Download Inclusive DIS in saturation models

Universal features of QCD dynamics in
hadrons & nuclei at high energies
Raju Venugopalan
DNP (APS/JPS) meeting, Hawaii, October 13, 2009
High Energy QCD
 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-the role of Glue
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Self-interacting carriers of
the strong force
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
Q   q  ( k   k  )
2
2
2
 
Q 2  4 Ee Ee sin 2  e 
2
pq
E
 
y
 1  e cos2  e 
pk
Ee
2
Q2 Q2
x

2 pq sy
quark+anti-quark
mom. dists.
Measure of
resolution
power
Measure of
inelasticity
Measure of
momentum
fraction of
struck quark
gluon mom. dists
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HERA data on inclusive DIS
Parton
Density
Gluon distribution from scaling
violations of F2
Proton is almost entirely glue by
x=0.01 for Q2 = 10 GeV2
x= fraction of momentum of hadron carried by parton
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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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Resolving the hadron in the Bjorken limit of QCD
-DGLAP evolution
increasing Q2
But… the phase space density
decreases-the proton becomes
more dilute
Important for precision/
beyond standard model physics
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Resolving the hadron in the Regge-Gribov limit of QCD
-BFKL evolution
- Large x
IMF picture:
Gluon phase space grows
- saturates at
occupation # f =
- Small x
Rest frame picture:
Scattering amplitude is
unity…
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Mechanism of gluon saturation in QCD
Gribov,Levin,Ryskin
Mueller,Qiu
Large x - bremsstrahlung
linear evolution (DGLAP/BFKL)
Small x -gluon recombination
non-linear evolution
(BK/JIMWLK)
p, A
Saturation scale QS(x) - dynamical scale below
which non-linear (“higher twist”) QCD dynamics
is dominant
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The Color Glass Condensate
McLerran, RV
Jalilian-Marian,Kovner,Weigert
Iancu, Leonidov,McLerran
In the saturation regime:
Strongest fields in nature!
CGC: Classical effective theory of QCD describing
dynamical gluon fields + static color sources in nonlinear 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
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Saturation scale grows with energy
unintegrated
gluon dist.
from NLL
RG evolution
Y=ln(x0/x)0,1,3,9
QS(x)
Bulk of high energy cross-sections:
a) obey dynamics of novel non-linear QCD regime
b) Can be computed systematically in weak coupling
Exact analogy to physics of “pulled” travelling wave
fronts in stat. mech.
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Munier,Peschanski
Evidence from HERA for geometrical scaling
Golec-Biernat, Stasto,Kwiecinski

