Physics beyond the standard model with kaons at NA62

The NA62 experiment at CERN Super Proton Synchrotron was designed to measure BR(K+ \u2192 \u3c0+\u3bdv\u304) with an in-fight technique, never used before for this measurement. This decay is characterised by a very precise prediction in the Standard Model. Its branching ratio, which is expected to be less than 10-10, is one of the best candidates to indicate indirect effects of new physics beyond SM at the highest mass scales. NA62 result on K+ \u2192 \u3c0+\u3bdv\u304 from the full 2016 data set is described. Also a search for an invisible dark photon A\u2032 has been performed, exploiting the efficient photon-veto capability and high resolution tracking of the NA62. The signal stems from the chain K+ \u2192 \u3c0+\u3c00 followed by \u3c00 \u2192 A\u2032\u3b3. No significant statistical excess has been identified. Upper limits on the dark photon coupling to the ordinary photon as a function of the dark photon mass have been set, improving on the previous limits over the mass range 60 - 110 MeV/c2

Externalities and the nucleolus

In most economic applications, externalities prevail: the worth of a coalition depends on how the other players are organized. We show that there is a unique natural way of extending the nucleolus from (coalitional) games without externalities to games with externalities. This is in contrast to the Shapley value and the core for which many different extensions have been proposed

Search for π⁰ decays to invisible particles

The NA62 experiment at the CERN SPS reports a study of a sample of 4 × 109 tagged π0 mesons from K+ → π+π0(γ), searching for the decay of the π0 to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of 4.4 × 10−9 is set on the branching ratio at 90% confidence level, improving on previous results by a factor of 60. This result can also be interpreted as a model- independent upper limit on the branching ratio for the decay K+ → π+X, where X is a particle escaping detection with mass in the range 0.110–0.155 GeV/c2 and rest lifetime greater than 100 ps. Model-dependent upper limits are obtained assuming X to be an axion-like particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs boson

Searches for lepton number violating K+ decays

The NA62 experiment at CERN reports a search for the lepton number violating decays K+→π−e+e+ and K+→π−μ+μ+ using a data sample collected in 2017. No signals are observed, and upper limits on the branching fractions of these decays of 2.2 x 10^-10 and 4.2 x 10^-11 are obtained, respectively, at 90% confidence level. These upper limits improve on previously reported measurements by factors of 3 and 2, respectively

Search for heavy neutral lepton production in K+ decays to positrons

A search for heavy neutral lepton (N) production in K+→e+N decays using the data sample collected by the NA62 experiment at CERN in 2017-2018 is reported. Upper limits of the extended neutrino mixing matrix element |Ue4|^2 are established at the level of 10^−9 over most of the accessible heavy neutral lepton mass range 144-462 MeV/c^2, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The |Ue4|^2 range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/c^2

Recent results in kaon physics

A review of the present experimental status of the K → πνν (Kπνν) and other kaon decay analyses at experiments NA62 (CERN) and KOTO (J-PARC) is given. The Kπνν decay is one of the best candidates among the rare meson decays for indirect searches for new physics in the mass ranges complementary to those accessible by current accelerators. The Standard Model (SM) prediction of the branching fraction (B) of the Kπνν decay is lower than 10−10 in both neutral and charged modes. The NA62 experiment aims to measure the B of the charged mode with better than 10% precision. Three candidate events, compatible with the SM prediction, have been observed from a sample of 2.12×1012 K+ decays collected in 2016 and 2017 by NA62. More than twice the statistics is available in the 2018 dataset currently being analysed. The KOTO experiment in Japan aims to measure B(KL → π0νν) using a technique similar to NA62, but with much lower momentum. In the first dataset taken in 2015 zero signal candidate events were observed. The current status of the analysis of the 2016-2018 dataset with 1.4 times more data is presented. Finally, the most recent results of other physics analyses at the NA62 experiment are summarised

Measurement of the very rare K + → π+νν¯ decay

The NA62 experiment reports the branching ratio measurement BR(K+→π+νν¯)=(10.6−3.4+4.0|stat±0.9syst)×10−11 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+→π+νν¯ decay, observed with a significance of 3.4σ. 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. This measurement is also used to set limits on BR(K+→ π+X), where X is a scalar or pseudo-scalar particle. Details are given of the analysis of the 2018 data sample, which corresponds to about 80% of the total data sample

Search for production of an invisible dark photon in π0 decays

The results of a search for π0 decays to a photon and an invisible massive dark photon at the NA62 experiment at the CERN SPS are reported. From a total of 4.12 x 10^8 tagged π0 mesons, no signal is observed. Assuming a kinetic-mixing interaction, limits are set on the dark photon coupling to the ordinary photon as a function of the dark photon mass, improving on previous searches in the mass range 60-110 MeV/c^2. The present results are interpreted in terms of an upper limit of the branching ratio of the electro-weak decay π0→γνν¯, improving the current limit by more than three orders of magnitude

An investigation of the very rare K+ → π+ vv¯ decay

The NA62 experiment reports an investigation of the K+→π+ν ̄ν mode from a sample of K+ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of (0.389±0.024)×10−10, corresponding to 2.2 events assuming the Standard Model branching ratio of (8.4±1.0)×10−11. Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of 1.78×10−10 for the K+→π+ν ̄ν branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of (0.48+0.72−0.48)×10−10, are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements

Search for heavy neutral lepton production in 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 vertical bar U-e4 vertical bar(2)and vertical bar U-mu 4 vertical bar(2)for heavy neutral lepton mass in the ranges 170-448MeV c(2) and 250-373MeV/c(2), respectively. This improves on the previous limits from HNL production searches over the whole mass range considered for vertical bar U-e4 vertical bar(2), and above 300MeV/c(2) for vertical bar U-mu 4 vertical bar(2)