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Energy Dependence of Nuclear Stopping and Particle production

Energy Dependence of Nuclear Stopping and Particle production. F. Videb œ k Physics Department Brookhaven National Laboratory. A Brahms Perspective. Overview. Stopping Baryon transport, stopping, longitudinal distributions, mechanism Experimental systematic

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Energy Dependence of Nuclear Stopping and Particle production

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  1. Energy Dependence of Nuclear Stopping and Particle production F. Videbœk Physics Department Brookhaven National Laboratory A Brahms Perspective

  2. Overview • Stopping • Baryon transport, stopping, longitudinal distributions, mechanism • Experimental systematic • AA (energy and centrality dependence) • A selection of comparison to models • Particle Production • Landau, Limiting Fragmentation, thermal aspects • Summary Bergen, Norge

  3. The net-baryon rapidity distributions are though to reflect the initial distribution of baryonic matter in the very first moment of the collisions. Due to the large mass subsequent expansion and re-scattering will not result in a significant rapidity change. What are the processes that governs the initial stopping of baryons? Goal to describe the space-time development of the HI reaction. J.D.Bjorken,PRD 27,140 (1983) Bergen, Norge

  4. pp collisions Early pp, and pA data lay the foundation for basics of baryon transport (stopping) .The systematic was established by the analysis of Busza and Goldhaber [Phys.Lett.139B,235(1984)] , Busza and Ledoux, Ann.Rev.Mod.Phys. based on FNAL data. • Estimated that dy would be ~2 for AA. • First systematic set of data came from ISR this lead to both the q-qq description and the later ideas of Baryon Junctions (and other mechanisms). • pp and p(d)A are important references in understanding baryon transport. • The recent data from NA49 at SPS is an important reference NA49 Bergen, Norge

  5. Transport Mechanisms • At very low energies (SIS, AGS) cascade and resonance excitations describe stopping and transverse behavior. • At higher energies string picture is relevant. • Di-quark-quark breaking corresponds to having the baryon number associated with the valence quarks. This is dominant process at lower energy. • Other mechanisms can carry the baryon number in a gluonic junction containing many low energy gluons; this will be increasing important at higher energy due to time-contraction of the projectile/targets at high energy. • These ideas were developed in early for pp • G.C.Rossi and G.Veniziano Nucl.Phys.B123(77)507 • B.Z.Kopeliovich and B.G.Zakharov Z.Phys.C43(1989) • D.Kharzeev Phys.Lett. B378(96) 238. Bergen, Norge

  6. What carries baryon number at high energies • Standard point of view • quarks have baryon charge 1/3 • gluons have zero baryon charge • When original baryon change its color configuration (by gluon exchange) it can transfer its baryon number to low x without valence quarks • baryon number can be transferred by specific configuration of gluon field (G.Garvey, B.Kopeliovich and Povh; hep-ph 0006325 [2002]) x Bergen, Norge

  7. Experimental Considerations • The net-protons are used as a measure for the net-baryons since rarely are all the particles that carries baryon number measured. • In almost all cases determined from protons, anti-protons that are easily accessible. • Net-Baryon = Net(p)+Net(L)+Net(Casade)+Net(neutrons), where each has to be corrected for feed-down. Only near mid-rapidity has the first two components been well determined well (at RHIC in Au-Au and at SPS in Pb-Pb collisions). • Studies of anti-baryon / baryon ratios is also a measure of the baryon transport. Bergen, Norge

  8. Au+Au collisions at AGS p+p picture is recovered in peripheral collisions In central collisions the rapidity distribution peaks at mid-rapidity Strong centrality dependence. Bergen, Norge

  9. Central Pb-Pb from NA49 Rather large but not complete stopping. The rapidity loss dy ~ 1.75+-.05 for PbPb and for SS 1.63+-.16. Pb-Pb at 158 A.GeV/c Phys.ReV.Lett.82,2473(99) Bergen, Norge

  10. L contribution to net-baryons The development of stopping and onset of transparency is well illustrated by the L measurements by NA49. Net(L) = 9.3+-1 Net(p) ~ 28+-1 i.e. L/p ~0.30 at SPS At RHIC Phenix, Star have shown that L/p ~0.9 Na49, PRL Bergen, Norge

  11. Net-p energy systematic At RHIC the mid-rapidity region is almost net-proton free. Pair baryon production dominates at RHIC. • AGS->RHIC : Stopping -> Transparency • Net proton peak > y ~ 2 Bergen, Norge

