1pm - 2pm: How strong exactly is the strong nuclear force?

At the fundamental level the strong nuclear force is described by QCD, the theory of quarks and gluons and their interactions. The strength of their coupling, alpha_s(m_Z), is one of the least precisely known fundamental parameters of the Standard Model. This is due to confinement and the resulting impossibility to carry out experiments on free quarks and gluons. While the Standard Model should be enough to describe experiments involving the strong interactions, phenomenological approaches often rely on additional assumptions, such as models for hadronization,  or "quark hadron duality''.  This induces systematic errors that are difficult to quantify  and not reducible by increased statistic. Methods based on numerical simulations of lattice QCD bypass the confinement problem and are therefore best placed to determine alpha_s(m_Z) with high precision, given well-measured hadron masses and decay constants as experimental input. I will first explain the challenges for the lattice QCD approach and then the  strategies used by the ALPHA collaboration in our recent work. The final result is  alpha_s(m_Z) = 0.11876(58), with a statistics-dominated relative error of 0.5 percent, about half the error of the PDG average of all non-lattice methods combined.