Claudia Marcelloni/CERN
On October 21, 2008, in accordance with some overly optimistic scheduling, 1,500 physicists and world leaders gathered outside Geneva to celebrate the inauguration of the biggest, most international, most expensive, most energetic, most ambitious experiment ever built. I enjoyed the day, which was filled with speeches, music, and—as is important at any European cultural event—good food. And despite anxieties (more on that later), 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.
The machine in question is, of course, the Large Hadron Collider (LHC). The name is literal, though admittedly uninspired. The LHC is indeed large, containing a 27-kilometer circular underground tunnel that stretches between the Jura Mountains and Lake Geneva near the French-Swiss border. This tunnel’s depth varies from 50 to 175 meters underground; the uneven terrain was in fact an interesting constraint on the tunnel’s depth and location. Electric fields inside this tunnel will accelerate two beams of protons (which belong to a class of particles called hadrons, hence the collider’s name) as they go round and round, more than 10,000 times each second. Then—and here’s where all the action happens—magnets will guide the two proton beams so that they collide in a region smaller than the width of a human hair. When this collision happens, some of the energy of the accelerated protons will be converted to mass (that’s what Einstein’s famous formula, E = mc2, tells us). In fact, the energy will be so high that the ingredients inside the proton—particles called quarks and gluons—will collide and convert to energy. And with these collisions and the energy they release, new elementary particles, heavier than any seen before, can be created.
The day’s events did not yet celebrate discovery but instead recognized the potential of the LHC and the triumph of the many countries that participated in its creation. An international community of scientists and officials began planning the LHC more than 20 years ago at CERN (the acronym stems from the original name, Conseil Européen pour la Recherche Nucléaire). CERN is a miracle of international cooperation, with scientists from 85 countries participating. The cost of the LHC is about $10 billion, of which CERN has paid two-thirds; CERN’s 20 member countries contribute according to their means, ranging from 20 percent from Germany to 0.2 percent from Bulgaria. Although the United States isn’t officially part of CERN, many American physicists work there, and we’ve put in $531 million.
You might remember that on September 10 last year 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 traversed the enormous tunnel in opposite directions. This involved commissioning the injection elements, starting the controls and instruments in the ring, checking that the magnetic field would keep the protons in the ring, and making sure all the magnets worked to spec and could be run simultaneously. Amazingly, the first time that could be done was the evening of September 9! Yet everything worked as well as or better than planned.
When I visited last October, everyone had stories about the excitement of September 10. Millions of people all around Europe tuned in to watch the graphs of the protons’ progress, which on the screen simply looked like two dots traversing a ring. The beams started slightly off direction, but people sat mesmerized as the path was modified so the protons could successfully circulate around the full circumference of the ring. Not everyone knew what he or she was watching, but everyone with eyes glued to the screen knew that something significant was in store. Meanwhile, inside CERN the thrill was palpable as physicists and engineers gathered in auditoriums to watch the same thing. The first beam went around the ring for a few turns. Each successive burst of protons was adjusted slightly so that soon the beams were circulating hundreds of times. At this point the LHC outlook seemed extremely promising.
But a little more than a week later the mood was seriously dampened. On September 19 engineers were preparing to attempt the first collision of the two beams. Sadly, this was a lot less successful. Before the collision could happen, as scientists were trying to ramp up the current and energy, something went wrong with a connection of the bus bar between magnets, creating an electrical arc that punctured the helium enclosure and caused large quantities of liquid helium to be released (helium is necessary to cool the superconducting magnets that guide the beams around the ring). This created a large amount of pressure, which in turn displaced the magnets that focus the protons, destroyed what needs to be a vacuum, damaged insulation, and contaminated the beams with soot—not exactly what we had been hoping for.
I learned more about the backstory during my visit. Keep in mind that the ultimate goal for collisions is a center of mass energy of 14 TeV, or trillion electron volts. I realize these might be unfamiliar units by which to measure energy, so to give some perspective, it is seven times the energy of the Tevatron particle accelerator at Fermilab in Illinois, which is presently the highest-energy machine, and 15,000 times the energy contained in the mass of a single proton at rest.
