## I. INTRODUCTION 

Wave functions are key objects of the quantum world, specifying the structure of com- posite states in terms of their fundamental constituents. The conceptual extension of the non-relativistic wavefunctions of Schr¨ odinger theory to relativistic hadron physics are the frame-independent light-front wave functions (LFWFs) of hadrons Ψ H n ( x$_{i}$, ⃗ k$_{i}$$_{⊥}$, λ$_{i}$ ) where x$_{i}$ = k + i P + = k 0 $_{i}$+ k z i P $^{0}$+ P z are the light-front momentum fractions of the n constituents, k$_{i}$$_{⊥}$ the transverse momentum components, and λ$_{i}$ the parton helicities. The LFWFs are defined as constituent wave functions at fixed light-front time τ = x + = t + z/c and in the light- cone gauge A + = A 0 + A z = 0 where A µ represents the gauge field [1–4]. The LFWFs are obtained explicitly by computing the hadronic eigensolutions | Ψ$_{H}$ 〉 of the QCD light-front Hamiltonian H$_{LF}$ projected on the free Fock basis Ψ H n = 〈 n | Ψ$_{H}$ 〉 . 

Light-front wave functions in QCD describe the quark and gluon composition of hadron at a fundamental level, leading to a description of a wide range of hadronic and nuclear physics phenomena [2]. For example, the parton distribution functions measured in deep inelastic lepton-hadron scattering, including DGLAP evolution and their transverse mo- mentum extensions, are defined from the sum over squares of the light-front wave functions. Form factors are given by the sum of overlap matrix elements of the initial and final LFWFs with the electroweak currents. The gauge-invariant distribution amplitudes φ ( x$_{i}$, Q ) which control hard exclusive reactions are the valence LFWFs integrated over transverse momenta k 2 ⊥ < Q $^{2}$. 

Recent theoretical developments have shown that final and/or initial-state interactions can generate a phase in scattering amplitudes which lead to novel single transverse spin asymmetries in high energy hadronic reactions at leading twist. A prime example of this rescattering physics in QCD is the Sivers single-spin asymmetry measured in semi-inclusive deep inelastic scattering and spin-dependent Drell-Yan lepton pair production [5–7]. Double initial-state interactions lead to an anomalous cos 2 φ azimuthal dependence of the production plane in unpolarized lepton pair hadroproduction, corresponding to the breakdown of the Lam-Tung relation in PQCD [8, 9]. Similarly, diffractive deep inelastic lepton scattering ℓp → ℓ $^{′}$p$^{′}$X arises from the exchange of gluons in the final state which occurs after the hard lepton-quark interaction [10]. Since nuclear shadowing involves diffractive deep inelastic processes, nuclear distributions are also dependent on rescattering processes. 

