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Fouad RAMI Institut Pluridisciplinaire Hubert Curien, Strasbourg

Forward Rapidity Physics with the BRAHMS Experiment. Introduction The BRAHMS Experiment Overview of Main Results  Bulk observables  High p t observables Summary & Outlook. Fouad RAMI Institut Pluridisciplinaire Hubert Curien, Strasbourg.

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Fouad RAMI Institut Pluridisciplinaire Hubert Curien, Strasbourg

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  1. Forward Rapidity Physics with the BRAHMS Experiment • Introduction • The BRAHMS Experiment • Overview of Main Results Bulk observables High pt observables • Summary & Outlook Fouad RAMI Institut Pluridisciplinaire Hubert Curien, Strasbourg F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  2. Space-time evolution of a HI collision at RHIC energies Parton scatterings take place during first stages Emission of hadrons Initial State (v~c) Dense Medium RHIC Results consistent with the existence of a dense partonic state of matter characterized by strong collective interactions: sQGP Hints on high density gluon saturation → describe the initial state of the collision within the framework of the Color Glass Condensate: CGC F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  3. A possible scenario for Au+Au collisions at RHIC • Initial conditions of the collision provided by the CGC • The CGC matter will evolve and may eventually form a QGP (if the system thermalizes) Ludlam and McLerran, Physics Today, 2003 • Combines QGP and CGC → Colliding nuclei in the Initial State considered as CGC matter • The CGC matter is not only important for the formation of the QGP • But the study of CGC matter itself is of fundamental interest Initial State → Understanding of basic properties of strong interactions Gluons inside one nucleus appear to the other nucleus as a wall made mostly of gluons travelling at high velocities (v~c) F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  4. Independent of the hadrons which generated it  Can be explored in protons and in heavy nuclei using  probes :  electrons to probe the structure of protons (HERA) or nuclei (e-RHIC)  protons (or deuterons) to probe nuclei (RHIC, LHC) Advantage of nuclei  Saturation can be reached at lower energies (larger x) due to the effect of their thickness CGC: Universal form of matter • Saturation physics and the CGC → Object of intensive theoretical studies (Next Talk) F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  5. Low energy Gluon Density Large x Gluon density increases High energy Small x x Saturation at high density QS : Saturation momentum QS larger in A than in p Nuclei → Qs2 A1/3 High Density Gluon Saturation Gluon distribution function of the proton • e-p scattering at HERA x=fraction of E transfered to the gluon • At small-x,the gluon density increases very strongly • → driving force toward saturation • The gluon density cannot • grow indefinitely (unitarity) Saturation can be probed at larger x-values in nuclei → RHIC, LHC F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  6. Relativistic Heavy Ion Collider@BNL • 2000-2006: 6 runs • Several systems/energies Au+Au @ 200 GeV @ 130 GeV @ 63 GeV Cu+Cu @ 200 GeV @ 63 GeV d+Au @ 200 GeV (control experiment) • p+p @ 200 GeV • (reference data) BRAHMS PHOBOS PHENIX STAR • Rapidity coverage • Main focus → MR (y=0) (most interesting region for QGP) • But also some data at forward rapidities → very promising … • Results obtained from all 4 experiments F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  7. Front Forward Spectrometer Back Forward Spectrometer Global Detectors 0<<1 (MRS) Two Rotatable spectrometers → Broad rapidity coverage FS → well suited for Forward Physics (up to η~4) → Centrality (Event Multiplicity) F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  8.    Global Detectors & Collision Centrality Au+Au @ SNN=130GeV • Measured with Multiplicity Detectors (TMA and SiMA) Central b=0 Peripheral b large Central Peripheral • Define Event Centrality Classes  Slices corresponding to different fractions of the cross section • For each Centrality Cut  Evaluate the corresponding number of participants Npart (in nuclear overlap) and number of inelastic NN collisions NCOLL (from Glauber Model) F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  9. dNch/d- Comparison to Model Predictions Au+Au@SNN=200GeV BRAHMS, PRL88(2002)202301 AMPT Zhang et al, PRC61(2001)067901 Lin et al, PRC64(2001)011902 High density QCD gluon saturation KLN model Kharzeev, Nardi & Levin, PLB523(2001)79 dNch/d • Similar predictions • Both calculations reproduce • dNch/d (shape