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Measurement of the Gluon Polarization at COMPASS

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. Outline. Introduction The COMPASS experiment Direct measurements of  G

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Measurement of the Gluon Polarization at COMPASS

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  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 N. DOSHITA

  2. Outline • Introduction • The COMPASS experiment • Direct measurements of G 3 measurements with 2 processes • The 2006 run • Analysis improvement • Summary N. DOSHITA

  3. Introduction Nucleon spin 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 ?? N. DOSHITA

  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 N. DOSHITA

  5. 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 N. DOSHITA

  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 N. DOSHITA

  7. *g  cc  D0X  (K)X *g  qq  hh Photon-Gluon-Fusion Open charm • Clean channel • Low statistics • Kidentification by RICH High pT hadron pairs • Large statistics • Physical background N. DOSHITA

  8. Open charm D*  D0 soft (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 N. DOSHITA

  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 N. DOSHITA

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

  11. 10% of statistics High pT event selection For PGF contribution enhancement pT > 0.7 GeV/c pT12 + pT22 > 2.5 (GeV/c)2xF>0.1, z > 0.1m(h1,h2) > 1.5 GeV/c2 N. DOSHITA

  12. 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 N. DOSHITA

  13. Results from Q2<1 (GeV/c)2 (2002-2004) 30%(PGF) MC A||/D = RPGFG/GaLLPGF + RQCDCq/qaLLQCDC + Rqqq/qaLLgq (G/G) 50%(all) resolved photons + RqgG/GaLLqg (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     N. DOSHITA

  14. G/G results of all processes N. DOSHITA

  15. 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 N. DOSHITA

  16. Acceptance and event gains in 2006 (preliminary) Larger magnet acceptance RICH upgrade N. DOSHITA

  17. D* sample D0Kπ π0 Effective signal • large light leptonbackground found in πssample • 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 N. DOSHITA

  18. G/G analysis improvement Working on analysis of 2006 data Open charm • Simultaneous determination of the signal and background • D0Kππ0channel • 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 N. DOSHITA

  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. N. DOSHITA

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