# Presentation: 520272

## QCD Under Extreme Conditions at the Relativistic Heavy Ion Collider

- James Nagle

- UC Berkeley colloquium
- February 4, 2002

## Outline

**Physics goals of Relativistic Heavy Ions**
** ****Color Glass Condensate**
** ****Quark-Gluon Plasma**
**RHIC and the Experiments**
**Run I Results and Run II Preview**

- (1) Initial Conditions

- (3) Hard Processes

- (2) Plasma Properties

## Structure of the Proton

- Momentum transfer** **
**Q****2**** = 0.1 GeV****2**

**Q****2**** = 1.0 GeV****2**

**Q****2**** = 20.0 GeV****2**

- Wavelength  = h/p

- See the whole proton

- See the quark substructure

- See many partons (quarks and gluons)

## Color Glass Condensate

- Put many nucleons into a nucleus and
- Lorentz boost to the infinite momentum frame
- Creates a 2-dimensional sheet of very high density color charges.

- High density of gluons (saturation) allows for the simplification of Quantum Chromodynamics
- Color fields can be described as classical solutions to the Yang-Mills equation

## Breaking the Glass

- 10,000 gluons, quarks, and antiquarks
- are made physical in the laboratory !
- What is the nature of this ensemble of partons?

## Lattice QCD

- The nature of this bath of quarks and gluons cannot be calculated directly with Quantum Chromodynamics (QCD).
- Thus we rely on lattice QCD calculations.
- Lattice QCD predicts a phase transition to a Quark-Gluon Plasma with a large increase in the number of degrees of freedom.

- Phase Transition:
-    **T = 150-200 MeV ~ 10****12**** ****0****F**
**  ******** ****~  0.6-1.8 GeV/fm****3**
- Assumes thermal system.

******/T****4**

**T/T****c**

- (F. Karsch, hep-lat/0106019)

- hadrons quark/gluon

## Deconfinement

_**in vacuum**_
-  linear increase with distance
-     from color charge
-  strong attractive force
-  _**confinement of quarks **_to
-     hadrons baryons (qqq) and
-     mesons (qq)

_**QCD potential:**_

_**in dense and hot matter**_
-  screening of color charges
-  potential vanishes for large
-     distance scales
-  _**deconfinement of quarks !**_

## Phase Diagram of Nuclear Matter

## Where to Study Extreme QCD?

## Relativistic Heavy Ion Collider

- Au + Au collisions at **200** GeV/u
- p + p collisions at 500 GeV
- spin polarized protons
- lots of combinations in between

- Equivalent Collider Energy (GeV/u)

- Luminosity (cm-2 sec1)

- Central Collisions/sec

## Four Major Experiments

## PHENIX People

- Mickey Chiu
- Justin Frantz
- Chun Zhang
- Sotiria Batsouli
- Sean Kelly
- Cheng-Yi Chi
- Brian Cole
- Jamie Nagle
- Bill Zajc

## Two Large Experiments

_**STAR**_
- Hadronic Observables over a Large Acceptance
- Event-by-Event Capabilities
- Solenoidal magnetic field
- Large coverage Time-Projection Chamber
- Silicon Tracking, RICH, EMC, TOF

_**PHENIX**_
- Electrons, Muons, Photons and Hadrons Measurement Capabilities
- Focus on Rare Probes:  J/, high-pT
- Two central spectrometers with tracking and electron/photon PID
- Two forward muon spectrometers

## Two Small Experiments

_**BRAHMS**_
- Hadron PID over broad rapidity acceptance
- Two conventional beam line spectrometers
- Magnets, Tracking Chambers, TOF, RICH

_**PHOBOS**_
- Charged Hadrons in Central Spectrometer
- Nearly 4 coverage multiplicity counters
- Silicon Multiplicity Rings
- Magnetic field, Silicon Strips, TOF

- Ring Counters

- Paddle Trigger Counter

- Spectrometer

- TOF

- Octagon+Vertex

## Run I at RHIC

_**Performance**_**          **_**Au + Au **_**      **_**RHIC Design**_
******s****nn****	****             130 GeV       200 GeV**
**L [cm****-2**** s ****-1**** ]            ~ 2 x 10****25        ****2 x 10****26 **
** ****Interaction rates   ~ 100 Hz      1400 Hz**

- June 2000

- Running in August

## (1) Initial Conditions

-  What is the energy density achieved?
-  How does it compare to the expected phase   		transition value from lattice QCD?
-  What is the initial density of created partons?

## Energy Density

- Bjorken formula for thermalized energy density in
- terms of measured transverse energy ET

- PHENIX:  Central Au Au yields

- Thermalization time ?

## Charged Particle Multiplicity

**PHENIX**
**STAR**
**PHOBOS**
**BRAHMS**

- Agreement between all four RHIC experiments at 130 GeV
- First result from PHOBOS from Run II at 200 GeV
- Particle production rising faster than in pp (p pbar)
- Over 5000 charged particles produced in central collisions at 200 GeV
- _Do we see evidence of gluon saturation in the initial state_?

## Gluon Saturation

- Wavefunction of low x partons overlap and the self-coupling gluons fuse,
- thus saturating the density of gluons in the initial state

- Gluon number density:
- g = Ax GN(x,pT2)/ R2~A1/3

- Test this by varying the A of the colliding nuclei
- One can look for saturation effects at higher x than at HERA in Deep Inelastic Scattering (e+p) due to nuclear size.

- 1 J.P Blaizot, A.H. Mueller, Nucl. Phys. B289, 847 (1987).

## Collision Characterization

- In Run I, we only collided one nuclear species (Gold).
- However, we can vary the collision size by selecting
- different impact parameter events
- 	Different number of participating nucleons

- Participants = 2 x 197 - Spectators

**Spectators**

**Binary collisions**

**Participants**

**Impact Parameter (fm)**

## Experimental Tests

- Number of charged particles increases per participating nucleon as expected from hard processes.
- EKRT model of gluon saturation is ruled out by data, but more realistic Kharzeev model works quite well.

- Npart

*nucl-ex/0105011*

*Phys. Rev. Lett. 86, 3500 (2001)*

**dN****ch****/d********/(0.5N****p****)**

## Other Tests

- Kharzeev & Levin, nucl-th/0108006
- Schaffner-Bielich et al, nucl-th/0108048

- Saturation models can predict the scaling with centrality and energy of the momentum space distributions of _initial partons_.

## Surprising Agreement

- Color Glass Condensate (Saturation) models tell us about the initial density of gluons liberated

- Lots of dynamics are simplified into one parameter to relate the gluons to final state hadrons.

- Charm couples directly to the initial gluon density.  Strong interactions conserve flavor and thus signal is preserved through time evolution !

## Charm Production

- Heavy quark production measurement in heavy ion collisions is experimentally very challenging.

- High transverse momentum single leptons and back-to-back leptons are an excellent signature of charm.

- One must account for contributions:
-   0,  Dalitz
-  conversions
- Remaining signal is then from
-   **open charm** and **open beauty**
-   **Drell-Yan**
-   thermal production
-   new physics

**D****0****      K****-****  ********+**

**Charm    **

**Beauty**

**Drell-Yan   **

******0****,******** Dalitz**

****** ****conversions**

**Simulation of single electrons**

**D****0****      K****-****  e****+ ********e**

**D****0****      K****-****  ********+ **********

## PHENIX Electrons

- PHENIX is designed to find
- 1 electron out of 10,000 pions !
- Electron identification:
- track reconstructed (momentum p)
- ring found in Ring Imaging Cherenkov Counter
-    (velocity threshold)
- cluster found in Electro-magnetic Calorimeter (energy E)

**PHENIX Spectrometer**

## Electron Results

- Charm cross section
- ppcc ~ 380  60  200 b
- and binary collision scaling consistent with our data.

- After subtracting the photonic and light hadron decay background, we see a clear electron signal.

- (2) Plasma Properties

- How can we measure the thermal history?

- ,  e+e-, +

- Kpnd,...

- Real and virtual photons from quark scattering is most sensitive to the early stages.  Run II measurement.

