The Accelerator Neutrino Neutron Interaction Experiment (ANNIE), housed at Fermilab in Batavia, Illinois, is closer to understanding one of the universe’s most abundant yet mysterious particles: neutrinos.
These particles are tiny, everywhere, and are constantly passing through everything, yet scientists know very little about how they behave when they interact with matter.
The primary goal of the ANNIE project is to improve understanding of these interactions.
“[Neutrinos] have a very tiny cross-section, meaning they usually fly directly through matter as if it wasn’t there,” said Matthew Wetstein, associate professor in Iowa State University’s Department of Physics and Astronomy and one of ANNIE’s founding scientists. “This makes them very hard to detect, as they rarely interact with anything, and you can only see them when they do interact.”
Inside Fermilab – how ANNIE works
(contributed photo)
Fermilab is a U.S. National Laboratory aimed at answering big questions about what the universe is made of. Using advanced particle accelerators, researchers study the core concepts in physics, including neutrinos, dark matter, and dark energy. The lab is an international collaboration site and a hub for visiting scientists worldwide.
Fermilab houses several particle accelerators – machines that use electric fields to speed up charged particles (electrons, protons, and ions) and collide them with a target, or with other charged particles. Presently, Fermilab uses these collisions to create beams of neutrinos. The ANNIE experiment is situated on one of those beam lines.
For ANNIE, a large tank of water sits on this invisible beam. The tank contains Large Area Picosecond Photodetectors (LAPPDs), an advance new light detection technology, and gadolinium – an earth metal.
“I like to call what we do `neutrino nuclear billiards` – we study what happens when the neutrino collides with the nucleus,” Wetstein explained. “When it hits, it’ll produce a shower of particles that come out and we can do the ballistics, reconstructing what its energy and direction were from the particles that fly out.”
To detect these interactions, ANNIE uses LAPPDs, which are advanced sensors capable of measuring the arrival of individual photons (particles of light) with unprecedented precision in both time and space. Scientists can then track the path of the particles that emitted the light.
“This photodetector can measure the exact time and location that light hits its,” Wetstein said. “LAPPDs can measure photon arrival times with 50 picosecond resolution – that’s a trillionth of a second, about the time it takes light to travel just one and a half centimeters.”
Wetstein said ANNIE utilizes gadolinium to capture free neutrons and emit light that is detected by the photosensors in the water.
A vessel can be deployed within the tank, containing another special liquid medium, called Water-based Liquid Scintillator (WbLS), that produces additional light when particles pass through it. This combination helps the environment stay clear of background noise, making it easier to spot the tiny signals neutrinos leave behind.
Worldwide partnerships
ANNIE is a collaborative effort, with contributions from researchers across 15 universities and six countries. One of the project’s key objectives is to train the next generation of scientists and engineers.
“Beyond the research, how we develop people, how we educate and train, the technology we push, all of these things are beneficial,” Wetstein said.
For early-career scientists, the ANNIE collaboration has led to faculty positions, leadership roles in major research initiatives, and further contributions to the scientific community.
While ANNIE is a relatively small-scale experiment by global standards, Wetstein said its impact has been significant in terms of career development and technological innovation.
“We have a huge track record of top-notch scientists who can trace their lineage back to what we do,” Wetstein said.
Unity in the science community
The ANNIE project’s commitment to transparency and quality ensures the integrity of its results. Individuals check the stats of the machinery online 24 hours a day, seven days a week.
“There are a lot of people involved and a lot of eyes,” Wetstein said. “There’s a shared sense of mutual responsibility, and we cross-check ourselves.”
Wetstein emphasized that collaboration and review are essential in any research, where precision is critical. The shared responsibility within the community helps catch errors, refine methods, and strengthen the conclusions drawn from the research.
“I think it’s important for people to understand that scientists aren’t individuals, just like sitting alone, we are a community, and we work very hard to help each other’s work. We love the give and take of that community,” Wetstein said.
(contributed photo)