Measurements of Charged Kaon Semileptonic Decay Branching Fractions K^+- -> pi^0 mu^+- nu and K+- -> pi^0 e^+- nu and Their Ratio

TL;DR

This paper reports precise measurements of charged kaon semileptonic decay ratios, which suggest a larger |V_{us}| CKM matrix element than previously thought, supporting CKM unitarity.

Contribution

First precise measurement of K^+- decay rate ratios using NA48/2 data, impacting CKM matrix element determinations.

Findings

Measured ratios are larger than PDG values.

Results support CKM matrix unitarity.

New Kμ3/Ke3 ratio compared with theoretical predictions.

Abstract

Measured ratios of decay rates for Ke3/K2\pi, K\mu3/K2\pi and K\mu3/Ke3 are presented. These measurements are based on K^+- decays collected in a dedicated run in 2003 by the NA48/2 experiment at CERN. The results obtained are Ke3/K2\pi = 0.2470 +/- 0.0009 (stat) +/- 0.0004 (syst) and K\mu3/K2\pi = 0.1637 +/- 0.0006 (stat) +/- 0.0003 (syst). Using the PDG average for the K^+- -> \pi^+-\pi^0 normalisation mode, both values are found to be larger than the current values given by the Particle Data Book and lead to a larger magnitude of the |V_{us}| CKM element than previously accepted. When combined with the latest Particle Data Book value of |V_{ud}|, the result is in agreement with unitarity of the CKM matrix. In addition, a new measured value of K\mu3/Ke3 = 0.663 +/- 0.003(stat) +/- 0.001(syst) is compared to the semi-empirical predictions based on the latest form factor measurements.