Collider: The Search for the World's Smallest Particles - Paul Halpern (2009)
The Future of High-Energy Physics
The International Linear
Collider and Beyond
The future of particle physics is unthinkable without intense international collaboration.
—LEV OKUN (PANEL DISCUSSION ON THE FUTURE OF PARTICLE PHYSICS, 2003)
Where to go from here? After more than seventy-five years of breathtaking progress, the future of high-energy physics is by no means certain. Much depends on what is found at the Large Hadron Collider (LHC).
In the most disappointing case, if no new physics were found at the LHC, the physics community would have to rethink its priorities. Would pressing collider energies even higher to probe greater particle masses be worth the cost? In an era of tight budgets, could governments around the world even be convinced to fork over the colossal sums needed to construct new ultra-powerful machines for a possibly illusive search? If the LHC came up empty, mustering political support for an even larger device would be an unlikely prospect. “It will probably be the end of particle physics,”1 said Martinus Veltman, referring to the possibility of the Higgs existing but not turning up at the LHC.
There’s no reason to expect such a bleak outcome, however. Assuming that the LHC does discover new particles, such as the Higgs or supersymmetric companions, heralds potential sources of dark matter, opens the door to an exploration of new dimensions, and/or finds something altogether unexpected, the theorists would work through the data and determine which models the results support. Then they would assess what new information would be needed to fill in any gaps.
Ideally, novel findings at the LHC would help decide to what extent the Standard Model is an accurate depiction of nature at a wide range of energy levels. It could also rule out the purest version of the Standard Model in favor of supersymmetric theories or other alternatives. Whittling down theories to the likeliest possibilities would be a happy outcome indeed. If past experience is any judge, however, given the creativity of theorists, there could well be more alternatives than ever before. What to do then?
Because of the Superconducting Super Collider (SSC) debacle, the prospect of American laboratories picking up where CERN leaves off are extremely poor. Aside from contributions to European and international projects, which have proven extremely vital, American high-energy physics looks cloudy in general. No new accelerator labs are in the planning, and the existing ones are grappling with severe reductions in funding.
Fermilab has been living on borrowed time for more than two decades. When the decision was made to locate the SSC in Texas, researchers in Batavia braced themselves for the final spins of their beloved particle carousel—even before the ride truly started. Fate would extend the merry-go-round’s run, however. The SSC’s cancellation in 1993 and the discovery of the top quark at the Tevatron in 1995 showed why the latter was so critical to particle physics. Instead of the machine shutting down permanently, it was temporarily closed for a thorough upgrade.
From 1996 until 2000, a $260 million makeover aspired to transform the Tevatron into an even more stunning collider. After the surgical stitches were taken out, a second series of experiments began, called Run II, and researchers examined the results of the face-lift. Run II produced many notable achievements, including enhanced measurements of the top quark mass, lower bounds on the Higgs mass, and examinations of hadrons containing bottom quarks. Yet, during the early part of the 2000s, its collision rate was not as high as hoped. Fermilab’s directorship realized that to maximize the chances for important discoveries before the LHC went online, they would have to get the machine running even more efficiently.
Fortunately, further efforts during scheduled shutdowns in winter 2004-2005 and in spring 2006 boosted the Tevatron’s luminosity to record levels. To accomplish this extraordinary feat, machine experts integrated the Recycler antiproton storage ring (a method for accumulating antiprotons) more effectively into the Tevatron workings and used the method of electron cooling to tighten up the antiproton beam. This granted the Tevatron yet a few more years of life.
Once the LHC is fully operational, though, the impetus to preserve the Tevatron will largely disappear. With a maximum energy of 2 TeV, it is unlikely that discoveries would be made with the Tevatron that aren’t found first with the LHC. The only way to significantly increase the Tevatron’s energy would be to build a new ring, which is not in the cards. Furthermore, the Tevatron’s use of antiprotons impedes its luminosity compared to proton-proton colliders such as the LHC. Antiprotons are much harder to engender than protons, given that the latter are easily mass-produced from ordinary hydrogen gas. In short, after midwifing particle events far longer than expected, the Tevatron could well be on the brink of retirement. As postdoctoral researcher Adam Yurkewicz jokingly remarked, the Tevatron has been “running so long, puffs of steam are coming out.”2
Given the uncertain future of U.S. laboratories, young American researchers planning to enter the field of high-energy physics had best expect to spend much time in Europe—or alternatively to conduct all of their research remotely. Either possibility has potential drawbacks. Traveling back and forth to Europe can be hard on families and also—if unfunded by stipends—on the wallet. To be safe, researchers anticipating spending time in Geneva might wish to choose partners and friends who are international diplomats, bankers, or fondue chefs—wealthy ones, preferably.
The alternative, conducting all of one’s research remotely, also has its perils. If researchers-in-training are based at CERN during a time in which hardware is being installed or repaired, they might gain valuable knowledge about instrumentation. But if they are spending their prime educational years at a remote institution that happens to be connected to CERN’s computational Grid, they might never have a chance. Suppose a graduate student never has hardware experience and specializes exclusively in computer analysis. He or she becomes a postdoctoral researcher and continues to concentrate in perfecting software packages. Then comes time for a professorship. Would a university be willing to gamble on hiring an experimentalist who doesn’t know a thing about calibrating calorimeters or wiring up electronics?
