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- Herausgeber: Icon Books
- Kategorie: Wissenschaft und neue Technologien
- Serie: Hot Science
- Sprache: Englisch
The Higgs boson is the rock star of fundamental particles, catapulting CERN, the laboratory where it was found, into the global spotlight. But what is it, why does it matter, and what exactly is CERN? In the late 1940s, a handful of visionaries were working to steer Europe towards a more peaceful future through science, and CERN, the European particle physics laboratory, was duly born. James Gillies tells the gripping story of particle physics, from the original atomists of ancient Greece, through the people who made the crucial breakthroughs, to CERN itself, one of the most ambitious scientific undertakings of our time, and its eventual confirmation of the Higgs boson. Weaving together the scientific and political stories of CERN's development, the book reveals how particle physics has evolved from being the realm of solitary genius to a global field of human endeavour, with CERN's Large Hadron Collider as its frontier research tool.
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CERN AND THE HIGGS BOSON
The Global Quest for the Building Blocks of Reality
JAMES GILLIES
CONTENTS
ABOUT THE AUTHOR
James Gillies began his career at CERN as a graduate student in 1986. After eight years in research and a brief stint at the British Council in Paris, he joined the laboratory’s communications group in 1995, heading the group from 2003 to 2015. He is now a member of CERN’s strategic planning and evaluation unit.
AUTHOR’S NOTE
This book gives just a glimpse of the fantastic journey of discovery that is particle physics. It would be impossible in a book of this kind to tell the whole story, with all its twists and turns, dead ends and new beginnings. As a result, there are whole areas of physics, giants of the field, technological advances and major laboratories that are missing or only hinted at. Instead, I have focused on the electroweak physics to which CERN has contributed so much over the years, and included just the physics necessary to tell the story of the Higgs. I have tried to give an idea of how extraordinary it is that human intellect has delivered the theories and the machines that allow us to understand the workings of the universe at such an intricate and intimate level. There are those who say that science of this kind diminishes nature’s beauty. On this point, I can only concur with the great Richard Feynman who offered the opposite view: science can only add to our sense of wonder. I hope that I have managed to convey some of that wonder in these pages.
I would like to thank Austin Ball, Stan Bentvelsen, Tiziano Camporesi, Dave Charlton, Jonathan Drakeford, Rolf Heuer, John Krige, Mike Lamont, Michelangelo Mangano, John Osborne and David Townsend, all of whom know much of this story far better than I, and generously gave up their time to read and improve the draft. Any remaining errors are my own. My thanks also go to series editor Brian Clegg, along with Duncan Heath and Robert Sharman at Icon Books for their many constructive comments and sensitive editing of the manuscript. Finally, I would like to thank my wonderful family for their patience and support.
1
BREAKING NEWS
15 June 2012
It was mid-afternoon when the phone rang. I was in the garden searching among weeds for the vegetables I’d planted a couple of months earlier. ‘What I’ve just seen is not going away,’ said the voice on the other end. It was Austin Ball, an old friend from the days when we were both working on the OPAL experiment at CERN, the European particle physics laboratory near Geneva. Earlier that day he’d seen the results of his experiment’s search for the elusive Higgs particle. He’d been in the room when the physicists working on one of the big LHC (Large Hadron Collider) experiments had taken their first look at their results, and what they had seen had set hearts racing.
Such moments are few and far between: occasions on which a scientist, or in this case a roomful of scientists, can be the first to know something completely new to humankind. What I would have given to be in that room – but I’d traded my research career 20 years ago for a job in CERN’s public communications team. Austin had thought long and hard before calling me, and for good reason: new results are closely guarded secrets until the experimenters are absolutely sure they are ready to go public. I felt honoured to be trusted enough to be brought into this privileged inner circle; and now, sworn to secrecy, I knew we had a job to do. We had to get ready for the biggest announcement in the laboratory’s history. And we had to do it with the utmost discretion.
The Large Hadron Collider is CERN’s flagship research instrument. It had risen to notoriety some four years earlier for all kinds of reasons. As the world’s largest scientific instrument, with a price tag to match, and host to global collaborations involving thousands of scientists and engineers of around 100 nationalities, it had grabbed the popular imagination. For many, CERN’s quest to understand the weird and wonderful universe we inhabit represented the true spirit of humanity; a model of what people can do when they put aside their differences and work together to achieve a common goal. To others, however, it was irresponsible, dangerous, or even redolent of the biblical story of Babel: an arrogant affront to the divine.
