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High Rapidity Physics with the BRAHMS Experiment

High Rapidity Physics with the BRAHMS Experiment. Ramiro Debbe. Presentation layout. BRAHMS has measured nuclear modification factors at different rapidities in AuAu collisions at full energy and we do not see a rapidity dependence in those ratios.

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High Rapidity Physics with the BRAHMS Experiment

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  1. High Rapidity Physics with the BRAHMS Experiment Ramiro Debbe CIPANP2006 May 30 -June 3 PR

  2. Presentation layout BRAHMS has measured nuclear modification factors at different rapidities in AuAu collisions at full energy and we do not see a rapidity dependence in those ratios. This we consider as a “puzzle” to be solved, our measurements do not extend to high values ot pT, they cover an intermediate range where many effects may appear with different strengths depending on rapidity. I will make an attempt to characterize the system at different rapidities using the ratio of anti-proton to negative pion. And depending on the behavior of that ratio, I will hypothesize what effects may conspire to produce the absence of rapidity in the nuclear modification factors. CIPANP2006 May 30 -June 3 PR

  3. The ratios (p/ -) have been measured at RHIC for many collision systems, energies and rapidities. The magnitude of this ratio changes from a high value at mid-rapidity in Au+Au collisions at 200 GeV to the small values measured at high rapidities in p+p collisions at the same energy. These last values are similar in magnitude to the ones measured in e+e- annihilation around the Z pole, where fragmentation in the vacuum. I will make the claim that the magnitude of the p/ - is driven by the partonic density of the medium (and the degree of thermalization). The correlation between this ratio and the measured RAA and RCP will allow me make statements about the physics at different rapidities in Au+Au collisions. CIPANP2006 May 30 -June 3 PR

  4. Nuclear modification factor for charged particles in Au+Au at 200 GeV Within errors, we can state that there is no rapidity dependence in these measurements, they all show similar pT dependence. CIPANP2006 May 30 -June 3 PR

  5. Possible scenarios Energy loss proportional to the medium density must certainly be present around and above intermediate value of pT (~3GeV/c) it may follow a behavior according to: E ~ density*length or E ~ density*length2 The parton density of the medium must have a rapidity dependence. (hadron-parton duality). Daniese et al. Point to a possible surface emission bias in all RHIC data. But more recent work by Wick et al. Show that light quarks can originate from deeper in region of the medium. Suppression can also be the result of much earlier modification of the wave functions of the beams. The effects would then be amplified by quantum evolution (evolution as function of ln(1/x)). CIPANP2006 May 30 -June 3 PR

  6. Medium modified fragmentation Energy loss in a medium formed after the collision : X. N. Wang et al. Energy loss is encoded in the fragmentation of the final state parton. The number of interactions (each emitting a gluon) depends on the density of the medium. Gluon density of the formed medium (rapidity dependent) CIPANP2006 May 30 -June 3 PR

  7. Rapidity dependence of the parton density In the parton-hadron duality approach, one can relate the measured distributions of produced particles to the distribution of partons in the early stages of the system. There is a factor of ~2 between mid-rapidity and y=3 Energy loss should be affected by similar amount. CIPANP2006 May 30 -June 3 PR

  8. The surface emission bias is not so strong Initial jet prod. distribution Density of the medium Wick et al. : Surviving parton with pT =15 GeV/c and all length fluctuations included in the calculation. CIPANP2006 May 30 -June 3 PR

  9. Rapidity dependence of the pbar/ - ratio Contrary to the absence of rapidity dependence of the RAuAu factor, the pbar/ - ratio does show a marked reduction as the rapidity increases. The shapes of the pT dependence have been reproduced by calculations involving recombination. CIPANP2006 May 30 -June 3 PR

  10. Recombination affecting fragmentation R. Hwa et al. Phys. Rev C70 Recombination function Product of parton densities Wave function details R. J. Fries et al. Phys. Rev. C68 CIPANP2006 May 30 -June 3 PR

