Search for the decay $K_{s} \to \pi^{0}e^{+}e^{-}$

Physics Letters B, 2002

Physics Letters B, 2003

A search for the decay K S → π 0 e + e − has been made by the NA48/1 experiment at the CERN SPS accelerator. Using data collected during 89 days in 2002 with a high-intensity K S beam, 7 events were found with a background of 0.15 events. The branching fraction BR(K S → π 0 e + e − , m ee > 0.165 GeV/c 2 ) = (3.0 +1.5 −1.2 (stat) ± 0.2(syst)) × 10 −9 has been measured. Using a vector matrix element and a form factor equal to one, the measurement gives BR(K S → π 0 e + e − ) = (5.8 +2.9 −2.4 ) × 10 −9 .

Physics Letters B, 2003

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 < 3.3 × 10 −7 has been obtained at 90% confidence level, where z = m 2 γγ /m 2 K 0 .

Physics Letters B, 2002

Physics Letters B, 2000

We present the rst observation of the decay K S ! + ; e + e ; based upon the data collected in 1998 by the NA48 experiment at CERN. We h a ve identi ed a clean sample of 56 events with negligible background contamination. Using K L ! + ; 0 D decays as normalization sample, the branching ratio is measured to be BR(K S ! + ; 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.

Proceedings of The 22nd International Workshop on Neutrinos from Accelerators — PoS(NuFact2021)

The NA62 experiment reports the branching ratio measurement BR(K + → +) at 68 % CL, based on the observation of 20 signal candidates with an expected background of 7.0 events from the total data sample collected at the CERN SPS during 2016-2018. This provides evidence for the very rare K + → + decay, observed with a significance of 3.4. The experiment achieves a single event sensitivity of (0.839 ± 0.054) × 10 −11 , corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10 −11. The result represents the most accurate measurement achieved so far of this ultra-rare decay. Future prospects and plans for data taking from 2021 will also be presented.

Physics Letters B, 2006

We present a measurement of the branching ratio of the CP violating decay K L → π + π − performed by the KLOE experiment at the φ factory DAΦNE. We use 328 pb −1 of data collected in 2001 and 2002, corresponding to ∼ 150 million tagged K L mesons. We find BR(K L → π + π − ) = (1.963 ± 0.012 stat. ± 0.017 syst. ) × 10 −3 . This branching ratio measurement is fully inclusive of final-state radiation. Using the above result, we determine the modulus of the amplitude ratio |η +− | to be (2.219 ± 0.013) × 10 −3 and |ǫ| to be (2.216 ± 0.013) × 10 −3 .

Physics Letters B, 2004

Phys. Lett. B, 2007

The decay rate of K S → πeν relative to the rate of K L → πeν has been measured by the NA48 collaboration in a neutral Kaon beam originating from 400 GeV proton-Be interactions at the CERN SPS. The result is 0.993±0.026 stat ±0.022 syst , compatible with 1 in agreement with the Standard Model prediction at tree level. It implies BR(K S → πeν) = (7.05 ± 0.18 stat. ± 0.16 syst. ) × 10 −4 .