11.5 The standard model  (Page 3/9)

In this process, the proton emits a ${\text{W}}^{+}$ and is converted into a neutron (b). The ${\text{W}}^{+}$ then combines with the electron, forming a neutrino. Other electroweak interactions are considered in the exercises.

The range of the weak nuclear force can be estimated with an argument similar to the one before. Assuming the uncertainty on the energy is comparable to the energy of the exchange particle by $\left(E\approx m{c}^2\right),$ we have

The maximum distance d that the exchange particle can travel (assuming it moves at a speed close to c ) is therefore

For one of the charged vector bosons with $m{c}^2\approx 80\text{GeV}=1.28\times {10}^{\mathrm{-8}}\text{J},$ we obtain $mc=4.27\times {10}^{\mathrm{-17}}\text{J}·\text{s}\text{/}\text{m}.$ Hence, the range of the force mediated by this boson is

Strong nuclear force

Strong nuclear interactions describe interactions between quarks. Details of these interactions are described by QCD. According to this theory, quarks bind together by sending and receiving gluons. Just as quarks carry electric charge [either $\left(+2\text{/}3\right)e$ or $(\mathrm{-1}\text{/}3)e]$ that determines the strength of electromagnetic interactions between the quarks, quarks also carry “color charge” (either red, blue, or green) that determines the strength of strong nuclear interactions. As discussed before, quarks bind together in groups in color neutral (or “white”) combinations, such as red-blue-green and red-antired.

Interestingly, the gluons themselves carry color charge. Eight known gluons exist: six that carry a color and anticolor, and two that are color neutral ( [link] (a)). To illustrate the interaction between quarks through the exchange of charged gluons, consider the Feynman diagram in part (b). As time increases, a red down quark moves right and a green strange quark moves left. (These appear at the lower edge of the graph.) The up quark exchanges a red-antigreen gluon with the strange quark. (Anticolors are shown as secondary colors. For example, antired is represented by cyan because cyan mixes with red to form white light.) According to QCD, all interactions in this process—identified with the vertices—must be color neutral. Therefore, the down quark transforms from red to green, and the strange quark transforms from green to red.

As suggested by this example, the interaction between quarks in an atomic nucleus can be very complicated. [link] shows the interaction between a proton and neutron. Notice that the proton converts into a neutron and the neutron converts into a proton during the interaction. The presence of quark-antiquark pairs in the exchange suggest that bonding between nucleons can be modeled as an exchange of pions.

In practice, QCD predictions are difficult to produce. This difficulty arises from the inherent strength of the force and the inability to neglect terms in the equations. Thus, QCD calculations are often performed with the aid of supercomputers. The existence of gluons is supported by electron-nucleon scattering experiments. The estimated quark momenta implied by these scattering events are much smaller than we would expect without gluons because the gluons carry away some of the momentum of each collision.