The largest scientific expedition ever mounted is preparing to set out. The team has gathered the best equipment it can muster, debated the challenges it will face, and drafted the most detailed maps it can of the terrain ahead. It aims to explore the “inner space” of matter, thousands of times smaller than a proton. Now, with the world’s most powerful particle accelerator nearing completion at CERN, near Geneva, two decades of planning are about to be put to the test.

The Large Hadron Collider (LHC) will tackle some of the biggest challenges in particle physics. It hopes to find the elusive Higgs boson, supposedly responsible for the masses of all other particles; it may see a “supersymmetric” world, where a myriad of new particles mirror our familiar matter; or it may even find a fifth dimension, curled up into a tiny loop at the heart of the atom. Perhaps most excitingly, it could find something that no-one has yet imagined, explaining at a stroke some of the long-standing questions that our current “Standard Model” of particle physics cannot answer.

To reveal the secrets that lie at the high-energy frontier we need an exceptional machine. Starting next year, protons will be smashed into one another at unprecedented energies of 14 TeV (14 × 10¹² electron-volts) at four different points around a 27 km-long circular tunnel deep under the Swiss and French countryside. Some of the quarks and gluons within the protons will collide head on, releasing enough energy to create showers of new particles. And waiting to record the crucial debris from these collisions will be four massive detectors, together employing about 6000 scientists from all over the world.

Of these, it is the two giant all-purpose detectors – ATLAS and CMS – where the main hopes of discovering the Higgs boson lie. The detectors have been designed with that goal very much in mind, but this also puts them in a great position to see a range of brand new physical processes beyond the Standard Model.

A massive problem

The Standard Model of particle physics describes subatomic matter in terms of quarks and leptons, which interact with one another via the strong, weak and electromagnetic forces. These forces are carried by “gauge bosons”: the electromagnetic force by photons; the strong force by gluons; and the weak force by the W and Z bosons. Quarks are affected by the strong force, which binds them together to form “hadrons” such as protons and neutrons that are only 10^–15 m across. Leptons, on the other hand, only feel the electromagnetic and weak forces, meaning that electrons, for example, can roam outside atomic nuclei in orbits 10^–10 m across. At such tiny scales, there is really no such thing as solid matter: the Standard Model is a “quantum field theory” in which both the particles of matter and the forces between them are described as vibrations of fields in a vacuum.

The great triumph of the Standard Model is that it unites the weak and electromagnetic forces, which behave very differently in everyday “low energy” conditions, into a single electroweak force at high energies. The difference between the electromagnetic and weak forces arises because of their interactions with another quantum field – the Higgs field. These interactions cause the photons of the electromagnetic field to be massless, while the W and Z bosons associated with the weak force have a large mass. Within the Standard Model, the Higgs field is also responsible for giving quarks and leptons their masses, and it is a vibration in the Higgs field that appears as the famous Higgs boson.

Without the Higgs boson, the Standard Model simply does not work; but despite previous experiments confirming the predictions of electroweak unification with great precision, this particle has not yet been discovered. The Standard Model does, however, tell us roughly where we should look for it. By putting the measured masses of the W and Z bosons into the electroweak theory we can calculate that the mass of the Higgs must be less than about 1 TeV/c².

As the energy of the proton–proton collisions at the LHC will be 14 TeV, it should be easy to produce a particle of this mass. You might imagine, therefore, that finding or ruling out the Higgs is a simple task – turn on the machine, see if a Higgs pops out, then go off to do something else. Unfortunately, life is not so simple.

Even if the Higgs field does exist, it seems to contain the seeds of its own destruction. This is because Higgs bosons must themselves move through the vacuum, interacting with all the fields there, including the Higgs field itself. Theoretical calculations taking into account all these interactions predict a Higgs mass comparable to the “Planck mass” of 10¹⁶ TeV/c², 16 orders of magnitude higher than required by the Standard Model. This huge discrepancy is known as the hierarchy problem, and it leads theorists to conclude that either the Higgs boson does not exist and something else will be found at the TeV energy scale to replace it; or that there is some as-yet unseen effect that modifies the theory to allow the Higgs mass to fall into the TeV range.

