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Drell-Yan Process and flavor asymmetry of u and d sea quarks

Drell-Yan Process and flavor asymmetry of u and d sea quarks. Ting-Hua Chang. Ling-Tung University, Taiwan. • Parton distribution functions in nucleons Drell-Yan cross section and sea quark distributions Drell-Yan angular distributions and Boer-Mulders functions

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Drell-Yan Process and flavor asymmetry of u and d sea quarks

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  1. Drell-Yan Process and flavor asymmetry of u and d sea quarks Ting-Hua Chang Ling-Tung University, Taiwan • Parton distribution functions in nucleons • Drell-Yan cross section and sea quark distributions • Drell-Yan angular distributions and Boer-Mulders functions • Future prospects for Drell-Yan experiments (FNAL E906) The 7th Circum-Pan-Pacific Symposium on High Energy Spin Physics, Yamagata, Sept 15 – 18, 2009 Outline

  2. Introduction

  3. Parton distributions • Scaling behavior in DIS indicated point-like objects (partons) in nucleons. • Non-vanishing F2 when x ~0 gave the hints of existence of sea quarks. • Now we know partons in nucleons are gluons, sea quarks, and valence quarks.

  4. u valence gluon (x 0.05) d sea (x 0.05) Unpolarized Parton Distributions (CTEQ6) • After 40 years DIS experiments, unpolarized structure of the nucleon reasonably well understood. • High x valence quark dominating

  5. What is the distribution of sea quarks? In the nucleon: • Sea and gluons are important: • 98% of mass; 60% of momentum at Q2 = 2 GeV2 • Not just three valence quarks and QCD. • Significant part of LHC beam. • What are the origins of the sea? CTEQ6m In nuclei: • The nucleus is not just a sum of protons and neutrons • EMC effect • PDFs in nuclei are modified • Binding via virtual mesons could affect antiquarks distributions

  6. Modification of Parton Distributions in Nuclei EMC effect observed in DIS F2Fe / F2D X F2 contains contributions from quarks and antiquarks How are the antiquark distributions modified in nuclei?

  7. Is in the proton? = Test of the Gottfried Sum Rule New Muon Collaboration (NMC), Phys. Rev. D50 (1994) R1 SG = 0.235 ± 0.026 ( Significantly lower than 1/3 ! )

  8. Explanations for the NMC result: • Uncertain extrapolation for 0.0 < x < 0.004 • Charge symmetry violation • in the proton Need independent methods to check the asymmetry, and to measure its x-dependence !

  9. Light Antiquark Flavor Asymmetry: Brief History • Naïve Assumption: • NMC (Gottfried Sum Rule) • NA51 (Drell-Yan) NA 51 Drell-Yan confirms d-bar(x) > u-bar(x)

  10. Light Antiquark Flavor Asymmetry: Brief History • Naïve Assumption: • NMC (Gottfried Sum Rule) • NA51 (Drell-Yan) • E866/NuSea (Drell-Yan)

  11. Drell-Yan Process

  12. Complimentality between DIS and Drell-Yan Drell-Yan DIS Both DIS and Drell-Yan cross sections are well described by NLO calculations

  13. xtarget xbeam Drell-Yan scattering: A laboratory for sea quarks Detector acceptance chooses xtarget and xbeam. • Fixed target : high xF = xbeam – xtarget • Valence Beam quarks at high-x. • Sea target quarks at low/intermediate-x.

