[Jlab-seminars] Theory Center Seminar

[Mary Fox]

Theory Center Seminar
Friday, May 11, 2018
2:00 p.m. (coffee at 1:45 p.m.)
CEBAF Center, Room L102

Paul Hoyer
University of Helsinki

*Bound States and QCD*

There are many formally equivalent perturbative approaches to QED bound 
states (atoms),
because even a first approximation has a non-polynomial wave function. 
Requiring that the
gauge field be classical at lowest order selects the \hbar expansion 
with a stationary action.
This principle allows to derive the SchrÃdinger equation from QED. 
Higher order corrections
are defined as in the Interaction Picture, but with the in- and 
out-states being eigenstates of
the Hamiltonian that includes the classical field. Features of hadron 
data indicate that the \hbar
expansion is relevant also for QCD bound states. The QCD scale can arise 
from a homogeneous,
O(\alpha_s^0) solution of the field equations. Given basic physical 
requirements the solution
appears to be unique (up to the scale). It implies a linear potential 
for mesons and a related
confining potential for baryons. At lowest order in 1/N_c mesons lie on 
linear Regge trajectories
and their daughters. There are massless (M=0) states which allow an 
explicit realization of
spontaneous chiral symmetry breaking, through mixing of the 0^{++} sigma 
state with the
perturbative vacuum. Chiral transformations of the sigma condensate 
generate massless 0^{-+}
pions. For a small quark mass m the pion gets a mass M \propto \sqrt{m}. 
The pion is annihilated
by the axial vector current as expected for a Goldstone boson.


The link to attend remotely via BlueJeans is available at 
https://www.jlab.org/div_dept/theory/seminars/2018-spring-cake-seminar.html
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