Research at Boston University

The Measure
of a Muon

By Neil Savage

Finding the Higgs boson took the collective efforts of thousands of scientists working on different pieces of the task—developing theoretical models, building the particle accelerator, analyzing the data. Professor of Physics Steve Ahlen, working on the project for the last 16 years, helped design and build one of the detectors that would capture the fleeting results of near-light-speed particle collisions so analysts could determine if they had actually found the Higgs.

The Large Hadron Collider (LHC) works by sending two beams of protons racing in opposite directions along a circular path at close to the speed of light. When bunches of protons smash into each other, about every 50 billionths of a second, some of the collisions should, according to theory, create the Higgs boson. The Higgs almost immediately breaks apart into other subatomic particles, and some of those decay even further, into more particles. It’s the job of various detectors to pick out the interesting few hundred traces of millions of collisions every second and measure the momentum and mass of the particles they see. Scientists then work backward from the results, tracing the particles’ source to see if some of them came from the decay of a Higgs.

Ahlen works on the muon chamber, the outermost of a series of detectors in the ATLAS experiment at the LHC. Detectors closer to the collision capture other particles—photons, electrons, pions. The only remnants to make it to the outer chamber are muons, which Ahlen calls a sort of heavy electron, and neutrinos, which can pass all the way through the planet.

The muon detector contains about 350,000 aluminum tubes, averaging four meters long, each filled with pressurized argon and carbon dioxide and with a wire running down the middle. When a muon passes through a tube, it knocks electrons off the gas atoms, which are drawn to the wire, producing an electrical signal. By measuring how long after the collisions it takes for the signal to occur, scientists can tell how far the muon is from the wire. Comparing that measurement as the muon passes through at least three groups of tubes tells them if its path is straight or slightly curved, which gives them its momentum.

“Imagine something traveling 30 to 40 feet and we have to measure if that’s straight or not, with the difference being the diameter of a human hair,” Ahlen says.

Now he’s developing better versions of the detector that can handle the higher energies and intensities the LHC will use in several years to look for other particles. “We’re still operating in its infancy,” he says. “We still haven’t gone to full energy, so discovering the Higgs boson is just the beginning.”

He says the field of particle physicists has been dominated since about 1980 by theorists trying to fit the results of earlier experiments into a coherent whole. Now, with the LHC up and running, the data gatherers are getting their turn in the spotlight. “We’re back into an observation-based regime,” Ahlen says. “It’s an exciting time to be an experimentalist.”

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