  12. Corrections to observedp and p-bar yields • These data are not feed-down corrected. • The estimated factor due to decay corrections, and assuming that p/n=1 is 2.03 leading to a net-baryon yield of ~14 at mid-rapidity. Bergen, Norge

  13. Rapidity loss: 2.03  0.16 2.00  0.10 Total E=25.72.1TeV Rapidity Loss Gaussians in pz: 6 order polynomial Bergen, Norge

  14. Even (unphysical) extreme approximations don’t change conclusions: Linear Increase in dy seems to saturate at RHIC. dy vs. ybeam E/B=25.72.1 GeV 47 < DE < 85 GeV Bergen, Norge

  15. net-neutrons no pt -dependence The assumption pbar/p = nbar/n is consistent with the data. Taking the values and Phenix deduce a Slightly lower ratio of nbar/n ~ 0.64. Thus the net-neutron yield is equal or slightly higher than net proton yield. Phenix Au-Au 200 GeV . nucl-ex0406004 Bergen, Norge

  16. Centrality Dependence The p-bar/p ratios has no or little centrality dependence as seen in data from NA49 and Phenix. The net-proton / Npart is also nearly constant with centrality. Bergen, Norge

  17. Data and Model Comparisons How do the data for pp, dA and AA constrain models? Are there clear evidence for new mechanisms? • String models • Parton cascade • Models involving Baryon Junctions Bergen, Norge

  18. Model Comparison d+Au • Models agree with the expectation that baryon transport increases • with increasing  thus resulting in a decreased p/p ratio • Data does not exhibit this behavior (nucl-ex/0309013 ) Bergen, Norge

  19. AMPT describes the net baryons and particle ratios quite well. Hijng on other hand underestimates the net yield at mid-rapidity. At the largets rapidity the staus is unclear. The <E>/Baryon distributions are quite different resulting in significant different energy loss. Rapidity and Energy Loss Bergen, Norge

  20. Baryon Junction was first into Hijing by Vance and Gyulassy (PRL 83,1735) to explain stopping and hyperon production at SPS energies • Recently V.Topor Pop et. Al (PRC70,064906) has further developed by adding intrinsic kT to study in particular the the pT dependence of baryon production. From Topor Pop et al. Red Hijing 1.37 Blue HijingBB 2.0 Green rqmd Bergen, Norge

  21. Bass,Muller, and Srivasta ;parton cascade model (AA)Phys.Rev.Lett 91,052302(2003) Bergen, Norge

  22. Brahms vs. UrQMD Bergen, Norge

  23. BRAHMS pp and AA at 200 GeV general similarity between pp and AA over a wide rapidity range. There are though significant difference at mid-rapidity where p-bar/p|pp > p-bar/p|AA from 0.73 to 0.78 Data from Phobos has a value of 0.83. The calculations with Pythia fails while hijing BB describes the magnitude and rapidity dependence well. Bergen, Norge

  24. pT Spectra : p, pbar BRAHMS Preliminary 0-10% 10-20% 20-40% 40-60% Bergen, Norge

  25. Yield and <pT> vs Rapidity AuAu 5% Bergen, Norge

  26. Fit: exponential Kaon Spectra Top 5% central collisions AuAu 200 GeV AuAu 62.4 GeV Bergen, Norge

  27. Kaon Slopes Top 5% central collisions Bergen, Norge

  28. Rapidity Densities Integrated multiplicities @ 200 GeV (Gaussian fit) N(K+) ~ 290 N(K) ~ 240 Bergen, Norge

  29. Landau hydrodynamics along beam axis Assumptions: • Isentropic expansion driven by equation of state • Mass-less particles • Pt and rapidity factorize Implications: • dN/dy Gaussian •  = log (√SNN/2mp) • ≈ log (beam) • Model consistent with “limiting fragmentation” Bergen, Norge

  30. Bergen, Norge

  31. Bergen, Norge

  32. Y < 1 : consistent with Hadron Gas Stat. Model K+/+ : 15.6  0.1 % (stat) K/ : 14.7  0.1 % (stat) [Phys. Lett. B 518 (2001) 41] Divergence at higher y : Associated K+ production No single source with unique T and B Kaons vs Pions RAPIDITY DEPENDENCE Bergen, Norge

  33. BRAHMS, PRL90 (2003) 102301 T~constant, Bvaries with y T~ constant, mB drives ratios In y or beam energy (?) Kaons vs mB ENERGY DEPENDENCE Net-kaon and net-proton distributions at 3 different beam energies Bergen, Norge