Whatever people thought, the net result was that the eyes of the world were on CERN, and when the time came to announce this particular result, it would not be a quiet affair in front of an exclusive audience of physicists in the lab’s main auditorium. This would be much bigger.
Timescales are long in particle physics. The LHC was first imagined in the late 1970s, and one of its main research goals went back even further, to 1964. That was the year that Robert Brout and François Englert, and independently Peter Higgs, published papers in the journal Physical Review Letters proposing a mechanism that would give mass to fundamental particles. Why should anyone care about that? Because we, and everything we can see in the universe, are made of fundamental particles, and without mass those particles would be unable to form anything solid. In other words, we would not exist.
From the early 1960s, understanding mass ranked among the most pressing of riddles in fundamental physics, and it would take almost half a century to solve. Thankfully, physicists are usually blessed with a great deal of patience. Before any experiments would be ready to deliver the experimental evidence to confirm the idea of Brout, Englert and Higgs, a decade of theoretical work would be needed. It would be several decades before technology delivered the instruments that would eventually crack the enigma.
Research and development for the LHC began in the mid-1980s, while experimental collaborations started to form in the early 90s. The project was fully approved by 1996, and construction began soon after. By 2008, the machine was ready to go, and under the eyes of the global media, a beam of particles was circulated for the first time on 10 September 2008. It was a day of great excitement at CERN. ‘Just another day at the office, eh?’ said LHC project leader Lyn Evans as I headed for home at the end of the day. But the elation was short lived. Just nine days later, the LHC suffered a setback from which it would take a year to recover: a helium leak led to extensive damage to the machine. Meanwhile, at Fermilab in the United States, another remarkable particle collider, the Tevatron, a venerable machine first switched on in 1985, was limbering up for one last push to discover the particle that had come to be known as the Higgs. Discovering the Higgs particle would bring confirmation that Brout, Englert and Higgs were right. The race was on.
Although particle theory was very clear that a mechanism for mass was needed, and would have to appear at the particle collision energies of the LHC, there was one key feature of the Higgs that it did not predict: the particle’s mass. It could well be in range of the Tevatron – nobody knew. But if the Higgs existed at all, it would definitely be in range of the LHC. The currencies of particle physics are mass and energy, with the exchange rate being the speed of light squared. That’s what Einstein’s famous equation E=mc2 tells us, and it’s why particle accelerators concentrate energy in a tiny space, converting it to mass in the form of new particles. The higher the energy of the accelerator, the higher the mass of the particles that can be produced, and the LHC was designed for a collision energy some seven times higher than that of the Tevatron.
By the end of 2009, the LHC was back in the race – and with a vengeance. Records rapidly fell, and on 30 March 2010, data collection began. The days of sudden realisations leading to ‘Eureka!’ moments in fundamental physics research are long gone. In modern particle physics research, discovery often comes through a painstaking analysis of vast quantities of data, looking for subtle signals that known physics can’t explain. Like everything else in modern research, Eureka requires patience.
Data came rolling in fast as the LHC performed better and better, but nobody was looking at what the data were saying. The main analyses run shielded from the view of human eyes until the time is deemed right to take a look. The reason that scientists do this is that humans are very good at seeing things that aren’t really there, and then skewing their interpretations to match their preconceptions. Algorithms know no such bias, and can be trusted to conduct the analyses free from prejudice.
Nervous eyes were scanning the horizon for hints of what might be happening across the Atlantic at Fermilab, but all was quiet there as well. By spring 2011, combined analyses from CERN and Fermilab had shown where the Higgs particle was not. They had narrowed down the range of masses it could have to 114–157 GeV, with a small window up at 185 GeV. A GeV – or Giga electron Volt – is a unit of mass used in particle physics. In everyday terms, it’s tiny. There are over 500 billion trillion GeVs in 1 gram. But in the world of fundamental particles, the Higgs, if it existed at all, would be a very heavy thing. To put it in context, the basic building blocks of atomic nuclei, protons and neutrons, weigh in at just about 1 GeV, and by the time we reach 185 GeV, we’re looking at atoms of heavy metals like tungsten.
In 2011, the Higgs was running out of places to hide, and everyone in the global particle physics community knew that representatives of the two LHC experiments spearheading the search, named ATLAS and CMS, would have to say something at the big summer conference in Mumbai, and so would their rivals at D0 (dee-zero) and CDF at Fermilab.
