| M | 2 = e 4 { 1 t 2 [ u ¯ ¯ ( p 3 ) γ γ μ μ u ( p 1 ) ] [ u ¯ ¯ ( p 1 ) γ γ ν ν u ( p 3 ) ] [ u ¯ ¯ ( p 4 ) γ γ μ μ u ( p 2 ) ] [ u ¯ ¯ ( p 2 ) γ γ ν ν u ( p 4 ) ] + 1 u 2 [ u ¯ ¯ ( p 3 ) γ γ μ μ u ( p 2 ) ] [ u ¯ ¯ ( p 2 ) γ γ ν ν u ( p 3 ) ] [ u ¯ ¯ ( p 4 ) γ γ μ μ u ( p 1 ) ] [ u ¯ ¯ ( p 1 ) γ γ ν ν u ( p 4 ) ] − − 1 t u [ u ¯ ¯ ( p 3 ) γ γ μ μ u ( p 1 ) ] [ u ¯ ¯ ( p 2 ) γ γ ν ν u ( p 3 ) ] [ u ¯ ¯ ( p 4 ) γ γ μ μ u ( p 2 ) ] [ u ¯ ¯ ( p 1 ) γ γ ν ν u ( p 4 ) ] − − 1 t u [ u ¯ ¯ ( p 3 ) γ γ μ μ u ( p 2 ) ] [ u ¯ ¯ ( p 1 ) γ γ ν ν u ( p 3 ) ] [ u ¯ ¯ ( p 4 ) γ γ μ μ u ( p 1 ) ] [ u ¯ ¯ ( p 2 ) γ γ ν ν u ( p 4 ) ] } .