  14. The Drell-Yan Process: • The x-dependence of can be directly related to cross section ratio in leading order with XF>0 by assuming • Charge symmetry • Dominance of u-ubar annihilation term

  15. Fermilab E866 Measurements

  16. Drell-Yan Cross Section Ratio and d-bar/u-bar

  17. Extraction of HERMES: Semi-Inclusive DIS

  18. The valence quarks affect the Dirac vacuum and the quark-antiquark sea

  19. Models for asymmetry Meson Cloud Models Chiral-Quark Soliton Model Instantons • Quark degrees of freedom in a pion mean-field • nucleon = chiral soliton • expand in 1/Nc These models also have implications on • asymmetry between and • flavor structure of the polarized sea

  20. Spin and flavor are closely connected • Meson Cloud Model • Pauli Blocking Model A spin-up valence quark would inhibit the probability of generating a spin-down antiquark • Instanton Model • Chiral-Quark Soliton Model • Statistical Model

  21. D-Y Angular Distributions

  22. Drell-Yan decay angular distributions Θ and Φ are the decay polar and azimuthal angles of the μ+ in the dilepton rest-frame Collins-Soper frame A general expression for Drell-Yan decay angular distributions: In general :

  23. Decay Angular Distribution of Drell-Yan Data from Fermilab E772 McGaughey, Moss, Peng; Annu. Rev. Nucl. Part. Sci. 49 (1999) 217 (hep/ph-9905409)

  24. Drell-Yan decay angular distributions Θ and Φ are the decay polar and azimuthal angles of the μ+ in the dilepton rest-frame Collins-Soper frame A general expression for Drell-Yan decay angular distributions:

  25. Decay angular distributions in pion-induced Drell-Yan NA10 π- +W Z. Phys. 37 (1988) 545 Dashed curves are from pQCD calculations

  26. Nuclear Effect? NA10 Z. Phys. C37, 545 (1988) Open: Deuterium Solid: Tungsten Nuclear effect seems not to be the dominant contribution.

  27. Decay angular distributions in pion-induced Drell-Yan E615 Data 252 GeV π- + W Phys. Rev. D 39 (1989) 92

  28. Decay angular distributions in pion-induced Drell-Yan Is the Lam-Tung relation violated? 140 GeV/c 194 GeV/c 286 GeV/c Data from NA10 (Z. Phys. 37 (1988) 545)

  29. QCD vacuum effects • Brandenburg, Nachtmann & Mirkes, Z. Phy. C60,697(1993) • Nontrivial QCD vacuum may lead to correlation between the transverse spins of the quark (in nucleon) and the antiquark (in pion). • The helicity flip in the instanton-induced contribution may lead to nontrivial vacuum. • Boer,Brandenburg,Nachtmann&Utermann, EPC40,55(2005). 0=0.17, mT=1.5

  30. Boer-Mulders function h1┴ 1=0.47, MC=2.3 GeV Boer, PRD 60 (1999) 014012 ν>0 implies valence BM functions for pion and nucleon have same signs

  31. Motivation for measuring decay angular distributions in p+p and p+d Drell-Yan • No proton-induced Drell-Yan azimuthal decay angular distribution data • Provide constraints on models explaining the pion-induced Drell-Yan data. (h1┴ is expected to be small for sea quarks. The vacuum effects should be similar for p+N and π+N) • Test of the Lam-Tung relation in proton-induced Drell-Yan • Compare the decay angular distribution of p+p versus p+d

  32. Decay angular distributions for p+d Drell-Yan at 800 GeV/c p+d at 800 GeV/c No significant azimuthal asymmetry in p+d Drell-Yan!

  33. Azimuthal cos2Φ Distribution in p+d Drell-Yan Lingyan Zhu et al., PRL 99 (2007) 082301 With Boer-Mulders function h1┴: ν(π-Wµ+µ-X)~ [valence h1┴(π)] * [valence h1┴(p)] ν(pdµ+µ-X)~ [valence h1┴(p)] * [sea h1┴(p)] ν>0 suggests same sign for the valence and sea BM functions

  34. What does this mean? • These results suggest that the Boer-Mulders functions h1┴ for sea quarks are significantly smaller than for valence quarks. • These results also suggest that the non-trivial vacuum correlation between the sea-quark transverse spin (in one hadron) and the valence-quark transverse spin (in another hadron) is small.

