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The Role of Semi Inclusive DIS Data in Determining Polarized PDFs

Saclay, Paris, 17 September 2010. The Role of Semi Inclusive DIS Data in Determining Polarized PDFs. E. Leader (London) , A. Sidorov (Dubna) , D. Stamenov ( Sofia). OUTLINE. A combined NLO QCD analysis of inclusive and semi inclusive DIS world data is presented.

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The Role of Semi Inclusive DIS Data in Determining Polarized PDFs

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  1. Saclay, Paris, 17 September 2010 The Role of Semi Inclusive DIS Data in Determining Polarized PDFs E. Leader (London), A. Sidorov (Dubna),D. Stamenov(Sofia)

  2. OUTLINE A combinedNLO QCD analysis of inclusive and semi inclusive DIS world data is presented The recent COMPASS data on A1p and A1dπ(+/-), A1dK(+/-) areincluded The higher twist corrections (HT) to g1 are accounted for (in contrast to the other analyses) Impact of SIDIS on polarized PDFs and higher twist A quark flavor decomposition of the polarized sea Summary

  3. The main goalto answer the question how the helicityof the nucleon is divided up among its constituents: Sz = 1/2 =1/2 DS(Q2) + DG (Q2) + Lz (Q2) DS = the parton polarizations Dq and DG arethe first moments of the helicity densities: To determine the shape of the polarized parton densities

  4. Inclusive DIS one of the best tools to study the structure of nucleon Q2 = -q2 = 4EE`sin2(q/2) l` k` x = Q2/(2Mn) n = E – E` l k q g* DIS regime==>Q2 >> M2, n >> M N P Fi(x, Q2)gi(x, Q2) preasymptotic region unpolarized SF polarized SF

  5. InclusiveDIS Cross Section Asymmetries Measured quantities where A1,A2 are the virtual photon-nucleon asymmetries the best quantity to test QCD and to determine PDFs If A||andare measured Ifonly A||is measured - kinematic factor NB. g cannot be neglected in the JLab, SLAC andHERMESkinematic regions

  6. In QCD Theory dynamical HT power corrections (t =3,4) => non-perturbative effects(model dependent) target masscorrections which are calculable in QCD A. Piccione, G. Ridolfi In NLO pQCD logarithmic in Q2 polarized PD evolve in Q2 according to NLO DGLAPeqs. Nf (=3) - the number of flavors

  7. IMPORTANT Due to the lack of the charged current neutrino data only the sums can be determined from polarized inclusive DIS data !

  8. An important difference between the kinematic regionsof theunpolarized and polarized data sets A half of the present inclusive data are atmoderateQ2 and W2: preasymptotic region The HT corrections to g1 are NOT negligible in the preasymptotic region and have to be accounted for LSS, Phys. Rev. D75 (2007) 074027

  9. x-Q2 range of F2 and g1 structure functions Polarized data

  10. Semi-inclusive processes allow to separate and Fragmentation functions ef2Dqf(x,Q2)Dfh (z,Q2) In LO QCD ef2 qf(x,Q2) Dfh (z,Q2) InNLOQCDthe Wilson coefficients have to be included

  11. Fit to the data Inclusive DIS Semi-inclusive DIS N.B. It is NOT known at present how to account for the HT and TMC corrections in SIDIS processes. Fortunately, they should be less important due to the kinematic region of the present SIDIS data.

  12. Input parton densities at Q20 = 1 GeV2 (more general expressions than those in our previous analyses) Sum Rules 16 free parameters

  13. ef2Dqf(x,Q2)Dfh (z,Q2) ef2 qf(x,Q2) Dfh (z,Q2)

  14. For the fragmentation functions Dqh(z, Q2) the DSS (de Florian, Sassot, Stratmann) ones have been used Unpolarized NLO MRST’02 PDFs have been used to calculate F1Nh The inverse Mellin transformation method has been used to calculate g1N(x, Q2),g1Nh (x, z, Q2) and F1Nh (x, z, Q2) from their moments. The positivity constraints on polarized PDFs are imposed Nr of the freeparameters – 26 (16 for PDFs and 10 for HT) The systematic errors are added quadratically to the statistical ones DATA Inclusive DIS – 841 experimental points Semi inclusive DIS – 202 experimental points Total – 1043 exp points

  15. Fit to the data A good description of both the DIS and SIDIS data DIS: SIDIS:

  16. Results:Longitudinal polarized PDFs and higher twist

  17. LSS’10 PDFs – this fit, LSS’06 – a fit to DIS data alone (PR D75, 2007) Error bands Δχ2 = 1 A flavor decomposition of the polarized sea due to SIDIS data are determined without additional assumptions Changing in sign very unexpected Changing in signΔG(x) - such a solution has been already found from inclusive DIS data N.B. In the QCD analyses of inclusive DIS data:

