Abstract The NA48/2 experiment at CERN reports the first observation of the K± → π0π0μ±ν decay based on a sample of 2437 candidates with 15% background contamination collected in 2003–2004. The decay branching ratio in the kinematic region of the squared dilepton mass above 0.03 GeV2/c4 is measured to be (0.65 ± 0.03) × 10−6. The extrapolation to the full kinematic space, using a specific model, is found to be (3.45 ± 0.16) × 10−6, in agreement with chiral perturbation theory predictions.

The NA48/2 experiment at CERN reports the first observation of the K-+/- -> pi(+/-)pi(0)e(+)e(-) decay from an exposure of 1.7 x 10(11) charged kaon decays recorded in 2003-2004. A sample of 4919 candidates with 4.9% background contamination allows the determination of the branching ratio in the full kinematic region, BR(K-+/- -> pi(+/-)pi(0)e(+)e(-)) = (4.24 +/- 0.14) x 10(-6). The study of the kinematic space shows evidence for a structure dependent contribution in agreement with predictions based on chiral perturbation theory. Several P- and CP-violating asymmetries are also evaluated. (C) 2018 The Author(s). Published by Elsevier B.V.

A bstract A 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 e 3 ± ) and 2.3 × 10 6 K ± → π 0 μ ± ν μ ( K μ 3 ± ) 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 e 3 ± and K μ 3 ± modes is also presented.

The NA48/2 experiment collected data in 2003 and 2004 with the main purpose to study direct CP violation in charged kaons decaying in three pions. Thanks to the huge statistics collected, other studies are also possible. In this paper two recent results, in the framework of QCD studies, are reported: the measurement of the K± → π0e±ν Form Factors and the first observation of the rare decay K± → π±π0e+e−.

The NA62 experiment at CERN collected a large sample of charged kaon decays with a highly efficient trigger for decays into electrons in 2007. The kaon beam represents a source of tagged neutral pion decays in vacuum. A measurement of the electromagnetic transition form factor slope of the neutral pion in the time-like region from $1.05\times10^6$ fully reconstructed $\pi^0$ Dalitz decay is presented. The limits on dark photon production in $\pi^0$ decays from the earlier kaon experiment at CERN, NA48/2, are also reported.

The NA62 experiment collected a large sample of charged kaon decays in 2007 with a highly efficient trigger for decays into electrons. A measurement of the pi(0) electromagnetic transition form factor slope parameter from 1.11 x 10(6) fully reconstructed K-+/- -> pi(+/-) pi(0)(D) , pi(0)(D) -> e(+) e(-) gamma events is reported. The measured value a = (3.68 +/- 0.57) x 10(-2) is in good agreement with theoretical expectations and previous measurements and represents the most precise experimental determination of the slope in the time-like momentum transfer region. (C) 2017 The Author. Published by Elsevier B.V.

A measurement of the branching ratio of the rare leptonic kaon decay $K^\pm \rightarrow \mu^\pm \nu_{\mu} e^+ e^- $ is presented using data collected by the NA48/2 experiment in 2003 and 2004. The measurement is performed in the region $M_{ee} > 140$~MeV/c$^2$. In this particular region low energy QCD contributions become important and can be calculated in the framework of Chiral Perturbation Theory (ChPT). From a total number of $1.56 \times 10^{11}$ recorded kaon decays, the branching ratio is measured to be $\mathcal{B} \left( K^\pm \rightarrow \mu^\pm \nu_{\mu} e^+ e^- | M_{ee} > 140~ MeV/c^2 \right) = (7.8 \pm 0.2) \times 10^{-8} $.

The NA48/2 experiment at CERN collected a large sample of charged kaon decays to final states with multiple charged particles in 2003–2004. A new upper limit on the rate of the lepton number violating decay K±→π∓μ±μ± is reported: B(K±→π∓μ±μ±)<8.6×10−11 at 90% CL. Searches for two-body resonances X in K±→πμμ decays (such as heavy neutral leptons N4 and inflatons χ) are also presented. In the absence of signals, upper limits are set on the products of branching fractions B(K±→μ±N4)B(N4→πμ) and B(K±→π±X)B(X→μ+μ−) for ranges of assumed resonance masses and lifetimes. The limits are in the (10−11,10−9) range for resonance lifetimes below 100 ps.

