Proving Quark Gluon Plasma via
Baryon Production at RHIC

Tatsuya Chujo
University of Tsukuba
Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

T.ChujoOutline�
1. Introduction
2. Overview of bulk properties at RHIC
3. Systematic study of baryon enhancement
4. What’s the origin of baryon enhancement?
5. Exploring the QCD phase diagram at RHIC
6. Summary

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1. INTRODUCTION�
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Why baryons* ?
(* protons and antiprotons in this talk)�

•Heavier mass than the light mesons,sensitive to the collective phenomena,such as a radial flow.
• Sensitive to the baryo-chemical propertyof the matter.
• Different number of constituent quarks
from that for mesons, test of recombination
models.�STAR: arXiv:0808.2041v1�

PHENIX: PRL 91, 172301 (2003), PRC 69, 034909 (2004)

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A lots of data and publications on
baryons from RHIC experiments; 1(spectra, yields, and jet correlations)�
•BRAHMS–•

Nuclear Stopping in Au+Au Collisions at √sNN = 200 GeV, PRL 93, 102301 (2004).

PHENIX
–
–
–
–
–
–
–
–
–

Scaling Properties of Proton and Antiproton Production in √sNN = 200 GeV Au AuCollisions, PRL 91, 172301 (2003). [TC]
Identified charged particle spectra and yields in Au+Au collisions at √sNN=200 GeV,PRC 69, 034909 (2004). [TC]
Nuclear effects on hadron production in d + Au collisions at √sNN = 200 GeV revealedby comparison with p + p data, PRC 74, 024904 (2006). [(TC)]
Jet structure of baryon excess in Au+Au collisions at √sNN =200 GeV, PRC 71, 051902(R) (2005).
Particle-Species Dependent Modification of Jet-Induced Correlations in Au+Au
Collisions at √sNN =200 GeV, PRL 101, 082301 (2008).
Correlated production of p and pbar in Au+ Au collisions at √sNN = 200 GeV, PLB 649(2007) 359-369.
Au+Au 62.4 GeV (preliminary) [TC], Cu+Cu 200 GeV (preliminary),
to be published
before QM09
Cu+Cu 22.5, 62.4 GeV (preliminary) [TC], p+p 62.4 GeV (preliminary) [TC](hopefully)�
p+p 200 GeV (new data)5* note: not the complete list.�
Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

T. Chujo

A lots of data and publications on
baryons from RHIC experiments; 2(spectra, yields, and jet correlations)�
•PHOBOS–•

Identified hadron transverse momentum spectra in Au+Au collisions at √sNN = 62.4
GeV, PRC 75, 024910 (2007).

STAR
–
–
–
–

Identified Baryon and Meson Distributions at Large Transverse Momenta from Au+Au
Collisions at √sNN = 200 GeV, PRL 97, 152301 (2006).
Energy dependence of π±, p and p-bar transverse momentum spectra for Au+Au
collisions at √sNN = 62.4 and 200 GeV, arXiv:nucl-ex/0703040.
Identified hadron spectra at large transverse momentum in p + p and d + Au collisionsat √sNN = 200 GeV, PLB 637 (2006) 161-169.
Systematic Measurements of Identified Particle Spectra in pp, d+Au and Au+Au
Collisions from STAR, arXiv:0808.2041.

* note: not the complete list.�

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T. ChujoA recent STAR publication(systematic study of PID spectra in p+p (200 GeV), d+Au (200 GeV),
Au+Au (62, 130, 200 GeV), arXiv:0808.2041)�

••π±, K±, p, pbar pT spectra (low pT region only, dE/dx by TPC).
A nice full paper (60 pages)!�

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2. BULK PROPERTIES AT RHIC�
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What are the bulk properties (EOS)?�
•
•

•

•
•
•
•
•
•
•
•
•
•
….

Energy density (ε)
Temperature (T):– critical temperature (Tc), initial temperature (Tini), chemical freeze-out
temperature (Tch), kinetic freeze-out temperature (Tkin)
Chemical potential (µ):– baryon chemical potential (µB), strangeness chemical potential (µs),
strangeness suppression factor (γs)
Collective flow velocity (<βT>)
Pressure gradient (ΔP), particle emission anisotropy (v2)
Particle multiplicity (dN/dy, N)
Transverse energy (dET/dy, ET)
Transverse momentum distribution (Ed3N/dp3)
Particle abundance and ratio
Average transverse momentum (<pT>)
HBT radii (Rout, Rside, Rlong, λ)Velocity of sound (vs)Shear viscosity – entropy ratio (η/s)
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What are the bulk properties (EOS)?�
•
•

•

•
•
•
•
•
•
•
•
•
•
….

