1. Forward Physics with BRAHMS at RHIC University of Bergen & CERN
Dieter Roehrich
University of Bergen & CERN
BRAHMS collaboration
Physics at forward rapidities
How well does pQCD work
p+p at 200 and 62 GeV
Probing the initial condition
d+Au collisions at 200 GeV
Final state effects
Au+Au collisions at 200 GeV

2. Introduction
Forward rapidity at RHIC collider offers insight into pp, dA and AA:
low-x region (for targets like p, A)
probing larger xF region where kinematic constraints may be important
opportunities to study if pQCD works at RHIC energies at large rapidities
energy loss of partons in dense matter (central AA collisions)

3. Kinematics RHIC example 21 process 108 106 104 102 M2 (GeV2) 100
At 4° (y~3 for pions) and pT=1 GeV/c one can reach values as low of x2 ~ 10-4
This is a lower limit, not a typical value: most of the data collected at 4° would have x2~0.01
y=rapidity of (xL, k) system x
108
106
104
102
100
M2 (GeV2) 2
22 process
Guzev, Strikman, and Vogelsang (hep-ph/ )

4. Parton Distribution Function
Measurements at high rapidity set the dominant parton type:
projectile (x1 ~1) mostly valence quarks
target (x2<0.01) mainly gluons
How well does NLO pQCD work at RHIC and at large rapidities?
Are there effects from small-x at large y?
x2 range
x1 range

5. pp at 200 GeV – forward rapidity
PRL 98 (2007)
Calculations done by W. Vogelsang. Only one scale =pT and the same fragmentation functions as used for the PHENIX comparison.
KKP has only 0 fragmentation. Modifications were needed to calculate charged pions. KKP FF does a better job compared to Kretzer,  and kaon production still dominated by gg and gq at these rapidities apart from the highest pT
No agreement with proton data

6. pp at 62 GeV – forward rapidity
 spectra at forward rapidities
Comparison of NLO pQCD calculations (Vogelsang) with BRAHMS data at high rapidity. The calculations are for KKP and a scale factor of =pt (DSS with CTEQ5 and CTEQ6.5 are also shown).
The agreement is surprisingly good in view of analysis of slightly lower ISR data at large y which failed to describe 0 at larger xF.
DeFlorian, Sassot and Stratman, PRD 75, (2007)

7. pp at 200 GeV – stopping Net-proton rapidity distribution
Despite large systematic uncertainties better agreement with the baryon transport in HIJING/B

8. pp – stopping and longitudinal scaling
Net-proton rapidity distribution

9. p/+ in p+p and Au+Au systems at 62.4 GeV
The overall difference between the ratio measured in pp and AuAu is consistent with the favored interpretation based on coalescence in Au+Au
Large value of ratio in p+p collisions may be driven by proximity to beam rapidity.
The similarity of the ratio in both systems may be driven by incoherent proton fragmentation.

10. Initial and final effects – dAu at 200 GeV
Initial effects
Wang, Levai, Kopeliovich, Accardi
Especially at forward rapidities:
Eskola, Kolhinen, Vogt, Nucl. Phys. A696 (2001)
HIJING
D.Kharzeev et al., PLB 561 (2003) 93
Others
B. Kopeliovich et al., hep-ph/
J. Qiu, I, Vitev, hep-ph/
R. Hwa et al., nucl-th/
D.E. Kahana, S. Kahana, nucl-th/
“Cronin effect”
Initial state elastic multiple scattering
leading to Cronin enhancement (RAA>1)
broaden pT
Anti
Shadowing
Nuclear shadowing
depletion of low-x partons
r/
ggg
Gluon saturation
depletion of low-x gluons
due to gluon fusion
”Color Glass Condensate (CGC)”
Suppression due to
dominance of projectile valence quarks, energy loss, coherent multiple scattering, energy conservation,
parton recombination, ...

