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The NA62 experiment at CERN collected the world's largest dataset of charged kaon decays in 2016–2018, leading to the first measurement of the branching ratio of the ultra-rare K+ → π+vv− decay, based on 20 candidates. Recent results from analyses of K+ → π0e+vγ, K+ → π+µ+µ− and K+ → π+γγ decays, using a data sample recorded in 2017-2018, are reported. Preliminary results of the first observation and analysis of the K± → π0π0µ±v decay, based on the NA48/2 data collected in 2003-2004, are also shown. [ABSTRACT FROM AUTHOR]

I will review recent developments in rare and radiative kaon decays from the theory side, with emphasis on those modes that are actively analyzed by the experimental collaborations. [ABSTRACT FROM AUTHOR]

The aim of the NA62 experiment is to study the extreme rare kaon decay K+ ? π+vv and to measure its branching ratio with a 10% accuracy. In order to do so, a very high intensity beam from the CERN SPS is used to produce charged kaons whose decay products are detected by many detectors installed along a 60 m decay region. The NA62 Data Acquisition system (DAQ) exploits a multi-level trigger system; following a Level0 (L0) trigger decision, 1 MHz data rate from about 60 sources is read by a PC-farm, the partial event is built and then passed through a series of Level1 (L1) algorithms to further reduce the trigger rate. Events passing this level are completed with the missing, larger, data sources (~400 sources) at the rate of 100 KHz. The DAQ is built around a high performance ethernet network interconnecting the detectors to a farm of 30 servers. After an overall description of the system design and the main implementation choices that allowed to reach the required performance and functionality, this paper describes the overall behaviour of the DAQ in the 2017 data taking period. It then concludes with an outlook of possible improvements and upgrades that may be applied to the system in the future. [ABSTRACT FROM AUTHOR]

KLOE-2 extends the physics program of the forerunner KLOE experiment, especially in the field of discrete symmetries tests with neutral kaons. KLOE and KLOE-2 have collected together the largest sample of electron-positron collisions at an energy equal to the φ-meson mass, corresponding to about 2.4×1010 produced φ mesons. The latest results on neutral kaon physics at KLOE will be reviewed, together with the status and prospects of the analyses of KLOE-2 data. A new measurement of the charge asymmetry in KS semileptonic decays with 1.7 fb−1 of KLOE data, which improves the sensitivity of previous measurements of about a factor two, will be presented. Furthermore, the status of the analysis devoted to directly test T and CPT symmetries in neutral kaons transitions, as well as the search of the pure CP-violating KS → 3π0 decay using part of the recently acquired KLOE-2 dataset, will be presented. [ABSTRACT FROM AUTHOR]

We review the state-of-the-art theoretical analyses of tau decays into a pair of mesons and a neutrino. The participant vector and scalar form factors, f+ (s) and f0(s), are described in the frame of Chiral Perturbation Theory with resonances supplemented by dispersion relations, and the physical parameters of the intermediate resonances produced in the decay are extracted through the pole position of f+,0(s) in the complex plane. As a side result, we also determine the low-energy observables associated to the form factors. We hope our study to be of interest for present and future experimental analyses of these decays. [ABSTRACT FROM AUTHOR]

The precise measurement of the branching ratio of an ultrarare decay K+ → π+v⊽ (~10-10 according to the calculation within the Standard Model) allows to probe New Physics via indirect effects at mass scales higher than those accessible at the LHC. The NA62 experiment at the CERN SPS is aimed at measuring this branching ratio with the 10% precision. To achieve such level of precision, a novel decay-in-flight technique is used. The statistics collected during the first NA62 physics run in 2016 allowed to demonstrate the proof of the experimental method and obtain O(10-10) single event sensitivity. The preliminary results based on the 2016 data set are described. [ABSTRACT FROM AUTHOR]

The NA48/2 experiment at CERN has collected large samples of charged kaons decaying into a pion and two muons for the search of heavy nuetrinos. In addition, its successor NA62 has set new limits on the rate of charged kaon decay into a heavy neutral lepton (HNL) and a lepton, with l = e; μ, using the data collected in 2007 and 2015. New limits on heavy neutrinos from kaon decays into pions, muons and positrons are presented in this report. [ABSTRACT FROM AUTHOR]

We compare and review the theoretical predictions for the K̅N and ηN elastic amplitudes where sizable variations are found among the considered approaches, especially at subthreshold energies relevant for studies of kaonic atoms and meson-nuclear quasi-bound states. Conditions for an appearance of dynamically generated states in meson-baryon multi-channel interactions are established and discussed for the K̅N and ηN systems. [ABSTRACT FROM AUTHOR]

