1. Measurement of the Gluon Polarization at COMPASS
Norihiro DOSHITA
Yamagata University, Japan
On behalf of the COMPASS Collaboration
The 6th Circum-Pan-Pacific Symposium on High Energy Spin Physics
2018/12/31
N. DOSHITA

2. The COMPASS experiment Direct measurements of G
Outline
Introduction
The COMPASS experiment
Direct measurements of G
3 measurements with 2 processes
The 2006 run
Analysis improvement
Summary
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3. Introduction DS  0.3 DG ?? Nucleon spin DS + DG + Lq + Lg1 1 =
DS + DG + Lq + Lg 2 2
DS : Quarks spin
DG : Gluons spin
DS  0.3
smaller than predicted
Lq, Lg : Orbital angular
momentum
DG ??
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4. The COMPASS experiment
Spin dependent DIS with muon beam and nucleon target
Muon beam
- 2 x 108 muons/spill
- 160 GeV/c
- ~80% polarization
RICH
- K  separation up to about 50 GeV/c
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5. Polarized target (2002-2004) Dilution refrigerator Magnet
~50 mK
Magnet
2.5T sol. and 0.6T dip.
70 mrad
Muon beam
Target LiD, ~50% polarization
2 x 60cm long
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6. Direct measurements of G
Experimental asymmetry
: Direction of beam polarization 
: Direction of target polarization
G : Photon-Gluon-Fusion process
Open Charm
High PT hadron pairs Q2 <1 (GeV/c)2
High PT hadron pairs Q2 >1 (GeV/c)2
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7. Photon-Gluon-Fusion Open charm High pT hadron pairs
*g  cc  D0X  (K)X
Clean channel
Low statistics
K identification by RICH
High pT hadron pairs
*g  qq  hh
Large statistics
Physical background
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8. Open charm D*  D0 soft D0  K  Selection D0 kinematics
(BR  68%)
D0  K 
(BR  4%)
Selection
D0 kinematics
mom. fraction ZD0 > 0.2 (0.25)
decay angle |cos*|<0.85 (0.5)
D* tag : mass difference m
3.1 MeV/c2 < (m-m) < 9.1 MeV/c2
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9. Results of Open charm (2002-2004) PGF/tot = S/(S+B):
determined by the fit
aLL calculated by MC using AROMA
parametrisation with (y, Q2, ZD0, PTD0)
New
Preliminary result
main sources  0.10 : false asymmetry
0.09 : fitting
0.07 : background asym.
2(scale) ~ 13(GeV/c)2
<xG> ~ 0.15
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10. High pT hadron pairs + + + .. qg
Several possible contributions to the measured asymmetry
MC needed to determine R and aLL q q qg g g
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11. High pT event selection
10% of
statistics
For PGF contribution enhancement
pT > 0.7 GeV/c
pT12 + pT22 > 2.5 (GeV/c)2 xF>0.1, z > 0.1 m(h1,h2) > 1.5 GeV/c2
analysis
Q2 < 1 (GeV/c)2
Q2 >1 (GeV/c)2
statistics
90%
10%
background
QCDC, LO
Resolved photons
MC generator
PYTHIA
LEPTO
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12. Contributions of LO and QCDC can be neglected
Results from Q2>1 (GeV/c)2
(2002,2003(28k events))
LEPTO Monte Carlo
<aLL/D> = /- 0.05
RPGF = /- 0.07
Contributions of LO and QCDC can be neglected
at xB < (A10)
Preliminary result
main sources  False asymmetry
2(scale) ~ 3(GeV/c)2
<xG> ~ 0.13
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13. Results from Q2<1 (GeV/c)2
( )
30%(PGF) MC
A||/D = RPGF G/G aLLPGF
+ RQCDC q/q aLLQCDC
+ Rqq q/q aLLgq (G/G)
50%(all)
resolved
photons
+ Rqg G/G aLLqg (q/q) 
+ …
Negligible： LO-DIS and Low-PT
Main background : resolved photons
Fraction of various processes
Preliminary result
2(scale) ~ 3(GeV/c)2
<xG> ~ 0.085
0.052 : Monte Carlo
0.014 : false asymmetry
0.013 : resolved photons
VMD term of the polarized PDFs of photon (fVMD) is not known.
possible range: -fVMD < fVMD < fVMD    
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14. G/G results of all processes
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15. Improvement in 2006 run New target system RICH upgrade
New magnet and wave cavity
larger acceptance
good magnetic field homogeneity
good wave distribution
180mrad
New 3 target cells
reduce false asymmetry
RICH upgrade
MAPMTs in the central area
Significant increase in number of photons
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16. Acceptance and event gains in 2006
(preliminary)
Larger magnet acceptance
RICH upgrade
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17. D* sample  Seff (D*)2006 /Seff (D*)2004 = 1.8 x 1.25 = 2.25
D0Kπ π0
Effective signal 
large light lepton background found in πs sample
rejected by RICH
Seff increases by 25 % .
~ half of 2006 data
Normalized
by the beam flux
Seff (D*)2006 /Seff (D*)2004 = 1.8 x = 2.25
New magnet
RICH upgrade
Improvements in the reconstruction
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18. G/G analysis improvement
Working on analysis of 2006 data
Open charm
Simultaneous determination of the signal and background
D0Kππ0 channel
Plan for NLO extraction
High pT (large Q2)
Lowering the sum PT2 cut
considerable gain in statistics while RPGF is still reasonable
Working on the analysis to include 2004/2006
High pT (low Q2)
Plan to divide xg into 2 bins
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19. Summary Direct measurements results indicate small G around xg 0.1.
Direct measurements can not discriminate between G>0 and G<0 solutions of QCD analysis.
The statistical errors will be reduced by the 2006 data.
The analysis improvements are going on.
2018/12/31
N. DOSHITA