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