Baryon Transport in Relativistic Heavy Ion Collisions

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Baryon Transport in Relativistic Heavy Ion
Collisions

• F. Videbk
• Physics Department
• Brookhaven National Laboratory

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

• 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

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• Goal to describe the space-time development of
  the HI reaction.

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?
J.D.Bjorken,PRD 27,140 (1983)
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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.

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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)/(BbarB) 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.
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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
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• 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(neutron
  s), 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.

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AuAu collisions at AGS
pp picture is recovered in peripheral
collisions In central collisions the rapidity
distribution peaks at mid-rapidity Strong
centrality dependence.
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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)
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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
Na49, PRL
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Net-p energy systematic
At RHIC the mid-rapidity region is almost
net-proton free. Pair baryon production dominates
at RHIC.

• AGS-gtRHIC Stopping -gt Transparency
• Net proton peak gt y 2

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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/n1 is 2.03 leading to a
net-baryon yield of 14 at mid-rapidity.
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Rapidity Loss
Gaussians in pz
6 order polynomial
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dy vs. ybeam
Even (unphysical) extreme approximations dont
change conclusions Linear Increase in dy seems
to saturate at RHIC.
E/B25.7?2.1 GeV 47 lt DE lt 85 GeV
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net-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
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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.

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pp 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
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More data and Model Comparisons

• Do the data for pp, dA and AA constrain models?
• Are there clear evidence for new mechanisms?

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d-Au Phobos
Au-Au

• AuAu proton ratio is (significantly) lower
  than dAu ratios

• All dAu particle ratios appear to be
  independent of centrality

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Model Comparison
dAu

• 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 )

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Baryon 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.

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HIJING/B

• A prediction from 98
• Strong proton stopping as well as enhanced
  strange baryon production.
• Over-predicted actual measurements

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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 ltEgt/Baryon distributions are quite different
  resulting in significant different energy loss.

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• 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 1.37 Blue
HijingBB 2.0 Green rqmd
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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.
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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/ppp gt
p-bar/pAA 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.
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Outlook

• 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 Dy of 8-9.6 in
  AA and pp are crucial for the understanding of
  processes other than quark-diquark breaking.

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Central region at LHC
Asymmetry AB 2 (B anti-B) / (B
anti-B) May allow to distinguish further between
various processes with slow energy / rapidity
dependence
H1 (HERA) ?? 7
in
9.61(8.63) ? ?
at LHC
(B. Kopeliovich)
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Summary

• AA collisions at RHIC show a large rapidity loss
  dy 2.0.
• In contrast the ltEgt is not (yet) as well
  constrained. Several models that describe the
  net-proton distributions have a range of energies
  ltEgt 25-37 GeV/nucleon.
• The finite net-baryon and p-bar/p lt 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 ltEgt 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.