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Flavor Asymmetry of the Nucleon Sea and the Connection with the Five-Quark Components

Explore the flavor asymmetry of sea quarks in the nucleon and its connection with five-quark components. Covering evidence, intrinsic sea quark models, ongoing/future experiments, and theoretical interpretations. Investigate Valence Quarks, Sum Rules, Strange Quark Content, and Light Antiquark Flavor Asymmetry in Deep-Inelastic Scattering. Examine the origin of u(x) ≠ d(x) through perturbative and non-perturbative QCD effects. Spin-Flavor connections and flavor non-singlet quantities with predictions from non-perturbative models.

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Flavor Asymmetry of the Nucleon Sea and the Connection with the Five-Quark Components

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  1. 8th Circum-Pan-Pacific Symposium on High Energy Spin Physics June 20-24, 2011 in Cairns, QLD, Australia Wen-Chen Chang Institute of Physics, Academia Sinica Flavor Asymmetry of the Nucleon Sea and the Connection with the Five-Quark Components

  2. Outline • Evidences for the Existence of Sea Quarks • Flavor Asymmetry of Sea Quarks • Theoretical Interpretations • Intrinsic Sea Quark & Light-cone 5q Model • Current & Future Experiments • Conclusion

  3. Deep Inelastic Scattering Q2 :Four-momentum transfer x : Bjorken variable (=Q2/2Mn) n : Energy transfer M : Nucleon mass W : Final state hadronic mass • Scaling • Valence quarks • Quark-antiquark pairs

  4. Sum Rules J.I. Friedman, Rev. Mod. Phys. Vol. 63, 615 (1991)

  5. Constituent Quark model • Axial vector current matrix elements: • Scalar density matrix elements: The simplest interpretation of these failures is that the sQM lacks a quark sea. Hence the number counts of the quark flavors does not come out correctly. - Ling-Fong Li and Ta-Pei Cheng, arXiV: hep-ph/9709293

  6. Sum Rules J.I. Friedman, Rev. Mod. Phys. Vol. 63, 615 (1991)

  7. Gottfried Sum Rule Assume an isotopic quark-antiquark sea, GSR is only sensitive to valance quarks.

  8. Measurement of Gottfried Sum New Muon Collaboration (NMC), Phys. Rev. D50 (1994) R1 SG = 0.235 ± 0.026 ( Significantly lower than 1/3 ! )

  9. 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 !

  10. Drell-Yan Process Acceptance in Fixed-target Experiments

  11. Light Antiquark Flavor Asymmetry • Naïve Assumption: • NMC (Gottfried Sum Rule) • NA51 (Drell-Yan, 1994) NA 51 Drell-Yan confirms d(x) > u(x)  

  12. Light Antiquark Flavor Asymmetry • Naïve Assumption: • NMC (Gottfried Sum Rule) • NA51 (Drell-Yan, 1994) • E866/NuSea (Drell-Yan, 1998)

  13. Deep-Inelastic Neutrino Scattering

  14. Strange Quark in the Nucleon CCFR, Z. Phys. C 65, 189 (1995)

  15. Strange Quark and Antiquark in the Nucleon NuTeV, PRL 99, 192001 (2007)

  16. Strange Quarks from SI Charged-Kaon DIS Production  x(s+s) HERMES, Phys. Lett. B 666, 446 (2008)

  17. HERMES vs. CCFR and CT10

  18. Nontrivial QCD VacuumAnimation of the Action Density in 4 Dimensions http://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/QCDvacuum/welcome.html

  19.  Origin of u(x)d(x): Perturbative QCD effect? • Pauli blocking • guu is more suppressed than gdd in the proton since p=uud(Field and Feynman 1977) • pQCD calculation (Ross, Sachrajda 1979) • Bag model calculation (Signal, Thomas, Schreiber 1991) • Chiral quark-soliton model (Pobylitsa et al. 1999) • Instanton model (Dorokhov, Kochelev 1993) • Statistical model (Bourrely et al. 1995; Bhalerao 1996) • Balance model (Zhang, Ma 2001)   The valence quarks affect the gluon splitting.

  20.  Origin of u(x)d(x): Non-perturbative QCD effect? • Meson cloud in the nucleons (Thomas 1983, Kumano 1991): Sullivan process in DIS. • Chiral quark model(Eichten et al. 1992; Wakamatsu 1992): Goldstone bosons couple to valence quarks. n   The pion cloud is a source of antiquarks in the protons and it lead to d>u.

  21. Meson Cloud Model (Signal and Thomas, 1987) • Chiral Field (Burkardt and Warr , 1992) • Baryon-Meson Fluctuation (Brodsky and Ma , 1996) • Perturbative evolution (Catani et al., 2004)

  22. Spin and Flavor are Connected J.C. Peng, Eur. Phys. J. A 18, 395–399 (2003)

  23. HERMES (PRD71, 012003 (2005)) • COMPASS (NPB 198, 116, (2010)) • DSSV2008 (PRL 101, 072001 (2008)) Light quark sea helicity densities are flavor symmetric.

