# Presentation: 656714

## Zooming in on the QGP?Heavy-Ion Collisions: an Overview 

- Thomas Ullrich
*Hard Probes 2005*
- June 10, 2006

## Slide 2

## Azimuthal Anisotropy: Elliptic Flow

- dN/d ~ 1+2 v2(pT)cos(2) + ....         = atan(py/px)            v2 = cos2
- v2:  2nd harmonic Fourier coefficient in dN/d with
-       respect to the reaction plane

**EoS** & geometry-driven
- momentum anisotropy

**Density** & geometry-driven
- absorption anisotropy

- R

- Elliptic flow observable sensitive to early evolution of system
- Large v2 is an indication of ***early*** thermalization

## v2: RHIC Measurements

- Rich set of data on v2 at RHIC:
-  h±, ±,0, K±,0, p,p, ,,  , Ω, φ
- Experiments: dN/dy|y=0 → dN/dpT → v2
- v2: Magnitude, pT, centrality and mass dependence,  dependence (h± only)
- v2 stronger than at SPS, AGS
- v2() > v2(K) > v2 (p) ~ v2()

## The Method Matters: v2{EP}, v2{2} versus v2{4} 

- In more central Au+Au collisions the difference between v2{2} and v2{4} increases from 10% at low-pt to about 40-50% at intermediate-pt
- Difference smaller for  > 2
- Huge for Cu+Cu  (reason unknown)

***W. Gang QM’05***

***Phys. Rev. C 72 (2005) 014904***

- v2{EP}:  Assume all correlations between particles due to flow
- v2{2}: Non flow correlation contribute order 1/N
- v2{4}: Non flow correlation contribute order 1/N3

## Ideal hydro overpredicts flow 
init. conditions, viscosity?
Afterburner (Teaney, Hirono, Bass ...)
Adds dissipative hadron phase
“Late” viscosity in hadron phase
Break up thermalization: viscosity effect
Need to fix initial conditions
CGC conditions → need “early” viscosity
BGK/Glauber OK? 

- init. conditions, viscosity?
- Afterburner (Teaney, Hirono, Bass ...)
- Adds dissipative hadron phase
- “Late” viscosity in hadron phase
- Break up thermalization: viscosity effect
- Need to fix initial conditions
- CGC conditions → need “early” viscosity
- BGK/Glauber OK?

- Hydrodynamics: Modeling High-Density Scenarios

**Full 2(3)-d Hydrodynamics**
-  EoS :1st order phase transition
-   QGP + excluded volume model

- Cooper-Frye
- formula

**RQMD, UrQMD, **
**JAM, ...**

*t* fm/c

- final state
- interactions

- Monte Carlo

**Hadronization**

*T**C*

*T**SW*

*The “era” of the afterburner*

## Elliptic & Radial Flow: Non-Viscous Hydro Works

***P. Huovinen***

***P. Huovinen***

- all: full hydro, all compare to v2{EP}

- Au+Au central , √s = 200 GeV

- Hydro             pQCD

- RHIC:    Tfo~ 100 MeV,  T  ~ 0.55 c

***D. d'Enterria, D. Peressounko , EPJ .C46:451-464***

## Hydro Limits: Like a Perfect Liquid?

- Hydro- dynamics without any viscosity describes heavy ion reactions at RHIC (QGP+H EoS)
- Thermalization time  = 0.6 fm/c
- Energy Density: =20 GeV/fm3

- =spatial eccentricity = y2-x2/y2+x2
- S=overlap area

***Hydro: Kolb, Sollfrank, Heinz  PRC62:054909***

- _Issues_
- No consistent set of hydro calculation that describes all observables
- Lattice inspired EoS in ideal hydro does as poorly as a hadron gas EoS
- Before we can make a connection to the EoS using v2(pT,m) much more work needed in theory (test different EoS, viscosity, hadronic phase)

## Another Word of  Caution ...  v4/(v2{4})2

- Incomplete thermalization?
- Need theory input how this would look in microscopic model

- Ratio v4/v22 is sensitive to **degree of thermalization**
***Borghini, Ollitrault nucl-th/0506045:***

