$K^+ o \pi^+ u\bar u$: a rising star on the stage of flavour physics

摘要

Motivated by the new experimental information reported by the BNL--E787Collaboration, we analyse the present impact and the future prospects opened bythe measurement of $\br(K^+\to\pi^+\nu\bar\nu)$. Although still affected by alarge error, the BNL--E787 result favours values of $\br(K^+\to\pi^+\nu\bar\nu)$ substantially larger than what expected within the StandardModel. As a result, this data already provide non-trivial constraints on theunitarity triangle, when interpreted within the Standard Model framework. Westress the importance of the clean relation between $\br(K^+\to\pi^+\nu\bar\nu)$, $\sin2\beta$ and $\Delta M_{B_d}/\Delta M_{B_s}$ that in thenext few years could provide one of the deepest probes of the Standard Model inthe sector of quark-flavour dynamics. A speculative discussion about possiblenon-standard interpretations of a large $\br(K^+\to \pi^+\nu\bar\nu)$ is alsopresented. Two main scenarios naturally emerge: those with direct new-physicscontributions to the $s \to d \nu\bar\nu$ amplitude and those with directnew-physics effects only in $B_d$--$\bar B_d$ mixing. Realistic modelsoriginating these two scenarios and possible future strategies to clearlyidentify them are briefly discussed.