The concentration of the field of high-energy physics in just a handful of labs—and soon perhaps in a single site—coupled with the rise of larger, more complex detectors has effectively reduced the possibilities for direct experience with the tangible aspects of the field. The days of sitting in trailers parked near tunnels waiting for telltale signals—an emblem of experimental work during the latter decades of the twentieth century—have come to a close. Instead, except for those lucky enough to be present during the building or upgrading of detectors, high-energy physics is largely becoming a hands-off occupation. Given that physical measurements are now conducted in supercooled chambers hundreds of feet beneath the ground, where radiation exposure can be perilous, such a progression is logical. Yet, will sitting in front of computer monitors, either in Geneva or elsewhere, and running statistical software be an exciting enough enterprise to attract the next generation of high-energy physicists?
In the mid-2010s, hands-on expertise will once again be key, when the LHC completes a planned upgrade to what is sometimes called the Super Large Hadron Collider. The main purpose of the enhancement is to boost the machine’s luminosity and increase the rate of productive collisions even further. When the collider is shut down for the upgrade, the detectors will also be taken apart. Burned-out electronics, baked by years of radiation damage, will be replaced and other instrumentation upgraded to improve the detectors’ performance.
Aside from the Super LHC, the next great hope for particle physics is an exciting new project called the International Linear Collider (ILC). As its name suggests, it is the first collider to be planned and funded by the international community, rather than mainly by the United States or Europe. The Superconducting Super Collider (SSC) was supposed to be international, but that never quite worked out. CERN accepts funding from beyond the European community specifically to support detector projects (ATLAS, CMS, and so forth) but not for the machines themselves. Thus if the ILC transpires, it would represent a milestone for global scientific endeavors.
The ILC is planned to be twin linear accelerators facing each other—one energizing electrons and the other positrons—housed in a tunnel more than twenty miles long. The reason for its linearity is to avoid energy losses due to synchrotron radiation—a major problem for high-speed orbiting electrons and positrons but not for those traveling in a straight path. To accelerate bunches of these particles close to light-speed, more than eight thousand superconducting niobium radio frequency cavities (perfecting conducting metal sheets used to transfer radio frequency energy to particles), each more than three feet long, will deliver a series of thirty million volt kicks. All told, these will boost the electrons and positrons up to 250 GeV each. Hence when they collide they will yield 500 GeV of energy, some of which will transform into massive particles. A vertex detector at the collision site will track the decay products of anything interesting that is produced.
Electron-positron collisions are relatively clean and thus ideal for precise measurements of mass. Consequently, though the ILC will be much less energetic than the LHC, its utility will be in pinning down the masses of any particles discovered at the more energetic device. For example, if the LHC produces a potential component of dark matter, the ILC will weigh it and thus inform astronomers what chunk of the cosmos might consist of that ingredient. Knowing the density of the universe would then offer clues as to its ultimate fate. Thus, the ILC would offer a valuable high-precision measuring device—a kind of electronic scale for the world of ultraheavy particles.
So far, the ILC is still in its early planning stages. A site has yet to be chosen—with countries such as Russia making offers. Coordinating the efforts to attract funding and design the project is Barry Barish, the ILC’s director, who formerly led the GEM (Gammas, Electrons, and Muons) project (for the aborted SSC) and the Laser Interferometer Gravitational Wave Observatory. After an initially enthusiastic response to the ILC from many different countries, he has been dismayed that some have started to back away from their prior commitments.
In 2007, after initially supporting research and development of the ILC at the level of $60 million, Congress abruptly reduced funding to $15 million. By October of that year, the ILC had spent much of the allocation, even though the funding was supposed to last until 2008. In a December news release, Barish noted, “The consequences for ILC are dire.”3
Many Europeans are frustrated that American support for scientific projects is unreliable. “In the U.S. everything has to be approved on a year-to-year basis,” said physicist Venetios Polychronakos. “Nobody is going to trust the U.S. to be a partner.”4
After years of reliability, U.K. funding for science has also gone wobbly. In December 2007, Britain’s Science and Technology Facilities Council (STFC) issued a report with dreary news for the ILC. “We will cease investment in the International Linear Collider,” it stated. “We do not see a practicable path towards the realisation of this facility as currently conceived on a reasonable timescale.”5
Recalling, perhaps, a happier age when British nuclear scientists were the “champions of the world,” Queen guitarist-turned-astronomer Brian May decried the funding cuts. Addressing a ceremonial gathering honoring his appointment as chancellor of Liverpool John Moores University, he said, “I think it is a big mistake and we are putting our future internationally at risk in science. . . . We need support for the great scientific nation we have been.”6
Because of the U.S. and U.K. decisions, the ILC is by no means a sure thing. Much depends on a restoration of the commitment by wealthier countries to pure science. Given the global economic crisis, funding for basic research has been a tough sell. Perhaps discoveries made by the LHC will attract enough interest to bolster support for a new collider. If the ILC is to avoid the same fate as ISABELLE and the SSC, its proponents will need to make the strongest possible case that precise measurements of massive particles will be critical to the future of physics.
Though there is much speculation, there are no concrete plans in the works for colliders more energetic than the LHC. Conceivably, the CERN machine will prove the end of the line. In the absence of new accelerator data, physicists would lose an important means of testing hypotheses about the realm of fundamental forces and substances. Astronomical measurements of the very early universe, through detailed probes of the microwave background—higher-precision successors to the Wilkinson Microwave Anisotropy Probe survey perhaps—would become the main way of confirming field theories. Perhaps the ultimate secret of unifying all the forces of nature would be found that way.
Until the day when colliders are a thing of the past, let’s celebrate the glorious achievements of particle physics and wish the LHC a long and prosperous life. We herald the extraordinary contributions of Rutherford, Lawrence, Wilson, Rubbia, and so many others in revealing the order and beauty of the hidden subatomic kingdom. May the LHC open up new treasure vaults and uncover even more splendor. Like Schliemann’s excavations of Troy, the deeper layers it unearths should prove a sparkling find.