Transcript Lessons from RHIC: predictions vs. reality

Lessons from RHIC:
predictions vs. reality
Summary of the Institute for Nuclear Theory
workshop on the first two years of RHIC
December 2002
not 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 generators
Compare the predictions from the competition
Draw some physics conclusions
Confront, also, a few newer theoretical ideas…
Charged particle yields
dNch/dh = 640
Rises somewhat faster than Npart
Rapidity distribution
Longitudinal dynamics
PHOBOS
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
PHENIX preliminary
ET/particle
~ 0.9 GeV
PHENIX preliminary
Similar cent.
dependence
as <pt>
But <pt> goes
up with s by
20% while
ET is constant
 particle mix
is changing
STAR
Can models reproduce the net baryons?
Fritiof 1.7
Venus 4.02
RQMD
Fritiof 7.2
Net baryon central
plateau (y=0 to ~ y=2)
Cannot (yet) differentiate
AMPT vs. HIJING/BJ
Event generator entry #1:
AMPT (C.M. Ko, et al.)
 Ingredients:
HIJING, Zhang’s parton cascade, ART hadronic rescatting
 Get dNch/dh 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 pp
Must add a partonic phase with large scattering cross
sections to reproduce v2 and HBT
To reproduce K-/K+, f need additional hadronic rescattering
channels
v2 from AMPT
“Minimalist” event generator entry: LEXUS
J. Kapusta and collaborators
 Ingredients: parameterized p+p collision results + Glauber
NN hard collision probability l = 0.6 (works OK at SPS)
Total multiplicity is fixed by energy conservation
Baryon density fixed by Dy 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 ones
energy conservation  Ncoll or Npart scaling of yields?
Particle Spectra @ 200 GeV
BRAHMS: 10% central
PHOBOS: 10%
PHENIX: 5%
STAR: 5%
QM2002 summary slide (T. Ullrich)
<pT> [GeV/c]
<pT> [GeV/c]
<pT> vs. Npart
•Systematic error on
200 GeV data
p (10 %), K (15 %),
p (14 %)
open symbol :
130 GeV data
• <pT> increases with Npart ; tends to saturate
• p < K < proton (pbar): consistent with radial expansion
Next event generator contestant: UrQMD





Bleicher, et al.
Ingredients: excitation and fragmentation of color strings,
formation and decay of hadronic resonances, hadronic
rescattering
Predict dET/dh = 600, dNch/dh = 750, ET/Nch = 0.85 GeV
Data say 495, 640, 0.9
Get ET to within 20%, Nch to 17%
At y=0 expect: 12 net protons, 400 p-, 45 K+
Data: 7, 230, 40
Predict <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…
We learned that
 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
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
STAR
v2=0.05
STAR
Preliminary
200 GeV: Preliminary
- Consistent results
- At 200 GeV better pronounced decrease
of v2 for the most peripheral collisions.
QM2002 summary slide (Voloshin)
Preliminary
v2 of mesons & baryons
Au+Au at sNN=200GeV
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, hydrodynamics
QGP EOS with transition to resonance gas
 Predictions:
Thermalization in 0.6 fm/c at RHIC
Get v2(pion multiplicity density) - must fix initial conditions
v2 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 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.
Does viscosity increase in hadron gas phase?
HBT – lots of questions
Panitkin, Pratt
•Data on HBT seem to
prefer fast freezeout
•How to increase R
without increasing
Rout/Rside?
 EOS, initial T and r
profiles (Csőrgó),
emissivity?
How do the initial conditions come about?
Denes Molnar, et al
 calculate parton transport, fixing s (i.e. transport opacity c)
 Predictions & insights:
ET loss due to pdV work so (ET)cent < (ET)peripheral
the ET data 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!
And the inputs are not free for the choosing…
pQCD fixes dNgluon/dy at large pT
pQCD fixes parton s at large Q2
 Picture doesn’t want to hang together!
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”
GLV “Thin” Plasma Limit
L/ lg Opacity Expansion
BDMS “Thick” Plasma Limit
No Shadow,
No Quench
No Shadow,
dEg/dx=0.5 GeV/fm
Default: Shadow,
dEg/dx=2.0
Baryons at high pT
protons
p0, h
Baryon yields scale with
Ncoll near pT = 2 – 3 GeV/c
Then start to fall
Meaning of Ncoll scaling?
Accident? Complex hard/soft interplay?
Quark coalescence? Medium modified jet fragmentation function?
Other penetrating probes
 J/Y
 Open Charm
 Di-jets vs. mono-jets
Need (a lot) more statistics in the data
But getting a first sniff of physics already
J/Y
Energy/Momentum
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 – Ziwei Lin
Data & predictions
within factor 2
(with or w/o
energy loss)
no x4 suppression
seen from periph.
to central, as
predicted for
dE/dx=-0.5GeV/fm
But Is 40-70% peripheral
enough? error bars
still big!
Away-side Jet Suppression
D. Hardtke
 trigger-jet
not 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…
Parton saturation
Measure forward y in p+A
(Qs larger, CGC is magnified)
clarify initial vs. final state
effect in AA!
Au+Au / pp
Dima Kharzeev, Jamal Jalilian-Marian
 Hadron multiplicities imply a coherent initial state
Initial 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 already
at s ~ 20GeV?
I doubt this!
Mini-Jets
conclusions
 Have early pressure buildup – high dNg/dy & they scatter!




