A Wagner

A new measurement of the (K+ → e+v)(K+ → μ +v) branching ratio

The branching ratio R = Γ(K+ → e+v)Γ(K+ → μ+v) has been measured on the basis of 534 observed K+ → e+v decays. The K+ decay at rest; the momenta of the e+ and μ+ are measured in a magnetic spectrometer using multiwire driftchambers, and the electrons are identified in a gas Cerenkov counter. The result is R = (2.37 ± 0.17_× 10−5. The value predicted for pure axial-vector interaction and μ-e-universality is R = 2.57 × 10−5.

Beta-decay of polarized Λ hyperons I. Measurement of the decay rate☆☆☆

Abstract The branching ratio R = Γ ( Λ → pe ν )/ Γ ( Λ → N π ) has been measured in a counter-spark chamber experiment in which 1078 ± 11 beta-decays of polarized Λ hyperons were observed. The value R = (8.4 ± 0.4) × 10 −4 is obtained.

A measurement of Kevγ+ decay

The decay K+ → e+ vγ has been observed. In a counter experiment at CERN, 56 events of this type have been identified by detection of a γ with an energy > 100 MeV and of an e+ with an energy between 236 MeV and the maximum e+ energy, 247 MeV. The angle between γ and e+ was > 120°. Thus, the experiment was sensitive only to the structure decay (SD) term proportional to the squared sum of vector- and axialvector amplitudes, |νK + aK|2, corresponding to the emission of right handed γ. We find Δ+(SD)/Δ(Ke2) = 1.05−0.30+0.25 and Δ_(SD) < 85 (90% CL). Δ+ is in agreement with theoretical predictions.

Construction and performance of the new ALEPH Vertex Detector

A new Silicon Vertex Detector was developed for the ALEPH experiment and first installed for the high energy run at 130 GeV at the end of 1995. The detector has an active length of 40 cm and consists of two concentric layers of silicon wafers with double-sided readout. It extends the angular coverage, has only half the passive material as the former detector in the tracking volume and is radiation hard to cope with the higher level of radiation background expected for the LEP2 phase. The construction and the performance of the detector is described.

Beta-decay of polarized Λ hyperons II. Determination of the vector and axial vector form factors☆☆☆

Abstract In the analysis of 1078 ± 11 beta decays of polarized Λ hyperons, the asymmetry parameters α e , α p , and α eν were determined. From these data and the transition rate Γ(Λ → peν the following values for the form factors were obtained: g 1 / f 1 = 0.62 −0.08 +0.09 , f 2 / f 1 = 0.89 ± 0.61 and ¦f 1 ¦= 0.32 ± 0.02; g 2 = 0 is assumed. The results agree with the Cabibbo theory.

Beta decay of polarized Λ hyperons III. Correlations in the Λ centre-of-mass system and the proton recoil spectrum

Abstract From the analysis of 817 kinematically resonstructed beta decay events of polarized Λ hyperons for the coefficient α ν of the neutrino correlation with respect to the Λ spin, α ν = 0.89 ± 0.08, for the coefficient α T of the T-odd correlation σ Λ ( p e × p ν ), α T = −0.14 ± 0.13. The proton recoil spectrum yields … g 1 / f 1 … = 0.64 ± 0.06.

A new determination of the Λ0 mean life

Abstract From 6582 kinematically-reconstructed Λo → pπ decays observed in an optical spark c amber, the Λ0 lifetime was evaluated to be τΛ = (2.69 ± 0.05) × 10−10 sec. This value is a factor of 1.067 ± 0.020 higher than the present world average.

Monitoring the Stability of the ALEPH Vertex Detector

Abstract The ALEPH Silicon Vertex Detector features an optical fibre laser system to monitor its mechanical stability. The operating principle and the general performance of the laser system are described. The experience obtained during 1997 and 1998 operations confirms the important role that such a system can have with respect to the detector alignment requirements. In particular, the laser system has been used to monitor short-term temperature-related effects and long-term movements. These results and a description of the laser-based alignment correction applied to the 1998 data are presented.