Classical notions such as the center of charge and mass are, however, hard to make precise for a quantum elementary particle. In practice the definition used by experimentalists comes from the form factors F i ( q 2 ) appearing in the matrix element ⟨ ⟨ p f | j μ μ | p i ⟩ ⟩ = u ¯ ¯ ( p f ) [ F 1 ( q 2 ) γ γ μ μ + i σ σ μ μ ν ν 2 m e q ν ν F 2 ( q 2 ) + i ϵ ϵ μ μ ν ν ρ ρ σ σ σ σ ρ ρ σ σ q ν ν F 3 ( q 2 ) + 1 2 m e ( q μ μ − − q 2 2 m γ γ μ μ ) γ γ 5 F 4 ( q 2 ) ] u ( p i )