My three least favourite quarks

Also at The Guardian.

Quarks are what dragged me into this mess. I’ve done about 50 hours work in the last 4 days and have run out of clean cutlery. My fridge contains only an empty tub of hummus and an out-of-date bottle of chilli sauce. I last saw my daughter on Tuesday, scrabbling around in the forest all covered in dandelion feathers, looking for edible foliage.

I found about quarks when I was an undergraduate at UCL, in the first term of my first year, when a wonderful lecturer called Andrew Fisher gave us a course called modern physics. He was (*is*, I’m sure) one of those unusual lecturers who tells you things you remember years later, as opposed to seconds.

I found a book in the science library that gave me what I craved: a table of the quarks and all of their properties: charge, mass.. then weird stuff. Some of them had distinctly worrying quantum numbers. Strangeness? Yeah, right. But it was still there when I woke up in the morning. Some quarks had strangeness. Obviously I had to look into this further.

It turned out that the quantum `number’ strangeness is exactly what it sounds. Some people were taking data from some very primitive cave-man style detector, and they found that some particles were behaving strangely. Being a physicist, okay, being a particularly `special’, creative, brilliant person (Murray Gell-Mann) this strange behavior was assigned a measure. Strangeness, of course.

The strange behavior turned out to be down to the strange quark.

So, I’m starting out with the strange quark because there was no real concept of quarkness before that. We knew that the proton had structure through all sorts of wonderful experiments which are now known as deep inelastic scattering, or DIS for short. The deep means that we are firing things at the proton that go deep, further than its ‘surface’ which is actually nothing more than a spherical(ish) force field. The ‘inelastic’ means that it doesn’t just bounce. Inelastic means that something has to break up.

The first DIS experiments told us that there was something going on inside the proton, it wasn’t smooth or solid all the way through. If you could get something (an electron) through the force-field that gave it a kind of surface, then you could clearly see that there was something inside. The electron would bounce of the innards of the proton in all sorts of directions, and when we measured these very carefully and added them all up, or integrated over them all (same thing) we thought that there almost certainly isn’t a ‘hard center’ to this thing. It is made of parts (three parts in the case of the proton), but it doesn’t have a center.

We had the parton model: partons were the things that lived inside protons and neutrons, which in turn lived inside atoms.

But that was it really, for a while. And its no wonder, when we think of the absolutely unimaginable mind-power that was required to take the next step.

Murray Gell-Mann having a little think, as potrayed by Toya Walker,

Murray Gell-Mann having a little think, as potrayed by Toya Walker

Murray Gell-Mann (his name should be pronounced Gell. Mann. Not Gellman, or Jellmann. He is a keen linguist and this is apparently important to him.) constructed a space in his mind that was not our usual way of doing this, which if we are honest consists mainly of measuring up a part of our living room with our eyes and wondering if that lovely old chesterfield in the skip down the road would fit, then going to get the tape measure. He decided that one direction (say width) would be strangeness and depth would be another thing he constructed, called isospin, and height would be good old simple charge. When he put the all the newly discovered particles on this 3-D graph where you and I would put our sofa and it probably wouldn’t quite fit, they not only fit but fell into a sort of pattern. Well now that all sounds a bit hippy, because I’ve explained it all in a rather vague way and we all know we can make patterns appear meaningful when they are not.

But he really did have something. He looked and listened, and he noticed that these three things: strangeness, isospin and charge, seemed to be able to classify this new world they he was observing pretty well. If there were three of these quarks (up, down and strange), then he could explain all of the new particles’ properties by combining them in various ways. It took a while for anyone to be convinced, of course. Murray was at CERN watching a talk on recent discoveries when he realized that the latest particle filled a hole in his diagram that made it very unlikely that there was a better way of explaining these proton-innards than his way.

He called it the quark model. Quark should be pronounced “kwork” not “kwark”. He named it after a line in a book by James Joyce called Finnigan’s wake. “Three quarks for muster Mark”. I know, Mark should be rhyme with quark, but Murray felt it was kwork, and in fact if you are from that part of the country where you pronounce mister muster then you probably pronounce kwark kwork too.

So there we have it: the first three quarks: up, down and strange. There are in fact six quarks now, but the others are much more exciting and deserve heir own stories. All of matter is made of the first two and electrons. Every single atom in the Universe has nothing in it other than some combination of up quark, down quark and electron. The strange ones inhabit particles that only exists transiently, before decaying to something stable that contains only normal up and down quarks.

So I will leave you with that: strangeness is not conserved.

The picture for this post was provided by the magnificent Toya Walker.

The link for deep inelastic scattering takes you to the wikipedia page. I don’t agree with the second sentence on this page regarding the reality of quarks, but the www isn’t exactly awash with explanations of DIS that won’t make most people want to die, so wikipedia will have to do.


About lilyasquith

I am a particle physicist working on sonification of the data output from the ATLAS detector at CERN. The project I'm working on is called LHCsound and is funded by the STFC. It is based at University College London where I have just finished my PhD on the search prospects for a low mass standard model Higgs Boson. I have now moved to Chicago to start an ATLAS postdoc with Argonne National Laboratory.
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