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Energy Dependence of Nuclear Stopping and Particle production Energy Dependence of Nuclear Stopping and Particle production F. Videbœk Physics Department.

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Presentation on theme: "Energy Dependence of Nuclear Stopping and Particle production Energy Dependence of Nuclear Stopping and Particle production F. Videbœk Physics Department."— Presentation transcript:

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

2 April 2, 2005Bergen, Norge2 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

3 April 2, 2005Bergen, Norge3 Goal to describe the space-time development of the HI reaction. J.D.Bjorken,PRD 27,140 (1983) 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?

4 April 2, 2005Bergen, Norge4 pp & pA collisions 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  y 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

5 April 2, 2005Bergen, Norge5 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.

6 April 2, 2005Bergen, Norge6 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

7 April 2, 2005Bergen, Norge7 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(  )+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.

8 April 2, 2005Bergen, Norge8 p+p picture is recovered in peripheral collisions In central collisions the rapidity distribution peaks at mid-rapidity Strong centrality dependence. Au+Au collisions at AGS

9 April 2, 2005Bergen, Norge9 Central Pb-Pb from NA49 Rather large but not complete stopping. The rapidity loss  y ~ 1.75+-.05 for PbPb and for SS 1.63+-.16. Pb-Pb at 158 A.GeV/c Phys.ReV.Lett.82,2473(99)

10 April 2, 2005Bergen, Norge10  contribution to net- baryons The development of stopping and onset of transparency is well illustrated by the  measurements by NA49. Net(  Net(p)  i.e.  /p ~0.30 at SPS At RHIC Phenix, Star have shown that  /p ~0.9 Do also note that  changed significantly over +-1 unit of y. Na49, PRL

11 April 2, 2005Bergen, Norge11 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

12 April 2, 2005Bergen, Norge12 Corrections to observed p 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.

13 April 2, 2005Bergen, Norge13 2.03  0.16 Rapidity Loss Rapidity loss : 6 order polynomial Gaussians in p z : 2.00  0.10 Total E=25.7  2.1TeV

14 April 2, 2005Bergen, Norge14  y vs. y beam Even (unphysical) extreme approximations don’t change conclusions: Linear Increase in dy seems to saturate at RHIC. E/B=25.7  2.1 GeV 47 <  E < 85 GeV

15 April 2, 2005Bergen, Norge15 net-neutrons no p t -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

16 April 2, 2005Bergen, Norge16 Centrality Dependence The p-bar/p ratios has no or little centrality dependence as seen in data from NA49 and PHENIX. The net-proton / N part is also nearly constant with centrality.

17 April 2, 2005Bergen, Norge17 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

18 April 2, 2005Bergen, Norge18 Model Comparison 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 ) d+Au

19 April 2, 2005Bergen, Norge19 Rapidity and Energy Loss 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 /Baryon distributions are quite different resulting in significant different energy loss.

20 April 2, 2005Bergen, Norge20 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 k T to study in particular the the p T dependence of baryon production. From Topor Pop et al. Red Hijing 1.37 Blue HijingBB 2.0 Green rqmd

21 April 2, 2005Bergen, Norge21 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.

22 April 2, 2005Bergen, Norge22 Brahms vs. UrQMD M.Bleicher et. al

23 April 2, 2005Bergen, Norge23 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. BRAHMS pp and AA at 200 GeV

24 April 2, 2005Bergen, Norge24 p T Spectra :  p T Spectra :  BRAHMS Preliminary 0-10% 10-20% 20-40% 40-60%

25 April 2, 2005Bergen, Norge25 Kaon Spectra Fit: exponential Top 5% central collisions AuAu 62.4 GeV AuAu 200 GeV

26 April 2, 2005Bergen, Norge26 Yield and vs Rapidity AuAu 5%

27 April 2, 2005Bergen, Norge27 Kaon Inverse Slopes (T) Top 5% central collisions

28 April 2, 2005Bergen, Norge28 Integrated multiplicities @ 200 GeV (Gaussian fit) N(K + ) ~ 290 N(K  ) ~ 240 Rapidity Densities

29 April 2, 2005Bergen, Norge29 Landau hydrodynamics along beam axis Isentropic expansion driven by equation of state Mass-less particles P t and rapidity factorize Assumptions: Implications: dN/dy Gaussian  = log (√S NN /2m p ) ≈ log (  beam ) Model consistent with “limiting fragmentation” (P.Steinberg,..

30 April 2, 2005Bergen, Norge30 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

31 April 2, 2005Bergen, Norge31 Kaons vs  B Net-kaon and net-proton distributions at 3 different beam energies

32 April 2, 2005Bergen, Norge32 Are there multiple sources that should be considered for the thermal descriptions we have discussed so much here? Even at SPS p/p and  /  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.

33 April 2, 2005Bergen, Norge33 Limiting Fragmentation Collision view in rest frame of projectile nucleus.

34 April 2, 2005Bergen, Norge34 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.

35 April 2, 2005Bergen, Norge35 Kaon inverse slope Kaons are convenient to test m T dependence Is this significant?

36 April 2, 2005Bergen, Norge36 We see a similar effect for kaons Kinematic limit means production does not go all the way to beam rapidity

37 April 2, 2005Bergen, Norge37 Summary AA collisions at RHIC show a large rapidity loss  y ~ 2.0. In contrast the is not (yet) as well constrained. Several models that describe the net-proton distributions have a range of energies ~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 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.

38 April 2, 2005Bergen, Norge38 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/d  seems to follow this idea. Identified pi, K, 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.

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