Post on 28-Aug-2018
An Update on Science Studies for the Electron Ion Collider
Thomas UllrichJuly 8, 2008
RHIC S&T Review 7-9 July 2008
LQCD = q̄(i!µ"µ !m)q ! g(q̄!µTaq)Aaµ ! 1
4Gaµ!Gµ!
a
EIC: Study of Glue That Binds Us All
2
• Gluons‣ Self-interacting force carries‣ Determine essential features of QCD ‣ Dominate structure of QCD vacuum‣ Responsible for >95% if visible mass in
universe G. SchierholzAction density in 3q system (lattice)
Action (~energy) density fluctuations of gluon-fields in QCD vacuum (Derek Leinweber)
Despite this dominance, the properties of gluons in matter remain largely unexplored
⇒ Electron Ion Collider = EIC
d2!ep!eX
dxdQ2=
4"#2e.m.
xQ4
!"1! y +
y2
2
#F2(x, Q2)! y2
2FL(x, Q2)
$How Glue is Measured (so far)
3
HERA F 2
0
1
2
3
4
5
1 10 10 2
10 3
10 4
10 5
F 2
em
-lo
g 10
(x)
Q 2 (GeV
2 )
ZEUS NLO QCD fit
H1 PDF 2000 fit
H1 94-00
H1 (prel.) 99/00
ZEUS 96/97
BCDMS
E665
NMC
x=6.32 10-5 x=0.000102
x=0.000161 x=0.000253
x=0.0004 x=0.0005
x=0.000632 x=0.0008
x=0.0013
x=0.0021
x=0.0032
x=0.005
x=0.008
x=0.013
x=0.021
x=0.032
x=0.05
x=0.08
x=0.13
x=0.18
x=0.25
x=0. 4
x=0.65
xf(
x,Q
2)
x
H1 PDF 2000
ZEUS-S PDF
CTEQ6.1
xuV
xg (!1/20)
xS (!1/20)
xdV
Q2 = 10 GeV
2
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
010
-410
-310
-210
-11
Scaling violation: dF2 /dlnQ2 and linear
DGLAP Evolution ⇒ G(x,Q2)
d2!ep!eX
dxdQ2=
4"#2e.m.
xQ4
!"1! y +
y2
2
#F2(x, Q2)! y2
2FL(x, Q2)
$How Glue is Measured (so far)
3
HERA F 2
0
1
2
3
4
5
1 10 10 2
10 3
10 4
10 5
F 2
em
-lo
g 10
(x)
Q 2 (GeV
2 )
ZEUS NLO QCD fit
H1 PDF 2000 fit
H1 94-00
H1 (prel.) 99/00
ZEUS 96/97
BCDMS
E665
NMC
x=6.32 10-5 x=0.000102
x=0.000161 x=0.000253
x=0.0004 x=0.0005
x=0.000632 x=0.0008
x=0.0013
x=0.0021
x=0.0032
x=0.005
x=0.008
x=0.013
x=0.021
x=0.032
x=0.05
x=0.08
x=0.13
x=0.18
x=0.25
x=0. 4
x=0.65
Scaling violation: dF2 /dlnQ2 and linear
DGLAP Evolution ⇒ G(x,Q2)
What Do We Know About Glue?
