Answering the Biggest Questions About the Smallest Matter
One way to talk about what particle physicists are up to when they smash together beams of subatomic particles is to say that they are probing the structure of matter at infinitesimal distances. Another way to describe it is to say that they’re going back in time, creating energies that existed a fraction of an instant after the big bang, when the universe’s fundamental forces and particles were forming. And in the summer of 2008, when the Large Hadron Collider (LHC), located in a 27-kilometer circular tunnel deep beneath the Franco-Swiss border near Geneva, starts colliding two beams of protons traveling at nearly the speed of light, physicists will be able to look back farther than ever before.
The massive LHC is the flagship project of CERN (Conseil Européen pour la Recherche Nucléaire), a joint venture now officially called the European Organization for Nuclear Research and recognized by the scientific community as the world’s largest particle physics center.
The LHC could help resolve theoretical disputes about the structure and evolution of the universe that have gone unresolved for years, and even decades, because of a shortage of the kind of experimental evidence that could determine a winner. And when the proton smashing begins, several Boston University scientists will be in the middle of the fray. Some are the developers or refiners of the competing theories. And some will be there as experimentalists who helped build and operate the massive particle detectors within the LHC to sift through mountains of data looking for signs of new particles — either those predicted by theories, or more likely, something totally unexpected. Whether they are involved in theory or experimentation, anticipation of the LHC has been career-long for most particle physicists.
The long-standing mysteries revolve around shortcomings in the Standard Model of particle physics, which lists the simplest particles known to exist (such as electrons, muons, and quarks) and describes how three fundamental forces — electromagnetism, the strong force that holds together the nuclei of atoms, and the weak force that underlies radioactive decay — act on them. But the Standard Model neglects gravity, and it offers no explanation for “dark matter,” a phenomenon indicating that most of the universe’s mass is invisible because it doesn’t emit light.
In addition, experiments have validated a theory developed in the 1970s by BU’s Arthur G. B. Metcalf Professor of Physics and Nobel Laureate Sheldon Glashow (then at Harvard) and other theorists that at the high energy levels of the early universe, the electromagnetic and weak force would have behaved similarly, both mediated by massless elementary particles. The Standard Model does not satisfactorily explain why, as the universe cooled, this symmetry disappeared: the particles of the weak force (the W and Z bosons, discovered in the 1980s by James Rohlf, a College of Arts and Sciences physics professor, then at Harvard, and others at CERN) acquired mass, while the particle of electromagnetism (the photon) did not.
“The LHC is a big microscope,” says Andrew Cohen, a CAS professor of physics. It is seven times more powerful than today’s highest energy particle collider, the Tevatron at the Fermilab in Illinois. And the higher the energy, the more the types of particles that can be created, particles predicted by the competing theories that go beyond the Standard Model to explain how the W and Z acquired mass. “We know at what energy scale we should start seeing the physics associated with this,” says Cohen, “and that’s where the LHC is going to get us.”
The Standard Model’s explanation for how the W and Z bosons got their mass is called the Higgs mechanism, developed in the 1960s. A nonscientific explanation of the mechanism, offered by Steven Ahlen, a CAS physics professor, begins with the notion that “mass is associated with the difficulty of getting things moving.” Thus, says Ahlen, one can think of subatomic particles affected by the Higgs field as marbles moving through molasses, making them harder to move, giving them the appearance of mass.
But experiments have been unable to find the new elementary particle predicted by the Higgs theory, the Higgs boson. Indeed, the hunt for this boson will be one of the LHC’s first tasks. In addition, the Higgs theory runs into trouble at higher energies. The laws of physics impose definite limits on a Higgs boson’s mass, but at higher energies the mass would have to exceed these limits to counteract instabilities caused by other particles. It’s called the “hierarchy problem,” and most of the theories to be tested at the LHC are attempts to deal with it.
One of the oldest such theories, called technicolor, was developed partly by Kenneth Lane, a CAS physics professor, in the 1970s. In place of the Higgs particle, technicolor proposes that particle masses are conferred by a fifth force carried by a spectrum of techni-particles. All of these techni-particles should be within reach of the LHC, says Lane, who has been waiting a long time for experiments that could comprehensively put his theory to the test. In the early 1980s, he coauthored a paper for Reviews of Modern Physics outlining the energy range particle beam density needed to investigate the mystery of how particles got their mass, parameters adopted in the proposed Superconducting Super Collider that was under construction in Texas when the project was killed by a cost-wary Congress in 1993. All eyes in the physics world then turned to the proposed LHC at CERN.
“I’ve basically spent my whole career on this,” says Lane. “So I’m definitely anxious for it to start working and for physics to come out of it.”
A newer theory than technicolor is known as supersymmetry. This idea posits that every particle in the Standard Model has a “superpartner” with opposite spin that cancels out the high-energy instabilities that give the Higgs theory so much trouble. Electrons are paired with super electrons (or selectrons), for instance, while quarks have super quarks (or squarks), and so forth. In fact, a few of these superpartners have been proposed as candidates for the elusive dark matter. These particles, none of which has yet been seen, should be visible in experiments at the LHC, says Martin Schmaltz, a CAS physics associate professor, who has spent many years working to refine supersymmetry.
