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The Color Glass Condensate
Outstanding questions:
What is the high energy limit of QCD?
How do gluons and quarks arise in hadrons?
What are the possible forms of high density matter?
Claim: CGC is a universal form of strongly interacting matter which controls the high energy
limit of QCD, and form which the gluons and quarks arise in this limit.
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How do we think about a high energy hadron?
Work in fast moving frame:
High Energy Limit is Small x Limit
Wavefunction has:
3 quarks
3 quarks plus 1 gluon
3 quarks plus 2 gluon
…….
3 quarks plus many gluons
The Gluon Wall:
Important matrix elements at high energies have lots of gluons in them 3
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In RHIC CollisionsAu-Au at 100GeV/Nucleon in each beam
About 1000 slow moving (small x) particles aremade in central collisions
Where do all the gluons go?
Cross sections for hadrons rise very slowly with energy
But the gluon density rises
much more rapidly!The high energy limit is the high
gluon density limit.
Surely the density must saturate for fixed sizes of
gluons at high energy.
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What is the Color Glass Condensate?
Glue at large x generates glue at small x
Glue at small x is classical field
Time dilation -> Classical field is glassy
High phase space density -> Condensate
Phase space density:
Attractive potential Repulsive interactions
Density as high as it can be
Because the density is high is small
is big
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There must be a renormalization group
The x which separates high x sources from small x fields is arbitrary
Phobos multiplicity data High energy QCD “phase” diagram
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Why is the Color Glass Condensate Important?
It is a new universal form of matter:
Matter: Carries energy; Separation of gluons is small compared to size of system; Number of gluons is large
New: Can only be made and probed in high energy collsions
Universal: Independent of hadron, renormalization group equations have a universal solution.
Universality <=> Fundamental
It is a theory derived from first principles in QCD of:
Origin of glue and sea quarks in hadrons
Cross sections
Initial conditions for formation of Quark Gluon Plasma in heavy ion collisions
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What does a sheet of Colored Glass look like?
On the sheet is small
Independent of
big
small
Density of gluons per unit area
Lienard-Wiechart potentials
Random Color
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The Color Glass Condensate Explains Growth of Gluons at Small x
Renormalization group equation predicts:
Gluon pile up at fixed size until
gluons with strength
act like a hard sphere
Once one size scale is filledMove to smaller size scale
Typical momentum scale grows
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The CGC Explains Slow Growth of Total Cross Section
Transverse distribution of gluons:
Transverse profile set by initial conditions
Size is determined when probe sees a fixed number of particles at some transverse distance
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CGC Explains Qualitative Features of Electron-Hadron Scattering
Function only of a particular combination of Q and x
Scaling relation
Works for
Can successfully describe quark and gluon
distributions at small x
and wide range of Q
Q is resolution momentum of photon, x is that of struck quark
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CGC Gives Initial Conditions for QGP in Heavy Ion Collisions
Two sheets of colored glass collide
Glass melts into gluons and thermalize
QGP is made which expands into a mixed phase of QGPand hadrons
Mystery: The QGP is very strongly interacting:
Arnold and Moore suggest heating may be due to
instabilities in melting CGC
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CGC predicted particle production at RHIC
Proportionality constant can be computed.
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CGC provides a theory of shadowing (modification of quark and gluon distributions in nuclei)
Two effects:
Multiple scattering: more particles at high pT
CGC modification of evolution equations => less particles
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Data from dA collisions at RHIC Consistent with CGC
Look for fragments of deuteron since they measure them smallest x
properties of the nucleus
Back to back jet correlations seen in STAR?
Detailed studies of x dependence?
What are the Quasi Particles of the Glass?
Regge (with Gorbachov)
Pomerons: Two gluon C even stateReggeons: Multiple gluon, multiple quark states
Odderon: C odd three gluon state; Pomeron gives imaginary part of T matrix and odderon real part at high energy
Quasi-particles are excellent probes of properties of a media.
Quasi particles of the glass are pomeron, odderon, reggeon etc.Were inferred long ago from scattering matrix theory.
Computable small fluctuation of color glass
Can be probed in pp, pA and eA interactions.
Polarized scattering a good probe.
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Exciting times for theory:
Beginning of a complete description of high energy limit of QCD
Must understand the collective excitations of the CGC: pomerons, reggeons, odderons…
Need to understand interactions of these collective excitations: ploops or Pomeron
loops
Relationships with universal behavior of nucleation models.
Solitons
More……..
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