Universal features of QCD dynamics in hadrons & nuclei at high energies Raju Venugopalan DNP (APS/JPS) meeting, Hawaii, October 13, 2009
Jan 14, 2016
Universal features of QCD dynamics in hadrons & nuclei at high energies
Raju Venugopalan
DNP (APS/JPS) meeting, Hawaii, October 13, 2009
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QCD is the “nearly perfect” fundamental theory of the strong interactions (F.Wilczek, hep-ph/9907340)
We are only beginning to explore the high energy, many body dynamics of this theory
What are the right effective degrees of freedom at high energies?
-- gluons & sea quarks, dipoles, pomerons, strong fields?
How do these degrees of freedom interact with each other and with hard probes? -- Multi-Pomeron interactions, Higher Twist effects, Saturation/CGC -- Rapidity Gaps, Energy loss, Multiple Scattering, Color Transparency -- Glasma, Quark Gluon Plasma
What can this teach us about confinement, universal features of the theory (infrared fixed point?) -- hard vs soft Pomerons, Strong Fields, in-medium hadronization
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High Energy QCD
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High Energy QCD-the role of Glue
Self-interacting carriers of the strong force
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are needed to see this picture. Localized energy fluctuation of Gluon Field(D. Leinweber)
Dominate structure of QCD vacuum
MILC Coll.:hep-lat/0304004
Hadron mass spectrum vs quenched lattice results
Quenched QCD full QCD
Nearly all visible matter in the universe is made of Glue
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Measuring Glue: what are our options?
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Gluon jet event in e+e- collisions at LEP
p+A and e+A provide complementary information on role of glue
Need both to test what’s universal and what’s not in QCD processes
Some final states differ dramatically (diffractive/exclusive states)
Nothing matches DIS for precision-vast majority of world data for pdfs (especially for glue) from DIS
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Inclusive DIS
sy
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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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HERA data on inclusive DIS
x= fraction of momentum of hadron carried by parton
PartonDensity
Gluon distribution from scaling violations of F2
Proton is almost entirely glue by x=0.01 for Q2 = 10 GeV2
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HERA data on inclusive DIS
# partons per unit rapidity
For Q2 ≤ 5 GeV2, leading twist (“parton gas”) description problematic. Sign of higher twist (multi-parton correlation) effects?
Recent HERA data on FL (H1: Q2 = 12-90 GeV2 ; ZEUS Q2 = 24-110 GeV2)
EIC can add significantly to world FL data set -- even for protons. Important test of QCD evolution
Golec-Biernat, Stasto, arXiv: 0905.1321
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increasing Q2
But… the phase space density decreases-the proton becomes more dilute
Resolving the hadron in the Bjorken limit of QCD -DGLAP evolution
Important for precision/beyond standard model physics
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- Large x
- Small x
IMF picture:Gluon phase space grows- saturates at occupation # f =
Resolving the hadron in the Regge-Gribov limit of QCD-BFKL evolution
Rest frame picture:Scattering amplitude is unity…
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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 (“higher twist”) QCD dynamics is dominant
Gribov,Levin,RyskinMueller,Qiu
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CGC: Classical effective theory of QCD describingdynamical gluon fields + static color sources 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!
McLerran, RVJalilian-Marian,Kovner,WeigertIancu, Leonidov,McLerran
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Saturation scale grows with energy
Bulk of high energy cross-sections:a) obey dynamics of novel non-linear QCD regimeb) Can be computed systematically in weak coupling
unintegratedgluon dist.from NLL RG evolution
QS(x)
Y=ln(x0/x)0,1,3,9
Exact analogy to physics of “pulled” travelling wave fronts in stat. mech.
