Theη−x0Mixing Angle and theκMeson

The SU(3) singlet-octet mixing angle between $\eta$ and $x^0$ is expressed in terms of the masses of the members of the pseudoscalar meson nonet and a scalar strange meson $\kappa$. Using the recent experimental value $m_{\kappa }=1100\mathrm{}$ MeV, one obtains $\theta =23\mathrm{}°$, or $\theta =14\mathrm{}°$, depending on whether the angle is defined for mesons being at rest or moving with infinite momentum.