On the 4th of July 2012, it was announced by physicists at CERN (The European Organisation for Nuclear Research) that two experiments at the Large Hadron Collider (LHC) had discovered a particle that looked a lot like the Higgs boson, the key signature for the existence of the Higgs field. In a previous post, I explained a bit about what the Higgs field was, how it gave mass to other particles and what exactly was the difference between the Higgs field and the Higgs boson. What I didn’t say, was why any of us should care. Alright, let’s lower our sights a little, why should physicists care?
Why is the Higgs even necessary?
No-one has actually asked me this question, so I’ll ask it myself – why is the Higgs field necessary to give particles mass? Why can’t mass simply be an ordinary input parameter – a property of a particle that just is? After all, even after we’ve introduced the idea of the Higgs, we don’t know why different particles couple to it with different strengths (ie. Why the electron has a different mass from that of muon etc). The answer is inextricably linked to the question of how forces work at the particle level, so let’s first take a step back and consider an electromagnetic field (remembering from the previous post that ripples in fields correspond to particles, which in this case is the photon).
An EM field diminishes with distance from any source, but in principle its range is infinite. This property ultimately depends on the fact that the photon is massless (more on this and virtual particles at a later date). Electromagnetism was the first force to be understood in quantum field theory (and remains mankind’s greatest intellectual achievement). When physicists eagerly tried to apply the same techniques to the weak force (the force responsible for radioactive decay), they hit a problem – the weak force is not infinite, it actually has a very tiny range (about 1/1000th the size of a proton). The implication of this was that the particles responsible for the weak force couldn’t be massless, they actually had to be (in relative terms) very heavy. This may not sound like a problem to you, but for physicists this was a nightmare. Massive gauge bosons (as they’re known) cause hellish problems for the mathematical consistency of quantum field theory; the probabilistic interpretation goes all out the window and nothing makes sense any more (inasmuch as quantum theory has ever made sense). This really becomes a problem as we start looking at processes involving energies greater than the mass-energy of the weak bosons, at which point the theory really breaks down.
Alright, big deal, so your “perfect” theory just turns out to work for some limited energy range; it get replaced by something more fundamental at higher energy.
Well, quite. This is where the Higgs comes in. By giving the bosons mass via the Higgs mechanism, we’re not really giving them mass, we’re just giving them the appearance of mass. Put differently, while they may have mass at low energies, by the time we start probing the troublesome energies where our predictions blow up, the Higgs field is no longer constant and the appearance of mass disappears.
So that’s why force particles need the Higgs boson; what about matter particles? A similar problem plagues them. What we think of as one single electron actually consists of two different particles – we call these different particles “left-handed” and “right-handed” electrons. If the electron were massless, these two particles would remain completely distinct, but the presence of mass causes them to turn into each other, so you can’t be sure which one you’re looking at. The problem with this is that only the left-handed version feels the weak force, so it can’t just change into a right-handed one without causing some serious trouble (this is a weak version of ordinary conservation of charge). What is needed is for the Higgs to swing to the rescue once more. The Higgs field feels the weak force in the same way as a left-handed electron. By interacting with the Higgs field, a left-handed electron can turn into a right-handed one if the Higgs carries away the “weak charge”, and then gives it back when the right-handed electron turns back into a left-handed one. Thus, we get the appearance of mass.
So why’s everyone so excited about finding the Higgs?
Higgs phenomena were first elucidated almost fifty years ago (and, controversially, by more than just Peter Higgs). It wasn’t long before a specific model was proposed for how the Higgs field might appear in nature, a model that met with such success that it quickly became the standard model for particle interactions (and was soon regarded as so prestigious and important that it became capitalised to Standard Model). By the late seventies and early eighties, Nobel prizes had been awarded and predictions confirmed by experiment. The last matter particle (the “top quark”) was discovered in 1994 [edit: silly me, it was actually the “tau neutrino”, finally discovered in 2000], leaving just one piece of the puzzle outstanding – not just any piece, but the crucial element that tied it all together and formed the basis of the most successful theory we’ve ever had. It is perhaps wrong, though, to say that that missing piece was the Higgs boson. What was really missing was a proper understanding of the nature of the Higgs field and whether or not it would have an associated particle. A great many rival theories have been proposed in the intervening years and now, like athletes training for years before finally competing at the Olympics, they are facing their hour of judgement before the LHC. Most, if not all, will fail. The next few years will tell us which theory will win gold.
More generally, to probe the Higgs energy scale is to knock on the gateway to a new world of physics. We know that the Standard Model, for all its glorious success, is not complete. Gravity, while dominant on astrophysical scales, is utterly irrelevant to particle physics, being 1032 times weaker than any other interaction. Why is there such an enormous disparity? Is supersymmetry realised in nature? Neutrinos only appear to come in left-handed versions, so where do they get their mass from? Where does the structure come from in the periodic table of elementary particles? What is dark matter, the invisible material which makes up 84% of the matter in the universe? What is dark energy, the even more mysterious substance that accounts for 73% of the energy in the universe? Not all of these questions will find their answers in studying the Higgs field, but by probing that energy regime, we hope that nature has left at least some crumbs of information that will guide us towards the next stage of our understanding of this awfully big universe. We have never faced more compelling and fundamental physical mysteries than right now – and with the LHC, we have never before had a machine better equipped to help us solve them.
To answer a couple of questions that have come my way…
Why do some particles interact with the Higgs and others don’t?
The only massless particles we know of are the photon and the gluon (the particle associated to the strong nuclear force). At some point, I may talk about electroweak unification (the way in which it turns out that the weak force and electromagnetism are two parts of the same thing) – suffice to say for now that in the process of separating the two forces, the photon gets spewed out and stops interacting with the Higgs. The gluon, on the other hand, does not interact with the Higgs because the Higgs field, like the electron and the neutrino, does not have any “strong” charge (I appreciate that that is something of a tautology). As to why these particles don’t have any strong charge – that is an open question…
The Higgs gives particles mass and gravity is caused by mass… so presumably they must be connected, right?
Not in any direct way, as far as we know. As I noted before, the hierarchy problem is the name given to the problem of why gravity is so much weaker than any other force and this is inextricably linked to the Higgs (specifically, gravity becomes significant at energies about 1017 times greater than the Higgs scale, but the two scales should naturally be the same, so what’s going on?). Nevertheless, the Higgs should not be confused with the graviton (the hypothesised particle associated to gravity). Moreover, it’s not mass that causes gravity, it’s energy (remember E=mc2; energy and mass are equivalent) and that is a far more general property than just mass. In fact, it’s more complicated than that: gravity actually feeds on a jambalaya of energy, momentum and pressure called the stress-energy tensor. It’s this more complicated behaviour which allows dark energy to have an apparently repulsive gravitational effect. More on that, perhaps, later…