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Determination of the gluon polarisation

Determination of the gluon polarisation. in experiment @ CERN. Adam Mielech , INFN Trieste on behalf of the COMPASS collaboration. Nucleon spin structure. quarks. gluons. orbital momentum. QED. g 1 (x,Q 2 ), g 2 (x ,Q 2 ). Q 2 =-q 2 =-( k-k’) 2. n = E-E’.

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Determination of the gluon polarisation

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  1. Determination of the gluon polarisation in experiment @ CERN Adam Mielech, INFN Trieste on behalf of the COMPASS collaboration

  2. Nucleon spin structure quarks gluons orbital momentum

  3. QED g1(x,Q2), g2(x ,Q2) Q2=-q2=-(k-k’)2 n=E-E’ Assuming Quark Parton Model: lab x=Q2/2Mn Proton momentum fraction carried by quark g1(x,Q2) world data (HERMES 2002) Quarks polarisation Evaluated from the spin structure function g1(x) measured in Deep Inelastic Scattering of polarised lepton on the polarised nucleon : e,m (k’) e,m (k,s) g*(Q2, n) P (P,S)

  4. “The spin crisis” Measured at CERN, SLAC, DESY Result from SMC fit to proton world data At Q2 =5GeV 2 Phys Lett B320 405 (1994) experiment theory Ellis –Jaffe sum rule prediction According to triangle anomaly, there is no unambiguous way to separate the quark contribution and the anomalous gluonic contribution

  5. p p Accesing gluon polarisation From QCD evolution of g1(x) stucture function: • or from direct measurements • Photon Gluon Fusion(SMC, HERMES,COMPASS) • jets from polarised pp2jets scattering (RHIC) Fit to the g1(x,Q2) world data, by Blümlein & Böttcher, Nucl. Phys. B636 (02) 225

  6. C 40% -76% 50% C DG/G from open charm

  7. q g* q q g* q QCD-Compton Leading Order DG/G from high pT hadrons Photon Gluon Fusion

  8. ps+ D+* K- D0 p+ C C h1 h2 q=c, Open Charm Production Looking for charged K and p coming from D0 and D±* decays Photon Gluon Fusion 62% 4% q =u,d,s,c at high pT Looking for high pT hadron pairs

  9. Luminosity: ~5 . 1032 cm-2 s-1 intensity:2.108 µ+/spill (4.8s / 16.2s) polarization: -76% momentum: 160 GeV/c Localisation • ~200 physicists from 12 countries • polarised muon beam • polarised target • particle tracking and momentum measurement • particle identification • calorimetry measurement LHC SPS N

  10. Experimental setup Target Spectrometers magnets μID HCALS m+ beam + 200 tracking planes RICH

  11. 3He – 4He dilution refrigerator (T~50mK) Superconducting Solenoid (2.5 T) Dipol (0.5 T) Two 60 cm long target cells with opposite polarisation Reconstructed interaction vertices Polarised 6LiD target Polarisation: 50% Dilution: 40% μ

  12. p, m K p 3 m 6 m vessel q(mrad) p(GeV/c) mirror wall photon detectors: CsI MWPC 5 m hadrons radiator: C4F10 m Particle identification RICH m – hadron separation is also possible using hadron calorimeters

  13. Tracking detectors

  14. ps+ D+* C K- D0 p+ C Open charm – selection in COMPASS ~300 COMPASS 2002 data

  15. High pT hadrons- selection h1 h2 • Current fragmentation • xF>0.1 • z>0.1 • 2 high pT hadrons • pT > 0.7 GeV/c • pT12+ pT22>2.5 (GeV/c)2 • m(h1h2) > 1.5 GeV/c2

  16. DG/G from high pT hadrons QCD-Compton Leading Order Photon Gluon Fusion fractions of cross section determined by Monte Carlo

  17. Additional background for Q2<1(GeV2) Resolved Photon VMD - Pomeron

  18. DG/G at COMPASS –present status Open charm • Theoretically clean DG/G extraction, experiment challenging because of difficulty of the charm reconstruction in the large target. • We are able to reconstruct charmed mesons. • We are still collecting the data. • Projected error on DG/G from 2002-2004 data: 0.24 High pT hadron pairs • Experimental signature easy, background subprocesses extraction based on Monte Carlo • Measured asymmetry from 2002 data: • + 2003, 2004 → stat. < 0.018 • Up to now systematic error contains only studies on false asymmetries due to target or spectrometer effects

  19. SUMMARY • Quarks spin distribution is known. • Gluon spin contribution is going to be measured soon in different processes. • Orbital momentum components of the spin → next generation experiments. • Nucleon spin puzzle is still very exciting subject.

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