Download - The Glue that binds us all
The Glue that binds us all
Phases of Matter Town Hall meeting, Jan. 12th, 2007
Probing the nature of gluonic matter with EIC: the world’s first eA collider
Raju Venugopalan
Brookhaven National Laboratory
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Talk Outline:
Outstanding questions in QCD at high energies
Lessons and open questions from HERA and RHIC
How these are addressed by measurements with EIC
The discovery potential of eA at EIC
Summary
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QCD explains ~ 99% of the mass of the visible universe
Quenched QCD (no dynamical quark-antiquark pairs)explains hadron mass spectrum to 10%
hep-lat/0304004
Hadron mass spectrum vs quenched lattice results Quenched QCD full QCD
The dynamics of glue is central to our understanding of the structure of matter
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The DIS Paradigm
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θμμ Measure of
resolution power
Measure of inelasticity
Measure of momentum fraction of struck quark
quark+anti-quarkmom. dists.
gluon mom. dists
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Where is the glue ?
The proton is dominated for x < 0.01 by glue- which grows rapidly…
What happens when the density of gluons becomes large ?
# of partonsper unit rapidity
momentum fraction of hadron
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Mechanism of gluon saturation in QCD
p, A
Large x - bremsstrahlunglinear evolution (DGLAP/BFKL)
Small x -gluon recombinationnon-linear evolution(BK/JIMWLK)
Saturation scale QS(x) - dynamical scale below which non-linear QCD dynamics is dominant
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CGC: Classical effective theory of QCD describingdynamics of gluon fields in non-linear regime
o Novel renormalization group equations (JIMWLK/BK) describe how the QCD dynamics changes with energy
o A universal saturation scale QS arises naturally in the theory
The Color Glass Condensate
In the saturation regime: Strongest fields in nature!
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Saturation scale grows with energy
Typical gluon momenta are large
Bulk of high energy cross-sections:a) obey dynamics of novel non-linear QCD regimeb) Can be computed systematically in weak coupling
Typical gluon kT in hadron/nuclear wave function
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Saturation scale grows with A
High energy compact (1/Q < Rp) probes interact coherently across nuclear size 2 RA - experience large field strengths
Enhancement of QS with A => non-linear QCD regime reached at significantly lower energy in A than in proton
Pocket formula:
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New window on universal properties of the matter in nuclear wavefunctions
ACan we quantify the various regimes ?
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Evidence of non-linear saturation regime at HERA ?“Linear” pQCD describes inclusive observables well-however hints of non-linear (“higher twist”) at small x and Q2
# partons per unit rapidity
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Kowalski et al.,hep-ph/0606272
Also see Forshaw et al.hep-ph/0608161
Saturation Models-excellent fits to HERA data
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Typical sat. scale is rather low...QS
2 << 1 GeV2
Caveat: Saturation scale extracted from HERA data inconsistent with model assumptions
Model assumes
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Evidence of non-linear saturation regime at RHIC ?
Global multiplicity observables in AA described in CGC models:
Kharzeev,Levin,NardiKrasnitz, RV
Au-Au mult. at eta=0
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DA:
Kharzeev,Kovchegov,TuchinAlbacete,Armesto,Salgado,Kovner,Wiedemann
D-Au pt spectra compared toCGC predictionHayashigaki, Dumitru, Jalilian-Marian
Talk by M. Leitch
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A
Estimates of the saturation scale from RHIC
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Outstanding questions in high energy QCD(QCD Theory Workshop, DC, Dec. 15th-16th, 2006)
What is the nature of glue at high density ? How do strong fields appear in hadronic or nuclear wavefunctions at high energies ?
How do they respond to external probes or scattering ?
What are the appropriate degrees of freedom ?
Is this response universal ? (ep,pp,eA, pA, AA)
An Electron Ion Collider (EIC) can provide definitive answers to these questions.
