First results on the K + → π + vv ¯ decay search from NA62

The precise measurement of the branching ratio of an ultrarare decay K + → π + vv ¯ (~10 -10 according to the calculation within the Standard Model) allows to probe New Physics via indirect effects at mass scales higher than those accessible at the LHC. The NA62 experiment at the CERN SPS is aimed at measuring this branching ratio with the 10% precision. To achieve such level of precision, a novel decay-in-flight technique is used. The statistics collected during the first NA62 physics run in 2016 allowed to demonstrate the proof of the experimental method and obtain O(10 -10 ) single event sensitivity. The preliminary results based on the 2016 data set are described.