Baryon Transport in Relativistic Heavy Ion Collisions

Baryon Transport in Relativistic Heavy Baryon Transport in Relativistic Heavy Ion CollisionsIon Collisions

F. Videbœk

Physics Department

Brookhaven National Laboratory

A mainly experimental overview of stopping and baryon transport in HI and pp

February 10, 2005 ICPAQGP 2005, Kolkata 2

OverviewOverview• Introduction

– Baryon transport, stopping, longitudinal distributions, mechanism

• Experimental systematic – AA (energy and centrality dependence)

• A selection of comparison to models– AA, dA and pp– Energy loss at RHIC

• 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?


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.


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.


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

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 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


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.


pp collisionspp collisions• 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.

NA49


More data and Model More data and Model ComparisonsComparisons

Do the data for pp, dA and AA constrain models?

Are there clear evidence for new mechanisms?


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


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


Baryon JunctionBaryon Junction

• 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.


HIJING/BHIJING/B

• A prediction from 98• Strong proton stopping

as well as enhanced strange baryon production.

• Over-predicted actual measurements


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.


• HijingBB(2.0) describes the net baryon distributions well.

• The rapidity loss is small and has a rather large E/B

From Topor Pop et al.Red Hijing 1.37Blue HijingBB 2.0Green rqmd


P/p vs pt is experimentally rather flat

The inclusion of BJ describes this quite well.

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


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


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


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.

Documents

Baryon Transport in Relativistic Heavy Ion Collisions