Transcript Physics Opportunities at an Electron-Ion Collider (EIC) Thomas Ullrich Phases of QCD Matter Town Meeting Rutgers University January 12, 2006 Lots of hard work from and violent.

Physics Opportunities at an
Electron-Ion Collider (EIC)
Thomas Ullrich
Phases of QCD Matter
Town Meeting
Rutgers University
January 12, 2006
Lots of hard work from and violent discussion with:
A. Bruell (JLAB), J. Dunlop (BNL), R. Ent (JLAB), D. Morrison (BNL),
P. Steinberg (BNL) , B. Surrow (MIT), R. Venugopalan (BNL),
W. Vogelsang (BNL), Z. Xu (BNL)
Crouching Quarks, Hidden Glue
Gluons: mediator of the strong interactions


Responsible for > 98% of the visible mass in universe
Determine all the essential features of strong interactions


QCD w/o quarks 
QCD w/o gluons 
QCD vacuum has non-perturbative structure driving:



Color confinement
Chiral symmetry breaking
In large due to fluctuations in the gluon fields in the vacuum
Hard to “see” the glue in the low-energy world


Does not couple to electromagnetism
Gluon degrees of freedom “missing” in hadronic spectrum


but dominate the structure of baryonic matter at low-x
are the dominant player at RHIC and LHC
2
What Do We Know About Glue in Matter?
Established Model:
linear
DGLAP
evolution
scheme
Deep
Inelastic
Scattering:


works well for quarks 
2
2
2

Q


q


(
k

k
)


cannot simultaneously describe gluons 
 negative at low Q2 ?
2
2   e 

Q  4 Ee Ee sin  
 explosion of G(x,Q2) at low-x
2
 violation of unitarity
pq
Ee
2   e 
y
 1
cos  
 problems in describing diffractive events
pk
Ee
2
Measure of
resolution
power
Measure of
inelasticity
(HERA)
2
2
Q
Q
New picture: BK based models introduce
x

2 pq sy
non-linear effects




 saturation
“Perfect” Tomography
characterized by a scale Qs(x,A)
grows with decreasing x and increasing A
arises naturally in the CGC framework
Measure of
momentum
fraction of
struck
quark
3
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?
How do fast probes interact with the gluonic medium?
Do strong gluon fields effect the role of color neutral
excitations (Pomerons)?
What system to use?
1. e+p works, but more accessible by using e+A
2. have analogs in e+p, but have never been measured in e+A
3. have no analog in e+p
4
eA: Ideal to Study Non-Linear Effects
Scattering of electrons off nuclei:


Small x partons cannot be localized longitudinally to better than size of nucleus
Virtual photon interacts coherently with all nucleons at a given impact parameter
 Amplification of non-linear effects at small x.
k’
e+A Collisions are Ideal
for
Studying “Glue”
k


W
Gain deeper understanding
of QCD
p
Terra incognita: Physics of Strong Color Fields
Nuclear “Oomph” Factor:
Pocket Formula :
1
Qs2
A3
  (where   1 3 )
x
hence
Qs2
 A
 
x
1
3
5
eA Landscape and a new Electron Ion Collider
The x, Q2 plane looks well
mapped out – doesn’t it?
Except for ℓ+A (nA)
many of those with small A and
very low statistics
Electron Ion Collider (EIC):
Ee = 10 GeV (20 GeV)
EA = 100 GeV
seN = 63 GeV (90 GeV)
High LeAu ~ 6·1030 cm-2 s-1
Terra incognita: small-x, Q  Qs
high-x, large Q2
6
How EIC will Address the Important Questions