 success of hydro, need for string melting, large s…
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, bT) 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….
 BUT
Serious conclusion should await
results from the “control” experiment d+Au
theoretical description(s) which hangs together
And the winners of the wine …
 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 teaching us v2 analysis)
 Much promise for understanding properties of QGP
Jet energy loss (Gyulassy,Wang, Vitev, Levai)
The wine is history…
Statistical models
 Johanna: chemical equilibrium with T=170 MeV, mB = 10 MeV
 Johann: sudden freezeout with incomplete chemical equilib.
Predictions (200 GeV)
Exptl. (130 GeV)
Exptl. (200 GeV)
0.75
0.076
0.95
0.15
0.66
0.074
0.90
0.15
T=177 MeV
mB = 29 MeV
0.75
STAR
0.58
0.66
0.021
0.19
0.0015
PHENIX
0.89
0.95
mB lower than SPS,
but not as low as
predicted
No anomalous
strangeness
enhancement…
Anti-particle/particle ratios vs. y vs. p+p
Au + Au
chemistry, stopping…
p+p collisions
BRAHMS 200 GeV
Yields at mid-rapidity:
Net-protons: dN/dy  7
Protons
: dN/dy  29
 ¾ from pair-production
Ratios similar to those in p+p!
ISR
extrapolation
Nbinary
PHENIX 130
hch
BRAHMS
PRL88(02)
STAR 130
Npart/2
is dE/dx =2 GeV/fm or 0.5 GeV/fm or not linear with x?
have a definite prediction for d+Au!
? 2003 ?
Charged Hadron Spectra
200 GeV results
from all experiments
Shape changes from peripheral  central
Preliminary sNN = 200
GeV
Preliminary sNN = 200 GeV
C. Roland,
PHOBOS
Parallel Saturday
p/p at high pT
Higher than in p+p
collisions or fragmentation
of gluon jets in e+ecollisions
Vitev & Gyulassy nucl-th/0104066
Can explain by combination of
hydro expansion at low pT with
jet quenching at high pT
Vitev: they can get v2 right
• There is a quantitative difference
Calculations/fits with flat  v2 = const  .
or continuously growing  v2  ln pT / m  .
Check against high-pT data (200 AGeV)
b~7 fm
Same for 0-50%
b=7 fm
C. Adler et al. [STAR Collab.],
arXiv: nucl-ex/0206006
• The decrease with pT is now
K. Filimonov [STAR Collab.],
arXiv: nucl-ex/0210027
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
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
Yield central /  N binary  central
Yield pp
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
Centrality is in top 30%
Recent hydrodynamic calculation
by U.Heinz and P. F. Kolb
(hep-ph/0204061)
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 Rout/Rside
A. Enikizono
QM2002
C.M. Kuo, QM2002 poster (PHOBOS) 200 GeV:
1.16  0.09  0.25( syst .) @0.25 GeV/c
HBT PUZZLE
Small Rout implies small Dt
P.Kolb
Large Rside implies large R
Small Rbeam implies
small breakup t, ~10 fm/c
Jet Evidence in Azimuthal Correlations at RHIC
 near-side correlation of
 also seen in g (p0) triggered
charged tracks (STAR)
events (PHENIX)
trigger particle pT = 4-6
trigger particle pT > 2.5
GeV/c
GeV/c
Df distribution
for summary
pT > 2
Df distribution for pT = 2-4
QM2002
slide (Peitzmann)
GeV/c
GeV/c
 signature of jets

