Petar Maksimović, On the Hunt for New Physics

By Gabe Popkin

In 2007, when Petar Maksimović started to work on the world’s most powerful particle accelerator, the Johns Hopkins physicist faced a choice: look for the Higgs boson, or look for things even stranger. The Higgs, at the time the field’s holy grail, needed to fill a major hole in the otherwise amazingly successful theory of fundamental particles known as the Standard Model.

But already, physicists knew that even with the Higgs, the Standard Model had to be incomplete. For one thing, it can’t explain gravity. Moreover, observations beginning in the 1970s showed that the model accounts for only around 5% of the universe’s energy; a mysterious substance called dark matter makes up another 25%, and an even more mysterious “dark energy” fills in the remaining 70%. Over the following decades, theorists developed a set of theories known collectively as “supersymmetry” suggesting that the Large Hadron Collider (LHC), then nearly completed at the European Organization for Nuclear Research, or CERN, in Geneva, Switzerland, could turn up never-before-observed dark matter particles. Those particles, and others similarly not predicted by the Standard Model, are where Maksimović set his sights.

Professor Maksimović arrived at Johns Hopkins in 2001, after completing a Ph.D. at MIT and a postdoc at Harvard. He began his research career working on the Tevatron at FermiLab, a U.S. Department of Energy facility outside of Chicago. There Maksimović led a team that in 2006 discovered a particle called “sigma-sub-b,” an “exotic” heavy cousin of the familiar proton and neutron. Though the finding largely confirmed what theorists had already suspected, it was still one of the highlights of the Tevatron’s 18-year run.

Maksimović continued to work part-time on the Tevatron until it shut down in 2011. But even before then he had turned most of his attention to the LHC, where he works on the Compact Muon Solenoid, or CMS, one of the LHC’s two main experiments. The CMS is a five-story-high ring of mind-boggling complex electronics whose major components are layers upon layers of sensors similar to those in a digital camera. The sensors are designed to capture information about the collision produced when near-light-speed protons or ions crash at CMS’s center. Typically, several jets of newly created particles spray outward from the collision site; many of those particles are unstable and decay almost immediately into other, lighter particles. The CMS’s sensors capture snapshots of the decay products as they zoom away. Using sophisticated algorithms that they spend years developing, physicists like Maksimović and his JHU colleague Morris Swartz, among others, can then reconstruct what the original particles’ masses and spins were and compare them to known particles or new ones predicted by theorists.

So far, new physics has proven elusive. While the LHC found the Higgs boson between 2010 and 2013, operating at partial power, Maksimović and his colleagues saw no hint of any new supersymmetric particles. For some this was disappointing, but Maksimović holds out hope for the theory. “Clearly many people expected the new particles associated with supersymmetry—if they exist—to be lighter than they apparently are,” he says, “but the fact that they aren’t light is not necessarily fatal."

Maksimović and his colleagues around the world are now eagerly anticipating the LHC’s second act. The machine is finally running at full power, meaning that particles are smashing into each other at energies up to 13 tera-electron-volts, in particle physicists’ preferred units. That gives Maksimović and his colleagues access to an additional 5 tera-electron-volts—a vast expanse of unexplored energy range.

And already some hints have appeared that the collider could be producing never-before-seen heavy particles, which would decay to pairs of other, lighter bosons. Such a finding would require new physics beyond the Standard Model. But Maksimović is staying cool until enough data come in to determine if the signal reaches the “five-sigma” level—the physics gold standard for a discovery. A five-sigma event happens by chance less than one in 10 million times, whereas right now the signal from the CMS is only around three-sigma, or one in a thousand. “You don’t get excited” about three-sigma, Maksimović says.

Still, he will be “waiting with bated breath” as new data pour out of the LHC in the coming months. “It’s a very exciting time because this is like starting a new collider,” he says. “It’s also somewhat stressful, because one has to really pay attention and be involved, because discovery can come fairly quickly.”

At the same time Maksimović is conducting cutting-edge research, he is also teaching introductory electricity and magnetism. Maksimović approaches his teaching with just as much energy and intellectual rigor as he does his research. He exposes his introductory physics students not just to the standard 19th-century physics curriculum but also to the modern physics that excites him and his colleagues. He even invites outstanding undergraduates to take part in his current research—a rare opportunity for young scientists. To date, he has worked with 22 undergraduate research assistants. And currently he is having teaching assistants lead discussions directly in the lecture hall, rather than the standard problem solving sessions. He also collects data to determine what is working in his classes, and what needs improvement. His students give him consistently excellent reviews, and in 2012 he was recognized with the JHU Alumni Association Award for Excellence in Teaching.

Maksimović also wins praise for his advising and people skills—a good strength to have in a project that involves collaborating with thousands of other scientists. “He’s amazing; I can’t rave enough,” says Alice Cocoros, a third-year graduate student in Maksimović’s group, who recently won a prestigious National Science Foundation graduate research fellowship to support her work at the LHC. She says Maksimović stands out for caring not just about his students’ research but also about factors like work-life balance and family decisions. “Sometimes in academia it seems like…you’re just supposed to be some physics machine that churns out physics, and [personal concerns] are supposed to be secondary. And it’s so not secondary for him. He really treats his students as full people with full lives.”

