Could we eliminate any particles from the standard model of physics?

Have you ever wondered whether we strictly need all of the particles in the standard model of particle physics? If I were making a fundamental particle ingredients list for the most common forms of visible matter in the universe, I’d say we primarily need electrons, up quarks and down quarks. But this list completely ignores important elements of how matter works: how it sticks together, what happens when it gains or loses energy and whether particle identities are stable. For example, as atoms gain or lose energy, they emit photons – particles of light. So now we’re up to electrons, two types of quarks and photons. And we know quarks can change flavours – an up quark can become a down quark and vice versa. That is made possible by the weak nuclear force, through interactions with three particles called the W+, W- and Z bosons. In these weak nuclear interactions, neutrinos are emitted – almost always as electron neutrinos. So that’s eight key particles, for anyone keeping track. The W and Z bosons are force carriers, just like the gluons I discussed in my last column. Gluons, which literally glue quarks to each other, are carriers of the strong nuclear force, and they make protons and neutrons possible. Electromagnetic interactions carried by photons also play a role in atomic structure. These force-carrying particles, rounded out by the mass-giving Higgs boson, are fundamentally different from what we might call “matter particles” because they tend to be so short-lived that we call them virtual. They exist only to do a specific job, and then they disappear. But we can’t do without them. Advertisement But there are particles that don’t seem necessary. Consider the electron’s two siblings, which are almost identical to it but much more massive: the muon is over 200 times more massive than the electron, while the tauon (or tau lepton) is more than 3400 times more massive than the electron. There are also four other types of quarks, in addition to the up and down: strange and charm, as well as top and bottom (also known as truth and beauty). In all cases, the less common particles are more massive than the more common ones. They are usually harder to make, more fleeting and therefore less likely to form stable forms of matter. That explains why most of the stuff we like to look at – whether it’s a painting or a star – is made of the stable stuff that doesn’t quickly fall apart. Most of these particles only come into existence through very high-energy collisions. For example, muons are formed when cosmic rays hit Earth’s atmosphere. Bottomonium, a combination of a bottom quark with its antimatter partner, is made when electrons are smashed into positrons in colliders – and was probably around in the primordial soup at the beginning of time. Even so, these unusual particles help make the more common stuff possible. The lepton family only works, mathematically, when it’s electrons, muons and tauons plus their three neutrino partners. All six quarks are necessary to explain the presence of the up and down quarks. They only work together. And studying the more exotic forms of matter can help us gain insight into the biggest questions of all. The Large Hadron Collider beauty (LHCb) experiment uses investigations of bottom and charm quarks to try to understand why there is more matter than antimatter in the universe – essentially, why there is anything rather than nothing. The researchers at LHCb are also creating new combinations of matter. Just this past March, they announced the discovery of something that we might call a charmed proton: instead of the proton’s usual two up quarks and one down, it has two charm quarks with a down. Which is to say, we are still learning about all the different ways our universe’s building blocks fit together, and that depends on understanding matter that, on first look, we don’t seem to need. For me, this is a cosmic reminder that, as Octavia Butler once said about predicting the future, we should count on the surprises.