Lessons from RHIC: predictions vs. reality

Summary of the Institute for Nuclear Theory workshop on the first two years of RHIC

December 2002not just about strangeness

And who won the wine??

Organization of this talk

MANY results and MANY predictions!

I’ll follow the data (being an experimentalist)Start with yields, y distributions, spectra

which were predicted by event generatorsCompare the predictions from the competitionDraw some physics conclusions

Confront, also, a few newer theoretical ideas…

dNch/d = 640

Rises somewhat faster than Npart

Charged particle yields

Rapidity distribution

PHOBOS

dN/dy ~ 220-230 per chargedNK+/dy ~ 40dNp/dy ~ 28Net baryon density at mid-y small, but not 0 B small

Longitudinal dynamics

PHENIX preliminary

PHENIX preliminary

Transverse energy

ET/particle~ 0.9 GeV

Similar cent.dependence as <pt>

But <pt> goesup with s by20% whileET is constant particle mixis changing

STAR

Can models reproduce the net baryons?

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

Cannot (yet) differentiate AMPT vs. HIJING/BJVenus 4.02

Fritiof 7.2RQMD

Fritiof 1.7

Event generator entry #1: AMPT (C.M. Ko, et al.)

Ingredients:HIJING, Zhang’s parton cascade, ART hadronic rescatting

Get dNch/d within 25%, meson & net baryon central plateau but spectra, ET off by 50% & baryon y loss insufficient

NOTE!To get reasonable particle yields must tweak model so it no

longer agrees with pp collisions.Modified fragmentation function to match lower s data,

rationale: fragmentation in dense matter NOT like ppMust add a partonic phase with large scattering cross

sections to reproduce v2 and HBTTo reproduce K-/K+, need additional hadronic

rescattering channels

v2 from AMPT

Ingredients: parameterized p+p collision results + Glauber NN hard collision probability = 0.6 (works OK at SPS)Total multiplicity is fixed by energy conservationBaryon density fixed by y in each collision

Create more hadrons in LEXUS than in wounded nucleon model, since wounded nucleons are not sterile in LEXUS. Some evidence for destructive interference among stopped nucleons at mid-y

Minimalist picture works ~ OK for the simple observables (dN/dy, <pt>) but not for more complex onesenergy conservation Ncoll or Npart scaling of yields?

“Minimalist” event generator entry: LEXUSJ. Kapusta and collaborators

Particle Spectra @ 200 GeV

BRAHMS: 10% centralPHOBOS: 10%PHENIX: 5%STAR: 5%

QM2002 summary slide (T. Ullrich)

<pT> vs. Npart

open symbol : 130 GeV data

•Systematic error on 200 GeV data (10 %), K (15 %), p (14 %)

• <pT> increases with Npart ; tends to saturate • < K < proton (pbar): consistent with radial expansion

<p

T>

[G

eV

/c]

<p

T>

[G

eV

/c]

Next event generator contestant: UrQMD

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

Predict dET/d = 600, dNch/d = 750, ET/Nch = 0.85 GeVData say 495, 640, 0.9 Get ET to within 20%, Nch to 17%

At y=0 expect: 12 net protons, 400 -, 45 K+Data: 7, 230, 40

Predict <pT> = 375, 500, 780 for , K, pData: 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…

Bleicher, et al.

Must have QGP-type equation of state to get the v2 and radial flow correctly!UrQMD has insufficient initial pressure as the strings

don’t scatter. AMPT “fixed” this by letting strings interact.

