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Spin physics with COMPASS

Spin physics with COMPASS. Overview and future plans. Stephane Platchkov Institut de Recherche sur les lois Fondamentales de l’Univers Saclay , France. 2009: Double celebration. 40 years ago – when all that begun…. First DIS experiment : Friedman, Kendall, Taylor, et al .

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Spin physics with COMPASS

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  1. Spin physics with COMPASS Overview and future plans Stephane Platchkov Institut de Recherchesurles loisFondamentales de l’Univers Saclay, France

  2. 2009: Double celebration Yamagata, Sept.15 2009

  3. 40 years ago – when all that begun… • First DIS experiment : Friedman, Kendall, Taylor, et al. • Parton model : Feynman • Scaling : Bjorken Yamagata, Sept.15 2009

  4. 20 years ago : the spin “crisis”… • EMC paper on the spin structure of the nucleon • Ellis-Jaffe sum rule violated • DS = ½ <Sz> is small: … instead of 0.5! Yamagata, Sept.15 2009

  5. COMPASS: nucleon spin studies Proposal : 1996, first measurements : 2002 • (Selection of) past measurements • Direct measurement of DG – talk of K. Klimazsewski, session 10 • DIS and Polarized Distribution Functions • Polarization of the strange quark sea • Asymmetry of the polarized sea • Transversity measurements • Future plans • Near future: transverse and longitudinal data • Study of GPD using DVCS and HEMP • Drell-Yan – talk of N. Doshita, session 6 Yamagata, Sept.15 2009

  6. Geneva lake Jura mountains COMPASS SPS LHC N NTU, Jan.22, 2009

  7. COMPASS experimental set-up • Beam energy: 100 – 200 GeV • Polarized muon beam (Pµ=80%) • Two-stage spectrometer • Large acceptance • Particle identification • Multi-purpose set-up Yamagata, Sept.15 2009

  8. DIS Deuteron asymmetries • Double-spin asymmetries • Main advantages of COMPASS : high energy, high Q2, low x • Data taking with muons: 2002 – 2004, 2006 – 2007 Data: 2002 - 2004 Yamagata, Sept.15 2009

  9. What does DIS says about DG? Yamagata, Sept.15 2009

  10. Deuteron and proton g1(x) world data Proton data - world Deuteron data - world COMPASS data: 2007 COMPASS data: 2002 – 2006 Yamagata, Sept.15 2009

  11. data are used to perform a NLO QCD fit Deuteron and proton g1(x) world data Yamagata, Sept.15 2009

  12. Compass deuteron + world data (proton 2007 not included) COMPASS analysis (2007) : both positive and negative solutions for DG(x) Is DG positive or negative? QCD fit results – Polarized parton distributions DG < 0 DG > 0 Yamagata, Sept.15 2009

  13. The shape of DG(x) is still unknown Dedicated measurements of DG(x) badly needed -- > talk of KonradKlimaszewski DG is calculated by many other groups… From E. Leader, Spin-2008 LSS-06 : Leader, Sidorov, Stamenov, Phys. Rev. D73, 2006 AAC’06 : Hirai, Kumano, Saito, Phys. Rev. D74, 2006 DSSV-08: De Florian, Sassot, Stratmann, Vogelsang, Phys. Rev. Lett. 101, 2008 + HERMES, Blumlein, Bottcher DG(x) can also change sign! Yamagata, Sept.15 2009

  14. Is the strange quark sea polarized? Yamagata, Sept.15 2009

  15. QCD fit to world DIS data (Compass, and many other groups…) Global analysis of G0, HAPPEx, E734 Slope of GsA for Q2 0 These data favor a negative Ds Ds is negative … Phys.Lett. B660(2008)458. Pate, McKee, Papavassiliou PR C78(2008)015207 Yamagata, Sept.15 2009

  16. Ds is close to zero, or positive! • DIS and SIDIS data from the deuteron (Hermes) • HERMES has measured Ds ≥ 0 • Is this result confirmed by COMPASS? • - Behavior at lower x? • - Why are SIDIS data and QCD fit results different? Hermes Coll., Phys. Lett. B666(2008)446 Also on Phys. Rev. D71, 2005 Leader, Sidorov, Stamenov, Phys. Rev. D73, 2006. Yamagata, Sept.15 2009

