The exacting work of mapping the subatomic world turned up all kinds of evidence that there are more things in heaven and earth than electrons and nucleons.
We discovered that we are bombarded with radiation from outer space, called cosmic rays. We saw hints of strange, almost invisible ghost particles (neutrinos), and the electron turned out to have a heavier brother (the muon).
Stranger still, we found out that we can ourselves make particles that exist for only a fraction of a second, and thanks to improvements in our measuring equipment, we learned more and more about the properties of these strange particles.
And eventually, someone discovered the underlying pattern concealed in all those measurements: the Standard Model.
I’ll give you an overview of the different ways that we particle physicists discovered new particles, using the analogy of a field biologist on an expedition. When a field biologist enters new territory in search for unknown animals he can discover new species in different ways.
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Suppose you’re a naturalist exploring a newly discovered island. Before you head off on your expedition, you can’t be sure you’ll discover anything new there. Maybe the plants and animals will be just the same as where you are from.
But once you’re on the island and you find an animal with strange features (say, a unicorn with wings) you don’t have to think long to realise that this is a new kind of animal you don’t have at home.
Still, before you can boast to your friends that you’ve discovered a new species, you’ll have to prove that the animal has unique traits. Now, in the case of a winged unicorn, that’s obviously not a difficult job.
That’s exactly what it was like when physicists first spotted the muon particle. They reacted with the same surprise: “Huh? What the heck is that?”
Looking at indirect evidence
As a field biologist, if you find tracks or hairs next to a watering hole that weren’t left by any familiar species of animal, then you know there must be a new species somewhere around. At that stage, you haven’t even seen it yet, but that doesn’t matter.
If it’s a very timid species, it could take a long time before you actually spot one. But even before then, you can find out many things about the animal.
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By studying its tracks, you can learn approximately how heavy it is and whether it lives alone or in groups. And its droppings can tell you whether it’s a carnivore or an herbivore.
This is exactly the procedure by which particle physicists discovered the neutrino particle. The particle itself was invisible, but the indirect clues left no doubt of its existence.
Fossils and remains
No one has ever seen a dinosaur, but thanks to their skeletons and other surviving fossils, we know they existed. If you go to a natural history museum, you can see those skeletons for yourself, and the many books on dinosaurs in the museum library will make it clear that we can figure out lots of things about the large, diverse range of dinosaur species.
In the subatomic world, many of the inhabitants, the elementary particles, survive for only a billionth of a billionth of a second. That’s much too short a time for us to see them, let alone study them carefully.
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But fortunately, when they vanish, they turn into particles that do stick around long enough for our detectors to see them. So we can reconstruct that there must have been a heavy particle around, just as we reconstruct facts about dinosaurs from their bones.
This technique has led to the discovery of hundreds of new particles—including the Higgs boson in 2012.