Electron and Photon Interactions at Intermediate Energies Proceeding
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C H R O M O D Y N A M I C S A N D THE QUARK M O D E L
N a t h a n Isgur D e p a r t m e n t of Physics U n i v e r s i t y of T o r o n t o Toronto, C a n a d a M5S IA7
INTRODUCTION
A l t h o u g h QCD seems to be the correct theory of the strong interactions I) , its a p p l i c a t i o n s to the m a i n p h e n o m e n a of strong i n t e r a c t i o n physics,
such as hadron masses and hadron decay and p r o d u c t i o n charac-
teristics,
is still in a r u d i m e n t a r y stage.
Lattice s i m u l a t i o n s have
now given a c o n v i n c i n g d e m o n s t r a t i o n that OCD confines,
but attempts
to c a l c u l a t e the masses of the l o w - l y i n g mesons and b a r y o n s on the lattice, w h i l e q u a l i t a t i v e l y encouraging, s a t i s f a c t o r y c o n c l u s i o n 2) .
have not yet r e a c h e d a
It is unclear, moreover,
how much further
these ab initio c a l c u l a t i o n s can be taken w i t h f o r s e e a b l e c o m p u t e r capacity. By contrast,
the quark m o d e l has had c o n s i d e r a b l e success in system-
a t i z i n g m a n y details of meson and b a r y o n spectra, couplings,
i n t r i n s i c moments,
and
e s p e c i a l l y after the naive q u a r k model was s u p p l e m e n t e d w i t h
certain simple d y n a m i c a l
features c h a r a c t e r i s t i c of QCD 3) .
In this form
the quark model has even been applied w i t h some success to such c o m p l i cated p r o b l e m s as d e r i v i n g the n u c l e o n - n u c l e o n p o t e n t i a l w h i c h u n d e r l i e s n u c l e a r physics. It must be stressed, however,
that the s p e c t r u m and d y n a m i c s of QCD
will i n e v i t a b l y be richer than that of the naive quark model.
For
example, even if we remove the quarks from 0CD, there w o u l d remain a nontrivial SU(3) states.
Y a n g - M i l l s theory w h i c h m u s t have its own s p e c t r u m of
These states, p o s s i b l y t r a n s f o r m e d in v a r i o u s w a y s by the
p r e s e n c e of quarks, will become the "glueball" states of QCD. u n c l e a r how the quark model should be e x t e n d e d to i n c o r p o r a t e gluonic degrees of freedom.
It is such
One w i d e l y a d o p t e d a p p r o a c h is to p r o c e e d
by a n a l o g y w i t h the "constituent quark" to posit the e x i s t e n c e of a "constituent
g l u o n " w i t h the q u a n t u m n u m b e r s of a gluon of w e a k c o u p l i n g
QCD p e r t u r b a t i o n t h e o r y
4) .
In the first part of this lecture I w i l l p r e s e n t an o p p o s i t e point of v i e w 5) b a s e d on OCD in the strong c o u p l i n g regime in w h i c h the gluonic degrees of f r e e d o m have c o n d e n s e d into c o l l e c t i v e
s t r i n g - l i k e flux tubes.
(It seems p l a u s i b l e that the strong c o u p l i n g limit in w h i c h quarks are c o n f i n e d may be more r e l e v a n t to an u n d e r s t a n d i n g of hadrons than the w e a k c o u p l i n g limit in w h i c h they are free.) model
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