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Atom Smashers

By Don Lincoln

Published Jul 1, 2015 8:00 AM

UCLA physicists study big questions at the large Hadron Collider.

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Illustration by Aaron Ross

What would you do if you could fire a bullet at a speed of nearly 186,000 miles per second? Shoot it into something just to see what happens? That’s pretty close to what some UCLA scientists are doing. More accurately, they take two beams of protons, each traveling so fast that they could circle the globe more than seven times in a single second, then smash them together in the center of a huge detector and take a picture of the debris. As if that weren’t challenging enough, they do it 40 million times a second.

So why would anyone do this? What scientific purpose does it serve? Well, for one, the temperature at the center of these collisions is the hottest ever achieved by mankind. It is 100,000 times hotter than the center of the sun. It is 10 times hotter than the center of a supernova, which is the explosion of a star with a flash bright enough to be seen halfway across the visible universe. A temperature like this was last common a scant trillionth of a second after the Big Bang. In a real sense, scientists are re-creating the conditions of the very birth of the universe and studying it in incredible detail. These collisions also enable scientists to peer deep inside matter, looking at things that are one-hundred-millionth the size of an atom, in an effort to understand the most fundamental building blocks of matter. By any measure, these studies probe the frontier of human knowledge.

The facility that enables scientists to do this is called the Large Hadron Collider, or LHC. It is located just outside Geneva, Switzerland, and is the largest particle accelerator ever built — a huge ring about 17 miles in circumference. Collisions occur inside four large detectors that are essentially large, fast cameras, frantically taking photos of the collisions. The particle detector on which UCLA scientists work is called the Compact Muon Solenoid, or CMS. This 100-megapixel camera stands 50 feet high, is 70 feet long, and weighs about 14,000 tons. It’s as big as a medium-size building.

Finding The Elusive Higgs Boson

Three UCLA physics professors, Robert Cousins, Jay Hauser and David Saltzberg, are in the business of asking and answering hard questions. This trio of scientific sleuths leads a group of UCLA researchers as part of an international collaboration to conduct research using the CMS detector. In 2012, that collaboration resulted in the discovery of the Higgs boson, an elusive subatomic particle that was the last missing piece of the wildly successful Standard Model of particle physics. The Higgs boson is the particle that gives mass to all subatomic particles, while the Standard Model is our best theory of the rules that govern the matter and energy of the universe. The discovery of the Higgs boson led to the 2013 Nobel Prize in physics for the team that predicted the particle’s existence back in 1964.

Hauser says, “When the LHC started colliding protons in 2010, we all knew that finding the Higgs particle would be very difficult. Performing better than we’d hoped, the accelerator delivered a lot of collisions, and the detector and the ingenuity of our data analysts exceeded our expectations. It really was a tour de force of experimental science.”

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The red circle shows the scale of the LHC. The area under which the tunnel for CERN’s LHC can be found is shown, near Geneva and Lac Léman. Seen in the background are the French Alps, with Mont Blanc. Photo courtesy of CERN.

The LHC ran from 2010 to February 2013, and CMS-affiliated scientists accumulated data that have led to more than 400 publications, a fraction of which were related to the discovery of the Higgs boson. The others investigated the behavior of other subatomic phenomena, which resulted in doctoral degrees for four UCLA graduate students. Saltzberg says, “The UCLA Ph.D. students on CMS were in the experimental trenches at CERN. They built apparatuses that recorded signals from the beam collisions. And they analyzed data, looking for rare anomalies that could have indicated a big discovery.”

While data already recorded and analyzed were an unqualified success, for the past two years the LHC has been shut down for refurbishments, retrofits and upgrades. The UCLA CMS group and their collaborators have used this time to work on the CMS detector to upgrade its capabilities.

While Cousins, Hauser and Saltzberg currently helm the group, they are assisted by about a dozen researchers, postdoctoral associates, and graduate and undergraduate students. The UCLA researchers built muon detectors, which played a central role in the discovery of the Higgs boson.

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Illustration by Aaron Ross

In order to be retrofitted, the CMS experiment required that both the LHC and CMS be substantially disassembled and reassembled. Greg Rakness, who was a UCLA researcher for 10 years before taking a fulltime position at Fermilab in April 2015, is the Co-Run Coordinator for the CMS experiment. It is his responsibility to ensure that all the components work together as designed.

