Notes

Outline

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

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

What is a color glass condensate?                     A layman’s view gluons ~ x-d ,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”         LCGC2 = (1/oR2)(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.

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 LCGC2 = (1/oR2)(dNgluon/dy) ~ 2-D gluon density

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”

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-d where d=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        è xA ~ 10-2

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/y 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? …..

pA at RHIC Run Various Nuclei to chart out effects Need pp to “Normalize” pA Needed for heavy ion program pAu   assume L=ÖL(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 2à1 Process (Sea quarks) e.g. Drell Yan, xF=x1-x2,, , M2=Sx1x For 2 à2 Process, e.g. (gluon distribution) Direct Photon, Open Charm, J/y 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

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 2à2 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 Æx2 ~10-3 Q2 ~ 5 GeV2

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 x2 will be at high y

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

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

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 h>1¡ Large acceptance photon detector in the forward region Q. Can we use the D-Y to get the gluon distribution?

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

Direct Photons  forward calorimeter (STAR?) Reguire Ec>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

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.

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