Question: Do you prefer to study quarks, leptons, or bosons? What's the most interesting fundamental particle?

Susan Cartwright answered on 18 Jun 2015:

Well, I’m a neutrino physicist, so I’m biased – obviously I prefer to study leptons, and I think neutrinos are the most interesting fundamental particle!!

Seriously, this doesn’t really reflect how particle physics works. Some of my colleagues who study the Higgs boson (yes, it’s a boson) do so by studying its decay into four (charged) leptons (e+e-e+e-, e+e-mu+mu-, or mu+mu-mu+mu-). So, although they are studying a boson, they are doing it by studying leptons, and they need to develop great expertise in exactly how leptons appear in the ATLAS detector and how to identify them accurately.

I will defend the claim that the neutrinos (there are three of them) are the most interesting fundamental particles that we currently know about (we haven’t discovered the dark matter particle yet, so we can’t really study it). All the particles in the Standard Model – even the Higgs – are behaving pretty much exactly as the original, mid-1970s, Standard Model expected them to – except the neutrinos.
– We thought in the 1970s that neutrinos were exactly massless. They aren’t.
– We thought in the 1970s that something that was produced as an electron-neutrino would always remain an electron-neutrino. It won’t – it might subsequently be observed as a muon-neutrino or a tau-neutrino.
– We thought in the 1970s that neutrinos and antineutrinos were different particles. This might be true, but it’s looking increasingly likely that it isn’t: neutrinos behave differently from antineutrinos because they have opposite spin, but deep down they’re fundamentally the same.
So, of the three things we thought we knew about neutrinos when the Standard Model was first constructed, two are definitely wrong, and the third is very probably wrong. You can’t say that about any other particle in the Standard Model – certainly not the Higgs, which is behaving exactly as we predicted it would. So that’s why I think the neutrinos are the most interesting fundamental particles. I don’t have a preference for one particular neutrino, but they all mix together anyhow!