Spin structure functions - some topics for a future EIC

Sebastian Kuhn

ODU

Things to measure:

• g1p,d,n: Evolution, low x extrapolation

=> Better determination of G, Bjorken and

Burkhardt-Cottingham sum rules,…

• SIDIS: s, u - d, …; kT dependence

• Spin EMC effect with spectator tagging

• Target fragmentation (e.g., self-analyzing

decay)

Existing Data for p and d

xbj

A1d ( D-state corrected ) HF perturbed QM

World Data Parm + d WF Q2 = 4.2 GeV2

World Data parm Q2 = 10 GeV2

Spin 3/2 suppression

Helicity 3/2 suppression

Symmetric Q Wave function suppression

SLAC - E143

SMC

HERMES

SLAC - E155

CLAS-EG1b Q2 = 1.0 - 4.52 GeV2

SU(6)

pQCD

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Isgur, PRD 59, 034013 (2003)

}

Close and Melnitchouk, PRC 68, 035210 (2003)

xbj

A1p

HF perturbed QM

World Data parm Q2 = 10 GeV2

Spin 3/2 suppression Helicity 3/2 suppression

Symmetric Q Wave function

SLAC - E143

SMC

HERMES

SLAC - E155

CLAS-EG1b Q2 = 1.0 - 4.52 GeV2

SU(6)

pQCD

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Proton

Deuteron

W > 2; Q2 > 1

Predicted Data from CLAS12Proton Deutero

nW > 2; Q2 > 1

Existing and proposed data for the neutron

Improvements in u, d, G, s expected before EIC

Note: Decreased uncertainties below x = 0.05 misleading (artifact of over-constrained fit)

Present knowledge ofs and G (ca 2008)

Kinematic Coverage (DIS)

Jlab at 12 GeV

DIS - low x

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

x

g1d

Q2 1-4

Q2 4-16

Q2 9-36

Q2 25-100

COMPASS '07

SLAC E155

Assume 10 GeV e- on 30 GeV d, Luminosity 1033/s/cm2, 100 days

Status of first moments

€

⇐ g1(x,Q2)dx

0

1

∫

€

g2(x,Q2)dx

0

1

∫ ⇒

Flavor Decomposition from SIDIS

QuickTime™ and a decompressor

are needed to see this picture.

(copied from a talk by Pawel Nadel-Turonski)

Concluding Thoughts

• Even at modest energy and luminosity, it looks like we can make significant progress on spin structure

• Obviously, I need to do some real work before making any firmer conclusions

• Obvious upgrade path towards lower x as cm energy increases

• We should stop thinking about DIS, SIDIS, PVDIS,… as different topics - instead, use unified EW theory as a comprehensive probe of inclusive nucleon structure, getting information from all structure functions F1 - F3, g1 - g5 (, -Z, W)