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About the Author
Jon Butterworth is a professor in the Department of Physics and Astronomy at University College London and a member of the ATLAS collaboration at CERN’s Large Hadron Collider in Geneva, Switzerland. He writes the Life and Physics blog for the Guardian, has written articles for a range of publications including the BBC and New Scientist, and is also the author of Most Wanted Particle, shortlisted for Book of the Year by Physics World. He was awarded the Chadwick Medal of the Institute of Physics in 2013 for his pioneering experimental and phenomenological work in high-energy particle physics. For the last fifteen years, he has divided his time between London and Geneva.
Read an Excerpt
Before the Data
1.1 Why So Big?
The Large Hadron Collider (LHC) sits in a tunnel 27km (nearly 17 miles) long and about 100m (almost 330 feet) underground. If you know London, it might help you to know that 27km is about as long as the Circle Line on the Underground, and the tunnel itself is similar in size to the Northern Line. If that doesn't help, then try this.
Imagine setting off from Meyrin, on the Swiss–French border near the airport, and driving towards the French countryside. The Jura Mountains are in front of you, Geneva Airport is behind. As you pass the border, you also pass the main site of the CERN laboratory on your left, and if you look to the right you will see a big wooden globe that looks like a sort of eco-nuclear reactor (it's not, it's an exhibition space, though it is eco-friendly, apparently), and you might catch a glimpse of the building housing the control room of the ATLAS experiment. You will know it if you see it, because it has a huge mural of the ATLAS detector itself on the wall.
Big though it is, the mural is painted to only one-third scale. ATLAS is very large, and is hidden underground, positioned at one of the interaction points of the LHC. These are the points where the two highest-energy particle beams in the world are brought into head-on collision. ATLAS is one of the two big general-purpose particle detectors designed to measure the results of these collisions.
Continue driving. You may imagine yourself in a nerdy little white van with a CERN logo on the side if this helps.
Pass through the village of St-Genis and continue into the Pays de Gex, in the foothills of the Jura Mountains. You are now surrounded by the LHC. If you are imagining yourself in winter, you might see the lifts of Crozet, the little Monts Jura ski resort, chugging away ahead of you. (Mont Blanc is behind you on the horizon, but keep your eyes on the road.) Keep driving, bear right towards Gex, maybe pass through the villages of Pregnin, Véraz and Brétigny. After about 25 minutes' driving through the French countryside – longer if you get stuck behind a tractor – you will get to the village of Cessy, near Gex. Here you will find the top of the shaft that leads down to CMS, the other big general-purpose detector on the LHC ring. ATLAS and CMS are independent rivals, designed differently by different collaborations of physicists, but with the same goal: to measure as well as possible the particles produced when protons collide in the LHC. They were designed to cross-check each other's observations, and to compete head-to-head for the quickest and best results.
All this time, on your journey from ATLAS to CMS, you have been inside the circumference of the world's biggest physics experiment. You entered it at the border when you passed ATLAS, and have now crossed its diameter.
The LHC is designed to collide subatomic particles at the highest energies ever achieved in a particle accelerator. We do this to study the fabric of the universe at the smallest distances possible, which for reasons to be described later also implies the highest energies possible. Given that the experiment is designed to look at very small things, it might be a surprise that it is so big. Building a long tunnel is very expensive, so why not make a smaller one?
In fact, it is the length of the tunnel that limits the energy of the colliding beams. If you accept the fact that to study small stuff you need high energies (please do, for now at least), you can understand why the LHC needs to be so big just from an understanding of fairly everyday physics.
Particles travel in a straight line at a constant speed, unless a force acts on them. This is one of Newton's laws of motion. In everyday life it isn't completely obvious (Newton was quite clever to work it out), but once you are aware of it, it is easy to see it in action.
The reason it is not completely obvious in everyday experience is that on Earth practically everything that moves has forces due to friction and air resistance acting on it, and everything experiences gravity. This is why if you set a ball rolling, it will eventually stop. Friction and air resistance act on it to slow it down. And if you throw a ball in the air, gravity will slow it down and eventually drag it back.
But in situations where friction or gravity can be ignored, things are clearer. Driving a fast car, or even a nerdy CERN van, you clearly have to apply a force, via the brakes, to slow it down. And more relevantly in the context of the LHC, if you want to change direction, to turn a corner at speed, this can only be done if there is sufficient friction between the tyres of the van and the road. Otherwise, you skid.
The driver and passengers experience a rapid turn of a corner as a sort of 'pseudo-force'. The van is turning, but your body wants to carry on in a straight line, so you feel as though you are being pressed against the sides of the van. It would be more true to our understanding of physics to think of the sides of the van as pushing against you, to force you to change direction, pushing you round the corner along with the vehicle.
The combination of speed and direction is called velocity. And if you combine the velocity and the mass of the object (the van, for example, or the passenger), you get the momentum. The bigger the mass, or the velocity, the bigger the momentum, and if you want to change the momentum of something, you need to apply a force to it.
