Collider: The Search for the World's Smallest Particles - Paul Halpern (2009)
Chapter 8. Crashing by Design
Building the Large
The age in which we live is the age in which we are discovering the fundamental laws of Nature, and that day will never come again
—RICHARD FEYNMAN (THE CHARACTER OF PHYSICAL LAW, 1965)
Compared to the wild flume ride of American high-energy physics, CERN has paddled steadily ahead like a steam-boat down the Rhone River. Each milestone has been part of a natural progression to machines of increasing might—able to push particle energies higher and higher. While American high-energy physics has become increasingly political—rising or falling in status during various administrations—the independence of CERN’s directorship and its commitment, cooperation, and collaboration to carrying out projects already proposed have enabled it to successfully plot the laboratory’s course for decades ahead.
One aspect of CERN’s impressive efficiency is its ability to recycle older projects into key components of state-of-the-art devices. The old Proton Synchrotron, upon its retirement as a stand-alone machine, became an injector for the Super Proton Synchrotron (SPS). The SPS, in turn, has been used for a variety of purposes, including serving as a preaccelerator for more powerful devices. Little at CERN ever truly goes to waste, and this keeps costs relatively low.
This tendency to adapt obsolete projects for reuse as parts of new ones reflects the European need to conserve space and vital resources. Europe is more crowded and doesn’t have the luxury of unbridled development. Therefore a venture like the SSC involving building a completely new facility from scratch in a region far from other labs would be much less likely to happen.
By making use of the old seventeen-mile tunnel for the Large Electron-Positron Collider (LEP), the Large Hadron Collider (LHC) serves as the perfect example of accelerator recycling. Digging the LEP tunnel was a colossal undertaking. From 1983 to 1988, it represented the largest civil-engineering project in Europe. Because the main ring had to be wedged between the Geneva airport and the Jura mountains, engineers had little room to maneuver. Tunnel diggers were forced to blast through thick layers of solid rock. To account for changes in topography, the ring had to be tilted by one and a half degrees. Amazingly, the tunnel lined up nearly perfectly (when its ends were joined, they were less than half an inch off) and was precisely the right size. It is therefore fortunate that the LHC hasn’t required a whole new tunnel but rather could be fit into the old one.
The decision to drop ultracool superconducting magnets into the LEP ring and turn it into a hadron collider—at the suggestion of Rubbia and others—was first discussed in the 1980s. (Hadrons, such as protons, are much more massive than electrons and thereby require far stronger magnets, such as superconducting magnets, to be steered through the same ring.) Reportedly, practically from the time the LEP opened, Rubbia was anxious to have the tunnel converted. While the SSC was under construction, many CERN physicists hoped that the LHC could be finished first. A frequent rallying cry was to plan on opening the LHC two years before the SSC—thus beating the Americans to the good stuff. The SSC’s cancellation added further impetus to the project, as it meant that the LHC would represent the main—or only—hope for finding certain massive particles. The model of international competition and vying labs confirming one another’s results, which served well during an earlier era of smaller machines, would need to be replaced with international cooperation, centered in Europe.
The final decision to build the LHC came little more than a year after the SSC was canned. On December 16, 1994, CERN’s nineteen member nations at the time voted to budget $15 billion over a two-decade span to build what would be the world’s mightiest collider. Through its leaders’ firm commitments, the continent that spawned Galileo and Kepler readied itself to be the vanguard of science once more.
Unlike with the SSC, politics played little discernable role in the LHC’s construction. Each European country that belongs to CERN contributes a specific annual amount that depends on its gross national product. Richer countries, such as Germany, France, and the United Kingdom, shell out the bulk of CERN’s budget—generally with no yearly debate over how much that amount will be. (The United Kingdom has recently become more cautious about future scientific projects, however.) Thus CERN administrators can rely on certain figures and plan accordingly.
Furthermore, unlike the American case, in Europe regional competitions have never decided where projects are built. The French communities of the Pays de Gex, the region where much of the tunnel is located, didn’t launch a “Don’t Mess with Gex” campaign to sway politicians one way or the other. Rather, they’ve quietly accepted CERN as a long-standing neighbor that shares the land with farmers, wine growers, cheese makers, and other producers. As the region’s motto proclaims, Gex is “Un jardin ouvert sur le monde” (a garden open to the world).
