Rolf Ent, ECT-Trento, October 28, 2008

Spin/Flavor Physics with a Future Electron-Ion

Collider

• Electron-Ion Collider: Options and Status (in US)• Gluons and Sea Quarks• Inclusive DIS Measurement Projections• Semi-Inclusive DIS Measurement Projections• Deep Exclusive Measurement Projections• Electroweak Measurements• Summary and Outlook

EIC science has evolved from new insights and technical

accomplishments over the last decade

• ~1996 development of GPDs• ~1999 high-power energy recovery linac technology • ~2000 universal properties of strongly interacting

glue • ~2000 emergence of transverse-spin phenomenon• ~2001 world’s first high energy polarized proton

collider• ~2003 tantalizing hints of saturation• ~2006 electron cooling for high-energy beams

NSAC 2007 Long Range Plan “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. EIC would provide unique capabilities for the study of QCD well beyond those available at existing facilities worldwide and complementary to those planned for the next generation of accelerators in Europe and Asia. In support of this new direction:

We recommend the allocation of resources to develop accelerator and detector technology necessary to lay the foundation for a polarized Electron Ion Collider. The EIC would explore the new QCD frontier of strong color fields in nuclei and precisely image the gluons in the proton.”

Electron Cooling

Snake

Snake

IR

IR

Four Electron-Ion Collider Facilities Considered

PHENIX

STAR

e-cooling (RHIC II)

Four e-beam passes

Main ERL (2 GeV per pass)

Add electron beam (COSY

ring) to GSI/HESR

eRHIC ELIC

MANUEL LHeC

Four Electron-Ion Collider Facilities ConsideredLHeC: L = 1.1x1033 cm-

2s-1 Ecm = 1.4 TeV

EICx2: L > 1x1033 cm-2s-

1 Ecm = 20-100+ GeV

• Add 70-100 GeV electron ring to interact with LHC ion beam• Use LHC-B interaction region• High luminosity mainly due to large ’s (= E/m) of beams

• Variable energy range• Polarized and heavy ion beams• High luminosity in energy region of interest for nuclear scienceNuclear science goals:• Explore the new QCD frontier: strong color fields in nuclei• Precisely image the sea-quarks and gluons to determine the spin, flavor and spatial structure of the nucleon.

High-Energy physics goals:• Parton dynamics at the TeV scale - physics beyond the

Standard Model - physics of high parton

densities (low x)

MANUEL@FAIR: L > 1x1033 cm-2s-

1? Ecm = 13 GeV

• Add 3 GeV electron accelerator to interact with FAIR ion beamNuclear science goal:• Precisely image the sea-quark and gluon structure of the nucleon.

Electron-Ion Collider – further info

• EIC (eRHIC/ELIC) webpage: http://web.mit.edu/eicc/

• Upcoming meeting: December 11-13, 2008 @ Berkeley

• 1st joint BNL/JLab EIC Advisory Committee meeting:February 16, 2009

• Next EIC meeting, joint with MANUEL project:May 2009 in Germany

Explore the 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 the 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?

Nuclear Science Goals: How do we understand the visible matter in our universe in terms of the fundamental quarks and gluons of QCD?

Gluons dominate QCD• QCD is the fundamental theory that describes structure and

interactions in nuclear matter.• Without gluons there are no protons, no neutrons, and no

atomic nuclei• Gluons dominate the structure

of the QCD vacuum

• Facts:– The essential features of QCD (e.g. asymptotic freedom, chiral

symmetry breaking, and color confinement) are all driven by the gluons!

– Unique aspect of QCD is the self interaction of the gluons– 98% of mass of the visible universe arises from glue– Half of the nucleon momentum is carried by gluons

9

The Low Energy View of Nuclear Matter• nucleus = protons + neutrons• nucleon quark model • quark model QCD

The High Energy View of Nuclear MatterThe visible Universe is generated by quarks, but dominated by the gluons

Removefactor 20

Exposing the high-energy (dark) side of the nuclei

CTEQ Example at Scale Q2 = 10 GeV2

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

),(2

),(2

14 2

22

2

2

4

2

2

2

QxFy

QxFy

yxQdxdQ

dL

eXep

(Thomas Ullrich, Dave Morrison)

Longitudinal Structure Function FL

• Experimentally can be determined directly IF VARIABLE ENERGIES!• Highly sensitive to effects of gluon

+ 12-GeV data+ EIC alone

FL at EIC: Measuring the Glue Directly

),(2

),(2

14 2

22

2

2

4

2

2

2

QxFy

QxFy

yxQdxdQ

dL

eXep

Explore gluon-dominated matter

At high gluon density, gluon recombination should compete with gluon splitting density saturation.

