Tanja Horn

International Conference on the Structure of Baryons, BARYONS 2013

Probing Sea Quarks and Gluons: The Electron-Ion Collider Project

Glasgow, Scotland, 24-28 June 2013

/ JLab

Nuclear Science: to discover, explore, and understand all forms of

nuclear matter and its benefits to our society

QCD: A fundamental theory for the dynamics of quarks and gluons

It describes the formation of all forms of nuclear matter

the nucleus

the nucleons

the quarks and gluons

Accounting for essentially all of the mass of the visible universe

Nuclear matter:

Rutherford exp’t SLAC “Rutherford”

1911 1968 Revolutionized our view

of atomic structure Open the gateway

to nucleon structure

1972

Discovery of QCD

Understanding QCD is a fundamental & compelling goal of Nuclear Science

QCD and the Origin of Mass

98% of the mass of visible matter is

dynamically generated by self-

generating gluon fields

– Higgs mechanism has no role here.

u + u + d = proton 0.003 + 0.003 +0.006 ≠ 0.938 GeV

The similarity of mass between the

proton and neutron arises from the

fact that the gluon dynamics are the

same

– Quarks contribute almost nothing.

How do gluons create energy/mass?

Hadron mass spectrum

from lattice QCD

At low energy:

At high energy:

Measure e-p at 0.3 TeV (HERA)

Predict p-p and p-p at 0.2, 1.96, and 7 TeV

Asymptotic freedom

+ perturbative QCD

Successes of QCD

Mysteries of the Strong Force

Gluon dynamics plays a large role in proton spin

– Spin=intrinsic (parton spin) + motion (orbital angular momentum)

3D structure of the nucleon is non-trivial

– Far more than the simple valence quark structure

– Need to understand confinement to know how

proton properties arise from constituents

Why do quarks contribute so little (~30%) to proton spin?

How do gluons bind quarks and themselves into 3D hadrons?

Dynamical balance between gluon radiation and

recombination

– How does the unitarity bound of the hadronic cross

section survive if soft gluons in a proton or nucleus

continue to grow in numbers?

Saturation of gluons?

Map the spin and spatial structure of quarks and gluons in nucleons

Discover the collective effects of gluons in atomic nuclei

Understand the emergence of hadronic matter from color charge

Opportunities for fundamental symmetry measurements?

• How much spin is carried by gluons?

• Does orbital motion of sea quarks contribute to spin?

• What do the partons reveal in transverse momentum and coordinate space

• What is the distribution of glue in nuclei?

• Are there modifications as for quarks?

• Can we observe gluon saturation effects?

• How do color charges evolve in space and time?

• How do partons propagate in nuclear matter?

Physics Program of an EIC

o Generalized Parton Distribution (GPDs)

o Transverse Momentum Distributions (TMDs)

• Can nuclei help reveal the dynamics of fragmentation?

Needs high luminosity

and range of energies

Unified view of nucleon structure

Wigner distributions:

EIC – 3D imaging of sea and gluons:

TMDs – confined motion in a nucleon (semi-inclusive DIS)

GPDs – Spatial imaging of quarks and gluons (exclusive DIS)

5D

3D

1D

JLab12 COMPASS

for Valence

HERMES JLab12

COMPASS

Only a small subset

of the (x,Q2)

landscape has been

mapped here:

terra incognita

Gray band: present

“knowledge”

Purple band: EIC (2 )

u u

An EIC with good

luminosity & high

transverse polarization

will be the optimal tool

3D

1D

Imaging the Transverse Momentum of the Quarks

No other machine in the world can do this!

Transverse Spatial imaging of sea quarks and gluons

GPDs Exclusive processes – DVCS or meson production:

Quark GPDs and its orbital contribution to proton’s spin:

First meaningful constraint

on quark orbital motion to

proton spin by combining the

sea from the EIC and

valence region from JLab 12

Fourier transform of the t-dep

Spatial imaging of glue density

t-dep

J/Ψ, Φ,.. Q

Example: Gluon Imaging from J/

Only possible at the EIC

Weiss et al, INT 10-3

(J/Ψ, φ)

ep → e'π+n ep → e'K+Λ

Horn et al. 08+, INT10-3

(DVCS)

Spatial Imaging through meson production

Are radii of quarks

and gluons different

at a given x? –

DVCS and J/

Are strange and non-

strange (light sea)

quark sizes different

at given x? - and K

production

Full image of the proton can be obtained by mapping t-distributions for different processes.

