m ρ refers to the pole mass of the ρ meson, whereas m ρ refers to the in-medium mass (or running mass in the medium) of the ρ meson according to QCD sum rules. The omega meson, sigma meson, and neutron are denoted by ω, σ, and N, respectively. The symbol f π denotes the free-space pion decay constant. (Decay constants have a "running time" and a "pole time" similar to the "running mass" and "pole mass" concepts, according to special relativity.) The symbol F π is also used to denote the pion decay constant.