Knocking on Heaven's Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World - Lisa Randall (2011)

Part III. MACHINERY, MEASUREMENTS, AND PROBABILITY

Chapter 9. THE RETURN OF THE RING

I entered graduate school for physics in 1983. The LHC was first officially proposed in 1984. So in some sense I’ve been waiting for the LHC for the quarter century of my academic career. Now, at long last, my colleagues and I are finally seeing LHC data and realistically anticipating the insights into mass, energy, and matter that the experiments could soon reveal.

The LHC is currently the most important experimental machine for particle physicists. Understandably, as it commenced operation, my physicist colleagues became increasingly anxious and excited. You couldn’t enter a seminar room without someone inquiring about what was happening. How much energy would collisions achieve? How many protons will beams contain? Theorists wanted to understand minutiae that had previously been almost an abstraction to those of us engaged in calculations and concepts and not machine or experimental design. The flip side was true as well. Experimenters were as eager as I’d ever seen them to hear about our latest conjectures and learn more about what they might look for and possibly discover.

Even at a conference that took place in December 2009, that was purportedly about dark matter, participants were eagerly commenting on the LHC—which had just completed its incredibly successful debut of acceleration and collisions. At the time, after the near despair of a little more than a year before, everyone was ecstatic. Experimenters were relieved they had data they could study to understand their detectors better. Theorists were happy they might get some answers before too long. Everything was working fabulously well. The beams looked good. Collisions had occurred. And experiments were recording events.

However, reaching this landmark was quite a story, and this chapter tells the tale. So fasten your seat belt. It was a bumpy ride.

A SMALL WORLD AFTER ALL

The story of CERN precedes that of the LHC by several decades. Soon after the end of World War II, a European accelerator center that would host experiments studying elementary particles was first conceived. At that time, many European physicists—some of whom had immigrated to the United States and some of whom were still in France, Italy, and Denmark—wanted to see cutting-edge science restored to their original homelands. Americans and Europeans agreed that it would be best for scientists and science if Europeans joined together in this common enterprise and returned research to Europe so they could repair the residue of devastation and mistrust remaining after the recently ended war.

At a UNESCO conference in Florence in 1950, the American physicist Isidor Rabi recommended the creation of a laboratory that would reestablish a strong scientific community in Europe. In 1952, the Conseil Européen pour la Recherche Nucléaire (hence the acronym CERN) was set up to create such an organization, and on July 1, 1953, representatives from twelve European nations came together to create the institution that became known as “the European Organization for Nuclear Research,” and the convention establishing it was ratified the following year. The CERN acronym clearly no longer reflects the name of the research center. And we now study subnuclear, or particle, physics. But as is often true with bureaucracy, the initial legacy remained.

The CERN facility was deliberately built centrally in Europe on a site crossing the Swiss-French border near Geneva. It’s wonderful to visit if you like the outdoors. The fabulous setting includes farmland and the Jura Mountains immediately nearby and the Alps readily accessible in the distance. CERN experimenters are on the whole a rather athletic bunch, with their easy access to skiing, climbing, and biking. The CERN site is quite large, covering enough territory for an exhausting run to keep those athletic researchers in shape. The streets are named after famous physicists, so you can drive on Route Curie, Route Pauli, and Route Einstein on a visit to the site. The architecture at CERN was, however, a victim of the time in which it was built, which was the 1950s with bland International Style low-rises, so CERN buildings are rather plain with long hallways and sterile offices. It didn’t help the architecture that it was a science complex—look at the science buildings on most any university and you will usually find the ugliest buildings on campus. What enlivens the place (along with the scenery) are the people who work there and their scientific and engineering goals and achievements.

International collaborations would do well to study CERN’s evolution and its current operations. It is perhaps the most successful international enterprise ever created. Even in the aftermath of World War II, when the countries had so recently been in conflict, scientists from twelve different nations joined together in this common enterprise.

If competition played any role at all, it was primarily directed against the United States and its burgeoning scientific endeavors. Until experiments at CERN found the W and Z gauge bosons, almost all particle physics discoveries had come from accelerators in America. The drunken physicist who walked into the common area at Fermilab where I was a summer student in 1982 saying how they “had to find the bloody vector bosons” and destroy America’s dominance probably expressed the viewpoint of many European physicists at the time—though perhaps somewhat less eloquently and definitely with poorer diction.

