First estimate of the NA62 sensitivity to the rare decay K+ → µ+νµX, X → γγ

Light, weakly coupled new particles with couplings to muons have been proposed which could resolve the discrepancy between measured and predicted values for the anomalous magnetic moment of the muon. They may also act as dark matter mediators. A muonphilic scalar particle X may be produced in rare kaon decays and could be observed by NA62. In this project, the branching ratio sensitivity for the process K+ → µ+νµX, X → γγ is evaluated, for masses mX between 20 and 300 MeV/c2. The best sensitivity is found to be of the order 10−7, for mX above 180 MeV/c2.