- v4/v22 = 0.5 for ideal hydro (more accurate for increasing values of pT)
- Observed integrated ratio larger than unity
- For more peripheral collisions increasing fast as function of transverse momentum

***STAR Preliminary***

## Constraints on Viscosity - What Do We Know?

- s = 4/3sT (sound attenuation length)
- pQCD: s/ = 0.18/(T) ~ 0.18
- (for s = 0.5 ?)
- AdS/CFT: s/ = 1/(3T)  ~  0.11

- What we urgently need is viscous
- hydro but experts tell us this is
- really hard  3 years away (always?)

- One attempt: D. Teaney
- 1st correction to thermal distribution function of an expanding gas
-  estimate viscous corrections to spectra and elliptic flow using boost invariant blast wave model

- Lack of Proof  ≠ Proof  of Lack

## Maybe a “Perfect” Fluid - But Not Ideal

- Maybe a “Perfect” Fluid - But Not Ideal

- What’s going on?
- hard EoS + strongly dissipative evolution
- softer EoS + negligible dissipative evolution (system stays in perfect local kinetic equilibrium – how, why?)
- What’s about 2↔3 processes?

- Same initial conditions for MPC & hydro
- Very opaque system but still dissipative even for gg  50 mb,  ~ 0.1 fm
- Still 30% smaller than ideal hydro (diff. established early ~ 1 fm/c)
- Note:  ~ 3 mb in pQCD (2 ↔ 2)
- Note:  ~ 1/

-  ~ 47 mb  as good as it gets  troublesome ?

## The gg ↔ ggg Puzzle

***Z. Xu and C. Greiner,  PRC 71, 064901 (2005)***
- 3+1 dimensional cascade calculation solving on-shell Boltzmann equations for partons including inelastic gg ↔ ggg pQCD processes

**new development**

- (Z)MPC, VNI/BMS

- Thermal (exp.) spectra after ~1 fm/c

## What’s Going On ?

- Old idea about the transport cross-section is wrong (Xu, HQ 2006)
- This issue addresses the questions on
- Fast thermalization (0 ~ 0.6 fm/c)
-  in parton cascade models to describe v2?
- Is it ***the*** solution or is something wrong ?
- Need to check more signatures (RAA ...)
- Issue needs to be solved!

## It Started 20 Years Ago ...

- _Key Idea:_ Melting in the plasma
- Color screening of static potential between heavy quarks in deconfined matter
- Suppression of states is determined by TC and their binding energy
- Color screening  Deconfinement
- QCD thermometer  Properties of QGP

##    Quarkonia – Lattice QCD  ...

- Recent developments:
- Heavy Quark potential?
- Singlet free energy: F1 (entropy term?)
- Singlet energy: V1
- When do states really melt?
- Neither F1 nor V1 are potentials, they are models!
-  spectral functions (results consistent with V1)
-  J/ melts at 1.5-2.5 TC
- Tdiss(y’)  Tdiss(cc)< Tdiss((3S)) < Tdiss(J/y)  Tdiss((2S)) < Tdiss((1S))

- F. Karsch, RHIC-II Science Workshop

- Is the sequential suppression pattern the smoking gun?

## Just When You Think You Start to Understand ...

- Charmonia correlators:  potentials vs. lattice   ***(Mócsy, Petereczky, hep-ph/0512156)***
- Lattice  reliable calculation of c and c correlators
- Taking F,V as plain (Cornell) potentials  calculate correlators

- Temperature-dependence of c and c lattice correlators is **not** explained with screened Cornell potential.
- Screening likely not responsible for quarkonia suppression
- time scale of screening not small compared to time scale of heavy quark motion
- Suppression from collisions with thermal gluons only?
- dNJ//dt = Ng dis

- Potential model with screening  :
- Á. Mócsy, P.P, hep-ph/0512156

## SPS: NA50 and NA60

-  Normalized to
- Drell-Yan
- Using Glauber &
- nuclear absorption  
- (from pA studies)
- Suppression beyond nuclear absorption
- observed in
- Pb+Pb and In+In
- at √s ~ 17 GeV

## Origin of Suppression at SPS ?

- J/absorption by produced hadrons (comovers)
- Capella and Ferreiro, Eur.Phys.J. C42 (2005) 419
- J/suppression in the QGP and hadronic phases
- including thermal regeneration and in medium properties of open charm and charmonium states
- Grandchamp, Rapp, Brown, NPA715 545; PRL. 92 212301; JPG 30 S1355
- Or is it much simpler?

