A.C. Schaffer

A new measurement of direct CP violation in the neutral kaon system

A new measurement of the ratio of the CP-violating amplitudes for KL → 2π0 and KL → π+π− is reported. The measured value for R = ∣η00/η+-∣2 is 0.9878±0.0026±0.0030, where the first error is the statistical uncertainty and the second is the estimate of the systematic uncertainty. This gives a value for the parameter describing direct CP violation: Re ϵ/ϵ = (2.0 ± 0.7) × 10−3.

A measurement of the phases of the CP-violating amplitudes in K0→2π decays and a test of CPT invariance

Abstract The phases of the CP-violating amplitudes in K0→π+π− and K0→2π0 decays, φ+−=46.9°±2.2° and φ00=47.1°±2.8°, have been measured in the same experiment, and a direct comparison gives the phase difference φ00−φ+−=0.2°±2.9°. This result leads to an upper limit on possible CPT violation in the K0 mass matrix, of |(m K 0 −m K 0 )/m K 0 | −18 at the 95% confidence level and is the most stringent test of the equality of particle and antiparticle masses.

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.

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.

Search for the decay KL→π0e+e−

A search for the decay KL→π0e+e− has been performed at the CERN SPS. One candidate event has been found, compatible with an expected background of 1.5 events. The corresponding upper limit for the branching ration is Γ(KL→π0e+e−)Γ(KL → all)<4×10−8 with 90% confidence.

Search for a neutral Higgs particle in the decay sequence KL0→π0H0 and H0→e+e−

Abstract We have searched for the sequence of decays K L 0 → π 0 H 0 and H 0 →e + e − at the CERN Super Proton Synchrotron (SPS), and have allowed for a non-zero H 0 lifetime. Three candidates have been seen, consistent with an expected background of 3.3. Limits on the branching ratio product in the range 10 −8 –10 −7 are presented as a function of the mass and lifetime of the H 0 . These can be used to restrict the neutral Higgs of the minimal standard model.

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 nR = 2.35 ± 0.75(stat.) ± 0.4(syst.) + 7.5 · (1 − BR(KL → γγ)5.86 × 10−4) nwhere 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.