Scattering particles: An interview with Penco

The equations that describe scattering particles are covered in your average introductory physics course, but there is a hidden richness to this otherwise simple problem. In recent years, there has been renewed interest in understanding scattering through the lens of "quantum field theory." One property of collisions is that, rather intuitively, the odds of any two particles scattering becomes smaller as you decrease the momentum of one of the particles. In the limiting case where the momentum is zero, the probability becomes zero as well.

Assistant Professor of Physics Riccardo Penco and two co-authors discovered something interesting about the "probability amplitudes" of phonon collision. Their finding was published in the journal "Physical Review Letters." In an interview with The Tartan, Penco explained how they "tweaked one of the main assumptions" of this problem by studying scattering in a medium, as opposed to a vacuum. "We looked at what happens when you scatter phonons off each other," he said.

Phonons are quasiparticles that represent sound, much like how photons carry electromagnetic radiation — light. As the momentum of the phonon became vanishingly small, the probability of phonon collision decreased proportionally to the particle's momentum raised to a fractional power, as opposed to the momentum to an integer power (essentially, it decreased according to the square root of x instead of x-squared). This means that the probability of collisions trends toward zero at a much slower rate. They are the first to demonstrate the probability curve can be proportional to a fractional power of momentum.

Penco described how co-author Tomáš Brauner invited him to a seminar in Stavanger, Norway, where they discussed the nature of particles scattered in fluid. They initially modeled this problem purely with code, but they kept noticing strange square-roots popping up in their probability curve.

"We spent a lot of time trying to find a bug in our code … no one had ever seen this before," Penco said. When Angelo Esposito visited Carnegie Mellon sometime later, he helped them realize there was no bug, and that this was a novel discovery. "Sometimes it's a matter of putting together the right people at the right time at the right place."

Penco's research involves "physical systems that range in size from laboratory to cosmological scales," and he's studied everything from sound-waves to tides to black holes. He acknowledges that his research may seem to involve disparate topics, but he links them together through the lens of "effective field theory." He explained how his breadth-first approach to research is encouraged by the interdisciplinary culture at CMU, and he's been given "free reign" to pursue different topics.

"To do research you need a certain mindset," Penco said. "There are people that like to get super deep in a subject and know every single thing written about it, and are very narrow. You need people like that, but you also need people to draw connections between things that researchers in isolation wouldn't put together."