The covariant structure of light - front wave functions and the behavior of hadronic form factors ∗

We study the analytic structure of light-front wave functions (LFWFs) and its consequences for hadron form factors using an explicitly Lorentz-invariant formulation of the front form. The normal to the light front is specified by a general null vector ω. The LFWFs with definite total angular momentum are eigenstates of a kinematic angular momentum operator and satisfy all Lorentz symmetries. They are analytic functions of the invariant mass squared of the constituents M 0 = ( ∑ k) and the light-cone momentum fractions xi = ki·ω/p·ω multiplied by invariants constructed from the spin matrices, polarization vectors, and ω. These properties are illustrated using known nonperturbative eigensolutions of the Wick–Cutkosky model. We analyze the LFWFs introduced by Chung and Coester to describe static and low momentum properties of the nucleons. They correspond to the spin-locking of a quark with the spin of its parent nucleon, together with a positive-energy projection constraint. These extra constraints lead to anomalous dependence of form factors on Q rather than Q. In contrast, the dependence of LFWFs on M 0 implies that hadron form factors are analytic functions of Q in agreement with dispersion theory and perturbative QCD. We show that a model incorporating the leading-twist perturbative QCD prediction is consistent with recent data for the ratio of proton Pauli and Dirac form factors.