CERN scientists did find those bosons. And now, with the LHC, CERN is the undisputed center of experimental particle physics. However, this was by no means predetermined when the LHC was first proposed. The American Superconducting Supercollider (SSC) that President Reagan approved in 1987 would have had almost three times the energy—had Congress continued its support. Although the Clinton administration initially didn’t support the project initiated by its Republican predecessors, that changed as President Clinton better understood what was at stake. In June 1993, he tried to prevent the cancellation in a letter to William Natcher, chairman of the House Committee on Appropriations, in which he said, “I want you to know of my continuing support for the Superconducting Super Collider (SSC)…. Abandoning the SSC at this point would signal that the United States is compromising its position of leadership in basic science—a position unquestioned for generations. These are tough economic times, yet our Administration supports this project as a part of its broad investment package in science and technology…. I ask you to support this important and challenging effort.” When I met the former president in 2005, he brought up the subject of the SSC and asked what we had lost in abandoning the project. He quickly acknowledged that he too had thought that humanity had forfeited a valuable opportunity.

Around the time that Congress killed the SSC, taxpayers ponied up about $150 billion to pay for the savings and loan crisis, which far exceeded the approximately $10 billion the SSC would have cost. The U.S. annual deficit in comparison amounts to a whopping $600 per American, and the Iraq War to more than $2,000 per citizen. With the SSC we would have had high-energy results already, and we would have reached far higher energies even than the LHC will achieve. With the end of the S&L crisis we left ourselves open to the financial crisis of 2008 and a bailout that was even more expensive to taxpayers.

The LHC’s price tag of $9 billion was comparable to the SSC’s proposed cost. It amounts to about $15 per European—or as my colleague Luis Álvarez-Gaumé at CERN likes to say, about a beer per European per year during the construction time of the LHC. Assessing the value of fundamental scientific research of the sort taking place at the LHC is always tricky, but fundamental research has spurred electricity, semiconductors, the World Wide Web, and just about all technological advances that have significantly affected our lives. It also inspires technological and scientific thinking, which spreads into all aspects of our economy. The LHC’s practical results might be difficult to anticipate, but the science potential is not. I think we can agree that the Europeans in this case are more likely to get their money’s worth.

Long-term projects require belief, dedication, and responsibility. Such commitments are becoming increasingly hard to come by in the United States. Our past vision in the U.S. led to tremendous scientific and technological advances. However, this type of essential long-term planning is becoming increasingly rare. You have to hand it to the European Community for their ability to continue to see their projects through. The LHC was first envisioned a quarter century ago and approved in 1994. Yet it was such an ambitious project that only now is it reaching fruition.

Furthermore, CERN has successfully broadened its international appeal to include not only the 20 CERN member states, but also 53 additional nations that have also participated in the design, construction, and testing of instruments—and scientists from 85 countries currently participate. The United States isn’t an official member state, but there are more Americans than any other single nationality working on the major experiments.

About 10,000 scientists participate in total—perhaps about half of the total number of particle physicists on Earth. One-fifth of them are full-time employees who live nearby. With the advent of the LHC, the main cafeteria has become so packed that you could barely order food without your tray hitting another physicist—a problem that a new cafeteria extension now helps alleviate.

With its international population, an American arriving at CERN will be struck by the many languages and accents reverberating in the cafeterias, offices, and hallways. The Americans will also notice the cigarettes, cigars, wine, and beer there, which also remind them they’re not at home. Some comment as well on the superior quality of the cafeterias, as did one of my freshman students who had worked there over the summer. Europeans, with their more refined palates, tend to find this assessment somewhat questionable.

The many employees and visitors at CERN range from engineers to administrators to the many physicists who actually do the experiments and the more than 100 physicists who participate in the theory division at any given time. CERN is structured hierarchically, with the chief officers and council responsible for all policy matters, including major strategic decisions. The head is known as the director general (DG) which perhaps has the ring of something out of Gilbert and Sullivan, though the many directorships under the DG account for the name. The CERN Council is the ruling body responsible for major strategic decisions such as planning and scheduling projects. It pays special attention to the Scientific Policy Committee, which is the major advisory board that helps evaluate proposals and their scientific merit.

The large experimental collaborations, with thousands of participants, have a structure of their own. Work is distributed according to detector components or types of analyses. A given university group might be responsible for one particular piece of the apparatus or one particular type of potential theoretical interpretation. Theorists at CERN have more freedom than experimenters to work on whatever is of interest to them. Sometimes their work pertains to CERN experiments, but many of them work on more abstract ideas that won’t be tested anytime soon.

