Vacuum angle theta in a strongly interacting hidden sector and its effect
Introducing a vacuum angle $\theta_h$ in a strongly interacting hidden sector like QCD, we study how $\theta_h$ affects physical quantities in a scale invariant extension of the standard model with that hidden sector. In this model the dynamical chiral symmetry breaking occurs in the hidden sector and generates a scale $ \vert \langle {\bar q} q \rangle \vert $ which triggers electroweak symmetry breaking. We find that the expression for a vacuum expectation value of the Higgs field depends on $\theta_h$, while the condition which determines the Higgs particle mass does not. This model contains the hidden-sector pions $\pi^0$ and $\pi^{\pm}$ which are candidates for cold dark matter, and we find the mass difference between the hidden-sector $\pi^0$ and $\pi^{\pm}$ will not be negligible for $\theta_h \approx \pi$.