SISD[p]
can be used as input for D-1-dimensional \sigma^{\mu } p_{\mu } with D-dimensional Lorentz vector p and is transformed into
PauliSigma[Momentum[p,D],D-1]
by
FeynCalcInternal
.
[p] SISD
\sigma \cdot p
[p] // FCI // StandardForm
SISD
(*PauliSigma[Momentum[p, D], -1 + D]*)
[p, q, r, s] SISD
(\sigma \cdot p).(\sigma \cdot q).(\sigma \cdot r).(\sigma \cdot s)
[p, q, r, s] // StandardForm
SISD
(*SISD[p] . SISD[q] . SISD[r] . SISD[s]*)
[q] . (SISD[p] + m) . SISD[q] SISD
(\sigma \cdot q).(m+\sigma \cdot p).(\sigma \cdot q)