#### October 18, 10:15 in E204

#### Paul Hoyer (Helsinki)

#### Bounds states in field theory

Abstract: Bound states form an important part of observable phenomena, and are a prominent feature of quantum field theories. Atoms provide precision tests of QED and serve as a conceptual model for hadrons, the relativistic bound states of QCD. The field theory description of bound states is nevertheless often omitted in standard textbooks and courses. Perhaps for this reason there are basic principles concerning relativistic bound states that receive little attention. I illustrate with three instructive questions:
1. What is the wave function of a QED atom which is in relativistic CM motion?
2. How can the single particle Dirac wave function describe bound states with an infinite number of pairs?
3. Is there a Born term (of lowest order in hbar) for bound states?
The answers provide guidance in addressing the more difficult question: Is there an expansion of QCD which at lowest order is similar to the Quark Model picture of hadrons?