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The first two years of RHIC: predictions vs. reality. Summary of the workshop: Who wins the wine, and why? And, by the way, What did we learn from the exercise?. Barbara V. Jacak Stony Brook December 15, 2002. Particle yields and spectra. global quantities hadron distributions.

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The first two years of rhic predictions vs reality

The first two years of RHIC: predictions vs. reality

Summary of the workshop:

Who wins the wine, and why?

And, by the way,

What did we learn from the exercise?

Barbara V. Jacak

Stony Brook

December 15, 2002

Particle yields and spectra

Particle yields and spectra

global quantities

hadron distributions

What do the data say
What do the data say?

G. Roland

dNch/dh = 640

Rises somewhat faster than Npart

Rapidity distribution
Rapidity distribution


dNp/dy ~ 220-230 per charge

dNK+/dy ~ 40

dNp/dy ~ 28

Net baryon density at mid-y

small, but not 0 mB small

Transverse energy
Transverse energy

PHENIX preliminary


~ 0.9 GeV

Similar cent.


as <pt>

But <pt> goes

up with s by

20% while

ET is constant

 particle mix

is changing

PHENIX preliminary

Anti particle particle ratios
Anti-particle/particle ratios

I. Bearden

Ratios similar to those in p+p!

p+p collisions


At mid-rapidity:

Net-protons: dN/dy  7

proton yield: dN/dy  29

 ¾ from pair-production



What model can reproduce the net baryons
What model can reproduce the net baryons?

Net baryon central plateau (y=0 to ~ y=2)

Cannot (yet) differentiate AMPT vs. HIJING/BJ

Ampt cheming ko
AMPT - CheMing Ko

Degree of difficulty = 3.5

  • Ingredients:

    • HIJING, ZPC parton cascade, ART hadronic rescatting

  • ET = 750 GeV at y=0 (50% off  *)

    • data say: 3.3 GeV x (300/2) = 495 GeV

  • 80 baryons at y= 3.9 (data say 34 at y=3.5)

    • at y=0: 14 p, 10 pbar; pbar/p = 0.6 (data say 29, 22, 0.74)

    • (ratio is within 25% of data  ***)

  • dNch/dh ~ 800 (dNch/dh within 25%  ***)

    • 430 p, 60 K per unit y at mid-y (data say 640 ,230, 40)

  • Central plateau |y|<1.5 for mesons

    (pion data says 1.5  *****)

Total score: 3.5 + 10.5 + 10.5 + 17.5= 42

What did we learn
What did we learn?

  • To get proper particle yields must tweak model so it no longer agrees with pp collisions

    • Changed fragmentation function to match lower s data, rationale: fragmentation in dense matter

  • Must add a partonic phase with large scattering cross sections to reproduce v2 and HBT

  • To reproduce K-/K+ need additional hadronic rescattering channels

    • Then get f K+K- correct in s = 130 GeV/A data

Lexus joe kapusta
LEXUS – Joe Kapusta

Degree of difficulty = 2

  • Ingredients: parameterized p+p collision results, Glauber, NN hard collision probability parameter l = 0.6

  • Minimalist approach, which works at SPS

  • Net proton density = 13 (data say 7  *)

  • dNch/dy = 1200, but should have been 950 using p+p at proper s

    • Correcting by 15% for yh, get 1020 or 800

    • (800 is within 25% of data, but –1 for p+p oops  **)

  • Particle spectra are too steep, but missing power law tail

  • proton <pT> ~ 0.925 GeV/c (data say 0.94  *****)

Total score: 2 + 4 + 10 = 16

(so their next model will be a Bentley…?)

What did we learn1
What did we learn?

  • Create more hadrons in LEXUS than in wounded nucleon model, since wounded nucleons are not sterile in LEXUS. Overprediction  some destructive interference among stopped nucleons at mid-y?

  • Total multiplicity is fixed by energy conservation

  • Baryon density fixed by Dy in each collision

  • Minimalist picture works ~ OK for the simplest observables, but not for more complex ones

  • Caution in interpreting scaling with Ncoll or Npart !

Particle spectra @ 200 gev
Particle Spectra @ 200 GeV

BRAHMS: 10% central



STAR: 5%

QM2002 summary slide (Ullrich)

Feed-down matters !!!

P t vs n part
<pT> vs. Npart

J. Velkovska

<pT> [GeV/c]

<pT> [GeV/c]

  • Systematic error on

  • 200 GeV data

  • p (10 %), K (15 %),

  • p (14 %)

open symbol :

130 GeV data

  • Increase of <pT> as a function of Npart and tends to saturate

  • p < K < proton (pbar)

  • Consistent with hydrodynamic expansion picture.