Nonetheless, all particle physicists at CERN and around the globe are excited about the LHC. They know their future research and the future of the field itself relies on the successful operation and discoveries of the next 10 to 20 years. They understand the challenges, but they also agree in their bones with the superlatives that go with this enterprise.

A BRIEF HISTORY OF THE LHC

Lyn Evans was the LHC’s chief architect. Though I’d heard him speak in his lovely lilting Welsh intonation the year before, I finally met him at a conference in California in early January 2010. This was an opportune time since the LHC was finally on track, and even for an understated Welshman, his pleasure was obvious.

Lyn gave a wonderful talk about the roller-coaster ride he’d had since first setting out to build the LHC. He began by telling us about the true inception of the idea in the 1980s, when CERN conducted the first official studies investigating the option of producing a high-energy proton-proton collider. He then told about the 1984 meeting that most people consider the idea’s official initiation. Physicists at that time met with machine builders in Lausanne to introduce the idea of colliding together proton beams with 10 TeV of energy—a proposal that was scaled down to 7 TeV beams in the final implementation. Almost a decade later, in December 1993, physicists presented an aggressive plan to the CERN Council, the governing body at CERN over major strategic decisions, to build the LHC during the next 10 years by minimizing all other experimental programs at CERN aside from LEP. At that time, the CERN Council turned it down.

Initially, one argument against the LHC had been the intense competition posed by the SSC. But that disappeared with the project’s demise in October 1993, at which time the LHC became the sole candidate for a very high-energy accelerator. Many physicists then became increasingly convinced of the significance of the enterprise. On top of that, machine research was extremely successful. Robert Aymar, who would ultimately head CERN during the LHC construction phase, chaired a review panel in November 1993 that concluded the LHC would be feasible, economical, and safe.

The critical hurdle in planning the LHC was developing strong enough magnets on an industrial scale to keep highly accelerated protons circulating in the ring. As we observed in the previous chapter, the existing tunnel size presented the biggest technical challenge, since its radius was fixed and magnetic fields therefore had to be very big. In his talk, Lyn happily described the “Swiss watch precision” of the first 10-meter-long prototype dipole magnet that engineers and physicists successfully tested in 1994. They reached 8.73 tesla on their first shot, which was their target and a very promising sign.

Unfortunately, however, although European funding is more stable than that of the United States, unforeseen pressures introduced uncertainties for CERN’s finances as well. The budget for Germany, which contributes the most to CERN, suffered from the 1990 reunification. Germany therefore reduced its contributions to CERN, and, along with the United Kingdom, didn’t want to see any major increase in the CERN budget. Christopher Llewellyn Smith—the British theoretical physicist who succeeded the Nobel Prize—winning physicist Carlo Rubbia as CERN director general—was, like his predecessor, strongly supportive of the LHC. By acquiring funding from Switzerland and France, the two host states that stood to benefit the most from the LHC’s construction and operation in their home territory, Llewellyn Smith partially alleviated the serious budget issues.

The CERN Council was appropriately impressed—both with the technology and with the budget resolution—and approved the LHC soon afterward on December 16, 1994. Llewellyn Smith and CERN furthermore convinced nonmember states to join and participate. Japan came on board in 1995, India in 1996, and soon after Russia and Canada, with the United States following in 1997.

With all the contributions from Europe and other nations, the LHC could override a proviso in the original charter that called for construction and operation in two phases, the first of which would involve only two-thirds of the magnets. Both scientifically and in terms of total cost, the reduced magnetic field would have been a poor choice. But the original intention was to allow budgets to balance every year. In 1996, when Germany again reduced its contribution due to its reunification costs, the budget situation again looked grim. However, in 1997, CERN was allowed to compensate for the loss by financing construction with loans for the first time.

After the budget history lowdown, Lyn’s talk turned to more happy news. He described the first test string of dipoles—a test of magnets combined together in a workable configuration—that took place in December 1998. The successful completion of this test demonstrated the viability and coordination of several of the ultimate LHC components and was a critical milestone in its development.

In 2000, when LEP, the electron-positron collider, had run its course, it was dismantled to pave the way for LHC installation. Yet even though the LHC was ultimately built in a preexisting tunnel and used some of the staff, facilities, and infrastructure that were already in place, a lot of man-hours and resources would be necessary before the transformation from LEP to the LHC could occur.

