1. Our Understanding of Sea Quarks in the Nucleon
Weekly Journal Club for Medium Energy Physics at IPAS 2011/1/10
Our Understanding of Sea Quarks in the Nucleon
Wen-Chen Chang 章文箴
Institute of Physics, Academia Sinica

2. Inelastic Electron Scattering
Q2 :Four-momentum transfer
x : Bjorken variable (=Q2/2Mn)
n : Energy transfer
M : Nucleon mass
W : Final state hadronic mass

3. Structure Function F2(Q2,x)

4. Describing F2 behavior with partons Lots of partons at small x!
J. G. Contreras
CTEQ School 2005

5. Unpolarized Parton Distributions (CTEQ6)
u valence
sea (x 0.05)
gluon (x 0.05) d

6. What is Origin of Sea Quarks?
Extrinsic: the sea quarks solely originate from the splitting of gluons, emitted by valence quarks.

7. Is in the proton? =
Gottfried Sum Rule

8. Experimental Measurement of Gottfried Sum
New Muon Collaboration (NMC), Phys. Rev. D50 (1994) R1
SG = ± 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: A laboratory for sea quarks
xtarget
xbeam
Detector acceptance chooses xtarget and xbeam.
Fixed target  high xF = xbeam – xtarget
Beam nucleon: valence quarks at high-x.
Target nucleon: sea quarks at low/intermediate-x.
Measure ratio of DY process from hydrogen and deuterium:

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

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

13. Advantages of 120 GeV Main Injector
The (very successful) past:
Fermilab E866/NuSea
Data in
1H, 2H, and nuclear targets
800 GeV proton beam
The future:
Fermilab E906
Data taking planned in 2010
1H, 2H, 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

14. 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.

15. Deep-Inelastic Neutrino Scattering

16. Adler Sum Rule

17. Strange Quark and Anti-quark in the Nucleon
CCFR, Z. Phys. C 65, 189 (1995)

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

19. Semi-inclusive DIS

20. Strange Quarks from Charged-Kaon DIS Production
HERMES, Phys. Lett. B 666, 446 (2008)

21. Asymmetry of W Production and Flavor Asymmetry of Nucleon Sea
p+p at sqrt(s)=14 TeV
Yang, Peng, and Groe-Perdekam,
Phys. Lett. B 680, 231 (2009)

22. CMS Measurement of W Asymmetry at sqrt(s)=7 TeV
Measured W-asymmetry is consistent with the prediction from the PDF with a flavor asymmetry of sea quarks.
CMS, arXiv:

23. Origin of u(x)d(x): Valence quark 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)
Bag model calculation (Signal, Thomas, Schreiber)
Chiral quark-soliton model (Diakonov, Pobylitsa, Polyakov)
Instanton model (Dorokhov, Kochelev)
Statistical model (Bourrely, Buccella, Soffer; Bhalerao)
Balance model (Zhang, Ma)
The valence quarks affect the Dirac vacuum and the quark-antiquark sea.

24. Balance Model
A physical hadron state is expanded by a complete set of quark-gluon Fock states
The parton numbers of quarks and gluons in the proton are
Splitting and recombination
qqg
gqqbar

25. Balance Model (Phys. Lett. B 523 (2001) 260-264)
More u valence quark in the proton leads to more recombination and thus less ubar.
Inclusion of gqqbar does not affect the result of flavor asymmetry.

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

27. Meson Cloud Model
Virtual  is emitted by the proton and the intermediate state is  + baryons.

28. Chiral Quark Model
Virtual  is emitted by the constituent quark.

29. Proton Structure: Remove perturbative sea
There is a gluon splitting component which is symmetric
Symmetric sea via pair production from gluons subtracts off
No Gluon contribution at 1st order in s
Nonperturbative models are motivated by the observed difference
A proton with 3 valence quarks plus glue cannot be right at any scale!!
Greater deviation at large-x
Paul E. Reimer, Physics Division, Argonne National Laboratory
24 February 2010

30. Intrinsic Sea Quark?
Brodsky et al. (1980) proposed an “intrinsic” (long time-scale) charm component in the proton (PLB 93,451; PRD 23, 2745).
A decomposition of |uudccbar> Fock state for proton. The intrinsic charm component is distributed at relatively large x region and could explain the large cross-section for charm production at large xf in hadron collisions.
Jen-Chieh and I are considering to extend this 5q model to describe the non-singlet distributions of (dbar-ubar) and (ubar+dbar-s-sbar), which are independent of the contributions from “extrinsic” sea quarks.

31. Sea quarks in

32. Sea quarks in

33. Sea quarks in

34. Sea quarks in
The light quark distribution in 5q configuration, which is assumed to be intrinsic,
is consistent with the non-singlet distribution of (dbar-ubar).

35. Sea quarks in

36. W production at the LHC is sensitive to the gluon distribution function.
Tevatron: W production is dominated by a LO process with two valence quarks.
LHC: The LO contribution must involve a sea quark; and the NLO contribution from a gluon is significant.

37. ?

38. Interesting Topics Missing in This Talk
Transverse spin structure of sea quarks.
Transverse momentum distribution of sea quarks.
The correlation of these two properties.
Flavor asymmetry of these two properties.
Interpretations from the Lattice QCD.

39. Conclusion
From DIS, DY and SIDIS processes, the structure of sea quarks in the nucleon are explored.
A large asymmetry between dbar and ubar was found at intermediate-x regions. The origin can be interpreted under the meson cloud model, chiral soliton model, intrinsic 5q model and etc. The intrinsic non-perturbative effect rather than extrinsic perturbative gluon-splitting seems more likely to be the cause.
No large asymmetry was observed for s and sbar.
The E906/FNAL and LHC experiments are expected to extend the measurement of sea quarks to the high-x regions where the existing uncertainties are large.
Precise understanding of sea quark distribution is important for the search of BSM in LHC.

40. References
C. Grosso-Pilcher and M. J. Shochet, Annu. Rev. Nucl. Part. Sci. 36 (1986) 1.
S. Kumano, Physics Reports 303 (1998) 183.
J.M. Conrad and M.H. Shaevitz, Rev. Mod. Phys. 70 (1998) 1341.
P.L. McGaughey, J.M. Moss and J.C. Peng, Annu. Rev. Nucl. Part. Sci. 49 (1999) 217.
G.T. Garvey and J.C. Peng, Prog. in Part. And Nucl. Phys. 47 (2001) 203.
J.C. Peng, Eur. Phys. J. A 18 (2003) 395.
J.T. Londergan, J.C. Peng, and A.W. Thomas, Rev. Mod. Phys. 82 (2010) 2009.