The conference came and went with no discovery in sight, and analysis continued apace. On 13 December, CERN organised a Higgs Update seminar to satisfy the demand for information coming from the global physics community. About ten times more people tuned into the webcast than there are particle physicists in the world, and they learned that the mass range for the Higgs had been squeezed to just 115–130 GeV, with both CERN experiments reporting that they might be seeing hints of something new hiding among the data with a mass of about 125 GeV. The signal was too weak for the experiments to be sure, but there was a new sense of excitement in the air. It was tantalising, but everybody was trying not to get too excited. Hints of new physics come and go, but as the seminar drew to a close, someone made the comment that if the Higgs existed, we’d know next year.
On 5 April 2012, the LHC resumed running, this time as the world’s only high-energy particle collider. Fermilab’s Tevatron had collected its last data on 29 September 2011, and although the D0 and CDF analyses were still ongoing, it was beginning to look like it would be up to the LHC experiments to prove the existence of the Higgs. Eyes were focusing ever more closely on CERN.
Spring and early summer were the calm before the storm. Data were coming in and analyses were running, but news from the experiments was scarce. Not only do analysis teams run their analyses blind, they also keep them to themselves for as long as possible to ensure that each analysis is independent. They do this because reproducibility is vital for science: if one experiment sees something and another does not, the chances are that someone’s made a mistake in their analysis, but if two completely independent experiments see the same thing, the chances are that it’s real.
Each experiment was reporting progress individually to CERN’s Director General, and as the summer conference season again approached, we had to decide what to do. That was when my old friend from OPAL, by this time working on the CMS experiment, disturbed me from my gardening.
22 June 2012
The International Conference of High Energy Physics (ICHEP) was scheduled to run from 4–11 July 2012 in Melbourne, Australia, and the party line from CERN that spring was that if a discovery were to be announced, we’d do it at CERN; anything else would be reported at ICHEP. The way things looked, we were working towards the latter option. Despite the big time difference, plans were put in place to relay the presentations back from Melbourne to CERN’s main auditorium so scientists there could take part. The media were clamouring for news of what was going to be said, but there was nothing to say. Even after what Austin had seen, if a discovery were to be announced, the experiments would need to be absolutely certain, and time was running out.
It looked as though the summer conference would come just a little too soon for 2011’s tantalising hints to crystallise into a strong enough signal to announce a discovery, and the CERN communications team started looking forward to a relatively tranquil summer. But then our plans were thrown into disarray. When members of the CERN Council gathered for their regular summer meeting on 21–22 June, they declared that whatever was to be announced, it would be announced at CERN. We issued a press release to that effect on 22 June – ‘CERN to give update on Higgs search as curtain raiser to ICHEP conference’ – fuelling speculation that a discovery announcement was on the cards.
The Melbourne plan was rapidly turned on its head. Instead of relaying the conference sessions to CERN, a second Higgs Update seminar would be held at CERN on 4 July – the only day compatible with CERN’s agenda and that of the conference – and it would be relayed to conference delegates arriving in Melbourne. Why had CERN changed its mind, people began to ask? Surely they’d only do that if they had a major announcement to make. The truth of the matter is that we still did not know whether there would be enough to announce a discovery or not.
Some people were taking no chances. On 26 June, I received an email from Carl Hagen asking whether he and his colleague Gerry Guralnik could attend. Hagen and Guralnik, along with the British physicist Tom Kibble, had conducted early work on the mechanism of mass in the 1960s, independently of Brout and Englert and of Higgs. We replied that they’d be welcome. The following day, I wrote to François Englert, Peter Higgs and Tom Kibble inviting them to attend. Robert Brout had sadly passed away on 3 May 2011. Higgs and Englert said they’d be delighted, while Kibble replied that he’d be attending a Higgs update event organised in Westminster to which the British Prime Minister and Science Minister had been invited.
The big day was fast approaching and still Rolf Heuer, the Director General of CERN, did not know whether he’d be presiding over a major announcement or another cliffhanger. By this time, each experiment was preparing to freeze its analysis for ICHEP, and the Director General had seen them both. Suddenly, it came to him: even if neither experiment was able to claim the five sigma significance required to announce a discovery, he knew that they were both close enough that when the data were combined, it would pass the threshold. He made the decision – it was going to be a discovery announcement.
Sigma: one small word that means a great deal to particle physicists in search of a discovery. Sigma gives a measure of the statistical significance of a measurement. In other words, it’s the likelihood that what appears to be a real phenomenon could just be the result of pure chance. For example, one sigma corresponds to a 32 per cent probability of a statistical fluke, two sigma 5 per cent, and three sigma 0.3 per cent.
