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

Quarks polarisation Evaluated from the spin structure function g1(x) measured in Deep Inelastic Scattering of polarised lepton on the polarised nucleon : g1(x,Q2) world data (HERMES 2002) e,m (k’) e,m (k,s) QED g1(x,Q2), g2(x ,Q2) g*(Q2, n) P (P,S) Q2=-q2=-(k-k’)2 n=E-E’ x=Q2/2Mn lab Proton momentum fraction carried by quark G1 plots…. Assuming Quark Parton Model:

“The spin crisis” experiment theory 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

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 p

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

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

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

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

Experimental setup Target μ ID HCALS m+ beam + 200 tracking planes Spectrometers magnets μ ID HCALS m+ beam + 200 tracking planes RICH

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

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

Tracking detectors

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

High pT hadrons- selection 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

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

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

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

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.