Beyond Feynman Diagrams (1/3)

<!--HTML-->The search for new physics at the LHC, and accurate measurements of Standard Model processes, all benefit from precise theoretical predictions of collider event rates, which in turn rely on higher order computations in QCD, the theory of the strong interactions. Key ingredients for such computations are scattering amplitudes, the quantum-mechanical transition amplitudes between the incoming quarks and gluons and the outgoing produced particles. To go beyond leading order, we need both classical tree amplitudes and quantum loop amplitudes. For decades the central theoretical tool for computing scattering amplitudes has been the Feynman diagram. However, Feynman diagrams are just too slow, even on fast computers, to be able to go beyond the leading order in QCD, for complicated events with many jets of hadrons in the final state. Such events are produced copiously at the LHC, and constitute formidable backgrounds to many searches for new physics. Over the past few years, alternative methods that go beyond Feynman diagrams have come to fruition. These new "on-shell" methods are based on a very old principle, unitarity. They can be much more efficient because they exploit the underlying simplicity of scattering amplitudes, and recycle physical building blocks. I'll give an overview of why the new methods are needed, followed by a qualitative explanation of how and why they work, along with some examples of state-of-the-art results obtained with them.