Small x issues in nucleon spin structure (focus: polarized gluon distribution)

Small x issues in nucleon spin Small x issues in nucleon spin structurestructure

(focus: polarized gluon (focus: polarized gluon distribution)distribution)

Abhay DeshpandeAbhay DeshpandeStony Brook University Stony Brook University

RIKEN BNL Research CenterRIKEN BNL Research Center

December 15, 16, 2006December 15, 16, 2006Washington DCWashington DC

December 15, 2006

[email protected]@Stonybrook.Edu 2

One of the Key QuestionsOne of the Key QuestionsPolarized gluons and its distribution…Polarized gluons and its distribution…• What was known before today’s premier facilities What was known before today’s premier facilities

existed?existed?• What will we know using today’s premier facilities?What will we know using today’s premier facilities?• What needs to be done to address the remaining What needs to be done to address the remaining

questions in future?questions in future?

Summary/Conclusion/MessageSummary/Conclusion/Message::•Need precise data at Need precise data at low xlow x•Require Require high CM and Qhigh CM and Q22

•As wide a As wide a range in Qrange in Q22 as possible as possible

December 15, 2006


Summary/Conclusion PRD (112002) 1998Summary/Conclusion PRD (112002) 1998Spin Muon Collaboration’s NA47 Experiment at CERNSpin Muon Collaboration’s NA47 Experiment at CERN

From COMPASS, PHENIX and STAR…. But…..From COMPASS, PHENIX and STAR…. But…..

Similar message from E143 PRD (112003) 1998… same issue of PRD, and any other Similar message from E143 PRD (112003) 1998… same issue of PRD, and any other theory or experimental effort that was concluding at the time!theory or experimental effort that was concluding at the time!

December 15, 2006


RHICRHIC >> gg collider @ high energy >> gg collider @ high energyBRAHMS & PP2PP

STAR

PHENIX

AGS

LINAC BOOSTER

Spin Rotators(longitudinal polarization)

Solenoid Partial Siberian Snake

Siberian Snakes

200 MeV Polarimeter AGS Internal Polarimeter

Rf Dipole

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC PolarimetersStrong AGS Snake

Helical Partial Siberian Snake

PHOBOS

Spin Rotators(longitudinal polarization)

Spin flipper

Siberian Snakes

Installed and commissioned during FY04 runCommissioned during FY05 run

Installed and commissioned during FY05 run

Run6:Run6:~60-65% beam polarizations~60-65% beam polarizations~2 x 10~2 x 103131/cm/cm22/sec/sec

December 15, 2006


RHIC Spin: inclusive RHIC Spin: inclusive 00 and jet and jet

Spin2006

December 15, 2006


G Prospects: 2009-2012G Prospects: 2009-2012

• Left:Left: A ALLLL ( (00) prospects by 2009 (65 pb-1 luminosity ) prospects by 2009 (65 pb-1 luminosity intregrated by intregrated by PHENXPHENX using using 00 double spin asymmetries double spin asymmetries

• Right:Right: A ALL LL (jets) prospects by 2012 by (jets) prospects by 2012 by STARSTAR with 500 GeV in with 500 GeV in Center of Mass dataCenter of Mass data

(From Research Plan for Spin Physics at RHIC, February 2005)(From Research Plan for Spin Physics at RHIC, February 2005)

December 15, 2006


Now in the x space… how does Now in the x space… how does this look?this look?

• Spin plan:Spin plan:– 65 pb65 pb-1-1 at at √√s=200GeV & 70% pols=200GeV & 70% pol– 309 pb309 pb-1-1 at at √√s=500GeV & 70% pols=500GeV & 70% pol

RHIC Spin Plan 2005, Feb. 11RHIC Spin Plan 2005, Feb. 11

Note the limited x range!Note the limited x range!

RHIC will constrain RHIC will constrain G(x) veryG(x) very significantly, but with limitedsignificantly, but with limited x reach.x reach.

Low x behavior will remain Low x behavior will remain unconstrainedunconstrained

December 15, 2006


One “recent” attempt of global One “recent” attempt of global analysis.... analysis....

• Asymmetry analysis Asymmetry analysis collaboration (AAC)collaboration (AAC)– Hirai, Kumano, SaitoHirai, Kumano, Saito

• Pre-print: Pre-print: hep-ph/0603213hep-ph/0603213

• Recent:Recent: hep-ph/0607063hep-ph/0607063

hep-ph/0607063: clearly says….hep-ph/0607063: clearly says….

December 15, 2006


W. Vogelsang

This should not be surprising….