On the Swiss side, Geneva is used to all manner of international enterprises. The city where the League of Nations was established and famous treaties were signed now houses a plethora of global organizations—the U.N. European headquarters, the World Health Organization, the International Labour Organization, the International Federation of Red Cross and Red Crescent Societies, and many others. CERN is well accepted along with its kindred cooperative institutions. The medley of researchers’ foreign languages—including English, Russian, and field theory—is nothing special; Genevese diplomats can match that Babel and more.
Furthermore, over the centuries Geneva has seen its share of groundbreaking movements. Compared to the impact of the Reformation and the Enlightenment, slamming particles together underground barely registers on the city’s Richter scale of history.
True, the French countryside west of Geneva is much quieter. To ensure a harmonious relationship, CERN has sought to minimize its impact on that region. The patchwork of pastures and vineyards that front the misty Jura mountains displays no visible indication that a giant particle-smashing ring lies hundreds of feet beneath them. Only the occasional road signs steering CERN vans to scattered laboratory buildings, and the power lines scratching through the green and golden tapestry, offer clues as to what lies below.
The latter represents possibly the biggest source of contention—CERN is a huge drain on the region’s electricity. Originally, this electricity was supplied by Switzerland; now it is furnished by France. When its machines are fully running, CERN expends about as much power as the entire canton of Geneva. Because of the predominance of electrical heating in the area, this usage would be felt mainly during wintertime. Consequently, as a considerate neighbor, CERN often adjusts its power needs to accommodate—for example, by scheduling shutdowns during the coldest time of year. Though this means less data collection, fortunately for the more athletic researchers, the winter closures are timed well with peak skiing season in the nearby Alps.
To prepare for the LHC project, the LEP tunnel needed to be completely gutted. After the final LEP runs took place in 2000, the refurbishing could finally begin. Orders went out for thousands of superconducting magnets of several different types. One kind, called dipole magnets, were designed to steer twin proton (or ion) beams around the loop. (A subset of the LHC experiments will involve accelerating ions rather than protons.) Dipoles tend to guide charged objects in a direction perpendicular to their magnetic fields—ideal for maneuvering. A second variety of magnets, called quadrupoles, were targeted at focusing the beams, to prevent them from spreading too much. To simplify the LHC’s design, these were placed at regular intervals. Other more complex magnet designs—called sextupoles, octupoles, and decupoles—were added to the mix to provide finer beam corrections. Like a delicate space mission, the orbit needed to be tuned just right.
Because the particles rounding the LHC would be alternatively steered and focused, with ample room for experimentation, the machine was not planned to be a perfect circle. Rather, it was divided into eight sectors, each powered separately. Sectors consist of curved parts and straight intervals—the latter used for a variety of purposes including injecting particles, narrowing the beams, and conducting experiments.
Researchers realized that two extreme conditions would need to be maintained to make the LHC a success. These requirements would bring some of the most hostile aspects of outer space down to Earth. First, the twin beam pipes, riding through the apertures of the magnets, would need to be kept as close to vacuum states as possible. That would allow the protons (and ions) to reach ultrahigh energies without bouncing off of gas molecules as in a pinball game. A pumping system was chosen that would maintain the pressure at 10-13 (one-tenth of one trillionth) that of the atmosphere at ground level. That’s far from as empty as the interplanetary void, but it’s closer to a pure vacuum than virtually anywhere else on Earth.
Second, the thousands of magnets would need to be supercooled well below the critical temperatures that maintain their superconducting states. This would keep their magnetic fields as high as possible—more than 8.3 Tesla, double the field used at the Tevatron. To keep the temperatures so low would require superfluid helium—a highly correlated ultracool state of that element—at 1.9 degrees Kelvin (above absolute zero). That’s even colder than the microwave background radiation detected by Penzias and Wilson in their confirmation of the Big Bang.
At first glance, it would seem to be prohibitively expensive to keep so many magnets so cold. Indeed, superfluid helium is very costly to produce. However, by surrounding each “cryomagnet” (as supercooled magnets are called) with an insulating vacuum layer, little heat from the outside would leak through. Emptiness is a great thermal blanket.
Another factor LHC designers had to reckon with has to do with lunar influences. Remarkably, the moon has a periodic lure on the region. No, there aren’t full-moon-crazed werewolves haunting the woods near Ferney-Voltaire and Meyrin, eager to rummage through supercooled containers looking for frozen steaks—at least as far as we know. Rather, the moon’s effect is purely gravitational. Just as it pulls on the oceans and creates the tides, the moon tugs on the ground, too. Rocks are certainly not as pliable as water, but they do have a degree of elasticity. Due to its lunar stretching, Earth’s crust in Geneva’s vicinity rises and falls almost ten inches each month. This creates a monthly fluctuation in the LHC’s length of about 1/25 of an inch.1 The effect was first noted when the tunnel was used for the LEP and has been accommodated through corrective factors in any calculations involving ring circumference.
