$K^{+}\to {\pi }^{+}\nu \bar{\nu }$: a rising star on the stage of flavour physics

Abstract

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 $\br(K^+\toπ^+ν\barν)$. Although still affected by a large error, the BNL--E787 result favours values of $\br(K^+\to π^+ν\barν)$ 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 $\br(K^+\to π^+ν\barν)$, $\sin 2\beta$ and $\Delta M_{B_d}/\Delta M_{B_s}$ 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 $\br(K^+\to π^+ν\barν)$ is also presented. Two main scenarios naturally emerge: those with direct new-physics contributions to the $s\to d\nu \bar{\nu }$ amplitude and those with direct new-physics effects only in $B_d$--${\bar{B}}_d$ mixing. Realistic models originating these two scenarios and possible future strategies to clearly identify them are briefly discussed.

13 pages, 3 figures; minor modifications, version to appear in Phys.Lett.B

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• B_{s,d} -> l^+ l^- and K_L -> l^+ l^- in SUSY models with non-minimal sources of flavour mixing
• K^+ -> pi^+ nu nu-bar and K_L -> pi^0 nu nu-bar Decays in the General MSSM
• Quark-Squark Alignment Revisited
• CP Violation: The CKM Matrix and New Physics
• The CKM Matrix and The Unitarity Triangle: Another Look
• KL -> pi0 nu nu-bar as a Probe of New Physics
• Shedding Light on the "Dark Side" of $B_d^0$--${\bar{B}}_d^0$ Mixing through $B_d\to {\pi }^{+}{\pi }^{-}$, $K\to \pi \nu \bar{\nu }$ and $B_{d,s}\to {\mu }^{+}{\mu }^{-}$
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• Iso-singlet Down Quark Mixing And CP Violation Experiments
• Kaon decays and the flavour problem
• Rare kaon decays in SUSY with non-universal A terms
• Two RICH Detectors as Velocity Spectrometers in the CKM Experiment
• Recent developments in rare kaon decays
• Estimate of B(K -> pi nu nubar) from Standard Model fits to lambda_t
• New Physics Search in Flavour Physics
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• Penguin diagrams in the charm sector in K+ --> pi+ nu anti-nu