P. Extermann

High energy cosmic muons and the calibration of the L3 electromagnetic calorimeter

Abstract A method to calibrate the L3 electromagnetic calorimeter with cosmic muons has been tested on a matrix of 100 tapered BGO crystals. Calibration constants in the energy range of 20–30 MeV were measured at the 2% level collecting about 200 muons per crystal. The results are in agreement with the calibration constant determined using a 10 GeV electron beam.

Observation of the K+ → π+e+e− decay

Abstract We report on the observation of 41 K + → π + e + e − decays resulting from a counter experiment on K e4 decay performed at CERN. The branching ratio assuming a uniform phase space distribution is found to be (2.6 ± 0.5) × 10 −7 .

Observation of a narrow state at 2.46 GeV/c2 — a candidate for the charmed strange Baryon A+☆

Abstract A narrow state has been observed in the reaction Σ − + Be → ( Λ K − π + π + ) + X in an experiment at the CERN SPS hyperon beam. At 2.46 GeV/ c 2 the effective ( Λ K − π + π + ) mass distribution shows an excess of 82 events above a background estimated to be 147, corresponding to a statistical significance of more than 6 standard deviations. The positive charge of the observed final state, which has strangeness −2, suggests the interpretation as a Cabibbo favoured decay of the charmed strange baryon, A + [quark content (csu)]. The cross section times branching ratio is measured to be σ · B = (5.3 ± 2.0) μ b/ (Be nucleus) for x > 0.6. The invariant production cross section is described by E d 3 σ /d p 3 ∞ (1 - x ) (1.7±0.7) exp[−(1.1 −0.4 +0.7 ) p T 2 ].

Evidence for narrow states decaying into (Λp + pions) at 3.1GeV / c2 with charges + 1, 0 and −1☆

Abstract Evidence for narrow states produced by Σ− interactions in beryllium, and decaying into ( Λ p + pions ), has been obtained in an experiment at the CERN SPS hyperon beam. At 3.1 GeV/c2, the ( Λ p π + π + ), ( Λ p π + π + π − ) and ( Λ p π + π − ) effective mass distributions show an excess of 45, 19 and 62 events above a background of 50, 28 and 187, respectively. The possibility that these three signals are statistical fluctuations is remote. The observed widths of the signals are compatible with the mass resolution of the apparatus, 24 MeV/c2 FWHM. We have not been able to find an interpretation of the quantum numbers of the final states, which are baryon number 0, strangeness −1 and charge +1, 0 and −1.

Measurement of the lifetime of the charmed strange baryon A

Abstract The lifetime of the charmed strange baryon A+ has been determined in an experiment at the CERN SPS hyperon beam from the distribution of distances between production and decay of A+ candidates of known momentum. The result is: τ = (4.8 −1.8 +2.9 ) × 10 −13 s .

Study of some rare decays of the K+ meson

Abstract In a search for very rare decays of the K + meson we observed the K + → μ + ν e + e − and K + →e + ν e + e − modes with branching ratios of (10±3) ×10 −7 and (2 +2 −2 )×10 −7 respectively. We also put new limits on a decay allowed by the alternate scheme of lepton conservation K + → π − μ + e + , on a decay with double charged weak current K + → π − e + e + and on other forbidden decays K + → μ − ν e + e + and K + → π + μ ± e ± .

Measurements of the total cross sections of Σ− and Ξ− on protons and deuterons between 74 and 137 GeV/c

The Σ− p and Σ− d total cross sections have been measured to a statistical accuracy of ±1% and ±0.5%, respectively, at five momenta from 74.5 to 136.9 GeV/c, using the hyperon beam at the CERN SPS. The Ξ−p and Ξ− d total cross sections have also been measured to the same statistical accuracy at 101.5 and 133.8 GeV/c. The systematic uncertainty at each momentum is estimated to be of the order of ±0.5%. The hyperon-nucleon cross sections are shown to be rising with energy, and the data are compared with various phenomenological models.

Observation of the π0 → e+e− decay

Abstract We have observed for the first time the very rare π 0 → e + e − decay. The tagged π 0 s come from K + → π + π 0 decays in flight. We find the branching ratio to be (2.23 −1.1 +2.4 ) × 10 −7 (90% C.L.). This value is well above the QED unitary limit and comes in rough agreement with a second-order electromagnetic process mediated by vector mesons.

Measurement of the slope of the π0-electromagnetic form factor

Abstract Using a magnetic spectrometer, we have studied 30 000 π 0 → e + e − γ events coming from K π2 + decays in flight. We find a positive value for the slope of the neutral-pion electromagnetic form factor: a = +0.10 ± 0.03 (or a = +0.05 ± 0.03 without radiative corrections). This value is higher than the predictions of vector meson models.

Study of the energy calibration of a high resolution electromagnetic calorimeter

Abstract A calibration study was performed on a prototype of the L3 BGO electromagnetic calorimeter. The accuracy and stability reached is a small fraction of the typical detector resolution.