I am being deliberately vague about how the velocity and mass combine to give momentum. At speeds much lower than the speed of light, it is good enough to just multiply – momentum is mass times velocity – and this is probably the right answer if you are taking a school course in physics. However, the exact expression is a little different, and the difference gets more and more important as speeds approach the speed of light. Then you need Einstein and relativity (of which more later), rather than Newtonian mechanics. But don't try this in a van.
Regardless of that, the larger the desired change in momentum, the bigger the force has to be. Hence the brakes on a lorry need to be able toexert more force than the brakes on a van, because even if the velocity is the same, the mass of the lorry is bigger so the change in momentum involved in making it stop is bigger.
This is the situation of the protons in the LHC tunnel. These are the highest-energy, and highest-momentum, subatomic particles ever accelerated in a laboratory. Even though the mass of a proton is tiny, their speed is tremendously high. They are really, really determined to travel in a straight line. So, to make the two beams of protons bend around the LHC and come into collision requires a huge force. The force is provided by the most powerful bending magnets we could build.
Given this maximum force, there is then a trade-off between how sharp the bend in the accelerator is and how high the proton momentum can be. Back to the van: this is exactly equivalent to the fact that there is a maximum speed at which you can take a given corner without skidding. If the corner is sharp, the speed has to be low, but for a gentle curve you can go faster. This, then, is why the LHC is so big. A big ring has more gentle curvature than a small one, and so the protons can get to a higher momentum without 'skidding'. Or, in their case, 'catastrophically escaping the LHC and vaporising expensive pieces of magnet or detector'. Something to be avoided.
The maximum bending power of magnets is thus the reason that proton accelerators need to be large if they are to get to high energies. For the other commonly collided particle, the electron, there is another reason that is worth looking at.
Before the LHC was installed, another machine occupied the 27km tunnel under the Swiss–French border. This was LEP – the Large Electron– Positron Collider. (Positrons are the antiparticle of the electron, carrying positive charge, in contrast to the electron's negative. LEP collided electrons and positrons together. Incidentally, people occasionally accuse particle physicists of hyping-up their equipment, but these are very descriptive, even dull, names.) LEP was turned off in the year 2000 because it had explored most of the physics within its reach and could not increase its energy further. The reason it could not go higher was, as with all the protons, also connected to the size of the tunnel, but in a different way.
This is to do with the fact that electrons have a mass about 1800 times smaller than the proton. Now, at the highest energies that doesn't make any significant difference to the force required to bend them round a corner. This is because, whether they are electrons or protons, they are moving very close to the speed of light, so you need the full special relativity expression for momentum, and the net result is that the mass they have when they are at rest is irrelevant for calculating the required force. So that wasn't the problem.
The problem was synchrotron radiation. This is the energy radiated by charged particles when they are accelerated. It is a universal phenomenon, roughly analogous to the wave a speedboat makes when it turns in the water. As a charged particle accelerates round a corner, photons fly off and carry away energy.
The effect is actually much more pronounced for particles with low mass. The amount of synchrotron radiation given off when a particle accelerates depends very strongly on the mass: if the particle mass drops, the energy loss increases by the mass-drop to the fourth power. So, as the proton mass is 1800 times bigger, the energy lost on the bends for electrons is (1800 x 1800 x 1800 x 1800) or about 11 trillion times larger than it is for protons.
As the electrons and positrons squealed round the corners of LEP, photons were radiated this way, and with every revolution of the beam around the ring, more energy had to be pumped in to compensate. This is done by radio-frequency electromagnetic waves confined in big metal structures at intervals around the ring. Electric and magnetic fields oscillate in these structures precisely in time with the passing of the bunches of electrons, so that every time a bunch arrives it gets a kick from the field. This is true in all such machines. But at some point you reach a beam energy where so much is lost in synchrotron radiation that the electromagnetic waves in those structures cannot replace it. That's your maximum collision energy. LEP hit that wall.
This is where the size of the tunnel comes in again, of course. A 27km tunnel has a rather gentle curve. If it were smaller, the bends would be sharper, the acceleration would need to be bigger, so the energy lost through synchrotron radiation would be greater, and the maximum collision energy would be lower.
As an aside, this synchrotron radiation is very useful in other contexts. The Diamond Light Source at Harwell in Oxfordshire, in South East England, for example, was built to produce it intentionally. The radiated beams of photons are used to study atoms, crystals, molecules, materials and surfaces. Many machines and laboratories originally built to study particle physics have been converted to become light sources once they have been superseded in the quest for higher energies. I have reason to be grateful for this personally, in fact. I did my doctoral work in Hamburg, at the DESY (Deutsches Elektronen-Synchrotron) laboratory. The particle physics of interest there at the time was the HERA electron–proton collider, where I worked in the ZEUS collaboration, the team of physicists responsible for one of the main particle detectors at the laboratory. But my then girlfriend was a crystallographer, using synchrotron light to work out the structure of proteins and other stuff. Because of the symbiotic relationship between particle-physics accelerators and synchrotron light sources, there is a branch of the European Molecular Biology Lab at DESY, and after a high-level discussion in the crowd at a St Pauli football match, Susanna managed to get her PhD supervisor to send her to Hamburg for most of her research. We've been married 20 years now, and it's all very fine and romantic. But synchrotron radiation is still a pain in the arse if you want a high-energy electron beam.
