Particle Accelerators and the Architecture of Matter

One of the most innovative ways in which physicists study the beginning of the Universe is to use gigantic pieces of equipment known as particle accelerators. This technology is the perfect synthesis of the very small, namely sub-atomic particles, with the very big, namely the entire Universe.

Inside these devices, streams of sub-atomic particles are accelerated to velocities close to the speed of light, which supplies them with similar quantities of energy that they would have possessed during the early Universe. To simulate the density of matter from those early times, highly accelerated beams of particles are then collided with each other.

The annihilations which result cause a liberation of energy which, in turn, leads to the creation of particles which must have populated the early Universe in large numbers but no longer exist naturally in our present day low energy cosmos.

Through the use of these accelerators to test their theories, particle physicists have developed a standard model which explains almost all particles and their interactions. The fundamental particles of nature which go to make up matter fall naturally into two families. These are known as the leptons and the quarks. Both families contain six particles which are split into three generations of pairs. The most familiar leptons are the first generation pair: the electron and the electron neutrino (usually referred to simply as the neutrino).

The other two generations are populated by heavier versions of both the electron (the muon and the tau) and the electron neutrino (the muon neutrino and the tau neutrino). The quark generations are composed of successively heavier versions of the up and down quarks; the charm and the strange, the truth and the beauty (see Fig. 2. 12).

Fig. 2. 12. Leptons and hadrons, the fundamental particles of the Universe. (Adapted from Big Bang Science, Particle Physics and Astronomy Research Council.)

These fundamental particles can combine in many ways to become the matter content of the Universe. For example, an electron has an electromagnetic charge of-1. The up quark has a charge of +2/3 whilst the down quark has a charge of -1/3. In order for the quarks to combine into the more familiar sub-atomic particles, neutrons and protons, they must come together in the right combinations. Two up quarks and one down quark make up a proton which possesses a charge of +1.

One up quark and two down quarks come together to form a neutron which carries no charge. These hybrid particles made from quarks are known as hadrons. Hadrons can be subdivided into quark triplets known as baryons, such as protons and neutrons and mesons which consist of a quark and its antimatter counterpart. Electrons, protons and neutrons then combine to form atoms. The exact number of electrons, protons and neutrons defines the atom’s chemical identity.

There are very few stable particles in the Universe today. Only the electron and the electron neutrino are truly stable (photons of radiation can also be considered stable). Neutrons will decay into an electron and a proton in about 896 seconds, if removed from an atomic nucleus. Protons may or may not be stable. If they are not stable they decay in lifetimes of around 10³¹ years and become a positron and a pi-meson which, itself, decays into two photons.

Although their vast lifespans make them stable for all current problems, the very long-term future of the Universe could be affected if protons do decay. All the other hadrons have lifetimes of a mere fraction of a second.

All the particles, be they leptons or quarks and their composites, interact with one another via the exchange of a third group of particles known as gauge bosons. It is these particles which carry the forces of nature. Throughout human history, what were once thought to be totally different forces have been shown to be different manifestations of the same force. For instance, Newton showed that the motion of the planets could be explained by the same force that caused the proverbial apple to fall to the Earth: gravity.

On the face of it, two totally different phenomena which, in reality , were manifestations of the same underlying principle. Earlier in this chapter we saw how electricity, magnetism and light were unified. Later, Einstein showed an interconnection between space and time in special relativity and then linked this work to gravity via his general theory of relativity.

Today we recognise only four forces, which we term the fundamental forces. They are gravity, electromagnetism, the weak nuclear force and the strong nuclear force. Each one of them acts differently from the others. They are carried by gauge bosons in the standard model. Electromagnetism is carried by the photon, which we have already met, and the weak nuclear force is carried by three different particles, the neutral Z° and the charged W" and W".

These particles can be thought of as being somewhat analogous to the photon except that the Ws possess a charge and they all possess mass. The strong nuclear force is also mediated by a gauge boson, known as a gluon, which transmits the force between quarks. Mesons then transmit the force between nucleons. As yet, there is no convincing quantum theory of gravity, although the hypothetical gauge boson has been named the graviton.

The mass of the gauge bosons affects the distance over which these fundamental forces can act. This is a consequence of the Heisenberg uncertainty principle which states:

Since the energy ΔЕ to create the gauge bosons is borrowed from the energy field of the space surrounding the interacting particles, that energy has to be paid back before equation (2.8) is violated. Equation (1.2) tells us how much energy needs to be borrowed and so the time the particle can ‘live’ for, Δt, is given by a combination of the two equations.

The range of the force, r, can therefore be shown to be inversely proportional to the mass of its gauge boson because, according to special relativity, nothing can travel faster than the speed of light in a vacuum.

The weak nuclear force is carried by massive particles which limit its range to approximately 10^-17m. The mesons which carry the strong nuclear force are less massive and can react over a distance of 10^-15m. Photons, being massless, can exist forever and so the range of the electromagnetic force is theoretically infinite.

Over large regions of space, however, electrical charge is neutral and so the electromagnetic force is, in fact, confined. Gravitons are theorised to be massless and, unlike electromagnetism, which can be positive or negative, gravity is only ever attractive, thus it cannot be cancelled out. This is the reason it shapes the Universe on its largest scales.

Particle accelerators can examine the behaviour of these particles and forces in conditions which mimic the early Universe. By doing so, a remarkable theory has been confirmed. A group of physicists had proposed that electromagnetism and the weak nuclear force would act in exactly the same fashion at sufficiently high energies.

Confidence in the Salem, Weinberg and Glashow theory was boosted enormously by the particle accelerator confirmations of their predictions. From now on, electromagnetism and the weak nuclear force would be forever linked as the electroweak force. This success has led physicists and cosmologists to believe that the strong nuclear force could also be unified with the electroweak force.

It is postulated to take place at even higher energies and is explained by a set of ideas known as the grand unified theories (GUTs). This is a set of theories which sets out to unify all the fundamental forces except gravity. A number of GUTs exist, but as yet none have been proven. GUTs predict that protons should decay.

Fig. 2. 13. Unification of forces. Central to most cosmological ideas is the notion that, as the temperature of the Universe was higher at earlier and earlier times in history, so the distinction between the separate forces of nature was less and less apparent. (Adapted from Silk, J., A Short History of the Universe, W.H, Freeman, 1994.)

Some physicists and cosmologists believe that, following a successful GUT, gravity would be unified and every interaction in the Universe would be described in terms of a single fundamental force of nature. This level of unification, if it even exists, is a long way into the future. A quantum theory of gravity is required first (see Fig. 2. 13).