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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

Forward Physics with BRAHMS at RHIC

Dieter RoehrichUniversity of Bergen & CERN

BRAHMS collaboration

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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 indense matter (central AA collisions)

310-6 10-4 10-2 100

x

108

106

104

102

100

M2 (

GeV

2)

Kinematics

RHIC example• 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

Guzev, Strikman, and Vogelsang (hep-ph/0407201)

y=rapidity of (xL, k) system

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2

21 process

22 process

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Parton Distribution Function

x2 range x1 range

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?

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pp at 200 GeV – forward rapidity

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

PRL 98 (2007) 252001

No agreement with proton data

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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, 114010 (2007)

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pp at 200 GeV – stopping

Net-proton rapidity distribution

• Despite large systematic uncertainties better agreement with the baryon transport in HIJING/B

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pp – stopping and longitudinal scaling

Net-proton rapidity distribution

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• 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.

p/+ in p+p and Au+Au systems at 62.4 GeV

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Initial and final effects – dAu at 200 GeV

• Initial effectsWang, Levai, Kopeliovich, Accardi

• Especially at forward rapidities:

Eskola, Kolhinen, Vogt, Nucl. Phys. A696 (2001) 729-746

HIJING

D.Kharzeev et al., PLB 561 (2003) 93

• OthersB. Kopeliovich et al., hep-ph/0501260

J. Qiu, I, Vitev,hep-ph/0405068

R. Hwa et al., nucl-th/0410111

D.E. Kahana, S. Kahana, nucl-th/0406074

“Cronin effect”Initial state elastic multiple scattering leading to Cronin enhancement (RAA>1)

Gluon saturationdepletion of low-x gluons due to gluon fusion”Color Glass Condensate (CGC)”

broaden pT

Nuclear shadowingdepletion of low-x partons Anti

ShadowingShadowing

r/ggg

Suppression due todominance of projectile valence quarks, energy loss, coherent multiple scattering, energy conservation, parton recombination, ...

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Charged hadrons – RdAu at different pseudorapidities

BRAHMS: PRL 93, 242303 (2004)

• Cronin-like enhancement at =0

• Clear suppression as changes from 0 to 3.2

RdAu d2Nd+Au/dpTd

<Ncoll> d2Nppinel/dpTd

where < < Ncoll> = 7.2> = 7.2±0.3

Nuclear Modification Factor

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RdAu: pions, kaons and protons (y=3)

• RdAu

– Suppression for and K – consistent with charged hadrons

– Less suppression for protons

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RdAu: pions, kaons and protons

central d+Au

peripheral d+Au

mid-rapidity forward rapidity

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Comparison pQCD vs CGC

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

LO pQCD calculation (nuclear shadowing) vs CGC

Global analysis of nPDFs including BRAHMS data

K.J. E

skola, H. P

aukkunena, C.A

. Salgadoc,

hep-ph 08020129v1 (2008)

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Final state effects – A+A collisions

Gallmeister et al., PRC67 (2003) 044905

Fries, Muller, Nonaka, Bass, nucl-th/0301078

Lin, Ko, PRL89 (2002) 202302

R. Hwa et al., nucl-th/0501054

Gyulassy, Wang, Vitev, Baier, Wiedemann…

e.g. nucl-th/0302077

Hadronic absorption of fragments

Energy loss of partons in dense matter

Parton recombination (up to moderate pT)

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Matter at forward rapidity

BRAHMS, Phys. Rev. Lett. 94 (2005) 162301

dn/dy drops by a factor of 3

radial flow drops by 30%

J.I. Jørdre (BRAHMS),PhD thesis (2004)

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RAuAu: identified hadrons – Au+Au at 200 GeV

• Strong pion suppression

• Enhancement for (anti-)protons

• NO change of RAuAu with rapidity

RAA = d2N/dpTd (A+A)

NColld2N/dpTd (p+p)

protons

BR

AH

MS P

relim

inarypions

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pQCD + GLV fit to RAA → L/λ

Opacity at forward rapidity

G. G. Barnafoldi et al. Eur. Phys. J. C49 (2007)333

• 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

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Surface emission

• 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 ?

Dainese, Loizides, Paic,Eur. Phys. J. C38 (2005) 461

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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

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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, France3Jagiellonian University, Cracow, Poland,

4Johns Hopkins University, Baltimore, USA, 5New York University, USA6Niels 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

The BRAHMS Collaboration