Making sense of the ‘particle zoo’
This is a great decade for the popularisation of science in general, and physics in particular. The detection of gravitational waves by LIGO, and the books of Carlo Rovelli, are just two things that have caught the public imagination.
But nothing has done more in this regard than the 2012 discovery, at CERN, of a particle that has all the hallmarks of being the Higgs boson, the so-called God Particle. Suddenly, particle physics became sexy (to the public anyway – particle physicists obviously already knew it was).
For the uninitiated, the world of particle physics can seem really daunting, at least partly due to the range of exotic nomenclature that abounds. Fermions, bosons, hadrons, leptons, quarks, gluons, baryons, mesons – it’s a lot to keep track of, especially while also trying to keep hold of the slippery concepts that necessitate such a range of names. No wonder that the phrase ‘particle zoo’ was coined – in the 1960s, for example, it wasn’t at all clear why there should be as many types of particle as there are. The theoretical framework developed to incorporate all this weirdness is known as the standard model of particle physics. Here is a picture of the particles in the standard model. You are not expected to understand it yet…
This two-part article is a primer on the standard model. We will approach it step by step, so that the picture builds up slowly. At the end you’ll have an understanding of the summary ‘map’ above. There are loads of great popular science books on particle physics. We are not trying to replicate them. We are trying to inspire you to read them, and give you a head start with the vocabulary to help you do so.
What are things made of? The particle picture in the 1920s
Hopefully, you are aware of the existence of atoms. In 1897 the electron was discovered – a tiny negatively charged particle, and the first particle found that is smaller than an atom (a ‘subatomic’ particle). The discovery of the positively charged proton followed in 1919. The charges on an electron and a proton, although opposite, are equal in magnitude, and all charges in this article are expressed relative to that unit ‘electron charge’, labelled [latex]e[/latex]. The masses of protons and electrons are extremely different, however. In the modern unit ‘MeV/c2’, the electron mass is 0.511 and the proton weighs in 1836 times more massive at a whopping 938.
Serious attempts were made to model atoms as consisting of just protons and electrons, but it became clear throughout the 1920s that atoms also ought to contain a different kind of particle, one that is ‘neutral’ (not electrically charged). It is now called the neutron.
Additionally, and for highly technical reasons that need not concern us, in 1928 Paul Dirac predicted the existence of a particle with the electron’s mass but with positive charge. It is the positron, the antiparticle of the electron.
Antimatter is the ‘mirror image’ of matter. It is created in particle accelerators, in radioactivity, or when cosmic rays interact with matter. A particle has the same mass and same lifetime as, and the opposite charge to, its antiparticle. If a particle and its antiparticle meet they are ‘annihilated’ and cease to exist. All their energy is released as gamma rays (a highly energetic form of electromagnetic radiation, like an invisible light).
But by 1930, although the neutron and positron had been predicted, they had not yet been discovered. So the sum total of subatomic particles discovered was as in the very simple diagram below. The eagle-eyed among you may have noticed that the proton (and also the positron) are not in the summary ‘map’ above. Don’t worry about this, all will become clear.
You might think that a simple picture consisting of two types of particle (four, if you include the predicted but not yet discovered neutron and positron) would not necessitate much in the way of classification.
However, theoretical physicists had already, in the 1920s, discovered that the statistical behaviour of hypothetical particles would depend on a quantity called their ‘spin’, and classified them into two types called fermions and bosons. So we need a small digression…
Spin
Angular momentum is momentum applied to motion in a circle. The Earth has an orbital angular momentum associated with its motion around the Sun. Its total angular momentum is greater than this because it is spinning on its own axis, and its spin is an extra source of angular momentum.
A particle is in a similar (but not identical) position. Its total angular momentum is generally different from its orbital angular momentum in any situation, and the difference is spin (sometimes more accurately, but less concisely called intrinsic angular momentum).