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

We learned that

Centrality dependence of v2

STAR

v2=0.05

130 GeV: 0.075< pt < 2.0 200 GeV: 0.150< pt < 2.04-part cumulants

200 GeV: 0.2< pt < 2.0

Preliminary

200 GeV: Preliminary

Note possible dependence on low pt cut

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

STARPreliminary

QM2002 summary slide (Voloshin)

Au+Au at sNN=200GeVv2 of mesons & baryons

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

2) Consistent between PHENIX and STAR

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

Ingredients: initial conditions, hydrodynamicsQGP EOS with transition to resonance gas

Predictions:Thermalization in 0.6 fm/c at RHICGet v2(pion multiplicity density) - must fix initial conditionsv2 value ~5% at RHIC due to phase transition softening EOS

(data say v2 ~ 6%)v2 vs. pT increases to 2 GeV/c v2(mesons) > v2 (baryons)spectra come out OK (once initial condition is fixed)

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

Hydrodynamics –Teaney & Shuryak

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

Predictions:RHIC should be near softest point in EOSs dependence of v2 correctly predicted for b=6 fm fixed initial conditions, then got spectra correctPredict particle yields without rescalingInitial entropy too high, HBT radii too large!

Lessons: hydro good to pT ~ 1.5 GeV/cViscosity corrections may be important; cause v2 to

bend over at 1 GeV/c pT (compared to ideal gas). Also helps reduce HBT radii.

Does viscosity increase in hadron gas phase?

HBT – lots of questions

•Data on HBT seem to prefer fast freezeout

•How to increase R without increasing Rout/Rside?

EOS, initial T and r profiles (Csőrgó), emissivity?

Panitkin, Pratt

calculate parton transport, fixing (i.e. transport opacity ) Predictions & insights:

ET loss due to pdV work so (ET)cent < (ET)peripheralthe ET data require small (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 el= 45 mbBut large opacity underpredicts HBT spectra!

And the inputs are not free for the choosing…pQCD fixes dNgluon/dy at large pT

pQCD fixes parton at large Q2 Picture doesn’t want to hang together!

How do the initial conditions come about?Denes Molnar, et al

Jet Quenching – Gyulassy, Wang, Vitev, Levai

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”

gL / OpacityExpansion

GLV “Thin” Plasma Limit

BDMS “Thick” Plasma Limit

No Shadow, No QuenchNo Shadow, dEg/dx=0.5

GeV/fmDefault: Shadow,

dEg/dx=2.0

Baryons at high pT

Meaning of Ncoll scaling?Accident? Complex hard/soft interplay? Quark coalescence? Medium modified jet fragmentation function?

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

Then start to fall

protons

, h

Other penetrating probes

J/ Open Charm Di-jets vs. mono-jets

Need (a lot) more statistics in the dataBut getting a first sniff of physics already

J/

Energy/Momentum

Data consistent with:Hadronic comover breakup (Ramona Vogt) w/o QGPLimiting suppression via surface emission (C.Y. Wong)Dissociation + thermal regeneration (R. Rapp)

Open charm – Ziwei Lin

Data & predictions within factor 2 (with or w/o energy loss)

no x4 suppression seen from periph. to central, as predicted fordE/dx=-0.5GeV/fm

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

Away-side Jet Suppression

trigger-jetnot much modification (the trigger particles from jets!)

Away side:strong jet suppression

jet quenching surface emission of jets?Color glass back-to-back jets simply not created…

D. Hardtke

Parton saturation

Hadron multiplicities imply a coherent initial stateInitial NN interactions are NOT independent!High parton density weak coupling Color Glass

hard parton scattering suppressed Nch scales with Npart, even at high pT ; monojets

saturation alreadyat s ~ 20GeV? I doubt this!

Dima Kharzeev, Jamal Jalilian-Marian

Au

+A

u /

pp

Mini-JetsMeasure forward y in p+A

(Qs larger, CGC is magnified)clarify initial vs. final state effect in AA!

conclusions

Have early pressure buildup – high dNg/dy & they scatter! success of hydro, need for string melting, large …

Freeze-out is fast 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 if right where does the

collectivity (v2, T) 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

So, are we seeing quark gluon plasma?If it looks like a duck, walks like a duck….