Topography played an even more important role when it came time to equip the LHC with detectors. Completely new caverns were excavated, with the largest, at “Point 1,” to accommodate the most sizable detector, ATLAS (A Toroidal LHC ApparatuS). Three other detectors, called CMS (Compact Muon Solenoid), ALICE (A Large Ion Collider Experiment), and LHCb (Large Hadron Collider beauty) were placed at additional points around the ring. The designs for each of these detectors took many years of planning. Their approval recognized their complementary roles in the overall LHC mission—each contributing a unique means of measuring particular types of collision by-products and thereby primed for different kinds of discoveries.
The ATLAS project has been in the planning for more than a decade. It represents a fusion of several earlier projects involving researchers from a number of different countries. Experiences at earlier collider projects—those completed along with those aborted—played a strong role in shaping the detector’s design.
Take, for instance, ATLAS’s electromagnetic calorimetry (energy-gauging) system. It relies on a method William Willis proposed in 1972 for the ill-fated ISABELLE collider: using liquid argon to convert radiation into measurable electrical signals through the process of ionization. When ISABELLE was canceled, Willis included liquid argon calorimetry again in the proposal he developed with Barish and others for the GEM detector at the SSC. In addition to Brookhaven, where Willis was based, the technique came to be used at laboratories such as Fermilab and SLAC. Now Willis is the U.S. project manager for ATLAS, where his liquid argon method forms a key component of the detector’s energy-measuring system.
If liquid argon is the blood flowing through the heart of ATLAS, silicon pixels and strips (wafers responsive to light, like digital cameras) offer the ultrasensitive eyes. Immediately surrounding its interaction point is a zone of maximum surveillance called the inner detector—where electronic eyes gaze virtually everywhere like a particle version of Big Brother. Except for the places where the beam line enters and leaves, the inner detector is completely surrounded by tiny light probes. In other words, it is hermetic, the ideal situation for high-energy physics where virtually all bases are covered. This state of maximum spy-camera coverage offers the optimal chance of reconstructing what happens in collisions.
To encapsulate the beam line in a symmetric way, most sections of ATLAS, the inner detector included, are arranged in a set of concentric cylinders, called the barrel, framed at the entrance and exit by disks perpendicular to the beam, called the end-caps. This geometry means that almost every solid angle from the beam line is recorded. The inner detector’s tracking system includes photosensitive pixels and strips covering the three interior layers of the barrel as well as the end-caps.
Between the inner detector and the calorimeters is a solenoid (coil-shaped) superconducting magnet with a field of approximately 2 Tesla. Cryostats (systems for supercooling) keep the magnet at less than five degrees above absolute zero. The purpose of the solenoid magnet is to steer charged particles within the inner detector—bending them at angles that depend upon their momenta (mass times velocity). Therefore, the electronic tracking system, in tandem with the magnet, enables researchers to gauge the momenta of collision products.
After particles breach the boundary of the inner detector, they enter the realm of the electromagnetic calorimeter. Bashing into lead layers, the electromagnetically interactive particles decay into showers and deposit their heat in the liquid argon bath, producing detectable signals. Delicate electronics pick up the signals from all of the energy lost, offering another major component of event reconstruction. Discerning the charge, momentum, and energy of a particle is like asking a soldier his name, rank, and serial number. Because each of these physical quantities is conserved, identifying each particle’s information optimizes the chances of figuring out which unseen carriers (such as neutral particles) might be missing.
Only some of the featherweight particles, such as electrons, positrons, and photons, are knocked out completely in the electromagnetic calorimeter; heavier (and nonelectromagnetic) particles can slip through. These bash into a thick layer of steel tiles interspersed with scintillators—the hadron calorimeter. Sensors abutting that layer record the heat deposited by any particles subject to the strong force. There protons, neutrons, pions, and their hadronic cohorts make their final stands.
The only charged particles that can evade both types of calorimeters without being absorbed are muons. To ensnare them, the outermost, and largest, layers compose the muon system. It operates in some ways like the inner detector, with magnets and a tracking system, only on a far grander scale—dwarfing the rest of ATLAS. Pictures of ATLAS taken after its completion inevitably showcase the muon system’s colossal end-cap: the Big Wheel.