There is a quantum number associated with spin, which gives a measure of its value (we’ve written about this previously). A particle can have a spin quantum number of 0, 1/2, 1, 3/2, 2, etc, but no intermediate values – spin is quantised (exists with only certain values), just like charge.
Fermi, Dirac, Bose, Einstein and others discovered that particles with half-integer spin obey one type of statistical behaviour (how they act when there are lots of them), and particles with integer spin obey another. In summary:
- Fermions are particles with half-integer spin: 1/2, 3/2, 5/2 etc. eg electrons.
- Bosons are particles with integer spin: 0, 1, 2 etc. eg photons.
The Pauli exclusion principle states that two identical fermions in the same system can not exist in the same state. That means that if everything else is the same their spins must be in different directions. The exclusion principle is the reason that electrons exist in shells in atoms and do not all crowd in to the lowest energy orbital. This in turn determines the chemical properties of all the elements. ALL of chemistry thus depends upon this single principle (sorry, chemists, couldn’t resist that one…). The exclusion principle does not apply to bosons – as many of those can be in the same quantum state as you wish…
Another way of describing this idea is given by Sean Carroll, in ‘The Particle at the End of the Universe’. He describes fermions as particles that take up space, and bosons as particles that can inifinitely pile on top of one another, so that they do not take up space.
Particle properties
So far, we have described particles in terms of several properties: mass, charge, and spin. There are others, but really not that many others. In other words, there are only a limited number of ways in which particles can differ in their characteristics, and in that sense they are not very complicated, unlike, say, human beings!
One other property that we will consider is a particle’s lifetime. Some particles, such as the proton and electron, are stable and remain a proton and electron indefinitely. Others spontaneously decay into other particles. The lifetime of a particle is linked to its mass. In general, heavier particles will decay into lighter particles – since energy and mass are equivalent ([latex]E=mc^2[/latex]), such decays result in a release of energy. In contrast, the creation of heavier particles from lighter ones requires an input of energy, and therefore is not a spontaneous process. That is what particle accelerators, such as the Large Hadron Collider at CERN, are for…
The mass, charge, spin and lifetime of a particle are the only properties we will consider for a particle, until we hit the section on ‘strangeness’!
The 1930s
The neutron and the positron were duly discovered within a few months of each other in 1932.
Meanwhile an important question was being pondered. All protons in the nucleus repel each other. Why doesn’t the nucleus fly apart?
In 1935 Hideki Yukawa proposed that there is another force acting between all the particles in the nucleus, whether protons or neutrons (we call them collectively ‘nucleons’). This force then holds the nucleus together, and overcomes the electrostatic repulsion. Because it must be stronger than the electrostatic repulsion, it is called the strong interaction (original, huh?). He also predicted the existence of an ‘exchange particle’ called a pion ([latex]\pi[/latex]), that would be exchanged between two nucleons and transmit the force between them.
Not all particles are subject to the strong interaction, and this leads to our next classification (remember, we have already seen fermions v bosons on the basis of their spins). We can categorise any particle as one of the following:
- Hadrons are particles which experience the strong interaction
- Leptons are particles which do NOT experience the strong interaction
Also in the 1930s, the study of cosmic rays (streams of high energy particles from outer space) led to the discovery of the muon. The muon is identical to the electron, except that its mass is 207 times greater. A muon is basically a heavy electron.
All this means that the picture in 1940 was as follows, with the pion still promised but not yet found…
Before we go further, remember that you are made of atoms. Atoms are made of protons, neutrons and electrons. Therefore, you are made of protons, neutrons and electrons. That is true. The existence of the positron and muon in the diagram above doesn’t stop it being true. I think it’s important to say this, because when you see the full timeline – which itself is just a selection of important particles, not all of them – it can be overwhelming and might cause you to wonder whether you are actually made of atoms (protons, neutrons and electrons) after all. You are! Don’t panic!
Also, don’t worry about the colour scheme. It follows a classification that we haven’t arrived at yet.
