COMPASS results on inclusive and semi inclusive polarised DIS

Transcription

1 COMPASS results on inclusive and semi inclusive polarised DIS Helena Santos LIP - Lisboa on behalf of the COMPASS Collaboration The nucleon spin The COMPASS experiment Longitudinal spin structure functions Valence quark polarisations Conclusions and outlook DSPIN - 07 XII WORKSHOP ON HIGH ENERGY SPIN PHYSICS

2 The Nucleon Spin naïve parton model: ΔΣ = Δu+Δd = gluons, sea and c quarks are important complete description: orbital angular momenta EMC (988): ΔΣ = 0. ± 0.09 ± 0.4 Δs+Δs = 0.4 ± 0.03 S N = ½ = ½ΔΣ + ΔG + L q + L g (h=)

3 Deep Inelastic Scattering μ (E,p ) μ(e,p) N(M,P) θ Q = q = (p p ) ν = E E x = Q /Mν y=ν/e z = E h /ν d σ dωde' = ( ) ( ) ( ) ( ) x,q + c F x,q + c g x,q + c g x,q c F spin independent 3 4 spin dependent

4 Polarised Deep Inelastic Scattering photon-nucleon asymmetry A e Δq(x) σ σ3 q q = = σ + σ3 e q(x) q q g (x) F (x) σ ~ + q Δq(x) = q(x) + q(x) q(x) = q(x) + + q(x) σ 3 ~ q + quark nucleon quark nucleon

5 The COMPASS Experiment at the CERN-SPS COmmon Muon and Proton Apparatus for Structure and Spectroscopy LHC SPS M N Luminosity: ~ cm - s - Beam intensity:.0 8 µ + /spill (4.8s/6.8s) Beam momentum: 60 GeV/c 30 physicists from countries

6 The COMPASS Spectrometer Trackers Magnets RICH Electromagnetic Calorimeters Hadronic Calorimeters Absorbers Target NIM A 577 (007) m SM SM 60 GeV μ+ polarised beam and target -80% 50%

7 The Target System See Y. Kisselev s talk Two 60 cm long target cells with opposite polarisation Target material: 6 LiD Polarisation ~ 50% Solenoid field:.5 T 3 He/ 4 He: T min ~ 50mK Field reversal every 8h From 006 on: New solenoid with 80 mrad acceptance Three target cells (better to face systematics)

8 Inclusive Asymmetry, Q < (GeV/c) Phys. Lett. B 647 (007) 330 A d asymmetry compatible with 0 at low x range (0.0005<x<0.0) At low x A d has been measured only by COMPASS and SMC Systematic errors are mainly due to false asymmetries

9 g The g (x) Structure Function ( x) = A ( x) R(x,Q ) = σ L x / σ ( x) ( + R) F T Q < (GeV/c) Knowledge of g at low Q is needed to test non-perturbative models: Regge and (G)VDM F (x) taken from SMC param. (SMC + JKBB: B. Adeva et al PRD60 (999) 07004; Erratumibid.D6:07990,000) x > 0. SLAC (PLB50 (990) 93; B5(999)994) < x < 0. NMC (R param. unpublished) x < ZEUS (EPJ7 (999) 609)

10 Inclusive DIS Asymmetry Phys. Lett. B 647 (007) 8 COMPASS 00, 03 & 04 data 89 x 0 6 events Q > (GeV/c) < x < < y < 0.9 A compatible with 0 for x < 0.05 Large asymmetry at large x Systematic errors: Multiplicative δ 0.0A (δp B, δp T, δf and δd) Additive rad. corrections ; A false < 0.4δA stat

11 Inclusive DIS Asymmetry Q > (GeV/c) Good agreement with previous experiments Improved significantly statistics at low x No tendency towards negative values at x < 0.03

12 The g d (x) Structure Function g ( x) = A ( x) x ( x) ( + R) F R(x,Q ) = σ L / σ T Q > (GeV/c)

13 QCD Analyses ΔΣ = Δu + Δd + Δs, Δq 3 = Δu - Δd, Δq 8 = Δu + Δd - Δs DGLAP equations: Input parameterisations (x dependence at a fixed Q 0 ): Minimization routine:

14 QCD Fits Two different approaches have been used: - Numerical integration in (x,q ) space (PRD58(998) 00) - Solution of DGLAP in space of moments (PRD70(004) 07403) Fits to world data 30 world data points, 43 from COMPASS NLO analysis (MS scheme) Data well described by two solutions: ΔG > 0 and ΔG < 0

15 Towards Structure Functions World data and QCD fits at Q 0 = 3 (GeV/c) g ( x,q ) = g ( x,q ) + 0 i fit fit [ g ( x,q ) g ( x,q )] Solutions with ΔG < 0 0 i