F2
 = Q2 / QS2
F2D
D
VM, DVCS V
Marquet, Schoeffel hep-ph/0606079
 Scaling seen for F2D and VM,DVCS for same QS as F2
Gelis et al., hep-ph/0610435
 Scaling confirmed by “Quality factor” analysis
 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
*
z
1-z
q
r
q
P
= 0.3; x0 = 3* 10-4
Kowalski, Teaney
Machado, hep-ph/0512264
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Saturation Models-excellent fits to HERA data
Kowalski et al.,
hep-ph/0606272
Also see Forshaw et al.
hep-ph/0608161
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Inclusive diffraction
MX
“Pomeron”
Rapidity
Gap
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
 Diffractive structure functions in DIS measure quark and
gluon content of Pomeron
Collins
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Caveat: Saturation scale extracted from HERA
data inconsistent with model assumptions ?
Model assumes
Typical sat. scale
is rather low...
QS2 << 1 GeV2
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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
c ~ 1 in saturation model
fit to HERA extrapolated to
nuclei Kowalski, Lappi, RV
Enhancement of QS with A => non-linear QCD regime
reached at significantly lower energy in A than in proton
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Evidence of geometrical scaling in nuclear DIS
Freund 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)
PHOBOS central Au+Au mult. vs models
600
Krasnitz, RV
1200
Kharzeev,Levin,Nardi
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DA:
Kharzeev,Kovchegov,Tuchin
Albacete,Armesto,Salgado,Kovner,Wiedemann
Blaizot, Gelis, RV
D-Au pt spectra compared to
CGC prediction
Hayashigaki, Dumitru, Jalilian-Marian
Forward pp @ RHIC as well
Boer, Dumitru, PRD 74, 074018 (2006)
Review: Jalilian-Marian, Kovchegov, hep-ph/0505052
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Natural explanation for limiting fragmentation
+ deviations in CGC
Jalilian-Marian
Extrapolation of BK-fit to RHIC LF data to LHC
dn/dy|_{y=0} = 1500-2250 in A+A at LHC
Gelis,Stasto, RV, hep-ph/0605087
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Why Physics at an e+A collider is interesting
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Not your grand aunt’s
e+A: world’s first such
collider, first measurements
of a range of final states…
eg. rapidity gaps, jets, impact
parameter dependent distributions
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?
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:
 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
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Gluon distribution from FL @ eRHIC
Eskola,Paukkunen,Salgado
EIC: 10 GeV + 100 GeV/n - estimate for 10 fb-1
Quark and
Glue contribution to
Hadron-hadron inclusive
Cross-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!
Large  = small MX
Small  = large MX
Interesting pattern of enhancement and suppression-can be tested
Kowalski,Lappi,Marquet,RV
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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
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to t = 10-4 GeV2
Caldwell-Kowalski
Universal gluodynamics & energy dependence of QS
A.H. Mueller, hep-ph/0301109
Small x QCD RG eqns. predict (fixed b) QS
approaches universal behavior with increasing energy
(Y) for all hadrons and nuclei
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-can the approach to this behavior be tested ?
In eA DIS, cleanly access cross-over region from weak field to
novel strong field QCD dynamics ?
Weak field
regime
Strong field
regime
Q2 >> QS2
Q2 << QS2
Qualitative change in final states: eg.,
1/Q6
1/Q2 change in elastic vector meson production
McDermott,Guzey,Frankfurt,Strikman; Review: Frankfurt, Strikman, Weiss
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Semi-inclusive DIS
Virtual photon with short coherence
length 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
Initial
Condensates
Singularity
Glasma
sQGP perfect fluid
Hadron
Gas 31
t
What does a heavy ion collision look like ?
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Color Glass
Initial
Condensates
Singularity
Glasma
sQGP perfect fluid
Hadron
Gas 32
t
Glasma flux tubes from small x dynamics
Krasnitz,Nara, RV; Lappi
Before: transverse E & B
“Weizsacker-Williams fields
parallel color E & B fields
Lappi,McLerran,NPA 772 (2006)
After: boost invariant Glasma flux tubes of
size 1/QS
generate Chern-Simons topological charge
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Kharzeev, Krasnitz, RV, Phys. Lett. B545 (2002)
Imagining the Glasma through long range
rapidity correlations
Causality dictates:
 < 1 fm for y > 4
Au+Au 200 GeV, 0 - 30%
PHOBOS preliminary
d2Nch
Ntrig dd
1
At LHC can probe color field
dynamics for  << 1 fm…
Very sensitive to gluon correlations
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in wave fn.
Theory @ EIC
Lattice
Gauge Theory
Condensed
Matter Physics
(B-E Condensates,
Spin Glasses
Graphene)
High Energy
Physics
(LHC,LHeC
Cosmic Rays)
Inclusive Diffraction-II
Impact 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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Estimates of the saturation scale from RHIC
A
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The unstable Glasma
Romatschke, RV:PRL 96 (2006) 062302
•
•
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
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
L or B
Kharzeev,McLerran,Warringa
Topological fluctuations
-sphaleron transitions in Glasma
=
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Chiral magnetic effect
+
External (QED) magnetic field
Effect most significant,
for transitions at early times
STAR Preliminary
Possible experimental signal of
39 2009)
charge separation (Voloshin, Quark Matter
Semi-inclusive DIS
At small x:
Hadron distributions and multiplicities sensitive to QS(x,A)
Kang,Qiu
Ratio to x = 10-2 proportional to
ratio of QS2(x,A)
Marquet,Xiao,Yuan
Need more quantitative studies
for 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
 Solution: for early times (t  1/QS) -- n-gluon production computed
in A+A to all orders in pert. theory to leading log accuracy
Gelis, Lappi, RV; arXiv : 0804.2630, 0807.1306, 0810.4829
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New window on universal properties of the
matter in nuclear wavefunctions
Iancu, RV, hep-ph/0303204
A
Can we quantify the various regimes ?
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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
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