Christian Schubert (Universidad Michoacana de San Nicolás de Hidalgo, Mexico)

String-based vs. string-inspired in Yang-Mills theory and gravity

It has been understood since the eighties that string theory amplitudes are in various aspects better organised than amplitudes in ordinary quantum field theory. One can try to exploit this fact either by representing field theory amplitudes as the infinite string-tension limit of string amplitudes (``string-based formalism’’), or by representing them by worldline path integrals and evaluating those in a way analogous to string theory (``string-inspired formalism’’). Here I review and compare both approaches with an emphasis on QCD and gravity amplitudes, on-shell as well as off-shell. A central role is played by the cycle-and-tail decomposition of the worldline integrands of the one-loop gluon and graviton amplitudes that recently has tuned out to be pivotal for color-kinematics duality and multi-particle states, and provides a natural route to the construction of Berends-Giele currents in BCJ gauge for Yang-Mills theory as well as gravity. Finally, I sketch a variant of the worldline path-integral formalism that has recently become popular as a tool for the calculation of classical black-hole scattering.