Post on 29-Mar-2019
V EMFN Roelof Bijker, ICN-UNAM 1
La Estructura del Nucleón• Introducción• La interacción electromagnética y débil• Los factores de forma del nucleón• El sabor del nucleón (cuarks u, d y s)• Dispersión de electrones• Modelos teóricos• Resultados
Roelof Bijker (ICN-UNAM)bijker@nucleares.unam.mx
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Primera Conferencia:- Introducción- Dispersión de Rutherford- Dispersión electrón-núcleo (e-A)- Dispersión electrón-protón (e-p) - Dispersión e-p con violación de paridad (PVES)
Segunda Conferencia:- Factores de forma- Modelo VMD- Los factores de forma electromagnéticos- La extrañeza del protón- Resumen y conclusiones
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Distance Scales
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La Física Nuclear
• La estructura nuclear (Hirsch, Velázquez)
• Reacciones nucleares (Aguilera, Rickards, Belmont, Chávez-Lomelí, Lizcano, Murillo)
• Aplicaciones (Andrade, Iturbe, Brandan, Cruz, Espinosa, Herrera, Policroniades)
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Aplicaciones
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El Futuro de la Física Nuclear
• El plasma de cuarks y gluones (CERN, RHIC) (Paic, Ayala, Sandoval)
• Haces radioactivos (TRIUMF, ORNL, MSU, GANIL, GSI, CERN, RIKEN) (Galindo)
• Nucleón, QCD (JLab, Mainz) (Bijker)• Neutrinos (Kamiokande, SNO) (Mondragón)• Astrofísica nuclear (Medina, Klapp)
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Nuclear Physics at the LimitsLimits of mass, stability, temperature, density,
energy, angular momentum, …
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Where the hell …?
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The Plum Pudding Model
J.J. Thomson (1856-1940)believed the electrons were embedded in a positively charged matrix-plum pudding (1898)
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The Structure of AtomsRutherford (1871-1937)showed that atoms contain a centralnucleus (1911)
Electrons orbit nucleuswith well-definedenergy and ill-definedpositions10-10 m
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Evidence for SubstructureMeasure size of struck objects(Rutherford 1911)
Substructure of protons and neutrons discovered using electrons as projectiles, MIT-SLAC collaboration (Friedman, Kendall, Taylor 1968)
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The Structure of Nuclei
Nucleus containsprotons with charge+e and unchargedneutrons
10-14
m
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Rutherford & Mott ScatteringNonrelativistic electron scatteringfrom a point-like nucleuswith Z protons and zero spin
Electron is a relativistic particle with spin 1/2
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Electron-Nucleus ScatteringFinite size of the nucleus
Form factor
Point-like charge distribution
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Differential Cross Section
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Uniform Charge Distribution
MinimaCharge radius
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Charge Distributions
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10-15 m
The Structure of NucleonsNeutrons and protons contain quarks (Friedman, Kendall, Taylor 1968)
Quark ModelProton: uudNeutron: udd
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<10-18 m
The Structure of Quarks?
There is no evidence for further structure
?
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Structure of the Nucleon• Anomalous magnetic moment (Stern, 1930’s)
• Radius (Hofstadter, 1950’s)
• Quark structure of the proton (Friedman, Kendall, Taylor, 1968)
• Spin crisis (1988)• Flavor asymmetry of the
nucleon sea (1991)• Strangeness (2000’s)
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Constituent Quark ModelsBaryons: qqq configurations
proton: uudneutron: udd
Ground state baryons: octet & decuplet
Ratio of nucleon magnetic moments
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Octet and Decuplet Baryons
Octet
Decuplet
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Quarks
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Electron-Proton ScatteringMott scattering from a point-like proton with spin 1/2
Finite size of the proton: Rosenbluth formula
Electric form factor
Magnetic form factor
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Rosenbluth SeparationResult: form factor scaling
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Polarization Transfer
Method: measure ratio of sideways (PT) tolongitudinal (PL) recoil polarization of proton
Result for form factor ratio
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Strangeness of the Proton
• Quark Model: proton = uud valence quarks• QCD : proton = uud valence + sea quarks• Sea of quark-antiquark pairs can be studied by
probing the strange quarks• Compares Quark Model to QCD
Kaufman, U. Mass.
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Strange Proton Form FactorNew data from parityviolating electron scattering
The strange (anti)quarkscome uniquely from thesea: there is no contamination from up ordown valence quarks
HAPPEX, PRL 98, 032301 (2007)
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Parity violatingParity conserving
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SAMPLE@MIT-BATESA4@MAMI-MainzHAPPEX@JlabG0@JLab
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Primera Conferencia:- Introducción- Dispersión de Rutherford- Dispersión electrón-núcleo (e-A)- Dispersión electrón-protón (e-p) - Dispersión e-p con violación de paridad (PVES)
Segunda Conferencia:- Factores de forma- Modelo VMD- Los factores de forma electromagnéticos- La extrañeza del protón- Resumen y conclusiones
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Strange Charge and Magnetism
Kaufman, U. Mass.
