NEXUS

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NEXUS
hep-ph/0007198 Physics Reports 350 (2001)
93-289
Hajo Drescher, Fuming Liu Sergej
Ostapchenko, Tanguy Pierog Klaus Werner
hep-ph/0102194 Phys. Rev. Lett. 86 (2001) 3506
Guideline theoretical consistency
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1Parton-basedGribov-ReggeTheory
Aim connecting properly parton model and
Gribov-Regge Theory
Extending work by Gribov, Kaidalov, Capella ...
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Reminder (Basic QM)

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Symbols full and dashed line ? elastic and
cut diagram
Very useful for nucleus-nucleus
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The elastic amplitude
soft
hard
semihard (one of three)
Soft parameterization - hard pQCD - semihard
convolution soft/hard
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Inelastic scattering in pp

• Amplitude

• Squared amplitude gt interference terms

gt Symbolic notation
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Inelastic scattering in AB
(Elastic and inelastic elem. Interactions)
Squaring amplitude ? sum over many
interference terms expressed via cut and uncut
elementary diagrams full energy conservation!!
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Classes of interference terms
- Number of cut diagrams for kth NN pair
- Momentum fractions of elementary interactions
We sum all terms in a class gt ?(K). The
inelastic cross section is a sum over classes
Symbol b impact parameter nuclear coordinates
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Interpretation

with
One can show
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? serves clearly as basis to calculate
(topological) cross sections but also particle
production conserving energy in both cases
!! (the only model which does so)
Consistency problem solved !!
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Comparing with conventional approach
Dashed conventional Full new approach

• Pomeron number distribution narrower than in
  conv. appr.
• Considerably less multiplicity fluctuations in pp
• comparison with data not so great

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2 Pomeron-Pomeron Interactions
?

• Shadowing
• Saturation

• Diffraction
• Screening

• Increasing mult. fluctuations
• Solving F2-?tot puzzle

One additional parameter triple Pomeron
coupling. Fixed from HERA diffractive data
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Parton language
Consider a cut Pomeron as a succession of parton
emissions
parton cascade
At high energies, more and more parton
cascades contribute
They overlap and interact
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Energy dependence
With increasing energy, higher and higher orders
have to be considered
We fix a maximal energy (so far LHC) and consider
all contributing orders
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Cutting diagrams
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Some consequences
Elastic scattering
Reduces increase of cross section with
energy (screening)
Cut diagrams
Increases multiplicity fluctuations
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Inclusive spectra
No effect on inclusive spectra relative
weight of diagrams 1 -4 2 ? the
three contributions cancel
The diagrams do not cancel. The
middle one is dominant. ? negative
contribution
? softening of inclusive spectra
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Consider the different contributions to inclusive
particle production in pp scattering at given
rapidity (?)
?
non-factorizable
Contribution zero (complete cancellation)
factorizable
? inclusive cross section is factorizable
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The different contributions to F2 in deep
inelastic scattering (DIS) are as well
factorizable
with the same function f as in pp scattering ?
So does this mean one can hide all these
complicated diagrams in a simple measurable
function f ?
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YES - if one is only interested in inclusive
spectra
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Structure function F2
Little difference !!!! because of
many cancellations
Red complete calculation Blue calculation
without Pomeron-Pomeron interactions
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Total and elastic cross section in pp
Red complete calculation Blue calculation
without Pomeron-Pomeron
interactions
Big difference!!! Important contributions from
nonfactorizable diagrams
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3 NEXUS Hydro

• Nucleus-nucleus collisions particle densities
  are too high for independent string fragmentation

• Use NEXUS for the initial stage (???0)
• Calculate energy density and velocity field at
  ??0
• Apply hydro evolution for ???0 (event by event!)

Efficient hydro code SPHERIO C.E. Aguiar, T.
Kodama U.F. Rio de Janeiro T. Osada,Y. Hama U.
São Paulo
Coupling O. Socolowski, KW Nantes
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Summary
Considerable improvement of the GRT approach by
considering energy conservation properly
Pomeron-Pomeron interactions are crucial but
contribute differently for inclusive spectra
and cross sections (eikonal approach does not
work)
Final stage hydro-evolution