The five phases of the LHC’s development included civil engineering to build caverns and structures for experiments, the installation of general services so that everything could run, the insertion of a cryogenic line to keep the accelerator cold, putting in place all the machine elements including the dipoles and all the associated connections and cables, and ultimately the commissioning of all the hardware to make sure everything worked as anticipated.

The CERN planners started off with a careful schedule to coordinate these construction phases. But as everyone knows, “the best laid plans o’ mice an’ men gang aft agley.” Needless to say, this applied all too well.

Budget issues were a constant nuisance. I remember the frustration and concern of the particle physics community in 2001 as we waited to find out how quickly some serious budget problems at the time could be resolved to allow construction to proceed. CERN management dealt with the cost overruns, but at a price in terms of CERN breadth and infrastructure.

Even after these funding and budget problems were resolved, LHC development still wasn’t entirely smooth sailing. Lyn in his talk described how a series of unforeseen events periodically slowed down construction.

Certainly no one involved in excavating the cavern for the CMS (Compact Muon Solenoid) experiment could have foreseen digging into a fourth-century Gallo-Roman villa. The property boundaries were parallel to the farm field boundaries that exist to this day. Excavation was halted while archaeologists studied buried treasure, including some coins from Ostia, Lyon, and London (Ostium, Lugdunum, and Londinium at the time the villa was occupied). Apparently the Romans were better at establishing a common currency than modern Europe, where the euro still hasn’t displaced the British pound and the Swiss franc as a means of exchange—particularly annoying for British physicists arriving at CERN who don’t have the currency required to pay for a taxi.

Compared to CMS’s travails, the 2001 excavation of the ATLAS cavern proceeded relatively uneventfully. Digging the cavern involved removing 300,000 metric tons of rock. The only problem they faced is that once the material was removed, the cavern floor began to rise slightly—at the rate of about a millimeter each year. This might not sound like much, but the movement could in principle interfere with the precise alignment of the detector pieces. So the engineers needed to install sensitive metrology instruments. They are so effective that they not only detect ATLAS movements, but are sufficiently sensitive to have registered the 2004 tsunami and the Sumatra earthquake that triggered it, as well as others that came later.

The procedure for building the ATLAS experiment deep underground was rather impressive. The roof was cast on the surface and suspended by cables while the walls were built up from below until the vault could sit on them. In 2003, the completed excavation was inaugurated with a celebration, notable for the presence of an alpine horn echoing inside, which in Lyn’s description was a source of great amusement. Installation and assembly of the experimental apparatus subsequently followed with the components lowered one by one until ultimately the ATLAS experiment was assembled with this “ship in a bottle method” in the excavated cavern belowground.

CMS preparations, on the other hand, continued to face rough seas. It once again got into trouble during excavation since it turned out that the CMS site was infelicitously placed not only over a rare archaeological site but also over an underground river. With the heavy rains that year, the engineers and physicists discovered to their surprise that the 70-meter-long cylinder they inserted into the ground to transport materials down had sunk 30 centimeters. To deal with this unfortunate hindrance, the excavators created walls of ice along the cylinder walls to freeze the ground and stabilize the region. Supporting structures to stabilize the fragile rock around the cavern also had to be installed, including screws up to 40 meters in length. Not surprisingly, the CMS excavation took longer than foreseen.

The only saving grace was that because of CMS’s relatively compact size, experimenters and engineers had already been considering constructing and assembling it on the surface. Constructing and installing components is a lot easier aboveground, and everything is faster since there is more room to work in parallel. This aboveground construction had the added critical benefit that the cavern problems wouldn’t further delay construction.

However, as you might imagine, it was a rather daunting prospect to lower this enormous apparatus—which is something I had a chance to think about when I first visited CMS in 2007. Indeed, lowering the experiment was no easy task. The largest piece began its 100-meter descent into the CMS pit, carried by a special crane, at the dauntingly low speed of 10 meters per hour. Since there was only a 10-centimeter leeway between the experiment and walls of the shaft, this slow descent and a careful monitoring system were critical. Fifteen large pieces of detector were lowered between November 2006 and January 2008—a brazen piece of timing as the final piece was delivered pretty close to the scheduled LHC start-up date.

Following the CMS water trouble, the next crisis in the construction of the LHC machine itself struck in June 2004, when problems were discovered in the helium distribution line known as the QRL. The CERN engineers who investigated discovered the French firm that had taken on this construction project had replaced the material designated in the original design with what Lyn described as a “five-dollar spacer.” The replacement material cracked, allowing thermal contraction of the inner pipes. This faulty component wasn’t unique, and all the connections had to be checked.

