# Presentation: 153691

##  Quark Gluon Plasma: The hottest plasma in the world

*Barbara Jacak*
*Stony Brook*
*Center of Excellence*
* **Symposium*
*Kyoto University*
*October 1, 2007*

## outline

**Heat nuclei: expect a plasma of quarks and gluons**
**Quantum Chromo Dynamics (QCD) predictions**
**Create and study it in the laboratory**
**expected QGP signatures**
**useful probes of plasma properties**
**p+p is a crucial benchmark – defines the expected**
**Surprise! evidence for strong coupling in QGP**
**it is very opaque (“jet quenching”)**
**it is a (~perfect) thermalized fluid **
**it catches even the heavy quarks**
**collective response to deposited energy**
**“****shines” brightly**

## QCD predicts a phase transition

**QCD predicts a phase transition**

**gluons carry color charge ******** gluons interact among themselves **
**theory is non-abelian **
**curious properties at large distance: **
**confinement of quarks in hadrons**

## Phase diagram of Nuclear Matter

**As with other material, nuclear matter has many phases.**
**Strong interaction ****→ strong coupling unless very dilute or hot**

## non-perturbative QCD – lattice gauge theory

## plasma

**ionized gas which is macroscopically neutral**
**exhibits collective effects**
**interactions among charges of multiple particles**
**spreads charge out into characteristic (Debye) length, ********D**
**multiple particles inside this length**
**	****they screen each other**
**plasma size > ********D**
**weakly or strongly coupled **
**	****depends on density (number of screeners)**
**	****can behave like a liquid or crystal if strongly coupled**
**“****normal” plasmas are electromagnetic (e + ions)**
**quark-gluon plasma interacts via strong interaction**
**color forces rather than EM, exchange g not ******
**non-abelian plasma **

##  probing a heavy ion collision

***  ******we focus on mid-rapidity (y=0)   y=1/2 ln[(E+p******L******)/(E-p******L******)]***
***   ******CM of colliding system          90******°****** in the lab at collider***

## RHIC at Brookhaven National Laboratory

**Collide Au + Au ions for maximum volume**
******s = 200 GeV/nucleon pair, p+p and d+A to compare**

## The Tools

**The Tools**

***STAR***
***specialty: large acceptance***
***measurement of hadrons***

***PHENIX***
***specialty: rare probes, leptons,***
***and photons ***

## events viewed by the 4 experiments

## Expected signatures of quark gluon plasma

**enhanced strange quark production**
**if T ****≥ m****s****, g + g → s + s**
**J/******** suppression**
**color screening breaks up cc pairs**
**phase transition signatures**
**if first order: long particle emission time**
**if second order: critical fluctuations in particles**
**copious thermal radiation**
**chiral symmetry restoration**
**jet quenching**

***NB: These were yes/no questions, subject to complicated S/B. But surprises lurk in the properties of the plasma!***

## a better way:plasma study by radiated  & “probe” particles

**a better way:****plasma study by radiated  & “probe” particles**

**as a function of transverse momentum**
**p****T**** = p sin ********with respect to beam direction)**
**	****90****° is where the action is (max T, ********)**
**	****	****	****p****L**** midway between the two beams!**
**p****T**** < 1.5 GeV/c**
**“****thermal” particles **
**radiated from bulk of the medium**
**“****internal” plasma probes**
**p****T**** > 3 GeV/c**
**jets (hard scattered q or g)**
**heavy quarks, direct photons**
**describe by perturbative QCD**
**produced early→“external” probe**

## 1st: benchmark probes in p+p collisions

**QCD works at RHIC!**
**can use perturbation theory to calculate high p transfer probe production**
**p+p gives baseline prediction for Au+Au**

