<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Reminder<br class=""><div><br class=""><blockquote type="cite" class=""><div class="">Theory Center Seminar</div><div class=""><div text="#000000" bgcolor="#FFFFFF" class=""><div class="moz-text-html" lang="x-unicode">
Friday, May 11, 2018<br class="">
2:00 p.m. (coffee at 1:45 p.m.)<br class="">
CEBAF Center, Room L102<br class="">
<br class="">
Paul Hoyer<br class="">
University of Helsinki<br class="">
<br class="">
<b class="">Bound States and QCD</b><br class="">
<br class="">
There are many formally equivalent perturbative approaches to QED
bound states (atoms), <br class="">
because even a first approximation has a non-polynomial wave
function. Requiring that the <br class="">
gauge field be classical at lowest order selects the \hbar
expansion with a stationary action. <br class="">
This principle allows to derive the SchrÃdinger equation from QED.
Higher order corrections <br class="">
are defined as in the Interaction Picture, but with the in- and
out-states being eigenstates of <br class="">
the Hamiltonian that includes the classical field. Features of
hadron data indicate that the \hbar <br class="">
expansion is relevant also for QCD bound states. The QCD scale can
arise from a homogeneous, <br class="">
O(\alpha_s^0) solution of the field equations. Given basic
physical requirements the solution <br class="">
appears to be unique (up to the scale). It implies a linear
potential for mesons and a related <br class="">
confining potential for baryons. At lowest order in 1/N_c mesons
lie on linear Regge trajectories<br class="">
and their daughters. There are massless (M=0) states which allow
an explicit realization of <br class="">
spontaneous chiral symmetry breaking, through mixing of the 0^{++}
sigma state with the <br class="">
perturbative vacuum. Chiral transformations of the sigma
condensate generate massless 0^{-+}<br class="">
pions. For a small quark mass m the pion gets a mass M \propto
\sqrt{m}. The pion is annihilated <br class="">
by the axial vector current as expected for a Goldstone boson.<br class="">
<br class="">
<br class="">
<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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