Of Quarks and Bosons31 August 2012
“We’re all just protons, neutrons, electrons that rest on a Sunday, work on a Monday,” or so The Cat Empire tells us. Yet the truth is always more complicated, as revealed on 4 July this year when the Higgs boson discovery was announced at the International Conference on High Energy Physics in Melbourne. Yes, we are all protons, neutrons, and electrons, but fundamentally we’re also up quarks, down quarks, Z and W bosons, photons, gluons, and now Higgs bosons too. Which is a bit of a mouthful for a lyricist.
The universe is made of the aforementioned particles plus taus, muons, three types of neutrinos and four more types of quarks. That’s the Standard Model of particle physics, the science determined to cut us all into as many tiny pieces as possible to discover what we’re made of. By now you’re probably scratching your head and wondering: how on earth did we end up with so many particles?
Everything we can touch (skin, rocks, tables) and some things we can’t touch but can feel (air) and some things we can’t do either but can at least see (stars, planets and spaghetti monsters) are made of atoms. Atoms have been around for a long time. They were first theorised in Ancient Greece and India and formalised by John Dalton and others in 1808. Later the atom, at first thought to be indivisible, became a tiny nucleus of protons and neutrons, surrounded by a lot of space and a cloud of electrons.
Then things took a turn for the strange. In 1964 Murray Gell-Mann and George Zweig independently dreamt up three new, smaller particles in an Inception-like shared dream. These particles make up protons and neutrons and have properties called ‘flavour’ and ‘colour charge’. They’re called quarks, which logically rhymes with storks.
Quarks illustrate a crucial point of particle physics: some particles can only be observed at energy levels higher than everyday life. Physicists use ‘observation’ and ‘existence’ interchangeably because if a particle isn’t observed it may not exist. The first generation of quarks are called ‘up’ and ‘down’, and these really are the building blocks of matter, forming everything we can drink, smoke and bang our heads against. The next quarks are called ‘charm’ and ‘strange’, which suggests the physicists were on something a little harder and watching Disney movies. The third and final quarks are called ‘top’ and ‘bottom’, and we shall leave the physicists to whatever they were doing when they thought of that.
Stuff doesn’t just sit there. It moves, falls, and bounces; emits light, heat, and radiation; and sometimes it explodes. That’s due to the forces that act on matter. There are four forces: electromagnetic, the weak and strong nuclear forces, and gravity. Electromagnetism allows lots of things that are useful to us, like: microwaves, radio, TV, electricity, magnetic fields. The strong nuclear force holds together protons and neutrons and blows things up, and the weak nuclear force causes radioactive decay and blows things up. Forces are conceptualised as fields (as in magnetic field), mediated and acted upon by particles. Electromagnetism is mediated by photons, the strong nuclear force by gluons and the weak nuclear force by two types of W bosons and one Z boson. All of these ‘force-mediating’ particles are known as bosons, as opposed to fermions, which are is what the hard stuff is made of.
On a side note gravity was, and still is, something of a mystery. It describes the way in which things with mass attract each other, nutted out in complex mathematical detail by Newton and reimagined by Einstein. This is called General Relativity, a theory of everything big in the universe. The only problem is that General Relativity completely breaks down at the level of particles. And the Standard Model is not so good at describing anything larger than an atom. Getting the two to mix in a Theory of Everything is the enduring problem of physics.
The ‘Higgs’, as it is known by those who are jiggy with it, was once again theorised in a shared dream. This time it was three different groups in1964; one of these dreamers was Peter Higgs. It was the particle to end all particles, the particle that would complete the Standard Model. The problem was that particles in the Standard Model don’t inherently possess mass. It’s a property that’s given to them by something else. Particles pass through a field called the Higgs Field and in doing so collect mass. The actual relationship between the field and the particle is mediated by the Higgs Boson.
A convenient metaphor for this is two people (particles) passing a ball back and forth between them (the Higgs Boson). The faster they pass the ball, the stronger the force, the greater the mass. Some particles like photons are completely un-co, drop the ball, and don’t gain any mass at all. The Higgs does this to all particles in the Standard Model, thereby creating mass. There’s another great metaphor involving Margaret Thatcher at a dinner party, but I’ll leave that one for you to google.
To find a Higgs boson in real life you have to get creative and spend a lot of money. Particles like the Higgs don’t appear by themselves under the normal, low energy conditions. To appear they need an energy boost. One way to do this is to drive lower energy particles round and round a tunnel as fast as possible and then smash them together. The energy released in such a collision is enough to generate a Higgs boson. That’s what scientists have been up to at the Large Hadron Collider (LHC), the 27-kilometre tunnel under France and Switzerland.
It is rare enough for science to make the news, let alone the front page of a metropolitan newspaper, or become a topic of popular discussion on entertainment television, but the Higgs achieved just that. As far as the general population was concerned it was a wrap; the problem of the universe was solved. What actually happened was more ambiguous, confusing, and much more exciting.
High energy physicists sadly never get to see the fruits of their labour. They approximate by measuring mass. When the LHC spiked with a reading of 125-126GeV, the two scientific bodies CMS and ATLAS knew they were on to something. ‘GeV’ stands for giga-electron-volts, which is how the mass of things of such small stature is measured. If you thought a volt was a measure of energy, you would be right. At such small scales mass and energy are equivalent and there is no point in distinguishing between the two.
A mass of 125-126GeV fits nicely with the Standard Higgs. To confirm the type of particle, the physicists look at what happens in the nanoseconds after the particle forms, when it becomes unstable and decays into myriad other particles. That’s when things got interesting. A Standard Higgs should decay into bottom quarks, photons, W and Z bosons and taus (which are a higher generation electron). This decay forms those unnaturally perfect patterns that have become a symbol for particle physics. The problem was the ratios were slightly off. There were a few too many photons, and the number of taus was barely more than the background rate of tau generation.
It could just be an anomaly. Many scientists think so. So far only about 12 Higgs bosons have been observed. With more experiments they will not only measure mass and decay, but the spin of the particle (which is the real clincher). Others, however, are going back to the desk and working out elegant solutions that fit the new particle, hoping to explain some of the most profound mysteries of the universe.
One such mystery is dark matter. You may recall that all matter in the universe is made of protons, neutrons and co. That needs a clarifier. All visible matter is made of protons, neutrons and co. And visible matter accounts for but four per cent of the universe. Ninety-six per cent of the universe is made of something called ‘dark matter’ and ‘dark energy’—so-called because it is undetectable. It comes from General Relativity and the fact that the universe is expanding rapidly. No one knows what it is made of. The Standard Model almost completely fails at this point. That was until the new Higgs came along and a new possibility emerged.
One theory that physicists are very keen on is called supersymmetry. That sounds like a weird combination of high school geometry and Marvel, proving once again that physicists are hopeless at coming up with titles that make any sense to normal people. What it actually refers to is an extension of the Standard Model that predicts a whole suite of superpartners for every particle in the Standard Model. That includes five types of Higgs Boson. Possibly the Higgs that appeared in the LHC was not the Standard Higgs, but one of the Super Higgs incarnations. Another solution involves the presence of other superparticles. If a super top quark is present—known as a ‘stop’—the anomalies cease to be anomalies and become predicted occurrences. A super tau, or ‘stau’ (which sounds like an eye disease), does a similar thing. To cut a long story short, these particles might be dark matter and could go towards explaining the rest of the universe.
Physicists are forever on the search for the Theory of Everything. The Theory of Everything explains the laws of physics that allow a universe to go from nothing to what we see now, including protons, neutrons, electrons, forces and dark matter. The discovery of the Higgs is a step forward, but it’s not Everything. Not yet.