A high precision determination of the strong coupling constant in the MS scheme at the Z-mass scale, using low energy quantities, namely pion/kaon decay constants and masses, as experimental input is presented. The computation employs two different massless finite volume renormalization schemes to non-perturbatively trace the scale dependence of the respective running couplings from a scale of about 200 MeV to 100 GeV. At the largest energies perturbation theory is reliable. At high energies the Schrödinger-Functional scheme is used, while the running at low and intermediate energies is computed in a novel renormalization scheme based on an improved gradient flow. Large volume Nf = 2 + 1 QCD simulations by CLS are used to set the overall scale. The result is compared to world averages by FLAG and the PDG. [ABSTRACT FROM AUTHOR]

The AMADEUS collaboration aims to provide unique quality results from K-hadronic interactions in light nuclear targets, in order to solve fundamental open questions in the non-perturbative strangeness QCD sector, like the controversial nature of the ⋀(1405) state, the yield of hyperon formation below threshold, the yield and shape of multi-nucleon K-absorption, processes which are intimately connected to the possible existence of exotic antikaon multi-nucleon clusters and to the role of strangeness in neutron stars. AMADEUS takes advantage of the DAΦNE collider, which provides a unique source of monochromatic low-momentum kaons and exploits the KLOE detector as an active target, in order to obtain excellent acceptance and resolution data for K-nuclear capture on H, 4He, 9Be and 12C, both at-rest and in-flight. [ABSTRACT FROM AUTHOR]

I review kaon decays. I introduce the flavor problem and possible solutions. Very rare kaon decays like K →πvv are very important to this purpose: we study also K →πl+l-, K →ππee where chiral dynamics is important to disentangle short distance effects. We have also studied lepton flavor (universality) violation in rare kaon decays and the Bardeen Buras Gerard approach to describe the K± →π±l+l- form factor. [ABSTRACT FROM AUTHOR]

At very-high energies (100 TeV - 1 PeV), the small value of Bjorken-x (≤ 10−3 − 10−7) at which the parton distribution functions are evaluated makes the calculation of charm quark production very difficult. The charm quark has mass (~1.5±0.2 GeV) significantly above the ΛQCD scale (~200 MeV), and therefore its production is perturbatively calculable. However, the uncertainty in the data and the calculations cannot exclude some smaller non-perturbative contribution. To evaluate the prompt neutrino flux, one needs to know the charm production cross-section in pN -> cc̄ X, and hadronization of charm particles. This contribution briefly discusses computation of prompt neutrino flux and presents the strongest limit on prompt neutrino flux from IceCube. [ABSTRACT FROM AUTHOR]

The NA48/2 and NA62-RK experiments at the CERN SPS collected large samples of charged kaon decays in flight in 2003–07. The data analysis is still on-going (with over 20 publications produced so far); the recent results from these experiments are presented. A new upper limit on the rate of a lepton number violating decay K± → π∓μ±μ± is reported: B < 8.6×10−11 at 90% CL. Searches for heavy sterile neutrino N,4 and neutral scalar resonances (χ) in K± → πμμ decays are reported. Upper limits on the products B(K± → μ±N4)B(N4 → π∓μ±) and B(K± → π±χ)B(χ → μ+μ-) are set in the range 10-10 to 10-9 for resonance lifetimes up to 100 ps. A preliminary measurement of the electromagnetic transition form factor slope of the π0 from 1.05 × 106 fully reconstructed π0 → γe+e- decays is presented: the obtained result a = (3.70 ± 0.53stat ± 0.36syst) × 10-2 represents the first observation of a non-zero slope in the time-like region of momentum transfer. [ABSTRACT FROM AUTHOR]

The KLOE-2 experiment at the INFN Laboratori Nazionali di Frascati (LNF) is currently taking data at the upgraded e+e- DAFNE collider. Present Run II follows a development phase to assess the feasibility of a long term acquisition program, Run I, which successfully ended in July 2015 with 1 fb-1 integrated luminosity collected in less than eight months. KLOE-2 represents the continuation of the KLOE experiment with a new physics program. The KLOE detector has undergone several upgrades including state-of-the-art cylindrical GEM Inner Tracker, electron-positron taggers for the γγ-physics studies and new calorimeters around the interaction point. In this article we briefly present the overview of the KLOE-2 experiment including the present status and achievements together with the physics plans. [ABSTRACT FROM AUTHOR]