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Nucleon Electromagnetic andWeak Vector Currents
Dirac and Pauli form factors
Sachs form factors
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Flavor Content
• Electromagnetic current
• Neutral weak vector current
i=1 : Dirac, i=2 : Pauli
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EM and Weak Form Factors
• Charge symmetry• Nucleon electromagnetic form factors
• Proton weak form factor
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Quark Form Factors
Kaplan & Manohar, NPB 310, 527 (1988)Musolf et al, Phys. Rep. 239, 1 (1994)
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Static Properties
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Parity violatingParity conserving
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SAMPLE@MIT-BATESA4@MAMI-MainzHAPPEX@JlabG0@JLab
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JeffersonLaboratory
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Theory• Lattice QCD• Chiral perturbation theory• Dispersion relations• Chiral quark-soliton model• Kaon loops• Quark models• Chiral bag models• Skyrme model• Vector Meson Dominance• .....
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Photon couples to nucleon via vector meson (ρ,ω,φ)
Vector Meson Dominance
Two-component model (IJL, 1973)
Intrinsic structure (valence quarks)Meson cloud (quark-antiquark pairs)
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Dirac and Pauli Form Factors
Intrinsic form factor
Bijker & Iachello, PRC 69, 068201 (2004)
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Electromagnetic form factors
Static properties
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Strangeness in the Proton• The strangeness distribution is a very
sensitive probe of the nucleon’s properties• The strange (anti)quarks come uniquely from
the sea: there is no contamination from up ordown valence quarks
• New data from SAMPLE, HAPPEX, PVA4 andG0 Collaborations
• φ meson mostly strange quarks
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Physical states
Ideally mixed states
Mixing angle
Bijker, JPG 32, L49 (2006) [arXiv:nucl-th/0607058]
Jain et al, PRD 37, 3252 (1988)
Strange Form Factors
Strange form factors
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• Meson-nucleon couplings
• Current-meson couplingsQuark in the vector mesons couples only tothe quark vector current of the same flavorwith a flavor-independent strength
Jaffe – PLB 229, 275 (1989)
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Isoscalar Couplings
Four parameters
One constraint
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Strange Couplings
One constraint: no net strangeness contributionto the electric charge
Reduction of the number of parameters!
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Isoscalar and Strange Couplings
• Couplings• Meson-nucleon couplings• Current-meson couplings: Quark in the
vector mesons couples only to the quarkvector current of the same flavor with a flavor-independent strength
• Static properties• Two independent couplings:
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Couplings
Bijker, JPG 32, L49 (2006)arXiv:nucl-th/0607058
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Summary• Two-component model for the electromagnetic
and weak vector currents
• Intrinsic structure, isoscalar and isovectormesons
• Coefficients taken from a previous study ofelectromagnetic form factors of the nucleon
• Strange form factors: parameter free !
Bijker & Iachello, PRC 69, 068201 (2004) +
Jaffe, PLB 229, 275 (1989)
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RESULTADOS
Factores de Forma Electromagnéticos
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Electromagnetic Form Factors
2004
1973
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Electric Form Factors
Andivahis (1994), Christy (2004), Qattan (2004)
Herberg (1999), Passchier (1999),Zhu (2001), Golak (2001), Schiavilla (2001), Madey (2003), Bermuth (2003), Warren (2004),
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Neutron Electric Form FactorElectric charge of the neutron (udd) is zero,
but the charge distribution is not
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Preliminary resultsfrom the CLAS collaboration (JLab)
Nucleon05: Vineyard
Neutron magneticform factor
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RESULTADOS
Factores de Forma Extraños
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Parity ViolatingElectron Scattering
Manohar & Kaplan (1988)Beck (1989)McKeown (1989)
First experiments
SAMPLE (>1997)HAPPEX (>2001)A4 (>2004)G0 (>2005)
HAPPEX Collaboration,Acha et al, PRL 98, 032301 (2007)
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Dirac and Pauli Form Factors
JPG 32, L49 (2006)arXiv:nucl-th/0607058
Limit of ideally mixedmesons
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Limit of ideally mixed mesons
Strange EM Form Factors
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G0 – PRL 95, 092001 (2005)
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Static PropertiesStrange magnetic moment
Strange charge radius
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Theoretical Values
Liu et al, arXiv:0706.0226 [nucl-ex]
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Summary and Conclusions• Strange form factors probe the sea quarks (Quark
Model vs. QCD)• The nucleon has a complex structure: valence quarks
and meson cloud• Electromagnetic form factors well reproduced• Strangeness contribution to form factors is small and
in agreement with the SAMPLE, HAPPEX, PVA4 andG0 data
• Future experiments to separate the strange electricand magnetic form factors
• Hyperon form factors
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