A search for the rare decay K+ -> pi+ nu nu

TL;DR

This paper reports on the search for the rare decay K+ -> pi+ nu nu, updates from the E787 experiment, and recent observations of related decay modes, aiming to measure the branching ratio and probe the CKM matrix element |Vtd|.

Contribution

It presents new experimental limits on the K+ -> pi+ nu nu decay, observations of K+ -> pi+ mu mu and K+ -> pi+ gamma gamma decays, and discusses detector upgrades and future sensitivity.

Findings

Branching ratio limit BR(pnn) < 2.4e-9 (90% CL)

Observation of K+ -> pi+ mu mu decay

Measurement of BR(SD+) = (1.33 +/- 0.12 +/- 0.18)e-5

Abstract

Recent results from the first phase of the E787 experiment and an update on the current status are presented. From the first phase the limit on the K+ -> pi+ nu nu (pnn) branching ratio is BR(pnn) < 2.4e-9 (90%CL). An observation of the decay K+ -> pi+ mu mu has been made using two separate techniques. An observation of the decay K+ -> pi+ gamma gamma has been made and the distribution of two photon invariant mass is inconsistent with phase space but consistent with chiral perturbation theory. A description of recent upgrades to the detector follows. With the upgraded detector the pnn decay should soon be observable. A discussion of the expected sensitivity for pnn during the current running period is presented. Possible improvements, in order to make a significant measurement of |Vtd|, are discussed. One result from the upgraded detector is a measurement of the structure dependent part (SD+) of the decay K+ -> mu+ nu gamma with a branching ratio of BR(SD+) = (1.33 +/- 0.12 +/- 0.18)e-5.