First search for K+ -> pi(+) nu(nu)over-bar using the decay-in-flight technique

The NA62 experiment at the CERN SPS reports the first search for K+ -> pi(+) nu(nu) over bar using the decay-in-flight technique, based on a sample of 1.21 x10(11) K+ decays collected in 2016. The single event sensitivity is 3.15 x10(-10), corresponding to 0.267 Standard Model events. One signal candidate is observed while the expected background is 0.152 events. This leads to an upper limit of 14 x10(-10) on the K+ -> pi(+) nu(nu) over bar branching ratio at 95% CL.
The cost of the experiment and of its auxiliary systems were supported by the funding agencies of the Collaboration Institutes. We are particularly indebted to: F.R.S.-FNRS (Fonds de la Recherche Scientifique - FNRS), Belgium; BMES (Ministry of Education, Youth and Science), Bulgaria; NSERC (Natural Sciences and Engineering Research Council), Canada; NRC (National Research Council) contribution to TRIUMF, Canada; MEYS (Ministry of Education, Youth and Sports), Czech Republic; BMBF (Bundesministerium für Bildung und Forschung) contracts 05H12UM5 and 05H15UMCNA, Germany; INFN (Istituto Nazionale di Fisica Nucleare), Italy; MIUR (Ministero dell'Istruzione, dell'Università e della Ricerca), Italy; CONACyT (Consejo Nacional de Ciencia y Tecnología), Mexico; IFA (Institute of Atomic Physics), Romania; INR-RAS (Institute for Nuclear Research of the Russian Academy of Sciences), Moscow, Russia; JINR (Joint Institute for Nuclear Research), Dubna, Russia; NRC (National Research Center) “Kurchatov Institute” and MESRF (Ministry of Education and Science of the Russian Federation), Russia; MESRS (Ministry of Education, Science, Research and Sport), Slovakia; CERN (European Organization for Nuclear Research), Switzerland; STFC (Science and Technology Facilities Council), United Kingdom; NSF (National Science Foundation) Award Number 1506088, U.S.A.; ERC (European Research Council) “UniversaLepto” advanced grant 268062, “KaonLepton” starting grant 336581, Europe. Individuals have received support from: Charles University (project GA UK number 404716), Czech Republic; Ministry of Education, Universities and Research (MIUR “Futuro in ricerca 2012” grant RBFR12JF2Z, Project GAP), Italy; Russian Foundation for Basic Research (RFBR grants 18-32-00072, 18-32-00245), Russia; the Royal Society (grants UF100308, UF0758946), United Kingdom; STFC (Rutherford fellowships ST/J00412X/1, ST/M005798/1), United Kingdom; ERC (grants 268062, 336581).