Large-momentum effective theory for parton physics

Partons are effective degrees of freedom describing the structure of hadrons involved in high-energy collisions. Familiar theories of partons are QCD light-front quantization and soft-collinear effective theory, both of which are intrinsically Minkowskian and appear unsuitable for classical Monte Carlo simulations. A new form of the parton theory has been formulated in terms of the old-fashioned, Feynman's infinite momentum frame, in which the parton degrees of freedom are filtered through infinite-momentum external states. The partonic structure of hadrons is then related to the matrix elements of static (equal-time) correlators in the state |Pz = \infty>. This representation lays the foundation of large-momentum effective theory (LaMET) which approximates parton physics through a systematic M/Pz expansion of the lattice QCD matrix elements at a finite but large momentum Pz, and removes the residual logarithmic-Pz dependence by the standard effective-field theory matching and running.