and absolute) • Differences for • peripheral Collisions • but Small effect! • Cannot discriminate • these models • Centrality dependence is well described  F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  10. dNch/dat Mid-Rapidity – Centrality Dependence  Saturation models reproduce also the energy dependence F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  11. dNch/d- d+Au @SNN=200GeV BRAHMS data PHOBOS data KLN Model:Kharzeev, Levin and Nardi, Nucl.Phys.A730(2004)448 • Good agreement except in the region of the Au fragmentation • where KLN model (dotted line) fails • CGC is not valid in this region (large-x)! • dN/dη = Npart dNpp/dη (solid line) → agreement in the Au • fragmentation region F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  12. Particle Production at RHIC vs. Saturation Models  Good description of particle production at RHIC Several features observed in the data are nicely reproduced - Rapidity dependence - Centrality dependence - Energy dependence - System dependence - Limiting Fragmentation phenomenon F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  13. BRAHMS PRL 88 (2002) PHOBOS PRL 91 (2003) • Also observed in pp, pp, p-emulsion, π-emulsion, A-A at SPS (Alner et al, Z.Phys.C33(1986)1, Deines-Jones et al, PRC(2000)4903) Limiting Fragmentation dNch/dh ¢/<Npart>/2 Au+Au 6% central • When shifted by ybeam (’    ybeam) → No Energy Dependence • Limiting behavior (LF) in the forward rapidity region (’ ~ 0) _ • Similar effect observed for v2 (PHOBOS) • Can be explained within the CGC (Jalilian-Marian, nucl-th/0212018) F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  14. F.Gelis, A.M.Stasto and R. Venugopalan Eur. Phys. J. C48 (2006) 489 Limiting Fragmentation in the CGC approach Au+Au ▲19.6 GeV (PHOBOS) ■ 130 GeV (PHOBOS) ● 200 GeV (PHOBOS) □ 130 GeV (BRAHMS) ○ 200 GeV (BRAHMS) • Reasonable agreement • (Fragmentation region) • Good agreement also for pp data (UA5) _ F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  15.  d+Au  Forward Rapidities Particle Production at RHIC vs. Saturation Models  Good description of particle production at RHIC Several features observed in the data are nicely reproduced - Rapidity dependence - Centrality dependence - Energy dependence - System dependence - Fragmentation phenomenon  Saturation effects seem to play an important role in particle production and dynamics at the early stages of A-A collisions at RHIC energies But other models can also reproduce most of the data! → Need for more “direct” evidence (experimental signatures)!  CGC theorists suggested to investigate the high pt region of hadron spectra If saturation effects are present at RHIC energies → should be seen as a suppression at high pt (relative to N-N reference) Most appropriate conditions F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  16. Forward measurements in d+Au collisions • No final state effects in d+Au If suppression → Only due to the Initial State MRS • Forward measurements → Access to small x in the gluon distribution of the Au nucleus BRAHMS measures in this side (d-fragmentation region) d Au FS xAu = mt/S e-y • Qs2 A1/3(Thickness effect) From y=0 to y=4  x values lower by ~10-2  Saturation momentum in Au larger than in p (saturation can be probed at larger x) F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  17. Parton Distributions Functions In the d-fragmentation region xAu = mt/S e-y xd = mt/S e+y xAu range xd range Mainly gluons Mostly valence quarks (Saturated wave function?) F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  18. High pt suppression in d+Au collisions at forward rapiditiesProbing the CGC matter at RHIC F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  19. Yield(AA) RAA = Yield(Cent)/NCOLL(Cent) RCP = Yield(Periph)/NCOLL(Periph) NCOLL(AA)  Yield(pp) Nuclear Modification Factor BRAHMS, PRL91(2003)072305 Nuclear Modification Factor Scaled N+N reference Central/Peripheral R<1  Suppression relative to scaled NN reference F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  20. d+Au shows very different behavior as compared to Au+Au • Au+Au → suppression • d+Au → Enhancement (Cronin effects) • Observed in all 4 experiments BRAHMS =0 Absence of suppression in d+Au data at MR Not necessarily inconsistent with CGC • No sensitivity to low-x at MR • Important to go forward (smaller x) Decisive test (control experiment) → Interpretation of Au+Au in terms of Energy Loss in dense partonic matter (Jet Quenching) Data: Nuclear Modification Factor at MR F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  21. BRAHMS Data: Going to Forward Rapidities (RdAu) x ~ 10-2 x ~ 510-4 For pt=2 GeV/c • Occurrence of suppression (relative to p+p collisions) at large rapidities • Gradual transition from Cronin enhancement to suppression  Consistent with the expected behavior for saturation effects BRAHMS, PRL 93 (2004) 242303 MRS MRS FS FS θ=90° θ=40° θ=12° θ=4° MB collisions F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  22. BRAHMS Data: Going to Forward Rapidities (RCP) • Same behavior as for RdAu • Onset of suppression: 1<η<2 • Centrality dependence: different behavior from η=0 → large η’s BRAHMS, PRL 93 (2004) 242303  Suppression mechanism depends on centrality → Larger effect in Central Collisions Consistent with saturation F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  23. Comparison to CGC calculations (RCP) ○ 0-20%/60-80% ● 30-50%/60-80% Kharzeev, Kovchegov and Tuchin, Phys. Lett. B599 (2004) 23 Good agreement with data → Transition from Cronin to suppression → Centrality dependence • Good agreement also for RdAu F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  24. STAR Results STAR, nucl-ex/0602011 → Clear suppression at large η → Good agreement with BRAHMS for charged hadrons Calculations that do not include saturation effects cannot reproduce data F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  25. Back-to-back Correlations in d+Au STAR, nucl-ex/0602011 Azimuthal correlation between forward π0 mesons (η=4) and Leading Charged Particles (LCP) detected at MR with pt>0.5GeV/c Suppression of the back- to-back peak in d+Au Qualitatively consistent with the CGC picture Kharzeev, Levin, and McLerran, Nucl. Phys. A748 (2005) 627 Additional argument in favor of saturation at RHIC Importance of correlation measurements and the need for quantitative understanding F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  26. Summary & Outlook RHIC results suggest the formation of CGC matter in the initial state of the collision • Saturation models provide a good description of particle production  dNch/dη, Energy and Centrality dependences well reproduced for both Au+Au and d+Au collisions  Limiting Fragmentation also described • Saturation effects provide an explanation to the high pt suppression observed in d+Au at forward y’s Supported by quantitative CGC model calculations  Transition from Cronin enhancement to suppression and Centrality Dependence • Confirmation of CGC requires further experimental tests Open charm, dileptons, photons Azimuthal correlations in the forward direction … • Main challenges in the future • Upgrades of RHIC experiments (including forward detectors) • LHC  much higher energies (smaller x) F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  27. BRAHMS Collaboration I. C. Arsene12, I. G. Bearden7, D. Beavis1, S. Bekele12, C. Besliu10, B. Budick6, H. Bøggild7, C. Chasman1, C. H. Christensen7, P. Christiansen7, H.Dahlsgaard7, R. Debbe1, J. J. Gaardhøje7, K. Hagel8, H. Ito1, A. Jipa10, E.B.Johnson11, J. I. Jørdre9, C. E. Jørgensen7, R. Karabowicz5, N. Katrynska5 ,E. J. Kim11, T. M. Larsen7, J. H. Lee1, Y. K. Lee4,S. Lindahl12, G. Løvhøiden12, Z. Majka5, M. J. Murray11,J. Natowitz8, C.Nygaard7 B. S. Nielsen8, D. Ouerdane8, D.Pal12, F. Rami3, C. Ristea8, O. Ristea11, D. Röhrich9, B. H. Samset12, S. J. Sanders11, R. A. Scheetz1, P. Staszel5, T. S. Tveter12, F. Videbæk1, R. Wada8, H. Yang9, Z. Yin9, I. S. Zgura2 1. Brookhaven National Laboratory, Upton, New York, USA 2. Institute of Space Science, Bucharest - Magurele, Romania 3. Institut Pluridisciplinaire Hubert Curien et Université Louis Pasteur, Strasbourg, France 4. Johns Hopkins University, Baltimore, USA 5. M. Smoluchkowski Institute of Physics, Jagiellonian University, Krakow, Poland 6. New York University, New York, USA 7. Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 8. Texas A&M University, College Station, Texas, USA 9. University of Bergen, Department of Physics and Technology, Bergen, Norway 10. University of Bucharest, Romania 11. University of Kansas, Lawrence, Kansas, USA 12. University of Oslo, Department of Physics, Oslo, Norway F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  28. Glass : In the gluon wall, gluons do not change their position rapidly because of Lorentz time dilatation  Will evolve on long time scale relative to their natural time scale  Similar property as in glasses • Condensate : High density  Coherent multi-gluon system (gluon condensate) If the phase space is filled with gluons  gluons from different nucleons will start to overlap (saturation effect) • Saturation is characterized by a saturation • scale below which recombination occurs • QS  Density of gluons in the transverse plane • Increases with s (1/x) and A Color Glass Condensate: Why? • Color : Composed of colored particles

