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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
2015/7/7
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Outline
• Introduction
• The COMPASS experiment
• Direct measurements of G
3 measurements with 2 processes
• The 2006 run
• Analysis improvement
• Summary
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Introduction
Nucleon spin
1
=
1
2
2
S + G + Lq + Lg
S : Quarks spin
G : Gluons spin
Lq, Lg : Orbital angular
momentum
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S  0.3
smaller than predicted
G ??
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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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Polarized target
(2002-2004)
Dilution refrigerator
~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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Direct measurements of G
Experimental asymmetry
Ae xp=
N
N
N
N
= P B PT f A||

: 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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Photon-Gluon-Fusion
Open charm
*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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Open charm
D*  D0 soft
D0  K 
(BR  68%)
(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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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
2(scale) ~ 13(GeV/c)2
<xG> ~ 0.15
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main sources  0.10 : false asymmetry
0.09 : fitting
0.07 : background asym.
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High pT hadron pairs
+
PGF
LL
A|| = RPGF a
+
q + R aLO q + resolved photons
QCDC
G + R
a
LL
LO
LL
QCDC
processes for lowQ
q
q
G
2
• Several possible contributions to the
measured asymmetry
• MC needed to determine R and aLL
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+ ..
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q
q
g
g
qg
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High pT event selection
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
10% of
statistics
analysis
Q2 < 1 (GeV/c)2
Q2 >1 (GeV/c)2
statistics
90%
10%
background
QCDC, LO
QCDC, LO
Resolved photons
MC generator PYTHIA
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LEPTO
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Results from Q2>1 (GeV/c)2
(2002,2003(28k events))
A||
= -0.015 + 0.080(stat.) + 0.013(syst.)
D
LEPTO Monte Carlo
• <aLL/D> = -0.75 +/- 0.05
• RPGF = 0.34 +/- 0.07
Contributions of LO and QCDC can be neglected
at xB < 0.05. (A10)
Preliminary result
2(scale) ~ 3(GeV/c)2
<xG> ~ 0.13
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main sources  False asymmetry
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Results from Q2<1 (GeV/c)2
(2002-2004)
MC A||/D = RPGF G/G aLLPGF
+ RQCDC q/q aLLQCDC
gq

resolved + Rqq q/q aLL (G/G)
+ Rqg G/G aLLqg (q/q) 
photons
+…
• Negligible： LO-DIS and Low-PT
• Main background : resolved photons
30%(PGF)
50%(all)
Fraction of various processes
Preliminary result
2(scale) ~ 3(GeV/c)2
<xG> ~ 0.085
VMD term of the polarized PDFs of
 ) is not known.
photon (fVMD



possible range: -fVMD < fVMD < fVMD
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0.052 : Monte Carlo
0.014 : false asymmetry
0.013 : resolved photons
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G/G results of all processes
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Improvement
in 2006 run
New target system
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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Acceptance and event gains in 2006
(preliminary)
Larger magnet acceptance
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RICH upgrade
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D* sample
D0Kπ π0
Effective signal
2
S
Seff =
S+B 
• large light lepton background
found in πs sample
• rejected by RICH
Seff increases by 25 % .
~ half of 2006 data
Seff (D*)2006 /Seff (D*)2004 = 1.8 x 1.25 = 2.25
Normalized
by the beam flux
• New magnet
• RICH upgrade
• Improvements in the reconstruction
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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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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.
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