Transcript Physics Opportunities with e+A Collisions at an Electron Ion Collider A New Experimental Quest to Study the Glue That Binds us All Thomas Ullrich,

Physics Opportunities with e+A
Collisions at an Electron Ion Collider
A New Experimental Quest
to Study the Glue That Binds us All
Thomas Ullrich, BNL
Hall C Meeting, JLAB
August 9, 2007
Theory of Strong Interactions: QCD
LQCD


a
Asymptotic Freedom & Color Confinement
In large part due to non-perturbative structure of QCD vacuum
Gluons: mediator of the strong interactions





“Emergent” Phenomena not evident from Lagrangian


1 a 
 q (i    m)q  g ( q  Ta q) A  G Ga
4

Determine essential features of strong interactions
Dominate structure of QCD vacuum (fluctuations in gluon fields)
Responsible for > 98% of the visible mass in universe
Hard to “see” the glue in the low-energy world

Gluon degrees of freedom “missing” in hadronic spectrum
 but drive the structure of baryonic matter at low-x
 are crucial players at RHIC and LHC
 Requires fundamental investigation via experiment
2
What Do We Know About Glue in Matter?
Deep Inelastic Scattering :
2


d 2 ep eX 4 e.m.
y 2 
y2
2
2

1 y  F2 (x,Q )  FL (x,Q )
2
4 
dxdQ
xQ 
2 
2



Scaling violation: dF2/dlnQ2 and
linear DGLAP Evolution  G(x,Q2)
Gluons dominate
low-x wave function
xG ( 1
20
xu v
)
xd v
xS ( 1
20
)
3
The Issue With Our Current Understanding
Established Model:
Linear DGLAP evolution scheme


Weird behavior of xG and FL from
HERA at small x and Q2
 Could signal saturation, higher twist
effects, need for more/better data?
Unexpectedly large diffractive crosssection
more severe:
Linear Evolution has a built in high
energy “catastrophe”


xG rapid rise for decreasing x and
violation of (Froissart) unitary bound
 must saturate
 What’s the underlying dynamics?
 Need new approach
4
Non-Linear QCD - Saturation

BFKL Evolution in x


proton
linear
explosion of color field?
N partons
any 2partons
new
partonsemitted
can recombine
as energy
into
increases
one
could be emitted off any of the N partons
Regimes of QCD Wave Function

New: BK/JIMWLK
based models
introduce non-linear effects
 saturation
 characterized by a scale Qs(x,A)
 arises naturally in the Color
Glass Condensate (CGC)
framework

5
e+A: Studying Non-Linear Effects
Scattering of electrons off nuclei:
Probes interact over distances L ~ (2mN x)-1
For L > 2 RA ~ A1/3 probe cannot distinguish
between nucleons in front or back of nucleon
Probe interacts coherently with all nucleons



 s xG(x,Qs2 )
Q ~
RA2
2
s
1
HERA : xG ~ 0.3
x
Nuclear “Oomph” Factor
Pocket Formula:
A dependence : xGA ~ A
1/3


A 2
2 A
(Qs )  cQ0  
x 
Enhancement of QS with A  non-linear QCD regime reached at
significantly lower energy
 in A than in proton
6
Nuclear “Oomph” Factor
Note :
Q Q
  s   s (Q )
Q2  Q
  s   s (Qs2 )
2
2
s
2
s
2

More sophisticated analyses  more detailed picture even exceeding the
Oomph from the pocket formula
(e.g. Armesto et al., PRL 94:022002, Kowalski, Teaney, PRD 68:114005)
7
Universality & Geometric Scaling
Crucial consequence of non-linear
evolution towards saturation:


Physics invariant along trajectories
parallel to saturation regime (lines of
constant gluon occupancy)
Scale with Q2/Q2s(x) instead of x and
Q2 separately
 Geometric Scaling

Consequence of saturation which
manifests itself up to kT > Qs
x < 0.01
8
Qs a Scale that Binds them All
Nuclear shadowing:
Geometrical scaling
proton  5
nuclei
Freund et al., hep-ph/0210139
Are hadrons and nuclei wave function universal at low-x ?
9
A Truly Universal Regime ?
Small x QCD evolution predicts:
QS approaches universal
behavior for all hadrons and
nuclei
 Not only functional form f(Qs)
“Research is what I'm doing
when
universal
but Ieven Qs becomes
the same
don't know what I'm doing.
“
?
A.H. Mueller, hep-ph/0301109


Wernher von Braun
Radical View:


Nuclei and all hadrons have a component of their wave function
with the same behavior
This is a conjecture! Needs to be tested
10
Understanding Glue in Matter
Understanding the role of the glue in matter involves understanding
its key properties which in turn define the required measurements:

What is the momentum distribution of the gluons in matter?



