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Gluon Polarisation Overview
quark contribution to nucleon spin. Why G ? G from scaling violations G from hadron production - Open charm - COMPASS
- High pT hadrons pairs & single - COMPASS/HERMES
G from pp collisions - RHIC A. Magnon (CEA-Saclay/IRFU & COMPASS)
A.Magnon IWHSS ‘08 – TorinoMarch 31, 2008
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Early measurements of (1)
SLAC Polarized electrons
large, “as expected” 1976-1983
g1p
xBj
SLAC
Compatible with =0.6
∫g1p
xBj
xBjg1p
Ellis-Jaffe = 0.6
EMC
EMC @ CERN polarized
Access lower x, = 0.12 ± 0.17
→ ” Spin crisis ’’ 1988
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Early measurements of (2)
HERMES, SLAC high precision, SMC @ CERN lower x
g1 for proton & neutron (deuteron) …
Bjorken Sum Rule relates proton & neutron g1=∫g1dx003.0181.0)(
6
1 2111 QC
g
g NS
V
Anp
024.0012.011 174.0
np
Theory, Q2=5 GeV2
SMC
= 0.2 - 0.3 confirmed to be small
Bjorken OK + s determination + first flavor separation …
1998
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COMPASS @ CERN, 160 GeV
HERMES @ DESY, e- 27 GeV
= 0.30 ± 0.01 (stat) ± 0.02 (evol)
=0.33 ± 0.011 (stat) ± 0.025 (theo) ± 0.028 (evol)
COMPASS fit to g1 p, n, d world data, MS scheme, Q2 = 3 (GeV/c)2
PLB 647 (2007) 8
HERMES from g1d data, MS scheme, Q2=5 (GeV/c)2,
neglecting x < 0.02 contrib., PRD75 (2007) 012007
s + s= - 0.08 ± 0.01 (stat) ± 0.02 (syst)COMPASS data alone
s + s= - 0.085 ± 0.013 (th) ± 0.008 (exp) ± 0.009(evol)
Recent measurements of
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Why measure G ?
½ = ½ΔΣ + ΔG + <Lq> + <Lg>
Measurement of G important :
1 – How are gluons polarized ? 2- Low value of a0 could be due to axial anomaly if G is large. (A. Efremov O.Teryaev, G. Altarelli – G. Ross)
3 – How large is parton orbital angular momentum
a0 =
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G from scaling violations G from hadron production - Open charm - High pT hadrons (pairs, single) G from pp collision
How to measure G ?
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Q2 = 3 GeV2
COMPASS NLO QCD fit
Comparison of fits - disagreement of data with previous QCD fits (LSS05, GRSV, BB)
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Q2 = 3 GeV2
New COMPASS g1d
G > 0 or G < 0, |G| ~ 0.3 a0 = 0.33 ± 0.03 ± 0.05 s = -0.08 ± 0.01 ± 0.02
COMPASS NLO QCD fit
Δ G
ΔG = - 0.31
ΔG = 0.34
G = 0.34
G = - 0.31
2006
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G from scaling violations G from hadron production (PGF) - Open charm - High pT hadrons (pairs, single) G from pp collision
How to measure G ?
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measure
calculate and
using Monte Carlo
Photon-gluon fusion (PGF)
Gluon polarisation is measurable in PGF
GGaRA
pgfpgfLL
LLA
pgfR
pgfa
N
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G/G from open charm
c
c
c -> D* -> s D0 -> Ks
cleanest process wrt physical bkgr
combinatorial bkgr, limited statistics
so far LO analysis, NLO in progress
Open charm, single D meson
N
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G/G from open charm
πKD0 ππKπDDs
0*
COMPASS Data: 2002,2003,2004 & 2006
160 GeV beam & 6LiD target
nD0 = 37398
nD* = 8675
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G/G from open charm
)(/gLLLLx
GGa
BS
SDA
Analysis uses both aLL and S/(S+B) weighting
aLL obtained from Neural Network trained on MC (AROMA):
input variables : Q2, xbj, y, pT, zD
S/(S+B) given by a parameterization: input variables : target cell,
fPμaLL, pK, θK, zD, cosθ*, pT, RICH Likelihoods
Weighting brings significant improvement in statistics due
to large variations of aLL and S/(S+B) in phase-space
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G/G from open charm
5 bins in = S/(S+B)
D0 -untagged
D* -tagged
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G/G from open charma
LL
gen
era
ted
aLL reconstructed
2006 aLL parameterization
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“ New ”
2002 – 2006 data D0 + D* G/G = -0.49 ± 0.27 (stat) ± 0.11 (syst) @ <xg> ~ 0.11, <2> ~ 13 (GeV/c)2
G/G from open charm
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G from scaling violations G from hadron production (PGF) - Open charm - High pT hadrons pairs, Q2 > 1 GeV/c2
G from pp collision
How to measure G ?