  34. Bergen, Norge

  35. Summary & Conclusions Transverse momentum spectra of kaons measured in rapidity range -0.1 < yK < 3.4 for central Au+Au collisions at 200 and 63 GeV Slopes: exponential in mT gives good description slopes at 200 GeV > 63 GeV, small step  Yields: N(+) ~ N(-) at mid-rapidity (47 and 44) but N(+) > N(-) at y > 2 due to associated K+ production K / : converge to ~ 15% at y ~ 0 (plateau y < 1) same within systematic errors for full phase-space ratios possible indication of strangeness equilibration at 200 GeV At 63 GeV, y = 0, ratios at “expected” values K vs mB mB seems to drive the kaon ratio in rapidity and energy with T~ constant, preliminary 63 GeV data consistent with this Bergen, Norge

  36. Limiting Fragmentation • Collision view in restframe of projectile nucleus. Bergen, Norge

  37. For pions we can actually plot y-ybeam Bergen, Norge

  38. We see a similar effect for kaons Kinematic limit means production does not go all the way to beam rapidity Bergen, Norge

  39. Summary • AA collisions at RHIC show a large rapidity loss dy ~ 2.0. • In contrast the <E> is not (yet) as well constrained. Several models that describe the net-proton distributions have a range of energies <E> ~25-37 GeV/nucleon. • The finite net-baryon and p-bar/p < 1 in both pp and AA at high energies seem to require additional baryon transport mechanism(s) over q-qq breaking. • Such mechanisms as the Baryon Junction will not decrease the <E> since only the BN is transported with the energy associated resides at large rapidities, and thus not available for particle production at mid-rapidity. • The connection between energy stopping and rapidity loss is broken at high energies. Bergen, Norge

  40. Landau Expansion • Limiting Fragmentation • Both seem to describe the bulk of data at AGS->RHIC energies. As Pointed out this may be resolved at LHC. • Thermal descriptions • Seem valid over rapidity as well as energy; minimal information content. Bergen, Norge

  41. Thats its folks Bergen, Norge

  42. d-Au Phobos Au-Au • Au+Au proton ratio is (significantly) lower than d+Au ratios • All d+Au particle ratios appear to be independent of centrality Bergen, Norge

  43. Theoretical Models PHENIX p/p ratio (0-10% central) R.J. Fries et al. (Duke) PRL90(2003)202303, PRC68(2003) 044902 - recombination dominates over fragmentation for an exponentially falling parton spectrum, but the fragmentation wins out, when the spectrum takes the form of a power law. - strictly separates soft and hard physics, allowing only the soft partons to recombine and only the hard partons to fragment. ReCombination Parton Coalescence R. Hwa et al. (Oregon) PRC70(2004)024904, PRC67(2003)034902 replaces fragmentation functions by a scenario where mini-jet partons develop a shower which subsequently recombines, i.e. recombination of soft partons with shower partons. V. Greco et al. (TAMU) PRL90(2003)202302, PRC68(2003)034904 added assumption that soft partons in QGP can coalesce with comoving hard partons from a mini-jet. Bergen, Norge

  44. pT Spectra : p BRAHMS Preliminary 0-10% 10-20% 20-40% 40-60% We are still working on… Bergen, Norge

  45. Over the full phase space: K+/+ = 16.6  1.5 % (syst) K/ = 13.7  2.0 % (syst) Why max AGS-SPS ? Net-Kaon distribution evolves like net-proton Kaons vs Pions ENERGY DEPENDENCE At y = 0, ratios converge to ~ 15 % Bergen, Norge

  46. p-/p+, pbar/p ratios BRAHMS Preliminary • No significant pT dependence up to 3GeV/c • Similar behavior at y~0 and y~1 Bergen, Norge

  47. Rapidity Densities Width after Gaussian fit: AGS ~ no dependence SPS-RHIC ~ strong dependence : longitudinal flow important Bergen, Norge

  48. p/p ratios BRAHMS Preliminary Hwa et.al, PRC70(2004)024905 Greco et.al.,PRL90(2003)202302 p/p ratios increase with pT up to 3GeV/c. L feed down correction applied. Bergen, Norge

  49. Landau hydrodynamics along beam axis Assumptions: • Isentropic expansion driven by equation of state • Mass-less particles • Pt and rapidity factorize Implications: • dN/dy Gaussian •  = log (√SNN/2mp) • ≈ log (beam) • Model consistent with “limiting fragmentation” Bergen, Norge

  50. Particle ratios in pp vs. AuAu B. H. Samset Poster Spec. 34 Bergen, Norge

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