Trace anomaly and non-scalar gluon EMT contributions in near-threshold quarkonium scattering on the light front

Abstract: We present a light-front formulation of the scalar trace-anomaly and non-scalar gluon energy-momentum-tensor contributions entering the compact-heavy-quarkonium chromoelectric interaction with a nucleon. The construction reformulates, in light-front variables, the separation between the scalar trace-anomaly contribution to the compact-quarkonium chromoelectric interaction and the forward normal gluon-energy contribution appearing in the QCD EMT mass decomposition. In light-front variables, the gluon momentum fraction follows from the $T_g^{++}$ matrix element, while the normal gluon-energy factor arises only after projecting the traceless EMT onto the threshold chromoelectric structure. In this way the forward light-front ratio reproduces the result obtained by combining the QCD EMT mass decomposition with the compact-quarkonium chromoelectric operator-product expansion (OPE), without introducing any spurious boost enhancement. We also construct the off-forward light-front form-factor combination controlling the non-scalar gluon-EMT contribution and show its equivalence to the corresponding covariant expression. Finally, the scalar form-factor radius is reinterpreted as a transverse light-front radius, providing a longitudinal-boost-invariant transverse-density interpretation in the Drell--Yan frame. The work is therefore not a new mass-radius extraction, but a light-front operator formulation connecting the QCD EMT mass decomposition, the compact-quarkonium chromoelectric OPE, modern gluon momentum fractions, and light-front transverse densities.