Wen-Chen Chang Institute of Physics, Academia Sinica 8th Circum-Pan-Pacific Symposium on High Energy Spin Physics June 20-24, 2011 in Cairns, QLD, Australia

 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 2

3 Q 2 : Four-momentum transfer x : Bjorken variable (=Q 2 /2 M ) : Energy transfer M : Nucleon mass W : Final state hadronic mass Scaling Valence quarks Quark-antiquark pairs

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

 Axial vector current matrix elements:  Scalar density matrix elements: 5 sQMexp 5/ /30.23 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/

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

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

8 New Muon Collaboration (NMC), Phys. Rev. D50 (1994) R1 S G = ± ( Significantly lower than 1/3 ! )

9 Uncertain extrapolation for 0.0 < x < Charge symmetry violation in the proton Uncertain extrapolation for 0.0 < x < Charge symmetry violation in the proton Need independent methods to check the asymmetry, and to measure its x-dependence !

10 Acceptance in Fixed-target Experiments

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

Naïve Assumption: 12 NA51 (Drell-Yan, 1994) E866/NuSea (Drell-Yan, 1998) NMC (Gottfried Sum Rule)

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

15 NuTeV, PRL 99, (2007)

16 HERMES, Phys. Lett. B 666, 446 (2008) x(s+s) 

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 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 )  Instanton model ( Dorokhov, Kochelev 1993 )  Statistical model ( Bourrely et al. 1995; Bhalerao 1996 )  Balance model ( Zhang, Ma 2001 ) 19 The valence quarks affect the gluon splitting.    

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

 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) 21

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

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

ExtrinsicIntrinsic Gluon splitting in leading twistGluon fusion & light quark scattering (higher-twist) Perturbative radiationNon-perturbative dynamics CP invariantPossible CP non-invariant Fast fluctuationWith a longer lifetime Of small xOf large x (valence like) Strong Q 2 dependentSmall Q 2 dependent 24  It is generally agreed that the observed flavor asymmetry mostly resulted from the intrinsic sea quarks.  For further investigation, it will be good to separate their contributions.

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

 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 P NS to the experimental Q 2 scale, make a comparison with the data. 26

 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 comoving light partons, more valence-like. 27 In the 1980’s Brodsky et al. (BHPS) suggested the existence of “intrinsic” charm (PLB 93,451; PRD 23, 2745).

ISR 28 Still No Conclusive Evidence….. CTEQ Global Analysis PRD 75, arXiv:hep-ph/

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30 In the limit of a large mass for quark Q (charm): m c =1.5, m s =0.5, m u, m d =0.3 GeV is obtained numerically.

 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 Q 2 =54 GeV W.C. Chang and J.C. Peng, arXiv:    

 The x(s(x)+s(x)) are from HERMES kaon SIDIS data at =2.5 GeV 2.  Assume data at x>0.1 are originated from the intrinsic |uudss> 5- quark state. 32    W.C. Chang and J.C. Peng, arXiv:

 The d(x)+u(x) from CTEQ 6.6.  The s(x)+s(x) from HERMES kaon SIDIS data at =2.5 GeV 2.  Assume  Probabilities of 5-q states associated with the light sea quarks are extracted. 33 W.C. Chang and J.C. Peng, arXiv: ,      

P(uu)P(dd)P(ss)P(cc)Reference Bag model; Donoghue and Golowich, PRD15, 3421 (1977) 0.003Light-cone 5q model; Hoffmann and Moore, ZPC 20, 71 (1983) Meson cloud model; Navarra et al., PRD 54, 842 (1996) Constituent 5q model; Riska and Zou, PLB 636, 265 (2006) Light-cone 5q model; Chang and Peng, this work (2011) 34    

 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 35

36 Fermilab E866/NuSea Data in H, 2 H, and nuclear targets 800 GeV proton beam Fermilab E906/SeaQuest Data taking planned in H, 2 H, and nuclear targets 120 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 Tevatron 800 GeV Main Injector 120 GeV

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. 37  

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

41 Kensuke’s talk on Monday

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

 COMPASS Polarized  -induced DY experiment at CERN: spin structure of sea quark.  MINER ν A at 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. 43  

 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. 44   

 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. 45      

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