- 0

****** ****mesons are qq bound states produced in the collisions**

## jet fragments in Au+Au vs. p+p

**jet fragments in Au+Au vs. p+p**

**in central collisions jets are quenched by the plasma**

## (colored) q & g lose energy, photons don’t 

**(colored) q & g lose energy, photons don’t**** **

## jet quenching was expected

**jet quenching was expected**

## Experimenters must ask questions!

**What happens to heavy quarks traversing QGP?**
**Prediction:  much less energy loss **
***large quark mass reduces phase space for radiated gluons***
**Measure via semi-leptonic decays**
**of mesons containing **
**charm or bottom quarks**

## c,b decays via single electron spectrum

**compare data to “cocktail” of hadronic decays**

## surprise #2: heavy quarks DO lose energy!

***BUT – what about e±  from B meson decays??!!***
***collisions also increase gluon & u,d quark Eloss***

**e****±**** from heavy quark decay**

## Plasmas have collective excitations. Do we?

- dN/d ~ 1 + 2 v2(pT) cos (2) + ...
-     “elliptic flow”

- momentum space

## v2 is large & reproduced by hydrodynamics

** ****large pressure ****	****buildup **
** ****anisotropy ******** ****	****happens fast **
** ****fast equilibration!**

**Surprise #3: must use viscosity ~ 0**
**“****perfect” liquid (D. Teaney, PRC68, 2003)**

## Elliptic flow scales with number of quarks

**transverse KE**

***implication: quarks, not hadrons, are the relevant degrees of freedom when the pressure is built up***

## hydrodynamic flow of quarks!

**Surprise 4: HOW can system thermalize in <0.6 fm/c???**
**  ****parton scattering insufficient (with pQCD cross section)**

**long expected, works at RHIC as long as:**
**viscosity per particle (********/S) vanishingly small**
**initial condition: thermalized system in < 0.6 fm/c**
**use QGP equation of state for first few fm/c**
***NB: learning properties by constraining hydrodynamical calculation + data constraint is standard in plasma physics!***

## Strong coupling suggests applying methods of string theory to QGP  

**Strong coupling suggests applying methods of string theory to QGP  **

*slide: R. Granier de Cassagnac*

*Policastro,Son, Starinets*

*Maldacena*

## AdS/CFT also used to calculate energy loss

**Gubser and collaborators**
**Liu, Rajagopal and Wiedemann**
**Teaney and Casalderrey-Solana**
**L. Yaffe and collaborators**
**Surprise #5: the matter is so strongly coupled that string theorists can have fun with it**

***→****** ******large drag force from plasma on heavy quark!***
***     ******NB: this plasma is dominantly made of gluons***

## do the heavy quarks equilibrate??

**This is like putting a rock in a stream**
**and watching if the stream  **
**  ****can drag it along...**
**Rate of equilibration gives**
**	****information on the viscosity**
**	****of the liquid!**

*analogy from J. Nagle*

## mechanism of energy loss?

**nucl-ex/0611018**
***accepted in PRL***

- Radiative energy loss alone:  fails to reproduce v2HF.
- Heavy quark transport model has better agreement with both RAA and v2HF.
- Small relaxation time  or diffusion coefficient *D**HQ* inferred for charm.
- But, agreement with data is not perfect...

## transport: diffusion is related to viscosity

**diffusion = brownian motion of particles**
**	****definition: flux density of particles J = -****D**** grad n**
**integrating over forward hemisphere:**
**	****D = diffusivity = 1/3 <v> ********  **
**	****	****so D = <v>/ 3n******
******D ******** collision time, determines relaxation time**
**	****data say: it is small!**

***particle concentration***

********* ******= mean free path***

**note:  viscosity is ability to transport momentum**
**     ******** ****= 1/3 ******** <v> ********     so D = ********S  ****→**** measure D get ********! **
**heavy quark + Langevin model ****→**** D ~ few times 1/(4********)**
**     ****NB:  ********/S**** ****≥**** 1/(4********from string ****↔****“QCD” correspondence**

## minimum  at phase boundary?

*Csernai, Kapusta & McLerran *
*PRL97, 152303 (2006)*

***quark gluon plasma***

## Color screening? the famous J/ suppression

**J/********bound state of c and cbar quarks)**
***Tests color screening length (************D******) : ***
***	******do bound c + ******c****** survive the medium?***
***    ******or does QGP screening kill them?***