  35. Extraction of Boer-Mulders functions from p+d Drell-Yan (B. Zhang, Z. Lu, B-Q. Ma and I. Schmidt, PRD 77(2008) 054011)

  36. Extraction of Boer-Mulders functions from p+d Drell-Yan (B. Zhang, Z. Lu, B-Q. Ma and I. Schmidt, PRD 77(2008) 054011) • It seems unlikely that p+d data alone can determine the flavor structure of BM functions! • A combined analysis of p+p and p+d, together with the π+p and π+d Drell-Yan cos(2Ф) data can lead to extraction of valence and sea Boer-Mulders functions.

  37. Sea-quark Boer-Mulders Functions 1) Use quark-spectator-antiquark model to calculate pion B-M functions. Pion-induced Drell-Yan data are well reproduced. (Lu and Ma, hep-ph/0504184) 2) Use pion-cloud model convoluted with the pion B-M function to calculate sea-quark B-M for proton. (Lu, Ma, Schmidt, hep-ph/0701255)

  38. Results on the Azimuthal cos2Φ Distribution in p+p Drell-Yan (Zhu, L.Y. et al, PRL 102(2009):182001) • p+p is similar to p+d • A global fit to ALL pion and proton data is needed

  39. Angular Distribution in E866 p+p/p+d Drell-Yan PT (GeV/c) p+p and p+d Drell-Yan show similar angular distributions. Should be analysed togther for better constraints on BM.

  40. E906 & Future Drell-Yan Exp.

  41. Main Injector 120 GeV Tevatron 800 GeV Advantages of 120 GeV Main Injector The future: Fermilab E906 • Data taking planned in 2010 • 1H, 2H, and nuclear targets • 120 GeV proton Beam The (very successful) past: Fermilab E866/NuSea • Data in 1996-1997 • 1H, 2H, and nuclear targets • 800 GeV proton beam • Cross section scales as 1/s • 7x that of 800 GeV beam • Backgrounds, primarily from J/ decays scale as s • 7x Luminosity for same detector rate as 800 GeV beam 50x statistics!! Fixed Target Beam lines

  42. Extracting d-bar/-ubar From Drell-Yan Scattering Ratio of Drell-Yan cross sections (in leading order—E866 data analysis confirmed in NLO) • Global NLO PDF fits which include E866 cross section ratios agree with E866 results • Fermilab E906/Drell-Yan will extend these measurements and reduce statistical uncertainty. • E906 expects systematic uncertainty to remain at approx. 1% in cross section ratio.

  43. Fermilab E906 dimuon experiment (Geesaman, Reimer et al., expected to run ~2010-2011) • BM functions can be measured at larger x

  44. Future prospect for Drell-Yan experiments • Fermilab p+p, p+d, p+A • Unpolarized beam and target • RHIC • Polarized p+p collision • COMPASS • π-p and π-d with polarized targets • FAIR • Polarized antiproton-proton collision • J-PARC • Possibly polarizied proton beam and target

  45. Transversity and TMD PDFs are also probed in Drell-Yan

  46. Outstanding questions to be addressed by future Drell-Yan experiments • Does Sivers function change sign between DIS and Drell-Yan? • Does Boer-Mulders function change sign between DIS and Drell-Yan? • Are all Boer-Mulders functions alike (proton versus pion Boer-Mulders functions) • Flavor dependence of TMD functions • Independent measurement of transversity with Drell-Yan

  47. END

  48. Alde et al (Fermilab E772) Phys. Rev. Lett. 64 2479 (1990) Structure of nucleonic matter: How do sea quark distributions differ in a nucleus? • EMC: Parton distributions of bound and free nucleons are different. • Antishadowing not seen in Drell-Yan—Valence only effect • Intermediate-xsea PDF’s absolute magnitude set by -DIS on iron. • Are nuclear effects the same for the sea as for valence? • Are nuclear effects with the weak interaction the same as electromagnetic? • What can the sea parton distributions tell us about the effects of nuclear binding?

  49. The Drell-Yan Process: The x-dependence of can be directly measured

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