By this time the cryogenic line had been partially installed and many other pieces had already been produced. To avoid blocking the supply chain and introducing further delays, the CERN engineers decided to repair what had already been produced while leaving industry to correct the problem before delivering the remaining parts. CERN’s factory operations and the need to move and reinstall large pieces of the machine cost the LHC a year delay. At least the delay was far less than the decade delay Lyn and others feared had lawyers been involved.

Without pipes and the cryogenic system, no one could install magnets. So 1,000 magnets sat around in the CERN parking lot. Even with the high-end BMWs and Mercedes that grace the lot at times, $1 billion worth of magnets probably exceeded the usual parking lot contents’ net worth. No one stole the valuable magnets, but a parking lot isn’t a great place to store technology, and further delays associated with restoring the magnets to their initial specification were inevitable.

In 2005, yet another near crisis occurred, having to do with the inner triplet constructed at Fermilab in the United States and in Japan. The inner triplet provides the final focusing of the proton beams before they collide. It combines three quadrupole magnets with cryogenic and power distribution—hence the name. This inner triplet failed during pressure tests. Although the failure was an embarrassment and an annoying delay, the engineers could fix it in the tunnel so the time cost wasn’t too severe in the end.

Overall, the year 2005 was more successful than its predecessor. The CMS cavern was inaugurated in February, though no horn graced the day. Another landmark event occurred in February—the lowering of the first cryodipole magnet. Magnet construction had been critical to the LHC enterprise. A close collaboration between CERN and commercial industry facilitated their timely and economical construction. Though designed at CERN, the magnets were produced at companies in France, Germany, and Italy. Initially, CERN engineers, physicists, and technicians placed an order for 30 dipoles in 2000, which they might then carefully examine to ensure quality and cost control before placing the final order for more than 1,000 magnets in 2002. CERN nonetheless maintained responsibility for procuring the main components and raw materials in order to maximize quality and uniformity and minimize cost. To do so, CERN moved 120,000 metric tons of material within Europe, employing an average of 10 big trucks a day for four years. And that was only one piece of the LHC effort.

After delivery, the magnets were all tested and carefully lowered through a vertical shaft into the tunnel near the Jura Mountains that overlook the CERN site. From there, a special vehicle transported them to their destination along the tunnel. Because these magnets are enormous and only a few centimeters separated the wall of the tunnel from the LHC installations, the vehicle was automatically guided by an optically detected line painted on the floor. The vehicle moved forward at a rate of only about a mile an hour in order to limit vibrations. That meant it took seven hours to get a dipole from the lowering point to the opposite end of the ring.

In 2006, after five years of construction, the last of the 1,232 dipole magnets was delivered. In 2007, the big news was the last lowering of a cryodipole and the first successful cooldown of a 3.3-km-long section to the design temperature of—271 degrees Celsius—which allowed the whole thing to be powered up for the first time, with several thousand amps circulating in the superconducting magnets in this section of the tunnel. As often happens at CERN, a champagne celebration marked the occasion.

In 2006, after five years of construction, the last of the 1,232 dipole magnets was delivered. In 2007, the big news was the last lowering of a cryodipole and the first successful cooldown of a 3.3-km-long section to the design temperature of—271 degrees Celsius—which allowed the whole thing to be powered up for the first time, with several thousand amps circulating in the superconducting magnets in this section of the tunnel. As often happens at CERN, a champagne celebration marked the occasion.

A continuous cryostat section was closed in November 2007 and everything was looking pretty good until yet another near disaster struck, this time involving the so-called plug-in modules, known as PIMs. In the United States, we didn’t necessarily follow all the reports about the LHC. But news spread about this one. A CERN colleague told me about the worry that not only had this piece failed, but it could be a ubiquitous problem all around the ring.

The problem is the almost 300-degree differential between a room-temperature LHC and a cool operating one. This difference has an enormous impact on the materials with which it is constructed. Metal parts shrink when cooled and expand when warmed. The dipoles themselves shrink by a few centimeters during the cooldown phase. This might not sound like much for a 15-meter object, but the coils must be accurately positioned to within a tenth of a millimeter to maintain the intense uniform magnetic field required to properly guide the proton beams.