Hints from HERA:

Area (q + q) > Area (g)

Dynamical models predict difference: pion cloud, constituent quark picture

-

• Q2 > 10 GeV2

for factorization

• Statistics hungry at high Q2!

x = 10-3

• DVCS asymmetries with a transversely polarized target

are sensitive to the GPD E, and open opportunities to

study spin-orbit correlations

F. Sabatie

Ei(x, ξ, t) = κi(t) Hi(x, ξ, t)

Model:

• Colliders provide an very good Figure-Of-Merit (FOM)

– Unpolarized FOM = Rate = Cross section x Luminosity x Acceptance

– Polarized FOM = Rate x (Polarization)2 x (Target dilution)2

Error bars shown only for κsea = +1.5

DVCS with a polarized “target”

(Lattice Calculation of the IP

density of up quark,

QCDSF/UKQCD Coll., 2006)

Proton Spin and Hadron Structure

Lq? Jg?

Measure g - Explore the “full” gluon and sea quark contribution

Measure TMD and GPDs - quantify the role of orbital motion

Two complementary approaches to resolve proton spin puzzle

Proton spin:

Over 20 years effort (following EMC discovery)

Quark (valence + sea) helicity (world DIS): of proton spin

Gluon helicity (RHIC+DIS): small?

It is more than the number ½! It is the interplay between the

intrinsic properties and interactions of quarks and gluons

current data

w/ EIC data

Q2 = 10 GeV2 Helicity PDFs at an EIC

G

Many models predict

u > 0, d < 0

Can constrain light quark sea polarization

(needs intermediate s and high luminosity)

and gluon polarization at small and large x. √

Precision measurement of Δg:

o Extend to smaller x regime

Gluons in nuclei

• HERA measured the longitudinal gluon distributions in the nucleon

– FL and dF

2/dln(Q2)

• Very little is known about gluons in nuclei

• Knowledge of gluon PDF

essential for quantitative

studies of onset of saturation

• EIC: access gluons through

FL (needs variable energy)

and dF2/dln(Q2)

Ra

tio

of g

luo

ns in

le

ad

to

de

ute

riu

m

pT

q

h

g* e

e

tproduction

>RA

pT2 vs. Q2

Hadronization – parton propagation in matter

tproduction

< RA

pT

q g*

e

e

h

• pT broadening strongly constrains theory

Accardi, Dupre

• Large range in υ at a collider allows

– Isolation of pQCD energy loss (large υ)

– Study of (pre)hadronization (smaller υ)

large υ smaller υ large υ

• Heavy flavors: B, D mesons, J/Ψ ...

• Jets above s = 1000 GeV2

– „real“ pQCD, IR safe

pT2 = pT

2(A) – pT2(2H)

Z. Kang

„If one could tag neutron, it typically

leads to larger asymmetries“

C. Hyde

• MEIC will provide longitudinal and transverse polarization for d, 3He, and other light ions

• Measurements of exclusive reactions like DVCS also greatly benefit from polarized neutron “targets“

Sivers asymmetry

EIC kinematics

• Polarized neutrons are important for probing d-quarks through SIDIS

Spectator Tagging with Polarized Deuterium

DpnnXLDRD for polarized light nuclei at the EIC

+

EIC forward detection design allows for separating the virtual photon

(current) and fragmentation regions in the final state

Final transverse momentum of the detected pion Pt arises from convolution

of the struck quark transverse momentum kt with the transverse momentum

generated during the fragmentation pt.

Du+(z,pt)

Rap

idity

Current

Target

Corr

ela

tions?

