CMU helps build Large Hadron Collider

Two weeks ago, the world’s largest particle supercollider, the Large Hadron Collider (LHC), officially celebrated a milestone in its development when the first particle beams were successfully accelerated across its 27-mile circumference. The real job of the LHC is smashing bits of our universe together in the hopes of discovering new elementary particles — notably the elusive Higgs Boson, and possible candidates for dark matter particles — which may help explain how our universe holds itself together.

The Higgs Boson is a hitherto-undetected elementary particle that the Standard Model predicts to exist and is crucial to explaining why particles have mass. Dark matter is a similarly undetected form of matter which is accountable for nearly 22 percent of the unseen mass in space holding galaxies together. Together these particles constitute some of the largest mysteries in physics today — mysteries the LHC may soon be able to unravel. The LHC essentially accelerates two beams of protons to energies of over seven trillion electron volts (TeV) in opposite directions, and then points them on a collision course. Upon impact, the immense energies released cause the constituents of the nuclei to interact, releasing elementary particles in a shower of radiation, which lasts for mere nanoseconds before decaying. These elementary particles created during collisions are the focus of massive amounts of research, as they are hoped to reveal what is truly at the heart of matter itself.

While smashing two clocks together may not seem like the smartest way to learn how clocks work, there appears to be little other recourse for high-energy physicists to learn how our world truly works.

Tom Ferguson, a physics professor at CMU, has been working on the LHC for over 14 years, from design to prototype to actual production. Currently, Carnegie Mellon has a team of 11 members led by Ferguson working on the LHC, including physics professors Helmut Vogel, Manfred Paulini, and James Russ, with the remainder being post-doctoral and graduate students.
Carnegie Mellon’s main contribution to the LHC lies in the construction of the Compact Muon Solenoid (CMS) — a 20-meter long, 18-meter high detector built for the specific purpose of detecting muons, elementary particles produced both during collisions and upon the decay of the collision products. Specifically, CMU constructed the end-cap muon chambers for the CMS — massive detector chambers which capture trails of muons as they fly through matter.

“The ability to detect muons and meson momentum is a crucial part of the CMS,” said Vogel, who recently began working on the CMS as a part of Ferguson’s team. “Many of the elementary particles have decay products that contain muons, and detecting these muons can give us a very good sense of what might have been produced inside,” he said.
The muon chambers are filled with a mixture of argon and carbon dioxide gas. As a muon passes through, it ionizes gas molecules around it, leaving a trail of ionized particles in its wake.

The freed electrons drift to a mesh of high-voltage wires strung across the chamber, which convert them into electrical pulses that are measured and recorded.The choice of gas for the muon chambers was an interesting one. “We could not use a gas like, say, oxygen, because it would be ‘poisonous’ to the electrons,” Vogel said. “Oxygen is highly electronegative — meaning the electrons would be absorbed before they could be detected,” he explained. “The gas also needs to be nonflammable, yet affordable — it is like a many-parameter optimization problem,” Vogel said.

“We built all the electronics that read out these chambers,” Ferguson said. “There is nothing on the market that comes even close to the precision we need for these measurements,” he said. All in all, CMU contributed to the fabrication of over 10,000 chips for 150,000 channels of detectors within the CMS alone.

The analysis team that the Carnegie Mellon contingent is involved in is currently researching a fuller understanding of upsilons, which are elementary particles that have been a subject of interest in high-energy physics for several years.

“It is clear we do not fully understand upsilon production,” Ferguson said. “We hope that by boosting to higher energies, we may be able to discover something new about these particles.”

Ferguson likened his work to those of chemists in the 19th century. “[Chemists] had all these elements that we know the atomic mass, the properties and characteristics of, but we do not know why they are there,” he said. “After Mendeleev put them in a table, he showed there was order to the system — and we were then able to create new elements as predicted by the Periodic Table.”
“Similarly, we now have these particles like the Higgs Boson that are predicted by the Standard Model, and we are trying to find them within these collisions,” Ferguson said. “Why are they there? What is underlying it all? We do not know yet, but we are hoping this will be the last piece of the puzzle.”

Whether or not the Higgs Boson is ever discovered, physicists at large are exceedingly excited about the prospects that the LHC brings. “Personally, I hope nobody is right,” Ferguson said. “I hope we discover something no one has predicted — now that would be exciting!”
Vogel, too, shares his optimism at the prospects of the LHC. “We expect to detect some intricate signatures or event topologies that tell us what kind of supersymmetric dark matter candidates might have been produced,” he said. For a rapper’s version of how the LHC operates, Vogel recommends the LHC Rap — which happens to be scientifically accurate — accessible by searching for “lhc rap” on YouTube, or at the following link: