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 My fridge contains only an empty tub of hummus and an out-of-date bottle of chilli sauce.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 (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.

In 1900, shortly after the electron and radioactivity were discovered, Lord Kelvin famously remarked:

“There is nothing new to be discovered in physics. All that remains is more and more precise measurement”

He would be proved horribly wrong. The discovery of the nucleus and then its constituents, the proton and neutron, revolutionised our view of what the world was made of. Our understanding of the world changed from the classical to the quantum and up to 1933 quantum mechanics went from success to success in describing experimental observations. This culminated in the Dirac equation, which predicted the existence of anti-matter, confirmed shortly afterwards by the discovery of the anti-electron (the positron). However, the physicists’ smugness was short-lived. Behind the scenes, all was not well. Quantum Mechanics was struggling to provide an explanation for particles that were raining down on earth from the cosmos at a rate of 10,000 per minute per m2. A veritable who’s-who of physics luminaries were trying to understand the nature of these “cosmic-ray” particles. Since at that time the only known particles were electrons, protons, neutrons, photons and (yet to be directly detected) neutrinos. It was assumed that these cosmic-ray particles arriving at the earth were electrons.

The problem with this (wrong) assumption was that the “electrons” raining down on the earth seemed to come in two varieties –1. those which were easily absorbed by blocks of lead and which created a secondary shower of electrons, positrons and photons when they interacted with the lead and 2. those that penetrated the lead blocks with aplomb.

At first, quantum mechanics had no explanation for why electrons should behave in either of these ways, but gradually the theory was modified (notably by Bethe, Carlson, Heitler and Oppenheimer). They found a way to describe type 1 (the “electron” that showered in lead) but, alas, they had no such luck finding an explanation for the type 2 penetrating particles. Theoretical physicists (having enjoyed so much success up to then) were in despair. Oppenheimer, always a bloke for adding a bit of gravitas to the situation and who generally preferred his glass half empty, wrote to his brother in 1934:

Oppenheimer: nicotine fuelled Quantum Mechanics.

“As you undoubtedly know, theoretical physics – what with the haunting ghosts of neutrinos, the Copenhagen conviction, against all evidence, that cosmic rays are protons, Born’s absolutely unquantizable field theory, the divergence difficulties with the positron and the utter impossibility of making a rigorous calculation at all – is in a hell of a way”

Quickly, the idea that the penetrating particles were protons was dismissed and the physics community was faced with a stark choice: a new particle or the acceptance that quantum mechanics was hopelessly flawed. For a time (now conveniently overlooked) they fudged the issue and started to speak sotto voce about the possibility of “red and green electrons” – one type being absorbed and the other penetrating.

Thankfully, the inspired development of new experimental techniques by teams in Europe and the USA meant the experimental observations of the penetrating particles became more precise. These experimental innovations (combined with new breakthroughs in the theory) allowed an interpretation that led to the inescapable truth: the penetrating particles were something like an electron, but significantly heavier.

The particle was originally given the name the “mesotron”. As is often the case in science, there was not a “Eureka moment” of discovery, but a gradual dawning of a new paradigm through the work of many people, both theoretical and experimental. Anderson got the credit (having already bagged a Nobel Prize for observing the positron, it was probably an easier sell..) but there was a considerable dramatis personae – Bethe, Heitler, Rossi, Neddermeyer, Street, Stevenson, Carlson and Oppenheimer – without whose contributions the “mesotron” would not have been discovered. The “mesotron” was quickly renamed the muon, and it became clear that the muon wasn’t a red or green electron, since if it were just a heavy or a more energetic electron it should decay to an electron and a photon, and this was not observed. The muon appeared to be its own distinct particle and so the muon (after the electron) was the second fundamental particle (i.e. one that doesn’t seem to be made of other particles) to be discovered. Its discovery thus heralded the start of particle physics as a subject.

Muons - I felt nowt

There are several hundred muons going through your head every second. Fortunately, their low energies (and high mass) mean they are harmless. These muons originate from the collisions of cosmic-rays (primarily protons spewed out by stars) with the atoms in our upper atmosphere. After their discovery it was observed that the number of these muons decreased as you got closer to the earth and the natural (and correct) conclusion was that they were not stable particles like the electron but a bit fly-by-night (and day), and they decayed to other more familiar particles (electrons and neutrinos) in about 2 millionths of a second. At this point it was known what the mass of the electron was and neutrinos were assumed massless, so by looking at the trajectory and energy of the electron from the muon decay (or measuring the time it took for the muon to decay) it became clear the the muon was a bit of a porker. It weighed in at about 200 times the mass of the electron.

