Bosons: The Force-carrying Particles

Within the Standard Model all interactions of fermions are mediated by the exchange of a gauge boson. So all the forces may be described by the interaction of fermions and bosons. Each force has it's own gauge bosons associated with it.

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The electromagnetic force is mediated by a single boson called the photon ($\gamma$). This boson is massless and chargeless. It couples to the charge of other particles, so only charged particles feel the effect of the electromagnetic force.

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The weak force has three mediating bosons. There is the chargeless ${\rm Z}^{0}$ which has a mass of 91.2 GeV and a width of 2.5 GeV [5,6]. Then there are the two charged bosons ${\rm W}^{\pm}$ which each have a mass of 80.45 GeV [7,8] and a width of 2.0 GeV [8].

The W bosons couple to the weak isospin of particles (t$_{3}$). Only left handed fermions and right handed anti-fermions have non-zero weak isospin. W bosons themselves also have non-zero weak isospin so can couple to each other.

The ${\rm Z}^{0}$ boson couples to a combination of both weak isospin and charge, as shown in equation 1.5. It has zero weak isospin itself.

$$C_{Z^{0}} = \frac{(t_{3} - \sin\theta_{w}{\rm Q})}{\sin\theta_{w}\cos\theta_{w}}$$ (1.5)

In equation 1.5, $\theta_{w}$ is known as the Weinberg or weak mixing angle and is related to the masses of the ${\rm Z}^{0}$ and ${\rm W}^{\pm}$ as follows:

$$\cos\theta_{w} = \frac{M_{W}}{M_{Z}} \simeq 0.87679$$ (1.6)

The weak force is the only force that couples to all the fermions. When a W boson interacts with a fermion it will always change the flavour of the fermion. There are no flavour changing neutral weak interactions.

A property of the weak force is that its bosons can couple to each other in certain combinations and also the photon. The coupling of a ${\rm W}^{+}$ and a ${\rm W}^{-}$ boson to a photon or ${\rm Z}^{0}$ boson is known as a Trilinear or Triple Gauge Coupling (TGC) and this process is the basis for this thesis.

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The Strong Force has eight mediating bosons called gluons. These are massless and chargeless and couple to the colour charge of particles. Of the fermions only quarks have non-zero colour charge. Gluons are the only bosons that have non-zero colour charge and can couple to other gluons.

The strong force is unlike the other two forces in that its strength actually increases with distance. This is the reason that no unbound quarks are seen. All quarks are bound as colourless baryons or mesons as discussed earlier.

A consequence of this property is that if two bound quarks are separated the potential energy between them increases until it reaches a level where two new quarks will form. Although each of these is a colour singlet, they effectively combine with the original object to form colourless objects. Continued separation will cause further quark pairs to form. Figure 1.2 shows a simple schematic of this process. If this separation occurs at high energy, the newly formed quark anti-quark pairs are seen as jets of particles. As the quarks in each jet form hadrons, this process is known as hadronisation.

Figure 1.2: Production of quark anti-quark pairs when a single pair are separated.