To accommodate the change, dipoles are designed with special fingers that straighten out to ensure electrical continuity when the machine is cooled down and that slide back when warmed. However, due to faulty rivets, the fingers collapsed instead of recessing. Worse yet, every interconnection was subject to this failure, and it wasn’t clear which ones were problematic. The challenge was to identify and fix each faulty rivet—without introducing a huge delay.

In a tribute to the ingenuity of the CERN engineers, they found a simple method of exploiting the existing electrical pickup located every 53 meters along the beam that was initially installed so that the electronics would be triggered by the beam passage. The engineers installed an oscillator into an object about the size of a Ping-Pong ball, which they could send around the tunnel along the path a beam would take. Each sector was three kilometers long and the ball could blow through, triggering the electronics each time it passed a pickup. When the electronics didn’t record a passage, the ball had hit the fingers. The engineers could then go in and fix the problem without having to open every single interconnect along the beam. As one LHC physicist joked, the first LHC collisions were not between protons, but between a Ping-Pong ball and a collapsed finger.

After this last resolution, the LHC seemed to be on track. Once all the hardware was in place, its operation could begin. In 2008, many human fingers crossed when at long last the first test took place.

SEPTEMBER 2008: THE FIRST TESTS

The LHC forms proton beams and after a series of energy boosts injects them into the final circular accelerator. It then sends those beams around the tunnel so that they return to their precise initial position, allowing the protons to circulate many times before being periodically diverted to collide with great efficiency. Each of these steps needs to be tested in turn.

The first milestone was to check whether the beams would actually circulate around the ring. And they could. Amazingly, after its long history of trials and tribulations, in September 2008, CERN fired up its two proton beams with so few hitches that the results exceeded expectations. On that day, for the first time, two proton beams in succession traversed the enormous tunnel in opposite directions. This single step involved commissioning the injection elements, starting the controls and instruments, checking that the magnetic field would keep the protons in the ring, and making sure all the magnets worked to spec and could run stimultaneously. The first time this sequence of events was ready was the evening of September 9. Yet everything worked as well as or better than planned when the tests took place the next day.

Everyone involved with the LHC describes September 10, 2008, as a day they will never forget. When I visited a month afterward, I heard many stories of the day’s euphoria. People followed the trajectory of two spots of light on a computer screen with unbelievable excitement. The first beam almost returned successfully on its first go-round, and with minor tweaking followed the exact path that was intended within the first hour of its being turned on. The beam at first went around the ring for a few turns. Then each successive burst of protons was adjusted slightly so that the beam was soon circulating hundreds of times. Not long after this, the second beam did the same—taking about one and a half hours to get exactly on track.

Lyn was just as happy that he didn’t know about the live video feed at the time from the control room, where the engineers were following the project, to the Internet, where the events were being broadcast for anyone to see. So many people watched those two dots on their screens that the sites were shut down for breaking capacity. People all over Europe—the CERN press office claims a couple of million—sat mesmerized as engineers modified the protons’ path to make them successfully circulate around the full circumference of the ring. Meanwhile, inside CERN, the thrill was palpable as physicists and engineers gathered in auditoriums to watch the same thing. At this point, the LHC outlook seemed more than extremely promising. The day was a wonderful success.

But a mere nine days later, euphoria transformed into despair. At the time, two important new features were to be tested. First, the beams were to be accelerated inside the LHC ring to higher energy than they had been during the first test, which used only the beam injection energy that protons have when first entering the LHC ring. The second part of the plan was to collide those beams, which would of course have been a huge milestone in LHC development.

However, at the last moment—on September 19—despite the engineers’ many considerations and precautions, the test failed. And when it did, it did so catastrophically. A simple soldering error in the copper casing connecting two magnets combined with too few functioning helium release valves caused a yearlong delay before protons would first collide.

The problem was that as scientists tried to ramp up the current and energy of the eighth and final sector, a joint between two magnets along the busbar that connects them broke. A busbar is a superconducting joint that connects a pair of superconducting magnets. (See Figure 27.) The splice that holds together a joint between two magnets was the culprit. The faulty connection created an electrical arc that punctured the helium enclosure and caused six metric tons of liquid helium—that would ordinarily be warmed up slowly—to be suddenly released. Superconductivity was lost in the quenching that occurred when the liquid helium heated up and reverted to gas.

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FIGURE 27 ] A busbar connects different magnets together. A faulty solder in one was responsible for the unfortunate incident in 2008.