  29. Evaluation of Npart and NCOLL Npart: Nucleons that interact inelastically in the overlap region between the two interacting nuclei NCOLL : Number of binary nucleon-nucleon collisions (one nucleon can interact successively with several nucleons if they are in its path) • Use Glauber Model Nucl.Phys.B21(1970)135 Main assumption : Independent collisions of part. nucleons Nucleons suffer several collisions along their incident trajectory (straight-line) without deflection and without energy loss • Nucleons inside nuclei distributed according to a Woods-Saxon density profile • Interaction probability between 2 nucleons is given by the pp cross section • Calculate the overlap integral at a given impact parameter

  30. HIJING: Heavy Ion Jet Interaction Event Generator Wang and Gyulassy, PRD44(91)3501 Parton cascade calculations where partons are treated as free particles and their evolution is studied taking into account QCD interactions and assuming that the initial distributions in phase space are given by the structure function of the nuclei.  provide detailed description at the partonic level of the early stages of nucleus-nucleus collisions  Two Component Model • Hard processes leading to minijet production are calculated using pQCD (PYTHIA) • pt p0=2GeV/c • Soft processes are calculated using the Lund String Model  Hadronization in Strings • Shadowing • Modification of parton structure • functions in the medium • Jet quenching • Energy loss of partons traversing • dense matter   Includes Nuclear effects   dNch/d = (1-x)Npart  xNcoll x=fraction of hard processes

  31. AMPT model Lin et al, PRC64(2001)011902 Zhang et al, PRC61(2001)067901 Hybrid model: • It uses HIJING to generate the initial phase space of partons. - It takes into account hadronic interactions in the final state (hadron rescattering) using a Relativistic Transport Model (ART).