Two main ideas have been put forward to solve the hierarchy problem, the most popular of which is supersymmetry (SUSY). In supersymmetric models, the interactions with normal matter that otherwise make the prediction of the Higgs mass comparable to the Planck mass are cancelled out by interactions with previously unseen supersymmetric particles or “sparticles”. SUSY predicts that there are sparticle partners for every type of normal particle, differing in the amount of spin that they carry. For example, electrons and quarks carry half a unit of spin, while their SUSY partners, dubbed selectrons and squarks, carry none. In order to keep the Higgs mass at the TeV scale, the sparticles must themselves have masses of about 1 TeV/c², and so they too are prime targets for the LHC. Indeed, supersymmetry is regarded as the least surprising type of new physics that could be discovered at the LHC.

The second approach to solving the hierarchy problem is more recent and more outrageous. If the calculations dictate that the Higgs mass is indeed comparable to the Planck mass, then instead of modifying the calculation, why not move the Planck mass down to the TeV scale? The Planck mass is a combination of fundamental constants that effectively sets the strength of gravity, so it seems ridiculous to alter it. But in fact, experiments have only measured gravity over distances longer than about 100 µm. Theorists have thus suggested that there could be extra spatial dimensions below this scale, curled up into tiny loops. Gravity could then be much stronger than we have measured, but with most of it being absorbed into the “invisible” extra dimensions.

Stronger gravity corresponds to a smaller Planck mass, lowering the predicted Higgs mass and thus solving the hierarchy problem. If this radical idea turns out to be correct, the collisions at the LHC will be energetic enough to excite the gravitational field in the extra dimensions, creating observable particles or even tiny black holes.

Reconstructing events

With the Higgs boson, supersymmetric particles and maybe even extra dimensions all predicted to occur at the scale of a few TeV, it is clear why the collisions at the LHC are of such great interest. Currently, the world’s highest energy collisions take place at the Tevatron accelerator at Fermilab in the US, but with its proton–antiproton collisions having energies of 2 TeV, new physics has remained tantalizingly out of reach.

At the LHC, bunches of some 10¹¹ protons at a time will be accelerated to 7 TeV. As the protons approach the detectors, they will be focused by electromagnetic fields to increase the chance of a collision. About 20 protons will collide each time the beams cross, most being glancing blows. But occasionally, two quarks in the incoming protons will meet head on, creating a “hard” collision in which the energy released can create new massive particles, including the Higgs boson and sparticles, if they exist.

The situation is complicated by the fact that we will never observe a Higgs boson or a sparticle directly because they are unstable and decay almost instantly into normal Standard Model particles. Instead, their fleeting existence must be inferred from the decay products that fly outwards from the collision and into the detector. Unfortunately, we do not have much time to do this. Travelling at close to the speed of light, particles pass through the entire ATLAS detector in about 30 ns, with the particles from the next set of collisions following just 25 ns behind. To make matters worse, the debris from each collision will contain 1000 or so particles, each of which must be tracked and measured to try to reconstruct what happened in the original collision, or “event”.

1 ATLAS cut-away

To this end, the huge ATLAS detector – weighing some 7000 tonnes and measuring over 26 m in length and 20 m in diameter – consists of a number of subdetectors, designed to identify and measure different types of particle (figure 1). Three main subsystems, are arranged in cylindrical layers around the vacuum pipe that carries the proton beams: the inner detector is devoted to measuring charged-particle tracks; the calorimeters measure the energy of particles; and the outermost subsystem is used to detect muons. Millions of electronic channels will read out the raw data from the detector systems – the first step in a chain that will allow physicists to deduce what happened in the collisions and hopefully to identify new physical processes.

Storing all the data read out from each of the billion or so collisions every second would be impossible, so the detectors themselves will be programmed to select only the most interesting events. Dedicated electronic “trigger” systems will inspect the data for high-energy deposits or interesting patterns of particles, accepting only about 0.25% of the events. Farms of powerful computers will be used to further process the surviving events, reducing the number passed onto waiting physicists for analysis to a more manageable 100 events per second.

Even after this massive filtering exercise, over a petabyte (10¹⁵ bytes, or a million CDs) of data will be recorded each year. Small teams of physicists will work together on their chosen topic, trying to pick out interesting “signal” events such as Higgs decays while removing “background” events that come from known Standard Model processes. In order to do this, we need to understand what the detectors should see from the Standard Model alone, and what the distinctive “signatures” of new particles will be. This is done using “Monte Carlo simulations”: computer programs that generate millions of random events obeying the underlying theoretical equations, starting from the hard collision and simulating the paths and decays of all the particles created by it.

These simulations also include a virtual model of the detector, which mimics the response of each detector element as the particles pass through it. The simulated data from this virtual detector are then fed into the same reconstruction program that physicists will use to inspect the real data, allowing them to compare the real data directly with the predictions of different theoretical scenarios. Such Monte Carlo studies have been carried out since 1984, when the case was first made to fund the LHC, and today the ATLAS database contains simulations of over 600 different scenarios.

Hunting the Higgs

Finding or eliminating the Higgs boson is so important for the LHC that the ATLAS detector was designed specifically with the Higgs search in mind. The first important factor to consider is how often Higgs bosons will be produced in the first place – given by multiplying the “production cross-section” of the Higgs and the “luminosity” of the LHC beams. The cross-section tells us how likely two colliding quarks are to produce a Higgs boson, while the luminosity tells us how many protons are in the LHC beams and how tightly they are focused. Using the design luminosity of the LHC and a Higgs mass of 250 GeV, we would expect to produce about 10 events containing Higgs bosons every minute. This is a promising start, especially as the detector is designed to run for up to 10 years.

But creating Higgs bosons in the first place is useless unless we can prove their existence by measuring their decay products in the detector. The Higgs boson can decay in many different ways, with the probability of a particular type of decay depending on the as-yet unknown mass of the Higgs. From previous experiments, we know that the Higgs mass must be above 114 GeV/c² – otherwise we would already have seen it – while theory requires it to lie below 1 TeV/c². If it has a “low” mass (about 120 GeV/c²), it will most often decay into a bottom quark and antiquark. Unfortunately, it is very hard to tell these apart from the quarks produced in collisions in which no Higgs is present. A low-mass Higgs can also decay into a pair of photons, but it does this much less often – only once in 1000 decays. However, this decay is much easier to pick out, and there are so many proton–proton collisions that there will still be thousands of these events produced.

If the Higgs has a higher mass (above 180 GeV/c²), then it is most likely to decay into pairs of W and Z bosons. The easiest signal to identify occurs when the Higgs decays into two Z bosons, which each decay in turn into a pair of electrons or muons. This signature of four leptons can be easily identified by the detector, and is not often produced by unknown Standard Model processes. Furthermore, by measuring the momentum of each of the leptons, it is possible to reconstruct the mass of the parent particle of each pair. Hence, particle physicists will home in on this type of decay by looking for two pairs of leptons in an event, the momenta of each of which “add up” to a value close to the known mass of the Z boson. Taking this idea further, the momenta of the two Z bosons can then be used to derive the Higgs mass.

In ATLAS, the precise momentum measurements required to do this are achieved using the inner detector, which is enclosed in a solenoidal magnet 2.4 m in diameter and 5.3 m long that provides a 2 T field. By measuring the path of a charged particle with sensors consisting of silicon wafers and gas-filled tubes, we can derive its momentum because the momentum determines the curvature of the path. The inner detector is designed to measure the positions of particle tracks with an impressive precision of about 10 µm. Importantly for the Higgs search, this also means that it can distinguish bottom quarks from other types of quarks, based on how far the hadrons they form travel before they decay.