“It’s an incredibly challenging task,” he says. “Pieces of equipment are shipped from over 100 institutions spread all over the world. We then have to connect them together in incredibly tight spaces. It’s like assembling a computer from scratch inside a bottle and guaranteeing that the computer will boot properly every time.”

Increasing Collisions

While particle physics research is typically the playground of graduate students and scientists who already have their doctorates, the CMS group at UCLA has a long history of involving undergraduates. Dozens of UCLA undergraduate students have made extremely important contributions, from assembling the huge muon detectors to building, testing and configuring thousands of electronic boards that enable the CMS detector to select the subset of data that has the best chance of leading to a discovery. These electronics have to work reliably and make a decision in about a millionth of a second. During the recent period of refurbishment, undergraduates Jacob Beres and Andrew Peck ’12 were stationed at CERN to help build and test additional muon detectors that will extend the experiment’s abilities.

“While making real contributions to frontier research, they get to experience firsthand the collaborative and international nature of work at CERN and learn about many aspects of detector hardware and electronics, while also getting a chance to enjoy the outdoor paradise that surrounds Geneva,” says Cousins. “It’s truly an extraordinary experience.”

Another challenging project in which UCLA undergraduates participated involved the invention of new technologies. For all the impressive capabilities of the CMS experiment, scientists expect that future LHC upgrades will increase the collision rate inside the CMS detector tenfold. Although this increased rate will provide additional opportunities for discoveries, it is accompanied by increased radiation damage to the central parts of the detector. In a few years, the battered heart of the detector will need to be replaced. This silicon-based detector will simply be worn out, and a new material will be required to survive the onslaught of collisions. UCLA undergraduates Charlie Schrupp and Taylor Barrella ’12 went to CERN as part of the team that tested a possible replacement technology made of diamond. Scientists hope that this new technology will be more robust against the punishing environment and could form the basis for a future upgraded detector.

“These undergraduate physics majors flew to CERN to be more than just spectators,” Saltzberg says. “By analyzing the beam data within minutes, they assured efficient use of the highly valuable and always all-too-brief beam time.”

Why Dark Matter Matters

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Illustration by Aaron Ross

Recommissioning of the LHC began in April, and the accelerator is now being turned over to researchers to explore new aspects of physics. And what incredible research it is! Around a thousand Ph.D. students will work on data analysis, trying to make a discovery that changes how we think about the universe. Pieter Everaerts, a UCLA postdoctoral researcher, leads a group that is exploring a branch of physics called supersymmetry, a proposed new theory of physics that could supplant the existing Standard Model. Supersymmetry predicts that all known particles have an asyet-undiscovered cousin. One of the cousins could well be a substance called dark matter, which is a form of matter that is invisible to our telescopes and which astronomers claim is five times more prevalent than the ordinary matter of molecules and atoms. Scientists speculate that the LHC might create dark matter and allow them to study that dark matter in the lab.

“Supersymmetry is the most compelling of a whole litany of proposed new theories,” Everaerts says. “It is incredibly exciting to be working at the LHC, where we combine many measurements to either verify the theory or maybe kill it entirely. Either way, we learn something crucial about the makeup of the universe.”

UCLA graduate student Eric Takasugi M.S. ’11 is taking another approach. He is studying the data for well-known phenomena and comparing these data to predictions from the Standard Model. His rationale is simple: In order to search for new physics, existing theory must describe the familiar physics processes that scientists have long known about. If the theory describes well-known physics, it is more likely that any discrepancy is a sign of something entirely new.

Because of the breadth of expertise among the UCLA physics faculty, Takasugi says he can compare his measurements to the predictions of the strong UCLA theory group headed by Professor Zvi Bern. That has been “an incredibly productive collaboration. It will be interesting to see if the data we take in 2015 and beyond continue to agree with predictions, because we can use known physics to provide insights into unknown physics. And, if not, well, that might be even more interesting.”

The next few years will be extremely exciting, with the prospect that this new data might lead to a new discovery, but the UCLA group is looking farther out. The LHC accelerators and detectors will require a constant stream of upgrades. UCLA researchers are already developing the technologies to confront this frantic pace.

“The LHC research program will run for probably the next 15 to 20 years,” Hauser explains. “It has already made one discovery that led to a Nobel Prize. With the enhanced beams and detectors, it will almost certainly make more. To quote Yogi Berra, it’s hard to make predictions, especially about the future, but the data recorded by the LHC will certainly cause us to rewrite the textbooks.”