Jon Butterworth’s book Smashing Physics is available as “Most Wanted Particle” in Canada & the US and was shortlisted for the Royal Society Winton Prize for Science Books. Also (10 April) fixed the same error in the sentence before that! Apologies – and thanks to Masahiro Kuze, one of the authors of the paper, for spotting the mistake. The data tell us that the radius of the quark is smaller than 43 billion-billionths of a centimetre (0.43 x 1016 cm). The original article said the quark radius limit is 20 times smaller than the proton, but it is 2000 times smaller I forgot they used cm not m. It is, as one might expect, very small indeed. Note added 8 April: I messed up my units, sorry. Quarks (along with electrons) remain the smallest things we know, and as far as we can tell, they could still be infinitely small. That’s 2000 times smaller than a proton radius, which is about 60,000 times smaller than the radius of a hydrogen atom, which is about forty times smaller than the radius of a DNA double-helix, which is about a million times smaller than a grain of sand. The data tell us that the radius of the quark is smaller than 43 billion-billionths of a centimetre (0.43 x 10 −16 cm). It is, as one might expect, very small indeed. Having spent a long time understanding their data – in particular, in understanding how the quarks and gluons are distributed inside the proton – the ZEUS experiment has just released a new limit on the size of the quark. Unexpected changes in the cross section could be a sign that we are beginning to see a finite (non-zero) size for the quark. That’s just the theory though, we always want to check with an experiment, especially given how many gaps the Standard Model leaves in our understanding of fundamental physics.

How small is a quark