How to parameterize a light and broad resonance (the σ meson)

We point out that a commonly used parameterization form to describe the $\sigma$ resonance is problematic by introducing a spurious singularity below two particle threshold, on the second sheet. The spurious singularity violates chiral symmetry and leads to sizable contribution when the resonance is light and broad -- as what happens to the $\sigma$ resonance, hence must be removed.