A particle of mass \(m\) moves along the \(x\)-axis in a potential energy field given by \(U(x) = -\dfrac{U_0}{\cosh^2(x/a)}\), where \(U_0\) and \(a\) are positive constants. The particle is displaced a small distance from its equilibrium position at \(x = 0\) and released from rest. What is the angular frequency \(\omega\) of the resulting small oscillations?