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  1. A bstract A search for the K + → π + X decay, where X is a long-lived feebly interacting particle, is performed through an interpretation of the K + → $$ {\pi}^{+}\nu \overline{\nu} $$ π + ν ν ¯ analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of X masses, 0–110 MeV /c 2 and 154–260 MeV /c 2 , and lifetimes above 100 ps are considered. The limits set on the branching ratio, BR( K + → π + X ), are competitive with previously reported searches in the first mass range,more »and improve on current limits in the second mass range by more than an order of magnitude.« less
  2. A bstract The NA62 experiment at the CERN SPS reports a study of a sample of 4 × 10 9 tagged π 0 mesons from K + → π + π 0 ( γ ), searching for the decay of the π 0 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 onmore »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 /c 2 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.« less
  3. A bstract The NA62 experiment reports an investigation of the $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ 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 amore »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 $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ K + → π + ν ν ¯ branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of ( $$ {0.48}_{-0.48}^{+0.72} $$ 0.48 − 0.48 + 0.72 ) × 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.« less
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