What is the space-time distributions of gluons in matter?



e+p and e+A
Unknown in e+A
How do fast probes interact with the gluonic medium?


e+p and e+A
Exploration of saturation regime better in e+A (Oomph Factor)
Strength of e+A
Do strong gluon fields effect the role of color neutral excitations
(Pomerons)?


e+p and e+A
Unknown in e+A
11
eA Landscape and a New Electron Ion Collider
Well mapped in e+p
Not so for ℓ+A (A)
Electron Ion Collider (EIC):
L(EIC) > 100  L(HERA)
Different EIC Concepts:
 eRHIC
 ELIC
Terra incognita: small-x, Q  Qs
high-x, large Q2
12
Electron Ion Collider Concepts

eRHIC (BNL): Add Energy
Recovery Linac to RHIC
Ee = 10 (20) GeV
EA = 100 GeV (up to U)
seN = 63 (90) GeV
LeAu (peak)/n ~ 2.9·1033 cm-2 s-1
eRHIC
(Linac-Ring)
PHENIX

ELIC (JLAB): Add hadron
beam facility to existing
electron facility CEBAF
Ee = 9 GeV
EA = 90 GeV (up to Au)
seN = 57 GeV
LeAu (peak)/n ~ 1.6·1035 cm-2 s-1
e-cooling
(RHIC
II)
Electron
Cooling
IR
Snake
Main ERL (2 GeV per pass)
STAR
Snake
Four e-beam
passes
Both allow for polarized e+p collisions !
IR
ELIC
13
What is the Momentum Distribution of Gluons?
Gluon distribution G(x,Q2)

Shown here:



Scaling violation in F2: dF2/dlnQ2
FL ~ s G(x,Q2)
Other Methods:



2+1 jet rates (needs jet algorithm and modeling of hadronization
for inelastic hadron final states)
inelastic vector meson production (e.g. J/)
diffractive vector meson production ~ [G(x,Q2)]2
14
F2 : Sea (Anti)Quarks Generated by Glue at Low x
F2 will be one of the first
measurements at EIC
nDS, EKS, FGS:
pQCD based models with
different amounts of
shadowing
Syst. studies of F2(A,x,Q2):
 G(x,Q2) with precision
 distinguish between
models

d 2 ep eX 4 2 
y 2 
y2
2
2

1 y  F2 (x,Q )  FL (x,Q )
2
4 
dxdQ
xQ 
2 
2

15
FL at EIC: Measuring the Glue Directly
FL requires s scan
Q2/xs = y
Here:
Ldt = 5/A fb-1 (10+100) GeV
= 5/A fb-1 (10+50) GeV
= 2/A fb-1 (5+50) GeV
statistical error only
 G(x,Q2) with
great precision

d 2 ep eX 4 2 
y 2 
y2
2
2

1 y  F2 (x,Q )  FL (x,Q )
2
4 
dxdQ
xQ 
2 
2

16
The Gluon Space-Time Distribution

What we know is mostly the momentum distribution of glue?
How is the glue distributed spatially in nuclei?
 Gluon density profile: small clumps or uniform ?


Various techniques & methods:

Exclusive final states (e.g. vector meson production r, J/, DVCS)


Deep Virtual Compton Scattering (DVCS)


color transparency  color opacity
Integrated DVCS cross-section: DVCS ~ A4/3
Measurement of structure functions for various mass numbers A
(shadowing, EMC effect) and its impact parameter dependence
17
Diffractive Physics in e+A
‘Standard
Diffractive
DISevent
event’
?
Detector activity in proton direction

HERA/ep: 15% of all events are hard diffractive

Diffractive cross-section diff/tot in e+A ?