11. Charged hadrons – RdAu at different pseudorapidities
BRAHMS: PRL 93, (2004)
RdAu =
d2Nd+Au/dpTdh
<Ncoll> d2Nppinel/dpTdh
where < Ncoll> = 7.2±0.3
Nuclear Modification Factor
Cronin-like enhancement at =0
Clear suppression as  changes from 0 to 3.2

12. RdAu: pions, kaons and protons (y=3)
Suppression for  and K – consistent with charged hadrons
Less suppression for protons

13. RdAu: pions, kaons and protons
mid-rapidity forward rapidity
central d+Au
peripheral d+Au

14. Comparison pQCD vs CGC LO pQCD calculation (nuclear shadowing) vs CGC
Global analysis of nPDFs including BRAHMS data
K.J. Eskola, H. Paukkunena, C.A. Salgadoc,
hep-ph v1 (2008)
d’Enterria
Conclusions (dAu)
The suppression and in particular the inversion vs. centrality of RdAu at high rapidity may be a signature for the gluon saturation and the small-x evolution.
Stronger gluon shadowing -> EPS08
Alternate explanations e.g. suppression due to large xF effects work quite well too

15. Final state effects – A+A collisions
Gallmeister et al., PRC67 (2003)
Fries, Muller, Nonaka, Bass, nucl-th/ Lin, Ko, PRL89 (2002)
R. Hwa et al., nucl-th/
Gyulassy, Wang, Vitev, Baier, Wiedemann…
e.g. nucl-th/
Hadronic absorption of fragments
Parton recombination
(up to moderate pT)
Energy loss of partons
in dense matter

16. Matter at forward rapidity
radial flow drops
by 30%
dn/dy drops by
a factor of 3
BRAHMS, Phys. Rev. Lett.
94 (2005)
J.I. Jørdre (BRAHMS),
PhD thesis (2004)

17. RAuAu: identified hadrons – Au+Au at 200 GeV
protons
BRAHMS Preliminary
pions
Strong pion suppression
Enhancement for (anti-)protons
NO change of RAuAu with rapidity
RAA =
d2N/dpTd (A+A)
NColld2N/dpTd (p+p)

18. Opacity at forward rapidity
G. G. Barnafoldi et al.
Eur. Phys. J. C49 (2007)333
pQCD + GLV fit to RAA → L/λ
Co-moving dynamics of jet and longitudinally expanding surface of the compressed matter
Initial geometry: longitudinally contracted dense deconfined zone
Less jet quenching but stronger initial state effects at forward rapidities

19. Surface emission
Dainese, Loizides, Paic,
Eur. Phys. J. C38 (2005) 461
Medium at RHIC is so dense that only particles produced close to the surface can escape
Can corona effect mask the lower parton density at =3.2 ?

20. Conclusions (AuAu) Nuclear modification
Strong pion suppression at all rapidities
Protons are enhanced at all rapidities (RAuAu) and moderate pT
No dependence of RAuAu on rapidity

21. The BRAHMS Collaboration
I.Arsene7, I.G. Bearden6, D. Beavis1, S. Bekele6 , C. Besliu9, B. Budick5,
H. Bøggild6 , C. Chasman1, C. H. Christensen6, P. Christiansen6, R. Clarke9, R.Debbe1,
J. J. Gaardhøje6, K. Hagel7, H. Ito10, A. Jipa9, J. I. Jordre9, F. Jundt2, E.B. Johnson10,
C.E.Jørgensen6, R. Karabowicz3, E. J. Kim4, T.M.Larsen11, J. H. Lee1, Y. K. Lee4,
S.Lindal11, G. Løvhøjden2, Z. Majka3, M. Murray10, J. Natowitz7, B.S.Nielsen6,
D. Ouerdane6, R.Planeta3, F. Rami2, C. Ristea6, O. Ristea9, D. Röhrich8,
B. H. Samset11, D. Sandberg6, S. J. Sanders10, R.A.Sheetz1, P. Staszel3,
T.S. Tveter11, F.Videbæk1, R. Wada7, H. Yang6, Z. Yin8, and I. S. Zgura9
1Brookhaven National Laboratory, USA, 2IReS and Université Louis Pasteur, Strasbourg, France
3Jagiellonian University, Cracow, Poland,
4Johns Hopkins University, Baltimore, USA, 5New York University, USA
6Niels Bohr Institute, University of Copenhagen, Denmark
7Texas A&M University, College Station. USA, 8University of Bergen, Norway
9University of Bucharest, Romania, 10University of Kansas, Lawrence,USA
11 University of Oslo Norway