24 research outputs found

    A Study of D*+ pi- production in semileptonic B decay

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    In a sample of 1.5 million hadronic decays of the Z collected by the ALEPH detector, a search is carried out for the decays and . The product branching ratio for D10 production is measured to be , and a 95% confidence level limit of is obtained for D2∗0 production. A topological search sensitive to the processes above, but also to wide resonances that decay to and to non-resonant production is also carried out, yielding . Direct evidence of D∗π production inconsistent with D10 and D2∗0 in semileptonic B decay is presented

    PRODUCTION OF CHARMED MESONS IN Z DECAYS

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    The production of charmed mesons D0(-), D+/-, and D*+/- is studied in a sample of 478,000 hadronic Z decays. The production rates are measured to be GAMMA(Z --> D*+/- X)/GAMMA(had) = 0.187 +/- 0.015 (exp.) +/- 0.013 (BR), GAMMA(Z --> D+/- X) = 0.251 +/- 0.026 (exp.) +/- 0.025 (BR), GAMMA(Z --> D0(-) X)/GAMMA(had) = 0.518 +/- 0.052 (exp.) +/- 0.035 (BR), where the errors from this analysis are separated from those coming from the D branching ratios (BR). The D *+/momentum distribution is extracted separately for Z --> cc and Z --> bbBAR events with the help of event shape variables. It is consistent with the prediction of the JETSET Monte Carlo program after adjustment of the charm fragmentation function. Constraining the shape of the Z --> bbBAR contribution, the average fraction of the beam energy taken by a D* meson produced in the fragmentation of a charm quark is extracted by a parametric fit to be [X(E)]c = 0.495 +/- 0.011 +/- 0.007. Evidence for D**0 (D1(2420)0 and/or D2*(2460)0) production is found in the D*+/-pi-/+ channel, accounting for a fraction (18 +/- 5 +/- 2)% of all D*+/- production. The relative production of vector and pseudoscalar mesons is dicussed, together with the possible effects of D** production. The c-quark forward-backward Z-pole asymmetry is determined from that of high momentum D*+/- to be A(FB)0,c = (7.7 +/- 4.4)%

    Production of charmed mesons in Z decays

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    The production of charmed mesons D0(−) ,D ± , andD *± is studied in a sample of 478,000 hadronicZ decays. The production rates are measured to be Γ(Z→D∗±X)Γhad=0.187±0.015(exp.)±0.013(BR),\hfillΓ(Z→D±X)Γhad=0.251±0.026(exp.)±0.025(BR),\hfillΓ(Z→D0(−)X)Γhad=0.518±0.052(exp.)±0.035(BR),\hfill where the errors from this analysis are separated from those coming from theD branching ratios (BR). TheD *± momentum distribution is extracted separately for Z→ccˉ and Z→bbˉ events with the help of event shape variables. It is consistent with the prediction of the JETSET Monte Carlo program after adjustment of the charm fragmentation function. Constraining the shape of the Z→bbˉ contribution, the average fraction of the beam energy taken by aD * meson produced in the fragmentation of a charm quark is extracted by a parametric fit to be c =0.495±0.011±0.007. Evidence forD **0 (D 1(2420)0 and/orD 2 *(2460)0) production is found in the D∗±π∓ channel, accounting for a fraction (18±5±2)% of allD *± production. The relative production of vector and pseudoscalar mesons is dicussed, together with the possible effects ofD ** production. Thec-quark forward-backwardZ-pole asymmetry is detrmined from that of high momentumD *± to beA FB 0,c =(7.7±4.4)%

    Search for CP violation in the decay Z ---> tau+ tau-

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    Data collected by ALEPH in the years 1990, 1991 and 1992 have been used to update a previous search for CP violation in the decay of the Z into tau(+)tau(-). The measurement of the weak dipole form factor of the tau lepton has been performed by studying correlations between the tau leptons. No signal of CP violation was found. The weak dipole form factor is found to be over tilde d(tau) = (+0.15 +/- 0.58(stat) +/- 0.38(sys)) 10(-17)e x cm, obtained with 19628 identified tau(+)tau(-) events. This gives an upper limit on the weak dipole form factor of |over tilde d(tau)| < 1.5 . 10(-17)e x cm at the 95% confidence level