The muon system’s enormous superconducting magnets have a much different shape from the central magnet. Rather than a solenoid, they are toroidal (doughnut-shaped) but stretched out. At one-quarter the length of a football field, they are the largest superconducting magnets in the world. Eight of them carve through the outer barrel—like an eight-way apple slicer. The sheer size of these magnets magnifies the bending of muons as they pass through. Thousands of sensors track the muons’ paths as they swerve—revealing those particles’ precise momenta.
The particles that survive the full range of detection systems are those that are insensitive to both the electromagnetic and strong interactions. The prime suspects among these are neutrinos. Because they interact solely through the weak and gravitational forces, neutrinos are very hard to detect. ATLAS does not make an attempt to catch these; rather, components of their momenta and energy are estimated through a subtraction process. Because the protons, before colliding, are traveling along the beam line, their total transverse (at right angles to the beam direction) momentum must be zero. According to conservation principles, the total transverse momentum after the collision—determined by adding up the momenta of everything detected—ought to be zero as well. If it isn’t, then subtracting that sum from zero yields the transverse momenta of unseen collision products. Therefore, the ATLAS researchers have a good idea of what the neutrinos have carried off.
A view of the ATLAS detector with its eight prominent toroidal magnets.
Halfway around the LHC ring, beneath the village of Cessy, France, is the other general-purpose detector, CMS. The “compact” in its name reflects the CMS’s aspiration to pursue similar physics to ATLAS with a detector a fraction of the volume—although still bigger than a house. Instead of an assortment of magnets, CMS is constructed around a single colossal superconducting solenoid (coil-shaped magnet) that puts out a field of 4 Tesla—approximately a hundred thousand times greater than the Earth’s. It surrounds the detector’s central silicon-pixel tracker and calorimeters, bending the routes of charged particles within those regions and extracting precise values of their momenta. Knowing the momenta helps the researchers reconstruct the events and deduce what might be missing, such as neutrinos.
Another difference between CMS and ATLAS concerns the way they force electromagnetically sensitive particles to “take a shower.” Instead of frigid liquid argon, the CMS electromagnetic calorimeter includes almost eighty thousand lead tungstate crystals (energy-sensitive materials) to measure the energies of electrons, positrons, and photons in particle showers. The hadrons encounter dense curtains of brass and steel, while muons are caught up in layers of drift chambers and iron that lie just beyond the magnet.
The CMS detector before closure.
The two collaborations have much in common: large teams of researchers from institutions around the world, ambitious goals, and the powerful data-capturing technologies required to carry out these bold objectives. Data from the millions of events recorded by each group—those passing the muster of the trigger systems designed to weed out clearly insignificant occurrences—will be sent electronically to thousands of computers in hundreds of centers around the world for analysis by means of a state-of-the-art system called the Grid.
Each team has an excellent shot at identifying the Higgs boson, assuming its energy falls within the LHC’s reach. If one team finds it, the other’s efforts would serve as vital confirmation. The research paper making the important announcement would literally contain thousands of names. Because of the shared credit, the Nobel committee would be hard-pressed to award its prize to an individual or small set of experimentalists. Unlike, for instance, Rubbia and Van der Meer’s winning science’s highest honor for the weak boson discoveries, there probably wouldn’t be obvious hands (aside from those of its namesake theorist) to confer the award.
Completing the quartet at the LHC’s interaction points are two sizable specialized detectors: the LHCb (Large Hadron Collider beauty) experiment and ALICE (A Large Ion Collider Experiment). Two other petite detectors will operate near the ATLAS and CMS caverns, respectively: the LHCf (Large Hadron Collider forward) and TOTEM (TOTal Elastic and diffractive cross-section Measurement) experiments.
The focus of the LHCb experiment is to produce B-particles (particles containing the bottom quark) and to examine their modes of decay. B-particles are extremely massive and would likely have a rich variety of decay products that could possibly furnish evidence of new phenomena beyond the Standard Model. In particular, the LHCb researchers will be looking for evidence of what is called CP (Charge-Parity) violation. CP violation is a subtle discrepancy in certain weak interactions when two reversals are performed in tandem: switching the charge (from plus to minus or minus to plus) and flipping the parity (taking the mirror image). Switching the charge of a particle makes it an antiparticle, which does not always behave the same in weak decays. Reversing the parity, as Lee and Yang demonstrated, similarly does not always yield the same results in weak decays. Physicists once believed that the combination of the two operations would always be conserved. However, in 1964, American physicists James Cronin and Val Fitch demonstrated that certain kaon processes subtly break this symmetry. Particular B-meson decays involving the weak interaction also violate CP symmetry—processes that the LHCb experiment hopes to study.
