A. Kreutz

Observation of the decay KL→π0γγ

Abstract The decay mode K L → π 0 γγ has been observed with a signal of 21 events and an expected background of 1.5±0.9 events. A branching ratio for decays with invariant γγ masses above 280 MeV of (2.1±0.6)×10 −6 is calculated. This result is compared with the values estimated from theoretical models and has implications for the CP conserving contribution to K L → π 0 e + e − decay.

Observation of the decay K(L) ---> pi0 gamma gamma

Abstract The decay mode K L → π 0 γγ has been observed with a signal of 21 events and an expected background of 1.5±0.9 events. A branching ratio for decays with invariant γγ masses above 280 MeV of (2.1±0.6)×10 −6 is calculated. This result is compared with the values estimated from theoretical models and has implications for the CP conserving contribution to K L → π 0 e + e − decay.

Measurement of the rate of the decay KL→e+e−γ and observation of a form factor in this decay

Abstract A large sample of Dalitz decays, K L → e + e − γ , has been observed. The branching ratio is Γ(K L →e + e − γ) Γ(K L → all )=(9.2±0.5±0.5)×10 −6 in good agreement with theoretical predictions. We observe an enhancement at high masses in the distribution of the invariant electron-positron pair mass, compared to the distribution expected from QED. This excess is interpreted as being due to virtual mesons contributing to the photon propagator.

A large-area transition radiation detector

Abstract The construction and the operation of a large-area transition radiation detector (TRD) for the NA31 experiment at CERN are described. The TRD incorporates several novel features for stabilizing the detector response. The density of the gas mixture (xenon+helium+methane) in the detection chambers is matched to the carbon dioxide gas in the surrounding radiators by tuning the helium concentration to avoid a hydrostatic pressure difference, which would deform the chamber walls. The chamber pressure is continuously regulated by computer control to maintain it to within 1 μbar of the radiator pressure. The gas gain of each of the four chambers is regulated to better than 0.2% by changing the high voltage under computer control, using the pulse-height spectra of 16 109 Ca sources mounted on the chambers. The results of performance studies are described. The detector has a pion efficiency of 98.7% with an electron rejection of a factor of 10.

A measurement of the decay KL→π0γγ

Abstract The full data set of the experiment NA31 at CERN has been used to analyse the decay mode K L → π 0 γγ . A signal of 63 events has been observed with an estimated background of 6.0±1.7 events, corresponding to a branching ratio of (1.7±0.3)×10 −6 consistent with our previous result based on partial statistics. The invariant mass spectrum of the two photons is found to be consistent with chiral perturbation theory, and the decay is dominated by the J = 0 two photon state.

PERFORMANCE OF AN ELECTROMAGNETIC LIQUID KRYPTON CALORIMETER

Abstract A prototype electromagnetic calorimeter containing 180 litres of liquid krypton has been tested in electron and muon beams at the CERN SPS. The main features of this detector are its active shower medium, a granularity of 2 cm with tower readout, and the use of the initial current readout technique with an effective shaping time of about 60 ns. An energy resolution for electrons of 4.1% √E[ GeV ] with a constant term of 0.4% and a noise contribution of 50 MeV has been achieved. The calorimeter has a spatial resolution of 4.5 mm √E[ GeV ] with a constant term of 0.3 mm and a time resolution of about 0.5 ns for energies larger than 10 GeV.

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.

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.

Observation of the decay KLO→ e+e−ep+e−

Abstract Two events with the characteristic of the decay K L O → e + e − e − have been observed with a negligible expected background from which a branching ratio of (4±3)×10 −8 is derived. This result is consistent with values estimated from theoretical models based on the double internal conventional of the decay K L O → γγ .