Theoretical Descriptions of the Nucleon
The Standard Model
The Standard Model of particle physics, a Quantum Field Theory, was developed between 1970 and 1973. The Standard Model describes all of the known elementary particles interactions except gravity. It is the collection of the following related theories quantum electrodynamics, the Glashow-Weinberg-Salam theory of electroweak processes and quantum chromodynamics.
As it is known the matter is made out of three types of elementary particles: quarks, leptons and mediators.In the Standard Model there are six quarks,including the up(u) and down(d) quarks, which make up the neutron and proton. They are classified according to charge(Q), strangeness(S), charm(C), beauty(B) and truth(T). All quarks are spin- 1/2 fermions
THE QUARK CLASSIFICATION
There are six leptons electron, muon and tau, with their partner neutrino. Leptons are classified by their charge(Q), electron number(
THE LEPTON CLASSIFICATION
There are six antileptons and antiquarks, all signs in the above tables would be reversed for them. Also, quarks and antiquarks can carry three color charges(red, green and ) that enables them to participate in strong interactions.
In the Standard Model the mediator elementary particles with spin-1 are bosons.
- The electromagnetic interaction is mediated by the photons, which are massless.
- The , and gauge bosons mediate the weak nuclear interactions between particles of different flavors.
- The gluons mediate the strong nuclear interaction between quarks.
THE MEDIATOR CLASSIFICATION
The Quark Parton Model
In The Parton Model partons are a hypothetical fundamental particles, which are a constituent of the hadron. It was shown that hadrons contain quarks and gluons. However, the Parton Model is used to describe short-distance interactions.
Bjiorken predicted that at very high energies the inelastic structure function dependence on decreases and it is function of , ||4 - momenta of the initial and final state leptons |- | , ||Polar and azimuthal angle of the scattered lepton |- | ||4 - momentum of the initial target nucleon |- | ||4 - momentum of the virtual photon |- | ||Negative squared 4 - momentum transfer |- | ||Energy of the virtual photon |- | ||Bjorken scaling variable |- | ||Fractional energy of the virtual photon |- | ||Squared invariant mass of the photon-nucleon system |- | ||4 - momentum of a hadron in the final state |- | ||Fractional energy of the observed final state hadron |}
Quark distribution Functions
define and describe
Quark distribution function q(x) is the probability(density) of finding a quark with fraction x of the proton momentum. It can be expressed as
It is known that the proton contains up(u) and down(d) quarks. Accordingly, We have up u(x) and down d(x) quark distribution functions in the proton. u(x) is the probability that momentum fraction x is carried by a u type quark and d(x) - for a d type quark. Moreover,
u(x)dx ( d(x)dx ) is the average number of up (down) quarks which have a momentum fraction between x and x+dx.
Actually, the proton can contain an extra pair of quark - anti quarks. The original(u, d) quarks are called valence quarks and the extra ones sea quarks.we are allowed to separate the quark distribution function into a valence and a sea part,
q(x) is the unpolarized distribution function and - the polarized.
The structure functions in the quark parton model can be written in terms of quark distribution functions,
The unpolarized structure function- measures the total quark number density in the nucleon, - the polarized structure function is helicity difference quark number density.
The unpolarized structure functions ( i= L, R, O ) should satisfy the following inequalities,
The structure functions in terms of the parton distributions can be written as,
where , , are the charge, left- and right-handed weak couplings of a ith type quark and , corresponding couplings for the electron.
Both models, pQCD and a hyperfine perturbed constituent quark model(CQD), show that as the scaling variable
The inclusive double polarization asymmetriesin the valence region, where the scaling variable can be written in terms of polarized and unpolarized valence quark distributions,
The semi-inclusive pion electro-production asymmetries can be written in terms of the valence quark distributions
where is the measured difference of the yield from oppositely charged pions. Using the first four equation (1), (2), (3) and (4) one can construct the valence quark distribution functions.
The semi - inclusive asymmetry can be rewritten in terms of the measured semi-inclusive and asymmetries:
where is the unpolarized structure function and the scaling polarized structure function.
The last equation can be expressed as
using the nomenclature of (6) equation, we have