constraining the polarized gluon pdf in polarized pp collisions at rhic frank ellinghaus university...
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![Page 1: Constraining the polarized gluon PDF in polarized pp collisions at RHIC Frank Ellinghaus University of Colorado (for the PHENIX and STAR Collaborations)](https://reader036.vdocuments.us/reader036/viewer/2022062515/56649d175503460f949ece9b/html5/thumbnails/1.jpg)
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
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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
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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 !!! ->
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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
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Frank Ellinghaus, University of Colorado
PHENIX and STAR
Large acceptanceAzimuthal symmetry
High rate capabilityLimited acceptance
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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.)
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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
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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)
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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
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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
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Frank Ellinghaus, University of Colorado
STAR – 0 and PHENIX -
STAR 0 consistent with PHENIX
PHENIX excludes G= G(x), -G(x) so far
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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
+
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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
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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
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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)