by Sean Carroll
During the progress of an engineering project of this magnitude, unanticipated roadblocks are going to pop up. While the LHC already had a waiting tunnel courtesy of LEP, new caverns had to be excavated for the four large experiments that would be used to measure the outcomes of the collisions. The CMS experiment sits on the far side of the ring from the main CERN site, near the town of Cessy, on the French side of the border. When workers set about digging a hole for the new experiment, they made an unanticipated discovery: the ruins of a fourth-century Roman villa. Jewelry and coins from what are today England, France, and Italy were found at the site. Fascinating for archaeologists, but a critical delay for the physicists; construction stopped for six months while the ruins were examined.
That was far from the end of it. The location of the CMS cavern turns out to sit beneath an underground river. The flowing water isn’t enough to disturb the experiment itself, but it posed problems for the excavation process. The construction team came up with a very physics-like solution: They sank pipes into the ground and filled them with liquid nitrogen, freezing the water into ice and giving the diggers solid ground to work with. “That was quite exciting,” Evans observed.
Evans, and the many other physicists and CERN staff working on the LHC, persevered. Apart from technical problems, skittish governments were constantly threatening to cut their contribution to CERN. At the highest levels, particle physics requires as much diplomacy and political savvy as scientific and technical know-how. A major step forward was achieved in 1997, when the United States agreed to contribute $2 billion to the project. All of the official member states of CERN are European: Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, the Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland, and the United Kingdom. The United States (along with India, Japan, Russia, and Turkey) is an “observer” state, allowed to participate in physics operations and attend meetings of the CERN council, but not to officially contribute to setting policy. Many other countries have agreements allowing their scientists to work at CERN. But the United States is the gorilla in the room, and securing a major commitment to the success of the LHC played a significant role in its development, as did earlier commitments from Japan and Russia. More than a thousand American physicists were soon working on the LHC.
Evans has a naturally easygoing style, and is more comfortable getting his hands dirty with a piece of equipment than demanding that underlings keep careful records of ongoing progress. While construction on the LHC proceeded according to plan, tiny cost overruns gradually accumulated. Matters came to a head in 2001, when it was realized that the accelerator was approximately 20 percent over budget. Against Evans’s judgment, Director General Luciano Maiani revealed the overrun in an open CERN council meeting, directly requesting that the member states pony up to cover the extra cost.
They were not happy. Robert Aymar, who would follow Maiani as director general in 2004, was instructed by the CERN council to undertake a close look at the management of their flagship machine. Some questioned whether Evans was the right man for the bureaucratic task, and whether a sterner hand wasn’t required. But Aymar understood that Evans’s unique understanding of the LHC was far more valuable than any looseness of style, and he was kept on as director of the project. Evans would later characterize this time as a low point in his work on the LHC. “I really got a grilling,” he said. “That was the worst year of all.”
On the September 19 incident after the machine had started up, Evans reflected, “This was the last circuit on the last sector, so it was a bitch. Fortunately, I’ve had some hard problems in the past.”
Accelerating particles
In a game of tetherball, one end of a rope is attached to a volleyball and the other to the top of a pole. Two combatants stand on opposite sides of the pole, whacking at the ball in an attempt to wind the rope around the pole. Now imagine there is just a single player, and that the rope can revolve freely around the top of the pole rather than get twisted up. On each revolution, the player pushes the ball in the same direction, nudging it toward ever-faster speeds.
In a nutshell, that’s the basic idea behind a particle accelerator such as the LHC. The role of the volleyball is played by a bunch of protons. The role of the rope that keeps the ball moving in a circle is played by strong magnetic fields that curve the protons around the ring. And the role of the player hitting the ball is played by an electric field that pushes the protons to increase their speed on each revolution.
Protons are extremely small by everyday standards, about one ten-trillionth of an inch across. You can’t just pick one up and throw it or whack at it with your hand as it passes by. To accelerate the protons in the LHC, a voltage generator creates a rapidly varying electric field that switches its direction as the protons pass, about 400 million times a second. The switching is timed very precisely, so that any given proton always sees an electric field pointing in the same direction as it traverses through the cavity, swiftly imparting greater velocity. This boost happens only at one point along the ring; most of the effort over the twenty-seven-mile course is simply spent keeping the protons turning in the appropriate direction, not making them go faster.
When the LHC is going full steam, there are a total of about 500 trillion protons circulating in two beams, one moving clockwise and the other counterclockwise around the ring. (Numbers are approximate because the machine’s performance gradually improves over time.) That’s a lot of protons, but it’s still a tiny number compared with any everyday object. All of the protons in the LHC come from a single unassuming canister of hydrogen, which looks for all the world like a fire extinguisher. A molecule of hydrogen has two atoms, each with just one proton and one electron. A bit of molecular hydrogen is extracted from the canister, then zapped with electricity to strip off the electrons, and the protons are sent on their way. Lyn Evans, who had been trained in fusion science rather than particle physics, got his start at CERN working on just such a process. There are about 1027 hydrogen atoms in the canister, which is enough to keep the LHC running for about a billion years. Protons are not a rare resource.