Now imagine rolling a die and getting sixes ten times in a row. It’s unlikely, but possible. By the time you get 100 sixes in a row, it’s looking increasingly likely that your die is loaded, but it could still be down to pure chance. It’s the same in particle physics, with a five sigma measurement being one that’s so unlikely to be down to chance that physicists feel comfortable to cry Eureka! In this case, five sigma corresponds to a chance of just one in about 3.5 million that the observed events are a statistical fluctuation and not a signal for the existence of the Higgs.
Things were moving fast. The science press started to call, trying to get a scoop. Some had been offered results from anonymous sources and were seeking confirmation from the CERN press office. They would have to wait. There was a physics summer school in the Sicilian hilltop town of Erice. Stan Bentvelsen, the Director of the Netherlands’ national laboratory for particle physics was there and so was Peter Higgs. Stan was a member of the ATLAS collaboration, and he was there to make a documentary film about particle physics with a Dutch filmmaker. In one scene, he’s pictured showing Higgs the already-published results from ATLAS. Soon after, Higgs got an invitation to a seminar at CERN. On the basis of what Stan knew, but was not saying, he told Higgs: ‘Take this invitation seriously and go.’
On 2 July, the CDF and D0 experiments published the final word on Fermilab’s search for the Higgs. They reported tantalising hints in the same mass range as the LHC experiments, but only at around the three sigma level, not nearly enough to be sure. All eyes were now firmly on CERN. On 3 July, the eve of the second Higgs Update seminar, a video interview of Joe Incandela, the CMS experiment’s spokesperson, was inadvertently made visible for just a few minutes on the CERN website. We’d recorded two interviews: one for a discovery, the other for a cliffhanger, and the plan was to use the appropriate one on the day. This was the discovery one, and the few minutes it was visible were all that were needed for half a dozen or so keen-eyed journalists to see it. Was it real, they asked? Wait until morning, we said.
The big day arrived. Those who had camped out overnight to ensure their place in the CERN auditorium rolled up their sleeping bags and waited for the doors to open. François Englert, Gerry Guralnik, Carl Hagen and Peter Higgs were ushered to their seats, while the crowds of journalists who had made the trip to CERN were shown to CERN’s council chamber, in which a press conference would follow, to watch the seminar on a big screen.
There’s a curious dichotomy in science. Papers begin with an abstract that summarises the main points of the paper, and then go on to explain in painstaking detail how the scientists got there. Scientific talks do it the other way round, taking the listener through step by step while saving the conclusion to the end. The auditorium was on tenterhooks, along with those gathered in Melbourne and the 500,000 people watching the webcast. By the time the ATLAS and CMS spokespersons, Fabiola Gianotti for ATLAS and Joe Incandela for CMS, concluded, each of the experiments reporting a five-sigma signal, Rolf Heuer declared: ‘As a layman, I would now say I think we have it. You agree?’ The deafening applause spoke for itself.
Somewhat ironically, by the time the CERN audience learned of the discovery, it was already headline news around the world. The UK Science Minister had turned up to the aforementioned event in Westminster, and John Womersley, the Chief Executive of the UK’s Science and Technology Facilities Council, decided not to keep him waiting. News of the Higgs discovery was broken in London, not Geneva after all, but for those in the CERN auditorium that day, there was no better place in the world to be.
The image of the day was of Peter Higgs wiping a tear from his eye as he learned that the idea he’d published half a century before had finally been shown to be right. ‘For me it’s really an incredible thing that it’s happened in my lifetime,’ he said. The emotion was no less profound for Carl Hagen and Gerry Guralnik and was perhaps all the more so for François Englert since his lifelong intellectual partner, Robert Brout, had not lived to see the day. As we escorted Peter Higgs to the council chamber for the press conference, he was jostled like a rock star and CERN’s press officers had to take on the role of bodyguards. When asked for comment, he was a picture of magnanimity, declaring that this was a day for the experiments. He was right: the theorists’ time would come, as he and Englert, who met for the first time at CERN on 4 July 2012, were invited to Stockholm the following year to receive the Nobel Prize.
Among the acres of media coverage of the discovery was a very poetic piece by Jeffrey Kluger in Time magazine that started with the line: ‘If physicists didn’t sound so smart you’d swear they were making half this stuff up’, and concluded with a wonderful summary of what had happened that day. ‘The boson found deep in the tunnels at CERN goes to the very essence of everything,’ Kluger wrote. ‘And in a manner as primal as the particles themselves, we seemed to grasp that, despite our fleeting attention span, we stopped for a moment to contemplate something far, far bigger than ourselves. And when that happened, faith and physics – which don’t often shake hands – shared an embrace.’
