UMass Amherst Christine Aidala Jacksonville, FL Measuring the Gluon Helicity Distribution at a Polarized Electron-Proton Collider APS April Meeting 2007.

UMass AmherstChristine Aidala

Jacksonville, FL

Measuring the Gluon Helicity Distribution at a Polarized Electron-Proton Collider

APS April Meeting 2007

C. Aidala, APS April Meeting April 15, 2007

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• One of the most common, stable components of everyday matter

• Fundamental object in QCD

• “If we understand the proton, we understand everything.” – F. Wilczek

• But we still don’t understand the proton!

q

q

g

Proton

u u

d

p

Proton Structure


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Proton Structure• Complex linear momentum

structure– Depends on energy scale at

which probed– Now well measured over a wide

range in x, Q2

• Can be described in terms of structure functions

• Or in terms of parton distribution functions (pdf’s)– f(x): Probability of finding a

quark of flavor f carrying momentum fraction x of the proton momentum

• Complex angular momentum structure!

• Discovered in late ’80’s by EMC experiment at CERN that quark spin contribution to proton spin only 20-30%!– “Spin crisis”

– Rest from gluon spin and orbital angular momentum

qGLG 2

1

2

1


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Electromagnetic probes of DIS don’t interact directly with gluons. Obtain gluon

distribution via Bjorken scaling violations.

),(log

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2 QxxgQ

F

World Data on F2p Structure Function

Next-to-Leading-Order (NLO) perturbative QCD (DGLAP) fits

Note sharp rise of gluon contribution below x~0.1.Gluons measured to carry ~50% of proton’s linear

momentum!


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World Data on g1p Polarized Structure Function

PolarizedUnpolarized

Very limited kinematic region currently measured

by fixed-target experiments. Extremely poor constraint on gluon helicity distribution from

scaling violations![Add xDg(x) figure?

Which?]

Polarized electron-proton collider could provide

kinematic coverage necessary!


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World Data on F2p Projected Data on g1

p

An EIC makes it possible!Region of existing g1p data

5 fb-1

A. Bruell


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g from g1 at the EIC5 fb-1

Note that positive g leads to negatively divergent g1 at low x, negative g to

positively divergent g1 at low x.

Excellent discrimination with EIC for lower Q2 bins.

GRSV std (g > 0)GRSV g = 0

GRSV g = +gGRSV g = -g

A. Bruell


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Polarized Gluon Distribution via Charm Production

c

c

D mesons

D mesons

LO QCD: asymmetry in D production directly proportional to G/G

Very clean process !


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Polarized Gluon Distribution via Charm Production: A First Study for EIC

Precise determination

of G/G for 0.003 < xg < 0.4at common Q2 of 10 GeV2

RHIC SPIN

DK10 fb-12.5 fb-1

A. Bruell


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Summary

• Proton a fundamental object in QCD. Decades of studies have revealed a rich linear momentum structure. Much remains to be understood of the proton’s spin structure!

• Polarized electron-proton collider would open up new kinematic regime and allow deeper understanding of proton spin structure, including greatly improved measurement of gluon spin contribution.

• Studies underway for two alternate EIC facilities, one at RHIC (BNL), the other at CEBAF (JLab)

• More info available at http://www.bnl.gov/eic


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Extra


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To Add?

• Add one-slide intro to EIC—eRHIC and ELIC designs, kinematic coverage, basic (minimum?) machine parameters. Cite also website.

• More details on charm

• Comments on RHIC spin program


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Polarized Parton Distribution Functions

• Polarized pdf--the difference in probability between scattering off of a parton with one spin state vs. the other– Function of xBjorken, the

momentum fraction of the proton carried by the parton

up quarks

down quarks sea quarks

gluon

EMC, SMC at CERN E142 to E155 at SLAC

HERMES at DESYPHENIX at RHIC

PRD74:014015 (2006)


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Comparison to Other Facilities

Luminosity vs. CM Energy

Q2 vs. x


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Future: Polarized Gluon Distribution from RHIC


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Very demanding detector requirements !


starting assumptions for EIC:

• vertex separation of 100m• full angular coverage (3<<177 degrees)• perfect particle identification for pions and kaons

(over full momentum range)• detection of low momenta particles (p>0.5 GeV)• measurement of scattered electron

(even at very small scattering angles)• 100% efficiency


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If: • We can measure the scattered electron even at angles close to 00

(determination of photon kinematics)• We can separate the primary and secondary vertex down to about 100 m• We understand the fragmentation of charm quarks ()• We can control the contributions of resolved photons• We can calculate higher order QCD corrections ()


