1. DIFFRACTION 2010, Sep. 10-15, Otranto, Italy
Forward Di-Hadron Correlations in d+Au Collisions in PHENIX Searching for Gluon Saturation with the MPC
Oleg Eyser
Beau Meredith, Mickey Chiu, Matthias Perdekamp
for the PHENIX Collaboration

2. The gluon density at low x
At small x we see an increase in gluons.
There must be a kinematic region in which the gluon density reaches a saturation!
Bremsstrahlung
~ asln(1/x)
Recombination
~ asr

3. The color glass condensate
log x-1
log Q2
DGLAP
non-perturbative
log LQCD2
Qs2(x)
Saturation
Scale
BFKL
Parton
Gas
Color Glass
Condensate
Mid Rapidity
Forward Rapidity
pT is balanced by gluons d Au
Hadronic probe
Couple to gluons
High A
Higher parton densities
Asymmetric
Probe low x 3

4. Signatures of the CGC
p0 meson
|h|<0.35
PHENIX
CGC expectations
Kharzeev, Kovchegov,Tuchin Phys.Rev.D68:094013,2003
BRAHMS STAR
increased h
Gluon saturation is not the only explanation for the reduced yields. There may be other effects.
PRL 97, (2006)
PRL 98, (2007)

5. Shadowing in jet correlations
pQCD 2→2 process = back-to-back di-jet
With shadowing in nucleus
forward jet
d+Au in HIJING (A. Ogawa) d p Au p
mid-rapidity jet
Vitev, hep-ph/ v2 5

6. Mono-jets from the CGC
In the saturation regime the jet pT is balanced by many gluons.
Kharzeev, Levin, McLerran (NPA748, 627) d Au
dense gluon field
The gluon field is increased in central vs. peripheral collisions. 6

7. The Nuclear Modification Factor
Correlation function
Two-particle distribution
Including two-particle acceptance
Conditional yield
Number of particle pairs per trigger particle
Including acceptance & efficiency
Nuclear modification factor
Conditional yield ratio d+A/p+p
Indicators of gluon saturation
IdA < 1
Angular de-correlation of widths

8. Gluon Saturation @ RHIC
PHENIX
STAR
BRAHMS
pp2pp
d+Au vs p+p
high-x valence quarks
0.25 < xq < 0.7
on low-x gluons
0.001 < xg < 0.1
Signatures of saturation
suppressions of nuclear modification factors
angular de-correlations of di-jet signals
saturation is expected at x≈10-4

9. Azimuthal Correlations in PHENIX
Trigger particle
Associate particle
Correlation
Near side peak
Away side peak
Central Arms | η | < 0.35
Muon Arms 1.2 < | η | < 2.4 Dj
Muon Piston Calorimeter
3.1 < | η | < 3.9

10. Azimuthal Correlations (prev.)
2003 data
Trigger hadron from the muon arms
Correlated hadron from the central arm
Phys.Rev.Lett.96:222301,2006

11. The Muon Piston Calorimeters
Electromagnetic Calorimeters at rapidity 3.1 < |h| < 3.9
2.2 x 2.2 x 18 cm3 PbWO4 crystals
220 cm from nominal interaction point
p0 merging effects
pT max ~ 2 GeV/c at outside ring
For higher pT
Reconstructed p0s for 7 GeV < E < 22 GeV
Clusters for E > 15 GeV (~ 85% p0)
gg invariant mass distribution
11 GeV < E < 15 GeV
Signal pairs
Mixed event pairs
p0 yield

12. Forward Di-jet Signal with Dh = 3.4
PHENIX
Central region
Side View
p0 or clusters
p0 or h+/- d Au
xgluon ~ 10-2
Aueffective xgluon ~ 10-3
peripheral to central

13. Forward correlations at h = 3.4
PHENIX
Central region
Side View
p0 or clusters
Work in progress
Area of away-side peak represents conditional yield
No efficiency correction
No uncorrelated background yet
Conditional yields, widths not shown d Au
xgluon ~ 10-3
Aueffective xgluon ~ 10-4

14. Forward correlations (minBias)
Trigger particle pT > 2 GeV/c
Associate particle pT > GeV/c

15. Forward correlations peripheral central
Peripheral d+Au seems more similar to p+p
Central d+Au shows a reduced yield, maybe broadening

16. pT,trigger
pT,associate

17. pT,trigger
pT,associate

18. Summary Central/forward di-hadron correlations
No significant broadening between d+Au vs pp
Reduced IdA in more central collisions
Model calculations are needed to understand this effect
Forward/forward di-hadron correlations
Qualitative indications of onset of gluon saturation hypothesis in reduced yields
Data also suggests width broadening
More work needed on uncorrelated background!