***note: it’s not so clear what to expect***
***other observations: ***
***strong coupling, collective motion, many collisions...***

## screening length: onium spectroscopy

***similar to that at SPS!***
***why??***
***more suppressed at y************ 0***

**screening length: onium spectroscopy**

## what does non-perturbative QCD say?

**Lattice QCD shows heavy**
**qq correlations at **
**T > T****c****, also implying that interactions are not zero**
**Big debate ongoing whether these are resonant states, or “merely” some interactions**
**Color screening – yes!**
**but not fully...**
**Some J/******** may emerge intact**

- Hatsuda, et al.

**J/******** is a mystery at the moment!**

**Others may form in final state if c and cbar find each other **

## so far we have seen

**medium is very opaque to light quarks and gluons**
**to heavy quarks too**
**system (dominated by gluons) thermalizes very quickly **
**collective hydrodynamic behavior**
**vanishingly small viscosity**
**i.e. doesn’t support shear stress**
**large cross sections & strong coupling**
**     ****forget about perturbation theory after 0.6 fm/c(!)**
**J/******** suppression does not follow the energy density**

***what happens to the energy deposited in the plasma?***

## experimental probe of the medium response

**study using hadron *****pairs***
**high p****T**** trigger to tag hard scattering**
**second particle to probe the medium**

## at high momentum, jets punch through

**STAR**

***central***
***collisions***

*Phys.Rev.Lett. 97 (2006) 162301 *

## Surprise #7: particles at lower pT look funny

**1 < p****T,a**** < 2.5 < p****T,t**** <4 GeV/c**

***peripheral***

***central***

## lost energy excites a sound (density) wave?

***FROM DATA PEAK LOCATION***
-  +/-1.23=1.91,4.37 → cs ~ 0.33
- (√0.33 in QGP, 0.2 in hadron gas)

***UNEXPECTED!  IS IT RIGHT?***
**relative excitation of sound and diffusion modes in intense study**
***data ******→****** sound mode very dominant***

## Is the medium hot enough to radiate? 

-  Enhancement for
- 0.2 < mee < 0.8 GeV
-  Also excess ******→******during** **hadron gas phase
-   Agree at 1.2 < m < 3  GeV and J/****** **by coincidence (J/**** scales  as ******0**** **due to scaling as Ncoll + suppression)

**Is the medium hot enough to radiate?**** **

- p+p and Au+Au normalized to ****0 region

- arXiv:0706.3034)

**measure ********* ****→ ****e+e- pairs **

## Low mass dilepton excess at RHIC is large

**yield excess grows faster than N******
**large excess below ******** **
**    ****q+q ****→*************→**** e+e- ?**
**    ****thermal radiation**

## Low-mass:  Comparison with theory 

**R.Rapp, Phys.Rev.C 63 (2001)**
**R.Rapp, nucl/th/0204003**

- low mass enhancement at
- 150 < mee < 750 MeV3.4±0.2(stat.) ±1.3(syst.)±0.7(model)

**Low-mass:  Comparison with theory **

- calculations: min bias Au+Au
- they include:
- QGP thermal radiation
- chiral symmetry restoration

***A. Toia***

## Plasma at RHIC is dense, hot and weird

***the quark gluon plasma is strongly coupled***
**very opaque to fast quarks & gluons, heavy quarks too**
**viscosity is vanishingly small**
**thermalization takes < 0.6 fm/c**
**can apply methods of string theory for QGP properties**
**partons seem to leave wakes in the plasma**
**plasma and/or hadron gas shines brightly**
**	****	****	****MYSTERIES:**
**what is the fate of B quarks in the plasma?**
**how can the system thermalize so fast?**
**interaction mechanism for heavy quarks? **
**what’s going on with J/******** and color screening?**

