Searching for Ghostly Neutrinos to Understand Why Matter Dominates the Universe

What's a neutrino and how do you detect one? Columbia physicist and neutrino hunter Georgia Karagiorgi explains.

Kim Martineau
September 08, 2021

Subatomic particles may sound mysterious, but they’re an essential, even beautiful, part of our world, says Georgia Karagiorgi, an associate physics professor at Columbia.  

“Everything is made of particles,” she said. “If it wasn’t for them, we wouldn’t be here. They provide us with a simple and elegant prescription—a set of building blocks and rules—from which we can derive everything around us.” 

With Columbia professor Michael Shaevitz, Karagiorgi is part of a long-running hunt for new types of neutrinos at Fermilab, a national particle accelerator laboratory near Chicago. Neutrinos are elementary particles created naturally, by nuclear reactions in stars like our Sun, or artificially, in nuclear reactors and particle accelerators. Neutrinos are similar to electrons, but with hardly any mass and no electrical charge. 

Georgia Karagiorgi, associate professor of physics at Columbia University

In the last two decades, physicists have established that neutrinos can flip, or oscillate, between three known weakly interacting states—muon-neutrino, tau-neutrino, and electron-neutrino. Hints of a fourth neutrino-type have been found in previous neutrino experiments at Fermilab and Los Alamos National Lab, and Karagiorgi and Shaevitz are now seeking to confirm or refute its existence. 

One thing is for sure: if a fourth neutrino-type exists, it wouldn’t interact with matter in the same way as its three known relatives, said Karagiorgi. Confirmation of a so-called sterile neutrino would challenge the “Standard Model” of particle physics, which has held for the last 60 years. Results from Fermilab’s MicroBooNE experiment is expected to shed more light on the mystery. Meanwhile, Fermilab is set to launch a related, more sensitive search for sterile neutrinos, the Short- Baseline Neutrino Program. Karagiorgi and Shaevitz are involved in that experiment, too.

What’s the Standard Model of particle physics and where do neutrinos fit in? 

Like the periodic table in chemistry, it’s a catalog of all known fundamental particles that make up matter. Particles are organized by their properties—like intrinsic spin, mass, and electric charge—which allows us to understand their underlying dynamics and how they interact with each other. Since the 1960s, this theoretical framework has been amazingly predictive. It has withstood the test of hundreds of experiments and measurements. 

However, it can’t explain our observations of dark matter in space or neutrinos morphing from one state to another. If neutrinos have no mass, as the Standard Model prescribes, they shouldn’t be able to oscillate. An extension to the Standard Model now accommodates neutrino oscillation, but sterile neutrinos would be one more challenge.

You can’t directly observe neutrinos. So how do you isolate and study them? 

Neutrinos traverse our detectors without leaving a trail, but every now and then they collide head-on with an atomic nucleus. The explosion that follows usually produces a spray of charged particles—electrons, muons, and/or protons. Each charged particle leaves behind a trail of scintillation light and ionization electrons produced by excited atoms along the charged particle's path. By detecting those trails, we can usually infer the presence of a neutrino, its identity, and how fast and in what direction it’s traveling. 

What fascinates you about this field?

Karagiorgi: It’s creative problem solving at its best. You start by carefully formulating a question, figuring out how to answer it, finding the answer (which can take years), and then proving you’re right. There’s no single way to find the right answer, but you do have to make sure it’s right. I knew this was the field for me when, as an undergraduate, I built a cosmic ray muon telescope from parts found in a storage closet. I spent the summer with a friend formulating questions about cosmic ray muons and augmenting our telescope to answer them. We built a rotating set of plywood pockets to measure muon flux as a function of azimuthal angle, and dragged our contraption up and down the fire escape of the tallest building at Florida Tech to measure how many muons were absorbed as a function of number of ceilings above us.

A collision between a neutrino and liquid argon created the red and green trails of charged particles seen here. Neutrinos can't be observed directly.

What’s your favorite particle and why?

There’s a reason why I’ve spent most of my career studying neutrinos. They’re elusive and hard to study, and we still have a lot of questions to answer about their properties. Neutrinos could hold the key to fundamental questions like why our universe is dominated by matter rather than antimatter. 

What’s the goal of MicroBooNE? 

MicroBooNE has two goals: to advance a novel detector technology, and to follow up on a long-standing anomalous signal observed by its predecessor experiment, MiniBooNE. If we can confirm this anomalous signal we may discover other neutrino states or properties that suggest the existence of a new class of particles or forces in nature.

What’s the hardest part about doing this research?

Patience. Neutrinos barely interact with other matter. A neutrino has a good chance of traveling through 200 Earths before interacting at all. So, to study them we need extremely intense neutrino beams and massive detectors running for many years, so as to collect data with enough interactions to make statistically meaningful conclusions. It can take more than a decade from the design of a neutrino experiment to making an actual discovery.

What made you come back to Columbia after finishing your postdoc and part of your PhD here? 

The sense of community, the freedom to be creative with the direction I take in my research, and the environment: New York City is a dynamic place. It’s always changing and reinventing itself. Like science itself, it’s there to teach us new things about our world.

Any advice for women considering a career in physics?

Do it.

You work at Nevis Laboratories, a short commute from Morningside Heights. What’s that like and what does a typical day look like?

To walk into the lab is to be transported back to the Golden Age of particle physics in the 1950s and 60s. It’s not unusual to find a CERN phone book from the 1990s in a desk drawer, or film from 1970s bubble chamber experiments in a cabinet. You can’t help but feel inspired walking the same hallways as legendary physicists like C.S. Wu, Leon Lederman, Melvin Schwartz, Jack Steinberger, James Rainwater, and many others. A typical day consists of the less popular but inevitable back-to-back Zoom calls, code debugging, emails to keep up with, tinkering with electronics boards, and, on the more fun days, data to play with and try to decipher!