Rainer Sommer - How strong are the strong interactions

The ALPHA Collaboration has computed one of the most elusive fundamental parameters of Nature: the strong coupling, alpha_s. It governs the interactions of quarks and gluons. At high energies, such as the ones reached at the Large Hadron Collider (LHC) at CERN, many processes can be computed in terms of a Taylor series in this coupling. A precise input value for these series is thus essential to make full use of this accelerator. We have performed lattice "simulations" of the fundamental theory of strong interactions called Quantum Chromodynamics (QCD) over a large range of energy scales in order to extract the coupling at LHC energies. Our experimental input comes entirely from the low energy properties of Pion and Kaon: masses and radioactive decay rates of these mesons.

I give elementary introductions to lattice QCD, to the question of a non-perturbative meaning and computation of alpha_s, and to the strategy used to bridge orders of magnitudes in scale from low energy meson physics to the high energy behavior of alpha_s.