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The Exploding Protons
During a number of milestone events in the recent history of our planet, Stefano Redaelli, a tall, thin, bearded thirty-three-year-old particle physicist from Milan with keen eyes and an easy smile, has been at the controls. Some would even say that on these occasions, when the gargantuan particle accelerator known as the Large Hadron Collider (LHC) is being powered to energy levels so immense they have never been seen before, Redaelli is not only the most powerful man who ever lived, but also the only person in history who, with a click of a mouse, could alter forever the fate of the world, and perhaps even of the entire solar system.
At 4:40 p.m. on Friday, March 5, 2010, Redaelli was once again the engineer in charge at the CERN Control Center outside the French village of Prévessin, just across the Swiss border from the headquarters of CERN, the European Organization for Nuclear Research. This is the place that governs the operation of the Large Hadron Collider, the most powerful particle accelerator in the world, as well as the series of smaller accelerators successively feeding the LHC with faster and faster protons (positively charged particles). It is from here that the LHC had just restarted after its winter break, incrementally increasing its power to new records.
This time the world’s news media had been kept away from the collider as it powered up, but by a stroke of luck I was allowed access to this nerve center of the entire LHC operation. I looked around me. I was in an ultramodern space about the size of a basketball court, one of whose walls had windows that reached all the way up to the ceiling, framing the snow-capped mountains of the French Jura in the near distance. Arrayed along the other walls were dozens of large, colorful display screens. Scientists and engineers clustered around four large knots of tables laden with computer consoles. The control center looked like a cross between the flight deck of the starship Enterprise and the floor of the New York Stock Exchange, but the big screens along the walls, on which Redaelli and his colleagues were now focused intently, were not displaying readouts from deep space or the latest stock prices. Instead they registered a stream of precise data that originated deep inside a circular tunnel measuring sixteen and a half miles in length, buried 300 feet below us. These measurements included: temperature—the lowest in the known universe, colder than the temperature of outer space; magnetic field strengths—among the most powerful ever created by man, some of them more than 200,000 times that of Earth’s magnetic field; and energy—at this moment 450 gigaelectron volts (GeV), an extraordinary level that would eventually ramp up to the almost inconceivable 7 teraelectron volts (TeV), which is more than fifteen times as high.1
As the engineer in charge, Redaelli was the man whose commands produced the energy increases inside the tunnel below us by raising electric power, now within the green range on one of the large screens, to yellow, and in unusual circumstances even to red, at hundreds of megawatts—the power consumption of a medium-size city. The electric current, fed into some 10,000 giant superconducting magnets and radio frequency devices, concentrates, bends, and accelerates the LHC’s twin proton beams, eventually raising their speeds to levels extremely close to that of light.
There were many other young scientists in the room, including Peter Sollander, a tall, bespectacled young technical expert from Sweden who was in charge of part of the infrastructure of the collider. Next to his area was the center controlling the liquid helium cooling the superconducting magnets in the tunnel. Each bar on a screen on the wall before us represented 154 magnets, and all the bars were now green, indicating that none of the temperature measurements from the magnets underground exceeded 1.9 degrees Kelvin (that is, 1.9 degrees Celsius above absolute zero, or -456.25 degrees Fahrenheit). This is the ambient temperature for superconducting magnets. Should the temperature in any magnet rise above its present level, its bar would turn red, and the entire operation would immediately have to shut down to prevent a disaster.
Other scientists were monitoring various aspects of the control of the most complicated scientific operation ever undertaken. On the left side of this large room was a subcenter for the feeder accelerators, which contributed power in stages. The first was a linear beam accelerator called Linac2, and it was followed by the more powerful Proton Synchrotron Booster, then by the Proton Synchrotron itself, and finally by the Super Proton Synchrotron (SPS)—a machine with a celebrated history of discoveries in particle physics in the 1980s. This last accelerator fed protons directly into the Large Hadron Collider. Another cluster of consoles controlled all technical aspects of the giant magnets underground and the electric power flowing into them. The last cluster on the right, where Redaelli was standing, was the control center for the LHC itself.
Right behind the young scientists huddled around the computer screens in this part of the room stood a stern-faced man in his sixties with wavy gray hair, wearing a light blue sweater and jeans, his eyes fixed on the third screen from the left on the wall above. Lyndon (“Lyn”) Evans was the silent power, the éminence grise of the control room. He was watching a blue line on the screen, which represented the power driving two opposing beams of protons racing around the 16.5-mile circuit underground at near light speed. Evans, a Welsh physicist known at CERN as “the father of the LHC,” represented the organization’s top management, but as is typical in this highly unusual international collaboration of more than ten thousand scientists from around the world, the actual decisions were often left to the young people here: the scientists and engineers who run the day-to-day operation of the collider.
