Current experimental upper bounds on the dipole moment give an upper bound of d N < 10 − − 26 e ⋅ ⋅ cm, which requires θ θ ¯ ¯ < 10 − − 10 . The angle θ θ ¯ ¯ can take any value between zero and 2 π π , so it taking on such a particularly small value is a fine-tuning problem called the strong CP problem.