Bound states in gauge theories, from QED to QCD
Paul Hoyer - University of Helsinki
Abstract
Perturbative methods allow accurate calculations of QED bound states (atoms). Hadrons have atom-like
features, even though their quark and gluon constituents are highly relativistic and confined. The
possibility that analytic Hamiltonian methods may be useful also for QCD bound states merits careful
attention.
The first lecture will be an overview of the motivations, challenges and status of a Hamiltonian
approach to gauge theory bound states. There is a tantalizing possibility that confinement is
described by a classical gauge field (Born approximation), with loop corrections being perturbatively calculable. The A^0 potential of mesons is linear when the boundary condition on the homogeneous
solutions of Gauss' law is fixed by \Lambda_{QCD}.
Bound states are spatially extended objects which transform non-trivially under boosts (c.f. the
classical Lorentz contraction). Relativistic dynamics involves pair creation, giving sea quark
distributions. The Born level bound states also feature parton - hadron duality. Scattering amplitudes
are defined using the bound states as zeroth order 'in' and 'out' states of the perturbative expansion.
Previous lecture notes can be found in arXiv:1402.5005 and a summary of results in arXiv:1409.4703