The structure of matter has since long revealed itself very different from our naïve, intuitive perception.
Additional pairs of particle-antiparticle
According to Field Theory, developed as a consequence of Quantum Theory, the quantum state of a single particle is an approximate description of reality. This is why Schwinger, Feynman and Tomonaga formulated Quantum Electrodynamics, and shortly after, Dyson demonstrated the equivalence of their formulation. It turns out that Dirac particles can be in several states at the same time, and some of these states are composed of additional particle-antiparticle pairs of the same kind. The electron, for example, exists both as a single electron, and as a superposition of 2 electrons and one positron, etc.
Measurements of the Hydrogen atom conducted in 1947 by Willis Lamb and Robert Rutherford showed a small shift in its energy levels compared to the predictions of Dirac’s equation for a single electron. This result refuted the single particle approach and supported the theory called Quantum Electrodynamics. The measured shift is called “The Lamb Shift”, and its magnitude is very small: about 1 to 106. This is because every state has a probability, and for the electron, the probability for deviating from a single-particle state is very small. A precise measurement of the electron’s magnetic moment lead to the same conclusions.
In Hadrons the Strong Interaction is much stronger than its electric counterpart, the existence of additional quark-antiquark pairs, as predicted by Field Theory, is easier to notice. And indeed, experiments successfully showed that there are antiquarks in the proton.
Theoretically, the existence of these antiquarks derives from the addition of one or several pairs of quark-antiquark to the proton’s wave function. Measurements show that there is in average an extra half – pair in every proton .
The antiquark is located in the proton’s periphery
Measurements showed that antiquarks tend to be located in the proton’s external part whereas quarks settle in a smaller volume of the proton . Why is that? According to Comay’s model, the core has a positive magnetic charge and the quarks, carrying a negative magnetic charge, are attracted to it. The antiquarks, carrying a positive magnetic charge, are therefore pushed away from the core, to the external part of the proton.
The Standard Model has no explanation for this phenomenon. Furthermore, the pi meson (called pion) is composed of a bound pair of quark-antiquark. Although the pion’s quarks are the same as those inside the proton, its radius is smaller than that of the proton. This means that a single quark can confine an antiquark in a sphere of dimensions smaller than the proton. According to the standard model and QCD, there are no other massive particles inside the proton except the quarks which attract each other. It is therefore not clear why 4 quarks (the 3 original quarks of the proton + the quark from the quark-antiquark pair) are not able to confine the antiquark at least within the part of the proton’s space in which they reside, and allow the antiquark to reside in a larger volume. This QCD problem becomes even clearer because here the Pauli exclusion principle does not impose any constraint on a single antiquark spatial confinement.
Electric charge distribution within the Neutron
Another interesting phenomenon is the propensity of negative electric charges inside the neutron to be located in more peripheral areas than the positive charges. The neutron contains a u-quark with a charge 2/3, and two d-quarks, each with a charge -1/3. Like in the proton, there are other quark-antiquark pairs inside the neutron.
Experiments show that neutrons contain more u-quark-antiquark pairs than d-quark-antiquark pairs. This is explained by the Pauli principle, according to which identical quarks cannot occupy the same quantum state. Therefore, because of the presence of 2 d-quarks as opposed to a single u-quark in the neutron, the addition of d-quarks is less likely. The situation in the proton is the exact opposite: there are more d-quark-antiquark pairs, because the proton contains 2 u-quarks and a single d-quark.
As we mentioned above, the antiquarks within the nucleon are pushed toward more peripheral areas. Thus the u-antiquark, with its negative electric charge of -2/3, will be located toward the external zone. The d-antiquark has a lower probability to be found in the neutron, and since its charge is smaller (1/3), it does not cancel out the contribution of the the negative charge of the u-antiquark.
The Standard Model has no explanation for this phenomenon as well.
 p. 281 of D. H. Perkins, Introduction to High Energy Physics, Addison-Wesley, Menlo Park, CA, 1987
 Look in the graph in . The x-width of antiquarks is smaller, and therefore has smaller Fermi motion and larger 3D volume