Authors: Sean Carroll
In retrospect, the accident in September 2008 helped the physicists and technicians at the LHC understand their machine much better, and as a result, the physics runs beginning in 2010 were stories of essentially uninterrupted progress. Given that operations didn’t start in earnest until that year, it came as a surprise to almost everybody that the experiments collected and analyzed enough data to discover the Higgs by July 2012. It’s as if you purchased an expensive car that breaks down almost immediately, and you have to spend a while combating some pesky maintenance problems. But once you finally get it on the road and hit the accelerator, the performance takes your breath away.
The Large Hadron Collider is Big Science at its biggest. The number of moving parts—human as well as mechanical—can sometimes be intimidating, or even depressing. In the words of Nobel Laureate Jack Steinberger, “The LHC is a symbol of just how difficult it is these days to make any progress. What a difference when compared to my thesis days, sixty-five years ago, when I, singlehandedly, in half a year, could do an experiment which marked an interesting step forward.” The LHC is the largest and most complicated machine ever built by human beings, and sometimes it’s a surprise that it works at all.
But it does work—spectacularly well. Over and over again, physicists I talked to while writing this book spoke of the awe-inspiring immensity of the operation, but also about how CERN could serve as a model for large-scale international collaboration. Experimentalist Joe Incandela said, “What’s amazing to me is that we have people from seventy countries around the world working—together. Palestinians and Israelis working side by side, Iranians and Iraqi scientists work together—such collaborations in the pursuit of Big Science shouldn’t be overlooked.” Joe Lykken, an American theoretical physicist at Fermilab, wistfully mused, “If only the United Nations could work like CERN, the world would be a much better place.”
If you believe that it’s a worthwhile task to pursue particles like the Higgs boson that require a huge amount of energy to create, Big Science is the only way to go. There is a tremendous amount of fantastic research to be done that can be tackled with relatively inexpensive tabletop experiments, but discovering new massive particles isn’t in that category. Right now the LHC is the only game in town, and its performance is a testimony to human ingenuity and perseverance.
Years of planning
The LHC is a marvel of planning and design. Physicists at CERN had been thinking about a giant proton collider for a while, but the first “official” discussions about what would eventually become the LHC were held at a workshop in Lausanne, Switzerland, in March 1984. The planners knew that the United States was contemplating what would eventually become the Superconducting Super Collider, so they needed to decide whether a European competitor was a sensible use of scarce resources. (They didn’t know, of course, that the SSC would eventually be canceled.) Unlike the SSC, which started from scratch building a new facility, the LHC would be limited in scope by the need to fit inside the already-constructed LEP tunnel. As a result, the target energy was set at 14 TeV, barely more than one-third of the 40 TeV target for the SSC. But the LHC would be able to produce more collisions per second, and was less expensive—and maybe all the good physics would be accessible at 14 TeV, rendering the higher energy of the SSC irrelevant.
Much of the impetus for moving forward with the LHC came from Italian physicist Carlo Rubbia, a brash and influential experimentalist who had collected a Nobel Prize in 1984 for his discovery of the W and Z bosons. Rubbia is a larger-than-life figure, as well-known for his forceful personality as for his accomplishments as a scientist (which are considerable). It was he who cajoled CERN into building the first proton-antiproton collider in 1981, a concept that would later be adopted by Fermilab’s Tevatron. (With the LHC we are back to colliding protons on protons, as it is too difficult to make a sufficient number of antiprotons to create the sought-after number of collisions.)
First as the chair of CERN’s Long Range Planning Committee, and later as director general of the lab from 1989 to 1993, Rubbia pushed strongly for the LHC at a time when LEP wasn’t yet finished and the United States was thought to be moving forward with the SSC. Europe was facing its own budgetary woes, especially in Germany, where the costs of reunification were running high. Rubbia was eventually able to convince the European governments that a hadron collider was the logical next step for the lab, regardless of what other countries might be doing. It wasn’t until 1991 that the CERN council adopted a resolution to officially study the LHC proposal, and not until December 1994 (after the SSC was canceled) that the project was finally approved. Lyn Evans was appointed director of the LHC, and the massive task of moving from idea to reality began in earnest.
The architect
In a project stretching over so many years, involving so many people and countries, and with such an intimidating number of significant subprojects, it would be unfair to give too much credit to any single person, downplaying the role of so many others. Nevertheless, if any individual is to be mentioned as having built the LHC, it would be Lyn Evans.
Evans comes across as an unassuming man, gray-haired and distinguished-looking but informal. Born to a mining family in Wales, his first love was chemistry; he took special joy making explosives, perhaps a fitting start for someone who would one day engineer the highest-energy particle collisions humans have ever achieved. In university he switched to physics because “physics was more interesting, and easier.” When the LHC project was approved, CERN needed someone with enough experience to manage the job, but young and energetic enough to see it through to completion. Evans was handed the daunting task of squeezing the highest possible physics return out of a machine with a fixed size, a limited budget, and an array of technological challenges that were unique in the history of experimental science. It was Evans who figured out how to take the original schematic plans for the LHC and modify them into a design that was compatible with financial realities.
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 10
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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.