81 research outputs found

    A study of the τ\tau Lepton polarization in Z0Z^0 decays

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    Tau polarization in the decay Z° -* t+t ~ channel is studied in this work. Z° events were collected by the ALEPH detector at the LEP collider at CERN, Geneva. The data collected by the detector were in a range of energy in the vicinity of the Z° pole. D ata analysis in the ALEPH Collaboration is achieved through the contribution of many people from various institutes spread all over the world, since the idea of LEP was born. The author's contributions to the experiment include work on the construction and testing of the electromagnetic calorimeter modules built at Glasgow University, as well as the analysis presented here. The analysis presented in this thesis is the individual work of the author within a team in the ALEPH Collaboration. No portion of this thesis has been subm itted in support of an application for another degree or qualification in this University, or any other institution of teaching

    Measurement of B - anti-B mixing at the Z

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    From more than 175 000 hadronic Z decays observed with the ALEPH detector at LEP, we select 823 events with pairs of leptons in the final state. From these we measure χ, the probability thata b hadron which is observed to decay originated as a hadron. We find χ=0.132−0.026+0.027

    Measurement of the B hadron lifetime

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    The average lifetime of B hadrons has been measured by the ALEPH experiment at LEP. Events containing B hadrons are selected by the identification of leptons with high transverse momentum in hadronic Z decays, and the lifetime is extracted from a fit to the impact parameter distribution of the lepton tracks. From a sample of 1.7x10^5 hadronic Z decays a lifetime of 1.29 +/- 0.06 +/- 0.10 ps is measured

    Evidence for b baryons in Z decays

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    In 160 000 hadronic Z decays recorded with the ALEPH detector at LEP, the yields of Λℓ− and Λℓ+ combinations have been measured. The observed excess of Λℓ− over Λℓ+ of 53±13 is interpreted as evidence for b baryons and their semileptonic decay. Assuming that three body decay processes such as dominate the semileptonic decay of b baryons, this ex cess corresponds to a product branching ratio , where ΛbandΛc+ denote the bottom and charm baryons respectively

    A Measurement of the b baryon lifetime

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    In 451000 hadronic Z0 decays, recorded with the ALEPH detector at LEP, the yields of lambda l- and lambdal+ combinations are measured. Semileptonic decays of b baryons result in a signal of 122 +/- 18(stat.)+22/-23(syst.) lambda l- combinations. From a fit to the impact parameter distribution of the leptons in the lambda l- sample, the lifetime of b baryons is measured to be 1.12+0.32/-0.29(stat.) +/- 0.16(syst.) ps

    Updated measurement of the average B hadron lifetime

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    An improved measurement of the average lifetime of b hadrons has been performed with the ALEPH detector. From a sample of 260 000 hadronic Z0 decays, recorded during the 1991 LEP run with the silicon vertex detector fully operational, a fit to the impact parameter distribution of lepton tracks coming from semileptonic decays yields an average b hadron lifetime of 1.49 ± 0.03 ± 0.06 ps

    Measurement of B - anti-B mixing at the Z using a jet charge method

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    Results on B0−0 mixing in e+e− annihilation at LEP are reported. A new method is used, where the charge of one b quark is tagged by a high-p, high-p⊥ electron or muon, and the charge of the other is extracted from the momentum-weighted average of the charges of jet fragments. Based on the analysis of 180 000 hadronic Z decays produced in the ALEPH detector, is obtained. Combining this result with the ALEPH dipleton measurement yields χ=0.129±0.022

    Measurements of mean lifetime and branching fractions of b hadrons decaying to J / psi

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    From a data sample of 450 000 hadronic events recorded with the ALEPH detector at LEP, 92±10 events are observed containing a J/ψ meson decaying to μ+μ− or e+e−. From these data the measured inclusive branching fraction for a b flavoured hadron to decay to a J/ψ is BR(b→ J/ψX) = (1.21±0.13 (stat.)±0.08 (syst.))%, and the average b hadron lifetime in the events tagged with a J/ψ is τb = 1.35+0.19−0.17±0.05 ps. Five events are observed consistent with the exclusive decay B± → J/ψK± and from these events the exclusive branching fraction is measured to be BR(B± → J/ψK±) = (0.22±0.10±0.02)%. Upper limits for other exclusive branching ratios are given

    Measurement of the b ---> tau- anti-tau-neutrino X branching ratio

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    Using a missing energy tag, evidence is presented for the decay b ---> tau- anti-tau-neutrino X, and its branching ratio is measured to be (4.08 ± 0.76 ± 0.62)%

    Measurement of the forward - backward asymmetry in Z ---> b anti-b and Z ---> c anti-c

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    From a sample of 150 000 hadronic Z decays collected with the ALEPH detector at LEP, events containing prompt leptons are used to measure the forward-backward asymmetries for the channels and , giving the results AFBb=0.126±0.028±0.012 and AFBc=0.064±0.039±0.030. These asymmetries correspond to the value of effective electroweak mixing angle at the Z mass sin2θW(mZ2) = 0.2262±0.0053
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