Scientists at Harvard and around the world held their breath Wednesday (Sept. 10), as colleagues switched on the most powerful particle accelerator ever built, the Large Hadron Collider at CERN, the particle physics laboratory in Geneva.
To their relief, the twin proton beams circulated in opposite directions around the 17-mile underground loop with no problems. Though the event, called “first beam” by physicists, was cause for celebration, the real fun begins in about a month when they direct those beams to cross, an event acutely anticipated because they’re not quite sure what will happen next.
Theories about the collisions of enormously powerful proton beams present strange possibilities, such as the formation of microscopic black holes, the opening of new dimensions, and the creation of elusive, theorized dark matter. Scientists involved talk about the “new physics” and peeking into a realm where our current theories about how the universe works break down.
One hope is that those peeks will help us understand the universe’s basic laws better, filling in glaring gaps in our knowledge and inspiring new theories that work equally well with all four of the universe’s most basic forces, thereby attaining the “Holy Grail” of theoretical physics.
Donner Professor of Science John Huth, who is coordinating one of the Large Hadron Collider’s (LHC) key experiments, called ATLAS, describes the upcoming proton beam collision as humanity’s “largest single scientific undertaking.” The uncertainty of what awaits, he says, is exciting.
“I’ve lived my life under this era of theoretical supremacy where everything worked out according to theory. Now we’re right at the point where theories break down,” Huth said.
The Large Hadron Collider is the most powerful ever built. Beams of protons traveling around the LHC’s underground loop will smash into each other with a combined energy of 14 tera-electron volts (TeV). Though a single electron volt is fairly tiny — on the scale of that used by a flying mosquito, according to CERN officials — the LHC generates 14 trillion of them and packs them into a proton, more than a million times smaller. The energy achieved by the LHC will be seven times more powerful than the current record holder, Fermilab’s Tevatron.
Accelerators like the LHC have been used to explore the mysteries of the atom for decades. Huth’s early work at Fermilab resulted in the discovery of the top quark, one of the 40 elementary particles predicted by theoretical physics’ Standard Model. Huth said that discovery was exciting, but is nothing compared to this.
“The size of the [top quark] experiment is dwarfed by ATLAS. The size of the collaboration is dwarfed by the ATLAS collaboration. And the kinds of physics we’re going to look at are also quite astonishing,” Huth said. “It’s almost a sensation of ‘pinch me, can this really be happening?’”
The Large Hadron Collider sits in a circular underground tunnel that crosses from Switzerland into France and back again. Along its circumference sit four detectors conducting experiments involving elementary particles.
ATLAS, which stands for “A Toroidal LHC Apparatus,” is a collaboration involving some 2,000 scientists around the world. Huth, project manager for physics and computing for the U.S. portion of ATLAS, said the Harvard team consists of himself, Professor of Physics Masahiro Morii, Mallinckrodt Professor of Physics Melissa Franklin, Assistant Professor of Physics Joao Guimaraes da Costa, and Senior Research Fellow in Physics George Brandenberg. Joining them is a team of eight graduate students and four postdoctoral fellows. In addition, Huth also oversees another 40 people working on software and computing to handle the enormous amount of data expected from ATLAS. Among other institutions, Huth said the Harvard team worked closely with researchers from the Massachusetts Institute of Technology, and Tufts, Brandeis, and Boston universities.
ATLAS, the largest and most complex detector ever built, is a cylindrical instrument measuring roughly 40 meters long by 22 meters in diameter. Sitting in an underground cavern at one of the LHC’s beam intersections, ATLAS was assembled underground of parts made in different locations. It is made up of concentric layers of tubing bundled so that they look, as one scientist described it, “like a bunch of straws glued together.”
Each of the hundreds of tubes, 3 centimeters in diameter, contains a high-voltage wire that creates a signal when a particle strikes it. The wires are located with extraordinary precision and placed within a tolerance of about 50 microns — the width of a human hair.
Morii said it is most likely that there will be several months of calibrations and adjustments, making sure first that the beam is working properly and producing collisions and then that the detectors are working well, so researchers know that the results they’re getting are real and not an artifact of misaligned or malfunctioning equipment.
“You turn it on — Wow! Then once it runs, you can sit down and say, ‘Now, let’s get down to business. What are we getting? How well is the machine performing?’” Morii said. “Typically the first three months to six months of data are wasted because nobody understands what’s going on. Within a year people have a good idea of how the machine works.
“I plan for a tedious process of poring through lots of data. I tell students ‘life is tough, you’re going to have to go through a lot of junk.’ But there is a chance that the signal from the new physics is so spectacular that when we get it we will know almost immediately that something’s happening.”
Breaking old theories to make new ones
The Large Hadron Collider was designed to burst the boundaries of known physics. For the past several decades, physics has been ruled by what’s called the Standard Model. The theory says, in essence, that everything in the universe is made up of 40 different elementary particles and it sets out rules by which those particles interact.
Most people understand that everything we see and handle in the everyday world is made up of atoms and that atoms consist of protons and neutrons bound together in a nucleus surrounded by a cloud of electrons.
The Standard Model takes that everyday understanding a bit further, breaking the protons and neutrons into even more basic particles called quarks, and positing a whole host of other particles with strange names like leptons and bosons, each with a particular characteristic and role to play in making the universe operate as it does.
The Standard Model has been so robust that its predictions have been verified by virtually every experiment carried out to test it. Just one particle predicted by the Standard Model — the Higgs boson — has yet to be seen in experiments. Scientists are eager to see whether the LHC’s energy levels can produce one.
Though the Standard Model has served the physics world well for decades, physicists know it’s not complete. For one, it encompasses just three of the four basic forces of the universe: electromagnetism and two less familiar forces, the “strong” and “weak” forces that operate inside the atomic nucleus, holding it together and fueling the reaction that powers the sun. The fourth basic force — gravity — operates under another set of theoretical rules entirely.
So physicists designed the Large Hadron Collider in the hope that what it reveals will help winnow the many theories competing to pick up where the Standard Model leaves off.
“As you stretch the theory to higher and higher energies, there is a point at which the theory cannot provide us with a consistent answer anymore. The theory itself stops working. That energy is just about the point we’re able to reach,” Morii said. “We knew that building this machine would have a huge impact on the history of particle physics. We have certain ideas about when the Standard Model will break and we want to break it. We fully expect something unexpected to happen.”
ATLAS, Huth said, was specifically designed to explore a subtheory of the Standard Model that describes the best-understood relationship between fundamental forces — that between electromagnetism and the weak force. Electroweak Theory says that electromagnetism and the weak nuclear force are different manifestations of a single force. ATLAS is designed to explore “symmetry breaking,” or how the single electroweak force can break into two forces with different characteristics.
Another important element of Electroweak Theory that needs fleshing out, Huth said, is how particles get mass. That, in turn, could provide the long-sought connection between gravity and the other three forces.
“That would be my fondest hope, that somehow there’s a gigantic ‘Aha!’ and we’re steered toward a theory that includes gravity as well as the other forces,” Huth said. “It’s the Holy Grail and, having been in this business since I was quite young, I’m not saying it will happen, but of all the places where there’s a chance it might happen, this would be it.”
When asked whether he has a favorite among the theories competing to explain what scientists will soon see, Huth said that, as an experimental physicist, he likes to remain neutral and let the results do the talking.
“My view of the best experiment is basically to create a blank slate and let Mother Nature write on it and tell you what’s going on,” Huth said.