Phenomenology of Glue

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Phenomenology of Glue
1. Quantum Chromodynamics (QCD) properties and
challenges.
2. Importance of gluons.
3. Experimental perspective.
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QCD
Part of the Standard Model describing the strong
interactions
The players Hadrons
protons,L
quarks
gluons
p, r, L
atomic nuclei
pions, L
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Why is QCD special ?

• A single theory which is responsible for
  phenomena at distance scales of
• the order of 10-15m as well as of the
  order 104m.

• It builds from objects (quarks and gluons) that
  do not exist in a common sense.

• Predicts existence of exotic matter, e.g.
  made from radiation (glueballs,hybrids)
• or novel plasmas.

• It is challenging !

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Properties of QCD

• Quantum Chromodynamics is a
• relativistic,
• 2. gauge,
• 3. local field theory,
• 4. of strongly interacting quarks and gluons.

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1. Relativity Particle number is not conserved,
N,H ¹ 0.
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2. Gauge symmetry
There are unphysical degrees of freedom.
Physical photon (gluon) has 2 polarizations,
but it is described by a 4-vector Am, m
0,1,2,3.
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Coupling constant in QCD
Asymptotic freedom
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4. Strongly coupled Confinement
r
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From high energy data
Gluons and quarks contribute equally to
protons momentum and spin distribution
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Canonical QCD in the Coulomb gauge

• Physical degrees of freedom.

• Natural for non-relativistic systems (EM,QM).

• Leads to quasi-particle (constituent)
  representation,
• Hadron i Valence quarks i
  small corrections.

• Amendable to standard many-body techniques
• (finite temperature and/or density).

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(Indirect)

• Gluons role in
• a) confinement,
• b) formation of constituent
  quarks,
• c) residual interactions.

(Direct)
2. Gluons role in determining hadron structure,
a) glueballs b)
hybrids.
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Confinement Lattice simulation
Ansatz wave function
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Where do constituent quarks come from
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Direct evidence for gluonic excitation
a) glueballs b) hybrids
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Glueballs
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Crystal Barrel
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Exotic hybrids
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Light exotic meson
Lattice predictions

• JPC 1- lowest state
• Higher masses difficult to resolve
• Chiral extrapolations 100-200 MeV

Thomas, Szczepaniak (02)
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Decays

• Unusual decay modes !

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Current phenomenology
Exotic candidates
Width (MeV)
Mass (MeV)
Experiment E852(BNL) Crystal Barrel(CERN)
1400
(hp)
350
(hp)
350
1600
E852
(rp)
170
1600
E852
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Is there an hp P-wave exotic 1- quantum
numbers ?
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Summary of hp0 and hp- experimental results

• The a0(980) and a2(1320) are clearly seen.
• The Breit-Wigner parameterization reproduces
  PDG values
• and production mechanisms are well understood.

The question remains what is the nature of
this wave.
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h-p0 P-wave phase shift
qhp MeV/c
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Finding the Exotic Wave
Double-blind M. C. exercise
An exotic wave (JPC 1-) was generated at level
of 2.5 with 7 other waves. Events were smeared,
accepted, passed to PWA fitter.
Statistics shown here correspondto a few days of
running.
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Outlook

• Development of canonical approach to QCD.
• Adaptation of many-body techniques in hadronic
  physics.
• Merger of QCD and the S-matrix theory.
• Phenomenology of exotic mesons.