seen in the four-jet analyses, where three high purity events are selected.

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arXiv2000

Search for Supersymmetric Particles in e+e- Collisions at sqrt(s) up to 202GeV and Mass Limit for the Lightest Neutralino

Searches for pair production of squarks, sleptons, charginos, and neutralinos are performed with the data collected by the ALEPH detector at LEP at centre-of-mass energies from 188.6 to 201.6 GeV. No evidence for any such signals is observed in a total integrated luminosity of about 410pb-1. The negative results of the searches are translated into exclusion domains in the space of the relevant MSSM parameters, which improve significantly on the constraints set previously. Under the assumptions of gaugino and sfermion mass unification, these results allow a 95% C.L. lower limit of 37GeV/c2 to be set on the mass of the lightest neutralino for any tan beta and sfermion mass. Additional constraints in the MSSM parameter space are derived from the negative results of ALEPH searches for Higgs bosons. The results are also interpreted in the framework of minimal supergravity.

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arXiv2000

Search for R-Parity Violating Decays of Supersymmetric Particles in e+e- Collisions at Centre-of-Mass Energies from 189GeV to 202GeV

Searches for the production of supersymmetric particles under the assumption that R-parity is violated via a single dominant LLE, LQD or UDD coupling were performed. These use the data collected by the ALEPH detector at LEP at centre-of-mass energies from 188.6 to 201.6 GeV. The numbers of candidate events observed in the data are consistent with Standard Model expectations. Upper limits on the production cross sections and lower limits on the masses of charginos, sleptons, squarks and sneutrinos are derived.

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arXiv2000

Searches for neutral Higgs bosons in e+e- collisions at centre-of-mass energies from 192 to 202GeV

Searches for neutral Higgs bosons are performed with the 237 pb-1 of data collected in 1999 by the ALEPH detector at LEP, for centre-of-mass energies between 191.6 and 201.6 GeV. These searches apply to Higgs bosons within the context of the Standard Model and its minimal supersymmetric extension (MSSM) as well as to invisibly decaying Higgs bosons. No evidence of a signal is seen. A lower limit on the mass of the Standard Model Higgs boson of 107.7 GeV/c2 at 95% confidence level is set. In the MSSM, lower limits of 91.2 and 91.6 GeV/c2 are derived for the masses of the neutral Higgs bosons h and A, respectively. For a Higgs boson decaying invisibly and produced with the Standard Model cross section, masses below 106.4 GeV/c2 are excluded.

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arXiv2000

Comparison of Deep Inelastic Electron-Photon Scattering Data with the Herwig and Phojet Monte Carlo Models

Deep inelastic electron-photon scattering is studied in the Q**2 range from 1.2 to 30 GeV**2 using the LEP1 data taken with the ALEPH, L3 and OPAL detectors at centre-of-mass energies close to the mass of the Z boson. Distributions of the measured hadronic final state are corrected to the hadron level and compared to the predictions of the HERWIG and PHOJET Monte Carlo models. For large regions in most of the distributions studied the results of the different experiments agree with one another. However, significant differences are found between the data and the models. Therefore the combined LEP data serve as an important input to improve on the Monte Carlo models.

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arXiv2000

Measurements of BR (b -> tau- nubar_tau X) and BR (b -> tau- nubar_tau D*+- X) and Upper Limits on BR (B- -> tau- nubar_tau) and BR (b -> s nu nubar)

Inclusive branching ratios involving b to tau transitions are measured in approximately four million hadronic Z decays collected by the ALEPH detector at LEP. The fully-inclusive branching ratio b -> tau nu X and the semi-inclusive branching ratio b -> tau nu D*+/- X are measured to be (2.43 +/- 0.20 +/- 0.25)% and (0.88 +/- 0.31 +/- 0.28)%, in agreement with the standard model predictions. Upper limits on the branching fractions b -> tau nu and b -> s nu nubar are set to 8.3 10**-4 and 6.4 10**-4 at the 90% C.L. These results allow a 90% C.L. lower limit of 0.40 (GeV/c**2)**-1 to be set on the tan(beta)/mH+/- ratio, in the framework of type-II two-Higgs-doublet mode

