Natalia Katryńska Smoluchowski Institute of Physics Jagiellonian University

1. Physics at Forward Rapidities • BRAHMS Experiment at RHIC Natalia Katryńska Smoluchowski Institute of Physics Jagiellonian University for the BRAHMS Collaboration

2. Outline • Overview of BRAHMS detector setup. • A few details of analysis process. • Nuclear Modification Factor in d+Au collisions for different centralities and rapidities @ 200 GeV. • 4. Forward like-particle ratios of pions, kaons and protons • @ 62.4 GeV and 200 GeV. • 5. The invariant yield distributions of identified hadrons. • 6. Limiting fragmentation in p+p collisions. • 7. Summary. 2

3. Broad RAnge Hadron Magnetic Spectrometers at RHIC Collidingsystems: @ 62.4 and 200 GeV Au+Au Cu+Cu BRAHMS PHOBOS η = 2.2 η = 3.2 PHENIX η = 0 STAR d + Au p + p RICH detector situated in Forward Spectrometer allows to identify high momentum particles (e-,µ-/+, π-/+, K-/+, p); ToF placed before RICH measures hadrons with lower velocity. 3

4. Some details of data analysis PID using RICH 1. Particle identification of pions, kaons and protons. 2. Applied corrections for: efficiency, weak-decays, absorption in materials . 3. The data was corrected for geomet- rical acceptance. CC vertex [cm] ZDC vertex [cm] m2 [GeV2/c4] PID using ToF p [ GeV/c] 4 corr. weak-decays&absorption

5. d+Au Collisions @ 200 GeV PRL 93 (2004) 242303 Nuclear medium effects: * Cronin enhancement for y ~ 0 ; * suppression at high pT; * expected Color Glass Condensate state (gluon saturation) at forward rapidity (low x) ? Nuclear Modififcation Factor: 5

6. Rd+Au for Identified Particles E.J.Kim, H.Yang, INCP2007 Although there is a limitation in pT to determine the nuclear modification factor at BRAHMS experiment the suppression of R d+Au is clearly visible for forward rapidities for pions, kaons and protons for 0-30% centrality bin. For peripheral centrality the value of factor is comparable with midrapidity region. At midrapidity, at pT ≈ 2 GeV/c , for both centralities R d+Au of protonos reaches value > 1.5. 6

7. Forward Like-Particle Ratios @ 200 GeV p + p collisions It seems that the slight decrease of π-/π+ with increasing pT at high rapidity reflects the superiority of valence quark fragmentation. The fact that the ratio of p/p is so small at high rapidity may be a strong indication that they are not produced by a common mechanism like g -> p fragmentation. BRAHMS preliminary BRAHMS preliminary 7

8. Pions and Protons Production p + p collisions @ 200 GeV The protons over antiprotons yield excess causes a noticeable difference between the p/π- and p/π+. That high p/π+at high pT is a puzzle , especially with reference to the baryon number transport considering as a soft low pT phenomena. BRAHMS preliminary BRAHMS preliminary 8

9. Particle Ratios @ 62.4 GeV p + p collisions BRAHMS preliminary The difference between protons and antiprotons yield seen @ 200 GeV, at lower energy increses by an order of the magnitude. BRAHMS preliminary 9

10. Comparison of p/π ratio p + p collisions A remarkable value for the p/π+(pT) ratio is observed at η = 3.2 in p+ p collisions @ 62.4 GeV. This value is considerably different from the value measured @ 200 GeV. The increase of p/π with increasing y is seen in the wide range of rapidity. BRAHMS preliminary 10

11. Invariant Cross Section vs. NLO Calculations p + p collisions @ 200 GeV hep-ph 0803.2768v1 PRL 98 (2007) 252001 factorization&normalization scales: µ≡µF≡µR≡pT ; CTEQ6 parton distribution functions; KKP modified to obtain FFs for specific charges: Dπ+u = (1+z)Dπ0u ; Dπ0u = 1/2Dπ- + π+u ; z = fraction of the parton momentum carried by the hadron; Dπ+u = function fragmenting u quarks into positive pions; AKK - FFs reproduce well STAR data at y~0 => at high y: gluons fragmenting into anti-/protons > 80% for pT < 5 GeV/c; 11

12. Invariant Cross Section vs. NLO Calculations F. Videbaek, Winter Workshop on Nuclear Dynamics, 2007 p + p collisions @ 62.4 GeV factorization&normalization scales: µ≡µF≡µR≡pT ; KKP modified as for p+p system @ 200 GeV; observed rapid changes with different rapidities at high pT (xF up to 0.5); although at high pT it seems that the fit falls down below data there is still good agreement; 12

13. Net-proton Distribution net-proton ≡ Taking into account also previous results it’s observed a tendency that it must be implemented a mechanism which allows for easy transport of baryon number toward midrapidity, while energy balance is maintained through an increased production of forward mesons. That’s included as baryon junction in HIJING/B model. J. Phys. G: Part. Phys. 8 August 2007 S619-S622 13

14. Net-proton dN/dy in d+Au Collisions @ 200 GeV H. Yang, QM2008, oral presentaion Going from midrapidity to forward rapidity there is no sigificant alteration - taking errors under consideration - for the most central d+Au collisions. For more peripherial reactions for rapidity y ~ 3 it is noticeable the difference of proton and antiproton production in comparison with midrapidity regime. 14

15. Limiting Fragmentation for Elementary Reactions A remarkable overlap of dN/dy distributions at all energies (p+p @ 200 GeV, ybeam ~ 5.3; p+p @ 62.4 GeV, ybeam ~ 4.2) appears when viewed in their projectile frame. 15

16. Summary d+Au collisions: * It is observed a significant suppression of the yield of charged hadrons compared to p+p collisions at forward rapidities. * Due to limitation in pT it is difficult to specify the character of particular particle in forward regime. * The results @ 200 GeV indicate that the simple quark-diquark breaking mechanism is not sufficient to characterize particles yields in the whole considered interval of rapidity. * It seems that NLO pQCD rationally describe the mesons (pions and kaons) production distributions at high rapidity in relativistic elementary collisions. * The updated AKK FFs reflect the invariant cross section for anti-/proton ; the unexpected excess of protons over antiprotons (p/p ~ 0.1 at y~3.3) yield suggests it may not be related to baryon transport number. * Forward particle production @ 62.4 GeV has a similar tendency, but with stronger effect. p+p collisions: 16

17. 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. Nielsen7, D. Ouerdane7, 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 , A. Qviller7 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. Smoluchowski 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 17

18. back-up slides 18

19. RCP for d+Au Collisions @ 200 GeV PRL 93 (2004) 242303 * at η = 0 central-peripheral ratio larger than semicentral-peripheral: increased role of Cronin-like multiple scattering effects ; * at η = 3.2 more central ratio is lower: suppression mechanism depends on the centraliy collision; 19

20. Limiting Fragmentation in Au+Au Collisions @ 62.4 and 200 GeV Viewing from the rest frame of one of the nuclei with simultaneously substracting calculated value of target baryons (grey bands) for heavy ions collisions at RHIC (BRAHMS) and SPS (NA49). 20