2 Extracting the Higgs

After passing outwards through the inner detector, the particles next encounter the calorimeters, which will measure their energy by stopping them in an absorbing material: liquid argon in the case of ATLAS. Electrons lose energy easily and create dense clusters of secondary particles, while hadrons scatter off nuclei and form much larger clusters. Accurate energy measurements are vital, for example in the search for the Higgs decay to two photons (figure 2). Furthermore, because energy is conserved in the collisions, we can infer the presence of “invisible” particles such as neutrinos – which pass through the detector without being measured directly – by checking whether the energy measurements add up or if some energy is “missing”.

3 Sister experiment: CMS

Muons, being relatively heavy, can pass through the calorimeter without losing much energy, and so they will be detected by the outermost subsystem. Detecting muons is vital to identify the important Higgs decay into two Z bosons, as either of them can subsequently decay into two muons. The muon system works by making a second momentum measurement of charged-particle tracks – eight massive superconducting magnets that surround the whole apparatus create a toroidal field in which the muons paths are bent, and 12,000 m² of detectors are used to measure the curvature of the track. Muons are distinguished by tracks that have roughly the same momentum before and after passing through the calorimeter.

ATLAS takes shape

The components of the ATLAS and CMS detectors have been designed and built by teams around the world. ATLAS is now beginning to take shape in its underground pit (see box on page 30), and, when protons finally begin to collide next year, its electronic eyes will be fully equipped to observe all the principal decay types of the Higgs boson. CMS, meanwhile is being assembled on the surface (figure 3).

The road to discovery

The Higgs search is so challenging in itself that the ATLAS detector does not need any extra features to explore other interesting signals. For example, supersymmetric particles decay by emitting high-energy quarks, electrons and muons that can be measured by the detector; and invisible “neutralinos” that produce missing energy in the same way as neutrinos. In models with extra dimensions, we may be able to infer the existence of a “graviton” – the hypothetical carrier of the gravitational force – either through measurements of missing energy or from pairs of electrons and muons the masses of which add up to the graviton mass.

4 Building significance

Indeed, there are so many theoretical possibilities that correctly interpreting the LHC data will therefore probably be more challenging than finding the signals themselves. The strategy will therefore be to look for deviations from the Standard Model predictions with as many different signatures as possible. To claim a discovery, we must be sure that our observation could not be merely a statistical fluctuation of the background processes (figure 4). If a significant discrepancy is found, it will be compared with the various theoretical models to see which type of new physics is the best match. However, we may end up in the position of announcing the discovery of physics beyond the Standard Model without being certain what that physics is!

The man behind the boson

Even if the experiment sees a strong signal, the team will want to be entirely convinced of its validity before announcing its discovery. The first events will undoubtedly look very strange, since the detector will still need to be fully calibrated, removing badly functioning electronic channels, misalignments and so on. The performance of the system will need to be understood in the real, rather than virtual, Monte Carlo world. For example, the illusion of large amounts of missing energy can be created when particles escape undetected through the space where the read-out cables leave the detector. Effects like this cannot be modelled confidently in advance, and so must be measured in situ.

Once we feel confident that we understand the detector, good data should be obtained routinely. Hundreds of terabytes will pour out each week, to be reconstructed on a worldwide network of computers. Like reconnaissance patrols, small teams of staff and students will search for their favourite signals, reporting their results to the full collaboration, which will focus on the most exciting results. Sometime in 2008, the first dispatches will arrive from beyond the frontier. Then we will know which fabulous beasts inhabit the land, and which are as mythical as the unicorn.