RHIC now

DIS now

pQCD workspQCD worksFor sureFor sure

pQCDpQCD““questionable”questionable”

RHIC in future

December 15, 2006


eRHIC vs. Other DIS FacilitieseRHIC vs. Other DIS Facilities• New kinematic regionNew kinematic region• EEee = 10 GeV (~5-12 GeV variable) = 10 GeV (~5-12 GeV variable)• EEpp = 250 GeV (~50-250 GeV = 250 GeV (~50-250 GeV

variable)variable)• EEAA= 100 GeV = 100 GeV • Sqrt[SSqrt[Sepep] = 30-100 GeV] = 30-100 GeV• Kinematic reach of eRHIC:Kinematic reach of eRHIC:

– X = 10X = 10-4-4 --> 0.7 (Q --> 0.7 (Q22 > 1 GeV > 1 GeV22))– QQ22 = 0 --> 10 = 0 --> 1044 GeV GeV22

• Polarization of e,p and light ion Polarization of e,p and light ion beams at least ~ 70% or betterbeams at least ~ 70% or better

• Heavy ions of ALL species at RHICHeavy ions of ALL species at RHIC– High gluonic densitiesHigh gluonic densities

• Luminosity Goal:Luminosity Goal:– L(ep) ~10L(ep) ~103333cmcm-2-2 sec sec-1-1

eRHIC

DIS

December 15, 2006


Low x Proton Spin StructureLow x Proton Spin StructureeRHIC 250 x 10 GeVLuminosity = ~85 inv. pb/day

Fixed target experiments1989 – 1999 Data

10 days of eRHIC runAssume: 70% Machine Eff. 70% Detector Eff.

Studies included statistical error & detector smearing to confirm that asymmetries are measurable. No present or future approved

experiment will be able to make this measurement

December 15, 2006


An Exercise:An Exercise:G from fits to eRHIC G from fits to eRHIC gg11(x,Q(x,Q22) )

Constrain better the shape and the first moment

G determined from the Scaling violations of g1

SMC Published 1998: First Moment of G(x) 1.0 +/- 1.0 (stat) +/- 0.4 (exp.syst) +/- 1.5 (low x)

-- one week eRHIC reduces statistical & low-x errors by ~3-5~3-5-- low x (~10-4 to 10-2)--> strong coupling, functional form at low -x, renorm. & fact. scales

€

G(x)dx =∫

December 15, 2006


SummarySummary• Importance of low x, high QImportance of low x, high Q22 and wide Q and wide Q22 range range

demonstrated for polarized gluons,demonstrated for polarized gluons, but there are other but there are other important measurements which also suffer due to lack important measurements which also suffer due to lack of low x dataof low x data

• Understanding the nucleon spin depends crucially on Understanding the nucleon spin depends crucially on exploring the low x spin structureexploring the low x spin structure

• A high luminosity polarized electron proton collider is A high luminosity polarized electron proton collider is required. required.

• Historically, low x and spin have been variables of high Historically, low x and spin have been variables of high return in terms of major discoveries that fundamentally return in terms of major discoveries that fundamentally changed our understanding of nature. Both together in changed our understanding of nature. Both together in an EIC certainly show high promise…..an EIC certainly show high promise…..

December 15, 2006


Spin & Low x Surprises…..Spin & Low x Surprises…..• Stern & Gehrlach (1921) Stern & Gehrlach (1921) Space Space

quantization associated with quantization associated with directiondirection

• Goudschmidt & Ulhenbeck Goudschmidt & Ulhenbeck (1926): (1926): Atomic fine structure & Atomic fine structure & electron spin magnetic electron spin magnetic momentmoment

• Stern (1933) Stern (1933) Proton anomalous Proton anomalous magnetic moment 2.79 magnetic moment 2.79 NN

• Kusch(1947) Kusch(1947) Electron Electron anomalous magnetic moment anomalous magnetic moment 1.001191.0011900

• Prescott & Yale-SLAC Prescott & Yale-SLAC Collaboration (1978) Collaboration (1978) EW EW interference in polarized e-dinterference in polarized e-d DIS, parity non-conservationDIS, parity non-conservation

• European Muon Collaboration European Muon Collaboration (1989) (1989) Spin Crisis/PuzzleSpin Crisis/Puzzle

• Elastic e-p scattering at SLAC Elastic e-p scattering at SLAC (1950s) (1950s) Q2 ~ 1 GeV2 Q2 ~ 1 GeV2 (Finite (Finite size of the proton)size of the proton)

• Inelastic e-p scattering at SLAC Inelastic e-p scattering at SLAC (1960s) (1960s) Q2 > 1 GeV2 Q2 > 1 GeV2 (Parton structure of the proton)(Parton structure of the proton)

• Inelastic mu-p scattering off Inelastic mu-p scattering off p/d/N at CERN (1980s)p/d/N at CERN (1980s) Q2 > Q2 > 1 GeV2 1 GeV2 (Un-polarized EMC (Un-polarized EMC effect, nuclear shadowing?)effect, nuclear shadowing?)

• Inelastic e-p scattering at Inelastic e-p scattering at HERA/DESY (1990s)HERA/DESY (1990s) Q2 > 1 Q2 > 1 GeV2 GeV2 Unexpected rise of F2 at Unexpected rise of F2 at low x, diffraction in e-p, low x, diffraction in e-p, Saturation/CGC (??)Saturation/CGC (??)

Small x issues in nucleon spin structure (focus: polarized gluon distribution)

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