- F. Karsch, D. Kharzeev, H. Satz, hep-ph/0512239

- Assume:
- NJ/(observed) = 0.6 NJ/ + 0.4 Nc
- 	(compatible w Hera-B data)
- J/ doesn’t melt
- c dissociation = ’ dissociation
- Right or wrong, it shows how important
- the missing c measurement is!

## Charmonium at RHIC: Screening and Regeneration

- General Observations:
- Models with regeneration, i.e., single charm quarks combining in the later stages to form J/’s – match the observed RHIC suppression better!

- Most actual models have suppression + various regeneration mechanisms
- Rapp - PRL 92, 212301 (2004)
- screening & in-medium production
- Thews - PRC73 (2006) 014904c
- pQCD NLO charm + recombination
- Andronic - PLB57, 136 (2003)
- statistical hadronization model with screening of primary J/’s + statistical recombination of thermalized cc’s
- Kostyuk – PRC 68, 041902 (2003)
- statistical coalescence + co-movers or QGP screening
- Bratkovskaya – PRC 69, 054903 (2004)
- hadron-string dynamics transport
- Zhu - PLB607, 107 (2005)
- J/ transport in QGP_ _co-movers, gluon breakup, hydro for QGP evolution no cold nuclear matter, no regeneration

- sum

- regeneration

- screening

***M. Leitch, SQM’06***

## Regeneration  Narrowing of pT and y?

- Recombined only

- Regeneration  Narrowing of pT and y?

- pT broadening lies in between Thews direct & in-medium formation suggesting some regeneration.
- Recombination predicts a narrower rapidity distribution with an increasing Npart.
- Strange: Going from p+p to the most central Au+Au : no significant change seen in the shape of the rapidity distribution.

- No Recombination

- Thews *et al.*

## The Promise of Jet Tomography

- Simplest way to establish the properties of a system
- Calibrated probe
- Calibrated interaction
- Suppression pattern tells about density profile
- Heavy ion collisions
- Hard processes serve as calibrated probe
- Suppression provides density measure

- +

- =

- p+p

- medium

## Calibrated Probe: Hard Interactions

- Hard interactions  pQCD
- Factorization holds
- PDF (initial) x NLO x FF (final)
- Example of probes well reproduced
-  in the forward direction, STAR PRL 92 (2004) 171801
- proton production in p+p,  STAR nucl-ex/0601033
- Direct  production, PHENIX Phys. Rev. D 71 (2005) 071102
- There are however “issues”
- (Strange) baryons, e.g. 
- Charm (at RHIC via D → e + X)
- Significant uncertainties in the calculations, so for precision the baseline needs to be measured
- N.B.: RHIC contributes substantially in improving FF (e.g. AKK)

**p+p/FONLL**

## Application to Heavy Ion Collisions: Initial Results 

- Strong suppression in Au+Au collisions, no suppression in d+Au:
- Effect is due to interactions between the probe and the medium
- Established use as a probe of the density of the medium

***PHENIX: Phys. Rev. Lett. 91 (2003) 072301***
***STAR: Phys. Rev. Lett. 91 (2003) 072304***
***PHOBOS: Phys. Rev. Lett. 91 (2003) 072302***
***BRAHMS: Phys. Rev. Lett. 91 (2003) 072303***

## Interpretation

- Gluon radiation: Multiple final-state gluon radiation off the produced hard parton induced by the traversed dense colored medium

