# Presentation: 505510

## Small-x valence quark production in pA collisions

**Javier L. Albacete**
*in collaboration with* **Yuri Kovchegov**

**Hard Probes 06, Asilomar, California, June 2006**

**hep-ph/0605053**

## MOTIVATION

-  Nucleons entering nuclear collisions suffer a large stopping that translates into a
-     net baryon production in the central rapidity region of the collision.

**MOTIVATION**

- 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

- Collision
- Energy

**HP06**

-  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)

## Quasi-classical approximation

-     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: s2A1/3
-     (McLerran-Venugopalan model).
-  Valence quark emission from the proton
-     or the nucleus have been treated  separately

**Quasi-classical approximation**

**HP06**

- +

- -

## The relevant diagrams in the high energy limit are:

**HP06**

## HP06

**HP06**

## Quasi-classical approximation

-  More sensitivity to the ultraviolet: Scales as                 for large momentum.

**Quasi-classical approximation**

_**Final result**_

**HP06**

## Cronin enhancement

- Comparison to pp collisions: **Nuclear modification factor**

**Cronin enhancement**

**HP06**

**Analogously to gluon production, **
**small-x valence quark production **
**presents a Cronin peak at ****k****T     **** Q****s**** **

- kt~Qs

## Quantum evolution

**Quantum evolution**

**HP06**

- At higher rapidities quantum effects become important:
- They have been included in the large-Nc limit of QCD (dipole model)

-  Harder gluons (y’>y) : linear DLA reggeon evolution (Itakura et al)

-  Softer gluons (y’<y) : Non-linear Balitsky-Kovchegov evolution

## Cronin enhancement

**Cronin enhancement**

**HP06**

- 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

## 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.

**SUMMARY**

**HP06**