Search for the bcb_c meson in hadronic Z decays

A search for the Bc meson decaying into the channels J/psi pi+ and J/psi l nu (l = e or mu) is performed in a sample of 3.9 million hadronic Z decays collected by the ALEPH detector. This search results in the observation of 0 and 2 candidates in each of these channels, respectively, while 0.44 and 0.81 background events are expected. The following 90\% confidence level upper limits are derived: Br(Z->Bc X)/Br(Z->q q )*Br(Bc->J/psi pi+) 3.6 10^-5 Br(Z->Bc X)/Br(Z->q q )*Br(Bc->J/psi l nu) 5.2 10^-5 An additional Bc->J/psi(e+e-) mu nu candidate with very low background probability, found in an independent analysis, is also described in detail

Measurement of the W mass in e+e−e^+e^- collisions at production threshold

In June 1996, the LEP centre-of-mass energy was raised to 161 GeV. Pair production of W bosons in e+e- collisions was observed for the first time by the LEP experiments. An integrated luminosity of 11 pb-1 was recorded in the ALEPH detector, in which WW candidate events were observed. In 6 events both Ws decay leptonically. In 16 events, one W decays leptonically, the other into hadrons. In the channel where both Ws decay into hadrons, a signal was separated from the large background by means of several multi-variate analyses. The W pair cross-section is measured to be sigma_WW = 4.23 +-0.73 (stat.) +- 0.19 (syst.) pb From this cross-section, the W mass is derived within the framework of the Standard Model: MW = 80.14 +- 0.34 (stat.) +- 0.09 (syst.) +- 0.03 (LEP~energy) GeV/c2

Study of muon-pair production at centre-of-mass energies from 20 to 136 GeV with the Aleph detector

The total cross section and the forward-backward asymmetry for the process $e^+ e^- \rightarrow \mu^+ \mu^- (n \gamma)$ are measured in the energy range 20-136 GeV by reconstructing the effective centre-of-mass energy after initial state radiation. The analysis is based on the data recorded with the ALEPH detector at LEP between 1990 and 1995, corresponding to a total integrated luminosity of 143.5 $\mathrm{pb}^{-1}$. Two different approaches are used: in the first one an exclusive selection of events with hard initial state radiation in the energy range 20-88 GeV is directly compared with the Standard Model predictions showing good agreement. In the second one, all events are used to obtain a precise measurement of the energy dependence of $\sigma^0$ and $A_{\mathrm{FB}}^0$ from a model independent fit, enabling constraints to be placed on models with extra Z bosons

Study of Fermion Pair Production in e+e−e^{+}e^{-} Collisions at 130-183 GeV

The cross sections and forward-backward asymmetries of hadronic and leptonic events produced in e+e- collisions at centre-of-mass energies of 130-183 GeV are presented. Results for ee, mumu, tautau, qq, bb and cc production show no significant deviation from the Standard Model predictions. This enable constraints to be set upon physics beyond the Standard Model such as four-fermion contact interactions, leptoquarks, Z' bosons and R-parity violating squarks and sneutrinos. Limits on the energy scale Lambda of eeff contact interactions are typically in the range from 2-10 TeV. Limits on R-parity violating sneutrinos reach masses of a few hundred GeV for large values of their Yukawa couplings

Measurement of the branching fraction for d0→K−π+d^0 \rightarrow K^-\pi^+

The branching fraction for D0 -> K- pi+ is measured with the statistics collected by ALEPH from 1991 to 1994. The method is based on the comparison between the rate for the reconstructed D*+ -> D0 pi+, D0 -> K- pi+ decay chain and the rate for inclusive soft pion production at low transverse momentum with respect to the nearest jet. The result found is B(D0 -> K- pi+) = (3.90 +- 0.09 +- 0.12)