A test of lepton flavour conservation in kaon rare decays by the NA48/2 experiment at CERN SPS is presented. The improved upper limit on the rate of the lepton number violating decay K± → π± μ±μ± is established with the data sample collected in 2003-2004. A precision test of lepton flavour universality in kaon rare decays by the successor NA62 experiment at CERN SPS is reported. The helicity suppressed ratio RK = Γ(K± → e± ν)/Γ(K± → μ± ν) is measured with a 0.4% precision using the data sample collected in 2007-2008 and is consistent with the prediction of the Standard Model. The main goal of the NA62 experiment is to measure the branching ratio of the ultra-rare K+ → π+ ν ν decay with 10% accuracy. This will be achieved by collecting ~1013 K+ decays in the fiducial volume. The NA62 estimated sensitivities to rare and forbidden decays of K+ and π0, including lepton flavour and number violating modes, and the NA62 potential to improve on several experimental limits are discussed. [ABSTRACT FROM AUTHOR]

The NA62 experiment at CERN aims at measuring the K+ →+ ν ν decay Branching Ratio (BR) with unprecedented precision, by collecting O(100) events with ~10% background in 2 years' data-taking. In this paper the measurement strategy adopted by the NA62 collaboration is described, as well as all the main detectors. The trigger system, a crucial item for the experiment, will also be discussed. [ABSTRACT FROM AUTHOR]

The NA62 and NA48/2 experiments at CERN collected a large sample of charged kaon decays with a low intensity beam and minimum bias trigger conditions in 2004 (NA48/2) and 2007 (NA62). This allowed measurements of a number of rare decays that are difficult to address in conventional high intensity experiments with highly selective trigger conditions. In particular, large samples of K± → π ± γ γ decays have been collected, allowing precision tests of the Chiral Perturbation Theory. [ABSTRACT FROM AUTHOR]

The results of the most recent analyses using kaon data collected by the KLOE experiment at the Frascati φ-factory, DAΦNE, are presented: a new improved measurement of BR(K+ → π+π-π+(γ)), a new improved search of the rare CP violating decay KS → 3π0, and new stringent limits on the CPT violating parameters Δaμ for neutral kaons in the context of the Standard Model Extension framework. [ABSTRACT FROM AUTHOR]

The NA48/2 and NA62 experiments at CERN study rare kaon decays using the decay in flight technique. In this paper two selected topics will be presented: new measurements performed by the two experiments of the K± → π±γγ decay which rep- resents a stringent test of the Chiral Perturbation Theory and the measurement of the Branching Ratio of the very rare decay K+ → π+v... at 10% level, that will provide a test of the Standard Model and possible extensions beyond it. [ABSTRACT FROM AUTHOR]

The available data on the D0 → KS0π+π- decays are analysed in the framework of the quasi two-body QCD factorization approximation. The annihilation, via W- exchange, amplitudes are added to the weak-decay tree amplitudes. The doubly Cabibbo suppressed parts of the amplitudes are also considered. The strong interactions between the kaon-pion and pion-pion pairs in the S -, P- and D-final states are described in terms of the corresponding form factors. The kaon-pion or pion-pion scalar and vector form factors are constrained by other experimental data. Unitarity, analyticity and chiral symmetry are also used to obtain their functional forms. We go through a minimization procedure to reproduce the KS0π-, KS0π+ and π+π- effective mass projections of the Dalitz plot distributions. The large number (27) of non-zero amplitudes leads to a large number of parameters. The resulting model distributions and branching fractions are compared to the accurate Belle Collaboration data. [ABSTRACT FROM AUTHOR]

Kl4 are for several reasons an especially interesting decay channel of K mesons: Kl4 decays allow an accurate measurement of a combination of S -wave ππ scattering lengths, one form factor of the decay is connected to the chiral anomaly and the decay is the best source for the determination of some low energy constants of ChPT. We present a dispersive approach to Kl4 decays, which takes rescattering effects fully into account. Some fits to NA48/2 and E865 measurements and results of the matching to ChPT are shown. [ABSTRACT FROM AUTHOR]

The KS → 3π0 decay is a pure CP violating process which, assuming CPT invariance, gives access to the CP - violating phase of the weak decay amplitude into I=1 final states. This decay has not been observed so far, and the best upper limit on the branching ratio BR(KS → 3π0) < 1.2 ⋅ 10-7 is two orders of magnitude larger than predictions based on the Standard Model. In this article we present the search for the KS → 3π0 decay performed with the KLOE detector operating at the DAΦNE φ - factory at the Frascati Laboratory. We describe the analysis techniques used for the background rejection and signal events selection leading to the result of an almost five times lower upper limit on the KS → 3π0 branching ratio. We also discuss the perspectives for a new measurement using the KLOE - 2 apparatus equipped with a new inner tracker and the calorimeters at low δ angle. [ABSTRACT FROM AUTHOR]