The theory of hadron interaction is quantum chromodynamics (QCD). However, at low energies perturbative methods cannot be applied since the coupling constant becomes very large. One method uses a low energy effective model, Chiral Perturbation Theory (CHPT), which is a rigorous methodology for QCD in the lowenergy regions. The predictive power of ChPT can be stringently tested by the form factors of the simplest semileptonic decay K_{l3}[1]. For the calculation of the K_{l3} form factors, a parameterization in terms of the form factors f_{+} and f_{0} which are associated with vector and scalar exchange, respectively, are convenient. The form factor f_{0} is related to f_{+} and f_{} through 

The K_{μ3} and K_{e3}
events were identified
by measuring μ^{+}s and e^{+}s
masses, as shown in a figure.
The Γ(K_{μ3})/Γ(K_{e3})
ratio can be written as,
Γ(K_{μ3})/Γ(K_{e3})
= N(K_{μ3}) / N(K_{e3}) •
Ω(K_{e3})/Ω(K_{μ3})
,
where N and Ω are the number of accepted events and the detector acceptance obtained by a Monte Carlo simulation, respectively. The Γ(K_{μ3})/Γ(K_{e3}) value and the associated λ_{0} parameter with the μe universality assumption were determined to be [3]
which is consistent with the ChPT prediction. 
