by Marcus Chown
A quick word on what a boson is. In nature, particles can carry an intrinsic, or ‘quantum’, spin, and it can only be a whole-number multiple of a basic spin (such as 0 or 1) or a half-integer multiple of that spin (such as ½ or ). The first type of particles are known as bosons and the latter are fermions. Force-carrying particles such as photons and gluons are bosons, while building-block particles such as quarks and electrons are fermions. There is a profound connection between the spin of particles and how they behave en masse. According to the ‘spin-statistics theorem’, two bosons with identical properties can be in the same place at the same time, but two fermions cannot. This is why photons are happy to travel together in their countless quadrillions in a laser beam, whereas electrons do their best to avoid each other. It is this unsociability of electrons that explains why they occupy separate orbits in atoms, thus making matter extended and solid bodies possible.
In the theory of Weinberg and Salam, the electromagnetic force exists to maintain a local gauge symmetry known as U(1); basically, to keep a number – the ‘complex’ phase of a quantum wave of an electron – the same at every point in space–time. The weak force exists to maintain a slightly more complex symmetry known as SU(2), which involves a 2 × 2 matrix that is similar to, but not quite the same as, the one used by Paul Dirac in his celebrated equation (see chapter ‘Mirror, mirror on the wall’). Together, the two symmetries are known as U(1) × SU(2). They mix with each other, as was also realised by Glashow, so the electroweak force carriers turn out to be the photon and the Z0, which arise from mixtures of the two symmetries, and the W– and W+. The Z0 is nothing more than a massive photon, which we can think of as ‘heavy light’.
Since the gauge force carriers exist to enforce local gauge symmetry, they are all massless. This would have been the case in the super-high-energy conditions in the earliest moment of the Big Bang. Enter the Higgs mechanism, and the Higgs field in this situation is a little more complicated than the one illustrated with the sombrero hat. It is a so-called SU(2) state with four components, which spawn four Goldstone bosons, or ‘Higgses’. Three are cannibalised by the W+, W– and Z0, in the process giving them masses. (The photon does not participate in this game and remains massless.) That leaves one Higgs boson with intrinsic mass – this is the ‘left-over’ particle whose existence was predicted by Peter Higgs in August 1964.
By July 2012, strong evidence of all the fundamental particles of the ‘Standard Model’ – the quantum field theory of the three non-gravitational forces – had been found in experiments.15 They include six quarks, known as up, down, strange, charm, bottom and top; six leptons, known as the electron, electron neutrino, muon, muon neutrino, tau and tau neutrino; and the twelve force carriers. Of these, the photon mediates the electromagnetic force; the W+, W– and Z0 the weak force; and eight gluons the strong force.‡
It is not quite true that all the particles of the Model had been found. All of them had been found, except one: the Higgs.
Methodist Central Hall, London, 4 July 2012
On the screen at the back of the stage, the press conference at CERN was starting. The laboratory’s main auditorium was rammed full, with its steeply banked seats even more jam-packed than the cavernous Methodist Central Hall in London. Butterworth was already fielding questions from eager journalists, and to his frustration, was only able to glance intermittently at the giant screen.
At CERN, the spokespersons for the ATLAS and CMS experiments, Fabiola Gianotti and Joe Incandela, were setting the scene. The Large Hadron Collider is the most complex machine ever built. It fills a twenty-seven-kilometre circular tunnel beneath the Swiss–French border that is as long as the Circle Line on the London Underground. It was previously occupied by the Large Electron–Positron collider (LEP).
LEP was limited in the collision energies it could reach because whenever electrons and positrons are accelerated – something that happened when their paths were bent into a circle by the powerful magnets around the LEP tunnel – they broadcast electromagnetic radiation, which sapped them of energy. Crucially, however, such ‘synchrotron radiation’ is more of a problem for light particles than heavier ones. In fact, it depends on the ‘inverse fourth power’ of the mass of a particle and so, for protons, which are about two thousand times heavier than electrons, it is about ten trillion times less serious than for their lighter cousins. This is the reason why, when LEP was ripped out of the tunnel, it was replaced with a proton–proton collider: the LHC. (A ‘hadron’, incidentally, is any particle that experiences the strong nuclear force.)
