Atoms - the inside story. Antimatter.

4. Fundamental families

page 15

	Antimatter

		The electron was the first known fundamental particle. We now know that it is a member of a family of fundamental particles called leptons (see page 16). The other family of fundamental particles is the quark family (see page 17). In each family, every particle has an anti-particle.

	Antimatter is born . . .

		For every fundamental particle, there is an euivalent particle of antimetter – known as its antiparticle. Why do we think this? The existence of antimatter was predicted by the Cambridge physicist Paul Dirac in the late 1920s. He developed an equation that could account for the effects of Special Relativity in the (then new) theories of Quantum Mechanics. When he solved the equation, it predicted the behaviour of the electron. This showed that it was probably a successful analysis. However, his solution also predicted the existence of another particle with similar but completely opposite properties to the electron. It had the same mass, a positive charge and negative energy states. He interpreted the solution to mean that, as well as the electron, there must be another particle that we don't normally come across. He suggested that this was a new particle of antimatter. It was an anti-electron and he called it the positron. Soon afterwards, in 1932, Carl Anderson observed positrons in cloud chamber events that were triggered by cosmic radiation. He photographed a positively charged particle being deflected in a magnetic field. He showed that its mass was equivalent to that of the electron. Since then, the existence of antimatter has been verified many times. Indeed, there are particle accelerators that fire beams of positrons at electrons to create spectacular collisions. In 1995, scientists at CERN (the European particle accelerator near Geneva) managed to combine anti-electrons with anti-protons to make tiny amount of anti-hydrogen. In 2003, they were able to make it in larger quantities.

Animation of electron postiron annihilation

Picture 4.1 When an electron meets a positron, they annihilate and release energy.

	. . . and antimatter dies

		So what happens when a particle meets an antiparticle? The answer is, they annihilate and release energy in the form of electromagnetic radiation. The amount of energy released is, in line with Einstein's theory of Special Relativity, equivalent to 2m_ec², where m_e is the mass of an electron (see page 25).

	What does antimatter mean?

		As with many of the theories about these tiny particles, it is difficult to give a physical meaning to antimatter. All we know is how it behaves and what it does, not what it is. However, Richard Feynman gave one description of a positron. He suggested it is like an electron with negative energy travelling backwards in time (all its behaviours agree with this description). So, if there is an antimatter version of you, at least you will not see it coming. This is because, according to Feynman, it is in the future coming back towards you. When you meet, you will not know a thing about it (although anyone standing nearby will not be so fortunate, as the collision between you and your anti-you will release more energy than an atomic bomb).

	Evidence for antimatter

		In beta decay, a nucleus throws out a fast moving electron (a beta particle). However, unlike alpha decay, the beta particles from a given nuclear decay do not all have the same energy. They have a range of energies up to a maximum value. One explanation is that the electron is not the only particle given out in beta decay (see page 16). There is also a particle of antimatter – the anti-neutrino. The anti-neutrino has a corresponding particle of normal matter called the neutrino. The neutrino has a very small mass and might even have no mass at all – (neutrino means 'little neutral one'). It belongs to the lepton family, which we'll look at on the next page.

	Beta plus decay

		Positrons are given out by some radioactive atoms in a form of decay called beta plus decay. This is very similar to beta decay except that the fast moving particle is a positron rather than an electron. Also, a neutrino rather than an anti-neutrino is given out at the same time. In each type of decay, the atom throws out one particle and one anti-particle.