Precise determination

of G/G for 0.003 < xg < 0.4

at common Q2 of 10 GeV2


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ELIC Accelerator Design Specifications

Center-of-mass energy between 20 GeV and 90 GeV

with energy asymmetry of ~10, which yields

Ee ~ 3 GeV on EA ~ 30 GeV up to Ee ~ 9 GeV on EA ~ 225 GeV

Average Luminosity from 1033 to 1035 cm-2 sec-1 per Interaction Point

Ion species: Polarized H, D, 3He, possibly Li Ions up to A = 208

Longitudinal polarization of both beams in the interaction region (+Transverse polarization of ions +Spin-flip of both beams)

all polarizations >70% desirable Positron Beam desirable


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ELIC Layout

30-225 GeV protons30-100 GeV/n ions

3-9 GeV electrons3-9 GeV positrons


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Design Features of ELICDirectly aimed at addressing the science program:

“Figure-8” ion and lepton storage rings to ensure spin preservation and ease of spin

manipulation. No spin sensitivity to energy for all species.

Short ion bunches, low β*, and high rep rate (crab crossing) to reach unprecedented

luminosity.

Four interaction regions for high productivity.

Physics experiments with polarized positron beam are possible. Possibilities for e-e-

colliding beams.

Present JLab DC polarized electron gun meets beam current requirements for filling

the storage ring.

The 12 GeV CEBAF accelerator can serve as an injector to the electron ring. RF

power upgrade might be required later depending on the performance of ring.

Collider operation appears compatible with simultaneous 12 GeV CEBAF operation

for fixed target program.


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eRHIC•Integrated electron-nucleon luminosity of ~ 50 fb-1 over about a decade for both highly polarized nucleon and nuclear (A = 2-208) RHIC beams.

50-250 GeV polarized protons up to 100 GeV/n gold ionsup to 167 GeV/n polarized 3He ions

•Two accelerator design options developed in parallel (2004 Zeroth-Order Design Report):

ERL-based design (“Linac-Ring”; presently most promising design):

• Superconducting energy recovery linac (ERL) for the polarized electron beam.• Peak luminosity of 2.6 1033 cm-2s-1 with potential for even higher luminosities.• R&D for a high-current polarized electron source needed to achieve the design goals.

Ring-Ring option:• Electron storage ring for polarized electron or positron beam. • Technologically more mature with peak luminosity of 0.47 1033 cm-2s-1.


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ERL-based eRHIC Design

Electron energy range from 3 to 20 GeV Peak luminosity of 2.6 1033 cm-2s-1 in electron-hadron collisions; high electron beam polarization (~80%); full polarization transparency at all energies for the electron beam; multiple electron-hadron interaction points (IPs) and detectors; 5 meter “element-free” straight section(s) for detector(s); ability to take full advantage of electron cooling of the hadron beams; easy variation of the electron bunch frequency

to match the ion bunch frequency at different ion energies.

0

0.5

1

1.5

2

2.5

3

20 30 40 50 60 70 80 90 100 110 120 130 140 150

Center-Of-Mass Energy, GeV

Pe

ak

Lu

min

os

ity

, 1

033 c

m-2

s-1

3GeV(e)-50GeV(p) 20GeV(e)-50GeV(p)


0

0.5

1

1.5

2

2.5

3

20 30 40 50 60 70 80 90 100 110 120 130 140 150

Center-Of-Mass Energy, GeV

Pe

ak

Lu

min

os

ity

, 1

033 c

m-2

s-1



PHENIX

STAR

e-cooling (RHIC II)

Four e-beam passes

e+ storage ring 5 GeV - 1/4 RHIC circumference

Main ERL (3.9 GeV per pass)

5 mm

5 mm

5 mm

5 mm

Compact recirculation loop magnets


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Ring-Ring eRHIC Design

Based on existing technology

Collisions at 12 o’clock interaction region

10 GeV, 0.5 A e-ring with 1/3 of RHIC circumference (similar to PEP II HER)

Inject at full energy 5 – 10 GeV

Polarized electrons and positrons

RHIC

5 – 10 GeV e-ring

e-cooling(RHIC II)

5 -10GeV full energy injector

UMass Amherst Christine Aidala Jacksonville, FL Measuring the Gluon Helicity Distribution at a Polarized Electron-Proton Collider APS April Meeting 2007.

Documents

proton proton structure

g grsv g

negative g

positive g

polarized gluon distribution

protons spin structure

summary proton

bruell slide