Constraining the super-high-energy protons to circle their subterranean race track one hundred metres beneath the surface of Switzerland and France requires bending their paths with the strongest possible electromagnets, which requires their coils to be powered with the largest possible electric currents – 12,000 amps in the case of the LHC. Such high currents would normally generate huge amounts of heat, but the 1,232 bending magnets of the LHC, each fifteen metres long and weighing thirty-five tonnes, are formed from special ‘superconducting’ coils, which are cooled by liquid helium, the world’s best refrigerant. At –271.3ºC – a mere 1.9 degrees above the lowest temperature possible – the coils offer no resistance to an electric current and so dissipate no heat while they remain superconducting. However, in tests shortly after 10 September 2008, when the LHC’s proton beams were first switched on, a connection between two magnets lost its superconductivity. This led to a spark that punctured the twenty-seven-kilometre-long cooling vessel – the biggest refrigerator ever built – causing an explosion as the escaping liquid helium rapidly turned to gas, leading to extensive damage to 750 metres of the magnet ring. The accident set the programme back by more than a year.
But since restarting in November 2009, the LHC had worked without a hitch. At ATLAS and CMS, collisions between protons travelling at 99.9999991 per cent of the speed of light recreate, for the briefest of instants, conditions that last existed a hundredth of a billionth of a second after the birth of the universe, when the temperature of the Big Bang fireball was about ten million billion degrees. From the energy of the collisions – not strictly speaking between protons but between their constituent quarks and gluons – are conjured ‘jets’ of quarks and gluons. They spawn exotic particles, which live for the tiniest slivers of time before transforming into yet more subatomic debris. The hardware and software is primed to filter out the majority of events from this bewildering subatomic mayhem, leaving only the rarest of rare signatures the physicists are looking for: the signature of the Higgs.
If created, the elusive subatomic particle is expected to survive for too short a time to be directly detected; the trick is to look for particles into which it decays, which are distinguishable from particles generated by a myriad of other confusing ‘background’ processes. ATLAS looked for rare pairs of photons generated by the decay of W+ and W– particles, in turn spawned by the Higgs. CMS looked for rare pairs of Z0s, also from the decay of a Higgs. The physicists working on the ATLAS and CMS experiments were kept in the dark about each other’s progress as far as possible. What CERN wanted more than anything was one experimental result confirmed by a second, entirely independent, result.
Butterworth knew roughly what Gianotti would be saying because he had been at CERN’s Salle Curie conference room the day before, when she rehearsed her presentation in front of her colleagues. Nevertheless, he was keen not to be distracted when her presentation reached its crescendo. Thankfully, however, the mounting excitement in Switzerland had spread to London. The journalists in Methodist Central Hall had fallen silent and all eyes were on the giant screen.
Gianotti was showing a graph. It showed a hump at 126GeV, roughly 126 times the energy required to create a proton. It was exactly what would be expected if the decay products seen by ATLAS and CMS came from a particle which existed for the most fleeting of instances. A new particle, hitherto unknown to science. Gianotti said the magic words: ‘For both expe
riments, the discovery has now reached the “five-sigma” level of confidence.’§
At CERN, there was pandemonium. The audience, which for the past half-hour had listened patiently to the technicalities, erupted into raucous applause and cheering. The TV picture panned to a flushed and smiling Peter Higgs, who had been invited to attend and was sitting squeezed in the middle of the auditorium. People all around him were congratulating him and reaching across to shake his hand. A modest man of eighty-three, he looked slightly bewildered. He was pushing up his glasses and appeared to be wiping away a tear. François Englert, a member of the ‘gang of two’, was also there. In a year’s time, the two men would share the 2013 Nobel Prize in Physics.16
In Methodist Central Hall in London there was also pandemonium. People were on their feet, cheering. Butterworth had entirely forgotten his feelings when he arrived at the building earlier. This was a piece of history, and even though he was 750 kilometres from Geneva, he was a part of it. He had thought he was working in an esoteric field of physics and that nobody in the wider world really cared, but the evidence was all around him that he was wrong. Everyone, no matter how much physics they understood, realised that this was a key moment in the history of science. A key moment in the history of the human race.
