A new measurement of the properties of the rare decay K -> pi+ e+ e-

TL;DR

This paper reports a precise measurement of the rare decay K+ -> pi+ e+ e-, confirming its vector nature, determining the decay form factor, and testing chiral QCD predictions with a large data sample.

Contribution

It provides the first detailed measurement of the decay form factor and tests higher-order chiral QCD effects in the rare decay K+ -> pi+ e+ e-.

Findings

Branching ratio measured as (2.94 +- 0.05(stat.) +- 0.13(syst.) +- 0.05(model))*10^{-7}

Decay is confirmed to be of vector nature

Form factor parameter delta determined as 2.14 +- 0.13 +- 0.15

Abstract

A large low-background sample of events (10300) has been collected for the rare decay of kaons in flight K+ -> pi+ e+ e- by experiment E865 at the Brookhaven AGS. The decay products were accepted by a broad band high-resolution charged particle spectrometer with particle identification. The branching ratio (2.94 +- 0.05(stat.) +- 0.13(syst.) +- 0.05(model))*10**{-7} was determined normalizing to events from the decay chain K+ -> pi+ pi0; pi0 -> e+ e- gamma. From the analysis of the decay distributions the vector nature of this decay is firmly established now, and limits on scalar and tensor contributions are deduced. From the (e+ e-) invariant mass distribution the decay form factor f(z)=f0(1+ delta*z) (z=M(ee)**2/m(K)**2) is determined with delta=2.14 +- 0.13 +- 0.15. Chiral QCD perturbation theory predictions for the form factor are also tested, and terms beyond leading order O(p**4) are found to be important.