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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. Outline.

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

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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


  • 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

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

Sum Rules

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

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

Sum Rules

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

Gottfried Sum Rule

Assume an isotopic quark-antiquark sea,

GSR is only sensitive to valance quarks.

Measurement of Gottfried Sum

New Muon Collaboration (NMC), Phys. Rev. D50 (1994) R1

SG = 0.235 ± 0.026

( Significantly lower than 1/3 ! )

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 !

Drell-Yan Process

Acceptance in Fixed-target Experiments

Light Antiquark Flavor Asymmetry

  • Naïve Assumption:

  • NMC (Gottfried Sum Rule)

  • NA51 (Drell-Yan, 1994)

NA 51 Drell-Yan confirms

d(x) > u(x)

Light Antiquark Flavor Asymmetry

  • Naïve Assumption:

  • NMC (Gottfried Sum Rule)

  • NA51 (Drell-Yan, 1994)

  • E866/NuSea (Drell-Yan, 1998)

Deep-Inelastic Neutrino Scattering

Strange Quark in the Nucleon

CCFR, Z. Phys. C 65, 189 (1995)

Strange Quark and Antiquark in the Nucleon

NuTeV, PRL 99, 192001 (2007)

Strange Quarks from SI Charged-Kaon DIS Production


HERMES, Phys. Lett. B 666, 446 (2008)

HERMES vs. CCFR and CT10

Nontrivial QCD VacuumAnimation of the Action Density in 4 Dimensions


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.

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.


The pion cloud is a source of antiquarks in the protons and it lead to d>u.

  • 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)

Spin and Flavor are Connected

J.C. Peng, Eur. Phys. J. A 18, 395–399 (2003)

  • HERMES (PRD71, 012003 (2005))

  • COMPASS (NPB 198, 116, (2010))

  • DSSV2008 (PRL 101, 072001 (2008))

Light quark sea helicity densities

are flavor symmetric.

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.

: 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

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.

“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.

Experimental Evidences of IC



Still No Conclusive Evidence…..

CTEQ Global Analysis

PRD 75, 054029

“Intrinsic” Sea 5q Component

“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):

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

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

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

Comparison of 5q Probabilities

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

Main Injector 120 GeV

Tevatron 800 GeV

FNAL E906/SeaQuest Experiment


  • 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

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.

Longitudinal and Transverse View of E906 Experimental Area

Run Schedule

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)

Kensuke’s talk on Monday

20 GeV PT Results


Very preliminary, not part of publication on the topic

Only muons (no electrons)

Uncertified systematic errors

J. Mans :: CMS EWK Measurements

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.


  • 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.


  • 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.


  • 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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