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arXiv2000

Study of the CP asymmetry of B0 -> J/psi K0S decays in ALEPH

The decay B0 -> J/psi K0_S is reconstructed with J/psi -> e+ e- or mu+ mu- and K0_S -> pi+ pi-. From the full ALEPH dataset at LEP1 of about 4 million hadronic Z decays, 23 candidates are selected with an estimated purity of 71%. They are used to measure the CP asymmetry of this decay, given by sin 2beta in the Standard Model, with the result sin 2beta = 0.84 +0.82-1.04 +-0.16. This is combined with existing measurements from other experiments, and increases the confidence level that CP violation has been observed in this channel to 98%.

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arXiv2000

Measurement of the Bbar0 and B- Meson Lifetimes

The lifetimes of the Bbar0 and B- mesons are measured using a sample of about four million hadronic Z decays collected from 1991 to 1995 with the ALEPH detector at LEP. The data sample has been recently reprocessed, achieving a substantial improvement in the tracking performance. Semileptonic decays of Bbar0 and b- mesons are partially reconstructed by identifying events containing a lepton with an associated D*+ or D0 meson. The proper time of the B meson is estimated from the measured decay length and the momentum of the D-lepton system. A fit to the proper time of 1880 D*+l- and 2856 D0l- candidates yields the following results: tauBbar0 = 1.518 +- 0.053 +- 0.034 ps, tauB = 1.648 +- 0.049 +- 0.035 ps, tauB-/tauBbar0 = 1.085 +- 0.059 +- 0.018.

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arXiv2000

A Measurement of the b-quark Mass from Hadronic Z Decays

Hadronic Z decay data taken with the ALEPH detector at LEP1 are used to measure the three-jet rate as well as moments of various event-shape variables. The ratios of the observables obtained from b-tagged events and from an inclusive sample are determined. The mass of the b quark is extracted from a fit to the measured ratios using a next-to-leading order prediction including mass effects. Taking the first moment of the y3 distribution, which is the observable with the smallest hadronization corrections and systematic uncertainties, the result is: mb(MZ) = [3.27+-0.22(stat) +-0.22(exp)+-0.38(had)+-0.16(theo)] GeV/c2. The measured ratio is alternatively employed to test the flavour independence of the strong coupling constant for b and light quarks.

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arXiv2000

Search for gamma gamma decays of a Higgs boson produced in association with a fermion pair in e+e- collisions at LEP

A serach for gamma gamma decays of Higgs boson is performed in the data sample collected at LEP with the ALEPH detector between 1991 and 1999. This corresponds to an integrated luminosity of 672 pb-1 centre-of-mass energies ranging from 88 to 202 GeV. The search is based on topologies arising from a Higgs boson produced in association with a fermion pair via the Higgs-strahlung process e+e- -> Hffbar, with ffbar = nunubar, e+e-, mu+mu-, tau+tau- or qqbar... (CERN)

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arXiv2000

Search for charged Higgs bosons in e+e- collisions at energies up to sqrt(s) = 189GeV

The data collected at centre-of-mass energies of 188.6 GeV by ALEPH at LEP, corresponding to an integrated luminosity of 176.2 pb-1, are analysed in a search for pair-produced charged Higgs bosons H+/-. Three analyses are employed to select the taunutaunu, taunucs and cscs final states. No evidence for a signal is found. Upper limits are set on the production cross section as a function of the branching fraction BR(H+ to tau nu) and of the mass M(H+), assuming that the sum of the branching ratios is equal to one. In the framework of a two-Higgs-doublet model, charged Higgs bosons with masses below 65.4 GeV/c2 are excluded at 95% confidence level independently of the decay mode.

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