Le ‘Pensées’, testo da tradurre

I motivi per accingersi oggi, in Italia, a tradurre le Pensées di Blaise Pascal sono due. Anzitutto l’affezione a quel testo, la sua frequentazione, l’utile e il piacere che in tanti anni di lettura hanno procurato e l’ammirazione per il loro stile, tanto più sorprendente a chi trova ingombranti, spesso illeggibili i loro contemporanei italiani: una nudità greca a fronte di un panneggio tardoromano. Si ricavano lezioni morali anche dallo stile di Pascal. Secondo motivo, l’insoddisfazione, per..

Measurement of inclusive π0\pi^{0} production in hadronic Z0Z^{0} decays

An analysis is presented of inclusive \pi^0 production in Z^0 decays measured with the DELPHI detector. At low energies, \pi^0 decays are reconstructed by \linebreak using pairs of converted photons and combinations of converted photons and photons reconstructed in the barrel electromagnetic calorimeter (HPC). At high energies (up to x_p = 2 \cdot p_{\pi}/\sqrt{s} = 0.75) the excellent granularity of the HPC is exploited to search for two-photon substructures in single showers. The inclusive differential cross section is measured as a function of energy for {q\overline q} and {b \bar b} events. The number of \pi^0's per hadronic Z^0 event is N(\pi^0)/ Z_{had}^0 = 9.2 \pm 0.2 \mbox{(stat)} \pm 1.0 \mbox{(syst)} and for {b \bar b}~events the number of \pi^0's is {\mathrm N(\pi^0)/ b \overline b} = 10.1 \pm 0.4 \mbox{(stat)} \pm 1.1 \mbox{(syst)} . The ratio of the number of \pi^0's in b \overline b events to hadronic Z^0 events is less affected by the systematic errors and is found to be 1.09 \pm 0.05 \pm 0.01. The measured \pi^0 cross sections are compared with the predictions of different parton shower models. For hadronic events, the peak position in the \mathrm \xi_p = \ln(1/x_p) distribution is \xi_p^{\star} = 3.90^{+0.24}_{-0.14}. The average number of \pi^0's from the decay of primary \mathrm B hadrons is found to be {\mathrm N} (B \rightarrow \pi^0 \, X)/\mbox{B hadron} = 2.78 \pm 0.15 \mbox{(stat)} \pm 0.60 \mbox{(syst)}

Measurements of the leptonic branching fractions of the τ\tau

Data collected with the DELPHI detector from 1993 to 1995 combined with previous DELPHI results for data from 1991 and 1992 yield the branching fractions B({\tau \rightarrow \mbox{\rm e} \nu \bar{\nu}}) = (17.877 \pm 0.109_{stat} \pm 0.110_{sys} )\% and $B({\tau \rightarrow \mu \nu \bar{\nu}}) = (17.325 \pm 0.095_{stat} \pm 0.077_{sys} )\%$

Search for new phenomena using single photon events in the DELPHI detector at LEP

Data are presented on the reaction \epem~\into~\gamma + no other detected particle at center-of-mass energies, \sqs = 89.48 GeV, 91.26 GeV and 93.08 GeV. The cross section for this reaction is related directly to the number of light neutrino generations which couple to the \zz boson, and to several other phenomena such as excited neutrinos, the production of an invisible `X' particle, a possible magnetic moment of the tau neutrino, and neutral monojets. Based on the observed number of single photon events, the number of light neutrinos which couple to the \zz is measured to be N_\nu = 3.15 \pm 0.34. No evidence is found for anomalous production of energetic single photons, and upper limits at the 95\% confidence level are determined for excited neutrino production (BR < 4-9 \times 10^{-6}), production of an invisible `X' particle (\sigma < 0.1 pb), and the magnetic moment of the tau neutrino (< 5.2 \times 10^{-6} \mu_B). No event with the topology of a neutral monojet is found, and this corresponds to the limit \sigma < 0.044/\epsilon pb at the 95\% confidence level, where \epsilon is the unknown overall monojet detection efficiency

Measurement of the Bd0^{0}_{d} oscillation frequency using kaons, leptons and jet charge

A measurement of the mass difference, \Delta m_d, between the two physical \mbox{B}^0_d states has been obtained from the analysis of the impact parameter distribution of a lepton emitted at large transverse momentum (p_t) relative to the jet axis and from the analysis of the flight distance distribution of secondary vertices tagged by either a high p_t lepton or an identified kaon. In the opposite hemisphere of the event, the charge of the initial quark has been evaluated using a high p_t lepton, a charged kaon or the mean jet charge. With 1.7 million hadronic Z^0 decays recorded by DELPHI between 1991 and 1993, \Delta m_d is found to be: \Delta m_d = 0.531^{+0.050}_{-0.046} ~(stat.) \pm 0.078 ~(syst.) ~ {\mathrm{ps}}^{-1} \,

Search for Neutral Heavy Leptons Produced in Z Decays

Weak isosinglet Neutral Heavy Leptons ($\nu_m$) have been searched for using data collected by the DELPHI detector corresponding to $3.3\times 10^{6}$ hadronic~Z$^{0}$ decays at LEP1. Four separate searches have been performed, for short-lived $\nu_m$ production giving monojet or acollinear jet topologies, and for long-lived $\nu_m$ giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio $BR($Z$^0\rightarrow \nu_m \overline{\nu})$ of about $1.3\times10^{-6}$ at 95\% confidence level for $\nu_m$ masses between 3.5 and 50 GeV/$c^2$. Outside this range the limit weakens rapidly with the $\nu_m$ mass. %Special emphasis has been given to the search for monojet--like topologies. One event %has passed the selection, in agreement with the expectation from the reaction: %$e^+e^- \rightarrow\ell \bar\ell \nu\bar\nu$. The results are also interpreted in terms of limits for the single production of excited neutrinos

Investigation of the splitting of quark and gluon jets

The splitting processes in identified quark and gluon jets are investigated using longitudinal and transverse observables. The jets are selected from symmetric three-jet events measured in Z decays with the Delphi detector in 1991-1994. Gluon jets are identified using heavy quark anti-tagging. Scaling violations in identified gluon jets are observed for the first time. The scale energy dependence of the gluon fragmentation function is found to be about two times larger than for the corresponding quark jets, consistent with the QCD expectation TeX . The primary splitting of gluons and quarks into subjets agrees with fragmentation models and, for specific regions of the jet resolution TeX , with NLLA calculations. The maximum of the ratio of the primary subjet splittings in quark and gluon jets is TeX . Due to non-perturbative effects, the data are below the expectation at small TeX . The transition from the perturbative to the non-perturbative domain appears at smaller TeX for quark jets than for gluon jets. Combined with the observed behaviour of the higher rank splittings, this explains the relatively small multiplicity ratio between gluon and quark jets

Measurement of the Quark and Gluon Fragmentation Functions in Z0Z^0 Hadronic Decays

The fragmentation functions and multiplicities in $b\overline{b}$ and light quark events are compared. The measured transverse and longitudinal components of the fragmentation function allow the gluon fragmentation function to be evaluated