- Formalisms: BDMPS (thick plasma), GLV (thin plasma)
- Mean parton energy loss  medium properties:
-     Eloss ~ gluon  (gluon density)
-     Eloss ~ L2    (medium length)  ~ L with expansion
- Characterization of medium
- transport coefficient
- gluon density dNg/dy
- Deduced initial gluon density at  = 0.2 fm/c:
- dNglue/dy ≈ 800-1200
-  ≈ 15 GeV/fm3  (in static medium)

## Towards the Quantitative: Limitations of RAA

- Central RAA Data

- Increasing density

- Towards the Quantitative: Limitations of RAA

- Surface bias effectively leads to saturation of RAA with density
- Challenge: Increase sensitivity to the density of the medium

***K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, Nucl. Phys. A747 (2005) 511***

***A. Dainese, C. Loizides, G. Paic, Eur. Phys. J. C38(2005) 461 ***

- Distributions of parton production
- points in the transverse plane

## One Way: Increased Precision and Reach

- Successful RHIC runs in 2004 (Au+Au) and 2005 (Cu+Cu)
- Probe systematic with beam energy, system size
- Increased reach from larger datasets:
- e.g. 0 to 20 GeV/c with high precision  still flat

***H. Buesching, HQ06****** ***

## Black and White

- Medium extremely black to hadrons, limiting sensitivity to density
- Medium transparent to photons (white): no sensitivity
- Is there something grey?

***S.S. Adler et al, Phys. Rev. Lett. 94, 232301 (2005)***

## Grey Probes

- Not all models are completely black at the parton level
- Significant differences between predicted RAA, depending on the probe
- Experimental possibility: increase sensitivity to the properties of the medium by varying the probe

***Wicks et al, nucl-th/0512076 ***

## Distinguishing Power Between Gluon and Quark?

- In the NLO calculation that best describes p+p data, significant difference in the gluon contribution between proton and pion spectra
- Yet, no significant difference in suppression: no difference between gluon and quark energy loss?

***Ruan WW06, I. Vitev, nucl-ex/0603010.***

- pT [GeV/c]

***AKK, private communication***

## Recombination present in all systems? 
If the baryon enhancement is from a larger pT kick (or flow) for baryons, why doesn’t B/M increase when the spectrum is steeper? 

- RCP at Lower Energies

- Is the observed baryon/meson anomaly just due to , K suppression?

- No, *at √s=17.3 GeV, baryons enhanced w/o pion suppression*

- Recombination present in all systems?
- If the baryon enhancement is from a larger pT kick (or flow) for baryons, why doesn’t B/M increase when the spectrum is steeper?** **

- Baryon/meson splitting at SPS and RHIC (200 & 62 GeV ) is the same

## Getting Heavy: Heavy Flavor Energy Loss

- In 2001, Dokshitzer and Kharzeev proposed “dead cone” effect  charm quark small energy loss
- Recent: Heavy quark energy loss in medium, e.g.: Armesto et al, PRD 71, 054027, 2005; M. Djordjevic et al., PRL 94, 112301, 2005.
- Heavy quarks will be important to understand the Energy Loss mechanisms and the competition between them

## RAA,v2 of Non-Photonic Electrons

- Charm energy loss via electrons from semileptonic D, B decays
- Experimental challenge: subtraction of photonic electron background
- Evidence of large heavy quark energy loss!
- Substantial elliptic flow for pT < 3 GeV/c

***STAR, QM05***

**w.o. B meson (c flow)**

**w. B meson (c,b flow)**

## 1982 - Initial idea
Bjorken, Fermilab-Pub-82/59-THY
early estimates  < 0.3 GeV/fm

2005 - Collisional loss in expanding QGP for heavy quarks
Mustafa: PRC72, 014905, Mazumder et al., PRD71, 094061
rebirth of collisional energy loss

coll. loss + rad. loss  non-photonic electron (charm) RAA
Wicks et al. nucl-th/0512076 	      finite size effects: 
Yes - Peigne et al., hep-ph/0509185
No – Djordjevic, nucl-th/0603066

- Bjorken, Fermilab-Pub-82/59-THY
- early estimates  < 0.3 GeV/fm
- 2005 - Collisional loss in expanding QGP for heavy quarks
- Mustafa: PRC72, 014905, Mazumder et al., PRD71, 094061
- rebirth of collisional energy loss
- coll. loss + rad. loss  non-photonic electron (charm) RAA
- Wicks et al. nucl-th/0512076 	      finite size effects:
- Yes - Peigne et al., hep-ph/0509185
- No – Djordjevic, nucl-th/0603066

- The Return of the QCD Collisional Energy Loss

- Radiative Energy Loss

- ‘there must be something else ...’