What is the role of gluons and gluon self-interactions in nucleons and nuclei? NSAC-2007 Long-Range Plan Report.– The nucleus as a “gluon amplifier”

Color glass condensate

Oomph factor stands up under scrutiny.Nuclei greatly extend x reach:xEIC = xHERA/18 for 10+100 GeV, Au

Longitudinal Structure Function FL

(Antje Bruell, Thomas Ullrich)

Explore the structure of the nucleon • Parton distribution

functions• Longitudinal and transverse spin distribution functions• Generalized parton distributions•Transverse momentum distributions

The Gluon Contribution to the Proton Spin

at small x

Superb sensitivity to g

at small x!

(Antje Bruell, Abhay Deshpande)

Projected data on g/g with an EIC, via + p D0 + X

K- + +

assuming vertex separation of 100 m.

Access to g/g is also possible from the g1p

measurements through the QCD evolution, and from di-jet measurements.

RHIC-Spin

The Gluon Contribution to the Proton Spin

Advantage: measurements directly at fixed Q2 ~ 10 GeV2 scale!• Uncertainties in xg smaller than 0.01 • Measure 90% of G (@ Q2 = 10 GeV2)

g/g

(Antje Bruell)

• Luminosity of 1x1033 cm-2 sec-1

• One day 50 events/pb• Supports Precision Experiments

Lower value of x scales as s-1

• DIS Limit for Q2 > 1 GeV2 implies x down to 1.0(1.3) times 10-4

• Significant results for 200 events/pb for inclusive scattering

• If Q2 > 10 GeV2 required for Deep Exclusive Processes can reach x down to 1.0(1.3) times 10-3

• Typical cross sections factor 100-1,000 smaller than inclusive scattering

• Significant results for 20,000-200,000 events/pb high luminosity essential

Luminosity Considerations for EIC

eRHIC

ELIC(W2 > 4)

x

Q2 (G

eV

2)

W2<4

eRHIC: x = 10-4 @ Q2 = 1ELIC : x = 1.3x10-4

12 GeV: x = 4.5x10-2

Include low-Q2 region

Spin/Flavor Decompositionquark polarization q(x)first 5-flavor separation

DIS probes only the sum of quarks and anti-quarks requires assumptions on the role of sea quarksSolution: Detect a final state hadron in addition to scattered electron Can ‘tag’ the flavor of the struck quark by measuring the hadrons produced: ‘flavor tagging’

q parton distribution functiondf elementary -q sub-processDf

h fragmentation function

SIDIS

u > 0

d < 0

5 on 50 EIC projected dataFlavor Decomposition @ EIC

Lower x ~ 1/s

5 on 50 s = 1000

10 on 250 s = 10000

10-3 10-2 10-110-3 10-2 10-1xBj xBj

100 days at 1033

(Ed Kinney, Joe Seele)

Precisely image the sea quarksSpin-Flavor Decomposition of the Light Quark Sea

| p = + + + …>u

u

d

u

u

u

u

d

u

u

dd

dMany

models predict

u > 0, d < 0

RHIC-Spin region

Precisely image the sea quarksSpin-Flavor Decomposition of the Light Quark Sea

| p = + + + …>u

u

d

u

u

u

u

d

u

u

dd

dMany

models predict

u > 0, d < 0No competition foreseen!100 days at 1033

100 days at 1033

New Spin Structure Function: Transversity

• Nucleon’s transverse spin content “tensor charge”

• No transversity of Gluons in Nucleon “all-valence object”

• Chiral Odd only measurable in semi-inclusive DIS

first glimpses exist in data (HERMES, JLab-6)

Later work: more complicated COMPASS 1st results: ~0 @ low x Future: Flavor decomposition

-(in transversebasis)

q(x) ~

Need (high) transverse ion polarization(Naomi Makins, Ralf Seidl)

ELIC

Vanish like 1/pT (Yuan)

Correlation between Transverse Spin and Momentum of Quarks in

Unpolarized TargetAll Projected Data

Perturbatively Calculable at Large pT

-

(Harut Avakian, Antje Bruell)

Sivers effect: Pion electroproduction

•EIC measurements at small x will pin down sea contributions to Sivers function

S. Arnold et al arXiv:0805.2137 M. Anselmino et al arXiv:0805.2677

GRV98, Kretzer FF (4par)

GRV98, DSS FF (8par)

(Harut Avakian)

Sivers effect: Kaon electroproduction

•At small x of EIC Kaon relative rates higher, making it ideal place to study the Sivers asymmetry in Kaon production (in particular K-). •Combination with CLAS12 data will provide almost complete x-range.