Linear DGLAP evolution negative G(x,Q2) at low Q2 ?built in high energy “catastrophe”- xG rapid rise violates unitary bound
xG must saturate ⇒ new approach4
10-4 10-3 10-2 10-1
x
xg(
x,Q
2 )
20
15
10
5
0
Q2 = 200 GeV2
Q 2 = 20 GeV 2
Q 2 = 7 GeV 2
Q2 = 1 GeV2
ZEUS NLO QCD fittot. error (!s free)tot. error (!s fixed)
uncorr. error (!s fixed)
ZEUS, PRD 67, 012007
Y =
ln 1
/x
non-
pert
urba
tive
regi
on
ln Q2
ln Q2s(Y)
saturation region
BK/JIMWLK
DGLAP
BFKL
ln !2QCD
"s << 1"s ~ 1
BK/JIMWLK: non-linear effects ⇒ saturation• characterized by Qs(x,A) • believed to have properties of
a Color Glass Condensate
The Science Program of an EICEIC research will penetrate some of the most profound mysteries of questions of 21st century physics
• Explore new QCD frontier: strong color fields in nuclei‣ How do the gluons contribute to the structure of the nucleus?‣What are the properties of high density gluon matter?‣ How do fast quarks or gluons interact as they traverse nuclear
matter? • Precisely image sea-quarks and gluons in the nucleon‣ How do the gluons and sea-quarks contribute to the spin structure
of the nucleon? ‣What is the spatial distribution of the gluons and sea quarks in the
nucleon? ‣ How do hadronic final-states form in QCD?
5
International Advisory Committee• Jochen Bartels (DESY)• Allen Caldwell (MPI, Munich)• Albert De Roeck (CERN)• Walter Henning (ANL)• Dave Hertzog (UIUC)• Xiangdong Ji (U. Maryland)• Robert Klanner (U. Hamburg)• Katsunobu Oide (KEK)• Naohito Saito (KEK)• Uli Wienands (SLAC)
EIC WG Organization Chart
6
EICC Steering Committee• Antje Bruell, Jlab• Abhay Deshpande*, Stony Brook, RBRC• Rolf Ent, Jlab• Charles Hyde, ODU/UBP, France• Peter Jacobs, LBL• Richard Milner*, MIT• Thomas Ulrich, BNL• Raju Venugopalan, BNL• Werner Vogelsang, BNL* contact persons
ep Physics• Antje Bruell, JLAB• Ernst Sichterman, LBL• Werner Vogelsang, BNL• Christian Weiss, JLAB
Detector• Elke Aschenauer, JLAB• Edward Kinney, Colorado• Andy Miller, TRIUMF• Bernd Surrow, MITElectron Beam Polarimetry• Wolfgang Lorenzon, Michigan
Working Groups:eA Physics• Vadim Guzey, JLAB• Dave Morrison, BNL• Thomas Ullrich, BNL• Raju Venugopalan, BNL
2 collaboration meetings/year; steering committee meets once a month; regular WG meetings
Overall: 96+ Scientists, 28 Institutions, 9 countries
NSAC Long Range Plan 2007
“An Electron-Ion Collider (EIC) with polarized beams has been embraced by the U.S. nuclear science community as embodying the vision for reaching the next QCD frontier.”
NSAC Recommendation for EIC:“We recommend the allocation of resources to develop accelerator and detector technology necessary to lay the foundation for a polarized Electron-Ion Collider.”
EIC in 2007 - a good year
7
Documenting the Science Case• The Electron Ion Collider (EIC)
White Paper• The GPD/DVCS White Paper• Position Paper: e+A Physics at
an Electron Ion Collider
Current Science Studies: e+A Key Physics Studies and their implications on detector and machine requirements
The Nuclear Oomph• physics reach into saturation regime ⇒ machine
