Energy Dependence of Nuclear Stopping and Particle production

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

F. VidebœkPhysics 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– 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 collisionspp 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

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

dydNy

Nyyyy

0

)(2

6 order polynomial

Gaussians in pz:

2

2

2))sinh((

exppz

zN pym

y 2.00 0.10

p

p

y

y

BByT dyy

dydN

m cosh)(

Total E=25.72.1TeV


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

p

p

y

y

BByT dyy

dydN

m cosh)(

E/B=25.72.1 GeV47 < E < 85 GeV


net-neutronsnet-neutronsno pt -dependence

The assumption pbar/p = nbar/n is consistent with the data.

Taking the values and Phenix deduce aSlightly 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


Centrality DependenceCentrality DependenceThe 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 largets rapidity the staus 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 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)


Brahms vs. UrQMDBrahms vs. UrQMD


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 0.78Data 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 : p, pbar Spectra : p, pbarBRAHMS Preliminary

0-

10%

10-

20%

20-

40%

40-

60%


Yield Yield and and

<p<pTT> vs > vs RapidityRapidity

AuAu 5%


Kaon Spectra Kaon Spectra

Fit: exponential

TmmA Texp

Top 5% central collisions

AuAu 62.4 GeV

AuAu 200 GeV


Kaon Slopes Kaon Slopes 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”




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


ENERGY DEPENDENCE

Kaons vs Kaons vs BB

BRAHMS, PRL90 (2003) 102301

T~constant, B varies with y

T~ constant, B drives ratiosIn y or beam energy (?)

Net-kaon and net-protondistributions at 3 different beamenergies



Summary & ConclusionsSummary & ConclusionsTransverse 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 B B seems to drive the kaon ratio in rapidity and energy with T~ constant, preliminary 63 GeV data consistent with this


Limiting FragmentationLimiting Fragmentation• Collision view in restframe of projectile

nucleus.


For pions we can actually plot y-For pions we can actually plot y-ybeamybeam


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-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.


• 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.



d-Au Phobosd-Au Phobos

• Au+Au proton ratio is (significantly) lower than d+Au ratios• All d+Au particle ratios appear to be independent of centrality

Au-Au


Theoretical ModelsTheoretical Models

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.

R. Hwa et al. (Oregon)PRC70(2004)024904, PRC67(2003)034902replaces fragmentation functions by a scenario where mini-jet partons develop a shower which subsequently recombines, i.e. recombination of soft partons with shower partons.

Parton CoalescenceParton Coalescence

PHENIX 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.

ReCombinationReCombination


ppTT Spectra : Spectra : BRAHMS Preliminary

0-10%10-20%20-40%40-60%

We are still working on…


At y = 0, ratios convergeto ~ 15 %

ENERGY DEPENDENCE

Why max AGS-SPS ?Net-Kaon distributionevolves like net-proton

Over the full phase space:K+/+ = 16.6 1.5 % (syst)K/ = 13.7 2.0 % (syst)

Kaons vs PionsKaons vs Pions


--//++, pbar/p ratios, pbar/p ratios

No significant pT dependence up to 3GeV/c Similar behavior at y~0 and y~1

BRAHMS Preliminary


Rapidity DensitiesRapidity Densities

Width after Gaussian fit:AGS ~ no dependence

SPS-RHIC ~ strongdependence : longitudinalflow important


p/p/ ratios ratios

Greco et.al.,PRL90(2003)202302

Hwa et.al, PRC70(2004)024905

p/ ratios increase with pT up to 3GeV/c. feed down correction applied.

BRAHMS Preliminary


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”


Particle ratios in pp vs. Particle ratios in pp vs. AuAuAuAu

B. H. SamsetPoster Spec. 34


Can we link limiting fragmentation to thermal analysis


Data for Hera (H1,1998) in gamma-p collisions were analyzed by Kopelliovich and Vogh in Phys.Lett.B446, 321 (1999).A finite baryon asymmetry A = 2 * (Bbar-B)/(Bbar+B) is observed in the lepton hemisphere corresponding to transporting the BN over about 7 units of rapidity.One motivation for studying other mechanism than q-qq breaking and its implications for heavy ion collisions.


Yield and <pYield and <pTT> vs > vs Rapidity Rapidity

AuAu 5%


P/p vs pt is experimentally rather flatThe inclusion of BJ describes this quite well.

In particular well is the overall proton over pion enhancement vs pt.


HIJING/BHIJING/B• A prediction from 98• Strong proton stopping

as well as enhanced strange baryon production.

• Over-predicted actual measurements


OutlookOutlook• Additional Data from RHIC and LHC

– Extended rapidity coverage in Au-Au from run-4 data. Centrality dependence of net-protons

– Au-Au at 62.4 GeV where the net-proton maximum is within acceptance

– pp data from 500 GeV will extend the energy range considerably for baryon asymmetries in pp

– Careful measurements in ALICE for y of ~8-9.6 in AA and pp are crucial for the understanding of processes other than quark-diquark breaking.


Central region at LHCCentral region at LHCAsymmetry AB = 2 * (B – anti-B) / (B + anti-B)May allow to distinguish further between various processes with slow energy / rapidity dependence

in %

at LHC (B. Kopeliovich)

– 9.61(8.63) ← η

H1 (HERA)Δη ~ 7

Energy Dependence of Nuclear Stopping and Particle production

Documents

understanding baryon

basics of baryon transport

high energy

higher energy

low energy gluons

lower energy

norgeau au collisions

peripheral collisions