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A bstract A measurement of the K + → π + ν ν ¯ decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This dataset was recorded, after modifications to the beamline and detectors, at a higher instantaneous beam intensity with respect to the 2016–2018 data taking. Combining NA62 data collected in 2016–2022, a measurement of B K + → π + ν ν ¯ = 13.0 − 3.0 + 3.3 × 10 − 11 is reported. With 51 signal candidates observed and an expected background of 18 − 2 + 3 events, B K + → π + ν ν ¯ becomes the smallest branching ratio measured with a signal significance above 5 σ .

A bstract Results from the study of the rare decays K + → π + ν ν ¯ , K + → π + μ + μ − and K + → π + γγ at the NA62 experiment at CERN are interpreted in terms of improved limits for B K + → π + X and coupling parameters of hidden-sector models, where X is a mediator. World-leading limits are achieved for dark photon, dark scalar and axion-like particle models.

A measurement of the$$ {K}^{+}\\to {\\pi}^{+}\\nu \\overline{\\nu} $$K + → π + ν ν ¯ decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This dataset was recorded, after modifications to the beamline and detectors, at a higher instantaneous beam intensity with respect to the 2016–2018 data taking. Combining NA62 data collected in 2016–2022, a measurement of$$ \\mathcal{B}\\left({K}^{+}\\to {\\pi}^{+}\\nu \\overline{\\nu}\\right)=\\left({13.0}_{-3.0}^{+3.3}\\right)\\times {10}^{-11} $$B K + → π + ν ν ¯ = 13.0 − 3.0 + 3.3 × 10 − 11 is reported. With 51 signal candidates observed and an expected background of$$ {18}_{-2}^{+3} $$18 − 2 + 3 events,$$ \\mathcal{B}\\left({K}^{+}\\to {\\pi}^{+}\\nu \\overline{\\nu}\\right) $$B K + → π + ν ν ¯ becomes the smallest branching ratio measured with a signal significance above 5 σ .

A measurement of the ${K}^{+}\\to {\\pi}^{+}\\nu \\overline{\\nu}$ decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This dataset was recorded, after modifications to the beamline and detectors, at a higher instantaneous beam intensity with respect to the 2016–2018 data taking. Combining NA62 data collected in 2016–2022, a measurement $\\mathcal{B} ({K}^{+}\\to {\\pi}^{+}\\nu \\overline{\\nu}) = \\large{(}13.0^{+3.3}_{-3.0}\\large{)}$ x $10^{-11}$ of is reported. With 51 signal candidates observed and an expected background of $18^{+3}_{-2}$ events, $\\mathcal{B} ({K}^{+}\\to {\\pi}^{+}\\nu \\overline{\\nu})$ becomes the smallest branching ratio measured with a signal significance above 5σ.

A bstract Measurement of the ultra-rare K + → π + ν ν ¯ decay at the NA62 experiment at CERN requires high-performance particle identification to distinguish muons from pions. Calorimetric identification currently in use, based on a boosted decision tree algorithm, achieves a muon misidentification probability of 1 . 2 × 10 − 5 for a pion identification efficiency of 75% in the momentum range of 15–40 GeV/ c . In this work, calorimetric identification performance is improved by developing an algorithm based on a convolutional neural network classifier augmented by a filter. Muon misidentification probability is reduced by a factor of six with respect to the current value for a fixed pion-identification efficiency of 75%. Alternatively, pion identification efficiency is improved from 72% to 91% for a fixed muon misidentification probability of 10 − 5 .

A bstract A sample of 1 . 3 × 10 5 K + → π 0 e + νγ candidates with less than 1% background was collected by the NA62 experiment at the CERN SPS in 2017–2018. Branching fraction measurements are obtained at percent relative precision in three restricted kinematic regions, improving on existing results by a factor larger than two. An asymmetry, possibly related to T-violation, is investigated with no evidence observed within the achieved precision.

We report the results of a search for dark photon in-flight decays to μ + μ − pairs at NA62. Dark photons could be produced by dumping protons onto a copper-iron absorber, reach the NA62 decay volume more than 80 m downstream of the dump and decay therein. The measurement is based on data collected in 2021 which correspond to 1.4 × 10 17 dumped protons.

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 + → π 0 e + v γ, 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 ± → π 0 π 0 µ ± v decay, based on the NA48/2 data collected in 2003-2004, are also shown.

The interferometer diagnostics using Far Infrared (FIR) laser beams provide, by phase measurement techniques, precise line-electron density measurements of magnetic fusion plasmas. But the FIR beams still suffer the effect of refraction when traversing the plasma. This can cause, when the density gradients are strong, temporary losses of signal that induce the so called fringe jumps on the estimated phase. On the CEA Tore Supra tokamak, a Field Programmable Get Array (FPGA) electronics has been developed using a time delay method to calculate the phase and correct the fringe jumps at the frequency rate of the probe sinusoidal signal (100 KHz). To test the efficiency of the algorithm on various plasma scenarios, a prototype of the CEA electronics has been installed on the JET tokamak and the data have been compared with those issued from the present JET electronics, which calculates the phase by a different method. Statistical comparisons between the two methods on more than 1500 JET shots are reported in this article and show that the two methods give similar results but that none of them is 100% reliable as still some fringe jumps remain, in particular when Edge Localised Modes (ELMs), pellet injections or disruptions occur. To understand this phenomenon, an analysis of the fast changes of the 100 KHz raw input signals during ELMs and pellet injections has been done with a 1MHz numerical acquisition. The typical durations of signal losses have been found to be few hundreds of micro-seconds. Meanwhile, the line density can increase and then return to its original value. Simulations show that an algorithm that would block the phase calculation during a longer time (i.e. 500 μs) than the disturbed period would help to avoid fringe jumps.

The former all-carbon wall on JET has been replaced with beryllium in the main torus and tungsten in the divertor to mimic the surface materials envisaged for ITER. Comparisons are presented between Type I H-mode characteristics in each design by examining respective scans over deuterium fuelling and impurity seeding, required to ameliorate exhaust loads both in JET at full capability and in ITER.