Markov-Yukawa transversality on covariant light front: Pion form factor, gauge invariance and Lorentz completion

The Markov-Yukawa Transversality Principle (MYTP) on a 2-body Bethe-Salpeter kernel is formulated on a covariant Null Plane (NP) to reconstruct the 4D BS wave function for 2 fermion quarks in terms of 3D entities that satisfy a 3D BSE. This result is the light-front counterpart of a 3D-4D interlinkage for the 2-body BS wave functions found earlier by imposing MYTP covariantly in the instant frame (termed CIA) of the composite. This formulation yields a 3D BSE which is formally identical to its Instant form, thus fully preserving its spectral results, as well as covariance. Further, the reconstructed 4D vertex functions in the new formulation ensure that 4D quark-loops are now free from ill-defined time-like momentum integrations (which had plagued the earlier instant form vertex functions), while a simple prescription of `Lorentz completionʹ in the new description yields a manifestly Lorentz-invariant result. This is illustrated for the pion and kaon form factors with full QED gauge-invariance, showing a k−2 behaviour at large k2, and `correctʹ slopes at small k2. This method is compared with the Kadychevsky-Karmanov light-front formalism.