The 1940s
In the 1940s and 1950s cosmic rays were investigated in detail. Many totally new particles were found in them such as the K-meson (kaon), lambda particle ([latex]\Lambda[/latex]) and sigma particle ([latex]\Sigma[/latex]), and more recently the list stretches to over 100. How were they found to be new particles? I mean, these things are pretty small, right? Well, massive progress was made on determining the properties of particles using instruments called cloud chambers and bubble chambers, which track particles’ paths through a magnetic field. The curvature of their paths through the field gives you information about the mass, charge and lifetimes of the particles. If you find a particle with a new combination of mass, charge and lifetime, it might be a new particle!
The particles named in the previous paragraph might well be new to you. If so, that is because they do not play a part in the matter we are usually concerned with. You are not made of kaons, for example (you really are still made of protons, neutrons and electrons, I promise!).
The shortness of the lifetimes of the newly discovered kaons, lambdas and sigmas indicated that they must decay via the strong interaction and thus be hadrons. All hadrons were quickly found to lie in one of two groups (yet another classification):
- Baryons, which have half integer spin (1/2, 3/2, etc), named from the Greek ‘barus’, meaning ‘heavy’, because they were comparatively massive, like protons or heavier
- Mesons, which have integer spin (0, 1, etc), named from ‘meso’ meaning ‘intermediate’, because the first to be discovered were intermediate in mass between the electron and the proton
Baryons were given a ‘baryon number’ (a quantum number, like that for spin we met earlier) of 1, antibaryons -1, and non-baryons 0. It was quickly discovered that in all particle reactions baryon number is conserved. By a particle reaction, we mean that when two particles smash together, they sometimes create brand new ones. The conservation of baryon number just means that however many baryons you started with, you have to end with the same number. Conservation laws are incredibly important in physics – you may have met the law of conservation of energy, for example. Charge is another quantity that is conserved in particle reactions, so the total charge of the ‘before’ particles is the same as the total charge of the ‘after’ particles.
We have now hit three different classifications (fermion/boson, hadron/lepton, meson/baryon). A summary diagram is given below with what we know so far – just skip it if you are on top of all this!
Now let’s update the particle discovery picture, so see where we have got to by 1950…
We are now in a position to understand the colour scheme! It is shown in the key on the right hand side. Notice that the kaon has been labelled as the ‘first strange particle discovered’. We need to talk about that.
Strangeness
As Frank Close point out in ‘The Cosmic Onion’, if a negatively charged pion hits a proton, then charge conservation and baryon number conservation would permit
[latex]\displaystyle{\pi^-+p\rightarrow \pi^-+\Sigma^+}[/latex]
(This is the first particle reaction we have met written out in symbolic notation. It means ‘a negative pion and proton collide, cease to exist and turn into another negative pion plus a positive sigma particle – you get the idea.)
By watching the tracks of the particles in a bubble chamber or its more modern electronic counterparts, scientists have observed countless numbers of particle reactions. But they have never observed this one. Not even once. Ever.
In 1953 Murray Gell-Mann proposed that there must exist another (previously unthought-of) property of matter that needs to be conserved in particle reactions, in addition to charge and baryon number. The non-existence of the above reaction could then be explained by saying that it does not conserve this new quantity and therefore does not happen.
The new property was given the name strangeness. From knowledge of which reactions did and did not happen, hadrons could be assigned a strangeness of 0, +1, or -1. Strangeness is conserved in all reactions except reactions which proceed via the weak interaction. We haven’t met the weak interaction yet, and will not dwell on it. We will just say for now that it is weaker than the strong interaction (!) and is responsible for beta decay, one of the mechanisms of radioactivity.
More hadrons
More and more hadrons were steadily found, and attempts were made to find relationships between them, in much the same way as the Periodic Table shows the relationships between the chemical elements. In 1960 Gell-Mann discovered the Eightfold Way.
To be continued in the second and final part, where all the good stuff happens…