Determination of the quark mass ratio $Q$ from $\eta \to 3 \pi$
The decay $\eta \to 3 \pi$ proceeds exclusively through isospin violating operators and is therefore an excellent probe to examine the strength of isospin breaking in the strong interaction. The latter can be expressed through the quark mass ratio $Q^2 = (m_s^2 - \hat{m}^2)/(m_d^2 - m_u^2)$. The main object of this thesis is to analyse the decay $\eta \to 3 \pi$ using dispersion relations in order to extract $Q$ as well as other physical quantities. The dispersion relations are a set of integral equations that lead to a representation of the decay amplitude in terms of four unknown subtraction constants. We apply two different methods to determine these. Following the procedure of Anisovich and Leutwyler, we match our dispersive representation to the one-loop result from chiral perturbation theory, thus updating their old analysis. In addition, we also make use of the recent experimental determination of the Dalitz distribution by the KLOE collaboration to obtain information on the subtraction constants. In this way we find $Q = 21.31^{+0.59}_{-0.50}$.