Unlike ATLAS and CMS, the LHCb detector does not surround its whole interaction point. Instead, it consists of a row of subdetectors in the forward direction. The reason is that the B-particle decays to be studied generally fan out in a cone in front of the collision site. Several hundred researchers from more than a dozen countries are members of the LHCb collaboration.
ALICE is an experiment that involves the collision of lead ions rather than protons. The LHC will circulate ions for one month each year to accommodate this project. When the lead ions collide, the hope is that they will produce a state of matter called quark-gluon plasma—a free-flowing mixture of hadron constituents thought to resemble the primordial broth that filled the very early universe. Normally, quarks are confined to hadrons—grouped in pairs or triplets and strung together by gluons. However, under the energetic conditions of the LHC, equivalent to more than a hundred thousand times the temperature at the Sun’s core, physicists think such barriers would crumble—liberating the quarks and gluons. This freedom would be extraordinarily brief. The massive detector used to record the outcome has a layered barrel design with eighteen components, including various types of tracking systems and calorimetry. More than a thousand physicists from more than a hundred different institutions are contributing to the project.
The LHCf experiment, the smallest at the LHC, makes good use of some of the leftovers from ATLAS. Standing in the beam tunnel about 460 feet in front of the ATLAS collision point, it is intended to measure the properties of forward-moving particles produced when protons crash together. The goal is to test the capability of cosmic ray measuring devices. Several dozen researchers from six different countries are involved in the experiment.
Finally, TOTEM, a long, thin detector connected to the LHC beam pipe, is geared toward ultrahigh-precision measurements of the cross-sections (effective sizes) of protons. Located about 650 feet away from the CMS detector, it consists of silicon strips situated in eight special vacuum chambers called Roman pots. These are designed to track the scattering profiles of protons close to the beam line. TOTEM involves the work of more than eighty researchers, associated with eleven institutions in eight different nations.
To monitor the progress of the LHC experiments, members of each group conduct regular meetings. Particularly for the larger detectors, each instrumental component requires calibration and careful monitoring. Group members frequently apprise one another of the results of this testing to troubleshoot any potential problems.
One issue that sometimes arises with the more complex detectors is anticipating how one component might affect another’s results—for example, through electronic noise. The presence of ultrastrong magnetic fields further complicates matters, as they could exert disruptive influences. During testing at ATLAS in November 2007, for example, one of the toroid magnets wasn’t properly secured and it moved about an inch toward an end-cap calorimeter. Fortunately, there was no damage. If a problem is found within one of the hermetically sealed sections, often nothing can be done until it is unsealed and opened up. Typically, such opportunities arise when the LHC is temporarily shut down—close to the winter holidays, for example.
The “machine people,” those involved with the planning and operations of the accelerators, have their own separate meetings. Their primary concern is that the overall system is working smoothly. One of the trickiest issues they face is keeping the dipoles, quadrupoles, and other ring magnets at their optimal fields with maximum energies.
If the magnetic fields and energies are raised too high in too rapid a manner, an adverse phenomenon called quenching occurs. Quenching is when part of a superconducting magnet overheats because of moving interior components and destroys the superconductivity. At that point, the magnet becomes normally conducting and its field drops to unacceptable levels. To combat such a ruinous situation, the magnetic fields are ramped up slowly, then reduced, again and again, in a process called training. It’s a bit like placing your feet in a hot tub, pulling them out, then putting them back in again, until you are used to the heat.
The detector and the machine groups are well aware of the LHC’s limitations. Every machine has structural limits—for example, upper bounds on the beam luminosity due to the magnets’ maximum focusing power. Consequently, researchers take note and plan for upgrades well in advance. It is remarkable that while some team members of the various collaborations are readying current experiments, others are involved in developing scenarios for modifying aspects of the detectors and accelerators years in advance. A planned luminosity upgrade to turn the LHC into the “Super LHC” is already intensely under discussion. Modern particle physics requires envisioning situations today, tomorrow, and decades ahead—sometimes all wrapped together in the same group meetings.
Amid all of these preparations, researchers try to keep their eyes on the big picture. Results could take years, but the history of science spans the course of millennia. The identification of the Higgs boson and/or the discovery of supersymmetric companion particles could shape the direction of theoretical physics for many decades to come. Another field eagerly awaiting the LHC findings is astronomy. Astronomers hope that new results in particle physics will help them unravel the field’s greatest mystery: the composition of dark matter and dark energy, two types of substances that affect luminous material but display no hint of their origin and nature.