The NA62 experiment recorded a large sample of K+ -> mu(+)nu(mu)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 x10(-6) to 10(-5) have been set on the squared mixing matrix element |U-mu 4|(2) between muon and heavy neutrino states, for heavy neutrino masses in the range 300-375MeV/c(2). The result extends the range of masses for which upper limits have been set on the value of |U mu(4)|(2) in previous production search experiments. (C) 2017 The Author(s). Published by Elsevier B.V.

Thanks to a large sample of kaon decays collected in 2003-2004, the NA48/2 experiment at CERN is able to set an upper limit on the rate of the lepton number violating decay BR(K±→π∓μ±μ±)< 8.6 × 10-11 at 90%CL, and to set limits at the 10-10 — 10-9 level on the appearance of resonances (including heavy neutral leptons N4 and inflatons χ) in the K±→πμμ decays as functions of the resonance mass and lifetime.

The NA48/2 experiment at CERN collected a large sample of charged kaon decays into final states with multiple charged particles in 2003--2004. A new upper limit on the rate of the lepton number violating decay $K^{\pm}\toπ^{\mp}μ^{\pm}μ^{\pm}$ obtained from this sample is reported: $\mathcal{B}(K^{\pm}\toπ^{\mp}μ^{\pm}μ^{\pm})<8.6 \times 10^{-11}$ at 90\% CL. Searches for two-body resonances in the $K^{\pm}\toπμμ$ decays (including heavy neutral leptons~$N_4$ and inflatons~$χ$) in the accessible range of masses and lifetimes are also presented. In the absence of a signal, upper limits are set on the products of branching ratios~$\mathcal{B}(K^{\pm}\toμ^{\pm}N_4)\mathcal{B}(N_4\toπμ)$ and $\mathcal{B}(K^{\pm}\toπ^{\pm}χ)\mathcal{B}(χ\toμ^+μ^-)$ as functions of the resonance mass and lifetime. These limits are in the $10^{-10}-10^{-9}$ range for resonance lifetimes below 100~ps.

The NA48/2 experiment at CERN collected a large sample of charged kaon decays to final states with multiple charged particles in 2003–2004. A new upper limit on the rate of the lepton number violating decay K±→π∓μ±μ± is reported: B(K±→π∓μ±μ±)\u003c8.6×10−11 at 90% CL. Searches for two-body resonances X in K±→πμμ decays (such as heavy neutral leptons N4 and inflatons χ ) are also presented. In the absence of signals, upper limits are set on the products of branching fractions B(K±→μ±N4)B(N4→πμ) and B(K±→π±X)B(X→μ+μ−) for ranges of assumed resonance masses and lifetimes. The limits are in the (10−11,10−9) range for resonance lifetimes below 100 ps.

Recent results and prospects for precision tests of the Standard Model in kaon decay-in-flight experiments at CERN are presented. A measurement of the ratio of leptonic decay rates of the charged kaon at the level of 0.4% precision constrains the parameter space of new physics models with extended Higgs sector, a fourth generation of quarks and leptons or sterile neutrinos. Searches for heavy neutrino mass states and the dark photon in the ∼ 100 MeV/c2 mass range based on samples collected in 2003-2007 are in progress and prospects will be discussed. The NA62 experiment, starting in 2014, will search for a range of lepton number and lepton flavour violating decays of the charged kaon and the neutral pion at improved sensitivities down to ∼ 10-12, which will probe new physics scenarios involving heavy Majorana neutrinos or R-parity violating SUSY.