In the summer of 1964, a shy, modest man in an office at the University of Edinburgh added two sentences to a paper that had been rejected for publication, predicting the existence of a hitherto unexpected massive particle. Now, almost four decades later and at a cost of some five billion euros to construct and run the most complex machine ever built, there it was.17 Or was it?
‘Is this the Higgs, Professor Butterworth? Is this really the Higgs?’ came the question.
‘We have found a new and real particle,’ Butterworth replied, choosing his words carefully. ‘It’s consistent with the Higgs.’
‘But is it the Higgs?’
‘We think it’s the Higgs, but we need to do some more work to be sure. What we know is that it is a new particle. That’s what’s so exciting: a new particle!’
But that was not enough for the journalists. The question kept coming back. It was relentless, and Butterworth found it both funny and frustrating. ‘Have you found the Higgs?’
Butterworth was unwilling to go there yet. Just because something looked like the Higgs particle did not mean it was the Higgs particle. Butterworth and his colleagues needed to measure the properties of the new particle – its quantum spin and the precise details of its decay – to see whether it was the Higgs boson, as described by the Standard Model. But in his bones, in his heart of hearts, he knew. It looked like the Higgs. It smelled like the Higgs. At long last, they had found the Higgs.
*
The discovery of the Higgs was monumental. It is the last jigsaw piece of the Standard Model, the high point of 350 years of science. We have identified the fundamental building blocks of the universe and understand the forces that bind them together. Everything exists – you and me, digestive biscuits, snails, soap operas, giraffes, stars and galaxies – to enforce local gauge symmetry, one simple principle from which everything arises.
Nobody knows why nature has such a strong desire to enforce local gauge invariance. In the words of the great Italian physicist Enrico Fermi, ‘Before I came here I was confused about this subject. Having listened to your lecture I am still confused. But on a higher level.’ But the discovery of the Higgs is a dramatic confirmation of the power of science – its central magic. That people can see things in the mathematical equations they have concocted to describe nature and then go out and find them in the real world. ‘That equations written on paper can know nature, and that forty-eight years later experiments can prove this, is awesome,’ says Frank Close. ‘An overworked adjective but on this occasion justified.’18
The Higgs particle is unique in the Standard Model. It is the only elementary boson that is not a gauge particle – the only boson that is not a force carrier. It has no electric charge and no quantum spin. In fact, it is the first spin-0 particle ever discovered. The carriers of the electromagnetic, weak and strong forces all have spin 1, whereas the ‘graviton’, the hypothetical carrier of the gravitational force, is expected to have spin 2.
The Higgs, weighing in at 126 times the mass of the proton, is the heaviest subatomic particle ever detected. Being so heavy, it interacts most frequently with other heavy particles such as the top- and bottom-quark and the heavy tau lepton, and it appears to behave exactly as predicted by theory. There is no reason to believe that it will not interact as predicted with the lighter quarks and leptons. However, such interactions are rarer and it will take a lot more data to confirm them, just as it will to observe the Higgs interacting with itself.
The Higgs is not the ‘God particle’, as it was dubbed by Leon Lederman, but, to hijack the words of the Indian novelist Arundhati Roy, it is ‘the god of small things’.
But although the Higgs particle is important, it is merely a short-lived ripple of the Higgs field; its true significance is in confirming the existence of the field itself and in beginning to reveal its properties. The Higgs field was always the key thing, but it was a more esoteric entity to sell to the public than the prospect of discovering a new subatomic particle.
