Evidence for $K^+ \to \pi^+ \nu \bar{\nu}$

TL;DR

This paper reports the first evidence of the rare decay $K^+ o \pi^+ u ar{ u}$, with results consistent with the Standard Model, and discusses future experiments aiming for higher sensitivity.

Contribution

It provides the first experimental evidence for the decay $K^+ o \pi^+ u ar{ u}$ and reports on the initial measurement and future experimental plans.

Findings

Observed a single event consistent with the decay.

Measured branching ratio aligns with Standard Model predictions.

Future experiments aim for significantly improved sensitivity.

Abstract

The first observation of the decay $K^+ \to \pi^+ \nu \bar{\nu}$ (pnn) has been reported. The E787 experiment presented evidence for this decay, based on the observation of a single clean event from data collected during the 1995 run of the AGS (Alternating Gradient Synchrotron at Brookhaven National Laboratory). The branching ratio indicated by this observation, B(pnn) = 4.2$^{+9.7}_{-3.5}\times10^{-10}$, is consistent with the Standard Model expectation although the central experimental value is four times larger. The final E787 data sample, from the 1995--98 runs, should reach a sensitivity of about five times that of the 1995 run alone. A new experiment, E949, has been given scientific approval and should start data collection in 2001. It is expected to achieve a sensitivity of more than an order of magnitude below the prediction of the Standard Model.