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ΔG/G from high pT hadron pairs
Photon Gluon Fusion ~ 30%
Leading Order QCD Compton
Q2 > 1 (GeV/c)2
g q q +
Resolved
Q2 < 1 (GeV/c)2
q,g
q,g
BKGRPGFLLPGFLL
AGGaRA
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Analysis uses parameterization of RPGF, RQCDC, RLO, aLL
PGF,
aLLQCDC, aLL
incl, 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
ΔG/G from high pT hadron pairs
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ΔG/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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Leading hadron
Sub-leading hadron
ΔG/G from high pT hadron pairs
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ΔG/G from high pT hadron pairs
Probabilities (fractions) of LO, QCDC, PGF : Monte Carlo vs Neural Network
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“ New ”
2002 – 2004 data: High pT, Q2 > 1 GeV/c2 G/G = 0.08 ± 0.10 (stat) ± 0.05(syst) @ <xg> = 0.082, (range: 0.055 – 0.123) 2 ~ 3 (GeV/c)2
ΔG/G from high pT hadron pairs
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(Released 2 Oct. 2006, SPIN2006)
2002 – 2004 data: High pT, Q2 < 1 GeV/c2
G/G = 0.016 ± 0.058 (stat) ± 0.055 (syst)
@ <xg> = 0.085, 2 = 3 (GeV/c)2
ΔG/G from high pT hadron pairs
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New high pT
G/G, direct measurements
New open charm
GRSV, G
std, 0.6
min, 0.2
QCD Fits
|G| ~ 0.3
max, 2.5
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Accurate G/G from COMPASS data (2002 – 2004) from high pT hadron pairs, Q2 < 1 GeV/c2 and Q2 > 1 GeV/c2 (new) G/G small (~ 0) @ <xg> = 0.08
Significant improvement for G/G from open charm (2002 – 2004 + 2006) and aLL + S/B weighting. Also G/G (~ 0) @ <xg> = 0.11
Similar conclusion from new HERMES analysis
G/G, direct measurements
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G from scaling violations G from hadron production - Open charm - high pT hadrons (pairs, single) G from pp collision
How to measure G ?
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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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q
q
G
G
G
G
G
G
pp collisions @ PHENIX & STAR
Reactions pp -> X, jet X, X, cc X, probe gluon Measure always product of 2 observables MC required to determine fraction of process
q
q
q
q
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pp collisions @ 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
pp0 X : hep-ex-0704.3599
pp X: PRL 98, 012002
Good agreement between NLO pQCD calculations and data confirmation that theory can be used to extract spin dependent pdf’s from RHIC data
PHENIX data
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Log10(xgluon)NLO pQCD: 0 pT=29 GeV/c xgluon=0.020.3 GRSV model: G(xgluon=0.020.3) ~ 0.6G(xgluon =01 ) 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
From pT to xgluon (PHENIX, 0X)
√s=200 GeV
G.Bunce
Dubna Spin07
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Calc. by W.Vogelsang and M.Stratmann
GRSV “standard”, G(Q2=1GeV2)=0.4, is excluded
by data on >3 sigma level: 2(std)2min>9
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
From ALL to G (PHENIX, 0 with GRSV)
“3 sigma”
G.Bunce
Dubna Spin07
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From ALL to G (STAR, jetX with GRSV)
2005 STAR preliminary
Systematic error band
Measured Jet PT (GeV)
GRSV DIS
Large gluon polarisation scenario is not consistent with data
J.C.Dunlop
Dubna Spin07
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Different sensitivity of + and - to the sign of G …. e.g. G > 0
πLL
πLL
πLL AAA
ο
STARSTAR
No constraint on G yet …
STAR, inclusive ± production (mid - η)
J.C.Dunlop
Dubna Spin07
Dramatic increase in precision in Run 2006
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G from RHIC
High statistics available to constrain G in xg range (0.02 – 0.3)
G not large (consistent with zero)
“Standard” scenario, G (Q2=1GeV2) = 0.4, is excluded
by data on > 3 sigma level: 2(std)2min > 9
(PHENIX ?)
Theoretical uncertainties might be significant
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RHIC, prospects
Improve exp. (stat.) uncertainties, move to higher pT
- more precise G in probed x range - probe (lower) and higher x and constrain G vs x Different channels - different systematics - different x, - gq -> q (pp -> jet), sensitive to G sign, parton kinematics well constrained, theoretically clean Different √s, 62 GeV, 200 GeV, 500 GeV
Substantial FOM = P4L needed
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Conclusion, possible scenarios
G 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
From COMPASS & RHIC:
G =|∫G(xG)| ≤ 0.4 ?
≈ a0 = 0.3
a0 =
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Additional slides
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Measurements of G/G
xg binning
High pT
in 2006
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PHENIX, different s
s=62 GeV 0 cross section described by NLO pQCD within theoretical uncertainties
Sensitivity of Run6 s=62 GeV data collected in one week is comparable to Run5 s=200 GeV data collected in two months, for the same xT=2pT/s
s=500 GeV will give access to lower x; starts in 2009
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Method- parameterize polarised parton distributions at Q0
2
e.g. qi ~ xi (1-x)i(1+ix)
- DGLAP evolution to measured Q2 - calculate g1 and fit all existing g1 data together
DGLAP evolution equations rule ∂/∂ lnQ2
dependence of parton distribution functions
and G coupled in the evolution
→ Extract G(x)
G from scaling violations
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AAC – NLO, hep-ph/0603213 including g1 new data from HERMES, COMPASS and JLAB + PHENIX ALL 0
G = 0.31 ± 0.32 at Q2=1 GeV2
xGxuv
xdv
Global QCD analysis: AAC - NLO
xq
q = - 0.050 ± 0.32
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New COMPASS A1d data
PLB647 (2007) 8