Precision Measurements and Prospects with Kaons at CERN

Abstract

The NA62 experiment at the CERN SPS collected the world's largest dataset of charged kaon decays during 2016-2018. The dataset was used for the most precise measurement of the branching fraction of the ultra-rare decay 𝐾 + → 𝜋 + 𝜈 ν. In this proceedings, the NA62 experiment reports recent results from analyses of decays 𝐾 + → 𝜋 0 𝑒 + 𝜈𝛾 (𝐾 𝑒3𝛾 ), 𝐾 + → 𝜋 + 𝜇 + 𝜇 − (𝐾 𝜋 𝜇𝜇 ) and 𝐾 + → 𝜋 + 𝛾𝛾 (𝐾 𝜋𝛾𝛾 ). The radiative 𝐾 𝑒3𝛾 decay has a data sample of O (100𝑘) candidates with sub-percent background contamination. Published results with the most precise measurement of the branching fraction and T-asymmetry are presented. The 𝐾 𝜋 𝜇𝜇 sample comprises about 27k signal events with negligible background contamination, and the results include the most precise determination of the branching ratio and the form factor. The 𝐾 𝜋𝛾𝛾 sample contains about 4k signal events with 10 % background contamination, and the analysis improves the precision of the branching ratio measurement by a factor of 3 with respect to the previous measurements. Recent results from the NA48/2 experiment from the analysis of the decay 𝐾 ± → 𝜋 0 𝜋 0 𝜇 ± 𝜈 (𝐾 00 𝜇4 ) are also reported in this proceedings. The sample consists of 2437 events with 15 % background contamination. The result is compared with a newly calculated theoretical prediction to 1-loop level, including previously neglected 𝑅 1 form factor. The NA62 experiment is approved to take data until the CERN Long Shutdown 3. A proposal for next-generation kaon experiments at CERN, called HIKE, was submitted to the SPSC in November 2023 and is also presented in this proceedings. In its first phase, HIKE will continue to study decays of charged kaons at significantly higher beam intensities than NA62 using a new detector system. Neutral kaon decays will be then studied in the second phase of HIKE.