Radial flow
Radial flow

<pT> prediction with Tth

and <b> obtained from

blastwave fit (green line)


<pT> prediction for

Tch = 170 MeV

and <b>=0

pp no rescattering,

no flow

no thermal equilibrium


F. Wang

<pT> of X and W from exponential fits in mT

Do they flow ? Or is <pT> lower due to different fit function?

Does it flow fits to omega m t spectra
Does it flow? Fits to Omega mT spectra

M. van Leeuwen (NA49)

C. Suire (STAR)

STAR preliminary



bT is not well constrained !

  • At SPS  and  are now found to be consistent with common freeze-out

  • Maybe  and  are consistent with a blastwave fit at RHIC

  • Need to constrain further  more data & much more for v2 of 

Urqmd bleicher
UrQMD - Bleicher

Degree of difficulty = 2

  • Ingredients: excitation and fragmentation of color strings, formation and decay of hadronic resonances, hadronic rescattering

  • dET/dh = 600, dNch/dh = 750, ET/Nch = 0.85 GeV

    • Data say 495, 640, 0.9

    • Get ET to 20%, Nch to 17%  *** and ***

  • y=0: 12 net protons, 400 p-, 45 K+

    • Data: 7, 230, 40  *, *, and ****

  • <pT> = 375, 500, 780 for p, K, p

    • Data: 400, 650, 940  ****

    • not enough radial flow!

  • v2 ~ 1% (way too low as the strings don’t collide)

    • Dense set of non-interacting strings… a problem…

Score = 32

We learned that
We learned that

  • Need QGP-type equation of state to get the v2 and radial flow correctly

    • UrQMD has insufficient initial pressure as the strings don’t scatter.

  • Mass shifts of resonances very sensitive to breakup dynamics. Resonances are not dissolved  implies fast freeze-out

Statistical model summary magestro

Predictions (200 GeV)

Exptl. (130 GeV)

Exptl. (200 GeV)



















Statistical model summary - Magestro

Degree of difficulty = 1

  • Johanna: chemical equilibrium with T=170 MeV, mB = 10 MeV

  • Johann: sudden freezeout with incomplete chemical equilibrium

T=177 MeV

mB = 29 MeV


Johanna – within ~15%


Johann - within ~ 40%


Lessons from statistical analyses
Lessons from statistical analyses

  • See chemical equilibrium populations at RHIC as at SPS

    • mB is lower, but not as low as predicted

    • No anomalous strangeness enhancement

  • Simple thermal emission produces proton spectra flatter than pion spectra, so they must cross someplace!

    • Of course the big question is where and why there??

Centrality dependence of v2





Centrality dependence of v2

Note possible dependence on low pt cut

200 GeV: 0.2< pt < 2.0

130 GeV: 0.075< pt < 2.0

200 GeV: 0.150< pt < 2.0

4-part cumulants


200 GeV: Preliminary

- Consistent results

- At 200 GeV better pronounced decrease of v2 for the most peripheral collisions.

QM2002 summary slide (Voloshin)

A puzzle at high p t

Still flowing at pT = 8 GeV/c? Unlikely!!

A puzzle at high pT

Nu Xu

Adler et al., nucl-ex/0206006

v2 of mesons & baryons

Au+Au at sNN=200GeV

1) High quality M.B. data!!!

2) Consistent between


pT < 2 GeV/c

v2(light) > v2(heavy)

pT > 2.5 GeV/c

v2(light) < v2(heavy)

Model: P.Huovinen, et al., Phys. Lett.

B503, 58 (2001)


Hydrodynamics ulrich heinz peter kolb
Hydrodynamics – Ulrich Heinz, Peter Kolb

Predictions of major importance!

  • Ingredients: thermal with some initial conditions, QGP EOS early with transition to resonance gas, geometry + Glauber, hydrodynamics

  • Predictions:

    • Thermalization by 0.6 fm/c at RHIC

    • v2 as function of pion multiplicity density (to fix initial cond.)

    • v2 has a dip (~5%) due to phase transition softening EOS

    • RHIC is near this point (data says v2 ~ 6%)

    • v2 vs. pT increases to 2 GeV/c

    • v2(mesons) > v2 (baryons)

    • spectra (once initial condition is fixed)

  • Lessons: v2 requires early rescattering! Hadronization follows thermalization by 5-7 fm/c. But, final state decoupling needs work (get HBT wrong)

Hydrodynamics teaney shuryak
Hydrodynamics –Teaney & Shuryak

  • Ingredients: hydrodynamics + RQMD for hadronic state and freeze-out

  • Predictions:

    • RHIC should be near softest point in EOS

    • s dependence of v2 correctly predicted for b=6 fm

    • fixed initial conditions, then got spectra correct

    • Predict particle yields without rescaling

    • Initial entropy too high, HBT radii too large!

