How to find the values of $a$ where there is no real solution to $r$?

How to find the values of $a$ in

(-2 Sqrt[6] a^2 b^2 M - 2 Sqrt[6] a^2 Sqrt[1 - b^2] M) r -
2 Sqrt[6] M r^3 + r^2 (4 Sqrt[6] M^2 + 2 a^2 b^2 Sqrt[ϵ]) +
r^4 Sqrt[ϵ] + a^4 Sqrt[ϵ] Cos[θ]^4=0


where there is no real solutions to $r$.

• I think it would help if you clarify which variables are real, imaginary, or complex. @Jose E Calderon 's answer gives a general solution without making assumptions about the domains of the variables, but with more information it could easily be modified to do so. – nadlr Nov 22 '16 at 19:09

Solve[(-2 Sqrt[6] a^2 b^2 M - 2 Sqrt[6] a^2 Sqrt[1 - b^2] M) r -

• I don't think this answers my question. Because I want the values $a$ for the solutions of $r$ in the equation are real, i.e. the roots are not real. – gbd Nov 19 '16 at 7:10