“It’s not just energy that counts,” Schmaltz says of the LHC. The new machine will also have a denser proton beam, meaning more frequent collisions (about 800 million per second), and hence more data to mine for evidence of new particles. “The things that we’re looking for are rare. They probably only happen once in a billion collisions,” he explains.
Schmaltz, along with Cohen, has also worked on another set of theories that goes beyond the Standard Model — the notion that the divergence of fundamental particle masses and forces, including gravity, is due to extra dimensions beyond the three we know, which are curled up into such short distances that we remain unaware of them.
Cohen explains the idea using the analogy of an ant walking across a piece of paper that’s been rolled into a tight cylinder. “The ant doesn’t know it’s on a two dimensional sheet,” he says. “It just thinks it’s on a line.”
According to this theory, the apparent weakness of gravity compared to the other known forces, a puzzle ignored by the Standard Model, stems from the fact that gravity must leak over from another dimensional plane. It’s possible, says Cohen, that the LHC will have enough energy to probe the extremely short distance scales where effects of these extra dimensions would be visible.
Nevertheless, Cohen points out that the tighter these dimensions are curled, the higher the energy of any physics associated with them. And here the same energy/mass constraints that complicate the Higgs theory come into play, making extra dimensions, in Cohen’s opinion, look “not so terrific” for explaining the break in electroweak symmetry.
Indeed, when it comes to what “new physics” theories the LHC experimental data might support, Cohen is fond of one he helped pioneer a few years ago along with two Harvard physicists. It’s called the Little Higgs, and like supersymmetry, it predicts the existence of new particles to accompany and stabilize the particles known to the Standard Model. While the hypothetical particles of supersymmetry have a different spin from their conventional partners, an arrangement that Cohen terms “exotic,” the Little Higgs predicts a “perfectly conventional symmetry” between particles of the same spin.
However, while physicists may like some theories better than others, nobody is completely satisfied with any of them.
“None of these models that we know about actually seems to be fitting the data all that well,” says Schmaltz, which makes it a real possibility that the LHC data will hint at new particles that no one has yet anticipated.
“It’s like looking for a needle in a haystack when you don’t even know what the needle looks like,” says Ahlen. “But that’s what makes the LHC a very exciting experiment.”
Ahlen is one of several BU physicists, among nearly 2,000 collaborators worldwide, working on ATLAS, one of the LHC’s two large “general purpose” particle detectors. The detector, which is the size of a five-story building about 100 meters underground, contains superconducting magnetic cables, kept colder than space, that create a strong magnetic field to focus and guide the proton beams. Closest to the proton collision point is the silicon inner tracker, which measures the tracks of resulting charged particles as they fly by. Outside of this are two instruments called calorimeters that measure the energies of these particles. The calorimeters capture most particles, with the exception of neutrinos and muons (particles like electrons, but heavier), whose trajectories and energy are measured in a system of chambers that Ahlen helped design. He says that because most other particles are absorbed by the calorimeters, looking at muon data for signatures of new particles offers “a very clear signal, a very clean way of looking for fundamental physical processes.”
Back in the 1980s, Ahlen was working with James Rohlf on detectors for the ill-fated Superconducting Super Collider. Today, Rohlf is among those working on the LHC’s other detector, called CMS (Compact Muon Solenoid). Rohlf’s expertise is in calorimetry, and his team, which includes three people stationed permanently at CERN, has spent several years designing, building, and testing the digital electronics that will allow the CMS calorimeter to deduce the energy of unstable particles that decay quickly after they’re created by the proton collisions.
Typically, new, more massive particles are discovered by searching for the lighter particles into which they decay, what’s known as a particle’s signature. When the LHC experiments begin, the first task will be creating particles that are known to exist and are well understood (such as W and Z bosons) in order to calibrate the CMS and ATLAS detectors to ensure they are reading the particle signatures accurately. Then experimenters will begin sifting through billions of collisions in search of the telltale signs of new particles, a process sometimes called “bump hunting,” because researchers look for an excess, or “bump,” in the type of events characteristic of a new particle’s decay that stand out from meaningless “background” events.
“This is not an experiment where you’ll turn it on and it’ll all be over,” says Rohlf. “This experiment will run for 20 years. It’s how the field will sustain itself.”
As the scheduled May 2008 start-up of the LHC nears, the BU physicists talk about an excitement in their field unmatched for many years.
“We’ve had these theories for decades. We haven’t had an experiment capable of testing those theories, and that’s what we’re about to get,” says Cohen. “Chances are we’ll find something we’ve never thought of before. It’ll be totally puzzling and wonderfully infuriating.”
For more information, see http://physics.bu.edu/research.
This article originally appeared in Boston University’s Research 2007 magazine.
Chris Berdik can be reached at email@example.com.