  18. ∆s(x) Our Ds(x)differs from that one obtained by DSSV (less negative at x<0.03 and less positive for large x). Note that DSSV have used the assumption for Ds(x) In contrast to a changing in signDs(x) coming fromSIDIS, in all the QCD analyses of inclusive DIS data a negative[Ds(x)+Ds(x)]/2for any x in the measured region is obtained ¯ ¯ ∆s(x) is controversial ! May bethe assumption∆s(x)=∆s(x) in SIDIS is not correct ? ¯ The determination of Ds(x) from SIDISstrongly depends on FFs (COMPASS – PL B680 (2009) 217) and the new FFs (de Florian, Sassot, Stratmann) are crucially responsible for the unexpected behavior of Ds(x) Obtaining a final and unequivocal result for Ds(x) remains a challenge for further research on the internal spin structureof the nucleon

  19. ΔG(x, Q2) The present polarized DIS and SIDIS data cannot rule out the solution with a positive gluon polarization The sea quark densities obtained in the fits with positive and node xΔG are almost identical. SIDIS data do NOT help to constrain better the gluon polarization.

  20. 1. Via Open Charm production (q=c) c  D0 K-p+ and D*+D0p+ COMPASS: ΔG/G =-0.08 +/- 0.21 +/- 0.11 at <xg>=0.11and<µ2> = 13 GeV2 LO treatment. All deuteron data included (C. Franco, DIS 2010, Florence) 2. Via High-pt hadron pairs (q=u,d,s) -Detect 2 hadrons (mostly pions) COMPASS, HERMES - 2 determinations: Q2 > 1 GeV2 Q2 < 1 GeV2 Determination of ∆G from direct measurements ∆G/G – two methods, three measurements Photon- Gluon Fusion Unfortunately, the direct measurements give us information on DG in narrow range of x

  21. Comparison with directly measured DG/G µ2 = 3 GeV2 ΔG/G from high pt hadron pairs The most precise values of DG/G, the COMPASS ones, are consistent with both of the polarized gluon densities determined in our combined QCD analysis ΔG/G from open charm production Both of our solutions for ΔG/G are also in agreement with the COMPASS experimental value, especially the changing in sign xΔG. The direct measurements of ΔG/G at COMPASS cannot distinguish between the positive and node xΔG(x) obtained from our QCD analysis

  22. As expected the SIDIS data do not influence essentially the sums already well determined from the inclusive DIS data. Our densities are well consistent with those obtained by DSSV.

  23. Higher twist effects Compared to HT(LSS’06): The values of HT(p) are practically not changed while the new values of HT(n)are smaller and almost compatible with zero within the errors for x > 0.1 . We consider this change of HT(n) as a result of the new behavior of Δs(x), positive for x > 0.03. In addition, the newA1pCOMPASS data impact on the smallest x point of HT(p,n). g1N = (g1N)LT,TMC + hN(x)/Q2

  24. The first moments of higher twist Thanks to the very preciseinclusive DIS CLAS data the first moments of HT corrections are also well determined. In agreement with the instantonmodel predictions and the values obtained from the analyses of the first moment of g1(p-n) (Deur et al., PR D78, 032001, 2008. ; R. Pasechnik et al. PR D78, 071902, 2008) In agreement with 1/NC expansion in QCD (Balla et al., NP B510, 327, 1998) 

  25. Higher twist vs TMC Sidorov, Stamenov: Mod. PL A21, 1991 (2006)

  26. LSS10 predictions for the COMPASS A1pπ+(-) and A1pK+(-) data

  27. After the fit including the COMPASS A1pπ+(-) and A1pK+(-) data

  28. Impact of SIDIS COMPASS/p data on PDFs uncertainties

  29. Impact of the future DIS CLAS12 data on PDFs uncertainties Using the 11 GeV highly polarized electron beam of the energy-upgraded CEBAF at JLab very accurate datain 0.075 ≤ x≤ 0.775, 1.01 ≤ Q2≤ 12.05 A significant improvement of the data accuracy do NOT impact on Δs errors?!

