Searches for lepton flavour and lepton number violation in K+^+ decays

The NA62 experiment at the CERN SPS is designed to conduct precision tests of the Standard Model by studying rare decays of charged kaons. NA62 took data in 2016-2018 and collected a large sample of charged kaon decays into final states with multiple charged particles. The sensitivity to a range of lepton flavour and lepton number violating kaon decays provided by this data set is improved over the previously reported measurements. Results from the searches of $K^+→ π^-l^+l^+$ decays with a partial NA62 data sample are presented

Status of the NA62 Experiment

The decay ${K^ + } \to {\pi ^ + }v\bar v$, with a very precisely predicted branching ratio of less than 10−10 in the Standard Model framework, 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 the branching ratio of such decay with a decay-in-flight technique, novel for this channel. The main goal of NA62 is to measure such Branching Ratio of ${K^ + } \to {\pi ^ + }v\bar v$ with an accuracy better than 20%. This will be achieved by collecting up to 100 ${K^ + } \to {\pi ^ + }v\bar v$ events with a background contamination at the level of 10%. The NA62 detector was commissioned in 2014 and 2015 and the experiment took physics data from 2015 to 2018. NA62 has already published the result of the 2016 data analysis. The 2017 data analysis is in progress, the single event sensitivity reached and the evaluation of the main backgrounds will be shown in this contribution

The neutral pion form factor at NA62

In 2007 the NA62 experiment at CERN collected a large sample of charged kaon decays with a highly efficient trigger selecting events with electrons in the final state. The kaon beam represents a rich source of tagged neutral pion decays in vacuum. The electromagnetic transition form factor slope of the π0 in the time-like region has been measured from about 106 fully reconstructed π0 Dalitz decays collected in 2007. The preliminary result a = (3.68 ± 0.51stat ± 0.25syst) × 10-2 is the most precise to date. This value is compatible with theoretical expectations and consistent with the previous measurements