K+→π+νν̄

Motivated by the new experimental information reported by the BNL-E787 Collaboration, we analyse the present impact and the future prospects opened by the measurement of B(K+→π+νν̄). Although still affected by a large error, the BNL-E787 result favours values of B(K+→π+νν̄) substantially larger than what expected within the Standard Model. As a result, this data already provide non-trivial constraints on the unitarity triangle, when interpreted within the Standard Model framework. We stress the importance of the clean relation between B(K+→π+νν̄), sin2β and ΔMBd/ΔMBs that in the next few years could provide one of the deepest probes of the Standard Model in the sector of quark-flavour dynamics. A speculative discussion about possible non-standard interpretations of a large B(K+→π+νν̄) is also presented. Two main scenarios naturally emerge: those with direct new-physics contributions to the s→dνν̄ amplitude and those with direct new-physics effects only in Bd–B̄d mixing. Realistic models originating these two scenarios and possible future strategies to clearly identify them are briefly discussed.