Smallx valence quark production in pA collisions

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Small-x valence quark production in pA collisions
Javier L. Albacete in collaboration with Yuri
Kovchegov
hep-ph/0605053
Hard Probes 06, Asilomar, California, June 2006
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MOTIVATION
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• Nucleons entering nuclear collisions suffer a
  large stopping that translates into a
• net baryon production in the central rapidity
  region of the collision.

Collision Energy
BRAHMS PRL93_102301

• So far this phenomenon has been mainly
  approached in non-perturbative models
• for particle production (string junction
  models, gluonic mechanism).
• A first principle calculation in the
  CGC-saturation framework is presented

• Valence quarks are carriers of baryon number
• Quasi-classical approximation (Cronin
  enhancement)
• Quantum evolution (Suppression of Cronin
  enhancement)

.
Itakura, Kovchegov, McLerran, Teany NPA 730 160
(2004)
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Quasi-classical approximation
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• The calculation is done in the light-cone gauge
  of the proton, A0
• using light-cone perturbation theory
  (time-ordered diagrams).
• Nuclear effects become manifest in the form of
  multiple rescatterings.
• Eikonal approximation Recoil of energetic
  quarks and gluons in their
• interaction with the nucleus is neglected.
  Coordinate Space.
• Quasi-classical approximation no more than 2
  exchanged gluons per
• nucleon ressummation parameter as2A1/3
• (McLerran-Venugopalan model).
• Valence quark emission from the proton
• or the nucleus have been treated separately

-

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The relevant diagrams in the high energy limit
are
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Quasi-classical approximation
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Final result

• Suppressed by powers of the cme of the
  collision, s
• More sensitivity to the ultraviolet Scales as
  for large momentum.

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Cronin enhancement
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Comparison to pp collisions Nuclear modification
factor
Analogously to gluon production, small-x valence
quark production presents a Cronin peak at kT
Qs
ktQs
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Quantum evolution
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At higher rapidities quantum effects become
important They have been included in the
large-Nc limit of QCD (dipole model)

• Harder gluons (ygty) linear DLA reggeon
  evolution (Itakura et al)

• Softer gluons (ylty) Non-linear
  Balitsky-Kovchegov evolution

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Cronin enhancement
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Quantum evolution erases the Cronin enhancement
present at central rapidity when moving towards
the proton fragmentation function
The suppression is driven by the non-linear
evolution. Linear reggeon evolution cancels out
in the nuclear modification factor The rate of
suppression is similar to the one found for
inclusive gluon production JLA, Armesto, Kovner,
Salgado, Wiedemann PRL 92, 082001
(2004) Experimental measurement could provide an
extra insight in the small-x CGC dynamics
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SUMMARY

• The inclusive small-x valence quark cross
  section in pA
• collisions at high energies has been
  presented.
• At a quasi-classical level it shows Cronin
  enhancement at central
• rapidity.
• Inclusion of quantum effects via non-linear
  evolution equations
• has been included.
• Quantum evolution washes out the Cronin
  enhancement at
• central rapidity, turning into a relative
  suppression at more
• forward rapidities
• Comparison with data Implementation of
  hadronization
• mechanism.
• Generalization to AA collisions.