ToStandardMatrixElement[exp] wraps Dirac structures, color structures and polarization vectors with the head StandardMatrixElement.
The idea of having standard matrix elements stems from A. Denner’s “Techniques for the calculation of electroweak radiative corrections at the one-loop level and results for W-physics at LEP200”, cf. arXiv:0709.1075.
Overview, DiracSubstitute5, DiracGamma, ToDiracGamma67, Spinor.
Spinor[Momentum[k2, D], 0, 1] . GAD[\[Mu]] . Spinor[-Momentum[k1, D], 0, 1]*
Spinor[-Momentum[ps, D], SMP["m_s"], 1] . GAD[\[Mu]] . Spinor[Momentum[pd, D], SMP["m_d"], 1]
ToStandardMatrixElement[%](\varphi (\text{k2})).\gamma ^{\mu }.(\varphi (-\text{k1})) \left(\varphi (-\text{ps},m_s)\right).\gamma ^{\mu }.\left(\varphi (\text{pd},m_d)\right)
\left\| (\varphi (\text{k2})).\gamma ^{\mu }.\bar{\gamma }^6.(\varphi (-\text{k1})) \left(\varphi (-\text{ps},m_s)\right).\gamma ^{\mu }.\bar{\gamma }^6.\left(\varphi (\text{pd},m_d)\right)\right\| +\left\| (\varphi (\text{k2})).\gamma ^{\mu }.\bar{\gamma }^7.(\varphi (-\text{k1})) \left(\varphi (-\text{ps},m_s)\right).\gamma ^{\mu }.\bar{\gamma }^6.\left(\varphi (\text{pd},m_d)\right)\right\| +\left\| (\varphi (\text{k2})).\gamma ^{\mu }.\bar{\gamma }^6.(\varphi (-\text{k1})) \left(\varphi (-\text{ps},m_s)\right).\gamma ^{\mu }.\bar{\gamma }^7.\left(\varphi (\text{pd},m_d)\right)\right\| +\left\| (\varphi (\text{k2})).\gamma ^{\mu }.\bar{\gamma }^7.(\varphi (-\text{k1})) \left(\varphi (-\text{ps},m_s)\right).\gamma ^{\mu }.\bar{\gamma }^7.\left(\varphi (\text{pd},m_d)\right)\right\|