The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (13 page)

Speed and energy

The LHC’s protons have a lot 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 = mc
²
, 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 counter
clockwise.

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 collaboration. 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.

Quite a difference from September 10, 2008, when the inauguration of the LHC was an international event witnessed by hundreds of people live and thousands more watching by Internet feeds around the world. On that day, Evans served as master of ceremonies in an LHC control room packed with news media, famous scientists, and visiting dignitaries. Drawing out the suspense, they didn’t simply push protons all the way around the ring, but opened up the eight sectors one by one. After the first seven sectors had been successfully navigated, Evans counted down as they prepared the protons to make a full circle of the ring. At the appointed moment, two dots flashed on a gray computer screen, indicating that the beam had both successfully left and arrived back at the same point. The room broke into applause, and a new era in particle physics had begun.

Physicists rarely retire in the conventional sense, and for Evans the new phase of his life will involve joining the CMS experiment at the LHC and helping to plan the next generation of accelerators. After the seminars announcing the discovery of the Higgs, he took a moment to muse on what it had felt like. “I went to the CMS summer party the other day, and there were about five hundred people there. When I see all these young people, I suddenly realize what a weight has been on my shoulders. I mean, how many people are relying on this machine to perform?”

Now that the machine is zooming along, CERN hopes that it will continue on for decades to come. It took more than a year to recover from the September 2008 setback, but since coming back to life the machine has performed splendidly. Running at 7 TeV of total energy through 2010 and 2011, then at 8 TeV in 2012, enabled the discovery of the Higgs boson or something very much like it. Still, the ultimate goal is to hit 14 TeV, and to achieve that will require shutting down for two years while equipment is tested and improved. The shutdown was originally planned to begin in late 2012, but after the discovery the CERN council decided to keep it running at 8 TeV for another few months. It’s a natural reaction; whenever you get a new toy, you want to play with it right now.

SIX

WISDOM THROUGH SMASHING

In which we learn how to discover new particles by colliding other particles at enormous speeds, and watching what happens.

A
s a child, I was fascinated by all kinds of science, but only two subjects really captured my attention: theoretical physics and dinosaurs. (When I was twelve, I didn’t know the word “paleontology.”) I flirted with other sciences, but the relationships never went very far. My junior chemistry set was fun, mostly because I could set things on fire, but I was never entranced by the thrill of creating new compounds in carefully controlled conditions.

But dinosaurs! There was true romance. My grandfather would take my brother and me to the New Jersey State Museum in Trenton, where we would skip right past the boring artifacts and history exhibitions to gawk at the ominously looming skeletons. I never seriously considered paleontology as a career, but every scientist I know secretly agrees that dinosaurs are the epitome of cool.

Which is why I was thrilled when, as a grown-up faculty member at the University of Chicago, I got the chance to go on a dinosaur expedition. Most paleontological outings do just fine without bringing physicists along, but this expedition was organized by Project Exploration, a nonprofit outfit devoted to bringing science to children and underrepresented minorities. It was a special event for friends of the organization, and I was brought along to provide a different kind of science outreach. Didn’t really matter to me—they could have said I was brought along to wash dishes, all I cared was that I was going to dig up dinosaur bones.

And dig we did, in a region of the Morrison geological formation near Shell, Wyoming (population approx. 50). The Morrison is chock-full of fossils from the Jurassic, and we whiled away the daylight heat cheerfully digging up specimens of
Camarasaurus
,
Triceratops
, and
Stegosaurus
. “Digging up” might give an exaggerated sense of the accomplishments of our largely amateur crew; mostly we made some progress on sites that would eventually be covered up and left for another trip to finish.

This experience taught me a great deal—primarily that theoretical physics is a cushier job than paleontology. However, it also answered a question that had been bugging me for years: How do you tell the difference between a piece of fossilized bone and the rock matrix that surrounds it? Over the course of millions of years, the original skeleton absorbs mineral from the rock nearby, until eventually it is more rock than bone. How do you distinguish one from the other?