Jim Olsen, at the frontiers of physics and teaching

Friday, Mar 31, 2017

Professor Jim Olsen was part of the team that found the Higgs boson, and before that, he helped cinch a major experimental finding that explained why our universe did not self-annihilate in the first few moments after the Big Bang.

Now, after two years serving in a senior leadership role at the European Organization for Nuclear Research (CERN) in Switzerland where the Higgs was discovered, the professor of physics is back on campus. His research on elementary particles produced at CERN's Large Hadron Collider continues, but he is also back to teaching classes, including, last fall, a class for incoming freshmen.

Why anything exists at all

Olsen became interested in physics as an undergraduate at the University of California-Davis when he took a student job building detectors to track high-energy protons, the particles inside atoms. It was his first substantial experience with research, and he was hooked.

He earned his doctorate degree in physics at the University of Wisconsin-Madison. By 1998, Olsen was a postdoctoral researcher at the University of Maryland, working on his first really big question: Why anything exists at all.

The question arises because the most widely accepted theory of our universe's origin states that the Big Bang resulted in the creation of equal amounts of matter and its opposite, antimatter. When these two substances come into contact, however, they immediately annihilate each other. How did matter get the edge over antimatter to produce the universe we know today?

 Maximilien Brice, courtesy of CERN.In the 1990s, an experiment called BaBar at the SLAC National Accelerator Laboratory at Stanford University sought to answer that question. Olsen joined the project with his postdoctoral adviser, Hasan Jawahery. A physics professor at the University of Maryland, Jawahery was impressed when he saw how Olsen, in his first year out of graduate school, taught and led younger scientists at BaBar.

"Right away it was very clear he was capable of leading younger people," Jawahery said. “He knows that when you work well with others, it not only advances their work, but yours as well.”

Olsen’s leadership abilities also drew the eye of A.J. Stewart Smith, Princeton's Class of 1909 Professor of Physics and a scientific team leader at BaBar. “When we started taking data in the summer of 1999, there were all sorts of problems,” Smith said. "I told Olsen, 'Jim, if you get this done, we can get to the physics research quicker.'"

The BaBar experiment involved accelerating two beams of particles toward each other until they collided, then looking at the resulting newly created particles to observe how the laws of nature treat matter and antimatter differently, or asymmetrically. To get to the heart of the matter-antimatter question, researchers needed to look at certain particles called B mesons. The B meson is in turn made of two other particles – a bottom quark and a lighter antimatter counterpart.

During its short lifetime, the B meson rapidly oscillates between existing as itself and existing as its antimatter counterpart, the anti-B meson. Two competing theories made the same prediction for how the B meson would oscillate between matter and antimatter states, but differed on how the B meson would decay, or break down into other particles. 

To monitor this decay and determine the correct theory, the team built a detector capable of sensing B mesons. The 30-foot tall, disk-shaped structure contained layers of electromagnets, chips, crystals, and gas-filled detectors that work together to track and measure the particles as they form and then quickly decay into new particles.

Most of the time the particles coming from the decay of a B-meson included a quark that goes by the name "charm," but Olsen focused on looking for decay events called “charmless decays," comparing the number of the detected decays to the number predicted by various theories. The name "charmless" was appropriate not only because there are no charm quarks involved in the decay, according to Olsen, but also because the task of searching for them was challenging, as these events are hundreds of times less common than other decay events. The charmless decays turned out to be critical to distinguishing between the several theories of matter-antimatter asymmetry.

Once up and running, BaBar's precise measurements began to pay off. Finally, physicists were able to use BaBar's findings to determine which theory of asymmetry was correct. The theorists who created the correct theory, Makoto Kobayashi and Toshihide Maskawa, won the 2008 Nobel Prize in Physics, giving Olsen and the rest of the BaBar team the satisfaction that they had contributed to discovering a fundamental truth about the universe.

