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Spin Structure of the Nucleon in COMPASS

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Spin Structure of the Nucleon in COMPASS Spin structure of the nucleon in COMPASS C. Quintans, LIP-Lisbon th 19 December 2010, PASC Workshop COMPASS Co-financed by: Spin structure of the nucleon in COMPASS C. Quintans – p.1 Outline The nucleon spin puzzle Longitudinally polarized DIS ? d A1, g1 and the Bjorken sum rule ? Flavour separation ? Gluon polarization Transversely polarized SIDIS TMD PDFs from polarized Drell-Yan GPDs from DVCS Conclusions Spin structure of the nucleon in COMPASS C. Quintans – p.2 The COMPASS Experiment at CERN Studies of the nucleon spin structure (2002 – 2007, 2010 – 2011) ? with polarized muon beam (µ+ at 160 GeV/c, ≈80% polarized) and polarized 6 targets ( LiD and NH3) Studies of hadron spectroscopy (2008 – 2009) ? with h− beam at 190 GeV/c (95% π−, 4% K−, 1% p¯) and unpolarized targets (liquid H2, Ni, Cu, Pb, W) ? with h+ beam at 190 GeV/c (75% p, 24% π+, 1% K+) and unpolarized liquid H2 target Future studies on the nucleon structure (2013 – 2015) ? GPDs from DVCS: with polarized muon beam (µ at 100-190 GeV/c and long st nd unpolarized liquid H2 (1 phase) or transversely polarized NH3 target (2 phase) ? TMD PDFs from polarized Drell-Yan: with hadron beam (95% π−) and a polarized NH3 target Spin structure of the nucleon in COMPASS C. Quintans – p.3 COMPASS spectrometer and target Polarized target 1m 3 He−Precooler Superconducting Upstream Downstream Solenoid cell cell NU ND µ−beam Acceptance + target spin (180mrad) µ beam @160 GeV reversal every 8h -80% polarization Dilution refrigerator Targets N' N' U D 6 LiD or NH3 targets Spin structure of the nucleon in COMPASS C. Quintans – p.4 wn y small poorl wn unkno kno The nucleon spin puzzle Nucleon spin: 1 = 1 ∆Σ + ∆ + 2 2 G LZ quarks gluons orbital spin spin ang. mom. In the quark-parton model, including relativistic corrections, expect ∆Σ ≈ 0:6 1988: EMC measured the quarks contribution to the spin of the nucleon to be very small ! Present world data: ∆Σ = 0:30 0:01(stat) 0:02(evol) 7! COMPASS, PLB 647 (2007) 8-17 Spin structure of the nucleon in COMPASS C. Quintans – p.5 The nucleon spin puzzle Nucleon spin: 1 = 1 ∆Σ + ∆ + 2 2 G LZ wn y quarsmallks gluonspoorl orbitalunkno spin spinwn ang. mom. kno In the quark-parton model, including relativistic corrections, expect ∆Σ ≈ 0:6 1988: EMC measured the quarks contribution to the spin of the nucleon to be very small ! Present world data: ∆Σ = 0:30 0:01(stat) 0:02(evol) 7! COMPASS, PLB 647 (2007) 8-17 Spin structure of the nucleon in COMPASS C. Quintans – p.5 Spin asymmetries in DIS For its deep inelastic scattering (DIS) programme, COMPASS uses beam and target polarized to measures double spin asymmetries. 2 2 (E,p) (E',p') Q = −q µ µ 0 q ν = E − E γ* 2 Q x = 2Mν E N X h z = ν The µ-deuteron/proton asymmetry is measured from the difference between cross-sections from 2 oppositely polarized target cells: ) − ( − µN 1 N − N A = ) − ( − fPT PB N + N This measured asymmetry relates to the longitudinal and transverse ∗ AµN γ -deuteron/proton asymmetries: D ≈ A1 Spin structure of the nucleon in COMPASS C. Quintans – p.6 Longitudinal polarization: inclusive asymmetries deuteron (2002-2006 data) proton (2007 data) p p 1 0.7 1 EMC 0.02 0.12 d 1 A d 1 COMPASS A A 0.80.1 SMC 0.8 0.6 0 SMC A 0.08 E143 0.06 E143 0.5 E155 -0.02 0.04 E155 0.60.02 HERMES 0.6 0 CLAS W>2.5 -0.04 HERMES 0.4 -0.02 COMPASS 0.3 0.4 -2 0.4 -0.06 10 x 10-2 x 0.2 0.2 0.2 0.1 0 0 0 -0.1 -2 -1 10-2 10-1 x 1 10 10 x 1 COMPASS, PLB 647 (2007) 330-340. COMPASS, PLB 690 (2010) 466-472. Significant improvement of precision in the low x region. Large asymmetry observed at high x, in agreement with other experiments. Spin structure of the nucleon in COMPASS C. Quintans – p.7 Longitudinal spin-dependent g1 PDF g1 is obtained from the asymmetry A1, for proton or deuteron: F2 g1 = A1 2x(1 + R) L T F2 ! SMC parameterization; R = σ /σ ! SLAC parameterization 0.08 0.04 d 1 p SMC 1 0.07 EMC E143 SMC xg xg 0.03 E155 0.06 E143 deuteron E155 proton HERMES 0.05 0.02 CLAS W>2.5 HERMES COMPASS 0.04 CLAS W>2.5 COMPASS 0.03 0.01 0.02 0 0.01 0 -0.01 -2 -1 -2 -1 10 10 x 1 10 10 x 1 st d From the 1 moment of g1 (x): 2 2 ∆Σ = a0 = 0:33 0:03(stat) 0:05(syst) at Q = 3 (GeV/c) 1 2 ∆s + ∆s¯ = 3 (a0 − a8) = −0:08 0:01 0:02 at Q ! 