- Hadron ratios represent thermal
- properties when inelastic collisions stop (chemical freeze-out).

- Hydrodynamic flow is sensitive to the entire thermal history, in particular the early high pressure stages.

- Chemical Freeze-Out

- Baryonic Potential B [MeV]

- Chemical Temperature Tch [MeV]

- 0

- 200

- 250

- 150

- 100

- 50

- 0

- 200

- 400

- 600

- 800

- 1000

- 1200

- AGS

*SIS*

- SPS

- RHIC

*hadron gas*

*neutron stars*

*early universe*

- thermal freeze-out

- deconfinement
- chiral restauration

**Lattice QCD**

*atomic nuclei*

- Simple Grand Canonical Ensemble analysis fitting particle ratios to determine B, T

- preliminary

- Particle Ratios

- ?

- Hydrodynamic Flow

- Spatial anisotropy leads to momentum anisotropy in the presence of large re-scattering.

- v2:  2nd harmonic Fourier coefficient

## Large Parton Re-Scattering

- midrapidity : |h| < 1.0

*Peripheral  ****  Central*

- Hadronic re-scattering is insufficient to describe the data.
- Implies strong partonic re-scattering and high initial density
- 	 = 20 GeV/fm3 and  = 0.6 fm/c

- Strong elliptic flow near the hydrodynamic limit

- Hydro calculations: P. Huovinen, P. Kolb and U. Heinz

## Exploding Source

*T**th* = 120+50-25 [MeV]
- <***r*>= 0.52+0.12-0.08 [c]

- 2* /dof *= 30.4/35* *

- -

- K-

**solid : used for fit**

- Simple: T = Tfreeze-out + m  r 2
- where  r  = averaged flow velocity

- Limiting Temperature ?

## (3) Probes of the Plasma

- Quarks and quarkonium states are excellent
- probes of the plasma

- Beams of  hard probes:  colored quarks, J/,....

**Colorless**
**Hadrons**

**Colored**
**QGP**

- All probes must be auto-generated.

## Hard Scattering

- Hard scattering of partons (large Q2) have calculable rates in perturbative QCD.
- The partons manifest themselves as jets, fragmented hadrons in a collimated angular cone.

- Schematic View of Jet Production

- OPAL Event Display

## Ingredients

- Flux of incoming partons (structure functions) from Deep Inelastic Scattering

- In order to calculate the yield of high pT hadrons

- Perturbative QCD

- Fragmentation functions D(z) in order to relate jets to observed hadrons

## Heavy Ion Reactions

- Baier, Dokshitzer, Mueller, Schiff, hep-ph/9907267
- Gyulassy, Levai, Vitev, hep-pl/9907461
- Wang, nucl-th/9812021
- and many more.....

- Partons are expected to lose energy via gluon radiation in traversing a color deconfined quark-gluon plasma.

**q**

**q**

- Experimentally look for a suppression of high pT hadrons from jet fragmentation.

## Neutral Pions

- Significant suppression relative to p-p scaling observed in central collisions

- PHENIX measures 0 with two types of calorimeters

## Energy Loss from 0

- Energy Loss

- Shadowing
- + Cronin

- Scaled pp

- Central data

- X.N. Wang, Phys. Rev. C 61, 064910 (2000)
- (nucl-th/9812021)

********

- Calculation of X.N. Wang includes a particular shadowing parameterization for the structure functions and kT broadening (Cronin).
- Agreement with data implies:
-       **dE/dx > 0.75 GeV/fm ***

## Be Careful

- This is a very exciting result because it is a suppression relative to point-like scaling and is qualitatively in agreement with parton energy loss expectations.
- However, many caveats remain.
-  Cronin Effect and pT broadening
-  Nuclear shadowing of structure functions
-  Hadron ratios in the fragmentation
-  Energy loss in cold nuclear matter
-  Does the parton hadronize inside?
- _Run II data is crucial._

**q**

**q**

## Unidentified Charged Hadrons

## Compare Ratios

- Suppression relative to binary collisions scaling

- 0 shows a larger suppression, surprising since most expect 	     unidentified to be dominated by +/- over kaons and (anti) protons

## Jet Fragmentation?

- Unidentified charged hadrons have a dominant proton/antiproton contribution above pT > 2 GeV !

