IWHSS 08 Torino

1
Gluon Polarisation Overview

• DS, quark contribution to nucleon spin. Why DG ?
• DG from scaling violations
• DG from hadron production
• - Open charm - COMPASS
• - High pT hadrons pairs single -
  COMPASS/HERMES
• DG from pp collisions - RHIC
• A. Magnon (CEA-Saclay/IRFU COMPASS)

2
Early measurements of DS (1)
SLAC Polarized electrons DS large, as
expected 1976-1983
EMC _at_ CERN polarized m Access lower x, DS
0.12 0.17 ? Spin crisis 1988
3
Early measurements of DS (2)

• HERMES, SLAC high precision, SMC _at_ CERN lower x
  
• g1 for proton neutron (deuteron)
• Bjorken Sum Rule relates proton neutron
  g1?g1dx

DS 0.2 - 0.3 confirmed to be small
Theory, Q25 GeV2
SMC
1998
Bjorken OK as determination first flavor
separation
4
Recent measurements of DS

• COMPASS _at_ CERN, m 160 GeV

COMPASS fit to g1 p, n, d world data, MS scheme,
Q2 3 (GeV/c)2 PLB 647 (2007) 8
DS 0.30 0.01 (stat) 0.02 (evol)
Ds Ds - 0.08 0.01 (stat) 0.02 (syst)
COMPASS data alone

• HERMES _at_ DESY, e- 27 GeV

HERMES from g1d data, MS scheme, Q25 (GeV/c)2,
neglecting x 012007
DS 0.33 0.011 (stat) 0.025 (theo) 0.028
(evol)
Ds Ds - 0.085 0.013 (th) 0.008 (exp)
0.009(evol)
5
Why measure DG ?
½ ½?S ?G
Measurement of DG important 1 How are gluons
polarized ? 2- Low value of a0 could be
due to axial anomaly if DG is large.
(A. Efremov O.Teryaev, G. Altarelli G. Ross) 3
How large is parton orbital angular
momentum
a0
6
How to measure DG ?

• DG from scaling violations
• DG from hadron production
• - Open charm
• - High pT hadrons (pairs, single)
• DG from pp collision

7
COMPASS NLO QCD fit
Q2 3 GeV2
Comparison of fits - disagreement of data with
previous QCD fits (LSS05, GRSV, BB)
8
COMPASS NLO QCD fit
DG 0.34
Q2 3 GeV2
DG - 0.31
New COMPASS g1d ? G 0 or ? G ?G 0.3 a0 0.33 0.03 0.05
?s -0.08 0.01 0.02
2006
9
How to measure DG ?

• DG from scaling violations
• DG from hadron production (PGF)
• - Open charm
• - High pT hadrons (pairs, single)
• DG from pp collision

10
Photon-gluon fusion (PGF)

• Gluon polarisation is measurable in PGF

• measure
• calculate and
• using Monte Carlo

N
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DG/G from open charm
Open charm, single D meson
c - (D) - (ps) D0 - Kp(ps)
c

• cleanest process wrt physical bkgr
• combinatorial bkgr, limited statistics
• so far LO analysis, NLO in progress

c
N
12
DG/G from open charm
COMPASS Data 2002,2003,2004 2006
160 GeV m beam 6LiD target
nD 8675
nD0 37398
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DG/G from open charm

• Analysis uses both aLL and S/(SB) weighting
• aLL obtained from Neural Network trained on MC
  (AROMA)
• input variables Q2, xbj, y, pT, zD
• S/(SB) given by a parameterization input
  variables target cell,
• fPµaLL, pK, ?K, zD, cos?, pT, RICH
  Likelihoods
• Weighting brings significant improvement in
  statistics due
• to large variations of aLL and S/(SB) in
  phase-space

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DG/G from open charm
D0 -untagged
D -tagged
5 bins in S S/(SB)
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DG/G from open charm
2006 aLL parameterization
aLL generated
aLL reconstructed
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DG/G from open charm

• New
• 2002 2006 data D0 D
• DG/G -0.49 0.27 (stat) 0.11 (syst)
• _at_ 0.11, 13 (GeV/c)2

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How to measure DG ?

• DG from scaling violations
• DG from hadron production (PGF)
• - Open charm
• - High pT hadrons pairs, Q2 1 GeV/c2
• DG from pp collision

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?G/G from high pT hadron pairs
Q2 1 (GeV/c)2
g
q
q
Photon Gluon Fusion 30
Leading Order
QCD Compton
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?G/G from high pT hadron pairs

• Analysis uses parameterization of RPGF, RQCDC,
  RLO, aLLPGF,
• aLLQCDC, aLLincl, xg, xC, etc based on
  Neural Network
• trained on MC (LEPTO for Q2 1).
• No cut on NN which assigns to each evt. a
  probability to
• originate from LO, PGF or COMPTON.
• Dependence on PDFs studied
• Parton shower (NLO process) added
• Detailed studies of systematics

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?G/G from high pT hadron pairs
Two parameters O1 O2 to express fractions R
(PGF, LO or QCDC) for each high pT event
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?G/G from high pT hadron pairs
Leading hadron
Sub-leading hadron
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?G/G from high pT hadron pairs
Probabilities (fractions) of LO, QCDC, PGF
Monte Carlo vs Neural Network
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?G/G from high pT hadron pairs

• New
• 2002 2004 data High pT, Q2 1 GeV/c2
• DG/G 0.08 0.10 (stat) 0.05(syst)
• _at_ 0.082, (range 0.055 0.123) m2 3
  (GeV/c)2

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?G/G from high pT hadron pairs

• (Released 2 Oct. 2006, SPIN2006)
• 2002 2004 data High pT, Q2
• DG/G 0.016 0.058 (stat) 0.055 (syst)
• _at_ 0.085, m2 3 (GeV/c)2

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? G/G, direct measurements
GRSV, DG
max, 2.5
std, 0.6
New high pT
min, 0.2
QCD Fits DG 0.3
New open charm
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? G/G, direct measurements

• Accurate DG/G from COMPASS data (2002 2004)
  from
• high pT hadron pairs, Q2
  1 GeV/c2 (new)
• DG/G small ( 0) _at_ 0.08
• Significant improvement for DG/G from open charm
  
• (2002 2004 2006) and aLL S/B weighting.
• Also DG/G ( 0) _at_ 0.11
• Similar conclusion from new HERMES analysis

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How to measure DG ?

