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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

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

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

- 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

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

Assume an isotopic quark-antiquark sea,

GSR is only sensitive to valance quarks.

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

SG = 0.235 ± 0.026

( Significantly lower than 1/3 ! )

- 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 !

Acceptance in Fixed-target Experiments

- Naïve Assumption:

- NMC (Gottfried Sum Rule)

- NA51 (Drell-Yan, 1994)

NA 51 Drell-Yan confirms

d(x) > u(x)

- Naïve Assumption:

- NMC (Gottfried Sum Rule)

- NA51 (Drell-Yan, 1994)

- E866/NuSea (Drell-Yan, 1998)

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

NuTeV, PRL 99, 192001 (2007)

x(s+s)

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

http://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/QCDvacuum/welcome.html

- Pauli blocking
- guu is more suppressed than gdd 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.

- 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.

- 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)

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.

- 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.

- 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

- 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.

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.

arXiv:hep-ph/9706252

ISR

Still No Conclusive Evidence…..

CTEQ Global Analysis

PRD 75, 054029

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):

- 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

- 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

- 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

- 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

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

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.

p+p at sqrt(s)=500 GeV

Yang, Peng, and Groe-Perdekam, Phys. Lett. B 680, 231 (2009)

Kensuke’s talk on Monday

Caveats

Very preliminary, not part of publication on the topic

Only muons (no electrons)

Uncertified systematic errors

J. Mans :: CMS EWK Measurements

- 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!