Energy density (ε)
Temperature (T):– critical temperature (Tc), initial temperature (Tini), chemical freeze-out
temperature (Tch), kinetic freeze-out temperature (Tkin)
Chemical potential (µ):– baryon chemical potential (µB), strangeness chemical potential (µs),
strangeness suppression factor (γs)
Collective flow velocity (<βT>)
Pressure gradient (ΔP), particle emission anisotropy (v2)
Particle multiplicity (dN/dy, N)
Transverse energy (dET/dy, ET)
Transverse momentum distribution (Ed3N/dp3)
Particle abundance and ratio
Average transverse momentum (<pT>)
HBT radii (Rout, Rside, Rlong, λ)• red: directly measured by pT spectra
Velocity of sound (vs)• pink: indirectly measured by pT spectra �
Shear viscosity – entropy ratio (η/s)

How the bulk properties change as a function of10
systemand beam energy?�
Heavy Ion Café, Univ. of Tokyo centrality,(Dec. 6, 2008)
T. Chujo

Charged particle multiplicity at
RHIC
PHOBOS
Cu+Cu
Preliminary

3-6%, Npart = 100• Same number ofparticipants, ~samenumber of charged
particle density at RHIC.

Au+Au
35-40%, Npart = 99

• Focus at the mid-rapidity
to study the multiplicity
scaling of bulk properties.

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T. ChujoAverage pT vs. Nch
STAR, arXiv:0808.2041�

factor ~2�

factor ~2�

ST: Transverse overlap area [fm2] �<pT> scales with √((dNπ/dy)/S), p+p 200 GeV, Au+Au 62.4, 130, 200 GeV data.
Suggests that the kinetic freeze-out properties in Au+Au collisions are energy independent.
CGC (gluon saturation): small x gluons overlap and recombine, reducing the total numberof gluons and increasing their transverse energy.
Predicts a lower particle multiplicity and larger <pT>.
In CGC, <pT> scales with √((dNπ/dy)/S).
Data is consistent with CGC picture.�

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Antiparticle-to-Particle Ratios vs. Nch
STAR, arXiv:0808.2041�

STAR, arXiv:0808.2041�

π-/π+: Flat and unity.

pbar/p:• A slight decrease with centrality(130, 200 GeV)
• Considerable drop with centrality(62 GeV) indicating that larger baryon
stopping in central collisions.
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PHENIX 62.4 GeV Au+Au
Hum…
pbar/p ratio: seems
decreasing with
Npart in PHENIX
data too.
DNP2004 (TC)�

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T. ChujoBulk properties vs. Nch (1)�Tch, Tkin�
<βT>STAR, arXiv:0808.2041�

STAR, arXiv:0808.2041�

Tch: constant with dNch/dy.
close to the lattice QCD: Tc ~160 MeV.
universality at RHIC energies.
Tkin: decreasing with dNch/dy.
same trend for all systems at RHIC (with dNch/dy)
indicating strong expansion and cooling?
Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

T. Chujo<β>: incleasing with dNch/dy.�

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Bulk properties vs. Nch (2)�
µB,s

γs

STAR, arXiv:0808.2041�

STAR, arXiv:0808.2041�

• µB: finite value, weak centrality
dependence (baryon stopping at
central)• µs: close to zero.

• γs: approaching to unity with dNch/dy.

Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

Strangeness production is strongly suppressed in p+p,
dAu, peripheral Au+Au. In central Au+Au, implying thatstrangeness is as equally equilibrated as light quarks.
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T. ChujoBulk properties vs. Nch (3)�
HBT�

v2/ε

PHENIX

A. Enokizono

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T. ChujoBulk properties vs. Nch (4)�K-/π-�

STAR, arXiv:0808.2041�

“Only” Nch (or initial energy density)
determines the bulk properties at RHIC?
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3. SYSTEMATIC STUDY
OF BARYON
ENHANCEMENTBaryon enhancement at RHIC
PHENIX: PRL 91, 172301 (2003), PRC 69, 034909 (2004),
PRC 74, 024904 (2006)

In Au+Au √sNN = 200 GeV central
collisions:• RCP (or RAA)
• Pions: Strong suppression ofyields above pT ~ 2 GeV/c, due tojet quenching.
• Protons: No suppression atintermediate pT (2-5 GeV/c).
• p/π and pbar/π ratios• Factor ~3 more (anti) protons thanpions at intermediate pT (2-5 GeV/c).
• Strong centrality dependence.

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Systematic study of PID spectra:
Au+Au, Cu+Cu, p+p at √sNN = 22.5, 62.4, 200 GeV

π0 pT spectrain Cu+Cu 22.5 GeV 21
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T. Chujo√s dep. of pbar/π- ratio (central)* No weak decay feed-down correction applied.

p+p 62.4 GeV, set thebaseline for HI data.
PHENIX data agrees
with ISR data.

