Measurement of the very rare K+→π+ννˉK^+ \to \pi^+ \nu \bar\nu decay

The decay K+→π+νν¯ , with a very precisely predicted branching ratio of less than 10−10 , is among the best processes to reveal indirect effects of new physics. The NA62 experiment at CERN SPS is designed to study the K+→π+νν¯ decay and to measure its branching ratio using a decay-in-flight technique. NA62 took data in 2016, 2017 and 2018, reaching the sensitivity of the Standard Model for the K+→π+νν¯ decay by the analysis of the 2016 and 2017 data, and providing the most precise measurement of the branching ratio to date by the analysis of the 2018 data. This measurement is also used to set limits on BR(K+→π+X ), where X is a scalar or pseudo-scalar particle. The final result of the BR(K+→π+νν¯ ) measurement and its interpretation in terms of the K+→π+X decay from the analysis of the full 2016-2018 data set is presented, and future plans and prospects are reviewed

Recent NA48/2 results on rare kaon decays

NA48/2 results contributing to ChPT testing are presented. A sample of 1663 events of the rare decay K ± → μ ± v e + e − $K^ \pm \to \mu ^ \pm ve^ + e^ -$ has been selected with a minimum e +e −effective mass of 140 MeV. The measured model independent decay rate is in agreement with ChPT predictions. The branching ratio of the K ± → μ ± v e + e − $K^ \pm \to \mu ^ \pm ve^ + e^ -$ decay, never observed so far, has been obtained from a sample of about 5000 candidates with less than 6% background, also in agreement with ChPT predictions. In addition, a most precise measurement of the charged kaon semileptonic formfactors has been obtained from 4.4 million Ke3 and 2.3 million Kµ3 events collected in 2004

NA48/2 Results on Rare Kaon Decays

A most precise measurement of the charged kaon semileptonic form factors has been obtained from 4.4 million Ke3 and 2.3 million Kμ3 events collected in 2004. In addition, the branching ratio of the K± → π±π0e+e− decay, never observed so far, has been obtained from a sample of 4919 candidates with 4.9% background contamination. The study of the kinematic space is performed, and several P, CP-violating asymmetries are evaluated

Precise measurement of the Decay K±→π0π0μ±νK^\pm \to \pi^0\pi^0\mu^\pm\nu

We report the first observation of the decay $K^{\pm} \rightarrow \pi^{0} \pi^{0} \mu^{\pm} \nu$ ($K^{00}_{\mu4}$) by the NA48/2 experiment at the CERN SPS. From 2437 detected signal candidates with a S/B ratio of about 6, the branching ratio of the decay is determined with high precision. In the region of squared dilepton mass above $0.03$~GeV$^2/c^4$, the branching ratio is found to be $BR(K^{00}_{\mu 4},S_l>0.03) = (0.65 \pm 0.03) \times 10^{-6}$. The full phase space result $BR(K^{00}_{\mu 4}) = (3.4 \pm 0.2) \times 10^{-6}$, depending on the decay model extrapolation, is in a reasonable agreement with the R form factor prediction from 1-loop Chiral Perturbation Theory

Search for heavy neutrinos in K+→μ+νμK^+ \rightarrow \mu^+ \nu_{\mu} decays

International audienceThe NA62 experiment recorded a large sample of K+→μ+νμ decays in 2007. A peak search has been performed in the reconstructed missing mass spectrum. In the absence of a signal, limits in the range 2×10−6 to 10−5 have been set on the squared mixing matrix element |Uμ4|2 between muon and heavy neutrino states, for heavy neutrino masses in the range 300–375 MeV/ c2 . The result extends the range of masses for which upper limits have been set on the value of |Uμ4|2 in previous production search experiments

Measurement of the form factors of charged kaon semileptonic decays

International audienceA measurement of the form factors of charged kaon semileptonic decays is presented, based on 4.4 × 10$^{6}$K$^{±}$ → π$^{0}$e$^{±}$ν$_{e}$ (K$_{e3}^{±}$ ) and 2.3 × 10$^{6}$K$^{±}$ → π$^{0}$μ±ν$_{μ}$ (K$_{μ3}^{±}$ ) decays collected in 2004 by the NA48/2 experiment. The results are obtained with improved precision as compared to earlier measurements. The combination of measurements in the K$_{e3}^{±}$ and K$_{μ3}^{±}$ modes is also presented