***hot gluons show surprising features & their spin is weird too! ***

## Energy density of matter

***high***** energy density: **
****** ****> 10****11 ****J/m****3**
**P > 1 Mbar**
**I > 3 X 10****15****W/cm****2**** **
**Fields > 500 Tesla  **

**backup slides**

## are J/’s regenerated late in the collision?

**c + c ****coalesce at freezeout ****→ J/******

- R. Rapp et al.PRL 92, 212301 (2004)
-  R. Thews et al, Eur. Phys. J C43, 97 (2005)
- Yan, Zhuang, Xu, PRL97, 232301 (2006)
-  Bratkovskaya et al., PRC 69, 054903 (2004)
-  A. Andronic et al., NPA789, 334 (2007)

**J/******** is a mystery at the moment!**

## How hot is the medium, anyway?

**at RHIC T****init**** ~ 1.5 – 2 T****c**** (~ 300 MeV)**
**indirect! flow, energy loss constrain initial conditions**
_**will**_** measure T via radiation of ******** & ********* ****→**** e+e-**

**real photons**
**inclusive/hadron decays**

## Spin of the proton: the surprises continue!

**Spin of the proton: the surprises continue!**

- arXiv 0704.3599 (accepted for publication)

****

## must examine lower x

##  baryon puzzle...

**baryons enhanced for 1.5 < p****T**** <  5 GeV/c**

## excess continues to ~ 5 GeV/c

**STAR**

## formation of baryons – coalescence of quarks

## Are back-to-back jets there in d+Au?

**Yes!**
**no medium **
**	****↓**
**     ****no jet quenching**

## At CERN (√s = 17 GeV):

**  ****NA50 and NA60 show suppression in Pb+Pb & In+In**
** ****suppression follows system size**
**  ****Normal nuclear absorption from p+A data: ******** = 4.18±0.35 mb**

**At**** ****CERN (****√s = 17 GeV)****:**

## Is the energy density high enough?

********* *********** ****5.5 *****GeV/fm******3***** (200 GeV Au+Au)**
** ****	*****well above predicted transition!***

- PRL87, 052301 (2001)

- Colliding system expands:

**value is lower limit: **
**longitudinal expansion rate, formation time overestimated**

## 0 suppressed to high pT, direct  at very high pT? 

*********0****** ******suppressed to high p******T******, direct****** ************ ******at very high p******T******? ***

## The acid test for quark scaling

*********meson (bound state of s+sbar) ******  ***
***mass ~ m******proton****** but flows like the mesons***

- PHENIX

## plasma basics – Debye screening

**distance over which the influence of an individual charged particle is felt by the other particles in the plasma**
**charged particles arrange themselves so as to effectively shield any electrostatic fields within a distance of order ********D**
******D**** =   ********0****kT**
**           ****  ****-------**
**	****	****   ****n****e****e****2**
**Debye sphere = sphere with radius ********D**
**number electrons inside Debye sphere is large**
**N****D****= N/V****D****= ********V****D       ****V****D****= 4/3 ******** ********D****3****  **

**1/2**

**n****e ****= number density**
**e = charge**

## plasma frequency and oscillations

**instantaneous disturbance of a plasma ****→**** collective motions**
**plasma wants to restore the original charge neutrality**
**electrons oscillate collectively around the (heavy) ions**
**characterized by natural oscillation frequency **
**	****plasma frequency**
**	****it’s typically high****	**
**restoring force:**
**  ****ion-electron coulomb attraction**
**damping happens via collisions**
**if e-ion collision frequency < electron plasma frequency ********pe****/2******
**  ****then oscillations are only slightly damped**
**a plasma condition: electron collision time large vs. oscillation**

## For all distributions described by temperature T and (baryon) chemical potential : 
		dn ~ e -(E-)/T d3p

**	****	****dn ~ e ****-(E-********)/T**** d****3****p**

**Does experiment indicate thermalization?**

***T******f****** ~ 175 MeV***

## Strange quarks are indeed enhanced over pp

***as expected for a hot equilibrated system!***