  17. COMPASS: SIDIS asymmetries - deuteron Deuteron data: 2002 – 2004, 2006 • Pions and kaons are identified • Good agreement with HERMES Yamagata, Sept.15 2009

  18. COMPASS: SIDIS asymmetries - proton Proton data: 2007 (preliminary) • Next step: • Leading Order (LO) fit of the 10 asymmetries (2x5) • Determine 5 flavor separated PDFs : Yamagata, Sept.15 2009

  19. Compass results for Du(x), Dd(x), and Ds(x) • Data extend down to x=0.004 • Ds(x) compatible with 0 in the measured range DNS: De Florian, Navarro, Sassot, Phys. Rev. D71, 2005 Du, Dd : compatible with HERMES Ds: Integral value: Yamagata, Sept.15 2009

  20. Semi-Inclusive DIS SIDIS: Detect a hadron in coincidence with the scattered muon Fragmentation Function: z is the fraction of energy carried by h • Assumptions on FF : • factorization • charge and isospin invariance Asymmetry: Yamagata, Sept.15 2009

  21. DS and Fragmentation Functions • Strong dependence on the FF ratio • Better evaluation of the relevant FFs is necessary DSS: De Florian, Sassot, Stratman, Phys. Rev. D75, 2007 EMC: EMC collaboration, Arneodo et al, Nucl. Phys. B321, 1989 KRE: Kretzer, Phys. Rev. F62, 2000 • First moments: FF from DSS = -0.01±0.01±0.01 • FF from EMC = -0.05±0.03±0.01 • Ds=½(Ds+Ds̄) :from QCD fit = -0.045±0.01±0.01 Yamagata, Sept.15 2009

  22. As for DG, the shape of Ds(x) may be more complicated than previously assumed DS and Global analysis of DIS, SIDIS, pp data From De Florian, Sassot, Stratman, Vogelsang, PRL 101, 2008 constrained by SIDIS DNS constrained by SU(3): 3F-D GRSV Yamagata, Sept.15 2009

  23. Is the polarized sea flavor symmetric? Yamagata, Sept.15 2009

  24. Unpolarized DIS • Gottfried sum rule is violated • QM prediction: = 1/3 • Data = 0.22±0.01±0.02 NMC: Phys.Rev.Lett. 66, 1991 confirmed by HERMES and E886 NMC 1991 From J.C.Peng, Eur. Phys. J. A18, 2003 • The unpolarized sea is asymmetric ( ) • What about the polarized one? Yamagata, Sept.15 2009

  25. Through valence spin distribution • Use hadron asymmetries in semi-inclusive DI scattering • Combine h+ and h- in “difference” asymmetry –> valence quarks • First moment compatible with Du = -Dd • Advantages (LO): • Fragmentation functions cancel • No Particle Identification • Better statistical errors COMPASS 2007 Using G1 from DIS Yamagata, Sept.15 2009

  26. Flavor asymmetry of the polarized sea NLO from DSSV, arXiv-0904.3821 Yamagata, Sept.15 2009

  27. Transverse spin effects Yamagata, Sept.15 2009

  28. Transversity PDFs • Transversity • Third distribution function (twist-2): DTq(x) • Transverse quark spin distribution in transversely polarized nucleon • Measurement • Semi-inclusive DIS • transversely polarized target • azimuthal asymmetry q(x) Dq(x) DTq(x) Yamagata, Sept.15 2009

  29. Transversity: Collins results prior to 2009 HERMES proton Phys. Lett. B673, 127 2009 COMPASS deuteron • Signal from HERMES (p) not confirmed by COMPASS (d)? • Large compensation between u and d quarks?

