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The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π 0 into two photons (γγ), leading to its unusually short lifetime. We precisely measured the decay width Γ of the π 0 → γ γ process. The differential cross sections for π 0 photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding Γ ( π 0 → γ γ ) = 7.798 ± 0.056 ( stat . ) ± 0.109 ( syst . ) eV , where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weighted average of Γ ( π 0 → γ γ ) = 7.802 ± 0.052 ( stat . ) ± 0.105 ( syst . ) eV . Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.more » « less
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A bstract The NA62 experiment at CERN targets the measurement of the ultra-rare $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ K + → π + ν ν ¯ decay, and carries out a broad physics programme that includes probes for symmetry violations and searches for exotic particles. Data were collected in 2016–2018 using a multi-level trigger system, which is described highlighting performance studies based on 2018 data.more » « less
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A bstract A sample of 2 . 8 × 10 4 K + → π + μ + μ − candidates with negligible background was collected by the NA62 experiment at the CERN SPS in 2017–2018. The model-independent branching fraction is measured to be (9 . 15 ± 0 . 08) × 10 − 8 , a factor three more precise than previous measurements. The decay form factor is presented as a function of the squared dimuon mass. A measurement of the form factor parameters and their uncertainties is performed using a description based on Chiral Perturbation Theory at $$ \mathcal{O} $$ O ( p 6 ).more » « less
