nstar 2007roelof bijker, icn-unam1 flavor asymmetry of the nucleon sea in an unquenched quark model...
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NSTAR 2007 Roelof Bijker, ICN-UNAM 1
Flavor Asymmetry of the Nucleon Sea in an Unquenched Quark
Model• Introduction• Degrees of freedom• Unquenched quark model• Closure limit; Spin of the proton; Flavor asymmetry of the nucleon
sea• Summary and conclusions
Roelof Bijker (ICN-UNAM)Elena Santopinto (Genova)
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Constituent Quark Models
• Baryons: qqq configurations• OGE, chiral, collective U(7), hCQM, bags,
soliton, Skyrme, NJL, …• Nonrelativistic and relativized• Ground state baryons: octet & decuplet, ratio of
nucleon magnetic moments
• Photocouplings, helicity amplitudes, form factors, structure functions, strong decays, …
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Open Problems
• Relative mass of N(1440) and N(1535) in hoqm OK in collective U(7) and hCQM
• Spin-orbit splitting of (1405) and (1520)• Missing resonances• Photocouplings of (1232): ratio OK, but
absolute values not• Large decay widths of N(1535), (1750),
(1670)• Low Q2 behavior of helicity amplitudes• …
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N(1520) Resonance
Collective U(7) PRC 54, 1935 (1996)hCQM JPG 24, 753 (1998)
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EXOTIC (non q3) DEGREES
OF FREEDOM
Relative mass of N(1440) andN(1535)
Spin-orbit splitting of 1405) and 1520)
Photocouplingsof (1232)
Large decay widths of N(1535), (1750) and (1670)
Low Q2 behaviorof helicity amplitudes
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Exotic Degrees of Freedom
• Quark-antiquark pairs, e.g. pentaquark, meson cloud (Thomas, Speth, Kaiser, Weise, Oset, Brodsky, Ma, Isgur, …)
• Higher Fock components (Riska, Diakonov, …)
• Strange form factors of the proton• Proton spin crisis• Flavor asymmetry of nucleon sea
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Strange Form FactorsPV electron scattering
The strange (anti)quarks come uniquely from the sea: there is no contamination from up or down valence quarks
HAPPEX, PRL 98, 032301 (2007)
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Flavor AsymmetryGottfried sum rule
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Extend the CQM to include the effects of quark-antiquark pairs
in a general and consistent way
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Unquenched Quark Model
• Pair-creation operator with 3P0 quantum numbers of vacuum and geometry of flux-tube breaking model
• Sum over a large tower of intermediate states to preserve the phenomenological success of CQM’s
Strange quark-antiquark pairs in the proton with h.o. wave functions
Geiger & Isgur, PRD 55, 299 (1997)
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Diagrams
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Geiger & Isgur, PRD 55, 299 (1997)
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Extensions
• Valence plus glue dominance with quark-antiquark corrections
• To any initial baryon or baryon resonance
• To any flavor of the quark-antiquark pair
• To any model of baryons and mesons
Santopinto & Bijker, nucl-th/0701227Bijker & Santopinto, nucl-th/0703053
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Flux-tube Breaking Model
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Closure Limit
Proton Spin
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Closure Limit
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Flavor Asymmetry
• Flavor asymmetry of the nucleon sea• Gottfried sum rule
with
Flavor symmetric sea
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Results
Pauli blocking (Field & Feynman, 1977) too smallPion dressing of the nucleon (Thomas et al., 1983)Meson cloud models
Flavor asymmetryNonperturbative QCD
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Proton Spin Crisis1980’s 1990’s 2000’s
Naive parton model3 valence quarks
QCD: contributions from sea quarks and gluons
.. and orbital angularmomentum
HERMES, hep-ex/0609039
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Spin of the Proton
• Unquenched quark model• Creation of quark-antiquark pairs
• Proton spin
• Present model:
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Preliminary Results
HERMES Collaboration, hep-ex/0609039
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Summary and Conclusions
• Unquenched quark model:• Higher Fock components (Riska,
Diakonov, ...)• Flavor content of physical observables• Flavor asymmetry of the nucleon sea• Spin of the proton• First results very promising• Future: Electromagnetic and axial form
factors of the nucleon and its excited states
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Octet Baryons
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Decuplet Baryons