Momentum Distribution of Gluons G(x,Q2):• via scaling violation of F2 • directly via FL (~G(x,Q2))• through 2+1 jets• through diffractive events (~G(x,Q2)2)
Diffractive Physics• Tagging diffractive events in e+A ⇒ feasibility & detector• Measuring diffractive events ⇒ detector
8
Nuclear Oomph HERA e+p:Despite energy and low-x reach higher than EIC: no clear evidence for non-linear QCD effects (saturation phenomena)
9
ZEUS BPC 1995
ZEUS SVTX 1995
H1 SVTX 1995
HERA 1994
HERA 1993
10 -1
1
10
10 2
10 3
10 4
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
1
x
Q 2
V
e
G
( 2 )
predicted Q2s proton (min. bias)
Enhancement of Qs with A ⇒ non-linear QCD regime reached at significantly larger x (lower √s) in A than in proton e+A physics program relies on this nuclear enhancement
e+A @ EIC:Probes interact over distances L~1/(2mNx)For L > 2 RA ~ A1/3 probe interacts coherently with all nucleons
€
(QsA )2 ≈ cQ0
2 Ax
1/3
Nuclear Enhancement(Oomph):
lowest F2 measurement
Recent Studies on Nuclear EnhancementKowalski, Lappi and Venugopalan, PRL 100, 022303 (2008)More detailed state-of-the-art analysis:Using dipole model and extracting b and x dependence of Qs from fits to diffractive and exclusive HERA data ⇒ construct b dependent Qs(x,b) in the nuclei
10
factor 25b-Satb-CGC
1/x
QS 2 (
GeV
2)
10-1
1
102
103
104
105
106
b = 0 fm
0.2 fm
0.4 fm
factor 20 1/x
Q 2 (
GeV
2)
1
0.1
10
10 2
10 3
10 4
10 5
10 6
Au (c
entra
l)
Q2s,g Ca (c
entra
l)
proto
ns
Kowalski and Teaney
Phys.Rev.D68:114005,2003
20x25 ~500
b-dependence
Confirm pocketformula ~ A1/3:
Implication for Machine Requirements
11
EIC Beam Energy (GeV)
√s (GeV) low-x reach compared to HERA (e+p equivalent)
2+100 28 410+100 63 18
20+100 89 3620+130 102 50
30+130 125 71
Y =
ln
1/x
no
n-p
ert
urb
ative
re
gio
n
ln Q2
ln Q2
s(Y
)
HERA
saturation region
ln !2QCD
NMC
BCDMS
E665
SLAC
CCFR
1 = y
10 -1
1
10
10 2
10 3
10 4
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
1
x
Q 2
V
e
G
( 2 )
20 GeV + 100 GeV/n
10 GeV +100 GeV/n
9 GeV + 90 GeV/n
EIC
Q 2s proton
Q 2s Ca (central)
Q 2s Au (central)
Despite advanced theory:• We do not know for sure how far
HERA was away from the saturation physics regime
• We have to reach far into this regime and we need a safety margin:
• √s ≳ 63 GeV
Key Measurement: F2, FL ⇒ G(x,Q2)
Assume:L = 3.8 1033 cm-2 s-1 (100x Hera)T = 10 weeksduty cycle: 50%L ~ 1/A (approx) ∫Ldt = 11 fb-1
12
x=0.0006
10 GeV + 100 GeV/n
x=0.0006
x=0.006
x=0.006 x=0.02
nDS
EKS
CGC
x=0.02
FLA
u/F
LDF
2Au/F
2D
1 10 102 103 1 10 102 103 1 10 102 103 104
1 10 102 1 10 102 1 10 102 103
Q2 (GeV2)
0.6
0.8
1
1.2
0.6
0.8
1
1.2
FL ~ αs G(x,Q2) requires √s scan, Q2/xs = y
Plots above: ∫Ldt = 4/A fb-1 (10+100) GeV = 4/A fb-1 (10+50) GeV = 2/A fb-1 (5+50) GeVstatistical error only
Simulations to demonstrate the quality of EIC measurements
x
GP
b(x)
/Gd(
x)
Statistical errors for
!Ldt = 10 fb-1 " 2 year running
!Q2": 1.3 2.4 3.8 5.7 9.5 17 34 89
Color Glass CondensateHKM
FGS
RHICLHC
10-110-210-30.2
0.4
0.6
0.8
1
1.2Folded with EIC acceptance
This study: 1% energy-to-energy normalization (typical HERA values)
Systematic uncertainties exceedstatistical errors• We probably can do better
(conceptual design!)