The muon is not a great impresario and has a rather restricted repertoire best suited perhaps for a fleeting appearance on X-factor. In the last 75 years we have observed it do only two things: interact and produce a neutrino or decay and produce an electron and two neutrinos. But, we believe the muon will have the last laugh and is more than a two-trick pony. The muon has something up its sleeve which will help us understand physics at energy scales well beyond the LHC. Pertinently, we believe that it will reveal a new type of fundamental interaction (in addition to the ones we know (the electromagnetic, gravitational and weak and strong nuclear interactions) that can help explain one of the longest standing problems in physics: how was the majority of the anti-matter created in The Big Bang hoovered-up (or presumably Dysoned-up these days) at the start of the Universe, in the time it takes to make a cup of tea?

We are planning to produce a beam of muons of unparalleled intensity to observe this new type of interaction (which I’ll describe in the future). Indeed, after a bit of a slack period, this is an exciting time to be a muon and their penetrating and magnetic properties are being exploited for a range of applications beyond particle physics. They were used in the 1960s to X-ray the pyramids to search for hidden chambers and recent advances in charged particle detectors have opened up the possibility to utilize cosmic-ray muons to precisely image very large volumes (sea containers, cargo vehicles, train stations, etc.) to detect bombs, fissile material or things that go bang in the night.  Muons are being used to study the properties of new compound materials that have the potential to provide novel semiconductors for the electronics industry or room-temperature superconductors with a diversity of applications from levitated trains (could be tricky on The Northern Line) to lossless power transmission. Recently a beam of muons produced at the UK ISIS facility was used to observe the phenomena of “magnetricity” in “spin-ice” which is potentially the first step towards a magnetic version of electronics.

So stay tuned, the muon is a plodder but its 15 minutes of fame is nigh.

It is a beautiful thing that Peter Higgs did. He took a very complicated description of reality that explained some things so well it could not be ignored, and utterly failed to explain other things, things that we couldn’t ignore. He didn’t ignore the fact that the standard model was rendering all things massless, which would mean we could not possibly exist.

Higgs: something to scrub for.

He allowed us to keep the symmetry we obviously need, and he just gave it a little shake. He perturbed it. He took the maths that describes a ground state of something we see in mathematics, our gate to reality, and he shook it slightly and then it gave us all(ish) of the answers.

The Higgs mechanism may well be wrong, but if it is, I think it is still something. Something beautiful. I want to know what is really going on as much as anybody I can think of. Lots of us do. The truth is that 95% of our brain power is directed towards working out how wrong we might be, calculating the uncertainty in every measurement and combining these calculations in such a tedious way as to put most undergraduates on a direct road to the city: the wonderful world of hedge funds, nice shoes and restaurants in Soho.

Having the Higgs in the room is, for me, like having a fresh bunch of flowers on the table from a new lover. It is a pleasure to do housework around such a thing. It doesn’t even feel like work.

I started writing about neutrinos because I love them. They are quite magical really; the Universe is completely swarming with them (they are the second most abundant particle after photons) but we know practically nothing about them. And what we do know is really weird. I wrote down “My favourite particle: the neutrino” to fit in with the other particle articles. But it isn’t my favourite particle. I toyed with “Not my favourite particle: the neutrino” but that just felt disrespectful. Then I decided it doesn’t matter what I write at the top of the page. Why do I need this post to fall into line with the others? I suppose it’s because I want symmetry. I want that warm, comfortable-yet-exciting feeling you get when starting to read the next book in a trilogy.

The neutrino was postulated (imagined-up) for a similar reason: the desire for symmetry.

A hundred years (ish) ago there was lots of great experimental science going on. There was no clear idea what atoms were, but there was an understanding that they weren’t solid balls. Ernest Rutherford and others had established, through experiments, that there was a nucleus in the center of an atom and had postulated that electrons were ‘in orbit’ around this nucleus. He came up with this description based purely on his experimental results: there was no theory at the time that predicted this.

I feel quite nostalgic for this sort of experiment-driven theory. In the era of the LHC it is not the done thing to come up with a new theory to describe what we see experimentally. In general, our results have to fit some theory that has already been proposed. When they don’t (they don’t) we tune the theoretical predictions to match our data, like twiddling a load of knobs (most of these theory predictors, which we call Monte Carlo, have twenty or thirty knobs) until we get some agreement. I am moaning about this, but really we don’t have a choice.

Anyway, back to the good old days.

Something called Beta decay had been observed. They had a name for it before they knew it for what it was. It was a nice bit of real-world, useful, observational science. It was noticed that some atoms would emit radiation without any provocation to do so.

