introducción la interacción electromagnética y débil ...bijker/emfn/bijker.pdf · • la...

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)[email protected]


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


Distance Scales


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)


Aplicaciones


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)


Nuclear Physics at the LimitsLimits of mass, stability, temperature, density,

energy, angular momentum, …


Where the hell …?


The Plum Pudding Model

J.J. Thomson (1856-1940)believed the electrons were embedded in a positively charged matrix-plum pudding (1898)


The Structure of AtomsRutherford (1871-1937)showed that atoms contain a centralnucleus (1911)

Electrons orbit nucleuswith well-definedenergy and ill-definedpositions10-10 m


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)


The Structure of Nuclei

Nucleus containsprotons with charge+e and unchargedneutrons

10-14

m


Rutherford & Mott ScatteringNonrelativistic electron scatteringfrom a point-like nucleuswith Z protons and zero spin

Electron is a relativistic particle with spin 1/2


Electron-Nucleus ScatteringFinite size of the nucleus

Form factor

Point-like charge distribution


Differential Cross Section


Uniform Charge Distribution

MinimaCharge radius


Charge Distributions


10-15 m

The Structure of NucleonsNeutrons and protons contain quarks (Friedman, Kendall, Taylor 1968)

Quark ModelProton: uudNeutron: udd


<10-18 m

The Structure of Quarks?

There is no evidence for further structure

?


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)


Constituent Quark ModelsBaryons: qqq configurations

proton: uudneutron: udd

Ground state baryons: octet & decuplet

Ratio of nucleon magnetic moments


Octet and Decuplet Baryons

Octet

Decuplet


Quarks


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


Rosenbluth SeparationResult: form factor scaling


Polarization Transfer

Method: measure ratio of sideways (PT) tolongitudinal (PL) recoil polarization of proton

Result for form factor ratio


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.


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)


Parity violatingParity conserving


SAMPLE@MIT-BATESA4@MAMI-MainzHAPPEX@JlabG0@JLab


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


Strange Charge and Magnetism

Kaufman, U. Mass.


Nucleon Electromagnetic andWeak Vector Currents

Dirac and Pauli form factors

Sachs form factors


Flavor Content

• Electromagnetic current

• Neutral weak vector current

i=1 : Dirac, i=2 : Pauli


EM and Weak Form Factors

• Charge symmetry• Nucleon electromagnetic form factors

• Proton weak form factor


Quark Form Factors

Kaplan & Manohar, NPB 310, 527 (1988)Musolf et al, Phys. Rep. 239, 1 (1994)


Static Properties


Parity violatingParity conserving


SAMPLE@MIT-BATESA4@MAMI-MainzHAPPEX@JlabG0@JLab


JeffersonLaboratory


Theory• Lattice QCD• Chiral perturbation theory• Dispersion relations• Chiral quark-soliton model• Kaon loops• Quark models• Chiral bag models• Skyrme model• Vector Meson Dominance• .....


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)


Dirac and Pauli Form Factors

Intrinsic form factor

Bijker & Iachello, PRC 69, 068201 (2004)


Electromagnetic form factors

Static properties


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


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


• 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)


Isoscalar Couplings

Four parameters

One constraint


Strange Couplings

One constraint: no net strangeness contributionto the electric charge

Reduction of the number of parameters!


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:


Couplings

Bijker, JPG 32, L49 (2006)arXiv:nucl-th/0607058


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)


RESULTADOS

Factores de Forma Electromagnéticos


Electromagnetic Form Factors

2004

1973


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),


Neutron Electric Form FactorElectric charge of the neutron (udd) is zero,

but the charge distribution is not


Preliminary resultsfrom the CLAS collaboration (JLab)

Nucleon05: Vineyard

Neutron magneticform factor


RESULTADOS

Factores de Forma Extraños


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)


Dirac and Pauli Form Factors

JPG 32, L49 (2006)arXiv:nucl-th/0607058

Limit of ideally mixedmesons


Limit of ideally mixed mesons

Strange EM Form Factors


G0 – PRL 95, 092001 (2005)


Static PropertiesStrange magnetic moment

Strange charge radius


Theoretical Values

Liu et al, arXiv:0706.0226 [nucl-ex]


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

introducción la interacción electromagnética y débil ...bijker/emfn/bijker.pdf · • la...

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