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 QCD-related Results from Kaon Physics at CERN (NA48/2 and NA62)

The NA48/2 experiment presents a preliminary result of K ± l decays form factors measurement based on the 4 . 28 × 10 6 K ± e 3 and 2 . 91 × 10 6 K ± μ 3 selected decays collected in 2004. The result is competetive with other measurements in K ± μ 3 mode and has a smallest uncertainty for K ± e 3 , that leads to the most precise combined K ± l 3 result and allows to reduce the form factor uncertainty of | V US | . The NA62 experiment collected a large sample of charged kaon decays with a highly efficient trigger for decays into elec- trons in 2007. A final result of a new measurement of the electromagnetic transition form factor slope of the neutral pion in the time-like momentum region from the 1 . 11 × 10 6 fully reconstructed π 0 Dalitz decays is presented in the second part of this article

New NA48/2 Results on Rare Kaon Decays

NA48/2 results related to ChPT testing are presented. The most precise measurement of the charged kaon semileptonic form factors has been obtained from $4.4 \times 10^6$ the $K^{\pm} \rightarrow \pi^0e^{\pm}\nu_e (K^{\pm}_{e3})$ and $2.91 \times 10^6$ the $K^{\pm} \rightarrow \pi^{0} \mu^{\pm}\nu_{\mu} (K^{\pm}_{\mu 3})$ events collected in 2004. In addition, the branching ratio of the $K^{\pm} \rightarrow \pi^{\pm}\pi^0e^+e^-$ decay, that has not been observed so far, has been obtained from a sample of 4919 decay candidates with 4.9% background. The branching ratio in the full kinematic region is measured to be $(4.24 \pm 0.15) \times 10^{-6}$, which is in agreement with ChPT predictions

Recent measurements of

The NA48/2 experiment presents a final result of the charged kaon semileptonic decays form factors measurement based on 4.28 million Ke3± and 2.91 million Kμ3± selected decays collected in 2004. The result is competetive with other measurements in Kμ3± mode and has a smallest uncertainty for Ke3± that leads to the most precise combined Kl3± result and allows to reduce the form factor uncertainty of |VUS|

Recent results from the NA48 experiment at CERN

The NA48/2 experiment presents a final result of the charged kaon semileptonic decays form factors measurement based on 4.28 million $K^{\pm}_{e3}$ and 2.91 million $K^{\pm}_{\mu 3}$ selected decays collected in 2004. The result is competetive with other measurements in $K^{\pm}_{\mu 3}$ mode and has a smallest uncertainty for $K^{\pm}_{e3}$, that leads to the most precise combined $K^{\pm}_{l 3}$ result and allows to reduce the form factor uncertainty of $\left | V_{US} \right |$. The NA48/2 experiment at CERN collected a very large sample of charged kaon decays into multiple final states. From this data sample we have reconstructed about 1663 events of the very rare decay $K^{\pm} \to \mu^{\pm} \nu e^{+} e^{-}$ over almost negligible background in the region with $m(e^{+}e^{−}$) above 140 MeV, which is of great interest in Chiral Perturbation Theory. We present the $m_{ee}$ spectrum and a model-independent measurement of the decay rate for this region

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

Search for heavy neutral lepton production in K+K^+ decays

A search for heavy neutral lepton production in $K^+$ decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the $10^{-7}$ to $10^{-6}$ level are established on the elements of the extended neutrino mixing matrix $|U_{\ell 4}|^2$ ($\ell=e,\mu$) for heavy neutral lepton mass in the range $170-448~{\rm MeV}/c^2$. This improves on the results from previous production searches in $K^+$ decays, setting more stringent limits and extending the mass range.A search for heavy neutral lepton production in K+ decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the 10−7 to 10−6 level are established on the elements of the extended neutrino mixing matrix |Ue4|2 and |Uμ4|2 for heavy neutral lepton mass in the ranges 170–448 MeV/ c2 and 250–373 MeV/ c2 , respectively. This improves on the previous limits from HNL production searches over the whole mass range considered for |Ue4|2 , and above 300 MeV/ c2 for |Uμ4|2