# Mesure de la polarisation du baryon $\Lambda_{b}$ dans les évènements $Z\to$ $b\bar{b}$ à l'expérience ALEPH au LEP

Abstract : We present the first measurement of the $\Lambda_{b}$ polarisation at LEP. The $\Lambda_{b}$ baryons are exected to be polarised in $Z$ de cays. The value of this polarisation depends on the fragmentation proces yielding $b$ baryons from the polarised b quarks (P=o,94) produced in $Z$ decays. The $\Lambda_{b}$ semileptonic decays are selected using ($\Lambda \pi$) $- lepton$ charge correlations wiyh a good purity (65%). The ratio of the mean chraged lepton and neutrino energies is sensitive to the $\Lambda_{b}$ polarisation. This ratio obtained from the data is divided by the ratio value extracted from a Monte Carlo sample of unpolarised $\Lambda_{b}$ semileptonic decays and compared to the theoretical prediction. From 3,9 millions $Z$ hadronic decays collected by ALEPH during the period 1991-1995, the $\Lambda_{b}$ polarisation is measured to be : $p=- 0,31\ _{-0,19}^{+0,22}$ (stat.) $\pm$ 0,08 (syst.)
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http://hal.in2p3.fr/tel-01520917
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### Identifiers

• HAL Id : tel-01520917, version 1

### Citation

Diaconu C.. Mesure de la polarisation du baryon $\Lambda_{b}$ dans les évènements $Z\to$ $b\bar{b}$ à l'expérience ALEPH au LEP . Physique des Hautes Energies - Expérience [hep-ex]. Université de Provence - Aix-Marseille I, 1996. Français. ⟨tel-01520917⟩

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