  30. Transversity: global fit to the data • Fit includes: • COMPASS (deuteron) • HERMES (proton) • BELLE @KEK (e+e-p+p-X) Anselmino et al., Phys. Rev. D75, 2007 Soffer bound Yamagata, Sept.15 2009

  31. COMPASS confirms HERMES proton results: nice agreement And also: Q2 dependence of the results Transversity: COMPASS proton data 2007 Q2 dependence Yamagata, Sept.15 2009

  32. Sivers asymmetry (deuteron) Fs=fh-fS

  33. Sivers asymmetries (proton results) Global fit: on COMPASS deuteron and HERMES proton data Asymmetries are small, compatible with zero Marginal agreement between predictions and data – better statistics needed

  34. COMPASS future • Short term plans: 2010, 2011 • Mid term proposal: 2012 and beyond NTU, Jan.22, 2009

  35. 2010: Transversity, one year on a NH3 target • Goal: improve statistics on the proton by a factor of 3 • Over the whole x range E = 160 GeV I = 2x108 µ/spill h+ h- COMPASS 2010 est. COMPASS 2007 HERMES 2005 Collins Sivers NTU, Jan.22, 2009

  36. 2011: Longitudinal, one year on a NH3 target • Goal: improve statistics on proton • Proton and deuteron data with similar accuracy E = 200 GeV I = 2x108 µ/spill deuteron proton COMPASS 2011 est. Yamagata, Sept.15 2009

  37. 2011: Longitudinal, one year on a NH3 target • Precise test of the Bjorken sum rule • Improved full flavor separation • Light quarks • Strange quark sea • Polarized sea asymmetry Yamagata, Sept.15 2009

  38. Proposal for 2012 and beyond • Study of the Generalized Parton Distributions (GPDs) • Natural extension of: Form factors PDFs (spatial distributions) (momentum distributions) • Unified description: 3-dimensional picture of the nucleon • Access through: • - Deeply Virtual Compton Scattering (DVCS) • - Hard Exclusive Meson Production (HEMP): r, w, f, p, h Yamagata, Sept.15 2009

  39. Cross section 4 GPDs: for each quark flavor Experimental apparatus must detect Incident and scattered lepton Outgoing real photon Recoiling soft proton Generalized Parton Distributions and DVCS known dsDeep VCS + Bethe-Heitler2 g g* x+x x-x GPDs P2 P1 t Yamagata, Sept.15 2009

  40. ZEUS H1 COMPASS HERMES JLab GPDs at COMPASS – kinematical coverage • Unique features • Both µ+ and µ- polarized beams • Energy: 100 – 190 GeV • √s = 14 – 20 GeV • Apparatus: “almost” ready • + upgrades • First tests done in 2008 Luminosity limit 100 events Q2 > 1 GeV2 • 2008: Single day of data taking L=1032 with a 2.5m LH2 target • Complementary to HERA and HERMES/JLAB Yamagata, Sept.15 2009

  41. Beam charge and spin asymmetry No HT contribution 160 GeV muon beam 2.5m LH2target 140 days global = 10%, L=1.2 fb-1 Prediction for 1 year of data taking Yamagata, Sept.15 2009

  42. Beam charge and spin sum • Nucleon “tomography”, or transverse imaging • Integrate over f and subtract BH: • transverse size of the nucleon vs x Impact parameter X=0.05 X=0.3 X=0.5 B(x) = b0 + 2 a’ln(x0/x) COMPASS PREDICTIONS From fit to FF data, Diehl, Feldmann, Jakob, Kroll, 2005 160 GeV muon beam, 2.5m LH2target 140 days, L=1.2 fb-1,global = 10%, Yamagata, Sept.15 2009

  43. Spectrometer upgrades • New equipments (Phase I) • Large Recoil Proton Detector • 2.5 m long LH2 target • Large angle ECAL calorimeter/project • R&D (Phase II) • New polarized target/superconducting magnet (Transverse field) Prototype: 1.2 m long; target: 40 cm Used in 2008, 2009 Yamagata, Sept.15 2009

  44. Summary • COMPASS is a major player in the nucleon spin/structure physics • PAST (2002 – 2007) • A number of important results with µ+ beam already available • Lot of data are being analyzed, additional results to come • PRESENT (2008 and 2009) • Hadron spectroscopy and polarizabilities studies (hadron beams) • NEAR FUTURE (2010 and 2011) • Improved statistics on the proton (T and L) • MID-TERM ( >2012) • GPD studies (µ+ and µ- beams) • DRELL-YAN studies with p and p beams ( talk of N. Doshita) Yamagata, Sept.15 2009

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