M. Chiu, PHENIX Parallel Saturday
Identifying Jets - Angular Correlations
 Remove soft background
by subtraction of mixed event distribution
 Fit remainder:
Jet correlation in Df;
shape taken from
PYTHIA
Additional v2 component
to correct flow effects
raw differential yields
PHENIX Preliminary 2-4 GeV
Verify PYTHIA using p+p collisions
Df (neutral E>2.5 GeV + 1-2 GeV/c charged partner)
Make cuts in Dh to enhance
near or far-side correlations
Blue = PYTHIA
|Dh|<.35
|Dh|>.35
In Au+Au collisions
Df (neutral E>2.5 GeV + charged partner)
1-2 GeV partner
Correlation after mixed event background
subtraction
Clear jet signal in Au + Au
Different away side effect than in p+p
|Dh|>.35
1/Ntrig dN/dDf
1/Ntrig dN/dDf
|Dh|<.35
jets or flow correlations? fit pythia + 2v2vjcos(2f)
.6-1.0 GeV/c
1-2 GeV/c
2-4 GeV/c
1/Ntrig dN/dDf
partner = .3-.6 GeV
v2vj
Df
Jet strength
See non-zero jet strength as partner pT increases!
How do protons scale with Ncoll/Npart?
Scale with Ncoll (unlike p)?!
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??
Charm cross section at RHIC
Centrality dependence of charm
Charged hadron correlations - small Df
jT
Correlation width  jT/pT
Correlation width
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 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 / (0.5 Npart)
dN/dy
PHENIX Preliminary
Au+Au at sqrt(sNN) =200GeV
PHENIX Preliminary
Au+Au at sqrt(sNN) =200GeV
p+
p+
K+
open symbol :
130 GeV data
K-
p
Positive
pbar
Negative
Npart
Npart
• Similar centrality dependence 130 GeV and 200 GeV
Opaque, expanding source would mean:
R  R = b ( Dt ) + ( X  Y )  2 b s
2
o
2
s
2

2
2
2
Opaque
2
xt
Expanding
Y (side )
X (out ) 
Rischke RIKEN workshop (2002):
Such strong xt correlations probably
require a lack of boost-invariance...
Rs( half  shell)
5
=
= 1.29
( sphere)
Rs
3
Ro( half  shell)
5
=
= 0.65
( sphere)
Ro
12
Energy Dependence
PHENIX preliminary
Assumptions:
in Lab
in C.M.
dX dX

dy
dh
dX
dX
 1.2
dy
dh
Energy density (Bjorken):
1 dEt
= 2
pR t dy
PHENIX preliminary
R = 1.18 fm  A1 / 3
t = 1 fm / c
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
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
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

K*0/K
STAR Preliminary
suppressed in AA versus pp
 f/K*0 appears enhanced versus pp
pp  uncorrected for trigger bias
and vertex finding efficiency
STAR QM Talks: E. Yamamoto and P. Fachini
v2 at high pT
min bias 200 GeV Au+ Au
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 6091.4% of stotal
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
A puzzle at high pT
Adler et al., nucl-ex/0206006
 Still flowing at pT = 8 GeV/c? Unlikely!!
Nu Xu
Radial flow
STAR
preliminary
F. Wang
<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
<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 mT spectra
M. van Leeuwen (NA49)
C. Suire (STAR)
STAR preliminary
RHIC
SPS/NA49
bT is not well constrained !
• At SPS W and X are now found to be consistent with common freeze-out
• Maybe W and X are consistent with a blastwave fit at RHIC
• Need to constrain further  more data & much more for v2 of W