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Review of G in DIS and pp… a lot has happened since Kyoto
Frank Ellinghaus
University of Mainz / University of Colorado
October 2008
SPIN’08, Charlottesville, USA
Frank Ellinghaus, Spin’08, Charlottesville
e-p Spins antialigned
In the beginning….
– Electron polarization transfers to virtual photons– Compare DIS cross sections with aligned and antialigned ep spins
e-p Spins aligned~
g1 (proton) > 0-> Larger cross section for anti-aligned ep Spins -> Higher probability for aligned quark-proton Spins
212( , )B
B
dg x Q
dx dQ
• Polarize electrons and nucleons (started in mid 1970s at SLAC)
G. Baum et al, PRL 51, 1983
Frank Ellinghaus, Spin’08, Charlottesville
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 30% of the nucleon spin (0.3)
HERMES: PRD 75:012007 (2007)
COMPASS: PLB 647:8 (2007)
• 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
…try DIRECT measurements ->
Frank Ellinghaus, Spin’08, Charlottesville
G via direct measurement in DIS
||
1 1~ ( ) ...PGF PGF BG BGG
A
f a fGf A A
Photon-Gluon Fusion: * g -> qqbar
• ccbar production, detect D-mesons, hard scale provided by charm mass, clean channel (low BG), statistically limited
• detect hadrons (or hadron pairs) with Q2>1 GeV2(hard scattering) More BG sources with difficult to determine or/and model-dependent fractions and asymmetries. Larger sample…
• detect hadrons (or hadron pairs) using all Q2
Even more BG sources with difficult to determine or/and model-dependent fractions and asymmetries. Largest sample…
Frank Ellinghaus, Spin’08, Charlottesville
Fixed Target DISBeam: 27.6 GeV e+/e-; Beam: 27.6 GeV e+/e-; 50-55% polarization50-55% polarization
Target: H, D Target: H, D 80-85% polarization80-85% polarization
6/30/07
Beam: 160 GeV Beam: 160 GeV ; ; 75-80% polarization75-80% polarization
Target: Target: 66LiD; LiD; 50% polarization50% polarization
Frank Ellinghaus, Spin’08, Charlottesville
The “golden” channel
D*->D0 soft
2002-2004 analysis: hep-ex/0802.2160
F. KunneTuesday
2002-2006 preliminary analysis:
“tagged D0”
Frank Ellinghaus, Spin’08, Charlottesville
COMPASS: Hadron Pairs, Q2 > 1 (GeV/c)2
LEPTO with JETSET tuned to data…
• pT>0.7 GeV/c for both hadrons increases PGF contr.• Q2 > 1 (GeV/c)2 provides hard scale• At least two add. subprocesses to be considered
500k events
K. KurekTuesday
Frank Ellinghaus, Spin’08, Charlottesville
COMPASS: Hadron Pairs, Q2 < 1 (GeV/c)2
0.080.040.095gx
PYTHIA
Add. Sensitivity to G, but polarized PDFs of photon unmeasured
• pT>0.7 GeV/c for both hadrons increases PGF contr.• pT
2 > 2.5 (GeV/c)2 provides hard scale
• Add. contr. (about 50%) from resolved-photon processes
PGF
QCDC
LO
PLB 633 (2005) 25
preliminary2 23( / )GeV c
Frank Ellinghaus, Spin’08, Charlottesville
HERMES, hadrons, all Q2
PYTHIA (tuned to data)
2 0.1250.082/ ( , ) 0.078 0.034 0.011g g x
20.204 1.35 2 GeVx
Results from different (proton, deuterium) data samples and different event topologies are consistent.
Good sensitivity, but model dependentg/g=0 -> contribution of quarks
Frank Ellinghaus, Spin’08, Charlottesville
Summary Direct DIS measurements
g/g=0 is likely small with unknown sign!
Frank Ellinghaus, Spin’08, Charlottesville
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, Spin’08, Charlottesville
PHENIX and STAR
Large acceptanceAzimuthal symmetry
High rate capabilityLimited acceptance
Frank Ellinghaus, Spin’08, Charlottesville
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, Spin’08, Charlottesville
G via direct measurement in pp
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, Spin’08, Charlottesville
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, Spin’08, Charlottesville
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
M. SarsourFriday
• No inconsistency with DIS data due to generally large uncertainties on Dg(x).• Small or negative gluon contribution to nucleon spin favored in this model.• Data also consistent with GS-C ->
Frank Ellinghaus, Spin’08, Charlottesville
Model dependence of G
g integral between
GRSV
- 0
GRSV
- std
GS-C
0<x<1 0 0.4 1
0.02<x<0.3 0 0.25 0
• Measurement averages over certain x range• Shape of G(x) cannot be extracted -> Value for first moment model dependent
Frank Ellinghaus, Spin’08, Charlottesville
PHENIX --0 at 200 GeV
Run 5: Phys.Rev.D76:051106,2007Run 6: arXiv:0810.0694
GRSV: Glueck et al., PRD 63 (2001)
G = G(x), -G(x) excluded; GRSV-std slightly disfavored
K. NakanoTuesday
Run 5 + Run 6
Frank Ellinghaus, Spin’08, Charlottesville
STAR – 0, PHENIX – PHENIX J/Psi
PHENIX excludes G= G(x), -G(x)
J. Seeletoday
STAR 0 consistent with PHENIX
X. WangFriday
A. Hoffman
today
Frank Ellinghaus, Spin’08, Charlottesville
+, –, 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
s=62.4 GeV
PHENIX Preliminary
A. Dattatoday
A. MorrealeTuesday
Frank Ellinghaus, Spin’08, Charlottesville
Charged hadrons at STAR A. Kocoloskitoday
Frank Ellinghaus, Spin’08, Charlottesville
Direct Photons at s=200 GeV
Run-5
At the end of the day all these (and the DIS, SIDIS) asymmetry data need to go into a “global” QCD fit in order to extract G! ->
q
g q-> small unc. from FFs-> better access to sign of G (q times G)
Theoretically clean “Golden Channel” is luminosity hungry…
Dominated by qg Compton:
R. Bennetttoday
Frank Ellinghaus, Spin’08, Charlottesville
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 ->
• Strong impact of Star jets and Phenix 0 in measured range. • Shape of G(x) cannot be extracted -> All “missing” spin can be at low x…. • Next step: Mapping of x-dependence via di-jets, di-hadrons and gamma-jet
M. StrattmannTuesday
Frank Ellinghaus, Spin’08, Charlottesville
• 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)
• pQCD applicable for pT > 2 GeV/c…
Accessing different x-rangesPHENIX: 0 ALL at s=62.4 GeV
Increased sensitivity to larger x
K. NakanoTuesday
STAR: 0 ALL at forward rapidity
Increased sensitivity to smaller x
• Also: step towards gamma-jet correlation measurements to map out the x-dependence• STAR Di-Jet measurements promising
T. SakumaFriday
S. WissinkFriday
arXiv: 0810.0701
Frank Ellinghaus, Spin’08, Charlottesville
Summary & Outlook• PHENIX and STAR data 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)• Direct measurements by HERMES+COMPASS support this finding with
somewhat larger uncertainties.• No sensitivity to shape of G(x)
-> all the “missing” spin can still be at smaller x.
• HERMES: – final word (long paper) soon
• COMPASS: – Open Charm: Add 2007 data (small improvement)
– High-pT hadrons, Q2>1 (GeV/c)2: Add 2006+2007 data (significant improvement)
• PHENIX+STAR: – 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)