What is the momentum distribution of the gluons in matter?
2)
 Gluon
distribution
What
is the
space-timeG(x,Q
distributions
of gluons in matter?
2) (BTW: requires s scan)
 FL probes
~ as G(x,Q
How do fast
interact
with the gluonic medium?
 Extract from scaling violation in F2: F2/lnQ2
Do strong gluon fields effect the role of color neutral
 2+1 jet rates (needs modeling of hadronization)
excitations
(Pomerons)?
 inelastic vector meson production (e.g. J/)
7
F2 at EIC: Sea (Anti)Quarks Generated by Glue at Low x
F2 will be one of the
first measurements at
EIC
nDS, EKS, FGS:
pQCD models with
different amounts of
shadowing

d 2 epeX 4a 2 
y2 
y2
2
2
1  y   F2 ( x, Q ) 

FL ( x, Q )
2
4 
2
2
dxdQ
xQ 


EIC will allow to
distinguish between
pQCD and
saturation models
predictions
8
FL at EIC: Measuring the Glue Directly
EIC: (10+100) GeV
Ldt = 2/A fb-1

d 2 epeX 4a 2 
y2 
y2
2
2



1

y

F
(
x
,
Q
)

F
(
x
,
Q
)


2
L
2 
2
dxdQ2
xQ4 

Q2/xs = y
Needs s scan
EIC will allow to measure G(x,Q2) with great precision
9
How EIC will Address the Important Questions




What is the momentum distribution of the gluons in matter?
What is the space-time distributions of gluons in matter?
 Measurement of structure functions for various mass numbers A
How
do fast probes interact with the gluonic medium?
(shadowing, EMC effect) and its impact parameter dependence
Do strong
gluoncompton
fields effect
the(DVCS)
role of color neutral
Deep virtual
scattering
excitations
(Pomerons)?
 color transparency
 color opacity

exclusive final states (e.g. vector meson production r, J/, …)
10
How EIC will Address the Important Questions




What is the momentum distribution of the gluons in matter?
What is the space-time distributions of gluons in matter?
How do fast probes interact with the gluonic medium?
 Hadronization,
Fragmentation
Do strong
gluon fields
effect the role of color neutral
 Energy
loss (charm!)
excitations
(Pomerons)?
11
Charm at EIC
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
12
How EIC will Address the Important Questions




What is the momentum distribution of the gluons in matter?
What is the space-time distributions of gluons in matter?
How do fast probes interact with the gluonic medium?
Do strong gluon fields effect the role of color neutral
excitations (Pomerons)?
•
•
•
•
diffractive cross-section diff/tot
• HERA/ep: 10% of all events are hard diffractive
EIC/eA: 30%?
diffractive structure functions
shadowing == multiple diffractive scattering ?
diffractive vector meson production - very sensitive to G(x,Q2)
d
dt
( γ*A  VA)  a S2 [G A ( x, Q 2 )]2
t 0
13
Diffractive Structure Function F2D at EIC
xIP = momentum
fraction of the
Pomeron with respect
to the hadron
 = momentum
fraction of the struck
parton with respect to
the Pomeron
xIP = x/
EIC allows to distinguish between linear evolution and saturation models
14
Connection to RHIC & LHC Physics
Even more crucial at LHC:
Thermalization:


gluon thermalizes
distribution functions
for
Pb(t
versus
x from
different
At Ratios
RHIC of
system
(locally)
fast
~
0.6
fm/c)
FF modification
0
models at Q2 = 5 GeV2:
(parton energy loss)
We don’t know why and how? Initial conditions?
Jet Quenching:


Refererence: E-loss in cold matter
d+A alone won’t do


 need more precise handles
?
no data on charm from HERMES
Forward Region:

Suppression at forward rapidities
Color Glass Condensate ?
 Gluon Distributions ?

Accardi et al.,
hep-ph/0308248,
CERN-2004-009-A
15
Many New Questions w/o Answers …
Latest News:
Observe
direct photons
Many
(all?)“E-loss”
of theseofquestions
cannot be answered
 Are we A+A
seeingorthe
EMC
effect?
by studying
p+A
alone.