François Englert (left) with Peter Higgs at CERN on 4 July 2012.
CERN
To get to the heart of what it was that caused the world to stop that day, we need to go back in time, all the way to the Greek city of Miletus in the 5th century BC, to Democritus, and perhaps his master Leucippus. Democritus is widely credited as being the originator of the concept of atomism: the notion that there’s a smallest indivisible particle of a substance that is still identifiable as being that substance. And we need to revisit Isaac Newton in Cambridge in the 1680s, making the first inroads into understanding the basic forces of nature at work between those ‘atoms’ of matter. It’s a journey that spans centuries and continents and is a testament to the power of human ingenuity.
2
ATOMOS
Particles and forces
Take a piece of stuff – a pencil lead made of pure carbon, for example – and cut it in half. Is what remains still carbon? Do it again, and again, and again, each time asking yourself the same question. Is there a point at which you arrive at the smallest, indivisible object that can still be called carbon? This is the kind of question that the ancient Greek philosopher Democritus was asking himself in 5th century BC Athens. In the process, he established the notion of atomism. You could argue that he also unwittingly thereby established the field of particle physics, because particle physics is all about exploring the tiniest constituents of matter from which the universe and everything in it is made, along with the forces at work between them.
Particle physics aims to understand what it is that everything is made of, including ourselves, and why matter behaves the way it does. Why does it organise itself into things like Democritus’ atoms at tiny distance scales? Why does it form complex objects such as tables, chairs, human beings and planets at intermediate distance scales, and why does it form structures like solar systems and galaxies at larger distance scales? To address these questions, particle physics, which studies the very small, teams up with cosmology, the science of the very large. This book will concentrate on the tiny.
The word atom derives from the Greek atomos, which simply means things that can’t be cut into smaller pieces. It’s not only in ancient Greece that the concept arose. References appear in other philosophical traditions, each postulating that everything we see and experience is made up of different configurations of a relatively small number of basic building blocks existing in what is otherwise a void.
The basic idea of the early atomists has stood the test of time. What we now call an atom of carbon is indeed the smallest indivisible object that can still be called carbon, though today’s atoms are far from indivisible. Each is made up of a set of smaller particles arranged in particular configurations to make up the range of atoms we know.
In 1879, the Russian scientist Dmitri Mendeleev placed all the known atoms in order of their mass in his now famous periodic table of the elements. In doing so, he found that they formed groups of atoms sharing the same properties within the table. Gaps in the table allowed him to forecast the existence of yet-to-be discovered atoms, and to predict their properties. Over time, the holes were duly filled as the missing elements were found.
The fact that patterns appeared in the table suggested that underlying the atoms was some deeper structure. Mendeleev’s periodic table pointed towards the atoms being composed of constituent particles, and it would not be long before the first fundamental particle of matter was discovered. The modern field of particle physics was about to be born.
In 1906, the British physicist Joseph John Thomson was awarded the Nobel Prize in physics ‘in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases’. In other words, he had discovered a particle called the electron.
In announcing the prize, the President of the Royal Swedish Academy of Science referred to the supposed existence of an elementary charge that had been postulated ever since Michael Faraday had shown in 1834 that every atom carries a charge that’s a multiple of that of the hydrogen atom. Here, the word atom is used in its modern day sense to mean the smallest indivisible piece of any chemical element, but it was not long before people were referring to an ‘atom of electricity’: a smallest indivisible unit of electricity.
In experiments carried out at the Cavendish Laboratory in Cambridge in 1897, Thomson had quantified Faraday’s atom of electricity. In his experiments, he passed an electric current through a glass tube with the air pumped out. The end of the tube was coated with a fluorescent material, on which a glowing dot appeared. This phenomenon was already known, and attributed to mysterious cathode rays. By applying electric and magnetic fields to the tube at the same time, Thomson caused the glowing dot to move, and when the effects of the two fields cancelled each other out, leaving the dot’s position unchanged, he applied the equations of motion of cathode rays in electric and magnetic fields to work out the charge to mass ratio of the particles making up the cathode rays. His conclusion was revolutionary. These particles were much smaller than an atom, and he postulated that they emanated from the atoms themselves. Thomson had shown that atoms were not the fundamental building blocks of matter that people supposed them to be, but were themselves composed of smaller things. Thomson’s discovery not only opened up a new field of research, it also led to new devices ranging from oscilloscopes to televisions, which, until recently, used cathode rays to trace out patterns on the screen.
