# Presentation: 774257

## Azimuthal correlations of forward dijets in d+Au collisions

- Cyrille Marquet
- Columbia University &
- RIKEN BNL Research Center

## Outline and Motivation

- 	the spectrum	          	   and

- - after the first d-Au run at RHIC, there was a lot of new results on
-   single inclusive particle production at forward rapidities

- the suppressed production (*R**dA* < 1) was predicted in the
- Color Glass Condensate picture of the high-energy nucleus

- d Au → h X

*y* increases

- 	the modification factor				  were studied

## Measurements at RHIC with d-Au

- trustable theory difficult when the two particles
- are separated by a large interval in rapidity

- Kharzeev, Levin and McLerran (2005)

- qualitative features : OK, quantitative results : NO

- a first attempt:

- it is a simpler problem when the two particles have similar rapidities

## Gluon saturation and the CGC

*x *: parton longitudinal momentum fraction

*k**T** *: parton transverse momentum

- weakly-coupled regime

- the distribution of partons
- as a function of *x* and *k**T* :

- dilute/dense separation characterized by the saturation scale Qs(*x*)

- QCD linear evolutions:

- DGLAP evolution to larger *k**T* (and a dilute hadron) BFKL evolution to smaller *x* (and a dense hadron)

## The CGC wavefunction

- a functional equation for the
- small-*x* evolution of

- Jalilian-Marian, Iancu, McLerran, Weigert, Leonidov, Kovner

- - is used to compute physical observables:

- the *x* evolution of the cross-sections
- is encoded in the x evolution of

- functions of Wilson lines, describing
- the scattering of partons off the CGC

- - obeys the JIMWLK equation:

- we will use this formalism to
- compute forward particle production

## Single inclusive particle production

- final state: transverse momentum *k* and rapidity *y*

- involves

- the partons probed in the process have

- high energies and forward rapidities
- are needed to probe small values of  *x*

- and

## RdA and forward pion spectrum

*R**dA* and forward pion spectrum

- first comparisons to data:

- Kharzeev, Kovchegov and Tuchin (2004)
- Kharzeev, Levin and Nardi (2005)

- Dumitru, Hayashigaki and
- Jalilian-Marian (2006)

- more recent work:
- from qualitative to quantitative agreement

- shows the importance of both evolutions:
*x*A (CGC) and  *x*d (DGLAP)

- shows the dominance of the
- valence quarks of the deuteron

*R**dA*

*p**T *- spectrum

## Two particles at forward rapidities

- moderate values of *x**d*, typically 0.5
- dominant partonic process :

- feasible in d-Au collisions at RHIC

*|k*1|, |*k*2| >> QCD   collinear
- factorization of the quark density

- d+Au  h1+h2+X

-  *y*1 ~ *y*2 ~ 3 : both h1 and h2
- in forward hemisphere

- final state

*A*

## 2-  4-  and 6-point functions

- the scattering off the CGC is expressed through the following correlators of Wilson lines:

- Blaizot, Gélis and Venugopalan (2004)

- need more than the 2-point function: no kT factorization

-   same conclusions 	in heavy-quark production
- 			and two-gluon production

- Jalilian-Marian and Kovchegov (2004)

## MV model +BK evolution 

- - then I evolve the correlators to the value of
- needed using BK evolution

- - finally I can compute the cross-section

## RHIC vs LHC

-  and	           in the calculation will be needed

- the CGC picture would be even
- better tested:

- value of *x**A* at LHC << RHIC

- the value of *x**d** *at the LHC is slightly smaller than at RHIC

- LHC

- RHIC

- but at RHIC, one has *x*h ~ 0.5, and
- the dominant partonic process is

## Conclusions and Outlook

-  I obtained CGC predictions (MV model + BK evolution) for the two-particle spectrum, when both particles are produced at forward rapidities
-  need to include hadron fragmentation for final results
-     need comparison with standard NLOQCD calculations
- - the predictions for the fully differential spectrum are not directly comparable with the data (unless fully differential spectra can be measured)
-  need integrations over the experimental y and *p**T* ranges

- future measurements:

- - at the LHC with p+Pb collisions ?

- - at RHIC with the new d+Au run data