So, LEP was shut down in 2000 and dismantled, and installation of the LHC began. The LHC can get to higher energies because it collides protons with 11 trillion times less of a synchrotron radiation problem, but it requires the most powerful bending magnets you can make if you want to get to the highest possible momentum.
The formal approval for construction of ATLAS and CMS was given on 1 July 1997 by the then Director General of CERN, Chris Llewellyn Smith. LEP had been good, but the protons promised more.
1. Incidentally, a man who previously, as head of physics at Oxford and afterwards as provost of UCL, seems to have had a period of following me around and being my boss.
Glossary: The Standard Model Particles and Forces
If you just want to crack on with the story and don't mind the odd unfamiliar word, you can skip these 'Glossary' bits. But without knowing something about the Standard Model, some of it might not make much sense.
The Standard Model of particle physics is our current best answer to the question 'What is stuff made of, if you break it down into its smallest components?'
Start with anything – a rock, the air, this book, your head – and pull it into its component parts (I recommend this remains a thought experiment). You will find fascinating layers of structure, micro- and nano-scale bits and pieces: fibres, cells, mitochondria.
You will eventually find molecules. With enough energy you can break them apart into component atoms. Atoms consist of a dense nucleus surrounded by electrons.
With a bit more energy, you can separate the electrons from the nucleus. With more energy still, the nucleus can be broken into protons and neutrons. With still more energy (and now you do need a big collider!), you can see quarks inside those protons and neutrons.
We have never managed to see anything inside a quark, or break one into pieces.
If, at the 'atom-smashing' stage, we had ignored the nucleus and tried breaking up the electron, we'd have reached that point earlier. We have never managed to see anything inside an electron, or break one into pieces. This – the fact that we haven't managed to break one yet – is our working definition of what it means for a particle to be 'fundamental'.
And a key point is that wherever we had started, with whatever material, we would have ended up with electrons and quarks. In the Standard Model, they are the stuff that everything is made of, and they themselves are not made of other stuff.
You will come across a lot of particles in this book, but remember, there aren't many different kinds of fundamental ones when you get right down to it.
Electrons are an example of a class of particles called leptons. There are also muons and taus, which are just like electrons only heavier. The only other leptons are the three kinds of neutrino. Neutrinos do not interact much with other matter, but there are lots of them around. More than a trillion neutrinos pass through you from the Sun every second.
The other class of fundamental-matter particles consists of the quarks. There are six of them, too, just as there are six lepton types. They are called up, down, strange, charm, bottom and top, becoming more massive as you go (but peaking on whimsy in the middle).
Protons and neutrons are made of up and down quarks. Quarks are never found out on their own, they are always stuck together in bigger particles. These particles, the ones made of quarks, are generically called hadrons (hence the Large Hadron Collider, which mostly collides protons but occasionally collides atomic nuclei, which also have neutrons inside).
Those are all the matter particles we know of. They all have anti-particle partners, and they all interact with each other – attracting, repelling, scattering – via forces, which are carried by another kind of particle – vector bosons.
The electromagnetic force is carried by photons (quanta of light) and is experienced by all charged particles. That is, everything except the neutrinos.
The strong force, which holds protons, neutrons, and atomic nuclei together, is carried by gluons, and is only experienced by the quarks.
The weak force is carried by W and Z bosons, and all particles experience this. The weak force is responsible for radioactive beta decay, amongst other things, but because it is weak, it does not feature much in everyday life. Even so, it is crucial to how the Sun works.
To make the Standard Model work, and in particular to allow the fundamental particles to have mass, another unique and completely new object is also required – a Higgs boson. The hunt for this is, of course, the main topic of this story and I'll say much more about it later.(Continues…)
Excerpted from "Most Wanted Particle"
Copyright © 2014 Jon Butterworth.
Excerpted by permission of The Experiment Publishing.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
Foreword Lisa Randall ix
Preface to the Paperback Edition xiii
Chapter 1 Before the Data 3
Chapter 2 Restart 38
Chapter 3 High Energy 61
Chapter 4 Standard Model 100
Chapter 5 Rumours and Limits 132
Chapter 6 First Higgs Hints and Some Crazy Neutrinos 170
Chapter 7 Closing In 198
Chapter 8 Discovery 224
Chapter 9 What Next? 242
About the Author 288