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<div class="moz-text-html" lang="x-unicode"> Theory Center Seminar<br>
Friday, May 11, 2018<br>
2:00 p.m. (coffee at 1:45 p.m.)<br>
CEBAF Center, Room L102<br>
<br>
Paul Hoyer<br>
University of Helsinki<br>
<br>
<b>Bound States and QCD</b><br>
<br>
There are many formally equivalent perturbative approaches to QED
bound states (atoms), <br>
because even a first approximation has a non-polynomial wave
function. Requiring that the <br>
gauge field be classical at lowest order selects the \hbar
expansion with a stationary action. <br>
This principle allows to derive the SchrÃdinger equation from QED.
Higher order corrections <br>
are defined as in the Interaction Picture, but with the in- and
out-states being eigenstates of <br>
the Hamiltonian that includes the classical field. Features of
hadron data indicate that the \hbar <br>
expansion is relevant also for QCD bound states. The QCD scale can
arise from a homogeneous, <br>
O(\alpha_s^0) solution of the field equations. Given basic
physical requirements the solution <br>
appears to be unique (up to the scale). It implies a linear
potential for mesons and a related <br>
confining potential for baryons. At lowest order in 1/N_c mesons
lie on linear Regge trajectories<br>
and their daughters. There are massless (M=0) states which allow
an explicit realization of <br>
spontaneous chiral symmetry breaking, through mixing of the 0^{++}
sigma state with the <br>
perturbative vacuum. Chiral transformations of the sigma
condensate generate massless 0^{-+}<br>
pions. For a small quark mass m the pion gets a mass M \propto
\sqrt{m}. The pion is annihilated <br>
by the axial vector current as expected for a Goldstone boson.<br>
<br>
<br>
<span class="" style="font-size: 14px;">The link to attend
remotely via BlueJeans is available at <a
href="https://www.jlab.org/div_dept/theory/seminars/2018-spring-cake-seminar.html"
class="">https://www.jlab.org/div_dept/theory/seminars/2018-spring-cake-seminar.html</a></span>
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