The Hunt for Higgs

Olsen continued working with BaBar after taking a job as an assistant professor of physics at Princeton in 2002. He eventually became the project's "physics coordinator," a top leadership position, from 2006 to 2007. After this, Olsen began working with CERN's Large Hadron Collider's Compact Muon Solenoid (CMS) detector – which is five times larger in radius than the BaBar detector – where he would again contribute to Nobel Prize-winning research.

As at BaBar with charmless quarks, Olsen carved out a niche at CERN by looking to detect the Higgs boson in a different way. Rather than focus on the predicted decay of the Higgs particle into electrons and muons – an event so easy to detect that scientists call the process that occurs “the golden channel,” Olsen used his experience working at BaBar to detect the decay of the Higgs into bottom quarks. Compared to the “golden channel,” the signals from Olsen’s chosen search method were about ten times harder to detect.

As a result, when both CMS and ATLAS, the LHC's other detector, reported finding the Higgs boson in 2012, the CMS team was able to provide a more detailed description of the Higgs' decay pathways. "We knew we weren't going to discover it first by looking in that channel, but it was an important analysis to have as it would help tell us what kind of particle we had found," said Olsen. "I was proud of that."

Olsen's contributions to the search for the Higgs boson situated him to lead the write-up of the paper announcing the Higgs' discovery in 2012, and the process of further analyzing the data about the particle from 2013 to 2014.

"To know, while you're writing a paper, that it will be cited thousands of times and be one of the historic papers in our field — that was a very satisfying moment in my career," Olsen said.

In 2014, as the analysis of the first batch of Higgs data wound down and the LHC came back on-line for a new round of experiments at even higher energy, Olsen was appointed physics co-coordinator of CMS, one of two lead scientists charged with reviewing the entire scientific output of the CMS detector.

It was in this position that he dealt with a scientific disappointment. Last year, both ATLAS and CMS had noticed a signal — a "bump" in the data — appearing to show a new particle with a mass of about 750 giga-electron volts, which is about six times heavier than the Higgs boson and almost four times heavier than a gold atom.

Though neither ATLAS nor CMS had strong enough observations to be certain they were seeing a new particle, both were observing similar fluctuations at the same mass, so theorists wrote hundreds of papers trying to explain the possible new particle using theories old and new.

Olsen didn't let the hype get to him, according to Dan Marlow, Princeton's Evans Crawford 1911 Professor of Physics, who leads a collaboration at the LHC to measure a crucial performance factor known as luminosity. "What's important isn't whether you think a result is real," Marlow said, "but what you do about it – which is to collect more data."

Both ATLAS and CMS did just that, collecting data at double the rate of the previous year. Olsen took pains not to bias his team's analysis by avoiding talking with the scientists running the same experiment at ATLAS. "The whole point of having two experiments is having independent observations of the same thing," Olsen said. "If you start talking, it's not independent."

For Marlow, this attitude is the hallmark of Olsen's leadership. "Jim is seen very much as a straight shooter. If he says this is what we're going to do, people understand that it's because that's what needs to be done," Marlow said.

And when more data were collected, the bump went away.

Olsen was unfazed by the non-discovery. Instead, he said he was satisfied that the data re-confirmed the discovery of the Higgs, allowing further analysis of its properties. “That’s how it works,” said Olsen. “You only know by repetition.”

At Princeton, Olsen will continue to study the Higgs boson and review papers from other scientists studying other exotic particles, both observed and hypothetical, at CMS. But he has also returned to teaching, including a first-year undergraduate course that introduces math and physics concepts for engineers.

As in his search for particles, Olsen refuses to take the easy way out in his teaching. Early on in his faculty career, he took on the teaching of a Princeton undergraduate course called Physics for Future Leaders, which teaches physics to non-science students.

"We went from Newton to Schrödinger in a single 10-week course. It was fun," Olsen said. “I feel strongly that it is our responsibility as scientists to help educate the general public about the fundamentals of our field, how we think, and why we do what we do."