Download ppt - NSTAR 2007Roelof Bijker, ICN-UNAM1 Flavor Asymmetry of the Nucleon Sea in an Unquenched Quark Model Introduction Degrees of freedom Unquenched quark model

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)


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, …


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• …


N(1520) Resonance

Collective U(7) PRC 54, 1935 (1996)hCQM JPG 24, 753 (1998)


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


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


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)


Flavor AsymmetryGottfried sum rule


Extend the CQM to include the effects of quark-antiquark pairs

in a general and consistent way


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)


Diagrams


Geiger & Isgur, PRD 55, 299 (1997)


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


Flux-tube Breaking Model


Closure Limit

Proton Spin


Closure Limit


Flavor Asymmetry

• Flavor asymmetry of the nucleon sea• Gottfried sum rule

with

Flavor symmetric sea


Results

Pauli blocking (Field & Feynman, 1977) too smallPion dressing of the nucleon (Thomas et al., 1983)Meson cloud models

Flavor asymmetryNonperturbative QCD


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


Spin of the Proton

• Unquenched quark model• Creation of quark-antiquark pairs

• Proton spin

• Present model:


Preliminary Results

HERMES Collaboration, hep-ex/0609039


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



Octet Baryons


Decuplet Baryons