Nucleon spin and structure studies with COMPASS DAPNIA Stephane Platchkov CERN and CEA-Saclay On behalf of the COMPASS Collaboration 07/09/2019 S. Platchkov, PANIC 05

The COMPASS collaboration Composed of 230 physicists 28 institutions 11 countries Data taking 2002, 2003, 2004 2005: SPS stopped LHC SPS COMPASS 07/09/2019 S. Platchkov, PANIC 05

The nucleon spin puzzle Quark contribution to the nucleon spin 1990 - 2005 : EMC, SMC, Hermes, SLAC : DS  0.20 – 0.25 instead of ~0.60 (QPM) The nucleon spin is more complicated than anticipated ?  Study Gluon contribution to the spin  And also many other spin observables… 07/09/2019 S. Platchkov, PANIC 05

Physics program of COMPASS Experiments with muon beam Gluon spin contribution Quark spin contribution Flavor decomposition of spin distribution functions Transverse spin distribution functions Transversity – two hadron correlations Vector meson production Spin transfer in L-hyperon production Pentaquarks Experiments with hadron beams Pion and Kaon polarizabilities Diffractive production of exotic states Search for glueballs Light meson spectroscopy Production of double charmed baryons Other Compass talks at Panic05 J.Nassalski – Delta-G A.Bressan – Transversity-1h R.Joosten – Transversity-2h J.Friedrich – Pentaquarks, hadrons A.Sandacz – r production, DVCS 07/09/2019 S. Platchkov, PANIC 05

COMPASS “first” period (2002 – 2004) Large amount of data collected with muon beam Longitudinally polarized target About 78% of the total Transversely polarized target About 22% of the total First data with hadron beam “Pilot” hadron run in 2004 This talk: Quick overview of available COMPASS data Mainly 2002+2003 Data from 2004 being analyzed 07/09/2019 S. Platchkov, PANIC 05

The COMPASS Spectrometer Trigger-hodoscopes RICH ECal & HCal μ Filter Drift chambers Straws MWPC 50 m Silicon Micromegas SciFi Gems SM1 SM2 6LiD Target 160 GeV μ Two stage spectrometer Polarized beam and target ~75% ≥50% SAT, LAT, PID 0.003<x<0.5, 10-3<Q2<10 GeV2 07/09/2019 S. Platchkov, PANIC 05

Quark spin content: inclusive DIS asymmetry Data taken in 2002 and 2003  Good agreement with previous experiments  Improved statistics at low x (Factor of ~2) 07/09/2019 S. Platchkov, PANIC 05

Quark spin content: g1d(x) structure function Use Data taken in 2002 and 2003 Phys.Lett. B612(2005)154. Also: new precise proton data from HERMES New COMPASS QCD fit to the world data on g1(x) (Q2=3 GeV2, NLO): DS(MS) = 0.22 + 0.03 without COMPASS data DS(MS) = 0.25 + 0.02 with COMPASS data 07/09/2019 S. Platchkov, PANIC 05

Semi-inclusive DIS Objectives: Preliminary data 2002 and 2003 Measure semi-inclusive spin asymmetries for p+, p-, K+ K- Ks Determine individual flavor spin distributions (isoscalar target): h Preliminary data 2002 and 2003  Kinematics range down to x = 0.003, Q2>1  High accuracy data In progress… 07/09/2019 S. Platchkov, PANIC 05

Transversity – Collins and Sivers asymmetries Full description of the nucleon Nucleon PDFs: Unpolarized Helicity Transversity Collins: probes transverse spin quark distribution Dtq(x) Sivers: probes intrinsic transverse momentum Data taken in 2002 only Collins PRL 94 (2005) 202002 Sivers See A.Bressan’talk Session III.7 Stat accuracy of the same order as Hermes Small values of asymmetries could be due to opposite signs of the transverse du and dd distributions. Full data set: reduce the errors by a factor of 2-3 Also: take data with a proton target – separation of transnversity Using all hadrons increases the stat by 1.5  Asymmetries are compatible with 0  Possible explanation: cancellation of DuT and DdT in isoscalar targets? 07/09/2019 S. Platchkov, PANIC 05

Collins asymmetries – near future COMPASS transverse spin “polarimeters”: Collins asymmetry Two-hadron correlation L-polarization Covered by: A. Bressan R. Joosten In progress… COMPASS Deuteron Combined 2002-2004 data COMPASS Proton Expected 2006 data (30d)  Accurate data to come 07/09/2019 S. Platchkov, PANIC 05

Longitudinal polarization of L andL hyperons Physics in the current fragmentation region Spin transfer from q to L Test of the strange quark asymmetry of the sea Data from 2003 only  With 2004 data: much improved statistics 07/09/2019 S. Platchkov, PANIC 05

Determination of DG/G Select Photon Gluon Fusion process Via Open Charm production Detect D°  K-p+ and D*+D°p+ Clean channel (no c quarks) High combinatorial background Via High-pt hadron pairs Good S/B ratio Physics background (QCDC, LODIS, Resolved photons contributions Rely on MC estimates D*+→D0π+ (2003) 1100 events COMPASS preliminary 07/09/2019 S. Platchkov, PANIC 05

Determination of DG/G – Two methods Open Charm High-pt pairs Data 2002+2003 Calculation: Glück et al.,Phys. Rev. D63 (2001) 094005 DG=2.5 DG=0.6 DG=0.16 DG/G(xg=0.1) = 0.024 ± 0.089(stat) ± 0.057(syst) Most precise value of DG/G today Present accuracy: 0.73 With 2004 data  0.43 With 2006 data  0.24 (estimate) See talk by J.Nassalski, PDF-II 07/09/2019 S. Platchkov, PANIC 05

Search for Φ-- Pentaquarks NA49: Peak at 1860 ± 8 MeV Several “evidence papers” on pentaquarks published Motivated by NA49 findings Ali et al. PRL 92 (2004) 042003 Search for the process: Check of COMPASS analysis: Look for COMPASS : 1700 events NA49 : 150 events  COMPASS does not confirm NA49 See talk by J.Friedrich, Pentaquarks session 07/09/2019 S. Platchkov, PANIC 05

COMPASS upgrades for 2006 New solenoid magnet RICH upgrade Larger acceptance 70 mrad  180 mrad Expect +30% FOM RICH upgrade Central region: maPMT system More photons Improved S/N Outer region: New faster electronics Expect +60% FOM Other upgrades: Large Drift Chamber Full ECAL coverage Etc… 07/09/2019 S. Platchkov, PANIC 05

COMPASS Programme The near future (2006 – 2010) Improved COMPASS Factor of Merit New OD magnet 1.3 Upgraded RICH 1.5 Full ECAL coverage 1.2 Other… 1.1 Total FoM improvement: ≥2.3 Strategy Strategy: muon beam in 2006, hadron beam in 2007, … Precise data to come! Long term future (beyond 2010) Plans to use DVCS to measure GPD Benefit from the high muon energy (190 GeV) Try to access orbital momentum L Additional detectors being developed Compass Factor of Merit 07/09/2019 S. Platchkov, PANIC 05

Summary - Results New COMPASS results Better spectrometer in 2006 Improved measurement of the g1 deuteron structure function Most precise DG/G measurement up to now First COMPASS results on tranversity Accurate results on semi-inclusive DIS No evidence for pentaquarks And a lot of data being analyzed Better spectrometer in 2006 Expected Factor Of Merit increase: ≥ 2.3 More precise results are coming…. 07/09/2019 S. Platchkov, PANIC 05