At a Glance: ATLAS and CMS experiments

• The collisions between protons at the Large Hadron Collider will take physicists into a completely uncharted energy regime
• One of the main goals of the LHC is to discover the elusive Higgs boson, which is responsible for the masses of all other particles
• The huge general-purpose ATLAS and CMS detectors have been designed to make the hunt for the Higgs boson as easy as possible
• A variety of other types of “new physics” could be found at these detectors, including supersymmetry and extra dimensions
• The discoveries made at the LHC should solve the “hierarchy problem” – one of the biggest challenges in particle physics

Nick Chohan is looking forward to Christmas. For the last three years the amiable CERN physicist and his team have been working around the clock in a large metal hangar that straddles the Franco-Swiss border, carefully testing the 1232 superconducting magnets that will soon guide protons at almost the speed of light around the world’s most powerful particle accelerator. This is no mean feat – it takes three people up to 12 hours just to connect one of the 15 m long, 35 tonne cylindrical dipoles to the test rig, and each one costs close to a nerve-wracking SwFr1m (over €600,000). But this month the last magnet is due to arrive at Chohan’s lab, and by the end of the year all of them will have been cleared for installation underground.

Particle physicists are used to thinking big, of course. In their quest to understand how nature behaves at the most fundamental level, they have been building machines that smash particles together at ever higher energies for the best part of a century. But the Large Hadron Collider (LHC), currently being built at the European particle-physics laboratory CERN, near Geneva, is rewriting the rules of the game.

In control

“Few people fully appreciated the scale and complexity of the LHC at the beginning,” says CERN’s chief scientific officer Jos Engelen. “It has surprised many of us, myself included.” And with the 27 km-circumference machine scheduled to switch on next year after more than 20 years of preparation, the excitement among CERN physicists is palpable. Once the LHC reaches its full design performance, some time early in 2008, protons will smash into one another about one billion times per second at an energy of 14 TeV (14 × 10¹² electron-volts), recreating conditions that existed shortly after the Big Bang and placing CERN at the forefront of high-energy physics for at least a decade.

A journey into the unknown

Nobody really knows what lies at the high-energy frontier, but the LHC is far from a stab in the dark. Since the 1970s our understanding of matter at the tiniest scales has been enshrined in the Standard Model of particle physics, which describes the world in terms of six quarks (which are bound in hadrons such as protons and neutrons) and six leptons (such as electrons) that interact via force-carrying particles called bosons. It is this theory, and its shortcomings, that points experimentalists to the energy regimes worth studying.

The Standard Model has been verified with impressive precision, its finest hour being the discovery of the W and Z bosons at CERN’s Super Proton Synchrotron (SPS) in 1983. The appearance of these particles at precisely the collision energy that theorists expected (about 0.1 TeV) proved that two of nature’s four fundamental forces – the electromagnetic and the weak nuclear forces – are simply low-energy facets of a more general “electroweak” interaction. The problem is that this electroweak symmetry, which would have existed when the universe was much younger, requires quarks and leptons – and therefore all matter – to be massless.

One of the biggest challenges in particle physics is to find the missing piece of the Standard Model jigsaw that enables nature to break this symmetry and so give rise to the medley of particle masses we observe. The favoured candidate for this is the Higgs mechanism, which requires the existence of a new massive particle called the Higgs boson. As Engelen puts it: “Verifying electroweak symmetry breaking is the defining motivation for the LHC. If we see nothing at the TeV scale then there is something deeply wrong with our understanding of particle physics.”

Another major goal of the LHC is to search for physics beyond the Standard Model, in particular for signs of supersymmetry or SUSY – an even more general symmetry of nature that treats matter and force particles as two sides of the same coin. SUSY would not just unify the electromagnetic and weak interactions, but would also bring the strong nuclear force and even gravity in a single framework. If it is real, there should be a host of weird new “sparticles” waiting to reveal themselves in the LHC collisions.

