Introduction to Particle Physics Part 3: Order from chaos – the Standard Model arrives
Having thought our knowledge of the makeup of the universe complete, physics turned its attention elsewhere. Or at least, until something was found in the 1930s that shocked everyone: a new type of particle streaming in from space – the muon (created when cosmic rays hit our atmosphere). In the years that followed, even more strange particles were discovered – over 80 by the 1960s! It was even said that the Nobel Prize should go to the person who didn’t discover a new particle that year! And to top it off, antimatter was discovered in 1931 (which can annihilate normal matter particles in a burst of energy) and mysterious ghost particles called neutrinos (that can travel through a light year of lead) were seen in 1956. It was like the periodic table all over again, some called it the ‘particle zoo’, but there were tantalising signs of a new pattern in amongst the madness…
This all changed in 1964, when Murray Gell-Mann and George Zweig proposed that most of the particles observed could be built out of just three fundamental building blocks called quarks. Think of it like having 3 different types of Lego bricks – the particle zoo is then all the combinations you can build with those bricks. The three quarks were called: up, down and strange. At first, it wasn’t clear if they were just a nice way to understand the patterns, or if they were real physical objects, but over the following 10 years their existence was confirmed.
In the mid-1970s, a new theory was formulated to explain the fundamental building blocks of nature and the forces that affect them – the Standard Model of Particle Physics. Particles made of quarks are called hadrons (the most famous is the proton, made of two up quarks and one down quark). In addition to the quarks, there are particles called leptons (the electron is a familiar example), which differ from quarks in that they aren’t affected by the strong force. The Standard Model also explained that the fundamental forces are caused by the exchange of tiny particles we call bosons. Electromagnetism is caused by photons; the strong force is caused by gluons (which ‘glue’ the nucleus together), and the weak force is caused by the W and Z particles.
Over the following decades, 3 more quarks were discovered, the last being the top quark in 1995. The quarks and leptons seem to fit neatly into 3 ‘generations’ (the columns above), but we still do not know why. Could this structure be suggestive of a deeper theory, just like with the periodic table?
With the discovery of the Higgs boson (which explains why the W and Z have mass whilst photons and gluons do not) in 2012, the Standard Model is finally complete. But the story of Particle Physics is far from complete; we have learned by now that it’s dangerous to think we know everything…
(Next time: Beyond the Standard Model)