Protons aren’t continuously injected into the LHC; they come in the form of a “fill,” which is added all at once, and maintained for about ten hours (or until the beam degrades for some reason). The protons are moved with utmost care through a series of preliminary accelerators before they finally enter the main ring. There is no room for sloppiness. The protons in the two circulating beams aren’t spread uniformly—they are grouped into thousands of “bunches” per beam, with more than 100 billion protons per bunch. The bunches are about an inch long, twenty-three feet apart, and focused into a very thin needle. The beam is about one twenty-fifth of an inch across while traveling around the ring—about the width of the lead in a pencil—and gets further concentrated down to one one-thousandth of an inch as the bunches enter a detector in order to collide. Protons all have an equal positive electric charge, so their natural tendency is to push apart from one another, and keeping the beam under control is a major task.
Besides the energy of the colliding particles, the other important quantity in an accelerator is the luminosity, which is a way of measuring how many particles are involved. You might think we could just count the number of particles zooming around, but what really matters is the number of collisions, and a lot of particles only lead to a lot of collisions if the beam is focused very tightly. During 2010, the priority was on shaking down the machine and checking that everything was in working order, so the luminosity wasn’t very high. By 2011, the kinks were largely worked out, and they collected about one hundred times as many collisions as in the previous year. In 2012, the success continued, and during the first half of the year they had more collisions than in all of 2011. That blaze of data is what enabled the sooner-than-anticipated discovery of the Higgs.
Speed and energy
The LHC’s protons have a l
ot of energy because they are moving fast—very close to the speed of light. Every massive object, whether a person or a car or a proton, has some amount of energy when it’s sitting still, from Einstein’s formula E = mc2, and an additional “kinectic” energy that depends on how fast it’s moving. In the everyday world, the energy of motion is much, much less than the energy an object has even at rest, just because everyday velocities are much, much less than the speed of light. The fastest airplane in the world is a NASA experimental craft called the X-43, which reaches speeds of up to seven thousand miles per hour; at that velocity, the plane’s energy of motion adds only one ten-billionth of its energy at rest.
Protons in the LHC move quite a bit faster than the X-43. During its first 2009–2011 run, they were traveling at 99.999996 percent of the speed of light, or 670,616,603 miles per hour. At those velocities, the energy of motion is much greater than the energy at rest. The rest energy of a proton is just a shade under one GeV. The first run of the LHC featured protons with 3,500 GeV of energy each, or 3.5 TeV for short, so that when two of them collided there was a total of 7 TeV of energy to go around. The 2012 run collided protons with a total of 8 TeV of energy, and the eventual goal is to reach 14 TeV. Fermilab’s Tevatron, by contrast, maxed out at about 2 TeV of total energy.
At velocities this close to the speed of light, the theory of relativity becomes crucially important. Relativity teaches us that space and time change at high velocities: Time slows down compared to clocks at rest, and lengths get contracted along the direction of motion. As a consequence, the seventeen-mile trip around the ring would appear like a much shorter journey to one of the high-energy protons, if protons noticed such things. At 4 TeV, a proton would perceive one trip around the ring to extend only twenty-one feet. Once they get up to 7 TeV per proton, it will be only twelve feet.
How much energy is a TeV? Not that much—about equal to the energy of motion of a mosquito in flight, not something you would notice if it bumped into you. The amazing thing is not that 4 TeV (or whatever) is so much energy, it’s that all that energy is packed into a single proton. And remember that there are 500 trillion protons zooming around inside the LHC. If we take the beam as a whole, now we’re talking serious energy—about the same energy of motion as an onrushing locomotive engine. You wouldn’t want to get in the way.
Or would you? While the protons in the LHC pack a considerable punch, they are collimated into a very fine beam. Maybe most of them would pass right through you?
Yes and no. Nobody has ever stuck any body parts into the LHC beam, nor could they possibly; it’s tightly sealed in a vacuum tube, inaccessible to meddling humans. But in 1978, an unfortunate Soviet scientist named Anatoli Bugorski did manage to take a high-energy particle beam right in the face. (Safety standards at the U-70 Synchrotron in Protvino, Russia, were a bit more lax than they are at CERN.) The beam that hit Bugorski consisted of 76 GeV protons—much less than the LHC but still considerable. He was not instantly killed—indeed, he’s still alive today. Bugorski later testified that he saw a flash of light, “brighter than a thousand suns,” but he reportedly didn’t feel pain. He received significant radiation scarring, lost hearing in his left ear, and became paralyzed on the left side of his face; he still suffers from occasional seizures. But he survived without noticeable mental impairment, went on to finish his PhD, and continued to work at the accelerator complex for years afterward. Still, experts recommend avoiding beams of high-energy protons.