- This is not the expected jet fragmentation function D(z).

## Hydro + Jets

- Teaney et al.

- Vitev *et al.* predict that “hard” jet fragmentation eventually dominates over hydrodynamics for antiprotons above pT~5 GeV

- Hydrodynamic expansion may boost “soft” physics into what was previously thought to be only “hard” physics pT region.

## Cold Nuclear Matter

- E866 at Fermilab

- x

- *

- +

- -

- HERMES at DESY

- Need to separate shadowing from energy loss, and then
-  -dE/dx = 2.32  0.52  0.5 GeV/fm

- Drell-Yan production in proton-A collisions is sensitive to parton energy loss.

- hep-ex/0109014

- Deep-Inelastic Scattering on Nuclei is sensitive to parton energy loss and hadron formation.

- Modified fragmentation functions
-   -dE/dx ~ 0.3 GeV/fm

- hep-ex/0012049

## Back-to-Back Correlations

- At pT<1.5 GeV, hydrodynamic description is good.
- Above this, hard scattering may dominate which is _uncorrelated_ with impact parameter plane, unless....

- Parton energy loss could be correlated to reaction plane due to different  average path lengths

## Photon Correlations

- PHENIX Preliminary

**p-p**

**40-60%**

**60-92%**

**20-40%**

**0-20%**

- Back-to-back photons from jet fragmentation are sensitive to energy loss and can be measured out to large pT.

- Data from Run I.
- Measurement to high pT from Run II data !

- Thesis analysis of Mickey Chiu

**m’ = p****T1**** + p****T2**

**R = Prob(p****T1****,p****T2****) / Prob(p****T1****)********Prob(p****T2****)**

**q**

**q**

********

****

****

****

****

********

## Run II

- RHIC achieved full energy
- RHIC achieved ~50% of design luminosity in the last two weeks
- For example PHENIX recorded
- 	~ 170 million events
- 	~ 24 b-1
- High statistics for charm.
- Ability to resolve many open issues of parton energy loss.
- Multistrange baryon measures.
- First measurement of quarkonia production.

## Quarkonia

- Exciting new lattice QCD results show that the confining potential between heavy quarks is screened at high temperature.

- This screening should suppress bound states such as J/.
- PHENIX has sufficient data in p-p and Au-Au for a first measurement from Run II.

## Other Physics

- nucl-ex/0108018

- RHIC just completed 5 weeks of polarized proton-proton running.
- Program for measuring the gluon contributions to the nucleon’s spin.
- Also, crucial comparison data for the Au-Au program.

- Photo-production of vector mesons is measured in superperipheral Au-Au interactions.

## Conclusions

- RHIC appears to be creating a hot, dense and expanding state of deconfined QCD matter
- All results so far consistent with this interpretation
-  energy density - exceeds lattice QCD expectations
-  initial conditions - saturated gluon distributions from       				color glass condensate
-  initial state - large parton scattering for hydrodynamic 					expansion
-  final state - rapidly expanding, thermalized state
-  hard probes - parton energy loss from deconfined medium
- Experiments have two orders of magnitude more data in Run II, and with more detector capabilities.
- Polarized proton-proton data taking just finished.
- Exciting year ahead of physics discovery !

## RHIC Spin Potential

- 	Wide range of measurements in many channels to address
- G gluons
- 	Sea quark polarization

## Disappearing Density

- Stage 1:  Initial hard scattering occurs
-                Parton sees flux of other partons from
- 	   the remainder of the two nuclei
- 	   If parton starts at 900, then no net
- 	   energy loss, just kT scatter
-      Stage 2:  Parton has coherent energy loss as
- 	        it traverses the energy dense medium
- 	        created in the collision
- 	        However, the density of the medium
- 	        is dropping rapidly (eg.  ~ time-4/3)
- 	        Thus this parton does not see RAU
- 	        of the highest density deconfined
- 	        medium

## Phase Transition Order