September 30, 2013 | 2
The Large Hadron Collider has only just begun its explorations, so it might seem a little premature to begin thinking about what new particle projects might come next. But given how long these things take to plan, is it ever too soon? This summer, particle physicists held a huge planning retreat in Minneapolis, which Peter Woit of Columbia University blogged about. Woit is best known as a sharp critic of string theory—a subject on which he and I have agreed to disagree—but his post on future accelerators had nothing to do with that. Among the attendees he quoted was theorist Nima Arkani-Hamed of the Institute for Advanced Study, who has taken up the cause of a ginormous new accelerator with his customary gusto. I visited Arkani-Hamed last week to talk about his rethink of theoretical particle physics, but we also chatted about his soul-stirring vision for the future of experimental physics.
Arkani-Hamed thinks his profession needs to think big. Very big. Extremely very big. As in, four times the circumference of the LHC, capable of slingshotting particles with seven times as much energy. The preliminary results from the LHC demand nothing less. CERN, the Institute for High Energy Physics in Beijing, and, more quixotically, Fermilab have also been toying with the idea.
Right now, the leading proposal for a post-LHC project is the International Linear Collider, a pair of 11-kilometer-long electron guns pointing at each other as if in a subatomic duel. Earlier this year, planners staked out a site in the north of Japan. The Japanese government views the project as a post-tsunami economic stimulus for the region.
The ILC fits into a historical pattern of making discoveries using protons, as the LHC does, and following up with electrons. Protons are not just subatomic particles but whole subatomic worlds, bustling with smaller particles. (Indeed, physicists have yet to understand them in full.) Smash two of them together and you get an ungodly mess. But protons do have the virtues of their defects: being heavy, they tend to hold onto whatever energy you give them as they round a bend, so they’re easier to rev up. Electrons are just the opposite. They’re as simple as they come, which makes them well suited to making precision measurements, but so lightweight that accelerating them in a circular ring is a process of three steps forward, two steps back. The ILC would smash electrons with an energy of 0.5 teraelectron-volts (TeV), much lower than rated power of the LHC (14 TeV), although the energy would be used to greater effect.
What forces an update of this plan is that the LHC has found the Higgs boson—and the Higgs boson only. It hasn’t turned up a single exotic particle, such as the supersymmetric particles that are the hypothesized heavy partners of known particles. Those superpartners might still be out there, merely too heavy for the LHC to produce so far. But much of the rationale for them has already evaporated. Supersymmetric particles were supposed to set the mass of the Higgs. For that to work, their own masses should be comparable to the Higgs’s. Yet the null result suggests that any supersymmetric particles are several times heavier, at least.
Crudely speaking, the Higgs mass equals the difference between the masses of particles and of their superpartners. If the superpartners are as massive as the LHC now requires, this subtraction must occur with a precision of one part in 10—a contrived situation that physicists call “fine-tuning.” If the LHC continues to come up empty-handed when it restarts in 2015, the degree of fine-tuning will be one part in 100.
However you look at it, the situation creates an itch that the ILC can’t scratch. If superpartners exist, they are likely to be too heavy for the ILC to create. The ILC will still dissect the Higgs, which is great, but still a significant narrowing of its scope. If, on the other hand, superpartners don’t exist, then what accounts for the Higgs mass? Arkani-Hamed concludes that a precision follow-up machine isn’t enough. Physicists also need another discovery machine—one that would take another giant leap in energy and look for whatever it is that explains the Higgs.
You might worry that the science-loving peoples of the world would pay billions for an übercollider only to find nothing: another null result. That seems unlikely—physicists have yet to build a new machine in vain—but Arkani-Hamed says a null result might actually be the most fascinating outcome of all. If there’s nothing out there to explain the Higgs mass, maybe the Higgs is inexplicable. Its mass might have been set at random. And that is most naturally understood as a consequence of living in one universe among many (as depicted above). Each parallel universe locks in a different Higgs mass, and we got the value we did because we got the value we did. Such a prospect is already suggested by the observed density of dark energy, which seems to have no explanation other than that we couldn’t exist if it were much different.
If a collider reaches 100 TeV while finding nothing, the Higgs mass must be finely tuned to one part in 10,000. Though not proof positive of the multiverse, that would be as compelling circumstantial evidence as you could ever hope for.
Leaving aside the multiverse, which I’m personally less enamored of than I once was, what I find interesting about Arkani-Hamed’s argument is that particle physics is entering a new phase. With the discovery of the Higgs, the Standard Model is now complete. It predicts the outcomes of all known particle experiments. It has icky aspects—even the short form of its equations fill a page and are riddled with arbitrary parameters—but no specific loose ends for experimentalists to tie up. Therefore the next generation of accelerators will have a different theoretical goal than prior ones: not to fill in the structure, but to understand it.
No particle physicist would dispute that a 100 TeV accelerator would be a dream machine, but many do dispute that planners should be thinking so far ahead. “I share Nima’s enthusiasm for a 100 TeV collider,” says Michael Peskin of the Stanford Linear Accelerator Center. “But this is a project for the future, not for today.”
What is more, there’s still hope that the LHC could find supersymmetric particles with a suitably Higgsish mass. Barry Barish of Caltech, a co-author of Scientific American‘s article on the ILC in 2008, says, “As far as what has been done so far at LHC, it has less than 1 percent of the planned final integrated data in place, so they are looking at the tip of the iceberg.” Other theorists think you don’t need supersymmetry to explain the Higgs; the Standard Model can already do that.
Perhaps the main cause for skittishness, though, is that most American particle physicists over the age of 40 still feel burnt by Congress’s cancellation of the Superconducting Super Collider in 1993. The SSC would have reached an energy of 40 TeV and settled the questions that Arkani-Hamed has posed. One might argue that the cancellation was justified by cost overruns, but be that as it may, U.S. scientists dare not dream anymore—the U.S. Congress is too fickle a patron. Even no-brainers such as cancer research and climate monitoring are a tough sell these days. The future of fundamental physics lies in Europe and Asia. Chris Quigg of Fermilab tells me: “On my last long visit to CERN, a former director general came to my office and announced, ‘Chris, I’ve been thinking: you should have built the SSC. It was the perfect machine.’”
Images courtesy of Nima Arkani-Hamed, Institute for Advanced Study