Boys and toys

Designing and building the LHC – for which the total cost is €6.3bn – is the responsibility of Lyn Evans. When asked to describe the extent to which the job is taking over his life as switch-on draws near, the 61-year-old Welshman looks as if he has not understood the question. “For the past 13 years the LHC has been my life,” he exclaims, adding that he is relieved to see the bright blue accelerator finally take shape in its tunnel. Evans has nursed the LHC through numerous difficulties, notably problems with the cryogenic system that delivers liquid helium to the superconducting magnets in order to keep them at a chilly 1.9° above absolute zero. But he is confident the machine is on track for a 2007 switch-on.

The main task now is to get all the magnets from Chohan’s test facility safely connected up inside the LHC tunnel. In addition to the 1232 dipoles, which will bend the protons around the ring in opposite directions, there are also thousands of quadrupole, sextupole, octupole and numerous other corrector magnets that will allow the beams to be focused and manipulated. Since it is far too risky to deliver the magnets directly to each point around the ring, they instead make a short journey from the test lab by lorry to a special shaft at the main CERN site via a traffic-light system that Evans can control to minimize the chances of an accident occurring. More than half of the dipoles are in place and, with a 65-strong installation team working 24 hours a day, the job should be finished by the end of March.

Then the real fun begins, if you are an accelerator physicist that is. “The plan is to have beams circulating with an energy of 0.45 TeV each by the end of November next year, with a short data run just before Christmas,” says Evans. “Then we’ll tell the four experiments to switch off so that we can ramp the energy up and get into the record books.” To do this, the LHC will need to produce a collision energy of 2 TeV (1 TeV per beam) in order to beat the current record held by the Tevatron proton–antiproton collider at Fermilab in the US. But in early 2008, after a short winter shutdown, the machine should be ramped to 14 TeV within a couple of months, leaving the Tevatron standing in the dust.

It is vital that you know what you are doing when playing around with such a high-energy beam. “The LHC is the first accelerator ever built that has the ability to self-destruct,” says Steve Myers, who his head of accelerators and beams at CERN. “The stored energy of each beam is equivalent to 100 kg of TNT going off, which could put the machine out of action for months if the protons veer off course.” He quickly adds, however, that the chances of that happening are extremely small thanks to the stringent safety measures that are in place. If things start to go really wrong, for instance, the beams will automatically be sent down two 600 m-long tangential tunnels into a huge graphite block surrounded by some 1000 tonnes of steel and concrete shielding.

“You wouldn’t want to be having your lunch down there when the beam is dumped,” jokes machine operator Roger Bailey, who adds on a more serious note that this area would then remain highly radioactive for many months. Bailey will be one of the people responsible, from the comfort of CERN’s brand new €5m control room, for carefully “threading” the beams around the magnetic fields of the LHC when the first protons are injected from the SPS next year. The downside of such rigorous safety standards, he points out, is that they will make it much harder to tinker with the beam to optimize the performance of the LHC. And anyone who actually attempts the potentially fatal act of eating their sandwiches in the LHC tunnel once the machine switches on will first have to get through security doors at the surface employing the latest iris-recognition technology.

Eyes on the ground

While the LHC steadily takes shape underground, a whole other world of activity is hotting up elsewhere around the CERN site. In fact, on the day Physics World arrived, Jim Virdee and his colleagues on the CMS experiment were about to throw a party. CMS is one of four enormous detectors that will be positioned around the LHC ring where the proton collisions will occur. Their task is to track and measure the hundreds and thousands of particles that will be produced when a quark from one proton strikes a quark from another proton travelling in the other direction, a tiny fraction of which may produce a Higgs or a SUSY particle in the process (see “Expedition to inner space”).

With the CMS project comprising some 2000 physicists from 180 institutes in 34 countries, inviting the collaboration to an impromptu party is no easy task. “The big celebrations will come in due course,” explains Virdee, who is deputy spokesperson for the experiment. “But later today a few of us are going to be marking three recent milestones: closing the detector for the first time; testing the magnet at full strength; and taking cosmic-ray data.” Witnessing the scale of the CMS detector, which is being built in a vast building in France about 10 km away from the main CERN site, it is easy to understand why its creators have something to shout about.