  30. Impact of the future SIDIS CLAS12 data on PDFs uncertainties Kinematic region: 0.04 ≤ x≤ 0.76, 1.01 ≤ Q2 ≤ 10.16 GeV2

  31. Spin sumrulefor the nucleon

  32. First moments of Δs(x), ΔG(x) and ΔΣ(x) at Q2 = 4 GeV2  Sz = ½ =½DS(Q2) + DG (Q2) + Lq (Q2) + Lg(Q2) = -0.27 (0.36) +/- 0.43 (0.19) + Lq (Q2) + Lg(Q2) To be determined from forward extrapolations of generalized PDFs Due to the ambiguity of the gluon polarization the quark-gluon spin contribution to the total spin of the nucleon is still not well determined.

  33. SUMMARY A combinedNLO QCD analysis of inclusive and semi inclusive world DIS data is presented In contrast to the other analysesthe target mass and higher twist corrections to the spin structure function g1are taken into account Due to SIDIS data The sea quark densities Δu and Δd are determined ¯ ¯ Changing in sign Δs(x),butdifferent fromthe DSSV one - less negative at x < 0.03 and less positive for x > 0.03 Δs(x)SIDIS differs essentially from a negative Δs(x)DIS obtained from all the QCD analyses of inclusive DIS data. This behavior strongly depends on the kaon FFs used. A model independent extraction of kaon FFs would help to solve this inconsistency. ¯ ¯ ¯ The SIDIS data, as well the direct measurements of ΔG/G,cannot help to distinguish between the positive and changing in sign solutions for ΔG(x) – the ambiguity of the form of ΔG(x) remains still large.

  34. Additional slides

  35. KTeV experiment Fermilab: PRL 87 (2001) 13201 b-decay SU(3)fprediction for the form factor ratio g1/f1 Experimental result A good agreement with theexact SU(3)f symmetry ! SU(3) breaking is at most of order 20% From exp. uncertainties NA48 experiment at CERN  g1/f1 = 1.20 +/- 0.05(PLB 645 (2007) 36)

  36. Which inclusive data to chose for QCD fits – A1or g1/F1 ? A1= g1/F1 - g2g2/F1 Kinematatic factor g2 = 4M2x2/Q2 cannot be neglected for most of the data sets (JLab, SLAC, HERMES) The QCD treatment of g2 is not well known The best manner to determine the polarized PDFs is to perform QCD fits to the data on g1/F1 The approximation A1th≈ (g1/F1)thused by some of the groups in the preasymptotic region is not reasonable ! N.B.

  37. There are essentially two methods to fit the data (accounting or not accounting for the HT corrections to g1) (g1)QCD = (g1)LT + (g1)HT+TMC(F1)QCD = (F1)LT + (F1)HT+TMC LT(LO, NLO)1/ln(Q2/L2) HT L2/Q2, TMC  M2/Q2 PDFs GRSV, DSSV, LSS OK in pure DIS region where HT can be ignored The two methods are equivalent in the pre-asymptotic region only if the (HT+TMC) terms cancel in the ratio g1/F1 LSS 2x(F1)exp = (F2)exp(1 + γ2)/(1+ Rexp)

  38. Pre-asymptotic region Fits to g1/F1 data using for the ratio will lead to the sameresults if the condition is fulfilled If not (which is the case),the ignored HT terms in g1 and F1, according to the Ist method, will be absorbed into the extracted PDFs

  39. LSS’06 vs DSSV LSS DSSV In the DSSV analysis the TM and HT corrections are not taken into account for both g1 and F1 structure functions F1(x, Q2)LT/NLO was calculated using the NLO MRST’02 PDFs DSSV: A first NLO global analysis of DIS, SIDIS and RHIC polarized pp scattering data

  40. As expected, the curves corresponding to g1tot(LSS)/F1(exp) and g1LT(DSSV)/F1LT(MRST) practically coincide (an exception for x > 0.2 !) although different expressions for g1 and F1were used in thefit The difference between F1(exp) and F1(MRST)LT is a measure of the size of TMC and HT corrections which cannot be ignored in the pre-asymptotic region proton Surprisingly g1LT(LSS) and g1LT(DSSV) coincide for x > 0.1 although the HT+TMC, taken into account in LSS and ignored in the DSSV analysis, do NOTcancel in the ratio g1/F1 in the pre-asymptotic region PUZZLE ???

  41. In the DSSV fit, a factor is introduced for the data in the pre-asymptotic region (CLAS, JLab/Hall A and SLAC/E143). There is NO rational explanation for a such correction !! Except for the fact that it is impossible toachieve a good description of these data, especially for the CLAS one, without this correction. ??? It turns out that accidentally more or less (4-18%) accounts for the TM and HT corrections to g1 and F1 in the ratio g1/F1ONLY for x > 0.1

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