16 Towards Structure Functions World data and QCD fits at Q 0 = 3 (GeV/c) g ( x,q ) = g ( x,q ) + 0 i fit fit [ g ( x,q ) g ( x,q )] Solutions with ΔG > 0 0 i

17 Polarised Parton Distributions Very small sensitivity of x(δq+δq) to xδg

18 QCD Fits Results Quark polarisation: (world data) Phys. Lett. B 647 (007) 8 η G > 0 η G < 0 η Σ 0.7 ± ± 0.0 η = Δ K k dx 0 η Σ (error factor larger without COMPASS) = 0.30 ± 0.0(stat) ± 0.0(evol) Gluon polarisation (indirect determination via DGLAP): Solutions with η G > 0: Solutions with η G < 0: η η prog G prog G = = , , η prog G η prog G = = η G

19 Gluon Polarisation ΔG/G Comparison between direct measurement of gluon polarisation (Y. Bedfer s talk) and COMPASS NLO QCD fits to g Unpolarised G(x) from MRST Bands correspond to statistical errors of ΔG

20 First Moment of g (COMPASS data only) Phys. Lett. B 647 (007) 8 N ( Q = 3(GeV/ c) ) = g ( x) N Γ dx = 0.050± 0.008(stat) ± 0.000(evol) ± 0.005(syst) 0 0 in literature (S.A. Larin et al., PLB404 (997) 53): α ( ) s(q ) = + α Q O( ) a (Q ) + N Γ s 0 a8 9 π 4 (from Y. Goto et al., PRD6 (000) 03407: a 8 = ± 0.05) a0 (Q 0 = 3(GeV / c) ) = 0.35 ± 0.03(stat) ± 0.05(syst) extrapolating to Q = 0.33 ± 0.03(stat) ± 0.05(syst) â 0 Q ( Δs + Δs) = (â0 a8) = 0.08 ± 0.0(stat) ± 3 0.0(syst)

21 Semi-inclusive asymmetries CERN-PH-EP/ COMPASS kinematic domain: inclusive DIS + 0. < z < 0.85 Statistics: N + = 30x0 6, N = 5x0 6, corr(n +,N ) 0% Systematic errors: Multiplicative δ 0.08A (δp B, δp T, δf and δd) Additive: rad. corrections ; A false < 0.5δA stat

22 Difference asymmetry In LO QCD FF do cancel out in A +. For a deuteron target: The contribution of sea quarks to the nucleon spin can be obtained by combining the matrix elements a 0 and a 8 and the integral To disentangle between symmetric and asymmetric sea scenarios δγ v < Δs+Δs is needed

23 CERN-PH-EP/ Difference asymmetry The measured x range is < x < 0.7, as r becomes For the acceptance studies full chain of MC simulation (spectrometer + same cuts as for data) with default LEPTO settings was performed

24 CERN-PH-EP/ Valence quark polarisations (ω D = 0.05 ± 0.0) Unpol. sea contribution to F vanishes for x > 0.3 Δu+Δd < u+d Much better precision All points evolve to Q 0 = 0 (GeV/c) accordingly to DNS parameterisation (D. De Florian, G.A. Navarro and R. Sassot, Phys. Rev. D7 (005) 09408) LO DNS analysis, based on KKP param. of FF, includes: All DIS g prior to COMPASS 004 data; All SIDIS data from SMC and HERMES (Δu = Δd = Δs = 0 for x > 0.3) Unpolarised MRST 004 LO PDFs have been used

25 Comparison with other experiments

26 Comparison with other experiments

27 Comparison with other experiments

28 Comparison with other experiments DNS parameterisation predicts successfully COMPASS SIDIS data

29 Estimate for the first moments (LO) CERN-PH-EP/ Contribution from the unmeasured 0.7 < x < region is (DNS fit) SU(3) symmetric sea was assumed in SMC The estimated Γ v (SIDIS + DIS) is.5σ stat away from the symmetric sea scenario

30 Conclusions From the first moment of g d, we extract the quark contribution to the nucleon spin (COMPASS data only): â 0 ΔΣ = 0.33 ± 0.03(stat) ± 0.05(syst) ( Δs + Δs) = 0.08 ± 0.0(stat) ± 0.0(syst) QCD fits to world data give for quark and gluon contributions: η Σ (Q0 = 3(GeV / c) ) = 0.30 ± 0.0(stat) ± 0.0(evol) ΔG Δu v + Δd v have been extracted from difference asymmetry approach Increase of the precision at small x by a factor of ~6 as compared to SMC DNS parameterisation predicts successfully COMPASS SIDIS data SU(3) symmetric sea scenario is disfavoured

31 Prospects Flavour separation with deuteron data and 007 proton data COMPASS decisive in the understanding of polarised PDFs