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T. Chujo√s dep. of pbar/π- ratio (central)

Cu+Cu 22.5 GeV,
pbar/π- ratio in central
agrees with p+p.

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T. Chujo√s dep. of pbar/π- ratio (central)

Cu+Cu 62.4 GeV,
pbar/π- ratio larger
thanthose in p+p and Cu+Cu 22.5 GeV.

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T. Chujo√s dep. of pbar/π- ratio (central)

Cu+Cu 200 GeV,
similar to those in
Cu+Cu 62.4 GeV.

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T. Chujo√s dep. of pbar/π- ratio (central)

Au+Au 62 GeV,
pbar/π- is
unchanged from
Cu+Cu 200 GeV

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T. Chujo√s dep. of pbar/π- ratio (central)

Au+Au 200 GeV,
p-bar/π- isenhanced.

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T. Chujo√s dep. of pbar/π- ratio (peripheral)

Peripheral collisions for all
systems
Conversing to the same line

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Centrality dep. of pbar/π- (22 GeV vs. 62 GeV)

0.3
* No weak decay feed-down correction applied.

• In 22.5 GeV Cu+Cu: weak centrality dependence, pbar/π- ratios are ~0.3-0.4 at pT =2 GeV/c, which is close to the value in p+p.
• In 62.4 GeV Cu+Cu: pbar/π- ratio in central collisions reaches R=~0.6 at pT = 2
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GeV/c, decreasing towards the peripheral events.
Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

T. ChujoComparison with SPS data
T. Schuster, A. Laszlo (NA49)
nucl-ex/0606005(Pb+Pb 17.2 GeV)
(Au+Au 200 GeV)
(Au+Au 200 GeV)

Pb+Pb 17.2 GeV (central)

0.25
0

1.0
pT (GeV/c)

2.0

3.0

SPS Pb+Pb: consistent with Cu+Cu 22.5 GeV pbar/π.
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pbar/π- ratio (central): summary• Increasing as afunction of √s.

• Indicates theonset of baryon
enhancement is in
between 22 GeV
and 62 GeV.

* No weak decay feed-down correction applied.

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T. Chujoπ0 pT spectra in Cu+Cu and p+p at
22.4, 62.4, 200 GeV
PHENIX: PRL 101, 162301 (2008)

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π0 RAA in Cu+Cu: energy dep.
PHENIX: PRL 101, 162301 (2008)• Enhancement at 22 GeV.
• Consistent with no energy loss model.
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T. Chujo33

4. WHAT’S THE ORIGIN OFBARYON ENHANCEMENT?�
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Jet induced baryon enhancement?

Away side

Near side

Away side two peaks (shoulder)

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Sonic shock wave?
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Baryon/Meson effect?
T. ChujoJet-pair distribution for associated M/B�
PHENIX: PRL 101, 082301 (2008)

•

•

•

•

Trigger particle:
charged hadron (2.5
<pT,trig < 4.0 GeV/c)
Associate particle:
meson or baryons(1.0 – 2.0 GeV/c).
Near side:
substantially weaker
for associatedbaryons.
Away side: similar for
associated mesons
and baryons.
“Shoulder” structureappeared. �
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Conditional jet yields for M/B�•

PHENIX: PRL 101, 082301 (2008)

Mesons:
–
–

–
–

•

Baryons:–––––
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T. Chujo

exponential decrease withincreasing pT, assoc.
Yield increase fromperipheral to central, withdifferent slope
Incompatible with invacuum fragmentation
Due to contribution fromcorrelated soft partons,
softening of FF,
recombination, energy loss,
etc… ?
different strongly fromthose for mesons.
Not exponential shape.
Yield (away) > Yield (near).
Much stronger increase
with centrality than thosefor mesons.
Might be due to thecorrelated soft parton
37
recombination.�

Baryon/meson ratios associated withhigh pT hadron trigger (2.5 < pT < 4GeV/c)
PHENIX: PRL 101, 082301 (2008)• Peripheral ratios ~ vacuum fragmentation• In-jet ratios ~ inclusive p/(π+K)
Away-side “shoulder”, and baryon enhancement in single
spectra: might be the common origin.

