John Lajoie is a builder - not of buildings or everyday machines, but of detectors with a special purpose. A physicist at the Department of Energy's (DOE's) Oak Ridge National Laboratory (ORNL), Lajoie has shaped his career around finding hard-to-solve problems and coming up with new ways to tackle them.
Detectors are how physicists make the invisible visible. They capture signals produced in high-energy collisions, revealing essential features and behaviors that help us study the tiniest of quantum particles. Without detectors, even the most powerful accelerator cannot answer fundamental questions about matter. Detectors are the essential tools that turn collisions into insight and possibility into discovery.
"When there's cutting edge technology that's almost ready, and it might answer a Big Science problem, that's when ORNL physicists step in to do something really special: build something new that can find a solution," Lajoie said.
Designing detectors and inventing new approaches help create the tools scientists need to see deeper into matter itself. At ORNL, where he leads the Relativistic Nuclear Physics Group in the Physics Division, Lajoie works at the frontier where discovery and engineering intersect. His research seeks to understand matter at its most fundamental level: how protons and neutrons are built from quarks and gluons and how their interactions give rise to the visible universe. A Fellow of the American Physical Society, Lajoie is known for developing advanced data collection systems that help enable major discoveries in the field.
He is also spokesperson for the ePIC Collaboration , which is designing and building the first detector for the future Electron-Ion Collider (EIC) at DOE's Brookhaven National Laboratory. Scientists anticipate that the collider will transform how they study the strong force that binds the atomic nucleus. The U.S. long-range nuclear physics research plan named the construction of the EIC a top priority, and the associated ePIC collaboration consists of hundreds of scientists and engineers representing 183 institutions from 26 countries.
The ripple effect: One tool for many uses
For Lajoie, the value of building new detectors lies not only in enabling individual experiments but also in how those detectors can unexpectedly advance other pursuits, often not by design.
"New tech doesn't know what questions it was designed to answer," Lajoie said. "It only knows what it was designed to do." Because of this, the innovation from a new build does not stop with the intended data collection, analysis and discovery. Its utility bleeds into other questions and even other fields of science. He recalled an example from early in his career when he spent months on a biophysics side project that spawned from a hallway conversation about a hard problem and a possible solution from a detector he already worked on.
At ORNL, this ripple effect is common. Detectors built for fundamental science inquiry have enabled, for example, national security technologies for radiological monitoring. One of the newest detectors at ORNL, Timepix4, was originally designed at CERN, also known as the European Laboratory for Particle Physics, for use in collider science. However, today it is being integrated into microscopes at the Center for Nanophase Materials Sciences, a DOE Office of Science user facility at ORNL, to help with materials characterization.
Lajoie says this serendipity - when the right details come together for unexpected success - can drive science forward.
Moreover, the challenges that come up during development of detectors forge a strong workforce. "When people come through for degrees and internships, many of them don't stay in the field or even in science," said Lajoie. "We teach them how to tackle hard problems. Then they go out and apply this skillset to the benefit of society and our country."
Flavors of detection and the AI advantage
When particles collide at high energy, they scatter quantum fragments in every direction. The basic detector types for studying collisions come in broad categories. Trackers measure the trajectory of charged particles as they fly out. Calorimeters measure the particles' energy, which gives clues to their identity and behavior. Particle identification detectors determine the type of particle by measuring its characteristics.
ORNL works with all three detector types, combining them in innovative ways to get maximum knowledge, leveraging expertise across the Physics Division. Researchers also work to pack more capability into smaller spaces, increasing detector analyzing power while managing complexity and cost. This balancing act of physics, engineering and practicality is exactly the kind of challenge Lajoie finds compelling.
The EIC has three different trackers, seven different calorimeters and four different particle ID detectors, all optimized and tuned to help us answer some of the biggest questions in nuclear physics: how quarks and gluons shape the properties of protons and neutrons, how nuclear matter emerges from their interactions, and what happens inside dense gluon-rich environments.
Nestled inside the EIC, ePIC is being designed around a streaming data system, unlike traditional detectors that rely on triggers to activate data collection. Instead of waiting for a predefined signal to decide what to keep, ePIC will continuously collect information, filtering from the torrent of data in real time. That makes artificial intelligence and machine learning essential - not only later in analysis but also at the front end where "AI at the edge" can help decide which signals are most important.
Disruptive potential from disruptive tech
For Lajoie, the EIC represents a transformative opportunity to master a fundamental force of nature-the force responsible for binding the matter that makes up the world around us. He compares that possibility to the long arc that followed James Clerk Maxwell's unification of electricity and magnetism in the late 1800s: a new basic understanding eventually helped unlock electricity, computers, superconductors and other major breakthroughs of the last century. The full impact of the EIC, he believes, may similarly take decades to unfold.
"EIC could completely transform our society with its potential to unify nuclear physics," Lajoie said. "Investing in this science thus makes the past only the prologue to the ripple effects to come."
Lajoie has seen this kind of long-view science before. Prior to joining ORNL in 2023 from Iowa State University, he spent more than 26 years working at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. There, he contributed to the PHENIX detector and later managed construction of hadronic calorimeters for its upgrade, sPHENIX, helping build systems that advanced the study of quark-gluon plasma, a state of matter that existed in the early universe. In the years ahead, as the EIC comes to life, it will mark the second major "build" he has helped bring from idea to reality.
Lajoie looks back on RHIC and thinks of thumbnail plots immortalizing what the research team thought they would see - and how wrong they were. But according to Lajoie, that is the beauty of detector science: seeing what really happens and letting it point you in new directions. That is the genesis of true scientific progress.
"Seeing a problem that might be solved by something new, having an idea, working out all the problems - that's hard," Lajoie said. "But the inspiration is what keeps you going through the dark, ultimately toward success and the next great leap forward in discovery."
The DOE Office of Science (Nuclear Physics) funds Lajoie's work.
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science . - Emily Tomlin