Munier,Peschanski
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Evidence from HERA for geometrical scalingGolec-Biernat, Stasto,Kwiecinski
F2 F2D VM, DVCS = Q2 / QS
2 D V
Marquet, Schoeffel hep-ph/0606079
Scaling confirmed by “Quality factor” analysisGelis et al., hep-ph/0610435
Scaling seen for F2D and VM,DVCS for same QS as F2
Recent NLO BK analysis: Albacete, Kovchegov, hep-ph-0704.0612
Recent caveats: Avsar, Gustafson, hep-ph/0702087
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Inclusive DIS in saturation models
q
q P
* z
1-zr
= 0.3; x0 = 3* 10-4
Kowalski, Teaney Machado, hep-ph/0512264
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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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Inclusive diffraction
RapidityGap
Big surprise at HERA:~ 15% of all events are hard diffractive (MX > 3 GeV) events
In rest frame: 50 TeV electron hits proton - in 1/7 events proton remains intact
“Pomeron”
MX
Diffractive structure functions in DIS measure quark and gluon content of Pomeron
Collins
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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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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
Kowalski, Lappi, RV
c ~ 1 in saturation modelfit to HERA extrapolated tonuclei
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Evidence of geometrical scaling in nuclear DISFreund et al., hep-ph/0210139
Nuclear shadowing: Geometrical scaling
Data scale as a function of = Q2 / QS2
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Evidence of non-linear saturation regime @ RHIC ?
Global multiplicity observables in AA described in CGC models:Input is QS(x,A)
Kharzeev,Levin,Nardi600 1200
Krasnitz, RV
PHOBOS central Au+Au mult. vs models
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DA:
Kharzeev,Kovchegov,TuchinAlbacete,Armesto,Salgado,Kovner,WiedemannBlaizot, Gelis, RV
D-Au pt spectra compared toCGC predictionHayashigaki, Dumitru, Jalilian-Marian
Review: Jalilian-Marian, Kovchegov, hep-ph/0505052
Forward pp @ RHIC as wellBoer, Dumitru, PRD 74, 074018 (2006)
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Extrapolation of BK-fit to RHIC LF data to LHCdn/dy|_{y=0} = 1500-2250 in A+A at LHC
Gelis,Stasto, RV, hep-ph/0605087
Natural explanation for limiting fragmentation + deviations in CGC
Jalilian-Marian
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Why Physics at an e+A collider is interesting
Not your grand aunt’s e+A: world’s first such collider, first measurementsof a range of final states… eg. rapidity gaps, jets, impactparameter dependent distributions
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Terra Incognita
Large x and Q2 : Precision study of propagation of colored probes in extended QCD medium. QCD showering and fragmentation in nuclei
Small x: Explore physics of strong, non-linear color fields
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What are the measurements?
Precision inclusive measurements of structure functions
:
: (Inclusive diffraction)
Semi-inclusive measurements of final state distributions
Exclusive final states
Multiple handles: x, Q2, t, MX2 for light and heavy nuclei
Would like answers to: What is the momentum distribution of gluons in matter What is the space-time distribution of gluons in matter How do fast probes interact with the gluonic medium? What is the nature of color neutral exchanges (Pomerons)
Tools/Measurements:
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EIC: 10 GeV + 100 GeV/n - estimate for 10 fb-1
Gluon distribution from FL @ eRHICEskola,Paukkunen,Salgado
Quark and Glue contribution toHadron-hadron inclusiveCross-section
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Inclusive diffraction In pQCD, expect exponential suppression of large gaps -- parametrize data with diffractive structure functions
which obey QCD evolution…not universal
In CGC, diffractive structure functions depend on universal dipole cross-section squared. Diffractive cross-section ~ 25% of total cross-section!
Kowalski,Lappi,Marquet,RV
Interesting pattern of enhancement and suppression-can be tested
Large = small MX
Small = large MX
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Exclusive final states in DIS
Brodsky et al. Frankfurt,Koepf,Strikman
In the dipole model:Kowalski,Motyka,Watt
Extract b dist. of glue In nuclei?
Claim: can extract t dependence in photo-production of J/ down to t = 10-4 GeV2 Caldwell-Kowalski
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Universal gluodynamics & energy dependence of QS
Small x QCD RG eqns. predict (fixed b) QS
approaches universal behavior with increasing energy (Y) for all hadrons and nuclei-can the approach to this behavior be tested ?
A.H. Mueller, hep-ph/0301109
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In eA DIS, 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 productionMcDermott,Guzey,Frankfurt,Strikman; Review: Frankfurt, Strikman, Weiss
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Semi-inclusive DIS
Virtual photon with short coherencelength scatters off quark or gluon (photon-gluon fusion) in medium- jet propagates through medium
Vastly extended reach at EIC relative to HERMES, Jlab, EMC
Precision studies of heavy quark energy loss
Accardi,Dupre,Hafidi,
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What does a heavy ion collision look like ?
