What is a neutrino, and how can one be detected?

Georgia Karagiorgi, a physicist at Columbia University and a neutrino hunter, explains and illustrates what she expects to discover at the Fermilab accelerator near Chicago.

According to Georgia Karagiorgi, an associate physics professor at Columbia, subatomic particles may appear strange, but they are an important, even beautiful, component of our universe.

She stated, “Everything is comprised of particles.” “We wouldn’t be here if it weren’t for them. They provide us a basic and beautiful prescription—a collection of building blocks and rules—from which we may deduce everything in our environment.”

Karagiorgi is part of a long-running search for new forms of neutrinos at Fermilab, a national particle accelerator facility outside Chicago, with Columbia professor Michael Shaevitz. Neutrinos are elementary particles produced either naturally, in stars like our Sun, by nuclear processes, or artificially, in nuclear reactors and particle accelerators. Neutrinos are comparable to electrons in that they have very little mass and no electrical charge.

Neutrinos may flip, or oscillate, between three known weakly interacting states—muon-neutrino, tau-neutrino, and electron-neutrino—as physicists have discovered during the last two decades. Previous neutrino studies at Fermilab and Los Alamos National Lab have shown hints of a fourth neutrino type, and Karagiorgi and Shaevitz are now working to confirm or disprove its existence.

One thing is for sure: if a fourth neutrino-type existed, it wouldn’t interact with matter in the same manner as its three known cousins, said Karagiorgi. Confirmation of a so-called sterile neutrino would throw into question the 60-year-old “Standard Model” of particle physics. The MicroBooNE experiment at Fermilab is intended to provide further insight on the enigma. Meanwhile, Fermilab is preparing to begin the Short-Basline Neutrino Program, a similar, more sensitive hunt for sterile neutrinos. Karagiorgi and Shaevitz are also taking part in the trial.

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Q. What exactly is the Standard Model of particle physics, and how do neutrinos fit into it?

Karagiorgi: It’s an inventory of all known basic particles that make up matter, similar to the periodic table in chemistry. Particles are classified according to their characteristics, such as inherent spin, mass, and electric charge, which helps us comprehend their underlying dynamics and interactions. This theoretical paradigm has proven very predictive since the 1960s. Hundreds of trials and measurements have proven it to be reliable.

It cannot, however, account for our observations of dark matter in space or neutrinos morphing between states. Neutrinos should not be able to oscillate if they have no mass, as the Standard Model predicts. Neutrino oscillation is currently accommodated by an extension to the Standard Model, but sterile neutrinos would be a new problem.

Q. Neutrinos can’t be seen directly. So, what’s the best way to isolate and analyze them?

Karagiorgi: Neutrinos pass through our detectors without leaving a trace, although they do hit with an atomic nucleus every now and again. The subsequent explosion generally results in a spray of charged particles such as electrons, muons, and/or protons. Along the journey of each charged particle, a trail of scintillation light and ionization electrons created by excited atoms is left behind. We can typically deduce the presence of a neutrino, its identity, and how quickly and in what direction it’s going by detecting those traces.

Q. What about this field attracts you?

Karagiorgi: It’s the pinnacle of problem-solving creativity. You begin by carefully forming a question, determining how to answer it, locating the solution (which might take years), and demonstrating your correctness. There is no one-size-fits-all approach to determining the correct answer, but you must ensure that it is correct. When I was an undergraduate, I made a cosmic ray muon telescope out of pieces discovered in a storage closet, I knew this was the field for me. I spent the summer with a buddy developing cosmic ray muon questions and enhancing our telescope to address them. We constructed a revolving set of plywood pockets to measure muon flux as a function of azimuthal angle, and we carried our contraption up and down the fire escape of Florida Tech’s tallest building to count how many muons were absorbed as a function of the number of ceilings above us.

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Q. Which particle is your favorite and why?

Karagiorgi: There’s a reason I’ve spent the most of my professional life investigating neutrinos. They’re enigmatic and difficult to research, and we still have a lot of unanswered questions regarding their properties. Neutrinos may contain the answer to fundamental issues such as why matter, rather than antimatter, dominates our world.

Q. What is MicroBooNE’s goal?

Karagiorgi: MicroBooNE has two objectives: to create a revolutionary detector technology and to investigate a long-standing anomalous signal discovered by its predecessor experiment, MiniBooNE. If this aberrant signal can be confirmed, we may find other neutrino states or characteristics that point to the presence of a new class of particles or forces in nature.

Q. What is the most difficult aspect of this research?

Karagiorgi: Patience, Neutrinos scarcely interact with the rest of the universe. Before interacting with anything, a neutrino has a high probability of passing across 200 Earths. To investigate them, we’ll need highly powerful neutrino beams and huge detectors that can collect enough data with enough interactions to draw statistically significant conclusions. From the conception of a neutrino experiment to its successful completion, it can take more than a decade.

Q. What drew you return to Columbia after completing your postdoctoral work and a portion of your PhD here?

Karagiorgi: The sense of community, the flexibility to be creative in my study path, and the environment: New York City is a vibrant city. It is always evolving and renewing itself. It exists to teach us new things about our world, just as science does.

Q. Do you have any advise for women who want to pursue a career in physics?

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Karagiorgi: Go ahead and do it.

Q. You work at Nevis Laboratories, which is conveniently located near Morningside Heights. What’s it like there, and what does a normal day entail?

Karagiorgi: Entering the lab transports you back to the 1950s and 1960s, when particle physics was at its peak. A CERN phone book from the 1990s may be found in a desk drawer, while video from 1970s bubble chamber experiments can be found in a cupboard. Walking along the same corridors as renowned physicists like C.S. Wu, Leon Lederman, Melvin Schwartz, Jack Steinberger, James Rainwater, and others can’t help but inspire you. Back-to-back Zoom conversations, code debugging, emails to keep up with, messing with electronics boards, and, on the more enjoyable days, data to play with and try to interpret are all part of a normal day.

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