Measurement of the radiative Ke3 branching ratio

We present a measurement of the relative branching ratio of the decay K0 → π± e± νγ (Ke3γ) with respect to K0 → π± e± ν (Ke3 + Ke3γ ) decay. The result is based on observation of 19 000 Ke3γ and 5.6 × 10^6 Ke3 decays. The value of the branching ratio is BR(K0e3γ , E∗γ > 30 MeV, θ∗eγ > 20◦)/ Br(K0e3 ) = (0.964 ± 0.008+0.011-0.009)%. This result agrees with theoretical predictions but is at variance with a recently published result

Measurement of K0e3 form factors

The semileptonic decay of the neutral K meson, K0L → π± e∓ ν (Ke3 ), 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+(q2)), scalar (fS) and tensor (fT) interactions were measured. The linear slope of the vector form factor λ+ = 0.0284 ± 0.0007 ± 0.0013 and values for the ratios |fS/f+(0)| = 0.015 +0.007-0.010 ± 0.012 and |fT/f+(0)| = 0.05 +0.03-0.04 ± 0.03 were obtained. The values for fS and fT are consistent with zero. Assuming only vector–axial-vector couplings, λ+ = 0.0288 ± 0.0004 ± 0.0011 and a good fit consistent with pure V–A couplings were obtained. Alternatively, a fit to a dipole form factor yields a pole mass of M = 859 ± 18 MeV, consistent with the K∗(892) mass

A new measurement of direct CP violation in two pion decays of the neutral kaon

The NA48 experiment at CERN has performed a new measurement of direct CP violation, based on data taken in 1997 by simultaneously collecting KL and KS decays into pi0 pi0 and pi+ pi-. The result for the CP violating parameter Re(epsilon'/epsilon) is (18.5 +- 4.5(stat.) +- 5.8(syst.)) x 10^-4

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Investigation of K-L,K-s -> pi(+)pi(-)e(+)e(-) decays

The $K_{L} \to \pi^{+}\pi^{-}e^{+}e^{-}$ and $K_{S} \to \pi^{+}\pi^{-}e^{+}e^{-}$ decay modes have been studied in detail using the NA48 detector at CERN SPS. Based on the data collected during the 1998 and 1999 run periods, a sample of 1162 $K_{L} \to \pi^{+}\pi^{-}e^{+}e^{-}$ candidates has been observed with an expected background level of 36.9 events, yielding the branching ratio measurement $BR(K_{L} \to \pi^{+}\pi^{-}e^{+}e^{-})=(3.08 \pm 0.20) \times 10^{-7}$. The distribution of events in the sin$\phi$cos$\phi$ variable, where $\phi$ is the angle between the $\pi^{+}\pi^{-}$ and the $e^{+}e^{-}$ decay planes in the kaon centre of mass, is found to exhibit a large CP-violating asymmetry with the value $A_{\phi}(14.2 \pm3.6)$%. For the $K_{S} \to \pi^{+}\pi^{-}e^{+}e^{-}$ decay channel, 621 candidates have been identified in the 1999 data sample with an estimated background contribution of 0.7 event. The corresponding branching ratio has been determined to be $BR(K_{S} \to \pi^{+}\pi^{-}e^{+}e^{-})=(4.71 \pm 0.32) \times 10^{-5}$. The combined value of this measurement with the published 1998 result is $BR(K_{S} \to \pi^{+}\pi^{-}e^{+}e^{-})=(4.69 \pm 0.30) \times 10^{-5}$. No asymmetry is observed in this decay mode. Our results are in good agreement with theoretical predictions based on a phenomenological description of radiative kaon decays. The form factor parameters a$_{1}$/a$_{2}$ and $\tilde{g}_{M1}$ in the $K_{L} \to \pi^{+}\pi^{-}e^{+}e^{-}$ direct emission process as well as the value of the $K^{0}$ charge radius have been extracted from the data