For example, the CMS magnet, which provides a bending field of 4 T in order to allow physicists to measure the momentum of particles, is quite simply the largest ever built. And as one cylindrical segment of the 12,500 tonne detector is slowly rolled back on its air-cushioned feet with a deafening screech, Virdee reels off the convoluted international history of the various pipes, wires and components that make up each concentric layer of the detector. About 80 m beneath the floor on which we are standing lies the huge cavern into which CMS will be lowered in pieces later this year. Here, things are much quieter, with nothing but the sound of tinny French radio echoing off the walls as a few technicians go about their business in the buckets of bright yellow cherry-pickers.

On the opposite side of the LHC ring, just off the main CERN site, the scene in the ATLAS cavern could not be more different. Unlike CMS, the 7000 tonne ATLAS experiment is being assembled directly underground. It already fills so much of the available volume in the cavern that the only way to get a sense of its scale is by watching the puppet-like figures in helmets and harnesses scramble over it from the gantries on the sides.

“It gives me a lot of pleasure to see the detector being built, and it is having a positive effect on the mood of the collaboration,” says ATLAS spokesperson Peter Jenni. “But there is still much to do, and a large part of my job now is devoted to changing the mindset of the collaboration as we move from the construction phase through commissioning to eventual operation.”

Flanking ATLAS are the two “smaller” LHC experiments LHCb and ALICE. Although these have more specific physics goals than CMS and ATLAS, they are by no means less important. LHCb, for example, will study the decay of B-mesons in order to tackle the problem of why matter and antimatter did not immediately annihilate with one another in the Big Bang to produce a universe comprising nothing but photons. ALICE, meanwhile, will allow detailed studies of the strong nuclear force, which will test the non-electroweak sector of the Standard Model, quantum chromodynamics.

It is the two general-purpose detectors CMS and ATLAS, however, that the world will be watching closest when the first proton beams are brought into collision next year. Hundreds of physics analyses in the form of complex computer programs are currently waiting to be let loose on the tens of thousands of gigabytes of data that will stream out of the detectors’ several million read-out channels each day. Many of these will look for specific experimental signatures of a Higgs or SUSY decay, while others are destined to study even more exotic possibilities such as extra dimensions and mini black holes. In order to cope with this data deluge, the LHC requires a completely new computing infrastructure – known as the Grid – which is proving a major challenge in itself.

The rush to analyse the LHC data will also make and break hundreds of scientific careers. While having two independent detectors based on very different technologies is vital to any claim of discovery, the competition between CMS and ATLAS to have their name associated with it could place CERN’s unique open environment under unbearable strain. Indeed, the hunt for the Higgs at the LHC has parallels with the feisty race between the UA1 and UA2 experiments at the SPS to find the W boson two decades ago. When Nobel prizes are at stake, physicists can be pretty cut-throat.

“It is natural that people identify with their experiment, and I think that pushes all of us to do a better job,” says Jenni. “Of course, both collaborations will do their best to be first, and it is clear that there are some personal ambitions, but in the end the credit will – and should – go to CERN. There will certainly not be just a single person associated with a discovery like the Higgs,” he says. But some of those who are closer to the actual analysis teams are less idealistic. “Forget the rivalry between ATLAS and CMS”, one CERN physicist told Physics World, “there will be enough trouble within the collaborations themselves, especially once data start to arrive.”

This is not something that has passed the CERN management by. Head of communications James Gillies is currently working on a strategy with which to deal with a possible Higgs discovery. “The last thing we want to have to do is issue an ‘anti-press-release’ that says CERN hasn’t found the Higgs after all,” he says. Although there are clear scientific procedures in place to deal with the first signs of the Higgs or other particles, Gillies is concerned that a particular institute or even individual might make premature claims of discovery. The trouble is that it could take years after the first signs of a new particle to amass enough data for CERN to make a definitive “five sigma” announcement.