    First measurement of the quark to photon fragmentation function

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    Earlier measurements at LEP of isolated hard photons in hadronic Z decays, attributed to radiation from primary quark pairs, have been extended in the ALEPH experiment to include hard photon productioninside hadron jets. Events are selected where all particles combine democratically to form hadron jets, one of which contains a photon with a fractional energyz≥0.7. After statistical subtraction of non-prompt photons, the quark-to-photon fragmentation function,D(z), is extracted directly from the measured 2-jet rate. By taking into account the perturbative contributions toD(z) obtained from anO(ααs) QCD calculation, the unknown non-perturbative component ofD(z) is then determined at highz. Provided due account is taken of hadronization effects nearz=1, a good description of the other event topologies is then found

    PRODUCTION OF CHARMED MESONS IN Z DECAYS RID F-5127-2011 RID C-6238-2011 RID E-8846-2011 RID D-3706-2011

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    Search for high mass photon pairs in e+ e- ---> f anti-f gamma gamma (f = e, mu, tau, neutrino, q) at LEP

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    The result of a search for high mass photon pairs from the processes with the ALEPH detector is reported. The result for f = e, μ and τ is to be compared with the observation of 4 events by the L3 Collaboration with invariant masses, Mγγ, of the two photons near 60 GeV. From a data sample approximately twice as large taken from 1990 to 1992, 6 events are found with Mγγ distributed between 50 GeV and 72 GeV, while 4.9 events are expected from a QED calculation. There is no evidence for a mass peak; only one event (μ+μ−γγ) at Mγγ = 59.4 ± 0.2 GeV is compatible with the L3 observation. In addition, for Mγγ > 50 GeV, no event is found for and only one event is found consistent with ; this event has Mγγ = 58.5 ± 1.9 GeV. High mass photon pair events have also been searched for in γγ collisions. This allows one to set an upper limit of 50 MeV for the width of an assumed resonance decaying to photon pairs

    A Precise measurement of the average b hadron lifetime

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    An improved measurement of the average b hadron lifetime is performed using a sample of 1.5 million hadronic Z decays, collected during the 1991–1993 runs of ALEPH, with the silicon vertex detector fully operational. This uses the three-dimensional impact parameter distribution of lepton tracks coming from semileptonic b decays and yields an average b hadron lifetime of 1.533 ± 0.013 ± 0.022 ps

    Search for a nonminimal Higgs boson produced in the reaction e+ e- ---> h Z*

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    A data sample corresponding to 1.23 million hadronic Z decays collected by the ALEPH detector at LEP has been searched for signals of the production of a non-minimal CP-even Higgs boson h in the reaction e+e- --> hZ*. The h decay modes considered were: those of the minimal standard model Higgs boson, with modified branching ratios; decays into a pair of CP-odd Higgs bosons A; and decays into invisible final states. Only one event was found, a very acoplanar e+e- pair which could originate from the standard model background process e+e- --> e+e- nu (nu) over bar. Upper limits for the cross-section of the reaction e+e- --> hZ* have been derived as a function of m(h), the mass of the Higgs boson h. In the case of invisible decays, the 95% CL lower limit on m(h) is 65 GeV/c(2) for a production cross-section equal to that of a minimal standard model Higgs boson. When combined with previous ALEPH results on the reaction e+e- --> hA, these cross-section upper limits exclude a domain in the (m(h), M(A)) plane of the MSSM such that, if invisible h and A decays can be neglected, 95% CL lower limits of 44 and 21 GeV/c(2) result for m(h) and m(A), respectively, independent of the other parameters of the model

    Search for the Standard Model Higgs boson

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    Using a data sample corresponding to about 1 233 000 hadronic Z decays collected by the ALEPH experiment at LEP, the reaction has been used to search for the standard model Higgs boson, in association with missing energy when , or with a pair of energetic leptons when . No signal was found and, at the 95% confidence level, mH exceeds 58.4 GeV/c2
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