# Presentation: 430520

## Gluon Saturation and the “Color Glass Condensate”  A new experimental direction for “QCD at RHIC”

- Richard Seto
- University of CA, Riverside
- Workshop on pA Physics at RHIC/
- Heavy Ion Physics for the Next Decade
- BNL Oct 29, 200

- R. Seto

- 10/29/00

## A new regime of calculable phenomena in QCD?

- QCD is the right theory of Strong Interactions (Wilczeck)
- Notoriously hard to calculate
- Regimes where QCD simplifies – Calculations can be done
- High Q2
- Well Tested – pQCD
- High Temperature/Baryon Density
-  RHIC and Lower energy Heavy Ion Physics
- Observables often difficult to quantify and/or interpret
- Hopes for a different situation at RHIC!
- Cold high baryon density QCD - nice but not testable in the lab?
**High Gluon Densities at low-x**
- Reliable non-perturbative calculations of experimental observables
- Testable in the laboratory : lepton-A,** pA**

- R. Seto

- 10/29/00

## What is a color glass condensate?                    A layman’s view

- gluons ~ x- ,i.e. there are more  of
-       them as you go to lower x
- violates unitarity
- Idea (many theorists) : Gluons saturate,
-        and the distribution stops growing.
- Recently, a new way to look at this phenomena (McLerran, Venogopolan etc) Idea: at low x there are so many gluons, that the quantum occupation numbers gets so large that the situation looks classical.
- Can use renormalization group methods to do a calculation of this effect. Depends only on a “scale”
-         CGC2 = (1/R2)(dNgluon/dy) ~ 2-D gluon density
- Gluons are bosons (interacting) – a bose condensate!
- Gluons fill up the available states, so putting more gluons in means they have to go into a higher energy state – higher pt
- Higher pt -> smaller transverse size
- Probes of a particular Q2 go blind to these small partons. Fixes up unitarity for a fixed Q2.

- R. Seto

- 10/29/00

## Why the idea is attractive

- Continues the theme of bulk matter, high density/temperature – a “gluon plasma”
- Condensed matter type many body phenomena – condensates
- Gluons (bosons! Interacting bosons) are in a single quantum state
- Gluons form a glass
- Long time scale coming from “frustration”
- Robust calculations in QCD using reliable “renormalization group” methods
- A method used (and trusted) in all branches of physics
- Depends on a single scale
- CGC2 = (1/R2)(dNgluon/dy) ~ 2-D gluon density

- R. Seto

- 10/29/00

## Why a glass?

- Glass – ala condensed matter –
-        A glass is a material with
-  long time scale
- Think of Window glass, which is a liquid –                                                       but it takes years for it to “pour”
- induced by “frustration”
- E.g. Spin glass
- Neighboring red and blue are “happy”
- Neighboring red and red  are “frustrated”
- In Color Condensate we have “relativistic frustration”
- Model Break Nucleus into  Gluon Field, and Source
- “Source” – quarks and gluons at high-x, ***Lorenz time dialated*** ***clock runs slow***
- Gluon field at low-x. Clock runs fast, but motion is governed by “source”,  and a long time scale governs the motion of the gluons. They are “frustrated”

- Work Work
-  Work Work

- W....o...r... k ...  W . .

- Happy
- Hard working
- Gluons

- Lorentz Time
- Dialated Spokesman

- Frustrated
- Gluons

- R. Seto

- 10/29/00

## Nucleus as amplifier of gluon density

- xpG(xp)=A1/3xAG(xA) i.e. gluon density is 7x higher in Gold at high energies
- Assume xG(x)~x- where =0.2 to 0.5
- xp ~ 10-1.5 to 10-4 xA For A=197
- Calculation of saturation region (Golec-Biernat, Wusthoff PRD 59,014017, 1998)
- Need to reach ~10-4 for xp         xA ~ 10-2

**Saturation **
**Region**

- dN/dy ~ R3 ~ A

- proton

- Nucleus

- R. Seto

- 10/29/00

## How do you experimentally see this?

- Look at Gluon Structure Functions at low-x  (M. Brooks)
- At saturation  it should turn over
- Measure in pA
- Direct photons (Paul)
- J/ production
-  problems in interpretation
- Production mechanisms
- Suppression
- Open Charm
- Change of quark structure functions with Q2 – use Drell- Yan as a probe
- Diffractive cross section
- J.C Peng/S. White
- Other things?
- Pt Broadening?
- .....

