Exicted Baryons in Large Nc QCD

37 Pages·2010·0.61 MB·English

Is QCD a string theory?
arXiv:0901.0494
Schrödinger’s cat?
Is Large N QCD a string theory?
c
arXiv:0901.0494
Schrödinger’s cat?
Is Large N QCD a an
c
approximate string theory for
sufficiently excited states?
arXiv:0901.0494
Schrödinger’s cat?
QCD & the Hagedorn Spectrum
The Hagedorn Spectrum
• The Hagedorn spectrum refers to a hadronic
spectrum in which the number of hadrons with
mass less then m grows exponentially with m:
N(m)~m-2b exp(m/T ) , where T , the Hagedorn
H H
temperature is a mass parameter.
• It has been argued that hadrons have a
Hagedorn spectrum.
There is some phenomenological
support for of this
Get data from
the Particle
Data Book
Integrated #
of mesons
with mass
less then m Bugg 2004
PDG
From Broniowski, Florkowski and Glozman 2004
Some data should be viewed
as being from the
apocrypha
This apparent exponential growth
in the empirical hadron spectrum
may be misleading for reasons
which may become clear later in
the talk
Integrated #
of mesons
with mass
less then m Bugg 2004
PDG
From Broniowski, Florkowski and Glozman 2004
Phenomenology and QCD Strings
• Hadronic strings are thought to emerge in QCD
as flux tubes only for highly excited states.
– Picture becomes valid when excitations are high
enough so that the length of the flux tube is much
bigger than its width and string motion dominates
– Dynamics of flux tubes give excitation spectrum of
hadrons.
q
• Mesons are open strings.
q
• Glueballs are closed strings.
A Phenomenological success of
the hadronic string picture
• String theories naturally give rise to a Hagedorn
spectrum for the density of hadrons:
N(m)~m-2B exp(M/T ) , where N(m) is the
H
number of hadrons with mass less than m & T ,
H
the Hagedorn temperature is a mass parameter
The Large N Limit and QCD Strings
c
• Phenomenology and theory of QCD strings are
both only clean at large N .
c
– Spectral results for hadrons (Regge, Hagedorn) depend
on hadron masses which are well defined if decays are
suppressed.
• Meson & glueball widths are suppressed at large as
N -1 and N -2 respectively.
c c
– Flux tubes break due to the creation of quark-antiquark
pairs. This destroys a simple string picture.
• This is suppressed by a factor of 1/N .
c