176 research outputs found
Measurement of the very rare K + â Ï+ÎœÎœÂŻ decay
The NA62 experiment reports the branching ratio measurement BR(K+âÏ+ÎœÎœÂŻ)=(10.6â3.4+4.0|stat±0.9syst)Ă10â11 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 2016â2018. This provides evidence for the very rare K+âÏ+ÎœÎœÂŻ decay, observed with a significance of 3.4Ï. The experiment achieves a single event sensitivity of (0.839 ± 0.054) Ă 10â11, corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 ± 1.0) Ă 10â11. This measurement is also used to set limits on BR(K+â Ï+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 heavy neutral lepton production in K+ decays to positrons
A search for heavy neutral lepton (N) production in K+âe+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^â9 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 â ÏΜΜ (KÏΜΜ) and other kaon decay analyses at experiments NA62 (CERN) and KOTO (J-PARC) is given. The KÏΜΜ 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 KÏΜΜ decay is lower than 10â10 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 2.12Ă1012 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 â Ï0ΜΜ) 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
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 production of an invisible dark photon in Ï0 decays
The results of a search for Ï0 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 Ï0 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 Ï0âÎłÎœÎœÂŻ, improving the current limit by more than three orders of magnitude
An investigation of the very rare K+ â Ï+ vvÂŻ decay
The NA62 experiment reports an investigation of the K+âÏ+Îœ ÌÎœ 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)Ă10â10, corresponding to 2.2 events assuming the Standard Model branching ratio of (8.4±1.0)Ă10â11. 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 1.78Ă10â10 for the K+âÏ+Îœ ÌÎœ branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of (0.48+0.72â0.48)Ă10â10, 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
New measurement of the radiative decay Ke3g at the NA62 experiment at CERN
The NA62 experiment at CERN reports new results from the study of the radiative kaon decay K+âÏ0e+ÎœÎł
(Ke3Îł
), using a data sample recorded in 2017 and 2018. Preliminary results with the most precise measurement of the Ke3Îł
branching ratio, and a T-asymmetry measurement in the Ke3Îł
decay, are presented
Search for lepton number and flavour violation in K+ and pi0 decays
The NA62 experiment at CERN collected a large sample of charged kaon decays into ïŹnal states with multiple charged particles in 2016-2018. This sample provides sensitivities to rare decays with branching ratios as low as 10 â11 . Searches for the lepton number violating K + â Ï â ” + e+ decay and the lepton ïŹavour violating K + â Ï + ” â e + and Ï 0 â ” â e + decays are reported. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% conïŹdence level. These results improve by one order of magnitude over previous results for these decay modes
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)
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