Color neutralization – a correlated 3D problem

o May reveal dynamical pair correlations as expected by

dynamical breaking of chiral symmetry in QCD vacuum

Pt

kt

pt

Pt

P’t

US based EIC: Kinematic reach and machine properties

For e-N collisions at the EIC:

First polarized e-p collider

Polarized beams: e, p, d/3He

Variable center of mass energy

Luminosity Lep ~ 1033-34 cm-2s-1

HERA luminosity ~ 5x1031 cm-2 s-1

For e-A collisions at the EIC:

First e-A collider

Wide range in nuclei

Variable center of mass energy

Luminosity per nucleon same as e-p

s (GeV2)

Q2 ~ ysx

EIC – staging at BNL and JLab

Stage I Stage II eRHIC @ BNL

MEIC / EIC @ JLab √s = 13 – 70 GeV

Ee = 3 – 12 GeV

Ep = 15 – 100 GeV

EPb

= up to 40 GeV/A

√s = 34 – 71 GeV

Ee = 3 – 5 (10 ?) GeV

Ep = 100 – 255 GeV

EPb

= up to 100 GeV/A

√s = up to ~180 GeV

Ee = up to ~30 GeV

Ep = up to 275 GeV

EPb

= up to 110 GeV/A

√s = up to ~140 GeV

Ee = up to 20 GeV

Ep = up to at least 250 GeV

EPb

= up to at least 100 GeV/A

(EIC) (MEIC)

MEIC/EIC Layout

Pre-

booster

Ion linac

IP1 IP2

High Energy Ring

(Stage-II EIC)

CE

BA

F

Electron

Injection

Interaction Region

ultra forward

hadron detection low-Q2

electron detection large aperture

electron quads

small diameter

electron quads

central detector

with endcaps

ion quads

50 mrad beam

(crab) crossing angle

n,γ

e p

p

small angle

hadron detection

60 mrad bend

In general, e-p and e-A colliders have a large fraction of their science related to the detection of

what happens to the ion beams… spectator quark or struck nucleus remnants will go in the

forward (ion) direction this drives the integrated detector/interaction region design

e

Hadron detection in

three stages

Endcap with 50 mrad

crossing angle

Small dipole covering

angles to a few degrees

Ultra-forward up to one

degree, for particles passing

the accelerator quads

Roman pots Thin exit

windows

Fixed

trackers in

vacuum?

• Good acceptance for ion fragments (different rigidity from beam)

– Large downstream magnet apertures

– Small downstream magnet gradients (realistic peak fields)

– Roman pots not needed

• Good acceptance for recoil baryons (rigidity similar to beam)

– Small beam size at second focus (to get close to the beam)

– Large dispersion (to separate scattered particles from the beam)

– Roman pots important

• Good momentum and angular resolution

– Large dispersion (e.g., 60 mrad bending dipole)

– Long, instrumented magnet-free drift space

• Sufficient separation between beam lines (~1m)

Ultra-forward Hadron detection requirements

DpnnXDVCS on the proton

e

p

n

Ultra-forward detection concept

• Momentum resolution < 3x10-4

– 15 MeV resolution for 50 GeV/A deuterium beam

• Excellent acceptance for all ion fragments

• Neutron detection in a 25 mrad cone down to zero degrees

• Recoil baryon acceptance

– up to 99.5% of beam energy for all angles

– down to 2-3 mrad for all momenta

• 100 GeV maximum ion energy allows using large-aperture magnets with achievable field strengths

20 Tm dipole 2 Tm dipole

solenoid

n p e

The central detector

Similar solutions for both designs: interesting dual-solenoid idea for JLab (à la ILC4)

eRHIC

GEANT4 modeling has started

BNL

JLAB

Summary

• An EIC is required to fully understand nucleon structure and the role of gluons in nuclei

EIC is the ultimate tool for studying sea quarks and gluons

EIC offers a fully integrated interaction region

• Straightforward detection of recoil baryons, spectators, and target fragments

EIC is a maturing project

• Designs ongoing at JLab and BNL

• Funds for joint detector R&D projects and accelerator R&D have been allocated

• Promising collaboration on central detector with HEP

• Physics effort for polarized light nuclei and forward tagging ongoing