Rare kaon decay to missing energy: Implications of the NA62 result for a $Z′$ model

Meson decays offer a good opportunity to probe new physics. The rare kaon decay K+→π+νν¯ is one of the cleanest of them and, for this reason, is rather sensitive to new physics, in particular, vector mediators. The NA62 Collaboration, running a fixed-target experiment at CERN, recently reported an unprecedented sensitivity to this decay, namely a branching fraction of BR(K+→π+νν¯)=(11-3.5+4.0)×10-11 at 68% C.L. Vector mediators that couple to neutrinos may yield a sizeable contribution to this decay. Motivated by the new measurement, we interpret this result in the context of a concrete Z′ model, and put our findings into perspective with the correlated KL→π0νν¯ decay measured by the KOTO Collaboration, current, and future colliders, namely the High-Luminosity and High-Energy Large Hadron Collider.