Constraining the polarized gluon PDF in polarized pp collisions at RHIC

Frank EllinghausUniversity of Colorado

(for the PHENIX and STAR Collaborations)

July 2008ICHEP’08, Philadelphia, USA

Frank Ellinghaus, University of Colorado

Parton Distribution Functions (PDFs)

• From fits to F2 measurements, unpolarized PDFs

can be inferred

– q=u,d (s,c)

( ( ) ( )) 0.5B B B Bq

dx x q x q x

PDFs fitted using F2 at Q2=4U=u+cu, D=d+sd

• The total fraction of nucleon momentum carried by quarks:

– Gluons carry the other half!

( ), ( ), ( )B B Bq x q x g x

U=u+cu, D=d+sd

Frank Ellinghaus, University of Colorado

Results from Inclusive Polarized DIS

• Analogous to unpolarized (F2) case, g1 can be used to fit polarized PDFs:

Polarized PDFs extracted from fits to g1(proton, deuteron)

• Result: Quarks carry only about 30 % of the nucleon spin (0.3)

• Gluon contribution G not well constrained due to small range in xB,Q2 (no polarized ep collider)

( ), ( ), ( )B B Bq x q x G x

…but polarized pp Collider !!! ->

Frank Ellinghaus, University of Colorado

RHIC @ BNL

STARSTAR

Relativistic Heavy Ion Collider also provides longitudinally and transverselypolarized proton beams at s = 200 GeV, 62.4 GeV, (500 GeV, 2009+)

Polarimeter (H jet) pC Polarimeters

Spin Rotators Siberian Snakes

Frank Ellinghaus, University of Colorado

PHENIX and STAR

Large acceptanceAzimuthal symmetry

High rate capabilityLimited acceptance

Frank Ellinghaus, University of Colorado

PHENIX longitudinally polarized pp Runs

Year s [GeV] Recorded L Pol [%] FOM (P4L)

2003 (Run-3) 200 .35 pb-1 27 1.5 nb-1

2004 (Run-4) 200 .12 pb-1 40 3.3 nb-1

2005 (Run-5) 200 3.4 pb-1 49 200 nb-1

2006 (Run-6) 200 7.5 pb-1 57 690 nb-1

2006 (Run-6) 62.4 .10 pb-1 48 5.3 nb-1

(Similar numbers for STAR. Experiments can separately choose longitudinal or transverse polarization.)

Frank Ellinghaus, University of Colorado

G via direct measurement

2 2~LL gg qg qqA a a q G aG q

Access to polarized gluon distribution function via double helicity asymmetry in inclusive polarized pp scattering, e.g.,

Invariant mass spectrum of 2 photons in EMCal(M=135MeV)

Measure from DISpQCD, fragmentation fcts.

0p p X

NRN

NRN

PPA

YBLL

1

L

LR Relative Luminosity R using

beam-beam counters

Frank Ellinghaus, University of Colorado

First: Check unpolarized case!

Using a set of unpolarized PDFs ( + fragmentation functions in case of hadron (0) production) the cross section agrees with NLO pQCD calculations.

PHENIX -- 0

PRD76:051106,2007

STAR -- jets

PRL 97, 252001 (2006)

Frank Ellinghaus, University of Colorado

STAR -- jetsRun 6 preliminary

Run 5 (2005): PRL 100, 23 (2008)

G = G(x), -G(x) excluded;

GRSV-std excluded with 99% CL

Run 6 preliminary

Frank Ellinghaus, University of Colorado

PHENIX --0 at 200 GeV

Run 5: Phys.Rev.D76:051106,2007

Run 6 preliminary

GRSV: Glueck et al., PRD 63 (2001)

G = G(x), -G(x) excluded;

GRSV-std excluded on 3 sigma level: 2(std)2min> 9

Run 6 preliminary

Frank Ellinghaus, University of Colorado

STAR – 0 and PHENIX -

STAR 0 consistent with PHENIX

PHENIX excludes G= G(x), -G(x) so far

Frank Ellinghaus, University of Colorado

+, –, 0 and the sign of G

0

0 LL LL LLG A A A

0

d d dD D D

0

0 LL LL LLG A A A

Especially in the region where qg scattering is dominant (pT > 5 GeV),the increasing contribution of d quarks (d<0) leads to:

“Model independent” conclusion possible once enough data is available.

Fraction of pion production

-

ST

AR

--

Ru

n 5

+

Frank Ellinghaus, University of Colorado

First pol. PDF extraction using pp data

DSSV, arXiv: 0804.0422First “global” (DIS+SIDIS+pp) analysis!

G small in measured range (0.05 < x< 0.2).Contribution at small or large x?

Different ranges in x can be probed in:• 500 GeV (2009+, lower x) and 62 GeV running (larger x, larger scale unc.) ->• different rapidities ->

• Measurement averages over certain x range• Shape of G(x) cannot be extracted -> Value for first moment model dependent • Next step: Mapping of x-dependence via di-jets, di-hadrons and gamma-jet

Frank Ellinghaus, University of Colorado

• At fixed xT = 2pT/sqrt(s) cross-section is 2 orders of magnitude higher at 62.4GeV than at 200GeV

• Significant result at high xT from small data set at 62.4 GeV (0.04 pb-1) when compared to 200 GeV data (1.8pb-1)

Accessing different x-rangesPHENIX: 0 ALL at s=62.4 GeV

Increased sensitivity to larger x

STAR: 0 ALL at forward rapidity

Increased sensitivity to smaller x

• Also: step towards gamma-jet correlation measurements to map out the x-dependence

Frank Ellinghaus, University of Colorado

Summary• Measurements of double helicity asymmetries in polarized pp

scattering at PHENIX and STAR provided a significant constraint on the polarized gluon PDF in a global QCD fit to “all” DIS, SIDIS and pp data

• G(x) small in measured range 0.05 < x < 0.2

• Different beam energies (500 GeV, 2009+) and rapidities will give access to an order of magnitude smaller x

• Correlation measurements will provide sensitivity to shape of G(x)