Down a Quantum Hole

Undergrad gets a shot at the world's largest physics experiment
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Jul 27, 2011

Dressed in a navy blue t-shirt and jeans with a brownish-blonde crew cut, Joshua Loyal  appears to be a typical college junior. Yet as he chats about his freshman summer searching for elusive and undiscovered sub-atomic particles, it's clear that, at twenty-one, Loyal has moved beyond average.

He spent that summer untangling the paths of particle collisions created at the Large Hadron Collider, or LHC. This enormous atom smasher is celebrating its first year in operation and getting closer to finding the Higgs boson. This elusive particle helps explain the origin of mass in the universe, but it hasn’t been detected yet. 

(See a Slideshow from CERN

“Learning about the particles the LHC may make in its collisions, it’s clear we were dealing with the fundamental forces of physics, essentially the building blocks of all life and matter around us,” Loyal said.

Before beginning his research, Loyal said he had a “laymen’s knowledge” of particle physics and the LHC. He didn’t become hooked on the machine's research until he watched a Discovery Channel special on the search for the Higgs particle.

Then, in an introductory physics class, Loyal and his classmates learned that Duke physicists were involved in the LHC's search for the Higgs and other fundamental particles. Intrigued by the research, he and five other students contacted high-energy physicist Al Goshaw to see if they could work with him.

“I was initially surprised that so many students contacted me about our high energy physics and LHC research,” Goshaw said. (Learn about one student's visit to LHC here.)

High-energy physics, or HEP, isn’t as provocative as other research experiences available at Duke, and it’s a field the students had not quite reached in their introductory physics courses.  “I wanted to see if this group was really ready and committed to this type of research, so before the summer even began I held a series of crash-course HEP 101 lectures,” he said.

Goshaw used the talks to work through the jargon of high-energy physics, the basics of the LHC and its parent institution the European Organization for Nuclear Research, or CERN, its history and how researchers analyze data coming from the particle accelerator.

To Goshaw's surprise, the students learned so quickly, and he soon found himself struggling to keep up with them.

All six students were accepted as summer undergraduate researchers. The group first had to teach themselves the specific details of how the accelerator and the A Toroidal LHC ApparatuS, or ATLAS, detector worked.

It was a lot to get their minds around. The LHC is the world's largest and highest-energy particle accelerator and is contained in an underground circular tunnel that has a circumference of 27 kilometers, or 17 miles. Beams of protons are accelerated around the ring, in opposite directions, at only three meters per second slower than the speed of light. Eventually the beams are forced to collide, igniting quantum fireworks that bust the core atomic particles into even smaller elementary particles. These particles then shower into detectors such as ATLAS.

“Some of these particles exist for only hundredths of a fraction of a second," Loyal said. "After the collision, what’s left is like a crime scene of evidence. We have to piece the data together to see what particles were there.”

He was particularly interested in finding traces of a fundamental particle called the W boson and calculating its mass. 

The W boson has no substructure, meaning that it is one of building blocks of the universe. Loyal said that just as photons, the massless particles that make light, are responsible for the electromagnetic force, the W boson, along with its counterpart, the Z boson, is a carrier particle for, or creates, the weak force.  

If the W boson decays, it turns into other matter particles, such as a quark and a differently charged antiquark, or a charged electron-like particle and a neutrino or an antineutrino. “These are the particles we use to identify the bosons in the collisions and then calculate the W’s mass,” Loyal said.

Throughout the summer, he worked with mock collision data to learn how to untangle the complex collisions and work backwards to calculate the mass of the W boson, which is about 80 billion electron-volts (GeV/c2). That’s about eighty times the mass of the proton or neutron, or roughly the mass of a bromine atom.