Transcript Rapporteur 4: Theory summary (30) Larry McLerran

Theory Summary*
QM 2006
Shanghai, China
* Not comprehensive
Art due to Tetsuo Hatsuda
and Steffen Bass
(with some artistic
interpretation)
CGC
Initial
Singularity
Glasma
sQGP
Hadron Gas
Strong correspondence with cosmology.
How can ideas be tested?
What are the new physics opportunities?
The Initial Wavefunction for High Energy
Baryon:
3 quarks
3 quarks 1
gluon
…..
3 quarks and
lots of gluons
Density of Gluons Grows
becomes weak
Color Glass Condensate
Successes:
Geometric scaling in DIS
Diffractive DIS
Shadowing in dA
Multiplicity in AA
Limiting fragmentation
Long range correlations
Total cross section
Pomeron, reggeon, odderon
Break down of factorization of
pp to ep? Saturated hot spots?
The Initial Singularity and the Glasma
The hadrons pass through one another:
Before the collision only transverse E and B
CGC fields
Topological charge density is
maximal:
Anomalous mass generation
Color electric and magnetic monopoles
In cosmology:
Almost instantaneous phase change
to longitudinal E and B
Anomalous Baryogenesis
Production of gluons and quarks
from melting colored glass
The Initial Singularity and the Glasma
Before collision, stability
After collisions, unstable
Quantum fluctuations can become as
big as the classical field
Quantum fluctuations analogous to
Hawking Radiation
Interactions of evaporated gluons
with classical field is g x 1/g ~ 1 is
strong
Thermalization?
Growth of instability generates
turbulence => Kolmogorov spectrum
Analogous to Zeldovich spectrum of
density fluctuations in cosmology
Topological mass generation
Fluctuations in The Initial Singularity
During inflation: Fluctuations on scale
larger than even horizon are made
Late times: Become smaller than even
horizon => Seeds for galaxy formation
Fluctuations over many units in
rapidity in initial wavefunction
Instabilities driven by
momentum anisotropy
The sQGP
Energy density is high
enough:
Good agreement of “well
thought out” hydro
computations with radial and
elliptic flow data
Very large energy loss of jets
The evidence is strong that one has
made a system of quarks and
gluons which is to a good to fair
approximation explained by a
Quark Gluon Plasma
More evidence:
Is
its lower bound
?
Conclusion depends on initial
conditions?
Coalesence models
reproduce v2 at intermediate
pt.
Do they work too well?
Energy conservation?
Has led some to suggest that
we live in the best of all
possible worlds!
Water
Do we really need
huge cross sections in
transport to reproduce
flow data?
Hydro plus
CGC Initial
Conditions
Good
description of
multiplicity
and pT
distributions
Hydro +CGC + Jet
quenching: good
description of jets
(except for heavy
quarks!)
Good description
of v_2 when
dissipative effects
in hadronic matter
are included
CGC Initial
conditions without
viscosity in QGP
do less well
Can and will do better:
We do not yet properly treat:
Thermalization.
Initial conditions.
Viscosity in QGP not yet treated in
fully consistent way
Hadronization and coalesence not
fully self consistent
Next generation of
hydro, e. g. Spherio:
Fully 3-d with viscosity
Need more than just
running codes and fitting
data!
How Perfect is
the sQGP?
CGC Initial
Conditions allow
for higher hydro
limit.
LHC?
CGC Initial Conditions?
Large parton cross sections not
required for flow.
Thermalization through mutligluon
interactions?
Plasma Instabilities?
Viscosity effects are unknown,
computation is theoretical challenge.
Viscous Hydrodynamics:
Becoming practical
Jet Correlations:
Mach cones one of earliest proposals for heavy
ion collisions: Greiner, Stocker and Frankfurt
group
Cherenkov radiation and Mach cones
possible, but devil in the details
Possible explanation as Sudakov form
factor for jet emission by Salgado et. al?
Deflected jets al a Vitev?
Au+Au central 0-12% ZDC
Mach Cone:
Radiation and scattering: No cone
Δ2
Cerenkov: Wide angles
Δ1
Heavy Quark Energy Loss:
Charm to bottom ratio consistent with expectation.
QCD total cross sections off from data by factor of
2-5
What is the basic energy
loss mechanism:
Radiation?
Elastic scattering?
First principles computations are
hard:
Results depend on low density
region
Jet quenching computations not
strictly perturbative.
String Theory and the sQGP.
Bad Side:
Good Side:
Results are for N=4 SUSY Yang Mills
Many proponents are
shamelessly enthusiastic
No running coupling constant
No particle masses
Can generate new ways of
thinking about old problems
Strict infinite coupling limit
May or may not have any qualitative
relationship to QCD
No limit where theory is QCD
Brian Greene:
“data now emerging from the
Relativistic Heavy Ion Collider
at BNL appear to be more
accurately described using
string theory methods than with
more traditional approaches”
Was derived from string but
has simple interpretation:
Mean free path must be
bigger than De Broglie
wavelength
<<< Is it true?
String vs Conventional Computation of Energy
Density and Pressure
Perturbation Theory:
Conventional method is
Lattice Gauge Theory:
Errors of order several %
25% off from ideal gas value
Naïve perturbation theory not
good
String Theory: About 10% off for energy density
(scaled by number of degrees of freedom)
But………
AdSCFT Result >
Perturbative computations can be fixed. Good agreement
for relatively weak coupling above Tc
Difference between strongly interacting and strong coupling.
String computations require coupling limit.
Is the coupling large?
Lattice Monte Carlo
.81
2.3
.90
1.8
1.02
.7
1.5
.4
2.0
.3
Intermediate to weakly coupled,
but strongly interacting.
AdsCFT MUST be accountable to the same scientific standards
as are other computations,
or else it is not science.
Theoretical Issues:
Many problems of deep
significance.
Conceptual and
computational.
Issues must be scientific:
Controlled approximation
A result must be falsifiable.
“Theoretical physics
must be done with
passion and
enthusiasm”