K+ -> pi0 e+ nu branching ratio from E865

TL;DR

This paper reports a precise measurement of the K+ -> pi0 e+ nu branching ratio, finding a value slightly higher than previous estimates, with implications for the CKM matrix element V_us and unitarity tests.

Contribution

The study provides a new, more accurate measurement of the K+ -> pi0 e+ nu branching ratio using a novel detection method for pi0 decay products.

Findings

Measured BR(K+(e3(gamma))) = (5.13 +/- 0.02(stat) +/- 0.09(sys) +/- 0.04(norm))%

Result is 2.3 sigma higher than the PDG value

Implications for V_us and CKM unitarity discussed

Abstract

E865 at the Brookhaven National Laboratory AGS collected about 70,000 K+(e3) events to measure the K+(e3) branching ratio relative to the observed K+ -> pi+ pi0, K+ -> pi0 mu+ nu, and K+ -> pi+ pi0 decays. The pi0 in all the decays was detected using the e+e- pair from pi0 -> e+e-gamma decay and no photons were required. Using the Particle Data Group branching ratios for the normalization decays we obtain BR(K+(e3(gamma))=(5.13+/-0.02(stat)+/-0.09(sys)+/-0.04(norm))%, where $K+(e3(gamma)) includes the effect of virtual and real photons. This result is 2.3 sigma higher than the current Particle Data Group value. The implications of this result for the $V_{us}$ element of the CKM matrix, and the matrix's unitarity are discussed.