QCD Light-Cone Sum Rules

This chapter starts the discussion of QCD light-cone sum rules from expansion of the quark propagator near the light-cone. Differences arise at the next-to-leading order, when interaction of the virtual quark with a gluon field is switched on. The chapter presents a detailed derivation of the sum rule. The underlying object of the light-cone sum rules (LCSR) method is a correlation function containing the T-product of two quark currents sandwiched between the QCD vacuum and a hadron state. With a proper choice of external momenta, the vacuum-to-hadron correlation function is calculated to a certain accuracy, employing OPE near the light-cone in terms of hadron distribution amplitude (DAs). One should distinguish between virtual gluons in the loops and gluons flowing from the pion and absorbed by quarks. The latter are by default soft and have to be counted as a part of the pion DA.