At the same time that Redaelli and his colleagues were controlling the Large Hadron Collider from the CERN Control Center, still other scientists were manning the collider’s four ultramodern control hubs that govern the actual scientific experiments being carried out in the LHC. One of these state-of-the-art control rooms was located about five miles to the west, at “Point 5” of the LHC, right above a giant detector called CMS (for Compact Muon Solenoid). Here Dr. Guido Tonelli, a leading particle physicist from Pisa, was controlling the action as his group of scientists watched their screens and waited to hear from the CERN Control Center at Prévessin whether the protons accelerated in the tunnel would be allowed to crash at high energy in the superconducting detector right below their feet. Tonelli was scrutinizing information on a computer screen as if oblivious to the rest of the room—crammed with other monitors, cables, and sophisticated computer equipment.
The heaviest scientific instrument ever built, the CMS is a gigantic construct of steel, copper, gold, silicon, many thousands of lead-tungstate crystals, and miles of superconducting niobium-titanium coils, as well as a reservoir of liquid helium; it is densely packed with extremely sensitive complex electronics, and it weighs a total of 12,500 tons. Just the iron inside the CMS detector weighs 10,000 tons—more than the weight of the Eiffel Tower. The outer shell of the huge device is a very powerful magnet, a superconducting electromagnet that must be cooled by liquid helium to a temperature below that of outer space in order to maintain its superconductivity—the conduction of electricity without resistance—required to power the magnet to the very high level of 4 tesla (a hundred thousand times Earth’s magnetic field; some magnets performing other tasks in the LHC produce a magnetic field strength twice as high). The energies of the particles that explode inside the CMS detector have not been seen since a trillionth of a second after the Big Bang launched our universe 13.7 billion years ago.
I had gone to the CMS control center an hour earlier this day, and standing inside this room, I couldn’t help but wonder at the incongruity of it all. This control room was housed in a building standing all alone in the middle of the bucolic French countryside, surrounded by cow pastures and plowed tracts of land, half a mile from the small village of Cessy. The nearest town was four miles to the southeast: Ferney- Voltaire. (The name Voltaire had been added to Ferney to commemorate the fact that in the eighteenth century, the famous French writer and philosopher lived here, wrote Candide, and contributed greatly to the economy of the town.)
Just outside Ferney-Voltaire was “Point 8” of the LHC, the location of a special-purpose detector called LHCb (“b” stands for “beauty”). Farther to the southeast was the Swiss border, and beyond it the suburbs of Geneva. At Meyrin, a western suburb near the Geneva Airport, was “Point 1” of the LHC, the location of a detector named ATLAS (A Toroidal LHC ApparatuS), whose function was similar to that of CMS and whose team of scientists was pursuing similar experi- ments with crashing protons; and near it was the sprawling headquarters of CERN. If one continued west along the circular track of the LHC, again crossing into France, within a few miles one would reach “Point 2,” the location of the last main detector of the LHC, called ALICE (A Large Ion Collider Experiment), which, like LHCb, was designed for a special scientific purpose.
Some months earlier, on November 30, 2009, just before the operation of the LHC was stopped for a winter break, Tonelli and his team of young scientists had been following tracks on their screens that represented the passage of thousands of tiny particles cascading from the first head-on collisions of protons traveling toward each other at nearly the speed of light and then exploding with immense energies inside the giant underground Compact Muon Solenoid detector.
When the CMS detector is operating, billions of protons crash inside it every second. Tonelli explained to me that of these, only one in a hundred thousand represents an “unusual event” that could potentially be of great interest to science. Higher-level algorithms are used to further skim the sample, and only 300 events per second are permanently recorded to undergo complete reconstruction and physics analysis. Out of these, about 1 high-interest particle collision is shown on screen every second, since our eyes can’t perceive very well a complicated image that lasts less time than that.2 Scientists operate the control room around the clock, and every once in a while they see a spectacular cascade of particles—debris from the immense explosion of protons crashing head-on in the belly of the giant detector deep underground below them. In November and December 2009, before the machine was shut down, there were many such trails of particles from proton collisions at record high energies.
