Differential Cross-Section

[math]-i \mathfrak{M}_{e^-e^-}=-i \left ( \frac{e^2(\mathbf p_1+\mathbf p_{1'})_{\mu}(\mathbf p_2+\mathbf p_{2'})^{\mu}}{(\mathbf p_{2'}-\mathbf p_2)^2}- \frac{e^2(\mathbf p_1+\mathbf p_{2'})_{\mu}(\mathbf p_2+\mathbf p_{1'})^{\mu}}{(\mathbf p_{1'}-\mathbf p_2)^2} \right )[/math]

[math] \mathfrak{M}_{e^-e^-}= e^2\left ( \frac{\mathbf P_1 \mathbf P_2+\mathbf P_{1'} \mathbf P_{2'}+\mathbf P_{1'} \mathbf P_2+\mathbf P_1 \mathbf P_{2'}}{(\mathbf P_{2'}-\mathbf P_2)^2}- \frac{\mathbf P_1 \mathbf P_2+\mathbf P_{2'} \mathbf P_{1'}+\mathbf P_{2'} \mathbf P_2+\mathbf P_1 \mathbf P_{1'}}{(\mathbf P_{1'}-\mathbf P_2)^2} \right )[/math]

Using the fact that [math]P_1^{'*}P_2^{'*}=P_1^*P_2^*[/math]

[math] \mathfrak{M}_{e^-e^-}= e^2\left ( \frac{ (\mathbf P_1^{*2}-2 \mathbf P_1^* \mathbf P_2^{'*}+ \mathbf P_2^{'*2})-(\mathbf P_1^{*2}+2 \mathbf P_1^* \mathbf P_2^*+ \mathbf P_2^{*2})}{t}- \frac{(\mathbf P_1^{*2}-2 \mathbf P_1^* \mathbf P_1^{'*}+ \mathbf P_1^{'*2})-( \mathbf P_1^{*2}+2 \mathbf P_1^* \mathbf P_2^*+ \mathbf P_2^{*2})}{u} \right )[/math]

[math]\mathfrak{M}_{e^-e^-}=e^2 \left (\frac{u-s}{t}+\frac{t-s}{u} \right )[/math]