B. Michel

Heavy flavour production in Z decays

Abstract From an analysis of inclusive leptons in data collected by the ALEPH detector at LEP, we measure the fractions of b b and c c events in hadronic Z decays. The b b fraction times semileptonic branching ratio is measured to be Br ( b → e )·Γ b b /Γ had = 0.0224 ± 0.0016 ± 0.0010 . Assuming a b semileptonic branching ratio of 0.102 ± 0.010 gives Γ b b /Γ had = 0.220 ± 0.016 ± 0.024 , in good agreement with the standard model prediction of 0.217. The c c fraction times semileptonic branching ratio is measured to be Br ( c → e )·Γ c c /Γ had = 0.0133 ± 0.0040 −0.0031 +0.0038 . Assuming a c semileptonic branching ratio of 0.090 ± 0.013 gives Γ c c /Γ had = 0.148 ± 0.044 −0.038 +0.045 , in agreement with the standard model prediction of 0.171.

A precise determination of the number of families with light neutrinos and of the Z boson partial widths

More extensive and precise results are reported on the parameters of Z decay. On the basis of 20 000 Z decays collected with the ALEPH detector at LEP we find Mz=91.182±0.026 (exp.) ±0.030 (beam) GeV, Γz=2.541±0.056 GeV and σhad0=41.4±0.8 nb. The partial widths for the hadronic and leptonic channels are Γhad=1804±44 MeV, Γe+e−=82.1±3.4 MeV, Γμ+μ−=87.9±6.0 MeV and Γτ+τ−=86.1±5.6 MeV, in good agreement with the standard model. On the basis of the average leptonic width Γl+l−=83.9±2.2 MeV, the effective weak mixing angle is found to be sin2θw(Mz)=0.231±0.008. Usin g the partial widths calculated in the standard model, the number of light neutrino families is Nν=3.01±0.15 (exp.)±0.05 (theor.).

Search for the Neutral Higgs Boson from Z0 Decay

Abstract A search for the neutral Higgs boson, using the processes Z 0 → H 0 e + e − , Z 0 → H 0 μ + μ − , Z 0 → H 0 τ + τ − , Z 0 → H 0 v v and Z 0 → H 0 q q , is performed on data collected by the ALEPH detector corresponding to about 11 550 events of Z0 → hadrons. Combining all these processes, the mass range excluded is 32 MeV−15 GeV at 95% CL and 40 MeV−12 GeV at 99% CL. The result from this experiment is unambiguous in the context of the standard model.

Search for the neutral Higgs Boson from Z0 decay in the Higgs mass range between 11 and 24 GeV

Abstract A search for the neutral Higgs boson in the mass range above 11 GeV (above the H 0 →b b threshold), using the process Z0→H0e+e−, Z0→H0μ+μ− and Z 0 → H 0 v v , is performed on data collected by the ALEPH detector corresponding to about 25 000 events of Z0→ hadrons. Combining all these processes, the mass range excluded is 11 GeV at 95% CL. Together with a previously published result from ALEPH, the mass range excluded is 32 MeV to 24 GeV at 95% CL. This result also extends the excluded mass region for neutral Higgs bosons from supersymmetry.

Measurement of the charged particle multiplicity distribution in hadronic Z decays

The charged particle multiplicity distribution of hadronic Z decays was measured on the peak of the Z resonance using the ALEPH detector at LEP. Using a model independent unfolding procedure the distribution was found to have a mean 〈n〉=20.85±0.24 and a dispersion D=6.34±0.12. Comparison with lower energy data supports the KNO scaling hypothesis in the energy range GeV. At GeV the shape of the multiplicity distribution is well described by a log-normal distribution, as predicted from a cascading model for multi-particle production. The same model also successfully describes the energy dependence of the mean and width of the multiplicity distribution. A next-to-leading order QCD prediction in the framework of the modified leading-log approximation and local parton-hadron duality is found to fit the energy dependence of the mean but not the width of the charged multiplicity distribution, indicating that the width of the multiplicity distribution is a sensitive probe for higher order QCD or non-perturbative effects.

Study of the J/ψ decay into five pions

Abstract A high statistics study of the J/ψ → π+π−π+π−π0 decay is performed from the 8.6 million J/ψs produced by the DM2 experiment at DCI. ϱ- and ω-associated production is evidenced and branching ratios for ϱa2(1320), inclusive ωπ+π−, ωf2(1270) and b1±(1235)π∓ dynamics are given. In the ωπ+π− final state a bump is observed at a low (π+π−) mass and a possible ωf0(975) contribution is discussed. Moreover first measurement of J/ψ → ωπ0π0 is reported and branching ratios for this decay and for ωf2(1270) and b1(1235)π0 dynamics are given. Evidence is found of the low (ππ) mass bump observed in the charged mode.

SEARCHES FOR THE STANDARD HIGGS BOSON

Abstract A data sample corresponding to about 100 000 hadronic Z decays collected by ALEPH at LEP has been used to search for the standard Higgs boson produced in the reaction e + e − → H 0 Z 0∗ . No indication for any signal was found, and a 95% CL lower limit on the Higgs boson mass has been set at 41.6 GeV.

Construction and performance of an iron-scintillator hadron calorimeter with longitudinal tile configuration

Abstract The first prototype of a scintillator tile hadron calorimeter with longitudinal tile orientation and wavelength shifting fiber readout has been built and tested with pion, electron and muon beams at the CERN SPS. This innovative geometry combines good performance and a simple and cost effective assembly procedure. Calibration and monitoring of this detector have also been investigated.

The pion electromagnetic form factor in the time-like energy range 1.35 ⩽ s ⩽ 2.4 GeV

Abstract The e+e− → π+π− cross section has been measured from about 280 events (an order of magnitude more than the previous world statistics) in the energy interval 1.35 ⩽ s ⩽ 2.4 GeV with the DM2 detector at DCI. The pion squared form factor |Fπ|2 shows a deep minimum around 1.6 GeV/c2 and is better fit under the hypothesis of two ϱ-like resonance ⋍0.25 GeV/c2 wide with 1.42 and 1.77 GeV/c2 masses.

Measurement of the reaction e+e−→ηπ+π− in the center of mass energy interval 1350–2400 MeV

Abstract The e+e−→ηπ+π− reaction has been measured in the center of mass energy interval 1350–2400 MeV by the magnetic detector DM2 at the Orsay storage ring DCI. Under the hypothesis of only one large resonance the cross section is not fit in a satisfactory way. The branching ratio τ−→η−π0ντ=(0.13 ± 0.02)% is deduced via CVC from the above measurement.