This is what we call radioactivity. Active radiation. Nothing to do with the radio, other than that radio also works via radiation. Radio is carried by electromagnetic radiation (my first love- the photon). What radiation is is the loss of energy by something. That energy invariably travels somewhere else in some form or other. But the thing with radioactivity is it is not deliberate. It just happens “naturally”.

When first discovered it triggered a whole host of money-making schemes, similar to modern-day homeopathy I suppose, except homeopaths make their millions out of plain water and packaging, whereas back in the day the quacks were raking it in by selling people radioactive face-cream. The scientists rebelled, lots of people got cancer and the business end closed. I laughed at these adverts and showed my mates, and then I felt really sad because I thought about people in the future (okay, the present) laughing at my friends for buying 0% proof hogswart.

Anyway (again), the science continued, and we realized that what was happening was that part of the atom, in fact part of the nucleus, was ‘decaying’.

It was established that the radiation being emitted was electrons, because the measurement of the emitted particles’ charge and mass was exactly the same as for electrons. The electrons weren’t being thrown ‘out of orbit’, they were being emitted from the nucleus. This is interesting, because the atomic nucleus only contains protons (with a charge of +1) and neutrons (with a charge of zero). To get a negatively charged particle out of this we thought that the neutron must decay into a proton and an electron: zero charge= +1 and -1. Great. This fitted nicely with the experimental measurement of the atom before and after it went through Beta decay.

But this is where things stopped being comfortable. This picture predicts precisely the energy that the emitted radiation (the electron) must have. Depending on what kind of atom is doing the decay, we should be able to predict exactly how much energy the electron has.When they measured the energy carried by these emitted electrons, they found that the electrons could have any energy. This was a huge problem because conservation of energy was (and is) the closest we get to feeling comfortable with any knowledge in particle physics. It always works. We don’t have to appeal to the fact that the theory is beautiful and elegant, we just do cold, hard experiments. And warm, soft ones. Any kind of one. We always get the same answer: energy is conserved. Always. But not in radioactivity.

Wolfgang Pauli wasn’t having any of it. There must  be something else emitted, something that the instruments were not detecting. Pauli described the properties that this invisible particle must have; It is neutral (no charge), it has a small (or zero) mass and it has spin half like the electron. If the new particle was emitted along with the electron, then the electron could have any energy, because the total energy emitted could still add up to be the exact value predicted by the theory. The new particle would have to be a ghost, traveling through matter without interacting at all. Then we could explain why it was not detected.

Of course this was not a comfortable place to be, but it was slightly less uncomfortable than violating the law of conservation of energy.

Pauli was aware of his responsibility, famously saying  “I have done a terrible thing. I have postulated a particle that cannot be detected”.

Wolfgang Pauli: he knew his stuff.

Is it okay to make a prediction that we cannot test? Science is about trying to describe the way the world works; our understanding should be based on what we observe and our theories should be testable. This is why some experimentalists laugh at string theory. I for one am happy to ignore it until someone comes up with a way for us to test its validity. Okay, some of the reason I ignore it is because it is hard, but I would be much more inclined to make the effort if I thought it was any use.

Thankfully, the little neutral ones, neutrinos, can be detected. It is just very, very difficult to do. This is because they simply do not interact with the particles that make up matter. Neutrinos can fly past an atom as if it were not there. They are neutral, so they are not affected by electromagnetic fields. The only thing in nature they have anything to do with is the weak nuclear force, so they tend to just whizz through the universe, through planets and through us, leaving no trace.

We do detect the odd one. The first was 25 years after Pauli’s prediction, by Cowan and Reines. Back in 1987 we saw a whopping 24 of them in just a few seconds, thanks to a supernova going off in the vicinity. But generally they are a bugger to catch.

So what do we know about neutrinos now? Almost nothing, but the little bit we do know is very interesting indeed. The standard model of particle physics (the theory that we are most happy with when putting together all the little bits of knowledge we have gathered from thousands of experiments over a hundred years) tells us that neutrinos have no mass, like photons. We now know that this is not the case. Neutrinos do have mass. We know that the three kinds of neutrinos must all have different masses, so at least two of them cannot be zero. I feel like nobody really talks about this. Perhaps I hang out with the wrong kind of physicists. Neutrino experiments have shown the standard model to be deeply flawed, yet we still persist in calling it “the standard model”.

I have finished writing this now without actually saying a single thing about why I love the neutrino. I haven’t mentioned that they are able to change flavour (neutrino oscillations) or that they could be their own antiparticle (are they Majorana or Dirac particles?) or that they can help us understand dark matter, or that the fate of the universe is in their hands.

Each one of these things is completely deserving of its own post, written by someone who knows their onions and is able to write something that is not 90% digression.