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

The NA62 experiment at CERN offers the highest experimental sensitivity to rare and forbidden kaon decays. A search for lepton number violating processes K+ -> pi(-)e(+)e(+) and K+ -> pi(-)mu(+)mu(+), using a partial data set collected by NA62 in 2016-2018, is reported. No signals are observed and upper limits at 90% confidence level on the branching fractions of these decays are obtained: 2.2 x 10(-10) and 4.2 x 10(-11), respectively. These results improve on previously reported measurements by factors of 3 and 2, respectively.