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

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)

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 10-6 10-4 10-2 100 x 108 106 104 102 100 M2 (GeV2) 2 22 process Guzev, Strikman, and Vogelsang (hep-ph/0407201)

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

pp at 200 GeV – forward rapidity PRL 98 (2007) 252001 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

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)

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

pp – stopping and longitudinal scaling Net-proton rapidity distribution

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.

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) 729-746 HIJING D.Kharzeev et al., PLB 561 (2003) 93 Others B. 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) 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, ...

Charged hadrons – RdAu at different pseudorapidities BRAHMS: PRL 93, 242303 (2004) RdAu = 1 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

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

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

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 08020129v1 (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

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 Parton recombination (up to moderate pT) Energy loss of partons in dense matter

Matter at forward rapidity radial flow drops by 30% dn/dy drops by a factor of 3 BRAHMS, Phys. Rev. Lett. 94 (2005) 162301 J.I. Jørdre (BRAHMS), PhD thesis (2004)

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)

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

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 ?

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

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