BUTSerious conclusion should await

results from the “control” experiment d+Autheoretical description(s) which hangs together

And the winners of the wine …

Best predictions of general features by event generatorAMPT (Ko, Lin, Zhang)

Novel approach, theoretically intriguing (+ agrees with data)Baryon junctions (Kharzeev, Vance, Gyulassy, Wang)

Important prediction with potential great insights to QGPHydrodynamics (Heinz & Kolb, Teaney & Shuryak, Bass &

Dumitru, Ollitrault for teaching us v2 analysis)

Much promise for understanding properties of QGPJet energy loss (Gyulassy,Wang, Vitev, Levai)

The wine is history…

Statistical models

Johanna: chemical equilibrium with T=170 MeV, B = MeV Johann: sudden freezeout with incomplete chemical equilib.

0.58

0.75

0.90

0.66

0.660.890.95

STAR PHENIX

0.021

0.0015

Exptl. (130 GeV)

0.074

0.15

Predictions (200 GeV)

0.19

0.95

0.75Exptl. (200 GeV)

0.076

0.15

eVB MeV

B lower than SPS, but not as low as predictedNo anomalous strangeness enhancement…

Anti-particle/particle ratios vs. y vs. p+p

BRAHMS 200 GeV

Yields at mid-rapidity:Net-protons: dN/dy 7Protons : dN/dy 29 ¾ from pair-production

p+p collisions

chemistry, stopping…

ISRextrapolation

Ratios similar to those in p+p!

Au + Au

Npart/2

Nbinary

PHENIX 130

BRAHMSPRL88(02)

? 2003 ?

STAR 130

hch

is dE/dx =2 GeV/fm or 0.5 GeV/fm or not linear with x?have a definite prediction for d+Au!

Preliminary sNN = 200 GeV

Preliminary sNN = 200 GeV

C. Roland, PHOBOS Parallel Saturday

200 GeV results from all experiments

Shape changes from peripheral central

Charged Hadron Spectra

p/ at high pT

Vitev & Gyulassy nucl-th/0104066

Can explain by combination ofhydro expansion at low pT withjet quenching at high pT

Higher than in p+pcollisions or fragmentationof gluon jets in e+e-collisions

Vitev: they can get v2 right

C. Adler et al. [STAR Collab.], arXiv: nucl-ex/0206006

K. Filimonov [STAR Collab.],arXiv: nucl-ex/0210027

b=7 fmb~7 fm

• There is a quantitative difference Calculations/fits with flat or continuously growing

2 .v const 2 / .ln Tv p

Check against high-pT data (200 AGeV)

Same for 0-50%

• The decrease with pT is now supported by data• For minimum bias this rate is slightly slower

See: N.Borghini, P.Dinh, J-Y.Ollitrault, Phys.Rev. C 64 (2001)

yield in AuAu vs. p-p collisions

70-80% PeripheralNcoll =12.3 ±4.0

PHENIX Preliminary

pp

centralbinarycentral

Yield

NYield /

D. d’Enterria

Yield ratio s=200/130 GeVConsistent at at high pT withpQCD predictions (STAR)

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

In the most 30% central

Comparison with hydrodynamic model

Recent hydrodynamic calculation by U.Heinz and P. F. Kolb(hep-ph/0204061)

kT dependence of Rlong indicates the early freeze-out?

Hydro w/o FS

Hydro at ecrit

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

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

Centrality is in top 30%

kT dependence of Rout/Rside

A. EnikizonoQM2002

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

.)(25.009.016.1 syst @0.25 GeV/c

HBT PUZZLE

P.Kolb

Small Rout implies small

Large Rside implies large RSmall Rbeam impliessmall breakup ~10 fm/c

Jet Evidence in Azimuthal Correlations at RHIC

near-side correlation of charged tracks (STAR)trigger particle pT = 4-6 GeV/c distribution for pT > 2 GeV/c

signature of jets

also seen in (0) triggered events (PHENIX)trigger particle pT > 2.5 GeV/c distribution for pT = 2-4 GeV/c

M. Chiu, PHENIX Parallel Saturday

QM2002 summary slide (Peitzmann)

Identifying Jets - Angular Correlations

Remove soft background by subtraction of mixed event distribution

Fit remainder:Jet correlation in ; shape taken from PYTHIAAdditional v2 component to correct flow effects