- R. Seto

- 10/29/00

## pA at RHIC

- Run Various Nuclei to chart out effects
- Need pp to “Normalize”
- pA Needed for heavy ion program
- pAu   assume L=L(pp)xL(AA)
- 100(p) x 100(Au)  or 250(p) x 100(Au)
- As Example – PHENIX
- Would like to measure x1, x2,,Q2
- For 21 Process (Sea quarks) e.g. Drell Yan, xF=x1-x2,, , M2=Sx1x
- For 2 2 Process, e.g. (gluon distribution) Direct Photon, Open Charm, J/
- Need to measure Outgoing Jet -Tough – Perhaps add a jet detector to PHENIX
- For the purpose of this talk – assume present PHENIX, upgrades suggested at end

- Muon Arms
- (10-30)  (12-30)
-  E>2GeV
- Central Arms (70-110 not shown)
- Electrons/Photons

- R. Seto

- 10/29/00

## The Simulation (Pythia)

- Level of simulation work is primitive – only primary processes, perfect detector (I.e. only angle and energy cuts – note: for electron arm, I used full azimuth)
- For 22 Processes, since we only measure one of the outgoing partons, what do we do?
- Use correlation of x2 with y.
- Then compare to a model.
- Note: correlation is not as good for open charm when detecting only the leptons

- Coverage for Muon arms to y~2.5
- x2 ~10-3
- Q2 ~ 5 GeV2

- x2

- yJ/

- R. Seto

- 10/29/00

## Faking up the saturation

- Model a gluon distribution
- For proton use GRV94.
- For Nucleus start with GRV94. For x2<10-2 flatten gluon distribution.
- Note: the Nucleus side was modeled as a neutron
- In the exp’t, compare pp, pd, pA.
- x1, x2 – fraction of nucleon momentum carried by parton
- X2 Refers to Nucleus
**Low x****2**** will be at high y**

- xG(x)

- +y

- R. Seto

- 10/29/00

## Charm

- Detect only leptons
- Require + -, e in event
- Look at ylepton
- 1 Month running
- Plenty of statistics even if
- Saturation effect is less
- Hard cuts needed
- Need Study of Backgrounds

- x2

- ylepton

- pp

- pA

- All leptons from Charm

- Detected leptons

- pp-pA

- Muon
- North

- Muon
- South

- 300K

- R. Seto

- 10/29/00

## J/

- J/

- 250x100
- Look at yJ/
- Turns out going to 250 doesn’t help much unless one extends y coverage of detector

- R. Seto

- 10/29/00

## Improving the situation

- Measure associated jet to get x1 , x2 , Q2
- Very tough. Associated current jet is often at small angles and must me disentangled from the fragmentation jet which heads down the beampipe.
- Improve muon acceptance with a very forward detector located in the tunnel. x2~10-4 for >1
- Large acceptance photon detector in the forward region
- Q. Can we use the D-Y to get the gluon distribution?

- R. Seto

- 10/29/00

## Detecting Jet w/ Direct photon

- For x2=5x10-3
- Require Jet w/ direct  to be > 10 GeV.
- Jet angle < 20 degrees
- Can we reconstruct a 10 GeV Jet?
- Can we separate it from the beam fragments?

- Jet Energy vs Angle

-  Energy vs Angle

- GeV

- GeV

- R. Seto

- 10/29/00

## Direct Photons

-  forward calorimeter (STAR?)
- Reguire E>5GeV
- 100x100
- (note- to get counts/month, multiply by 35)

- Running at 250x100 allows one to get to x2~10-4 if calorimeter coverage to y=5

- 400K

- R. Seto

- 10/29/00

## Conclusions

- pA presents this community with a new vista of QCD research
- More specifically – the realm of high gluon density
- this region has the potential to give experimentalists, firm, experimentally verifiable non-perturbative predictions
- The simplest of these can be tested with the present machine/experiments
- There may be a host of new phenomena (ala condensed matter, many body physics) associated with this regime of QCD, which will become understandable as experiments progress.

- R. Seto

- 10/29/00

## Slide 17

- R. Seto

- 10/29/00

## Slide 18

- R. Seto

- 10/29/00

## Nucleus

- R. Seto

- 10/29/00

## Slide 20

- R. Seto

- 10/29/00