Predictions: ~25-40%?
Look inside the “Pomeron”


Diffractive structure functions
Diffractive vector meson production ~ [G(x,Q2)]2
18
Diffractive Structure Function F2D at EIC
2

d 4 eh eXh
4 e.m.
y 2  D y 2 D 
 2 4 1 y  F2  FL 
2
dxdQ ddt
 Q 
2 
2

= x/xIP

xIP = momentum fraction of the
pomeron w.r.t the hadron
 Distinguish between linear
evolution and saturation models
 Insight into the nature of pomeron
 Search for exotic objects (Odderon)
Curves: Kugeratski, Goncalves, Navarra, EPJ C46, 413
19
Hadronization and Energy Loss
nDIS:


Suppression of high-pT hadrons analogous but weaker than at RHIC
Clean measurement in ‘cold’ nuclear matter
Fundamental question:
When do colored partons get neutralized?
Parton energy loss vs.
(pre)hadron absorption
Energy transfer in lab rest frame
EIC: 10 <  < 1600 GeV HERMES: 2-25 GeV
EIC: can measure heavy flavor energy loss
zh = Eh/
20
Connection to p+A Physics
F. Schilling, hex-ex/0209001

e+A and p+A provide excellent
information on properties of gluons
in the nuclear wave functions

Both are complementary and offer the
opportunity to perform stringent checks
of factorization/universality


Issues:
 p+A lacks the direct access to x, Q2
Breakdown of factorization (e+p
HERA versus p+p Tevatron) seen
for diffractive final states.
21
Connection to RHIC & LHC Physics
Matter at RHIC:



thermalizes fast (t0 ~ 0.6 fm/c)
We don’t know why and how?
Initial conditions?  G(x, Q2)
Role of saturation ?


RHIC → forward region
LHC → midrapidity
 bulk (low-pT matter) &
semi-hard jets
LHC
RHIC
Jet Quenching:


Need Refererence: E-loss in
cold matter
No HERMES data for
 charm energy loss
 in LHC energy range
EIC provides new essential input:
• Precise handle on x, Q2
• Means to study exclusive effects
22
Experimental Aspects
I. Abt, A. Caldwell, X. Liu,
J. Sutiak, hep-ex 0407053
J. Pasukonis, B.Surrow, physics/0608290
Concepts:
 Focus on the rear/forward acceptance and thus on low-x / high-x physics

compact system of tracking and central electromagnetic calorimetry inside a
magnetic dipole field and calorimetric end-walls outside
 Focus on a wide acceptance detector system similar to HERA experiments
 allow for the maximum possible Q2 range.
23
EIC Timeline & Status

NSAC Long Range Plan 2007


Goal for Next Long Range Plan 2012


Recommendation: $6M/year for 5 years for machine and
detector R&D
High-level (top) recommendation for construction
EIC Roadmap (Technology Driven)








Finalize Detector Requirements from Physics
Revised/Initial Cost Estimates for eRHIC/ELIC
Investigate Potential Cost Reductions
Establish process for EIC design decision
Conceptual detector designs
R&D to guide EIC design decision
EIC design decision
High priority in Long Range Plan
2008
2008
2009
2010
2010
2011
2011
2012
24
Summary
The EIC presents a unique opportunity in high energy nuclear
physics and precision QCD physics

e+A




Study the Physics of Strong Color Fields
 Establish (or not) the existence of the saturation regime
 Explore non-linear QCD
 Measure momentum & space-time of glue
Study the nature of color singlet excitations (Pomerons)
Test and study the limits of universality (eA vs. pA)
e+p (polarized)