  11. p/ - ratio measured at two rapidities The ratio measured at mid-rapidity has a visible dependence on centrality that can be connected to the “size” of the system and hence the parton density. Distributions peak around 3 GeV/c for all centralities. At high rapidity, (y=3) the ratio is smaller, peaks at lower pT and has weaker centrality dependence. CIPANP2006 May 30 -June 3 PR

  12. The ratio measured in three different systems at the same energy Very interesting result for proton/pion At the same centrality the smaller CuCu system shows a somewhat smaller p/ - ratios compared to the larger AuAu system. CIPANP2006 May 30 -June 3 PR

  13. If one fixes the number of participants on both systems the p/ - ratio is the same y=0 and 3. Even though the “shapes” must be different. CIPANP2006 May 30 -June 3 PR

  14. Summary of the p/ - ratio measurements At the most central set the p/- “measures” a system similar to p+p at y=0 p+p 200 GeV CIPANP2006 May 30 -June 3 PR

  15. Back to nuclear modification factors If the statement made about the p/ - ratio measuring the partonic density of the medium (and its degree of thermalization) then: Energy loss in the medium may explain these “suppressed” ratios There must be different “physics” at high rapidity. CIPANP2006 May 30 -June 3 PR

  16. The absence of rapidity dependence is even present for the nuclear modification factor of identified particles. CIPANP2006 May 30 -June 3 PR

  17. A more dilute system at higher rapidity will show stronger volume dependence, hence the ratio at y=3.2 shows more Npart dependence, while the y=0 has a hint of saturation of energy loss. CIPANP2006 May 30 -June 3 PR

  18. Nuclear modification factor for identified particles at high rapidity in AuAu collisions at 200 GeV RAA and RCP factors for produced particles are very similar, but in the case of protons isospin can produce big differences. CIPANP2006 May 30 -June 3 PR

  19. The AuAu system at two energies As the events become more and more peripheral, the 200 GeV approaches nucleon-nucleon behavior. But at lower energy the more peripheral events show some type of Cronin enhancement. CIPANP2006 May 30 -June 3 PR

  20. The low energy AuAu system shows an even stronger “suppression”at high rapidity. CIPANP2006 May 30 -June 3 PR

  21. Comparing two systems at low energy near mid-rapidity These results are consistent with weak effects of a dense medium and a return of strong SPS Cronin type enhancements CIPANP2006 May 30 -June 3 PR

  22. The smaller CuCu system at 200 GeV Similar to AuAu stronger suppression for the most central events CuCu central = AuAu semi-central but geometries must be different. CIPANP2006 May 30 -June 3 PR

  23. The proton/+ ratio at forward rapidities Dramatic difference between y=0 and 3. There are ~2.5 more protons than pions at ~ 2.5 GeV/c “Forward production of protons is favored in recombination because of scarcity of anti-quarks” R. Hwa et al. nucl_th/0605037 CIPANP2006 May 30 -June 3 PR

  24. Summary BRAHMS has measured nuclear modification factor in different systems and energies. At the highest energies these factors do not show a clear rapidity dependence. The p/- ratio does show a rapidity dependence that was hypothesized as a “measure” of the parton density of the system. BRAHMS results may be an indication that energy loss in a dense medium does not extend to high rapidity, and the suppression seen there could be connected to the effects of quantum evolution on the gluon density of the colliding beams. CIPANP2006 May 30 -June 3 PR

  25. Backup slides CIPANP2006 May 30 -June 3 PR

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  28. p/p+ ratio integrated over 1.5<pT<2.5 GeV/c at forward rapidity increase with centrality; smaller dependence seen at midrapidity CIPANP2006 May 30 -June 3 PR

  29. pion kaon CIPANP2006 May 30 -June 3 PR

  30. CIPANP2006 May 30 -June 3 PR

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