A measurement of the $K^{+} \to \pi^{+} \mu^{+} \mu^{-}$ decay

TL;DR

This paper reports a highly precise measurement of the $K^{+} o \pi^{+} \mu^{+} \\mu^{-}$ decay's branching fraction and form factor, providing new insights into kaon decay dynamics and testing theoretical models.

Contribution

The study provides the most precise measurement to date of the decay's branching fraction and analyzes the form factor using Chiral Perturbation Theory at (6).

Findings

Branching fraction measured as (9.15 b1 0.08) d7 10^{-8}.

Decay form factor characterized as a function of squared dimuon mass.

Form factor parameters determined with uncertainties consistent with theoretical predictions.

Abstract

A sample of $2.8 \times 10^4$ $K^{+} \to \pi^{+} \mu^{+} \mu^{-}$ candidates with negligible background was collected by the NA62 experiment at the CERN SPS in 2017--2018. The model-independent branching fraction is measured to be $(9.15 \pm 0.08)\times 10^{-8}$, a factor three more precise than previous measurements. The decay form factor is presented as a function of the squared dimuon mass. A measurement of the form factor parameters and their uncertainties is performed using a description based on Chiral Perturbation Theory at $\mathcal{O}(p^6)$.