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Color Glass
Condensates
Initial
Singularity
Glasma sQGP - perfect fluid
Hadron Gas
t
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What does a heavy ion collision look like ?
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Color Glass
Condensates
Initial
Singularity
Glasma sQGP - perfect fluid
Hadron Gas
t
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Glasma flux tubes from small x dynamics
After: boost invariant Glasma flux tubes of size 1/QS
Krasnitz,Nara, RV; Lappi
Before: transverse E & B“Weizsacker-Williams fields
Lappi,McLerran,NPA 772 (2006)
parallel color E & B fields
Kharzeev, Krasnitz, RV, Phys. Lett. B545 (2002)
generate Chern-Simons topological charge
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Imagining the Glasma through long range rapidity correlations
Causality dictates:
Au+Au 200 GeV, 0 - 30%PHOBOS preliminary
ηΔφΔ dd
Nd
N
1 ch2
trig
< 1 fm for Δy > 4
At LHC can probe color field dynamics for << 1 fm…Very sensitive to gluon correlationsin wave fn.
Theory @ EIC
High EnergyPhysics
(LHC,LHeCCosmic Rays)
Lattice Gauge Theory
CondensedMatter Physics
(B-E Condensates, Spin Glasses Graphene)
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Inclusive Diffraction-IIImpact parameter dipole (CGC) models give -sq. fits ~ 1 to HERA e+p inclusive and diffractive cross-section: H. Kowalski, C. Marquet, T. Lappi and R. Venugopalan,
Phys. Rev. C 78, 045201 (2008).
= parton mom./Pomeron mom. xP= Pomeron mom./ Hadron mom.
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A
Estimates of the saturation scale from RHIC
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The unstable Glasma • Small rapidity dependent quantum fluctuations of the LO Yang-Mills fields grow rapidly as
• E and B fields as large as EL and BL at time
Romatschke, RV:PRL 96 (2006) 062302
Possible mechanism for rapid isotropization Problem: collisions can’t ‘catch up’
Turbulent “thermalization” may lead to “anomalously” low viscosities
Asakawa, Bass, Muller; Dumitru, Nara, Schenke, Strickland
Significant energy loss in Glasma because of synchroton like radiation?
Shuryak, Zahed; Zakharov; Kharzeev
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P and CP violation: Chiral Magnetic Effect
Kharzeev,McLerran,Warringa
L or B
+ External (QED) magnetic field
Chiral magnetic effect
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= STAR Preliminary
Possible experimental signal of charge separation (Voloshin, Quark Matter 2009)
Topological fluctuations-sphaleron transitions in Glasma
NCS = -2 -1 0
1 2
Effect most significant, for transitions at early times
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Semi-inclusive DIS
At small x:
Hadron distributions and multiplicities sensitive to QS(x,A)
Ratio to x = 10-2 proportional to ratio of QS
2(x,A)
Marquet,Xiao,Yuan
Kang,Qiu
Need more quantitative studiesfor EIC kinematics
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Forming a Glasma in the little Bang Glasma (\Glahs-maa\): Noun: non-equilibrium matter between Color Glass Condensate (CGC)& Quark Gluon Plasma (QGP)
Problem: Compute particle production in QCD with strong time dependent sources
Gelis, Lappi, RV; arXiv : 0804.2630, 0807.1306, 0810.4829
Solution: for early times (t 1/QS) -- n-gluon production computed in A+A to all orders in pert. theory to leading log accuracy
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New window on universal properties of the matter in nuclear wavefunctions
A
Can we quantify the various regimes ?
Iancu, RV, hep-ph/0303204
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Strong color fields may be more accessible in eA collisions relative to ep
Nuclear profile more uniform-can study centrality dependence of distributions
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Outline of talk
Whither the “perfect” theory ? - QCD at high energies
QCD coherence at small x => Universality - Saturation in hadrons & nuclei;
the Color Glass Condensate picture
Exploring the structure of high energy nuclei with EIC - entering terra incognita
From Glue to Glasma & QGP - how multi-parton correlations in nuclei generate extreme states of quark-gluon matter