K+ -> pi+ mu+ mu- in E865 at BNL

TL;DR

This paper reports the most precise measurement of the K+ -> pi+ mu+ mu- decay branching ratio and form factor, revealing a larger slope than simple models predict and challenging existing theoretical frameworks.

Contribution

It provides the first large dataset for K+ -> pi+ mu+ mu- and compares results with theoretical models, highlighting discrepancies with O(p^4) Chiral Perturbation Theory.

Findings

Branching ratio: (9.23 +/- 0.6 stat +/- 0.58 syst) x 10^-8

Form factor slope lambda = 0.182+/-0.01+/-0.007

Results are inconsistent with O(p^4) Chiral Perturbation Theory

Abstract

Preliminary values for the K+ -> pi+ mu+ mu- branching ratio and form factor are reported, based on 400 events, a factor of 2 more in total events and 100 times the present world sample of fully reconstructed events. The results are consistent with previous results on the pi+ e+ e- mode. However, the relatively large slope of the form factor in q^2, lambda = 0.182+/-0.01+/-0.007, required to fit the pi+ e+ e- data and to give consistency between the pi+ e+ e- and pi+ mu+ mu- branching ratios, is larger than expected in simple models of the decays. The K+ -> pi+ mu+ mu- branching ratio we find, (9.23 +/- 0.6 stat +/- 0.58 syst)x10^-8, is the most precise measurement of this mode and is approximately 3.2 sigma larger than the previous measurement. These pi+ l+ l- results are inconsistent with O(p^4) Chiral Perturbation Theory but compatible with O(p^6). Systematic studies for both modes are still in progress.