Living the dream

The way the LHC is perceived in the public domain is absolutely crucial, not just for CERN but for the future of particle physics. Indeed, you could argue that the LHC is the biggest gamble that physicists have ever taken. “The LHC is an absolute necessity if we are to take the field further,” says CERN’s director general Robert Aymar. “If nothing new turns up below a [quark–quark] collision energy of 1 TeV, this would be very bad for particle physics, and for humanity.” As Engelen, who is also deputy director general of the lab, puts it: “Seeing nothing at all is what would make the theorists really speechless!”

Almost everybody agrees, however, that the most probable outcome is also the best for particle physics: that something, whatever it is, will turn up. This would make it more likely that the next big collider project currently under consideration – the International Linear Collider, which would enable precision measurements to be made of, say, the Higgs boson – will get the green light. As Aymar points out: “Politicians are not going to back a machine that costs twice as much as the LHC without some hard evidence to go on.”

But such thoughts are far from everyone’s mind at CERN right now. From senior management to summer students returning to their home institutes, the mood at the lab is one of quiet confidence and raw excitement. “The decision to go ahead and build the LHC was taken in 1996,” recalls Aymar. “But if we had to take that decision tomorrow, the outcome would be exactly the same. Without this machine, we might as well dream.”

At a Glance: The Large Hadron Collider

The Large Hadron Collider

• Switching on next year, the LHC will guide protons at almost the speed of light in opposite directions around a 27 km underground ring before smashing them into one another with an energy of 14 TeV at four separate points
• Although 14 TeV is no more than the energy of 14 mosquitoes in flight, it is concentrated in such a tiny volume that the resulting energy density is the highest ever produced in a laboratory
• The several thousand magnets required to guide and focus the proton beams are currently being connected in a tunnel that lies between 50 and 150 m beneath the Franco-Swiss border and that originally housed the Large Electron Positron collider
• The protons start out on the main CERN site in “Linac2” before being gradually ramped up in energy by the Proton Synchrotron and then Super Proton Synchrotron (SPS), from where they will be injected into the LHC
• The protons will then be electromagnetically whipped up to 7 TeV, whereupon hundreds of billions of them will circulate the ring 11,245 times every second
• Waiting to catch the debris to fly out from the proton collisions will be four exceptionally large detectors: ATLAS, just across the road from the main CERN site; LHCb and ALICE on either side; and CMS some 10 km away
• The future of high-energy particle physics rests heavily on what is and is not revealed in these detectors in the next few years

The LHC in their own words

“What is nice about CERN is that as a Masters or a PhD student you are listened to and treated as an equal. I feel I can knock on anyone’s door and ask them a question.”
Thijs Versloot
CERN summer student, LHCb

“The LHC is an interesting sociological experiment – you can see people in the collaborations jostling for power. I’d like to work on something less mainstream than the Higgs, to avoid being caught in a huge fight.”
Anne-Sylvie Giolo
CERN fellow, CMS

“I like the idea that I am working on something so huge that it requires many people from lots of different countries. The downside of big collaborations is that there are lots of people competing to work on the sexiest measurements with only a few doing the necessary nitty-gritty stuff.”
Jamie Boyd
CERN fellow, ATLAS

“It’s fantastic to finally see the detector come together and know it has been made possible by the efforts of so many people around the world. It energizes you and builds morale.”
Jim Virdee
Deputy spokesperson, CMS

“The unprecedented scale of the LHC experiments has meant that I’ve had to pick up management skills on the job. Sitting back and thinking about the physics is more of a hobby these days.”
Peter Jenni
Spokesperson, ATLAS

“I am lucky that my career has coincided with a machine like the LHC. Having worked on all of CERN’s major colliders, I should be retiring just after it has reached its design performance.”
Steve Myers
CERN head, accelerators and beams

“There is nothing about my job that I do not like. For me it’s all about strategy. No matter what you are working on you need to have a vision, and to realize that vision you need to find and implement a strategy.”
Robert Aymar
CERN director general