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 K+ decays to a muon and invisible particles

The NA62 experiment at CERN reports searches for K+ → μ+N and K+ → μ+νX decays, where N and X are massive invisible particles, using the 2016–2018 data set. The N particle is assumed to be a heavy neutral lepton, and the results are expressed as upper limits of O(10−8) of the neutrino mixing parameter |Uμ4|2 for N masses in the range 200–384 MeV/c2 and lifetime exceeding 50 ns. The X particle is considered a scalar or vector hidden sector mediator decaying to an invisible final state, and upper limits of the decay branching fraction for X masses in the range 10–370 MeV/c2 are reported for the first time, ranging from O(10−5) to O(10−7). An improved upper limit of 1.0×10−6 is established at 90% CL on the K+ → μ+ννν¯ branching fraction

Measurement of the very rare K+->pi+nu nu<mml:mo stretchy="true"><overbar></mml:mover> decay

Abstract The NA62 experiment reports the branching ratio measurement BR K + → π + ν ν ¯ = 10.6 − 3.4 + 4.0 stat ± 0.9 syst × 10 − 11 $$\mathrm{BR}\left({K}^{+}\to {\pi}^{+}\nu \overline{\nu}\right)=\left({10.6}_{-3.4}^{+4.0}\left|{}_{\mathrm{stat}}\right.\pm {0.9}_{\mathrm{syst}}\right)\times {10}^{-11}$$ 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 + ν ν ¯ $${\pi}^{+}\nu \overline{\nu}$$ 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 pi(0) 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

Search for Lepton Number and Flavor Violation in K+ and pi(0) Decays

Searches for the lepton number violating K^{+}→π^{-}μ^{+}e^{+} decay and the lepton flavor violating K^{+}→π^{+}μ^{-}e^{+} and π^{0}→μ^{-}e^{+} decays are reported using data collected by the NA62 experiment at CERN in 2017-2018. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level: B(K^{+}→π^{-}μ^{+}e^{+})&lt;4.2×10^{-11}, B(K^{+}→π^{+}μ^{-}e^{+})&lt;6.6×10^{-11} and B(π^{0}→μ^{-}e^{+})&lt;3.2×10^{-10}. These results improve by 1 order of magnitude over previous results for these decay modes

Measurement of the very rare K+ → π+ν ¯ν decay at the NA62 experiment

The K+ -&gt; pi(+) nu(nu) over bar decay, with a very precisely predicted branching ratio of less than 10(-10), is among the best processes to reveal indirect effects of new physics. The NA62 experiment reports the branching ratio measurement BR(K+ -&gt; pi(+) nu(nu) over bar) = (1.06(-0.34)(+0.40)vertical bar(stat) +/- 0.09 vertical bar syst) . 10(-10) 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+ -&gt; pi(+)nu(nu) over bar decay, observed with a significance of 3.4 sigma. This measurement is also used to set limits on BR(K+ -&gt; pi(+) X), where X is a new feebly interacting scalar or pseudo-scalar particle, foreseen in several new physics scenarios

Measurement of the K+ → π+γγ decay

A sample of 3984 candidates of the K+→π+γγ decay, with an estimated background of 291±14 events, was collected by the NA62 experiment at CERN during 2017–2018. In order to describe the observed di-photon mass spectrum, the next-to-leading order contribution in chiral perturbation theory was found to be necessary. The decay branching ratio in the full kinematic range is measured to be (9.61±0.17)×10−7. The first search for production and prompt decay of an axion-like particle with gluon coupling in the process K+→π+a, a→γγ is also reported

Performance of the NA62 ring imaging Cherenkov detector

The Ring Imaging Cherenkov detector of the NA62 experiment at the CERN SPS is a key element of particle identification in the NA62 experimental strategy. According to the NA62 requirements, the RICH identifies π + and μ + in the momentum range between 15 and 35 GeV/c with a muon rejection factor of O(10 2 ), measures the arrival time of charged particles with a precision better than 100 ps and is one of the main components of the NA62 trigger system. After the commissioning in 2014–2015, the RICH detector has been successfully operated during the 2016, 2017 and 2018 data taking periods of NA62. The main design aspects and operational characteristics of the NA62 RICH are described and results on its performance, directly measured with data, are presented