A particle of mass \(m\) moves along the \(x\)-axis in a region where its potential energy is given by the function \(U(x) = ax^4 - bx^2\), where \(a\) and \(b\) are positive constants. The particle is initially located at one of its stable equilibrium positions. What is the minimum initial speed \(v_0\) that must be imparted to the particle so that it can reach the origin \(x = 0\)?