High-p T Suppression in Heavy Ion Collisions at BRAHMS

1. High-pT Suppression in Heavy Ion Collisions at BRAHMS Selemon Bekele for the BRAHMS Collaboration The University of Kansas Lake Louise Winter Institute, Alberta, Canada

2. Tracking Detectors Magnetes Lake Louise Winter Institute, Alberta, Canada

3. Suppression of high-pT particle production is described in terms of the nuclear modification factor or in terms of the ratio of central (C) to peripheral (P) yield Brahms PRL 91(2003) where η = - ln ( tan(θ/2) ) High-pT in AuAu @ = 200 GeV Lake Louise Winter Institute, Alberta, Canada

4. hadrons leading particle q q hadrons leading particle Heavy ion collision Nucleon-Nucleon collision (b) (a) hadrons leading particle q q hadrons leading particle Back to back correlation measurements at STAR indicated that the distribution of the away side jets is strongly suppressed in comparison with the near side jets at high-pT [STAR PRL 90 (2003) 082302] Lake Louise Winter Institute, Alberta, Canada

5. Reaction plane [STAR PRL 93 (2003) 0252301] more suppression of particle spectra out of the reaction plane than in the reaction plane Lake Louise Winter Institute, Alberta, Canada

6. Results from AuAu at mid-rapidity are consistent with partonic energy loss in the larger interaction volume corresponding to central collisions.  High-pT suppression might be due to final state interactions How can one be sure? Is it possible to look at situations where final state interactions are expected to be less important? Au d Look at dAu collisions If suppression is a final state effect RdAu ~ 1 If suppression is an initial effect RdAu < 1 Lake Louise Winter Institute, Alberta, Canada

7. High-pT suppression at η = 0 in dAu Brahms PRL 91(2003) No suppression has been seen in dAu collisions at η = 0 This observation seems to give credence to the hypothesis that high-pT suppression may indeed be a final state effect But wait, will this picture persist if one looks at forward rapidities? Lake Louise Winter Institute, Alberta, Canada

8. Why is forward rapidity important? • A very high energy hadron has contributions to its wave function from gluons, quarks and anti-quarks x = Eparton/Ehadron x ~ e-y • Low x  large rapidity • The low x phenomena are described by the theory of the color glass condensate (CGC) • Can be probed in the nuclear medium • pT suppression can be related to nuclear shadowing, gluon saturation Lake Louise Winter Institute, Alberta, Canada

9. Density of low x gluons • grows as energy increases • Leads to gluon saturation • Parton scattering centers • reduced as a result of • gluon-gluon fusion • Less number of hard • scatterings • Reduction in production • of high pT hadrons Au d Lake Louise Winter Institute, Alberta, Canada

10. [BRAHMS PRL 93 (2004) 242303] dAu @ = 200 GeV10% most central collisions -Cronin like enhancement at η = 0 -Clear suppression as η changes from 0 to 3.2 -Suggest high-pT suppression may be due to initial state effects Lake Louise Winter Institute, Alberta, Canada

11. Brahms PRL 93 (2004) 242303 Rcp in dAu @ = 200 GeV More suppression as one goes to forward rapidity Lake Louise Winter Institute, Alberta, Canada

12. Rcp in AuAu @ = 200 GeV Truls Martin Larsen, QM2005 Lake Louise Winter Institute, Alberta, Canada

13. Nucl-ex / 0602018 Rcp in AuAu @ = 62.4 GeV • More suppression at very forward ηcompared to • AuAu @ 200 GeV and little pT dependence Lake Louise Winter Institute, Alberta, Canada

14. Rcp in CuCu • CuCu serves as bridge between dAu and AuAu • Provides data to study system size dependence together with pp, dAu and AuAu Lake Louise Winter Institute, Alberta, Canada

15. Rcp in CuCu @ = 200 GeV, η = 3.2 Centrality dependence of Rcp in CuCu shows more suppression in central than peripheral collisions Lake Louise Winter Institute, Alberta, Canada

16. Similar Rcp in CuCu and AuAu for the same number of participants @ = 200 GeV centrality Npart ------------------------------------ CuCu 0-10% 96.8 40-60% 17.2 AuAu 33-43% 96.3 59-79% 15.7 η = 3.2 Lake Louise Winter Institute, Alberta, Canada

17. system size dependence of p/π @ = 200 GeV Eun-Joo Kim, QM2005 - p/π+ and anti-p/π- ratio increases with size of colliding system - p/π+ ratio larger at forward rapidity than that at mid-rapidity - anti-p/π- ratio smaller at forward rapidity than that at mid-rapidity Lake Louise Winter Institute, Alberta, Canada

18. Comparison of AuAu & CuCu Eun-Joo Kim, QM2005 anti-p/π- ratios in different collision systems are similar for comparable number of binary collisions Lake Louise Winter Institute, Alberta, Canada

19. Summary • Preliminary results on suppression of high-pT hadron production relative to binary scaling in Au+Au and Cu+Cu collisions at BRAHMS • Mid-rapidity results suggest high-pT suppression may due to final state effects • Forward rapidity results in dAu collisions suggest high-pT suppression may be due to initial state effects which may also play a role in Nucleus-Nucleus collisions • Rcp in CuCu similar to AuAu for the same number of participants at sNN = 200 GeV. What matters may be the geometry of the interaction region and not the over all size of the colliding system • anti-p/π- ratios in different collision systems are similar for comparable number of binary collisions More to come in the near future Lake Louise Winter Institute, Alberta, Canada