G(x,Q2) and Systematic Errors
13
Systematic Uncertainties• While statistical errors can be rather well evaluated
(acceptance, kinematics, L) the systematic uncertainties are the big unknown
• Hard to estimate: need at least a rough detector design
0.6
0.8
1
1.2
x=0.0006
1 102
10
0.6
0.8
1
1.2
x=0.02
x=0.002
1 10 210 10
nDS
EKS
CGCx=0.2
FLA
u/F
LDF
LAu/F
LD
Q2 (GeV2) Q2 (GeV2)
Current Focus: Diffractive EventsSurprising Discovery at HERA ep: 15% of all e+p events are hard diffractive (p intact)
Diffractive cross-section σdiff/σtot in e+A: 25-40%? Look inside the “Pomeron”: diffractive structure functions F2D, FLD
Diffractive vector meson production ~ G(x,Q2)2
14
1 Introduction
Diffractive processes such as ep ! eXp have been studied extensively in deep-inelastic elec-tron1-proton scattering (DIS) at the HERA collider [1–8], since understanding them in detail
is fundamental to the development of quantum chromodynamics (QCD) at high parton densi-
ties. The photon virtuality Q2 supplies a hard scale for the application of perturbative QCD, so
that diffractive DIS events can be viewed as processes in which the photon probes a net colour
singlet combination of exchanged partons. A hard scattering QCD collinear factorisation the-
orem [9] allows ‘diffractive parton distribution functions’ (DPDFs) to be defined, expressing
proton parton probability distribution functions under the condition of a particular scattered
proton four-momentum. The x and Q2 dependences of diffractive DIS can thus be treated with
a similar theoretical description to that applied to inclusive DIS, for example through the appli-
cation of the DGLAP parton evolution equations [10].
Within Regge phenomenology, diffractive cross sections are described by the exchange of
a leading pomeron (IP ) trajectory, as illustrated in figure 1. H1 diffractive DIS data [3] havebeen interpreted in a combined framework, which applies the QCD factorisation theorem to
the x and Q2 dependences and uses a Regge inspired approach to express the dependence on
the fraction xIP of the incident proton longitudinal momentum carried by the colour singlet
exchange. The data at low xIP are well described in this framework and DPDFs and a pomeron
trajectory intercept have been extracted. In order to describe the data at larger xIP , it is necessary
to include a sub-leading exchange trajectory (IR), with an intercept which is consistent [2] withthe approximately degenerate trajectories associated with the !, ", a2 and f2 mesons.
IP,IR
e
e
!"
}pp
(Q )2
X (M )X
#$%
(t)
}
(x )IP
Figure 1: Schematic illustration of the diffractive DIS process ep ! eXp and the kinematicvariables used for its description in a model in which the pomeron (IP ) and a sub-leading (IR)trajectory are exchanged.
In many previous analyses, including [3], diffractive DIS events are selected on the basis
of the presence of a large rapidity gap (LRG) between the leading proton and the remainder
1For simplicity, the incident and scattered leptons are always referred to in the following as ‘electrons’, although
the data studied here were obtained with both electron and positron beams.