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The Electron Ion Collider
Quantitative QCD studies in largely “terra incognita” small x-large Q2 regime
Variable ep c.m energy up to 100 GeV and high luminosity (~100 times HERA) unpolarized e-p scattering
pol. e-pol. p - highest energies and collider mode for the first time (parallel Town Hall discussion & tomorrow)
First eA collider with wide range of nuclear beams and c.m. energy up to 63 (90) GeV/ nucleon Precision studies of QCD in nuclear media & very high parton densities
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What are the measurements with EIC ?See Thomas Ullrich’s talk
Momentum distributions of gluons and quarks in nuclei
Space-time distributions of quarks and gluons in nuclei Extract space-time dist. of nuclear glue from exclusive final states
Interaction of fast probes with nuclear media
First semi-inclusive measurements: charm and bottom dists. & energy loss in nuclei
Role of color neutral (Pomeron) excitations in scattering off nuclei Semi-hard (M ~ QS
A ) diffractive final states predicted to be > 30 % of cross-section
Gluon dists. measured for x < 0.01 in nuclei for first time
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Strong color fields are vastly more accessible in eA at EIC relative to ep at HERA
Nuclear profile more uniform-study centrality dependence
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The nuclear oomph factor…
Saturation scale significantly enhanced in nuclei
~ 6 enhancement in central Aurelative to min-bias proton
Matchespocket formulato 10%
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EIC can cleanly access cross-over region from weak field to novel strong field QCD dynamics
Weak fieldregime
Q2 >> QS2
Strong fieldregime
Q2 << QS2
Qualitative change in final states: eg.,1/Q6 1/Q2 change in elastic vector meson production!McDermott,Guzey,Frankfurt,Strikman
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p/D-A and AA are complementary probes to eA
Universality:
Soft color exchange between protonand nucleus breaks factorization atorder 1 / Q4
RHIC DA and LHC AA/pA -significant discovery potential Universality => genuine discovery will require complementary probes
Qiu,Sterman
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Summary
EIC with variable energies, nuclear beams and high luminosity is a powerful tool to access and study universal properties of QCD at high parton densities
These studies have profound ramifications for our understanding of QCD dynamics at the LHC-especially in heavy ion collisions
The ability of EIC to distinguish between model predictions for measurements is discussed in thefollowing talk by Thomas Ullrich.
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EXTRA SLIDES
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Inclusive measurements
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==
⎟⎠⎞⎜⎝
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⎟⎠⎞⎜⎝
⎛ ′′=
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θ
θμμ Measure of
resolution power
Measure of inelasticity
Measure of momentum fraction of struck quark
quark+anti-quarkmom. dists.
gluon mom. dists
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Diffractive measurements
Color singlet multi-glue(Pomeron ) exchange
Very sensitive to glue mom. dists.
Extract spatial (impact parameter) dists. of gluon fieldsDeg. of freedom: classsical fields, Pomeron interactions?
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DIS highlights
Bjorken scaling: the parton model.
Scaling violations: QCD- asymptotic freedom, renormalization group; precision tests of pQCD.
Rapid growth of gluon density at small x, significant hard diffraction.
Measurement of polarized structure functions: the “spin crisis”.
QCD in nuclei: EMC effect, shadowing, color transparency,…
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II: Extracting gluon distributions in pA relative to eA
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Direct photons
Open charm
Drell-YanAs many channels…but more convolutions, kinematic constraints-limit precision and range.
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Dramatic breakdown of factorization between ep and pp for diffractive final states
At the Tevatron:
Predictions obtained with HERA diffractive pdfs overestimate CDF
data by a factor of about 10
Alvero,Collins,Terron,Whitmore
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A dependence of saturation scale - estimates from fits to HERA and NMC data
0.33
A dependence
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In pQCD, survival probability ~ 1
Dipole Survival Probability
Data from
Space-time dist. of strong Color Fields!
A 0.3 fm qq dipole survives only 20% of the time scattering off center of the proton!
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Dominant impact parameters in DIS scattering off a proton
b (GeV-1)
Strong color fields are localized here
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Hubble
Hubble is taking beautiful pictures of dark matter binding Galaxies…
Can EIC obtain similar pictures of glue bindingvisible matter ?