where δ [L is either 0 or 1 depending on whether α ≠ A or α = A, respectively. We also know that <〈e HH 〉 <〈E B 〉 = 0. Finally, for the in vacuo zero-temperature limit of µ L the terms <f IJGFK56 <〈E B 〉 and <〈Y〉e <〈E B 〉 are negligible. We can then simplify eq. ( ):