  • Lessons: hydro good to pT ~ 1.5 GeV/c

    • Viscosity corrections may be important; cause v2 to bend over at 1 GeV/c pT (compared to ideal gas). Also helps reduce HBT radii. Maybe small viscosity early, but increases in hadron gas phase?

Parton transport theory denes molnar
Parton transport theory – Denes Molnar

Degree of difficulty = 5

  • Next step beyond hydro – calculate parton transport, fixing s (i.e. transport opacity c)

  • Predictions & insights:

    • ET loss due to pdV work so (ET)cent < (ET)peripheral

    • ET results require small s (3 mb)

    • can’t easily fix up with inelastic collisions

    • need parton subdivision to avoid numerical “viscosity”

    • Can reproduce v2 if dNgluon/dy very large or sel= 45 mb

    • But large opacity underpredicts HBT spectra!

    • pQCD fixes dNgluon/dy at large pT

    • pQCD fixes parton s at large Q2

    •  Picture doesn’t want to hang together!

Next jets and high p t
Next, jets and high pT

summary from

Thomas Peitzmann,


Charged hadron spectra

Preliminary sNN = 200 GeV

Preliminary sNN = 200 GeV

Charged Hadron Spectra

200 GeV results from all experiments

Shape changes from peripheral  central

C. Roland,


Parallel Saturday

P p at high p t
p/p at high pT

Higher than in p+p

collisions or fragmentation

of gluon jets in e+e-


Vitev & Gyulassy nucl-th/0104066

Can explain by combination of

hydro expansion at low pT with

jet quenching at high pT

Jet quenching gyulassy wang vitev levai

No Shadow, No Quench

No Shadow, dEg/dx=0.5 GeV/fm

GLV “Thin” Plasma Limit

Default: Shadow, dEg/dx=2.0

Jet Quenching – Gyulassy, Wang, Vitev, Levai

Degree of difficulty = 5

  • HIJING: Beam jets @ pt<2 GeV (LUND), pQCD mini jets @ pt>2 GeV (PYTHIA), geometry (Glauber), 1D expansion, conservation laws; tuned to pp data 10-103 GeV

  • + nuclear shadowing and parton energy loss “knobs”

BDMS “Thick” Plasma Limit


? 2003 ?




STAR 130



15% too many particles, baryons over-quenched, but predicted the suppression

BUT: dE/dx =2 GeV/fm or 0.5 GeV/fm or not linear with x?

Vitev they can get v2 right
Vitev: they can get v2 right

  • There is a quantitative difference

  • Calculations/fits with flat

  • or continuously growing

Check against high-pT data (200 AGeV)

b=7 fm

b~7 fm

C. Adler et al. [STAR Collab.],

arXiv: nucl-ex/0206006

Same for 0-50%

  • The decrease with pT is now

  • supported by data

  • For minimum bias this rate is

  • slightly slower

K. Filimonov [STAR Collab.],

arXiv: nucl-ex/0210027

See: N.Borghini, P.Dinh, J-Y.Ollitrault,

Phys.Rev. C 64 (2001)

Other penetrating probes
Other penetrating probes

  • Open Charm

  • J/Y

  • Dileptons

Need (a lot) more statistics in the data

But getting a first sniff of physics already



Data consistent with:

Hadronic comover breakup (Ramona Vogt) w/o QGP

Limiting suppression via surface emission (C.Y. Wong)

Dissociation + thermal regeneration (R. Rapp)

Open charm lin
Open charm - Lin

about x2 within predicted curves

with or w/o

energy loss

no x4 suppression

from peripheral to central,

as predicted for


But -

Is 40-70% peripheral enough? error bars still big!

Some old things and some new things
Some old things and some new things

  • HBT

  • High pT baryons

  • Dijets vs. monojets

    • Well, there was a prediction but for 10x the pT

  • Parton saturation

Hbt lots of questions
HBT – lots of questions

Panitkin, Pratt

  • How to increase R without increasing Rout/Rside?

    • EOS, initial T and rprofiles (Csőrgó), emissivity?

  • Why entropy looks low?