The enormous amount of helium released created a huge pressure wave that effectively caused an explosion. In less than 30 seconds, its energy displaced some magnets and destroyed the vacuum in the beam pipe, damaged the insulation, and contaminated 2,000 feet of beam pipe with soot. Ten dipoles were totally destroyed and 29 more were so damaged they needed to be replaced. Needless to say, this was not exactly what we had been hoping for. And this was also something no one in the control rooms had any inkling of until someone noticed that a stop button in the tunnel for one of the computers had been triggered by the escaping helium. Soon afterward, they realized the beam had been lost.

I learned more about the backstory during a visit to CERN a few weeks after the mishap. Keep in mind that the ultimate goal for collisions is a center of mass energy of 14 TeV, or 14 trillion electron volts. The decision was made to keep the energy down to only about 2 TeV for the first run in order to ensure that everything functioned properly. Later the engineers planned to increase it to 10 TeV (5 TeV per beam) for the first actual data runs.

However, the plan became more ambitious following a small delay due to a transformer that broke on September 12. Scientists continued testing the tunnel’s eight sectors up to 5.5 TeV during the interval afforded by the short delay and had time to test seven out of the eight sectors. They verified those could run properly at higher energy, but they didn’t have the opportunity to test the eighth. They nonetheless decided to charge ahead and attempt higher-energy collisions since there didn’t seem to be any problem.

Everything worked fine until the engineers attempted to raise the energy of the last untested sector. The crippling accident occurred when its energy was being raised from about 4 to 5.5 TeV—which required between 7,000 and 9,300 amps of current. This was the last moment for something to go wrong, and it did.

During the year of the delay, everything was repaired at a cost of about $40 million. Although repairing the magnets and the beam took time, they were not impossible tasks. Enough spare magnets were on hand to replace the 39 dipole magnets that were beyond repair. In total, 53 magnets (14 quadrupole and 39 dipole) were replaced in the sector of the tunnel where the incident occurred. In addition, more than four kilometers of the vacuum beam tube were cleaned, a new restraining system for 100 quadrupole magnets was installed, and 900 new helium pressure release ports were added. In addition, 6,500 new detectors were added to the magnet protection system.

The bigger risk was the presence of 10,000 joints between magnets that could potentially cause the same problem. The danger had been identified, but how could anyone trust that this problem would not reemerge elsewhere in the ring? Mechanisms were needed to detect any similar problem before it could cause any harm. The engineers once again rose to the challenge. Their updated system now looks for minuscule voltage drops that might signal the presence of resistive joints, signaling a break in the closed system that houses the cryogenics that keeps the machine cold. Caution also dictated some delays to improve the helium release valve system and to further study the joints as well as the copper casings of the magnets themselves—which meant a delay in achieving the highest energies at which the LHC is designed to operate. Nonetheless, with all the new systems to monitor and stabilize the LHC, Lyn and others were confident that the kind of pressure buildups that caused the damage will be avoided.

In some sense, we are lucky that engineers and physicists were able to fix things before true operations began and filled the experiments with radiation. The explosion cost the LHC a year before they could even begin to test beams and aim for collisions again. That was a long time, but not so long on the scale of a quest for the underlying theory of matter that we have had for the last 40 years, and in many respects for thousands of years.

On October 21, 2008, the CERN administration did, however, stick to one piece of their initial plan. On that day, I joined 1,500 other physicists and world leaders outside Geneva to celebrate the official LHC inauguration, which had been optimistically planned well in advance—before anyone could have predicted the disastrous events that occurred a mere few weeks before. The day was filled with speeches, music, and—as is important at any European cultural event—good food. It was enjoyable and informative even with the premature timing. Despite anxieties about the September incident, everyone was filled with hope that these experiments would shed light on some of the mysteries surrounding mass, the weakness of gravity, dark matter, and the forces of nature.

Although many CERN scientists were unhappy about the infelicitous timing of the event, I saw the celebration more as a contemplation of this triumph of international cooperation. The day’s events did not yet honor discovery but instead recognized the potential of the LHC and the enthusiasm of the many countries participating in its creation. A few of the speeches were truly encouraging and inspirational. The French prime minister, Frangois Fillon, spoke of the importance of basic research and how the world financial crisis should not impede scientific progress. The Swiss president, Pascal Couchepin, spoke of the merit of public service. Professor Jose Mariano Gago, Portugal’s minister for science, technology, and higher education, spoke about valuing science over bureaucracy and the importance of stability for creating important science projects. Many of the foreign partners visited CERN for the first time for the day’s celebration. The person seated next to me during the ceremony worked for the European Union in Geneva—but had never set foot inside CERN. Having seen it, he enthusiastically informed me of his intention to return soon with his colleagues and friends.

