An Unquenched Quark Model of Baryons Introduction Degrees
An Unquenched Quark Model of Baryons • • 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) DPy. C 2007 Roelof Bijker, ICN-UNAM 1
Baryons • Baryons ) QCD qqq decuplet & octet (Gell-Mann, Zweig) Spin-statistics paradox in ++ ) color (Greenberg, Han & Nambu) Valence quark model (Dalitz, Becchi & Morpurgo, Isgur & Karl, …) • QCD ) VNN ? • Baryons ) VNN ? DPy. C 2007 Roelof Bijker, ICN-UNAM 2
Nuclear Physics DPy. C 2007 Roelof Bijker, ICN-UNAM 3
Constituent Quark Models • Baryons: qqq configurations • OGE, chiral, collective U(7), h. CQM, 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, … DPy. C 2007 Roelof Bijker, ICN-UNAM 4
Open Problems • Relative mass of N(1440) and N(1535) in hoqm OK in collective U(7) and h. CQM • 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 Q 2 behavior of helicity amplitudes • … DPy. C 2007 Roelof Bijker, ICN-UNAM 5
Baryon Masses Lambda Nucleon Bijker, Iachello & Leviatan, AP 284, 89 (2000) DPy. C 2007 Roelof Bijker, ICN-UNAM 6
Open Problems • Relative mass of N(1440) and N(1535) in hoqm OK in collective U(7) and h. CQM • 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 Q 2 behavior of helicity amplitudes • … DPy. C 2007 Roelof Bijker, ICN-UNAM 7
N(1520) Resonance Collective U(7) PRC 54, 1935 (1996) h. CQM JPG 24, 753 (1998) DPy. C 2007 Roelof Bijker, ICN-UNAM 8
Relative mass of N(1440) and N(1535) Low Q 2 behavior of helicity amplitudes EXOTIC (non q 3) DEGREES OF FREEDOM Large decay widths of N(1535), (1750) and (1670) DPy. C 2007 Spin-orbit splitting of (1405) and (1520) Roelof Bijker, ICN-UNAM Photocouplings of (1232) 9
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 DPy. C 2007 Roelof Bijker, ICN-UNAM 10
Strange Form Factors PV electron scattering The strange (anti)quarks come uniquely from the sea: there is no contamination from up or down valence quarks DPy. C 2007 HAPPEX, PRL 98, 032301 (2007) Roelof Bijker, ICN-UNAM 11
Strange EM Form Factors R. Bijker - JPG 32, L 49 (2006) ar. Xiv: nucl-th/0607058 DPy. C 2007 Roelof Bijker, ICN-UNAM 12
1980’s Proton Spin Crisis Naive parton model 3 valence quarks 1990’s QCD: contributions from sea quarks and gluons 2000’s . . and orbital angular momentum HERMES, hep-ex/0609039 DPy. C 2007 Roelof Bijker, ICN-UNAM 13
Flavor Asymmetry Gottfried sum rule DPy. C 2007 Roelof Bijker, ICN-UNAM 14
Extend the CQM to include the effects of quark-antiquark pairs in a general and consistent way DPy. C 2007 Roelof Bijker, ICN-UNAM 15
Strong QCD • Analogy with diatomic molecules • Starting point: Valence quarks + Flux tubes • Large NC limit of QCD • Quenched lattice QCD • Heavy quark limit • Flux-tube breaking: quark-antiquark pairs DPy. C 2007 Roelof Bijker, ICN-UNAM 16
Unquenched Quark Model Strange quark-antiquark pairs in the proton with h. o. wave functions Geiger & Isgur, PRD 55, 299 (1997) • Pair-creation operator with 3 P 0 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: hadron spectroscopy and OZI rule DPy. C 2007 Roelof Bijker, ICN-UNAM 17
Flux Tubes Derek Leinweber (Adelaide) DPy. C 2007 Roelof Bijker, ICN-UNAM 18
Diagrams DPy. C 2007 Roelof Bijker, ICN-UNAM 19
Geiger & Isgur, PRD 55, 299 (1997) DPy. C 2007 Roelof Bijker, ICN-UNAM 20
Extensions • Valence quarks 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/0701227 Bijker & Santopinto, nucl-th/0703053 DPy. C 2007 Roelof Bijker, ICN-UNAM 21
Flux-tube Breaking Model DPy. C 2007 Roelof Bijker, ICN-UNAM 22
Closure Limit Proton Spin DPy. C 2007 Roelof Bijker, ICN-UNAM 23
Octet Baryons DPy. C 2007 Roelof Bijker, ICN-UNAM 24
Decuplet Baryons DPy. C 2007 Roelof Bijker, ICN-UNAM 25
Closure Limit DPy. C 2007 Roelof Bijker, ICN-UNAM 26
Proton Spin Composition 1980’s Naive parton model 3 valence quarks 1990’s 2000’s QCD: contributions from sea quarks and gluons . . and orbital angular momentum HERMES, hep-ex/0609039 DPy. C 2007 Roelof Bijker, ICN-UNAM 27
Spin of the Proton • Unquenched quark model • Creation of quark-antiquark pairs • Proton spin • Present model: DPy. C 2007 Roelof Bijker, ICN-UNAM 28
Preliminary Results HERMES Collaboration, hep-ex/0609039 DPy. C 2007 Roelof Bijker, ICN-UNAM 29
Flavor Asymmetry • Flavor asymmetry of the nucleon sea • Gottfried sum rule Flavor symmetric sea with DPy. C 2007 Roelof Bijker, ICN-UNAM 30
Results Flavor asymmetry Nonperturbative QCD Pauli blocking (Field & Feynman, 1977) too small Pion dressing of the nucleon (Thomas et al. , 1983) Meson cloud models DPy. C 2007 Roelof Bijker, ICN-UNAM 31
Summary and Conclusions • • Unquenched quark model Higher Fock components (Riska, Diakonov, . . . ) Flavor content of physical observables Spin of the proton Flavor asymmetry of the nucleon sea First results very promising Future: Electromagnetic and axial form factors of the nucleon and its excited states DPy. C 2007 Roelof Bijker, ICN-UNAM 32
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Closure Limit DPy. C 2007 Roelof Bijker, ICN-UNAM 36
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