- b

- c

## In BJs collisional loss formula
(adaption of rel. Bethe-Bloch)

What is s in a QGP?

A fixed parameter?
Isn’t it running ?

- Take running coupling into account
- independent of jet energy
- for T > 1.5 TC considerably larger than previous estimates

- In BJs collisional loss formula
- (adaption of rel. Bethe-Bloch)
- What is s in a QGP?
- A fixed parameter?
- Isn’t it running ?

- The Question That Has Always Puzzled Me ...

## Beyond Single Particle Spectra

- Overlap zone has ellipticity
- path length dependence can be probed with dihadron correlations
- Dihadron correlations introduce geometric biases different from RAA
- Surface bias in trigger hadrons: longer path lengths
- Photon-hadron and beauty-hadron correlations have yet different biases
- No surface bias in trigger photons: full path length distribution?

***T. Renk, nucl-ex/0602045***

## Dijets - Dihadrons High Trigger and High Associated pT

- Dijets - Dihadrons High Trigger and High Associated pT

- Clear jet-like peaks seen on near and away side in central Au+Au – width not broadened
- Away-side yield strongly suppressed to level of RAA
- No modification of shape in the longitudinal (zT) or transverse ( width) directions
- Strong set of additional constraints on E-loss models

***STAR, nucl-ex/0604018***

*STAR Preliminary*

- dN/dzT

- Au+Au/d+Au

*STAR Preliminary*

- d+Au

- 1/Ntrig dN/d()

- Au+Au 20-40%

- Au+Au 0-5%

- 8 < pT(trig) < 15 GeV/c

- pT(assoc)>6 GeV

- D(zT)

**1**

**0**

## Where Does the Energy Go?

- Lower the associated pT to search for radiated energy
- Additional energy at low pT BUT no longer collimated into jets
- Active area: additional handles on the properties of the medium?
- Mach Shocks, Cherenkov Cones ...
- e.g. Renk and Ruppert, Phys. Rev. C 73 (2006) 011901

***PHENIX, QM05 and nucl-ex/0507004***

***STAR, Phys. Rev. Lett. 95 (2005) 152301***

## Zooming in on the *QGP
* your favorite plasma goes here s,w,bs,c,...

Soft and Medium Sector – Data Driven
RHIC, SPS: plenty of data – all you can eat
except low energy scan at RHIC (hunt for critical point)
need more theory (manpower)
bounds on viscosity ?
EoS ?
short thermalization time ? 
Hard Sector – Theory Driven ?
need more measurements (running time & upgrades)
heavy flavor sector (Quarkonia, B, D)
-jet
correlation studies (e.g. 3 particle)
theory
energy loss: collisional vs. radiative, what else?
LHC – the clock is ticking ...
will it simplify or complicate our picture ?

- Zooming in on the *QGP
- * your favorite plasma goes here s,w,bs,c,...
- Soft and Medium Sector – Data Driven
- RHIC, SPS: plenty of data – all you can eat
- except low energy scan at RHIC (hunt for critical point)
- need more theory (manpower)
- bounds on viscosity ?
- EoS ?
- short thermalization time ?
- Hard Sector – Theory Driven ?
- need more measurements (running time & upgrades)
- heavy flavor sector (Quarkonia, B, D)
- -jet
- correlation studies (e.g. 3 particle)
- theory
- energy loss: collisional vs. radiative, what else?
- LHC – the clock is ticking ...
- will it simplify or complicate our picture ?

- “*The good news: The sQGP may not be as difficult to understand as many have feared.”*
- Berndt Müller
- I hope this is true ...
- Let this conference be a success and let us a step further in this direction
- Get well soon Miklos !