The reason why Bugorski’s head was not blown to smithereens is that many of the protons did indeed simply pass through him. But at the LHC, it is often necessary to “dump” a fill, which means putting the entire energy of the beam somewhere. (If you could just slow the protons down they would harmlessly dissipate, but that’s not practical.) Another way of thinking of that total energy is that it adds up to about 175 pounds of TNT. And it all has to go somewhere, every ten hours or so at the end of a fill.
Experiments have demonstrated that the full brunt of the LHC beam would be sufficient to melt a ton of copper. You certainly don’t want it careening randomly into your finely tuned experimental apparatus. Instead, a dumped beam is deflected and diffused away from the normal beam line by special magnets, after which it travels half a mile before landing in a special graphite “dump block.” The graphite material is especially good at spreading the energy and not melting in the process, despite reaching temperatures of 1,400 degrees Fahrenheit. There are about ten tons of graphite in total, all of which are encased in one thousand tons of steel and concrete shielding. Give it a few hours to cool down, and you’re ready for the next beam dump.
Mighty magnets
We think of the LHC as a giant circular ring seventeen miles around, but it’s actually more like a curvy octagon, with the ring divided into octants. There are eight arcs, each over a mile and a half long, and the arcs are connected by straight sections about a third of a mile long. If you were to visit one of the arcs in the LHC tunnel, you’d see a series of big blue tubes stretching in either direction—the “dipole magnets” that guide the protons as they pass down the beam pipe. There are 154 of these tubes along each arc, each of them fifty feet long and weighing over thirty tons. The inside of each tube is mostly taken up by an ultracold superconducting magnet, and in the very center are two narrow beam pipes through which the protons move—one with particles moving clockwise, the other counterclockwise.
If a charged particle like a proton sits stationary in a magnetic field, it doesn’t feel any force at all; it can happily stay there at rest. But when a moving charged particle passes through a magnetic field, it gets deflected from a straight line. (Neutral particles would pass right through, unaffected.) Remember that the LHC beam has the energy of a moving train; we need such incredibly powerful magnets simply because it’s not easy to bend the protons in a tight curve.
The LHC magnets are as strong as they can be, to allow for the highest possible proton energies in a tunnel of fixed size. The earth has a magnetic field, which helps your compass tell the difference between north and south; the field inside one of the LHC dipoles is about 100,000 times stronger than the earth’s. So strong, in fact, that ordinary materials aren’t up to the job, and superconductors are required. The magnets contain almost five thousand miles of wound cable, made from a superconducting compound of niobium and titanium, cooled to ultralow temperatures by 120 tons of liquid helium. The inside of the LHC is actually colder than outer space: the magnet temperatures are lower than that of the cosmic background radiation left over from the Big Bang.
Temperature isn’t the only criterion by which the LHC compares favorably with outer space. The interior of the beam pipes, the tubes through which the protons actually travel, must be kept as empty as absolutely possible; if they were filled with air, the protons would constantly be running into the air molecules, destroying the beam. So the beam pipes are kept in a very strict vacuum, so much so that the pressure inside the pipe is about the same as the atmospheric pressure on the moon.
Before the machine was started for the first time, the LHC team worried about whether they had made the beam pipe as empty as required. When the Tevatron started up at Fermilab in 1983, the first attempts to circulate protons quickly fizzled out; the culprit was ultimately discovered to be a tiny piece of tissue clogging the pipe. But how do you easily check seventeen miles of accelerator? The beam pipes themselves are only about an inch across, which led to an ingenious idea: Technicians made a kind of “Ping-Pong ball” from impact-resistant polycarbonate, stuck a radio transmitter inside, and sent it rolling down the pipe. If the ball got stuck, technicians could track the transmissions and figure out where it had stopped. It was a neat idea, and someone was probably disappointed when the balls rolled through unscathed, giving the beam pipes a clean bill of health.
The LHC magnets are the biggest and bulkiest parts of the machine, and represent an extraordinary triumph of technological innovation as well as international collaborati
on. That level of precision doesn’t come cheaply. It’s hard to put an exact cost on the LHC, because many expenses go into the upkeep of the lab in general, but a figure around $9 billion gives a good feeling for the total budget. In the words of physicist Gian Giudice, “When expressed in euros per kilogram, the price of the LHC dipoles—the most expensive part of the accelerator—is the same as Swiss chocolate. Were the LHC built of chocolate, it would cost about the same.”
Chocolate might not sound very expensive; after all, we eat it. But usually not seventeen miles’ worth of the very best. It all adds up.
Passing the torch
Lyn Evans officially retired from CERN in 2010, after the machine was successfully up and running. He had first joined the lab in 1969, giving him more than four decades of experience, serving through ten different directors general. Back in 1981, he, Carlo Rubbia, and Sergio Cittolin, an Italian physicist with a penchant for decorating lab notebooks with Leonardo da Vinci–style sketches, were the only three people in the control room at 4:15 a.m., when they turned on the upgraded Super Proton Synchrotron and witnessed the first proton-antiproton collisions inside a particle accelerator.