  32. Wang & Gyulassy, PRL86(2001)3496 1 3  2 BRAHMS   |  | |  | 1 HIJING – Jet quenching 2 HIJING – No Jet quenching 3 EKRT (Gluon Saturation) dNch/d at Mid-Rapidity - Energy Dependence • Good agreement between all 4 RHIC experiments • Au+Au data much larger than pp  Not a simple superposition of pp  Evidence for collective behavior • Both models HIJING and EKRT reproduce the measured multiplicities =0 Small difference in the predictions of these models at RHIC energies F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  33. Forwardmeasurements in d+Au collisions Sensitivity to smaller-x values MRS d Au FS • BRAHMS spectrometers measure in the d-fragmentation region • To reach small x in the gluon distribution of the Au nucleus xAu = mt/S e-y  Go very forward  Larger saturation scaleQS : Qs2(x) = Q02 (x0/x)λ • Qs2 A1/3(Thickeness effect)  Saturation scale in Au larger than in p (saturation can be probed at lower x) • No final state effects in d+Au From y=0 to y=4  x values lower by ~10-2  One could hope to see the occurrence of a suppression effect D.Kharzeev et al, hep-ph/0307037

  34. What do we expect? D. Kharzeev et al, hep-ph/0307037 CGC at y=0 As y grows Very high energy RpA : Nuclear Modification Factor • At RHIC energies Cronin effects predominant at mid-rapidity • At more forward y’s •  Transition from • Cronin enhancement • to a suppression • effect • This is what one would expect if there is an effect of gluon density saturation in the initial state

  35. Jet Quenching effect (Final State effect) • Parton energy loss in the traversed dense medium  suppression in jet production (high pt hadrons) Origin of high–pt suppression • Saturation of gluon densities in the colliding nuclei (Initial State effect) • Jets do not lose energy but they are produced in a smaller number (due to saturation effects) • High pt Suppression clearly observed in central Au+Au collisions by all 4 RHIC experiments (Run1&2)

  36. PHOBOS Results PRC 70 (2004) 061901(R)

  37. PHENIX Results PRL 94 (2005) 082302

  38. Comparison to CGC calculations (RdAu) Kharzeev, Kovchegov and Tuchin, Phys. Lett. B599 (2004) 23 CGC calculations (different assumptions) F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  39. CGC calculations: Predictions for LHC LHC, =0 RHIC, =3.2 Predictions for LHC p-A collisions • Stronger suppression • at LHC (smaller x)

  40. Au+Au @ SNN=200 GeV BRAHMS, PRL91(2003)072305 Central Peripheral Central/Peripheral High pt Suppression in Au+Au • No clear Rapidity Dependence • Confirmed by more recent results atη= 1 and 3.2 and also in Cu+Cu (preliminary data) • Dense medium extends to high rapidity • Gluon saturation (larger contribution at Forward Rapidities)

  41. (a) R = Yield(AA) / <Nbinary> Yield (pp) y=3 y=2 (b) y=0 Rapidity Dependence in Au+Au Rapidity Dependence of high pt spectra (Polleti and Yuan (nucl-th/0108056)) Variation of the amount ofenergy loss (dE/dx) with the density of the traversed medium. Larger suppression ( small R) at y=0 than at higher rapidities  Reflects changes in the density of the traversed medium

  42. Schematic view of jet production Yield(AA) RAA = hadrons leading particle q q • In A-A, partons traverse the medium leading particle • If QGP  partons will lose a large part of their energy (induced gluon radiation)  Suppression of jet production  Jet Quenching Nuclear Modification Factor NCOLL(AA)  Yield(pp) Scaled pp reference High ptsuppression & Jet Quenching • Particles with high pt’s (above ~2GeV/c) are primarly produced in hard scattering processes early in the collision  Probe of the dense and hot stage • p+p experiments  Hard scattered partons fragment into jets of hadrons Experimentally  Suppression in the high pt region of hadron spectra (relative to p+p)

  43. BRAHMS Acceptance Transverse momentum [GeV/c] Rapidity Large rapidity coverage → Forward region covered by the FS F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

  44. Particle Identification Forward spectrometer MR spectrometer Particle Identification (BRAHMS RICH) MRS =0  / K separation2.5 GeV/c Proton ID up to4 GeV/c Ring radius vs momentumgives PID  / K separation25 GeV/c Proton ID up to35 GeV/c F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

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