  24. Origin of Sea Quarks • It is generally agreed that the observed flavor asymmetry mostly resulted from theintrinsic sea quarks. • For further investigation, it will be good toseparate their contributions.

  25. : Flavor Non-singlet Quantity • is a flavor-non-singlet (FNS) quantity. • Extrinsic sea quarks vanish at 1st order in s . • Non-perturbative models are able to describe the trend. • Greater deviation is seen at large-x valence region. • No model predicts

  26. Intrinsic Sea & Flavor Non-singlet Variables • Select a non-perturbative model with a minimal set of parameters. • Construct the x distribution of flavor non-singlet quantities: , , at the initial scale. • After a QCD evolution with the splitting function PNS to the experimental Q2 scale, make a comparison with the data.

  27. “Intrinsic” Charm in Light-Cone 5q Model In the 1980’s Brodsky et al. (BHPS) suggested the existence of “intrinsic” charm (PLB 93,451; PRD 23, 2745). • Dominant Fock state configurations have the minimal invariant mass, i.e. the ones with equal-rapidity constituents. • The large charm mass gives the c quark a larger x than the other comovinglight partons, more valence-like.

  28. Experimental Evidences of IC arXiv:hep-ph/9706252 ISR Still No Conclusive Evidence….. CTEQ Global Analysis PRD 75, 054029

  29. “Intrinsic” Sea 5q Component

  30. “Intrinsic” Sea 5q Component mc=1.5, ms=0.5, mu, md=0.3 GeV is obtained numerically. In the limit of a large mass for quark Q (charm):

  31.  Data of d(x)-u(x) vs. Light-Cone 5-q Model • The shapes of the x distributions of d(x) and u(x) are the same in the 5-q model and thus their difference. • Need to evolve the 5-q model prediction from the initial scale  to the experimental scale at Q2=54 GeV2.   W.C. Chang and J.C. Peng, arXiv: 1102.5631

  32. Data of x(s(x)+s(x)) vs. Light-Cone 5-q Model  • The x(s(x)+s(x)) are from HERMES kaon SIDIS data at <Q2>=2.5 GeV2. • Assume data at x>0.1 are originated from the intrinsic |uudss> 5-quark state.  W.C. Chang and J.C. Peng, arXiv: 1105.2381

  33.   Data of x(d(x)+u(x)-s(x)-s(x)) vs. Light-Cone 5-q Model   • The d(x)+u(x) from CTEQ 6.6. • The s(x)+s(x) from HERMES kaon SIDIS data at <Q2>=2.5 GeV2. • Assume • Probabilities of 5-q states associated with the light sea quarks are extracted.  W.C. Chang and J.C. Peng, arXiv: 1105.2381,1102.5631

  34. Comparison of 5q Probabilities    

  35. The Light-Cone 5-q Model • It is surprising that many FNS quantities can be reasonably described by such a naïve model with very few parameters (mass of quarks and the initial scale). • For completeness, this model should be extended to take into account: • Anti-symmetric wave function • Chiral symmetry breaking effect • Spin structure • Higher configuration of Fock states

  36. Main Injector 120 GeV Tevatron 800 GeV FNAL E906/SeaQuest Experiment FermilabE906/SeaQuest • Data taking planned in 2010 • 1H, 2H, and nuclear targets • 120 GeV proton Beam 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

  37. d/u 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.

  38. Longitudinal and Transverse View of E906 Experimental Area

  39. Run Schedule

  40. Charged Asymmetry of W at RHIC p+p at sqrt(s)=500 GeV Yang, Peng, and Groe-Perdekam, Phys. Lett. B 680, 231 (2009)

  41. Kensuke’s talk on Monday

  42. 20 GeV PT Results Caveats Very preliminary, not part of publication on the topic Only muons (no electrons) Uncertified systematic errors J. Mans :: CMS EWK Measurements

  43. Future Experiments • COMPASS Polarized -induced DY experiment at CERN: spin structure of sea quark. • MINERνAat FNAL: x-dependence of nuclear effects for sea and valance quarks. • JLAB-12 GeV: transverse spatial distribution of partons. • (Polarized) DY experiment at J-PARC: d/u at very large-x region. • EIC at RHIC: sea quark distributions and their spin dependence.  

  44. Conclusion • Using DIS, Drell-Yan and SIDIS processes, the structure of sea quarks in the nucleon are explored. • A large asymmetry between d and u was found at intermediate-x regions. • No large asymmetry was observed between s and s.   

  45. Conclusion • The observed large flavor asymmetry mostly resulted from the non-perturbative effects. • The measured x distributions of (d-u), (s+s) and (u+d-s-s) could be reasonably described by the light-cone 5q model. The probabilities of the intrinsic 5q states of light sea quarks are extracted.      

  46. Conclusion • The sea quarks are connected with the non-perturbative feature of QCD. They could be the key to understand the confinement!

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