EIC

CLAS12

(Harut Avakian)

Sivers effect: sea contributions

•Negative Kaons most sensitive to sea contributions. •Biggest uncertainty in experimental measurements (K- suppressed at large x).

GRV98, DSS FF

S. Arnold et al arXiv:0805.2137

M. Anselmino et al arXiv:0805.2677

GRV98, Kretzer FF

(Harut Avakian)

The Future of Fragmentation

TMDu(x,kT) f1,g1,f1T ,g1T

h1, h1T ,h1L ,h1

p

h

xTMD

D

Current Fragmentation h

qCan we understand the physical mechanism of fragmentation and how do we calculate it quantitatively?

Target Fragmentation

pM

h

dh ~ q fq(x) Dfh(z) dh ~ q

Mh/pq(x,z)

QCD Evolution

Correlate at EIC

Exclusive Processes: EIC Potential and Simulations

• Diffractive channels- data/experience from HERA: p (DVCS), 0p, p, J/p- DVCS simulations by A. Sandacz et al., see e.g.

http://web.mit.edu/eicc/SBU07/index.html- Found to be feasible with luminosity of 1033

• Non-diffractive channels- New territory for collider!- Much more demanding in luminosity- Physics closely related to JLab 6/12 GeV

- quark spin/flavor separations, nucleon/meson structure- Feasibility study of +n, 0p, K+

- A. Bruell, T. Horn, V. Guzey, and C. Weiss: in progress

GPDs and Transverse Gluon ImagingDeep exclusive measurements in ep/eA with an EIC:

diffractive: transverse gluon imaging J/, o, (DVCS) non-diffractive: quark spin/flavor structure , K, +, …

[ or J/, , 0

, K, +, … ]

Describe correlation of longitudinal momentum and transverse position of

quarks/gluons

Transverse quark/gluon imaging of nucleon

(“tomography”)

Are gluons uniformly distributed in nuclear matter or are there small clumps of glue?

GPDs and Transverse Gluon Imaging

gives transverse size of quark (parton) with longitud. momentum fraction x

EIC:1) x < 0.1: gluons!

x < 0.1 x ~ 0.3 x ~ 0.8

Fourier transform in momentum transfer

x ~ 0.001

2) ~ 0 the “take out” and “put back” gluons act coherently.

2) ~ 0 x - x +

d

GPDs and Transverse Gluon ImagingGoal: Transverse gluon imaging of nucleon over wide range of x: 0.001 < x < 0.1Requires: - Q2 ~ 10-20 GeV2 to facilitate interpretation

- Wide Q2, W2 (x) range - Sufficient luminosity to do differential measurements in Q2, W2, t

Q2 = 10 GeV2 projected data

Simultaneous data at other Q2-values

EIC enables gluon imaging!

(Andrzej Sandacz)

Statistical uncertainty in +n measurement

Luminosity= 1031

Γ d

σ/d

t (u

b/G

eV

2)

• High luminosity is essential to achieve the experimental goals

Ee=5 GeVEp=50 GeV Assume: 100 days(Tanja Horn, Antje Bruell, Christian Weiss)

Systematic uncertainty on +n rate estimate

• Data rates obtained using two different approaches are in reasonable agreement:• Ch. Weiss: Regge model• T. Horn: π+ empirical

parameterization

10<Q2<1515<Q2<2035<Q2<40

0.01<x<0.02

0.02<x<0.05

0.05<x<0.1

Assume: 100 days, Luminosity

= 1034

(Tanja Horn, Antje Bruell, Christian Weiss)

Λ

10<Q2<1515<Q2<20

35<Q2<40

0.01<x<0.02

0.02<x<0.05 0.05<x<0.1

Assume: 100 days, Luminosity = 1034

1H(e,e’K)Λ Momentum and

Angle Distributions

Rate estimate for KΛ

Using an empirical fit to kaon electroproduction

data from DESY and JLab

eA Landscape and a New Electron Ion Collider

Well mapped in e+pNot so for ℓ+A (nA)

Electron Ion Collider (EIC):L(EIC) > 100 L(HERA)

eRHIC (e+Au):Ee = 10 (20) GeVEA = 100 GeVseN = 63 (90) GeVLeAu (peak)/n ~ 2.9·1033 cm-2 s-

1

ELIC (e+Au):Ee = 10 GeVEA = 100 GeVseN = 63 GeVLeAu (peak)/n ~ 2.9·1034 cm-2 s-

1 (500 MHz operation)

Terra incognita: small-x, Q Qs

high-x, large Q2

Explore Electroweak Physics

• What are the unseen forces present at the dawn of the Universe but have disappeared from view as the universe evolved? precision electroweak experiments: sin2W , …

Questions for the Universe, Quantum Universe, HEPAP, 2004; NSAC Long Range Plan, 2007

“The task of the physicist is to see through the appearances down to the underlying, very simple, symmetric reality.”