* Recombination of correlated soft partons induced
via strong parton medium interactions?
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Intermediate pT ridge & Jet(near side only)(from SQM08, O. Barannikova, STAR)
pTtrig > 4.0 GeV/c
2.0 < pTAssoc < pTtrig

Au+Au 0-10%

3 < pt,trig< 4 GeV/cpt,assoc. > 2 GeV/c

p+p / π++π-STAR preliminary•

Jet+ridge yields follow similar
trend in pT for all trigger types(left bottom)•

Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

Au+Au: 2 < pTtrig < 3 GeV/c
Cu+Cu: 3 < pTtrig < 6 GeV/c

Production mechanisms forjet and ridge are different (p/π,Λ/K)
T. Chujo

Figure from “Future Science at theRelativistic heavy ion Collider (Aug. 25, 2006 version)”,by RHIC II Science Working Groups

5. EXPLORING THE QCD
PHASE DIAGRAM AT RHIC
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Excitation functions of freeze-out properties
STAR, arXiv:0808.2041�

Q: Freeze-out properties changes from
AGS to SPS (e.g. √sNN~10 GeV )? �• μB: falls monotonically.
• Tch: rapidly rises at SIS and AGS energy,
saturates at SPS and RHIC energies (a unique Tch~ Tc from lattice QCD).
• Tkin: decoupled at √sNN~10 GeV from Tch. Due tothe strong collective flow, matter is cooled 
prolong period of chemical freeze-out and kinetic
freeze-out.
• <βΤ> : rapid increase from SIS to AGS,
increasing slowly from SPS to RHIC.
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“hone” at SPS is true?
STAR, arXiv:0808.2041�

 K−/π− ratio : steadily
increases with √sNN,while K+/
π+ sharply increases at low
energies.
 A maximum K+/π+ value isreached at about √sNN ≈ 10
GeV.
 K+/K- vs sqrt(s): smooth
decrease (log scale).
 using the functional forms for
K+/K- and K-/π-, then make thefunction for K+/π+.
•

Generates a maximum at 10 GeV(“horn”)..

 More detail energy scan isneeded.

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Yet another onset at RHIC
PHENIX: PRL 101, 162301 (2008)

Emergence of opacity

Cu+Cu

Approach to constant v2and hydrodynamic limit?
Au+Au

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p-bar/π- ratio vs. √sNN

• Increasing as afunction of √s.

• Indicates theonset of baryon
enhancement is in
between 22 GeV
and 62 GeV.

* No weak decay feed-down correction applied.

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Search for QCD Critical Point (QCP)

From C. Nonaka (JPS2008 fall)�
Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

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From C. Nonaka (JPS2008 fall)�

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From C. Nonaka (JPS2008 fall)�

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From C. Nonaka (JPS2008 fall)�

Observable: look at pbar/p vs. pT.�Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

T. Chujo49

6. Summary• Baryons (protons and antiprotons) has a unique role tocharacterize the many bulk properties of matter,hadronization mechanism, and medium response.
• Bulk properties at RHIC (at mid-rapidity) : governed by
the charged particle multiplicity–

Relevant to the CGC gluon saturation picture.

• Systematic study of baryon enhancement:– qualitative difference between 22 GeV and 62.4 GeV on theproperty of baryon enhancement (while freeze-out properties
seems to be already changed at 10 GeV).
– Jet correlation: indicating the jet induced baryon enhancement.

• Towards the understanding of QCD phase diagram and
QCP search.
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Backup Slides

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T. Chujo√sNN dep. of p/π+ ratio (central)• decreasing

as a functionof √s.

* No weak decay feed-down correction applied.

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π0 RAA vs. √sNN

D. d’Enterria, nucl-ex/0504001
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STAR p+p 200 GeV data(xT scaling)�
STAR: PLB 637 (2006) 161-169•

In p+p collisions, xT (=2pT/√s)scaling works for both inclusive
charged hadrons and identified
hadrons (pions, protons, andantiprotons).

•

Invariant cross sections can be
expressed as the following
equation:
E

•

€

•

d 3σ
=
dp 3

1
s

n(x T , s )G(xT )The power “n” = 6.3-6.5
showed a good scaling in p+p
collisions (c.f. PPG023).
Indicates soft and hardtransition by data.
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Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

T. Chujo√sNN dep. RAA for antiprotons (by ISR fit)
Nuclear Modification Factor

RAA (pT ) =yield ( AuAu ) Ncoll
yield ( pp)• Used ISR data at
23 GeV and 63 GeV(Alper. NPB 100,237) for p+p
reference.
• Similar RAA for all
three systems.
* Note:p+p 62.4 GeV p+p data has
been measured by PHENIX,
still working on the trigger bias
and cross section seen in thedetector. Here we use ISR fit
56
to obtain RAA.

* No weak decay feed-down correction applied.

Heavy Ion Café, Univ. of Tokyo (Dec. 6, 2008)

T. Chujo√sNN dep. RAA for charged pions (by ISR fit)
• Used ISR fit (nucl-ex/
0411049, D. d’Enteria)
for p+p
parameterization.
• Moderate suppression
for Au+Au 62.4 GeV.

Error notations:- Error bars: statistical error for HI spectra- Boxes:1) sys. error for p+p reference.
2) sys. error for HI spectra.
- Lines: Ncoll error (1σ.)• Greater than unity for
Cu+Cu 62/22 GeV (pT >2.0 GeV/c).

* No weak decay feed-down correction applied.

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