NOVEMBER 2009: VICTORY AT LAST

The LHC finally came back online on November 20, 2009, and this time, it was a stunning success. Not only did proton beams circulate for the first time in a year, but a few days later, they finally collided, creating sprays of particles that would enter the experiments. Lyn enthusiastically described how the LHC worked better than he had expected—a remark that I found encouraging but a bit peculiar in light of his being in charge of making the machine run as successfully as it had.

What I hadn’t understood was how much more quickly all the pieces had fallen into place than would have been anticipated based on the experience with past machines. Maurizio Pierini, a young Italian CMS experimenter, explained to me what Lyn had meant. Tests that took 25 days in the 1980s for the LEP beams of electrons and positrons in the same tunnel were now completed in less than a week. The proton beams were remarkably on target and stable. And the protons stayed in line—very few stray particles were detected. The optics worked, the stability tests worked, realignments worked. The actual beams matched precisely the computer programs that simulated what should occur.

In fact, the experimenters were taken by surprise when they were told Sunday at 5:00 P.M., only a couple of days after the renewed beams began circulation, to expect collisions the next day. They had anticipated a little bit of time between first beams after the shutdown and the first actual collisions they could record and measure. This was now to be their first opportunity to test their experiment with actual proton beams, rather than the cosmic rays they had used while waiting for the machine to run. The short notice meant, however, that they had very little time to reconfigure their computer triggers that tell computers which collisions to record. Maurizio described the anxiety they all felt, since they didn’t want to foolishly fumble this opportunity. At the Tevatron, the first test had been mangled by an unfortunate resonance of the beam circulation with the readout system. No one wanted to see this happen again. Of course, in addition to unease, an enormous amount of excitement was shared by everyone involved.

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FIGURE 28 ] Brief outline of the LHC’s history.

On November 23, the LHC at long last had its first collision. Millions of protons collided with the injection energy of 900 GeV. These events meant that after years of waiting, experiments could begin taking data—recording the results of the first proton collisions in the LHC ring. Scientists from ALICE, one of the smaller experiments, even submitted a preprint (a paper before publication) on November 28.

Not too long afterward, a modest acceleration was applied to create 1.18 TeV proton beams, the highest-energy circulating beams ever. Only a week after the first LHC collisions, on November 30, these higher-energy protons collided. The net center of mass energy of 2.36 TeV exceeded the highest energies ever achieved before, breaking Fermilab’s eight-year-old record.

Three LHC experiments registered beam collisions and tens of thousands of such collisions occurred over the next few weeks. Those collisions won’t be used to discover new physical theories, but they were incredibly useful for determining that the experiments in fact worked and could be used to study Standard Model backgrounds—events that don’t indicate anything new, but could potentially interfere with real discoveries.

Experimenters everywhere shared the satisfaction of the LHC’s having reached record energies. Remarkably, the LHC did it just in the nick of time—the machine had been scheduled to shut down from the middle of December until March of the following year, so it was either December or several more months’ delay. Jeff Richman, a Santa Barbara experimenter who works on the LHC, joyfully shared this fact at the dark matter conference we were both attending, since he had made a bet with a Fermilab physicist as to whether the LHC would achieve higher energy collisions than Fermilab’s Tevatron before the close of 2009. His cheerful demeanor made it clear who had won.

On December 18, 2009, the wave of excitement was temporarily suspended when the LHC shut down after this commissioning run. Lyn Evans concluded his talk discussing the plans for 2010, when he promised a sizable increase in energy. The plan was to go up to 7 TeV before the end of the year—a substantial increase in energy over anything before. He was enthusiastic and confident—as turned out to be justified when indeed the machine came back on line at this higher energy. After so many ups and downs, the LHC was finally working according to plan. (See Figure 28 for an abbreviated timeline.) The LHC should continue to run through 2012 at 7 TeV, or possibly a bit higher energy, before shutting down for at least a year to prepare for raising the energy to as close as possible to the LHC’s 14 TeV target. During this and the following runs, the LHC will also try to raise the intensity of the beams to increase the number of collisions.

Given the smooth operation of the experiments and machines after turning back on in 2009, Lyn’s closing words for his talk resonated with the audience: “The adventure of LHC construction is finished. Now let the adventure of discovery begin.”