Presentation on theme: "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."— Presentation transcript:
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
Axial vector current matrix elements: Scalar density matrix elements: 5 sQMexp 5/31.26 10.6 1/30.23 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/9709293
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 = 0.235 ± 0.026 ( Significantly lower than 1/3 ! )
9 Uncertain extrapolation for 0.0 < x < 0.004 Charge symmetry violation in the proton 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 !
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 ) 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)
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, 054029 arXiv:hep-ph/9706252
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 2. 31 W.C. Chang and J.C. Peng, arXiv: 1102.5631
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: 1105.2381
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: 1105.2381,1102.5631
P(uu)P(dd)P(ss)P(cc)Reference 0.1980.1480.0930.011Bag model; Donoghue and Golowich, PRD15, 3421 (1977) 0.003Light-cone 5q model; Hoffmann and Moore, ZPC 20, 71 (1983) 0.250 0.0500.009Meson cloud model; Navarra et al., PRD 54, 842 (1996) 0.10 - 0.15 Constituent 5q model; Riska and Zou, PLB 636, 265 (2006) 0.1220.2400.024Light-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 1996-1997 1 H, 2 H, and nuclear targets 800 GeV proton beam Fermilab E906/SeaQuest Data taking planned in 2010 1 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
J. Mans :: CMS EWK Measurements 42 20 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