Valence quarks

It is found in experiments that the nucleon vector current can to a good approximation be described as the sum of the corresponding valence quark vector currents. For the vector currents of the proton (p) and the neutron (n) we obtain therefore... 

Level I. A nucleon (neutron, proton) consists of three (valence) quarks, clearly seen on the scattering image obtained for the proton. Nobody has yet observed a free quark. 

Each of the nucleons is composed of three quarks (called the valence quarks). 

Properties of nucleons (p, n). Quark structure symbol, electric charge (e), spin (A) and its direction. There are three valence quarks both in p and n, such that they are colorless... 

The weak interactions change quark and lepton flavors, e.g., a d-quark into -quark or a muon into an electron (this latter, e.g., in the p e Vet /i process). The quark structures of the proton and neutron as well as the properties of nucleons are presented in O Table 2.3. The baryons are built up from three (valence) quarks and massless gluons, but they contain also dynamical (or sea) quarks (quark-antiquark pairs) in a small quantity. The mesons are built up from quark-antiquark pairs and gluons. 

Fig. 3.4. (a) The electric field dose to the proton (composed of three quarks) is so strong that it creates matter and antimatter (shown as electron-positron pairs). The three quarks visible in scattering experiments represent the valence quarks, (b) One of the radiative effects in the QED correction of the c order (see Table 3.1). The pictures show the sequence of the events from left to the right A photon (wavy line on the left) polarizes the vacuum and an electron-positron pair (solid lines) is created, and the photon vanishes. Then the created particles annihilate each other and a photon is created, (c) A similar event (of the order in QED), but during the existence of the electron-positron pair the two particles interact by exchange of a photon, (d) An electron (horizontal solid line) emits a photon, which creates an electron-positron pair, that annihilates producing another photon. Meanwhile the first electron emits a photon, then first absorbs the photon from the annihilation, and afterwards the photon emitted by itself earlier. This effect is of the order c in QED. 

Fig. 3.5 The proton par-ton distribution functions measured at HERA at 2 = lOGeV, for valence quarks and xd, sea quarks xS, and gluons xg. The gluon and sea distributions are scaled down by a factor 20 [23]...

This process proceeds via a Cabibbo allowed transition and, although it is depressed by comparison with an interaction with a valence quark. 

Because, by definition, the valence quarks give the proton its correct 517(2) or 517(3) properties, we e jq>ect the sea to be basically neutral, i.e. singlet under these transformations. But the above symmetries are not perfect, and while 517(2) is well respected, in nature 517(3) is broken somewhat. It thus seems reasonable, for the proton, to insist in the simple picture that... 

We would naturally expect the structure of a hadron to be dominated by its valence quarks—u and d for a nucleon. So we might expect the contribution from the sea to be small. But this cannot be true for all x as we saw from the discussion of B x) in Section 16.2. And QCD arguments indicate that the sea should become increasingly important for small x. 

For small x the ratio is close to 1 suggesting little influence of valence quarks at small x and dominance of a symmetric sea contribution in which u + ufud + d. All this is nicely consistent with the conclusions following from the behaviom of B(x) discussed in Section 16.2. 

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