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

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 2018/12/31 N. DOSHITA

Introduction DS  0.3 DG ?? Nucleon spin DS + DG + Lq + Lg 1 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 ?? 2018/12/31 N. DOSHITA

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 2018/12/31 N. DOSHITA

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 2018/12/31 N. DOSHITA

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 2018/12/31 N. DOSHITA

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 2018/12/31 N. DOSHITA

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 2018/12/31 N. DOSHITA

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 2018/12/31 N. DOSHITA

High pT hadron pairs + + + .. qg Several possible contributions to the measured asymmetry MC needed to determine R and aLL q q qg g g 2018/12/31 N. DOSHITA

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 2018/12/31 N. DOSHITA

Contributions of LO and QCDC can be neglected Results from Q2>1 (GeV/c)2 (2002,2003(28k events)) LEPTO Monte Carlo <aLL/D> = -0.75 +/- 0.05 RPGF = 0.34 +/- 0.07 Contributions of LO and QCDC can be neglected at xB < 0.05. (A10) Preliminary result main sources  False asymmetry 2(scale) ~ 3(GeV/c)2 <xG> ~ 0.13 2018/12/31 N. DOSHITA

Results from Q2<1 (GeV/c)2 (2002-2004) 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     2018/12/31 N. DOSHITA

G/G results of all processes 2018/12/31 N. DOSHITA

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 2018/12/31 N. DOSHITA

Acceptance and event gains in 2006 (preliminary) Larger magnet acceptance RICH upgrade 2018/12/31 N. DOSHITA

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 1.25 = 2.25 New magnet RICH upgrade Improvements in the reconstruction 2018/12/31 N. DOSHITA

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 2018/12/31 N. DOSHITA

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