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.

First observation of the KS → π0γ γ decay

Using the NA48 detector at the CERN SPS, 31 KS → π 0γγ candidates with an estimated background of 13.7 ± 3.2 events have been observed. This first observation leads to a branching ratio of BR(KS → π0γγ) z>0.2 = (4.9 ± 1.6stat ± 0.9 syst) × 10-8 in agreement with Chiral Perturbation Theory predictions. © 2003 Elsevier B.V. All rights reserved.

Measurement of Kμ30 form factors

Abstract This Letter reports on a new high precision measurement of the form factors of the K L → π ± μ ∓ ν μ decay. The data sample of about 2.3 × 10 6 events was recorded in 1999 by the NA48 experiment at CERN. Studying the Dalitz plot density we measured a linear, λ + ′ = ( 20.5 ± 2.2 stat ± 2.4 syst ) × 10 −3 , and a quadratic, λ + ″ = ( 2.6 ± 0.9 stat ± 1.0 syst ) × 10 −3 term in the power expansion of the vector form factor. No evidence was found for a second order term for the scalar form factor; the linear slope was determined to be λ 0 = ( 9.5 ± 1.1 stat ± 0.8 syst ) × 10 −3 . Using a linear fit our results were: λ + = ( 26.7 ± 0.6 stat ± 0.8 syst ) × 10 −3 and λ 0 = ( 11.7 ± 0.7 s…

Measurement of the Ξ0→Λγ decay asymmetry and branching fraction

In data taken with the NA48 experiment at the CERN SPS in 1999, 730 candidates of the weak radiative hyperon decay Ξ0→Λγ have been found with an estimated background of 58±8 events. From these events the Ξ0→Λγ decay asymmetry has been determined to α(Ξ0→Λγ)=-0.78±0.18stat±0.06 syst, which is the first evidence of a decay asymmetry in Ξ 0→Λγ. The branching fraction of the decay has been measured to be Br(Ξ0→Λγ)=(1.16±0.05 stat±0.06syst)×10-3. © 2004 Elsevier B.V. All rights reserved.

A new measurement of the branching ratio of K(S) → γ γ

Abstract The decay rate of KS → γγ has been measured with the NA48 detector at the CERN SPS. A total of 149 KS → γγ events have been observed. The branching ratio is determined to be (2.58±0.36(stat)±0.22(sys))×10−6.

First measurement of the rate K0L → πμνγ

Abstract In a preliminary version of the NA48 experiment at the CERN SPS, in which an iron-scintillator sandwich calorimeter was used as a photon detector, the ratio of K L → πμνγ and K L → πμν decays was measured. The branching ratio BR(K L →πμνγ, E γ ∗ ≥30 MeV )/(K L → πμν ) is found to be (2.08±0.17 +0.16 −0.21 )×10 −3 , where the first error is statistical and the second systematic. The result is in agreement with theoretical predictions.

Measurement of K-e3(0) form factors

The semi-leptonic decay of the neutral K meson $K^{0}_{L} \to \pi^{\pm}e^{\mp}\nu (K_{e3})$, was used to study the strangeness-changing weak interaction of hadrons. A sample of 5.6 million reconstructed events recorded by the NA48 experiment was used to measure the Dalitz plot density. Admitting all possible Lorentz-covariant couplings, the form factors for vector $(f_{+}(q^{2}))$, scalar $(f_{S})$ and tensor $(f_{T})$ interactions were measured. The linear slope of the vector form factor $\lambda_{+} = 0.0284 \pm 0.0007 \pm 0.0013$ and values for the ratios $|f_{S}/f_{+}(0)|=0.015^{+0.007}_{-0.010} \pm 0.012$ and $|f_{T}/f_{+}(0)|=0.05^{+0.03}_{-0.04} \pm 0.03$ were obtained. The values fo…

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 KL→π0γγ. 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.

Measurement of the K-S mean lifetime from pi(+)pi(-) and pi(0)pi(0) decays using K-L decays to determine the acceptance

A precision measurement of the KS mean lifetime has been performed by comparing, in the same experimental setup, the distributions of two-pion decays of neutral kaons produced by high energy proton-beryllium interactions in two targets at a relative distance variable between 112 and 160 m. The value obtained combining the results from π+π- and π0π0 decays is τS = (0.8971 ± 0.0021) 10-10 s. © Springer-Verlag 1997.