• DG from scaling violations
• DG from hadron production
• - Open charm
• - high pT hadrons (pairs, single)
• DG from pp collision

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RHIC polarized pp collider
Year P L(pb-1) P4L() 2004 40 3
0.08 2005 50 13 0.8 2006 60 46
6
() G.Bunce Dubna Spin07
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pp collisions _at_ PHENIX STAR

• Reactions pp - pX, jet X, gX, cc X, probe
  gluon
• Measure always product of 2 observables
• MC required to determine fraction of process

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pp collisions _at_ PHENIX STAR
Considerable progress in pQCD NLO calculations
Jäger,Schäfer, Stratmann,Vogelsang de Florian
Jäger,Schäfer, Stratmann,Vogelsang Signer et
al.
Gordon,Vogelsang Contogouriset al. Gordon,
Coriano
Bojak, Stratmann
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Unpol. Cross Section in pp
Good agreement between NLO pQCD calculations and
data ? confirmation that theory can be used to
extract spin dependent pdfs from RHIC data
pp??0 X hep-ex-0704.3599
PHENIX data
pp?? X PRL 98, 012002
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From pT to xgluon (PHENIX, p0X)
vs200 GeV
Log10(xgluon)

• NLO pQCD ?0 pT2?9 GeV/c ? xgluon0.02?0.3
• GRSV model ?G(xgluon0.02?0.3) 0.6??G(xgluon
  0?1 )
• Each pT bin corresponds to a wide range in
  xgluon, heavily overlapping with other pT bins.
• These data is not much sensitive to variation of
  ?G(xgluon)
• within our x range.
• Any quantitative analysis should assume some
  ?G(xgluon) shape

G.Bunce Dubna Spin07
33
From ALL to DG (PHENIX, p0 with GRSV)
Calc. by W.Vogelsang and M.Stratmann
?
3 sigma

• GRSV standard, ?G(Q21GeV2)0.4, is excluded
• by data on 3 sigma level ?2(std)??2min9
• Only exp. stat. uncertainties are included (the
• effect of syst. uncertainties is expected to
  be
• small in the final results)
• Theoretical uncertainties are not included

G.Bunce Dubna Spin07
34
From ALL to DG (STAR, jetX with GRSV)
Large gluon polarisation scenario is not
consistent with data
J.C.Dunlop Dubna Spin07
35
STAR, inclusive p production (mid - ?)
Different sensitivity of p and p- to the sign
of ?G . e.g. ?G 0 ?
No constraint on DG yet
Dramatic increase in precision in Run 2006
J.C.Dunlop Dubna Spin07
36
DG from RHIC

• High statistics available to constrain DG in xg
  range
• (0.02 0.3)
• DG not large (consistent with zero)
• Standard scenario, ?G (Q21GeV2) 0.4, is
  excluded
• by data on 3 sigma level ?2(std)??2min 9
  (PHENIX ?)
• Theoretical uncertainties might be significant

37
RHIC, prospects

• Improve exp. (stat.) uncertainties, move to
  higher pT
• - more precise DG in probed x range
• - probe (lower) and higher x and constrain DG
  vs x
• Different channels
• - different systematics
• - different x,
• - gq - qg (pp - g jet), sensitive to DG
  sign, parton
• kinematics well constrained, theoretically
  clean
• Different vs, 62 GeV, 200 GeV, 500 GeV

Substantial FOM P4L needed
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Conclusion, possible scenarios

• From COMPASS RHIC
• DG ?DG(xG) 0.4 ?
• DS a0 0.3

a0
DS DG Lq
Lg ½ 1/2 0.3 0.35
0 0 ½ 1/2 0.3 0.0
0.35 ½ 1/2 0.3 - 0.35
0.70 COMPASS/RHIC
JLab/HERMES/COMPASS
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Additional slides
40
Measurements of DG/G
xg binning High pT in 2006
41
PHENIX, different ?s
?s62 GeV ?0 cross section described by NLO pQCD
within theoretical uncertainties
Sensitivity of Run6 ?s62 GeV data collected in
one week is comparable to Run5 ?s200 GeV data
collected in two months, for the same
xT2pT/?s ?s500 GeV will give access to lower
x starts in 2009
42
DG from scaling violations
DGLAP evolution equations rule ?/? lnQ2
dependence of parton distribution functions
Method - parameterize polarised parton
distributions at Q02
e.g. Dqi xai (1-x)bi(1gix) - DGLAP evolution
to measured Q2 - calculate g1 and fit all
existing g1 data together
DS and DG coupled in the evolution
? Extract DG(x)
43
Global QCD analysis AAC - NLO
xDuv
xDG
DG 0.31 0.32 at Q21 GeV2
xDq
xDdv
Dq - 0.050 0.32
AAC NLO, hep-ph/0603213
including g1 new data from HERMES, COMPASS and
JLAB PHENIX ALL p0
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New COMPASS A1d data
PLB647 (2007) 8