Cristina Lazzeroni (University of Birmingham)

Measurement of 𝐵𝑟(𝐾+→𝜋+𝜈𝜈¯) with the NA62 experiment at CERN SPS, and prospects for high-intensity kaon physics at the SPS

Abstract: Rare kaon decays that are strongly suppressed in the Standard Model, while their rates can be calculated with very small theoretical uncertainties, are potentially sensitive to high mass scales ad uniquely sensitive to new physics models where large deviations can be envisaged. The decay 𝐾+→𝜋+𝜈𝜈¯, with a very precisely predicted branching ratio of less than 10−10, is one of the best candidates to reveal indirect effects of new physics at the highest mass scales. The NA62 experiment at CERN SPS is designed to measure 𝐵𝑟(𝐾+→𝜋+𝜈𝜈¯) with a decay-in-flight technique, novel for this channel. NA62 took data in 2016, 2017 and 2018. Statistics collected in 2016 and 2017 allows NA62 to reach the Standard Model sensitivity for 𝐾+→𝜋+𝜈𝜈¯; the analysis of the 2018 data leads to the most precise measurement ever performed. The preliminary result of 𝐵𝑟(𝐾+→𝜋+𝜈𝜈¯) measurement from the analysis of the full data set 2016-2017-2018 is presented, and future prospects reviewed. Following upon NA62's successful application of the in-flight technique, a comprehensive program for the study of the rare decay modes of both K+ and KL mesons is being formed, to be carried out with high-intensity kaon beams from the CERN SPS in multiple phases, including both an experiment to measure BR(K+ à π+ νν) at the 5% level and measurements of KL decays with charged particles. In addition to the optimization of the design of the beam lines, the experimental program requires advances in precision tracking at high rate, high-efficiency calorimeters and veto detectors with excellent PID capabilities. The physics case for this program and the experimental challenges faced will be discussed.