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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 • Energy dependence • 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 & pA 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 Do also note that L/ L changed significantly over +-1 unit of y. 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 p/p = n/n is consistent with the data. Taking the values and Phenix deduce a Slightly lower ratio of n/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 largest rapidity the status 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 the model 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) • The transport from 2 phenomena • initial asymmetry in parton distribution function toward low x (0.01) • Multiple scattering (PCM) • The parton cascade model do not include spectator Baryons. Only about 50% are liberated in the initial partonic fragmentation. Bergen, Norge

  22. Brahms vs. UrQMD • M.Bleicher et. al 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 BRAHMS Preliminary 0-10% 10-20% 20-40% 40-60% Bergen, Norge

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

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

  27. Kaon Inverse Slopes (T) 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” (P.Steinberg,.. Bergen, Norge

  30. 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

  31. Kaons vs mB Net-kaon and net-proton distributions at 3 different beam energies Bergen, Norge

  32. Are there multiple sources that should be considered for the thermal descriptions we have discussed so much here? Even at SPS p/p and L/ L are not constant, not even at y=0. At Rhic the deviation are small within +-1 Should this not be considered, and what are the implications, if any, for the discussion and understanding of the horn. Bergen, Norge

  33. Limiting Fragmentation • Collision view in rest frame of projectile nucleus. Bergen, Norge

  34. Dn/dy for identified pions in the limiting fragmentation picture.Compilation from STAR in recent paper where y’s from pi’0 (corrected) represents 2* pi0. Bergen, Norge

  35. Kaon inverse slope Kaons are convenient to test mT dependence Is this significant? Bergen, Norge

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

  37. 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/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 to mid-rapidity and rapidity loss may be broken at high energies. Bergen, Norge

  38. The systematic studies from AGS, SPS to RHIC have yielded a wealth of high quality systematic dependencies • Landau Expansion • Seems at first hand to describe produced particle longitudinal expansion. • Limiting Fragmentation • Several properties apart from dN/dh seems to follow this idea. Identified pi, K, <mt> for Kaons.Is this coinci dental ? • 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. • Do we have to deal with multiple source descriptions to handle both the ‘central’ system and the influence from the fragmentation proton-rich region. • The new data from Run-4 and run-5 (Au.Au and Cu.Cu) will add important data for the ‘soft’ physics studies. Bergen, Norge

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