"Many people expected the new particles associated with supersymmetry—if they exist—to be lighter than they apparently are, but the fact that they aren’t light is not necessarily fatal."
—Professor Petar Maksimović

Riding a Terahertz Wave

Peter Armitage makes new discoveries in quantum phenomena

By Joe Sugarman

At first, Peter Armitage, an Associate Professor and principal investigator at the Johns Hopkins-Princeton University Institute for Quantum Matter, used technology developed by others to conduct his experiments. But as his investigations went deeper, he and his research team began to create the technology themselves. Armitage is primarily interested in studying electronic and magnetic properties that exhibit “exotic” phenomena at low temperatures like superconductivity and magnetism. “My background was not in lasers, but I saw physical questions I wanted answered,” he says. “So I had to make the instrumentation to get the answers. Some of the capabilities that we’ve created don’t exist anywhere else on the planet.”

Armitage and his group have created a device to explore quantum materials in the terahertz frequency range, and another that uses broadband microwave frequencies for material analysis. A third device developed by Armitage’s team employs Fourier transform infrared spectroscopy to record the entire infrared spectrum from near to far infrared at the same time.

“The resolution and sensitivity of the terahertz methods that Peter developed are ideally suited for probing the surface conductivity of a new class of topological quantum materials that is just now bursting onto the stage,” says Collin Broholm, the department’s Gerhard H. Dieke Professor and Director of the Institute for Quantum Matter. “His experiments have also opened a new window on exotic forms of magnetism and superconductivity with great technological potential.”

Associate Professor Peter Armitage and Postdoctoral Researcher LiDong Pan work with an ultrafast femtosecond laser in the terahertz spectroscopy lab

“I try to understand how it is that large ensembles of strongly interacting, but fundamentally simple particles like electrons in solids can act collectively to exhibit emergent macroscopic quantum phenomena like superconductivity,” Armitage says. “Like waves on the sea, the behavior I study is intrinsically collective and is not easily reduced to the properties of individual particles—it is emergent.

“One of the primary ways that we learn about physical systems in general is by shaking them at their natural frequency scales. This is true in cases as varied as masses on springs, to violin strings, to atoms. One might want to use light to probe the natural energy scales of materials. But it turns out that many of the natural frequency scales of these material systems—such as topological insulators, electronic glasses, and heavy fermions (a type of metallic alloy in which the superconducting electrons have unusually large effective masses)—in which I am interested are in parts of the electromagnetic spectrum below those that are easily accessible with conventional light methods. So my group has spent a lot of time developing very specific probes using the appropriate low-frequency terahertz waves. These techniques give us unique insight into these material systems.

“I saw lots of low-hanging fruit,” admits Armitage who arrived at Johns Hopkins after completing postdocs at U.C.L.A. and the University of Geneva. “There were techniques that existed and were developed by other groups that I thought were being underexploited and could be used in this area. So I very much changed my direction when I came to Hopkins because I saw there were new things that could be done. The materials I was interested in studying possess properties that make terahertz radiation an ideal probe.”

Terahertz spectroscopy—a method of using low-frequency terahertz radiation pulsed at super-cooled materials to examine their properties—was in its infancy when Armitage arrived at Johns Hopkins in 2006. Back then, Armitage says, most researchers in the field were electrical engineers or laser research groups, not necessarily interested in using terahertz spectrometers to explore material physics.

Part of Armitage’s lab looks like a high-tech erector set. But this tabletop contraption is no toy, with its complex lattice of optical supports, mirrors, lenses, photoconductive switches, and lasers. It’s a delicately arranged device that employs the relatively new field of terahertz spectroscopy to probe the properties of condensed matter.

“All of these things—the mirrors, the lenses, the supports—are exquisitely tuned to keep the terahertz radiation going around the table,” says Armitage. “It’s hard enough to tune lasers with precision, but when the lasers are invisible and you can’t see where they’re going, it’s a lot of work, a lot of trial and error.”

While Armitage has made a name for himself as a pioneer in a new field, he may be best known throughout the extended Johns Hopkins community as the organizer of the “Professor Extraordinaire” demonstration show at the department’s annual Physics Fair, a day-long event involving demonstrations, experiments, and physics contests for students in grades K-12. It’s a role that Armitage assumed in his very first year as a faculty member. “I was a newly arrived, untenured assistant professor and I just happened to be standing in the wrong line at lunch one day when I was asked,” he jokes.

Each year Armitage chooses a different theme for the fair—from light to pressure to temperature. Then with the department’s “demonstration czar,” Steve Wonnell, he brainstorms experiments often involving dry ice, various “controlled explosions,” infrared cameras, even the infamous “physics ninja,”to illuminate the theme. Armitage is likely the only professor in the department to have fired a Bernoulli “toilet paper cannon” at local TV newscasters on live morning television. “That was one of my earliest teaching innovations, if you will,” he says of the device jerry-rigged from two leaf blowers and several rolls of toilet paper used to demonstrate the Bernoulli principle. “In 30 seconds you can empty two rolls of toilet paper on someone.”

Armitage says he enjoys hosting the Physics Fair because its hands-on nature appeals to the experimenter within. “I started off wanting to do theoretical physics and moved into experimental. A major part of what I do is build things,” he says. “That tinkering around with stuff probably plays a role in what I do for the Physics Fair...and every year it’s gotten a little more goofy.”

His playful, upbeat attitude has made him a favorite among students over the years. It’s a trait that initially attracted doctoral student Grace Bosse to his lab. “He’s a very funny guy and very positive,” says Bosse. “I’m pessimistic. I know everything is going to go wrong, but Peter is the opposite. He’s like, ‘Well, of course it’s going to work. Just do it.’”

Armitage says he plans to expand on his work in terahertz spectroscopy, as new discoveries continue to drive the need to create new technology. “I think we’ve made a big impact in an underexploited area,” he says. “Now I have former postdocs off doing their own work in the field. I feel like we’ve established a bit of an niche, which has been very satisfying.”