EIC provides new level of precision:
• Handle on x, Q2
• Means to study effects exclusively
• RHIC is dominated by glue  Need to know G(x,Q2)
In short we need ep but especially eA  EIC
16
EIC Collider Aspects
Requirements for EIC:

ep/eA program




polarized e, and p
maximal ion mass A
s ~ 100 GeV
high luminosity (L > LHera)
There are two complementary concepts to realize EIC:

eRHIC



construct electron beam to collide with the existing RHIC ion complex
high luminosity (6·1030 cm-2s-1), ions up to U, s ~ 100 GeV
ELIC


construct ion complex to collide with the upgraded CEBAF accelerator
very high luminosity (4·1034 cm-2s-1/A), only light ions, s ~ 50 GeV
17
Experimental Aspects
J. Pasukonis, B.Surrow, physics/0608290
I. Abt, A. Caldwell, X. Liu,
J. Sutiak, hep-ex 0407053
Concepts:
1. 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
2. Focus on a wide acceptance detector system similar to HERA experiments
 allow for the maximum possible Q2 range.
18
Summary
eA collisions at an EIC allow us to:

Study the Physics of Strong Color Fields



Establish (or not) the existence of the saturation regime
In Short:
Explore non-linear QCD
EIC allows us to
Measure momentum & space-time of glue
expand and deepen our understanding of QCD
Study the nature of color singlet excitations (Pomerons)
 Study and understand nuclear effects


shadowing,
effect,time
Energy
Lossstarted!
in cold matter
NowEMC
is a good
to get
Test and study the limits of universality (eA vs. pA)
EIC White Paper: http://www.physics.rutgers.edu/np/EIC-science-1.7.pdf
 Cross-fertilization: DIS (Hera), RHIC/LHC, JLAB

Soon: EIC/eA Specific Position Paper: http://www.bnl.gov/eic
19
BACKUP
20
Structure Functions in DIS
Quantitative description of electron-proton scattering
Q 2   q 2  ( k   k  ) 2
2   e 

Q  4 Ee Ee sin  
2
pq
E
 
y
 1  e cos 2  e 
pk
Ee
2
2
Q2 Q2
x

2 pq sy
Measure of
resolution
power
Measure of
inelasticity
Measure of
momentum
fraction of
struck
quark
2

d 2 epeX 4a 2 
y2 
y
2
2
1  y   F2 ( x, Q ) 

FL ( x, Q )
2
4 
2 
2
dxdQ
xQ 

21
eA From a “Dipole” Point of View
In the rest frame of the nucleus:
Propagation of a small pair, or “color dipole”
k’
k
r : dipole size
p
valid in the small-x limit
Coherence length of virtual photon’s fluctuation intoqq: L∼ 1/2mN x
L >> 2R



Physics of strong color fields
Shadowing
Diffraction
L << 2R



Energy Loss
color transparency
EMC effect
22
Vector Meson Production
HERA: Survival prob. of qq
pair of d=0.32 fm scattering off
a proton from elastic vector
meson production.
Strong gluon fields in center of
p at HERA (Qs ~ 0.5 GeV2)?
Survival Probability
“color dipole” picture
color opacity
color transparency
b profile of nuclei more
uniform and Qs ~ 2 GeV2
23
What Do We Know About Glue in Matter?
Deep Inelastic Scattering:
Distribution functions G(x,Q2) evaluated through models
 rise steeply at low Bjorken x
Gluons and Quarks
Gluons
Is nature well-described by model evolution?
24
Diffractive DIS is …
… when the hadron/nuclei remains intact
momentum transfer
t = (P-P’)2 < 0
Pomeron
P
hadron
diffractive mass of the final state
MX2 = (P-P’+l-l’)2
P
Q2
Q2


2 (PP').(l l') M X2  t Q2
 ~ momentum fraction of the struck parton with respect to the Pomeron
xpom = x/
rapidity gap :  = ln(1/xpom)
xpom ~ momentum fraction of the Pomeron with respect to the hadron
HERA/ep: 10% of all events are hard diffractive
Black Disk Limit: 50%
EIC/eA: 30%?
25