SpinorUBarD[p, m] denotes a \bar{u}(p,m)-spinor that depends on the D-dimensional momentum p.
Overview, Spinor, SpinorUBar, SpinorU, SpinorV, SpinorVBar, SpinorUD, SpinorVD, SpinorVBarD.
SpinorUBarD[p, m]\bar{u}(p,m)
SpinorUBarD[p, m] // FCI // StandardForm
(*Spinor[Momentum[p, D], m, 1]*)SpinorUBarD[p]\bar{u}(p)
SpinorUBarD[p] // FCI // StandardForm
(*Spinor[Momentum[p, D], 0, 1]*)SpinorUBarD[p] . GSD[p]
DiracEquation[%]\bar{u}(p).(\gamma \cdot p)
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