L. Serin

A test of chiral perturbation theory from the measurement of the decay KS → γγ

Data from NA31 experiment at CERN have been used to measure the decay KS → γγ. From 69 candidate events of the type K0 → γγ, 52 events can be attributed to KL → γγ and one event is expected from background processes. The ratio of the relative decay widths R = Γ(KS → γγ)Γ(KL → γγ) is measured to be R = 2.2 ± 1.0(stat.) ± 0.3(syst.) ± 0.2(ext.) where the external systematic error is due to the experimental uncertainty in the branching ratio of the decay KL → γγ. This translates into a branching ratio of BR(KS → γγ) = (2.2 ± 1.1) × 10−6. If these data are combined with the published data from an earlier data taking period with the same experiment, the ratio R is determined to be R = 2.35 ± 0.75(stat.) ± 0.4(syst.) + 7.5 · (1 − BR(KL → γγ)5.86 × 10−4) where the last term accounts for the dependence on the measured KL → γγ branching ratio. The corresponding branching ratio for KS → γγ is BR(KS → γγ) = (2.4 ± 0.9) × 10−6. The measurement is a sensitive test of chiral perturbation theory and in good agreement with theoretical predictions.

Measurement of the decay rate of the radiative Ke30 decay

Abstract Data from the NA31 experiment at CERN have been used to measure the decay K L → π ± e ± νγ ( K e 3 γ 0 ). 1384 ± 53 events of this decay type have been observed. The decay rate relative to K e 3 0 is found to be Γ(K e3γ 0 , E γ ∗ ≥ 30 MeV , Θ e-γ ∗ ≥ 20°) Γ(K e3 0 ) = (0.934 ± 0.036 ( stat. ) −0.039 +0.055 ( syst. )) % which corresponds to a branching ratio of BR (K e3γ 0 , E γ ∗ ≥ 30 MeV , Θ e-γ ∗ ≥ 20°) = (0.361 ± 0.014 ( stat. ) −0.015 +0.021 ( syst. )) %. The result is in good agreement with theoretical predictions. The decay K L → π ± e ± νγ is found to be dominated by internal bremsstrahlung.

Search for the decay KL → π0π0γ

Abstract Data from the NA31 experiment at CERN have been used for a search for the decay mode K L → π 0 π 0 γ . After all cuts, three events remain in the signal region with an estimated background of 2.2 ± 0.9 events. The upper limit for the branching ratio of K L → π 0 π 0 γ is 5.6 × 10 −6 at the 90% confidence level.

Search for the decay KL → 3γ

Abstract Data from the NA31 experiment at the CERN SPS have been used for the first search for the decay mode KL → 3γ. Seven events have been found with an estimated background of 6.7 ± 1.5 events. The corresponding upper limit for the branching ratio is Γ(K L → 3γ) Γ(K L → all ) = 2.4 × 10 −7 at the 90% confidence level, assuming a phase-space decay distribution.

Measurement of the branching ratio of the double Dalitz decayK L →e+e−e+e− and the CP parity of theK L -meson

Data from the NA31 experiment at CERN have been used to measure the branching ratio of the double Dalitz decay of the long-lived neutral kaon. Eight events have been found with negligible background. The measured distribution of the angle between the planes of the two e+e−-pairs favours aCP=−1 state for the long-lived neutral kaon.

Lower Limit on the Mass of the Neutralino (LSP) at LEP with the ALEPH Detector

The large amount of data accumulated at LEP2 by the ALEPH experiment has been used to search for supersymmetric particles. No signal has been found therefore limits have been determined. Within the Constrained Minimal Supersymmet- ric Standard Model, the constraints from direct SUSY searches of charginos. sleptons and neutralinos. are combined to extract a lower limit on the mass of the neutralino considered to be the Lightest Supersymmetric particle. An improved limit is obtained when the limit on the Higgs mass is included. Neutralino masses up to 38 GeV/c2 are excluded at 95% confidence level.

Measurement of the decay KL→e+e−γγ

Abstract The full data set of the NA31 experiment at CERN has been used to analyse the decay mode K0L→e+e−γγ. A signal of 47 events has been observed with an estimated background of 6.9 events. The total number of kaon decays in the decay volume was determined to be (2.1±0.1stat)·109. This corresponds to a branching ratio of 8.0±1.5 +1.4 −1.2 ·10 −7 .

Determination of the branching ratiosΓ(KL→3π0)/Γ(KL→π+-0) andΓ(KL→3π0)/Γ(KL→πev)

AbstractImproved branching ratios were measured for theKL→3π0 decay in a neutral beam at the CERN SPS with the NA31 detector:

Determination of the branching ratios

\Gamma ({\rm K}_L \to 3\pi ^0 )/\Gamma ({\rm K}_L \to \pi ^ + \pi ^ - \pi ^0 ) = 1.611 \pm 0.037