A sample of 1.69×107 fully reconstructed π0→γe+e− decay candidates collected by the NA48/2 experiment at CERN in 2003–2004 is analyzed to search for the dark photon (A′) production in the π0→γA′ decay followed by the prompt A′→e+e− decay. No signal is observed, and an exclusion region in the plane of the dark photon mass mA′ and mixing parameter ε2 is established. The obtained upper limits on ε2 are more stringent than the previous limits in the mass range 9MeV/c2

A sample of $1.69\times 10^7$ fully reconstructed $\pi^0\to\gamma e^+e^-$ decay candidates collected by the NA48/2 experiment at CERN in 2003--2004 is analysed to search for the dark photon ($A'$) production in the $\pi^0\to\gamma A'$ decay followed by the prompt $A'\to e^+e^-$ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass $m_{A'}$ and mixing parameter $\varepsilon^2$ is established. The obtained upper limits on $\varepsilon^2$ are more stringent than the previous limits in the mass range $9~{\rm MeV}/c^2

A sample of 65210 K ± → π 0 π 0 e ± ν (K e4 00 ) decay candidates with 1% background contamination has been collected in 2003-2004 by the NA48/2 collaboration at the CERN SPS. A study of the differential rate provides the first measurement of the hadronic form factor variation in the plane (M ππ 2 , M eν 2 ) and brings evidence for a cusp-like structure in the distribution of the squared π 0 π 0 invariant mass around \( 4{m}_{\pi^{+}}^2 \). Exploiting a model independent description of this form factor, the branching ratio, inclusive of radiative decays, is obtained using the K ± → π 0 π 0 π ± decay mode as normalization. It is measured to be BR(K e4 00 ) = (2.552 ± 0.010stat ± 0.010syst ± 0.032ext) × 10−5, which improves the current world average precision by an order of magnitude while the 1.4% relative precision is dominated by the external uncertainty from the normalization mode. A comparison with the properties of the corresponding mode involving a π + π − pair (K e4 + − ) is also presented.

The NA48/2 Collaboration at CERN has accumulated and analysed unprecedented statistics of rare kaon decays in the K_e4 modes: K_e4(+-) (K^±→π^+ π^- e^±ν) and K_e4(00) (K^±→π^0 π^0 e^±ν) with nearly one percent background contamination. It leads to the improved measurement of branching fractions and detailed form factor studies. New final results from the analysis of 381 K^±→π^±γγ rare decay candidates collected by the NA48/2 and NA62 experiments at CERN are presented. The results include a decay rate measurement and fits to Chiral Perturbation Theory (ChPT) description.

A study of the dynamics of the rare decay K-+/- -> pi(+/-)gamma gamma has been performed on a sample of 232 decay candidates, with an estimated background of 17.4 +/- 1.1 events, collected by the NA62 experiment at CERN in 2007. The results are combined with those from a measurement conducted by the NA48/2 Collaboration at CERN. The combined model-independent branching ratio in the kinematic range z = (m gamma gamma/m(K))(2) > 0.2 is B-MI(z > 0.2) = (0.965 +/- 0.063) x 10(-6), and the combined branching ratio in the full kinematic range assuming a Chiral Perturbation Theory description is B(K-pi gamma gamma) = (1.003 +/- 0.056) x 10(-6). A detailed comparison of the results with the previous measurements is performed. (C) 2014 The Authors. Published by Elsevier B.V.

The NA48/2 experiment at CERN collected two data samples with minimum bias trigger conditions in 2003 and 2004. A measurement of the rate and dynamic properties of the rare decay K-+/- -> pi(+/-)gamma gamma from these data sets based on 149 decay candidates with an estimated background of 15.5 +/- 0.7 events is reported. The model-independent branching ratio in the kinematic range z = (m(gamma gamma)/m(K))(2) > 0.2 is measured to be B-MI(z > 0.2) = (0.877 +/- 0.089) x 10(-6), and the branching ratio in the full kinematic range assuming a particular Chiral Perturbation Theory description to be B(K-pi gamma gamma) = (0.910 +/- 0.075) x 10(-6). (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).