The Higgs field is something truly new. As mentioned earlier, every other field is zero in empty space. It may jitter a little because of quantum uncertainty, but it averages out to zero. The Higgs, however, is non-zero everywhere in space, and because it is ubiquitous, everything in the universe spends its life immersed in it. Until 4 July 2012, this was only a theoretical possibility. But now, because we have observed a ripple in the Higgs field – the Higgs boson – we know it is really there. And every fermion – every quark and lepton – interacts with it constantly. Like the W+, W– and Z0, they are intrinsically massless, but their mass depends on how strongly they interact with the Higgs field.19 What for centuries people have called ‘mass’ is now known to be a consequence of the interaction between the fundamental particles and the Higgs field.
And what of the Higgs boson itself? Well, it gets its mass – wait for it – by interacting with itself!
There is a proviso here. Although two separate mechanisms give particles mass – the devouring of the Goldstone bosons in the case of the W+, W– and Z0, and the interaction with the Higgs field in the case of the fermions (and even the Higgs) – these mechanisms turn out to be responsible for only about 1 per cent of the mass of your body. This is because its major building blocks are quarks, and the lion’s share of their mass is explained not by the Higgs but by Einstein’s special theory of relativity. Inside the protons and neutrons of atomic nuclei, the quarks fly around at close to the speed of light and, as Einstein showed, bodies become more massive as they approach light speed.¶
If the physicists had sold the LHC to politicians by saying it was going to find the reason for 1 per cent of mass, they probably would not have got very far. But that 1 per cent is of key importance because without it, the quarks and electrons in your body would be massless. This would mean they would travel at the speed of light and would be unable to settle into atoms. Everything would fly apart. Without the Higgs field, you and me, the stars and the galaxies, would not exist.
Of course, this is exactly the way it was in the earliest moments of the Big Bang. At the high energies that existed then, all particles were massless and travelled at the speed of light, interacting with each other in a completely different manner to the way in which they do in today’s low-energy universe. ‘The past is a foreign country: they do things differently there,’ as novelist L. P. Hartley observed in The Go-Between. It is our great triumph as human beings to have discovered this. It was the switching on of the Higgs field that made everything we see around us possible.
The Higgs field, being ubiquitous, may play some other as-yet-unsuspected role in controlling the universe. But even if it does not, its mere existence shows us that scalar fields are possible. Such fields, a
s already mentioned, have the key property that they appear the same to everyone no matter what their velocity, and therefore do not conflict with the requirement of Einstein’s special theory of relativity. The existence of the Higgs field raises the possibility that the universe may contain other scalar fields, and that these may explain some of its deeply puzzling features. For instance, the universe is believed to have gone through a brief phase of accelerated expansion, known as ‘inflation’, during its first split second of existence and, bizarrely, is undergoing a far weaker and more sustained form of accelerated expansion today, powered by mysterious ‘dark energy’. Theorists suspect that the former phase was driven by a scalar field which exerted repulsive gravity known as the ‘inflaton’, and that the latter may also be driven by a scalar field.
The truth about the Higgs field, however, is that we know that it exists but we do not know its origin, why it has a non-zero average value in empty space or whether it is actually fundamental. Conceivably, it could be a composite of fields like protons and neutrons, which are made up of three separate quark fields. The hope among physicists is that as they learn more about the Higgs, they will get new physical insights because, although the Standard Model is a triumph, there is so much about it that is arbitrary and mysterious.
Physicists do not know, for instance, why the fundamental particles have the masses they have – why are top quarks roughly a trillion times heavier than neutrinos? – and why the fundamental forces have the relative strengths they have. We do not know why the electromagnetic force is an extraordinary ten thousand billion billion billion billion times stronger than the gravitational force. And why are there three families of quarks and three families of leptons, with each generation more massive than the one before? Even more seriously, there is no place in the Standard Model for gravity or for ‘dark matter’, which is known to outweigh the visible stuff in the stars and galaxies by a factor of six. The Standard Model is an approximation of a deeper theory. And it is that deeper theory that everyone is desperate to find.