Using the Weinberg-Soper formalism we construct the front-form (j,O)@ (0, j) spinors. Explicit expressions for the generalized Melosh transformations up to spin two are obtained. The formalism, without explicitly invoking any wave equations, reproduces the spin-; front-form results of Melosh, Lepage and Brodsky, and Dziembowski.

We identify the source of a discrepancy between pPft /long and FP'~~~,,,, found by Isgur and Llewellyn Smith. We find that the discrepancy disappears when virtual qq pair production for longitudinal-momentum transfers is included. Our result rectifies the discussion by Isgur and Llewellyn Smith of soft nonperturbative effects at presently available values of Q2. PACS Number(s): 13.40.Fn, ll.lO.St, 1 2 . 3 8 . B~ While perturbative Q C D ( P Q C D ) [l-51 is expected t o be a n adequate tool t o study the asymptotic Qoo behavior of hard exclusive reactions, doubts have been raised [6,71 a b o u t t h e applicability of P Q C D a t presently available Q'. Currently a strong disagreement continues a b o u t a scale of momentum transfers characteristic for the transition from the nonperturbative to perturbative regime. While some argue [8-141 t h a t the transition may take place for Q2 as low as 4-6 (GeV/c)', or a t some 15 (GeV/c)' [15], others maintain [16, 171 t h a t the transition should take place for much higher values of Q 2 , perhaps as high as 100 (GeV/c)'. Recently Isgur and Llewellyn Smith [16] (ILS) reexamined their original arguments [7] against the applications of P Q C D in exclusive processes a t presently available Q2.

Covariant perturbation theory is formulated using a new set of space-time coordinates. This corresponds to a quantization on any flat spacelike surface in Minkowski space. One limit of the theory reproduces the usual instant (equal-time) dynamics, whereas a different limit gives light-front dynamics. Neither the infinite-momentum frame nor infinite momenta are involved. In particular a smootl~ parametric transformation from the instant to light-front picture is given for a system at rest.

_ We investigate the evolution equation for distribution amplitudes in the framework of a scalar theory quantized on the light cone. We End general solutions for the cases of 4 and 6 dimensions and use them to reconstruct two-body relativistic bound state wave functions at small distances. The relation between the light-cone bound state equation and the Bethe-Salpeter equation is discussed.