4
“Footprint” of Diffraction1. Outgoing proton with large xL = Ep’/Ep ≈ 1‣ typical t = (p-p’)2 smaller than 1 GeV2,
〈t〉≈0.15 GeV2
2. Produced system X must have small mass w.r.t γ*p center-of-mass (W)
3. Rapidity gap between p and X‣ Δη≈ln (1/xIP)
Identifying Diffractive Events
15
Large rapidity gap method‣ no information on t (limited XIP reach)
Proton Spectrometer• Identify leading proton
‣ low t ⇒ outgoing p scattered at low angles close to the beam axis (θ ≲ 1 mrad) ‣ Roman pots w Si-position detectors + beam optics‣ RHIC experience from pp2pp program
Challenge: Nuclei break up easily (compared to p)
Diffractive eA event
A → fragments (breakup) challenging!A → n + A-1 (Dipole Resonance) possible ?!A stays intact and θ > 0.1 mrad (P=?) best case
Current efforts: Estimate: Pbreakup, Pnon-breakup, Pn-emissionA-Spectrometer concept (beamline integration)Experience at RHIC from UPC program
Current Science Studies: e+p
Inclusive physics • unpolarized + polarized structure functions
Direct measurements of polarized gluon distribution ΔG • current studies: via charm production
Semi-inclusive physics • current quark fragmentation and flavor separation• pT dependent parton distributions• Sivers and Collins functions
Exclusive processes and diffraction• DVCS + meson production (pseudoscalar and vector) • 3 dimensional image of the proton & orbital momentum • General Parton Distributions (GPD)
16
While there’s lots of interesting e+p physics that does not need polarized electron and protons, it’s the polarized e+p program that constitutes a new energy frontier
Spin structure functions: g1(x,Q2)
x,Q2 reach appears sufficient at √s=100 GeV to distinguish models for g1 in a crucial x range as long as Q2 < 12 GeV2
Measurement of g1 at very small x could settle the ΔG problem.20/30+325 GeV (eRHIC) option gets you up to Q2=40 GeV2 at x=10-3
17
E155
E143
SMC
HERMES
0.01
0.1
1
10
100 101 102 103 104
Q2(GeV2)
gp 1(x
,Q2)
+ C
(x)
x = 0.0007
x = 0.0025
x = 0.0063
x = 0.0141
x = 0.0245
x = 0.0346
x = 0.0490
x = 0.0775
x = 0.122
x = 0.173
x = 0.245
x = 0.346
x = 0.490
x = 0.735
EIC
Coverage
x
2.5 < Q2 < 7.5 GeV
2
1
0
10-3 10-2 10-1-1
7+150 GeV
Inclusive ScatteringImpact on EIC on the uncertainties for NLO polarized PDFs
18
(LSS’06 derived from recent CLAS and Compass data)
q(x, Q2), G(x,Q2) are (anti) quark and gluon polarized densities
Exclusive Processes in e+p • Essential part of the EIC program‣ General Parton Distributions (GPD)‣ “Quark/gluon imaging” of nucleon
• Challenging measurement‣ High luminosity L ~ 1034 cm-2 s-1
‣ Detectors: coverage, resolution, particle ID
• Lessons from MC simulations‣ e+p → e’ π+ n, π0 p, KΛ
19
!, meson
p, np
Q
e
e’
2
GPD
QCD
10-10
10-9
10-8
10-7
10-6
10-5
10-4
0 0.2 0.4 0.6 0.8 1
d!
(ep"
e’n#
+)
/ dt
[µ
b /
GeV
2]
-t [GeV2]
ep" e’n#+, sep = 4000 GeV
2, L = 10
34 cm
-2 s
-1, 16 weeks
0.01 < x < 0.02
4 < Q2 < 6 GeV
2
10 < Q2 < 15 GeV
2
A. Bruell, T. Horn, G. Huber, C. Weiss (2008)
SummaryOngoing physics studies • Current focus e+A
‣ diffractive physics & detector requirements‣ next: jet physics
• Current focus e+p‣ exclusive processes (luminosity requirements)‣ various processes: kinematics & detector requirements
All studies & efforts still conducted by few enthusiast• Relatively broad interest but many are reluctant to get
further involved at this point• Most efforts centered around labs (BNL, JLAB, LBNL)• Need to strengthen the user base that is willing to get
their hand dirty20
EIC Roadmap
21
NSAC Long Range Plan 2007‣ Recommendation: $6M/year for 5 years for machine and
detector R&DGoal for Next Long Range Plan ~2012‣ High-level (top) recommendation for construction
EIC Roadmap (Technology Driven)‣ Finalize Detector Requirements from Physics 2008‣ Revised/Initial Cost Estimates for eRHIC/ELIC 2008‣ Investigate Potential Cost Reductions 2009‣ Establish process for EIC design decision 2010‣ Conceptual detector designs 2010‣ R&D to guide EIC design decision 2011‣ EIC design decision 2011‣MOU’s with foreign countries? 2012
Continuous effort: Strengthening the science case