Precisely image the sea-quarks and gluons to determine the spin, flavor
and spatial structure of the nucleon
For more see: http://web.mit.edu/eicc/
25
The EIC Collaboration
17C.
Aidala, 28E. Aschenauer, 10J. Annand, 1J. Arrington, 26R. Averbeck, 3M. Baker, 26K.
Boyle, 28W. Brooks, 28A. Bruell, 19A. Caldwell, 28J.P. Chen, 2R. Choudhury, 10E. Christy, 8B.
Cole, 4D. De Florian, 3R. Debbe, 26,24-1A. Deshpande, 18K. Dow, 26A. Drees, 3J. Dunlop, 2D.
Dutta, 7F. Ellinghaus, 28R. Ent, 18R. Fatemi, 18W. Franklin, 28D. Gaskell, 16G. Garvey, 12,241M. Grosse-Perdekamp, 1K. Hafidi, 18D. Hasell, 26T. Hemmick, 1R. Holt, 8E. Hughes, 22C.
Hyde-Wright, 5G. Igo, 14K. Imai, 10D. Ireland, 26B. Jacak, 15P. Jacobs, 28M. Jones, 10R.
Kaiser, 17D. Kawall, 11C. Keppel, 7E. Kinney, 18M. Kohl, 9H. Kowalski, 17K. Kumar, 2V. Kumar,
21G. Kyle, 13J. Lajoie, 3M. Lamont, 16M. Leitch, 27A. Levy, 27J. Lichtenstadt, 10K. Livingstone,
20W. Lorenzon, 145. Matis, 12N. Makins, 6G. Mallot, 18M. Miller, 18R. Milner, 2A. Mohanty, 3D.
Morrison, 26Y. Ning, 15G. Odyniec, 13C. Ogilvie, 2L. Pant, 26V. Pantuyev, 21S. Pate, 26P. Paul,
12J.-C. Peng, 18R. Redwine, 1P. Reimer,
15H.-G. Ritter, 10G. Rosner, 25A. Sandacz, 7J.
Seele, 12R. Seidl, 10B. Seitz, 2P. Shukla, 15E. Sichtermann, 18F. Simon, 3P. Sorensen, 3P.
Steinberg, 24M. Stratmann, 22M. Strikman, 18B. Surrow, 18E. Tsentalovich, 11V. Tvaskis, 3T.
Ullrich, 3R. Venugopalan, 3W. Vogelsang, 28C. Weiss, 15H. Wieman,15N. Xu,3Z. Xu, 8W. Zajc.
1Argonne
National Laboratory, Argonne, IL; 2Bhabha Atomic Research Centre, Mumbai, India; 3Brookhaven National
Laboratory, Upton, NY; 4University of Buenos Aires, Argentina; 5University of California, Los Angeles, CA; 6CERN, Geneva,
Switzerland; 7University of Colorado, Boulder,CO; 8Columbia University, New York, NY; 9DESY, Hamburg, Germany;
10University of Glasgow, Scotland, United Kingdom; 11Hampton University, Hampton, VA; 12University of Illinois, UrbanaChampaign, IL; 13Iowa State University, Ames, IA; 14University of Kyoto, Japan; 15Lawrence Berkeley National Laboratory,
Berkeley, CA; 16Los Alamos National Laboratory, Los Alamos, NM; 17University of Massachusetts, Amherst, MA; 18MIT,
Cambridge, MA; 19Max Planck Institut für Physik, Munich, Germany; 20University of Michigan Ann Arbor, MI; 21New Mexico
State University, Las Cruces, NM; 22Old Dominion University, Norfolk, VA; 23Penn State University, PA; 24RIKEN, Wako,
Japan; 24-1RIKEN-BNL Research Center, BNL, Upton, NY; 25Soltan Institute for Nuclear Studies, Warsaw, Poland; 26SUNY,
Stony Brook, NY; 27Tel Aviv University, Israel; 28Thomas Jefferson National Accelerator Facility, Newport News, VA
96 Scientists, 28 Institutions, 9 countries
26
Additional
Slides
27
Regimes of QCD Wave Function in 3D
28
Spin Physics at the EIC - The Quest for G
Spin Structure of the Proton
½ = ½  + G + Lq + Lg


quark contribution ΔΣ ≈ 0.3
gluon contribution ΔG ≈ 1 ± 1 ?
G: a “quotable” property of the
proton (like mass, charge)
Measure through scaling violation:
dg1
2

g(x,Q
)
2
d log( Q )
G 
x1
 g(x,Q )dx
2
x 0

Superb sensitivity to g
at small x!
29
Charm at EIC in e+A
Based on HVQDIS model, J. Smith
EIC:
allows multi-differential measurements of heavy flavor
covers and extend energy range of SLAC, EMC, HERA, and
JLAB allowing study of wide range of formation lengths
30
What Do We Know About Glue in Matter?
Deep Inelastic Scattering :
2


d 2 ep eX 4 e.m.
y 2 
y2
2
2

1 y  F2 (x,Q )  FL (x,Q )
2
4 
dxdQ
xQ 
2 
2

Deep Inelastic Scattering: Scaling
Measure
of resolution
violation:
dF2power:
/dlnQ2 and
~1/wavelength
linear
DGLAP Evolution  G(x,Q2)
Q 2  q 2  (k   k  ) 2
2
Measure of momentum fraction
of struck quark

x
Gluons
dominate
2
Qlow-x wave function
2 pqxG
( 1
20
xu v
)
Measure of inelasticity
xd v
E E
y
Ee
xS ( 1 ) '
e 20 e
“Perfect” Tomography
31