What's the smallest particle we can detect?

Chris asks Stuart Higgens to zoom in on this question from Katie. 

Stuart - Do you know what it’s a really difficult question to answer. I had to go back to my particle physics undergraduate notes to check. It’s very difficult because in our current understanding of physics - the standard model of particle physics - we treat the fundamental elementary particles as point like. That means they don’t really have a size and that causes a whole issue of problems both conceptually, and almost potentially mathematically. What we can think of though in terms of these is what kind of size they interact with? So if we were to fire one particle into another, what’s the kind of area that they would interact with? And that’s called a cross section in particle physics.

In that case, something like a quark, which is the constituent of a proton or a neutron - very, very very, tiny. I’m going to quote John Butterworth, who’s a Professor at UCL, who said “it’s about 43 billion billionths of a centimetre.” So absolutely incredibly tiny. That’s the upper limit it could be so it could be smaller, or it may not even have a size at all.

Chris - So there you go Katie, so the answer is pretty small.