    • Low entropy implies equilibrated QGP ruled out

Baryons at high p t


p0, h

Baryons at high pT

Jia, Sorenson

Yields scale with Ncoll near pT = 2 – 3 GeV/c

Then start to fall

Meaning of Ncoll scaling?

Accident? Complex hard/soft interplay?

Medium modified jet fragmentation function?

Away side jet suppression


not much modification

(the trigger particles from jets!)

Away side:

strong jet suppression

Away-side Jet Suppression

D. Hardtke

  • Strong jet suppression  surface emission of jets?

  • Color glass back-to-back jets simply not created…

Parton saturation
Parton saturation

Dima Kharzeev, Jamal Jalilian-Marian

  • Hadron multiplicities imply a coherent initial state

    • Initial NN interactions are NOT independent!

    • High parton density  weak coupling  CGC

  • Saturation at y=0, and even more so at forward y

    • affects QCD evolution, even at Q2 > Qs2

    • causes multiplicity to scale with Npart, even at high pT

    • hard parton scattering suppressed by CGC  monojets

  • does saturation set in already at s ~ 20GeV? I doubt this!

  • Should measure in forward y in p+A, where Qs is larger and CGC is magnified.

    • This should clarify initial vs. final state effect in AA!


  • Have early pressure buildup – high dNg/dy & they scatter!

    •  success of hydro, need for string melting, large s…

  • High pT, high mass data look like pQCD + something

    • Jet quenching works; surface emission??

    • Baryon flow is a nuclear effect!

    • Color glass is intriguing, but where does the collectivity come from?

  • Event generators (still) a valuable tool to investigate sensitivity of observables to physics ingredients

  • Integrated quantities are simple (conservation laws!)

    • Caution in interpreting scaling with Npart or Ncoll

    • e+e- scaling with Npart is arbitrary, agreement irrelevant

      Experiments: homework to allow quantitative comparisons (multiple 15% factors = sloppy interpretations!)

And the winners are
And the winners are…

  • Best predictions of general features by event generator

    • AMPT (Ko, Lin, Zhang)

  • Novel approach, theoretically intriguing (+ agrees with data)

    • Baryon junctions (Kharzeev, Vance, Gyulassy, Wang)

  • Important prediction with potential great insights to QGP

    • Hydrodynamics (Heinz & Kolb, Teaney & Shuryak, Bass & Dumitru, Ollitrault for “inventing” v2 analysis)

  • Much promise for understanding properties of QGP

    • Jet energy loss (Gyulassy,Wang, Vitev, Levai)

Yield in auau vs p p collisions
yield in AuAu vs. p-p collisions

D. d’Enterria

Yield ratio s=200/130 GeV

Consistent at at high pT with

pQCD predictions (STAR)

PHENIX Preliminary

70-80% Peripheral

Ncoll =12.3 ±4.0

K t dependence of r
kT dependence of R

Centrality is in top 30%

  • Broad <kT> range : 0.2 - 1.2 GeV/c

  • All R parameters decrease as a function of kT

  •  consistent with collective expansion picture.

  • Stronger kT dependent in Rlong have been observed.

kT : average momentum of pair

Comparison of kaon to pion
Comparison of kaon to pion

In the most 30% central

Comparison with hydrodynamic model
Comparison with hydrodynamic model

Centrality is in top 30%

Recent hydrodynamic calculation by U.Heinz and P. F. Kolb


Hydro w/o FS

  • Standard initialization and freeze out which reproduce single particle spectra.

Hydro at ecrit

  • Assuming freeze out directly at the hadronization point. (edec = ecrit)

kT dependence of Rlong indicates the early freeze-out?

Kt dependence of r out r side
kT dependence of Rout/Rside

A. Enikizono


C.M. Kuo, QM2002 poster (PHOBOS) 200 GeV:

@0.25 GeV/c

Hbt puzzle

Small Rout implies small Dt


Small Rbeam implies

small breakup t, ~10 fm/c

Large Rside implies large R

Jet evidence in azimuthal correlations at rhic

near-side correlation of charged tracks (STAR)

trigger particle pT = 4-6 GeV/c

Df distribution for pT > 2 GeV/c

signature of jets

also seen in g (p0) triggered events (PHENIX)

trigger particle pT > 2.5 GeV/c

Df distribution for pT = 2-4 GeV/c

Jet Evidence in Azimuthal Correlations at RHIC

QM2002 summary slide (Peitzmann)

M. Chiu, PHENIX Parallel Saturday

Identifying jets angular correlations

raw differential yields

PHENIX Preliminary

2-4 GeV

Identifying Jets - Angular Correlations

  • Remove soft background

    • by subtraction of mixed event distribution

  • Fit remainder:

    • Jet correlation in f;

    • shape taken from

    • PYTHIA

    • Additional v2 component

    • to correct flow effects

Verify pythia using p p collisions
Verify PYTHIA using p+p collisions

Df (neutral E>2.5 GeV + 1-2 GeV/c charged partner)

Make cuts in  to enhance

near or far-side correlations




In au au collisions
In Au+Au collisions

Df (neutral E>2.5 GeV + charged partner)

1-2 GeV partner

Correlation after mixed event background


Clear jet signal in Au + Au

Different away side effect than in p+p



1/Ntrig dN/d

1/Ntrig dN/d

Jets or flow correlations fit pythia 2v 2 v j cos 2
jets or flow correlations? fit pythia + 2v2vjcos(2)

1-2 GeV/c

partner = .3-.6 GeV

.6-1.0 GeV/c

2-4 GeV/c

1/Ntrig dN/d



Jet strength

See non-zero jet strength as partner pT increases!

How do protons scale with n coll n part
How do protons scale with Ncoll/Npart?

Scale with Ncoll (unlike p)?!

High p t baryons scale with n coll
High pT baryons scale with Ncoll!

J. Velkovska

Low pT near Npart scaling

But baryons with pT > 2 GeV/c

behave very differently!

From jets? Unsuppressed??

Homework assignment phenix star
Homework assignment (PHENIX & STAR)

Charged larger than p0

But difference not same as for RAA

PHENIX and STAR RAA not the same

Different reference in each case!

Systematic difference between experiments

Charged hadron correlations small df



Correlation width

Charged hadron correlations - small Df

Correlation width  jT/pT

  • Fit charged correlations with v2 + Gaussian (fixed pT)

  • Jet signal visible via s

    • Width of near-side Gaussian decreases with pT

    • No significant centrality dependence on near-side

How do high p t yields scale
How do high pT yields scale?

  • vs. binary collisions:

    • continuous decrease as function of centrality

    • factor ~ 3.5 from peripheral to central

  • vs. participants:

    • first increase, then decrease as function of centrality

    • for Npart > 100 have 3s change (scaling or no?)

    • surface emission?

    • re-interactions?

    • accident?

18% scaling uncertainty from corrections

Dn dy

PHENIX Preliminary

PHENIX Preliminary

Au+Au at sqrt(sNN) =200GeV

Au+Au at sqrt(sNN) =200GeV



dN/dy / (0.5 Npart)




open symbol :

130 GeV data






  • Similar centrality dependence 130 GeV and 200 GeV

Opaque expanding source would mean
Opaque, expanding source would mean:



Rischke RIKEN workshop (2002):

Such strong xt correlations probably

require a lack of boost-invariance...

Energy dependence
Energy Dependence


in Lab in C.M.

Energy density (Bjorken):

  • 2% most central at sNN=200 GeV:

  •   5.5 GeV/fm3

    From AGS, SPS to RHIC:

    Transverse energy and charged particle multiplicity densities per participant consistent with logarithmic behaviour

PHENIX preliminary

PHENIX preliminary

P k p spectra from star
p, K, P spectra from Star

  • High quality data over 9 centrality selections

  • Shape described by

    blast wave fit

K k and p p from ags to rhic
K-/K+ and p/p from AGS to RHIC

I. Bearden (BRAHMS)

Becattini caluclation using

statistical model:

T=170, gs=1 (weak dependency)

vary mB/T  K+/K- andp/p

K- /K+=(p/p)1/4 is

a empirical fit to the data points

K-/K+ driven by ms

~ exp(2ms/T)

p/p driven by mB

~ exp(-2mB/T)

ms = ms (mB) since <S> = 0

QM2002 summary slide (Ullrich)

BUT: Holds for y  0 (BRAHMS y=3)

The k 0 story
The K*0 story

  • K*0/K suppressed in AA versus pp

  • f/K*0 appears enhanced versus pp

STAR Preliminary

pp  uncorrected for trigger bias and vertex finding efficiency

STAR QM Talks: E. Yamamoto and P. Fachini

Centrality dependence of p pi
Centrality dependence of p/pi

  • Ratios reach ~1 for central collisions

  • Peripheral collisions lower, but still above gluon jet ratios at high pT

  • Maybe not so surprising 1)“peripheral” means 60-91.4% of stotal

  • 2) p/pi = 0.3 at ISR



Note pbar p behavior
Note pbar/p behavior

Centrality dependence only for pT > 3 GeV/c

Peripheral collisions have quite a few protons at mid-y