- S. Weinberg

The LHC is driving global interest in low energy tests of the Standard Model.

Relatively high x charge symmetry violation?

Preliminary - EIC (Roy Holt)

Explore Charge SymmetryAssumed: u = up = dn & d = dp = un

Valid at < 1% : (Mn – Mp)/Mp ~ 0.1%

Figure from:Rodionov et al., Int. J. Mod. Phys. Lett. A9 (1994) 1799[Similar to MRST, Eur. Phys. J. C35 (2004) 325]

)()(2)()(

),(),( 22

22

00

xcxsxuxdx

QxFQxF

vvvv

NWNW

Accessible by comparison of e+d with e-d charged current cross sections:

uv(x) = uvp – dv

n

dv(x) = dvp - uv

n

Explore Charge SymmetryAssumed: u = up = dn & d = dp = un

Valid at < 1% : (Mn – Mp)/Mp ~ 0.1%

uv(x) = uvp – dv

n

dv(x) = dvp - uv

n

For the sea alone, CSV may be large!MRST obtained:

]08.01)[()(

]08.01)[()(

xuxd

xdxupn

pn

1

0

1

0

2222 )()()()()()( xdxudxxFxFxFxFx

dx dWdWpWpW

Accessible through charge symmetry sum rule defined by Ma (Phys. Lett. B274 (1992) 111)

Projected A(W-) Assuming xF3 will be known

Parity-Violating g5 Structure Function

To date unmeasured due to lack of high Q2 polarized e-p possibility.

Assumptions:1) Q2 > 225 GeV2

2) One month at luminosity of 1033

Requires positron beam

(Jose Contreras, Abhay Deshpande)

JLab 12 GeV Upgrade: The 12 GeV Upgrade, with its 1038+ luminosity, is expected to allow for a complete spin and flavor dependence of the valence quark region, both in nucleons and in nuclei.

Electron Ion Collider (eRHIC/ELIC/MANUEL)Provide a complete spin and flavor dependence of the nucleon and nuclear sea, study the explicit role that gluons play in the nucleon spin and in nuclei, open the new research territory of “gluon GPDs”, and study the onset of the physics of saturation.

Personal View:

Spin/Flavor Physics with a Future Electron-Ion

Collider

Concluding Statement (from Roy Holt at May 2008 EIC

Workshop)

• EIC research can penetrate some of the most profound mysteries and questions of 21st century physics.

• Technology is improving at an astounding rate:– Accelerator design, cavity improvement, energy recovery, crab cavities, cooling, polarization, polarimetry, detectors,

petascale computing, …

• There are interesting new opportunities worldwide. The next 10 years will be even more exciting than the last 10 years.

• We must put forward a most compelling case for the EIC on the time scale of the next LRP, folding in the international plans:– EIC project may be “Global” due to size (OECD Global Science Forum)

Diffractive Surprises‘Standard DIS event’

Detector activity in proton direction

7 TeV equivalent electron bombarding the proton … but proton remains intact in 15% of cases …

Diffractive event

No activity in proton direction

Predictions for eA for such hard diffractive evens range up to: ~30-40%... given saturation models

Look inside the “Pomeron” Diffractive structure functions Diffractive vector meson production ~ [G(x,Q2)]2

GPDs and Transverse Gluon Imaging

k

k'

*q q'

p p'

e

A Major new direction in Nuclear Science aimed at the 3-D mapping of the quark structure of the nucleon.

Simplest process:Deep-Virtual Compton Scattering

Simultaneous measurements over large range in x, Q2, t at EIC!

At small x (large W): ~ G(x,Q2)2

A large community believes a high luminosity polarized electron ion collider is the ultimate tool to understand the structure of quark-gluon systems, nuclear binding, and the conversion of energy into matter to such detailed level that we can use/apply QCD.

An Electron Ion Collider will allow us to look in detail into the sea of quarks and gluons, to create and study gluons, and to discover how energy transforms into matter From DOE 20-year plan

• Spin Structure of the Nucleon- Gluon and sea quark polarization- The role of orbital momentum

• Partonic Understanding of Nuclei- Gluon momentum distributions in Nuclei- Fundamental explanation of Nuclear Binding- Gluons in saturation, the Colored Glass Condensate- Hard diffraction

• Precision Tests of QCD- Bjorken Sum Rule

Gluons in the Nucleus

Note: not all models carefully checked against existing data + some models include saturation physics

1H(e,e’K)Λ Momentum and

Angle Distributions