What did they represent? Could they have been the signature of the elusive Higgs boson, a particle that physicists believe is responsible for endowing all matter in the universe with its mass, the so-called God Particle? Was there a hint of the existence of the unseen and mysterious “dark matter” that physicists and astronomers believe permeates galaxies? Or did the detector record the telltale sign of a hidden dimension of the space we live in, one of six or seven additional dimensions suggested by string theory? Any one of these discoveries would represent a giant step forward in our understanding of nature, and all of them are among the goals that the Large Hadron Collider was built to achieve.
The intense amount of highly concentrated energy released by proton collisions inside the LHC takes science to an unexplored new level, a region of high energy the like of which has not been seen in our universe since a fraction of a second after the Big Bang. In this way the Large Hadron Collider is taking us back billions of years to conditions that prevailed in the universe shortly after its fiery birth. Thanks to the LHC, physical science will never be the same as we peer far deeper into the universe than ever before; uncover its structure, past and present; glimpse its future; and perhaps even decipher its meaning.
The head-on collisions of trillions of protons taking place deep under the ground of the border region of Switzerland and France turn energy into mass in the form of other particles that emanate from the collisions and fly off in various directions at high speeds. This process occurs because of Einstein’s famous equation, E = mc2, which says that mass and energy are merely two different manifestations of the same thing. Einstein’s incredibly powerful formula (actually, a variation that is somewhat more complicated and incorporates the speed of the particles) is what makes all research in particle accelerators possible.3 The idea is as follows.
Particles are accelerated to great speeds and then made to crash into incoming particles from the opposite direction. Energy is released from these collisions, and in accordance with Einstein’s formula, this energy then turns into other fast-moving particles.4 So from the released energy that resulted from the particle collisions, mass can be created. This new mass, born of pure energy, may constitute particles like those that existed when the universe was only a tiny fraction of a second old, and studying their behavior holds the key to our understanding of the forces and particles we see in the world today.
The LHC thus re-creates particles and natural phenomena that have never before been observed. It also takes us back in time to a very distant primordial past when the universe was an immensely dense and hot “soup” of particles, called the quark-gluon plasma. The collider also acts as a giant microscope: It can show us the inner workings of space-time.
The Large Hadron Collider was designed and constructed over a period of twenty years at immense expense—costs have by now exceeded $10 billion—by CERN scientists with one aim in mind: to uncover the ultimate laws of the universe. To discover these laws and see long-lost particles, forces, and interactions required an unprecedented effort that could only be undertaken through close international cooperation spanning many institutions and countries and areas of scientific expertise. The LHC project is the most advanced scientific cooperation in history.
How does the machine work? In the LHC experiment, two beams of protons obtained by ionizing hydrogen gas are made to travel in opposite directions with continuously increasing speed. The machine is called the Large Hadron Collider because protons are hadrons. A hadron (from the Greek word for “thick”) is a particle made of quarks. A proton is made up of three quarks, and it therefore belongs to a more specific category of hadrons, called the baryons. Hadrons that contain only two quarks are called mesons. The protons produced from the hydrogen gas are gradually accelerated in a successive series of CERN’s smaller accelerators, until they reach a speed at which they can be injected into the LHC. Here, powerful radio frequency devices in the tunnel “kick” the particles faster every time they pass by. Giant superconducting electromagnets, cooled to almost absolute zero in order to give them electrical conductivity with zero resistance and thereby endow them with the maximum possible power, bend the paths of the protons along the circular trajectory underground and concentrate and maintain the two opposing beams.
The LHC uses 9,593 superconducting electromagnets: There are 1,232 main magnets curving the trajectory of the protons along the underground racecourse, 392 magnets focusing the beams of protons around the tunnel, and 6,400 corrector magnets, making small adjustments in the paths of the protons so that they will crash at locations determined with a precision of a small fraction of a millimeter—much less than the width of a human hair. Still other magnets, some of them embedded inside the main magnets, perform related tasks.
When operated at its maximum energy level, the LHC keeps accelerating the protons until they reach the almost unimaginable speed of 99.9999991 percent of the speed of light (which is 186,282.397 miles per second). This happens when the LHC is run at an energy level of 14 TeV (teraelectron volts). One TeV is roughly the energy of the flight of a mosquito, which would seem like a tiny amount—but it is highly concentrated: The LHC generates fourteen times this energy in the volume of a pair of protons, which means it is packed into a space a trillion times smaller than a mosquito.5 This is by far the highest energy level per volume ever achieved.6 In this extremely high-energy realm, new particles and phenomena are likely to appear that until now have lived only in physicists’ imaginations.