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

Energy Dependence of Nuclear Stopping Energy Dependence of Nuclear Stopping and Particle productionand Particle production

F. Videbœk

Physics Department

Brookhaven National Laboratory

A Brahms Perspective

April 2, 2005 Bergen, Norge 2

OverviewOverview• 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


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?


pp & pA collisionspp & pA collisionsEarly 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


Transport MechanismsTransport 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.


What carries baryon What carries baryon number at high energiesnumber 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


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.


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 AGSAu+Au collisions at AGS


Central Pb-Pb from NA49Central 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)


contribution to net-contribution to net-baryonsbaryons

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


Net-p energy systematicNet-p energy systematicAt 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


Corrections to observedCorrections to observedp and p-bar yields 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.


y 2.03 0.16

Rapidity Loss Rapidity Loss

Rapidity loss:

py BB

partpp dy

dy

dNy

Nyyyy

0

)(2

6 order polynomial

Gaussians in pz:

2

2

2

))sinh((exp

pz

zN pym

y 2.00 0.10

p

p

y

y

BB

yT dyydy

dNm cosh)(

Total E=25.72.1TeV


y vs. yy vs. ybeambeam

Even (unphysical) extreme approximations don’t change conclusions: Linear Increase in dy seems to saturate at RHIC.

p

p

y

y

BB

yT dyydy

dNm cosh)(

E/B=25.72.1 GeV47 < E < 85 GeV


net-neutronsnet-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


Centrality DependenceCentrality 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.


Data and Model Data and Model ComparisonsComparisons

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


Model ComparisonModel 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


Rapidity and Energy Loss 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 <E>/Baryon distributions are quite different resulting in significant different energy loss.


• 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 1.37Blue HijingBB 2.0Green rqmd


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.


Brahms vs. UrQMDBrahms vs. UrQMD• M.Bleicher et. al


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


ppTT Spectra : Spectra : BRAHMS Preliminary

0-

10%

10-

20%

20-

40%

40-

60%


Kaon Spectra Kaon Spectra

Fit: exponential

T

mmA Texp

Top 5% central collisions

AuAu 62.4 GeV

AuAu 200 GeV


Yield Yield and and

<p<pTT> vs > vs RapidityRapidity

AuAu 5%


Kaon Inverse Slopes (T)Kaon Inverse Slopes (T) Top 5% central collisions


Integrated multiplicities @ 200 GeV (Gaussian fit)

N(K+) ~ 290 N(K) ~ 240

Rapidity DensitiesRapidity Densities


Landau hydrodynamics along Landau hydrodynamics along beam axisbeam axis

• Isentropic expansion driven by equation of state

• Mass-less particles

• Pt and rapidity

factorize

Assumptions:

Implications:

• dN/dy Gaussian • = log (√SNN/2mp) ≈ log (beam)• Model consistent with “limiting fragmentation” (P.Steinberg,..


Y < 1 : consistent with Hadron Gas Stat. ModelK+/+ : 15.6 0.1 % (stat)K/ : 14.7 0.1 % (stat) [Phys. Lett. B 518 (2001) 41]

Divergence at higher y :Associated K+ productionNo single source with

unique T and B

Kaons vs PionsKaons vs PionsRAPIDITY DEPENDENCE


Kaons vs Kaons vs B B

Net-kaon and net-protondistributions at 3 different beamenergies


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.


Limiting FragmentationLimiting Fragmentation• Collision view in rest frame of projectile

nucleus.


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.


Kaon inverse slope Kaon inverse slope

Kaons are Kaons are convenient to test convenient to test mmT T dependencedependence

Is this significant?Is this significant?


We see a similar effect for kaonsWe see a similar effect for kaons

Kinematic Kinematic limit means limit means production production does not go does not go all the way to all the way to beam beam rapidityrapidity


SummarySummary• AA collisions at RHIC show a large rapidity loss y ~ 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.


• 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/dseems 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.

Documents

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