PHENIX Preliminaryraw differential yields

2-4 GeV

Verify PYTHIA using p+p collisions

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

||<.35 ||>.35

ake cuts in to enhance near or far-side correlationsBlue = PYTHIA

In Au+Au collisions

1-2 GeV partner

(neutral E>2.5 GeV + charged partner)

||<.35 ||>.35

1/N

trig d

N/d

1/N

trig d

N/d

Correlation after mixed event background subtraction

Clear jet signal in Au + AuDifferent away side effect than in p+p

Jet strengthSee non-zero jet strength as partner pT increases!

jets or flow correlations? fit pythia + 2v2vjcos(2)

partner = .3-.6 GeV .6-1.0 GeV/c 2-4 GeV/c

1/N

trig d

N/d

v2

vj

1-2 GeV/c

How do protons scale with Ncoll/Npart?

Scale with Ncoll (unlike )?!

High pT baryons scale with Ncoll!

Low pT near Npart scaling

But baryons with pT > 2 GeV/cbehave very differently!From jets? Unsuppressed??

J. Velkovska

Charm cross section at RHIC

Centrality dependence of charm

Charged hadron correlations - small

•Fit charged correlations with v2 + Gaussian (fixed pT)•Jet signal visible via

Width of near-side Gaussian decreases with pT

No significant centrality dependence on near-side

Cor

rela

tion

wid

th

jT

pT Correlation width jT/pT

How do high pT yields scale?

vs. binary collisions:continuous decrease as

function of centralityfactor ~ 3.5 from

peripheral to central vs. participants:

first increase, then decrease as function of centrality

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

surface emission? re-interactions?accident?

18% scaling uncertainty from corrections

dN/dy

K+

p

Positive Negative

PHENIX Preliminary PHENIX Preliminary

• Similar centrality dependence 130 GeV and 200 GeV

open symbol : 130 GeV data

Au+Au at sqrt(sNN) =200GeV Au+Au at sqrt(sNN) =200GeV

K-

pbar

dN

/dy

/ (0

.5 N

pa

rt)

Npart Npart

Opaque, expanding source would mean:

2222222 2)()( xtso YXRR

)(outX

)(sideY

29.13

5)(

)(

spheres

shellhalfs

R

R

65.012

5)(

)(

sphereo

shellhalfo

R

R

Opaque Expanding

Rischke RIKEN workshop (2002): Such strong xt correlations probably require a lack of boost-invariance...

Energy Dependence

Assumptions: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

d

dX

dy

dX

d

dX

dy

dX2.1

dy

dE

Rt

2

1

cfm

AfmR

/1

18.1 3/1

PHENIX preliminary

PHENIX preliminary

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

I. Bearden (BRAHMS)

Becattini caluclation usingstatistical model: T=170, s=1 (weak dependency)

vary B/T K+/K- andp/p

K- /K+=(p/p)1/4 is a empirical fit to the data points

KK driven by s

~ exp(2s/T)

p/p driven by B

~ exp(-2B/T)

s = s (B) since <S> = 0BUT: Holds for y 0 (BRAHMS y=3)

QM2002 summary slide (Ullrich)

The K*0 story

K*0/K suppressed in AA versus pp /K*0 appears enhanced versus pp

STAR QM Talks: E. Yamamoto and P. Fachini

STAR Preliminary

pp uncorrected for trigger bias and vertex finding efficiency

min bias 200 GeV Au+ Au

v2 at high pT

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 total

2) p/pi = 0.3 at ISR

Note pbar/p behavior

Centrality dependence only for pT > 3 GeV/c

Peripheral collisions have quite a few protons at mid-y

Adler et al., nucl-ex/0206006

A puzzle at high pT

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

Nu Xu

Radial flow

<pT> prediction with Tth

and <> obtained from blastwave fit (green line)

<pT> prediction for Tch = 170 MeV and <>=0pp no rescattering, no flowno thermal equilibrium

STAR

preliminaryF. Wang

<pT> of and from exponential fits in mT

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

Does it flow? Fits to Omega mT spectra

M. van Leeuwen (NA49) C. Suire (STAR)

SPS/NA49

RHIC

STAR preliminary

T 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