Search for CP violation in K0→3π0 decays

Abstract Using data taken during the year 2000 with the NA48 detector at the CERN SPS, a search for the CP violating decay K S → 3 π 0 has been performed. From a fit to the lifetime distribution of about 4.9 million reconstructed K 0 / K ¯ 0 → 3 π 0 decays, the CP violating amplitude η 000 = A ( K S → 3 π 0 ) / A ( K L → 3 π 0 ) has been found to be Re ( η 000 ) = − 0.002 ± 0.011 ± 0.015 and Im ( η 000 ) = − 0.003 ± 0.013 ± 0.017 . This corresponds to an upper limit on the branching fraction of Br ( K S → 3 π 0 ) 7.4 × 10 −7 at 90% confidence level. The result is used to improve knowledge of Re ( e ) and the CPT violating quantity Im ( δ ) via the Bell–Steinberger relation.

Observation of the decay KS→π+π−e+e−

Abstract We present the first observation of the decay KS→π+π−e+e− based upon the data collected in 1998 by the NA48 experiment at CERN. We have identified a clean sample of 56 events with negligible background contamination. Using KL→π+π−π0D decays as normalization sample, the branching ratio is measured to be BR(KS→π+π−e+e−)=[4.5±0.7(stat)±0.4(syst)]×10−5. This result is in good agreement with the theoretical expectations from the mechanism of inner bremsstrahlung.

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 decay KL→e+e−γγ

The full data set of the NA31 experiment at CERN has been used to analyse the decay mode KOL → 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. © 1998 Published by Elsevier Science B.V.

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 a CP=-1 state for the long-lived neutral kaon. © 1995 Springer-Verlag.

Measurement of the KL→e+e−e+e− decay rate

Abstract The decay rate of the long-lived neutral K meson into the e + e − e + e − final state has been measured with the NA48 detector at the CERN SPS. Using data collected in 1998 and 1999, a total of 200 events has been observed with negligible background. This observation corresponds to a branching ratio of Br ( K L → e + e − e + e − ) = ( 3.30 ± 0.24 stat ± 0.23 syst ± 0.10 norm ) × 10 −8 .

FIRST EVIDENCE FOR DIRECT CP VIOLATION

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 ± 0.7…

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.

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

Improved branching ratios were measured for theKL→3π0 decay in a neutral beam at the CERN SPS with the NA31 detector: $$\Gamma ({\rm K}_L \to 3\pi ^0 )/\Gamma ({\rm K}_L \to \pi ^ + \pi ^ - \pi ^0 ) = 1.611 \pm 0.037$$ and $$\Gamma ({\rm K}_L \to 3\pi ^0 )/\Gamma ({\rm K}_L \to \pi ev) = 0.545 \pm 0.010.$$ From the first number an upper limit for ΔI=5/2 and ΔI=7/2 transitions in neutral kaon decay is derived. Using older results for the Ke3/Kμ3 fraction, the 3π0 branching ratio is found to beΓ(K L →3π0)/Γ tot =(0.211±0.003), about a factor three more precise than from previous experiments.

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.

Measurement of the quadratic slope parameter in the K-L -> 3 pi(0) decay Dalitz plot

A value of (-6.1 +- 0.9_{stat} +- 0.5_{syst})*10^{-3} is obtained for the quadratic slope parameter h in the K_L -> 3\pi^0 decay Dalitz plot at the NA48 experiment at the CERN SPS. The result is based on 14.7*10^6 fully reconstructed K_L -> 3\pi^0 -> 6\gamma decays. This is the most precise measurement of any of the Dalitz plot slope parameters in the charged and neutral kaon decays so far.

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 .

Search for the decay KS → π0γγ

Abstract A search for the decay K S → π 0 γγ has been made using the NA48 detector at the CERN SPS. Using data collected in 1999 during a 40-hour run with a high-intensity K S beam, an upper limit for the branching ratio BR ( K S → π 0 γγ , z ⩾0.2) −7 has been obtained at 90% confidence level, where z = m γγ 2 / m K 0 2 .

Search for a neutral Higgs particle in the decay sequence $K^{0}_{L} \to \pi^{0}H^{0}$ and $H^{0} \to e^{+}e^{-}$

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 new measurement of direct CP violation in the neutral kaon system

A new measurement of the ratio of the CP-violating amplitudes for $K_{L} \to 2\pi^{0}$ and $K_{L} \to \pi^{+}\pi^{-}$ is reported. The measured value for $\Re = |\eta_{00} / \eta_{ +-}|^{2}$ is $0.9878 \pm 0.0026 \pm 0.0030$, 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: $\Re$ $\epsilon'/\epsilon = (2.0 \pm 0.7) \times 10^{−3}$ .