1 Spin structure of the nucleon in COMPASS C. Quintans – p.8 Bjorken sum rule Combining deuteron and proton data, one can access the NS p n non-singlet spin structure function g1 = g1 − g1 . NS According to the Bjorken sum rule the first moment of g1 is proportional to the ratio of axial and vector coupling constants gA=gV : 1 NS 2 1 gA NS 2 dx g1 (x; Q ) = j j C1 (Q ) Zxmin 6 gV 1 NS ∫ gNS(x)dx 92% of the first moment of g comes from the 1 1 xmin 0.2 Bjorken sum rule measured region 0.18 0.16 At the lowest accessible x in COMPASS, 0.14 1 NS 0.12 R0:004 dx g1 = 0:180 0:009 0.1 0.08 The value expected from the Bjorken sum rule is COMPASS data g 0.06 A 0.188, taking j g j from neutron β decay 0.04 V 0.02 Validity of the Bjorken sum rule confirmed with 5% sta- 0 10-2 10-1 1 tistical precision. xmin Spin structure of the nucleon in COMPASS C. Quintans – p.9 Flavour separation From a semi-inclusive DIS analysis, using charged kaons and pions identified in the RICH detector, with both deuteron and proton polarized targets, full quarks flavour decomposition (in LO) can be done: 0.4 x∆u 0.4 x∆d Using the DSS fragmentation functions 0.2 0.2 (D.Florian, Sassot, Stratmann, Phys.Rev. 0 0 D75 (2007) 114010) -0.2 -0.2 -2 -1 -2 -1 The curves are predictions from the ∆ 10 10 x 10 10 x 0.05 x u x∆d 0.05 DSSV fit calculated at NLO 0 0 (D.Florian, Sassot, Stratmann, -0.05 -0.05 Vogelsang, Phys.Rev.D 80 (2009) 10-2 10-1 -2 -1 0.02 x∆s x 10 10 x 034030) 0 -0.02 The flavour asymmetry of the sea helicity ¯ -0.04 distributions ∆u¯ − ∆d is slightly positive, -2 -1 10 10 x compatible with zero. PLB 693 (2010) 227-235 Spin structure of the nucleon in COMPASS C. Quintans – p.10 µ µ γ∗ q High pT hadron pairs g q ? Large statistics available N ? Very dependent on Monte-Carlo Gluon polarization The direct measurement of ∆G is of crucial importance for the understanding of the spin puzzle. ,! Access it via the photon-gluon fusion (PGF) process. PGF events are selected by analysing: Open Charm production µ µ ? Very clean: almost no physics γ∗ c background ? Low statistics g c N Spin structure of the nucleon in COMPASS C. Quintans – p.11 µ µ γ∗ c g c N Gluon polarization The direct measurement of ∆G is of crucial importance for the understanding of the spin puzzle. ,! Access it via the photon-gluon fusion (PGF) process. PGF events are selected by analysing: Open Charm production ? Very clean: almost no physics µ µ background ? Low statistics γ∗ q High pT hadron pairs g q ? Large statistics available N ? Very dependent on Monte-Carlo Spin structure of the nucleon in COMPASS C. Quintans – p.11 ∆G from open-charm 5 channels with Do production are studied (data 2002-2007): Do ! Kπ (untagged) D∗ tagged : Do ! Kπ, Kππo, Kπππ, sub(K)π 0 0 COMPASS 2007 DKπ (D* tagged) COMPASS 2007 DKππ0 (D* tagged) 0 ) 1200 ) 2 2 COMPASS 2007 DKπ ) 0 2 N(D ) = 3100 Preliminary Preliminary 800 1000 ππ0 40000 Preliminary K N/10 (MeV/c N/10 (MeV/c N/5 (MeV/c 800 600 0 30000 N(D ) = 4600 600 400 20000 N(D0) = 19000 400 200 10000 200 0 0 0 -400 -300 -200 -100 0 100 200 300 400 -600 -400 -200 0 200 400 600 -600 -400 -200 0 200 400 600 2 2 2 M - M 0 (MeV/c ) M - M 0 (MeV/c ) M - M 0 (MeV/c ) Kπ D Kπ D Kπ D 0 COMPASS 2007 Dsub(K)π (D* tagged) ) 0 2 COMPASS 2007 DKπππ (D* tagged) ) Preliminary 2 400 Preliminary 1000 N/10 (MeV/c N(D0) = 1100 N/10 (MeV/c sub-threshold Kaons 300 S ∆G 0 Araw = PtPbfaLL + 500 N(D ) = 1000 200 S B G 100 0 0 -400 -300 -200 -100 0 100 200 300 400 -400 -300 -200 -100 0 100 200 300 400 2 2 M - M 0 (MeV/c ) M - M 0 (MeV/c ) Kπ D Kπ D Weighted events: all factors between Araw and ∆G=G enter in the weight. The method implies simultaneous extraction of the background asymmetry: 1 Abkg = B Araw PtPbfD S+B Spin structure of the nucleon in COMPASS C.
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