Measurement of K^0_e3 form factors

TL;DR

This paper measures the form factors in the semileptonic decay of the neutral K meson, providing precise values for vector, scalar, and tensor interactions, and testing the Standard Model's V-A coupling structure.

Contribution

The study presents the first comprehensive measurement of all possible Lorentz-covariant form factors in KL -> pi e nu decay using a large data sample.

Findings

Measured vector form factor slope lambda_+ = 0.0284+-0.0015

Scalar and tensor form factors are consistent with zero

Dipole fit yields a pole mass of 859+-18 MeV

Abstract

The semileptonic decay of the neutral K meson, KL -> pi e nu (Ke3), was used to study the strangeness-changing weak interaction of hadrons. A sample of 5.6 million reconstructed events recorded by the NA48 experiment was used to measure the Dalitz plot density. Admitting all possible Lorentz-covariant couplings, the form factors for vector (f_+(q^2)), scalar (f_S) and tensor (f_T) interactions were measured. The linear slope of the vector form factor lambda_+ = 0.0284+-0.0007+-0.0013 and values for the ratios |f_S/f_+(0)| = 0.015^{+0.007}_{-0.010}+-0.012 and |f_T/f_+(0)| = 0.05^{+0.03}_{-0.04}+-0.03 were obtained. The values for f_S and f_T are consistent with zero. Assuming only Vector-Axial vector couplings, lambda_+ = 0.0288+-0.0004+-0.0011 and a good fit consistent with pure V-A couplings were obtained. Alternatively, a fit to a dipole form factor yields a pole mass of M = 859+-18 MeV, consistent with the K^*(892) mass.