Newswise — When astronomers point their telescopes to the heavens, they tend to look toward the light. They may search for the pin pricks of shining stars, the billowing of cold gas clouds, the faint heat of a star nursery or the rhythmic blaze of a pulsar. These faint signals of light beckon in a universe that is mostly dark.

In fact, the visible bits only account for 5% of the universe, while the dark side makes up the other 95%.

Now, an ongoing experiment at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility is aiming a spotlight into this so-called hidden dark sector. It aims to uncover hints of particles that interact with the dark by catching their light. 

“You underestimate the power of the dark side.”

Most of our universe is currently thought to be composed of dark matter and dark energy. While the dark sector isn’t visible to light microscopes, its existence is inferred by how it affects the stars and galaxies we study.

Currently, it’s thought that fully 27% of our universe is dark matter. This is matter that, as far as we know, isn’t made of the ordinary particles found in the Standard Model, the theoretical framework that nuclear physicists use to describe the subatomic particles that build our visible universe, such as protons and neutrons.

According to Liping Gan, the Jefferson Lab Eta Factory (JEF) experiment aims to capture hints of particles that connect the light and dark sides. Gan is a professor at the University of North Carolina, Wilmington, and a spokesperson for the JEF experiment. She says one of the goals of the experiment is to produce and capture hints of dark matter particles.

“One of the first ones is to search for dark gauge bosons. And that will give us some clue about the properties of the dark sector,” she said. 

If they do exist, dark gauge bosons would be rare examples of particles that can interact with the ordinary particles described in the Standard Model, as well as the mysterious dark matter particles.

To search for these bosons, ironically, the physicists will be looking for faint signals of light. 

“There’s no such thing as ‘the unknown’, only things temporarily hidden, temporarily not understood.”

The experiment will be carried out with the Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility accessed by more than 1,700 nuclear physicists worldwide to study the nature of matter.

The original GlueX Forward Calorimeter, as installed in Jefferson Lab’s Experimental Hall D. (Aileen Devlin / Jefferson Lab)

In the experiment, the CEBAF accelerator will generate a beam of energetic electrons and direct them into a diamond crystal. There, the electrons stimulate the diamond to release highly energetic light particles, or photons. These tiny but mighty particles of light then barrel into a target made of hydrogen. 

Hydrogen atoms are the simplest element to explore: a single proton makes up the atom’s nucleus, while the motion of a single electron forms the atom’s outer perimeter. The interactions of those photons with hydrogen’s protons spawn additional particles that will then enter detectors, where many of them can be measured.

JEF experimenters are most interested in producing the eta (η) meson. Like the proton, the η meson is made of a combination of smaller particles called quarks and gluons. But unlike the proton, the η isn’t stable. Instead, it will fall apart, or decay, into other particles in about half of an attosecond (5×10-19 seconds).

“Then our goal becomes looking for a decay. There are very many different decay channels we are going to measure, because a different decay channel tells us a different part of the physics,” explained Gan. 

According to Simon Taylor, a Jefferson Lab staff scientist and spokesperson for the JEF experiment, the experimenters are looking for rare decays.

“And a particular rare decay that we’ve been focusing on is a four-photon final state,” he said. 

“Only 18 out of 10,000 will decay to this. Most of the etas will decay into two gammas. So that’s why we call it rare reaction or rare decays, it happens only for a small fraction of the experiment,” Gan said.

The most interesting decay channels in this experiment are when η may produce a dark gauge boson. Dark gauge bosons are thought to be both exceedingly rare and short-lived. They, too, will decay away into other particles in the tiniest of fractions of a second. 

“These other particles include photons of light. We don’t know if these dark gauge bosons exist or will be produced in these decays, but this decay mode will allow us to search for them,” said Taylor.

The researchers will sift through the data they collect to see if they can discover unexpected peaks in their data, where there are more four-photon groupings than they expect. This will trigger intensive analysis to determine if each of these “bumps” is a tell-tale sign of the η meson decay.

Justin Stevens is the Wilson & Martha Claiborne Stephens associate professor of physics at William & Mary and spokesperson for phase two of the Gluonic Excitations Experiment (GlueX-II). The JEF and GlueX-II experiments are running concurrently using the same apparatus. He explained that one of the biggest challenges the two experiments face is finding those tell-tale bumps of interest inside the ginormous amount of data that will be generated. 

For context, 50 million light photons slam into the hydrogen target every second during the run. Each one has the potential to generate a few to dozens of particles that will stream into the detectors, all at once.

“We are getting something like 5 to 10 times higher luminosity in total compared to the original phase of GlueX,” Stevens said.

Taylor said that there are also other issues that will make finding a rare bump a challenge.

“It can get confused with other reactions,” Taylor explained. “Photons are detected by reconstructing the electromagnetic showers they produce in a calorimeter.  Some tiny fraction of the time, one or more of the photons disappear down the beam line or in a hole in the detector. Or two very close showers of particles may merge in the calorimeter together.”

To ensure the researchers would be able to capture these rare reactions, they needed to optimize one of their main detectors. An upgrade would ensure the detector can measure the photons it catches to such accuracy, the physicists could be sure they are capturing signals from a potential dark gauge boson and not from the remnants of incoming photons or from other reactions.

“You can’t stop the signal, Mal. Everything goes somewhere.”

For this experiment, the JEF collaboration upgraded the existing GlueX Forward Calorimeter (FCAL) to improve its resolution and hardiness. 

A view of the FCAL system during the upgrade. (Aileen Devlin / Jefferson Lab)

FCAL originally consisted of 2,800 lead glass scintillating modules connected to photomultiplier tubes. Each module has a 4 cm square face to collect the incoming particles produced in the experiment. These modules were designed to capture incoming particles and convert them into showers of light as the particles travel down their 45 cm length. This light can then be measured by the photomultiplier tubes and converted into signals that are recorded as data.

The upgrade removed 440 of the lead glass modules at the center of the detector and replaced them with 1,596 smaller lead tungstate (PbWO4) crystals. The new crystals feature only a 2 cm square face and extend 20 cm in length. They form a central square inside the new Eta Calorimeter (ECAL) that is surrounded by 2,360 of the original lead glass modules. 

A lead glass scintillating module (left) with a lead tungstate (PbWO4) crystal (right) are shown here inside Experimental Hall D. (Aileen Devlin / Jefferson Lab)

Zisis Papandreou is a JEF experiment spokesperson and a professor and head of the physics department at the University of Regina in Saskatchewan Canada. He compared the upgraded detector to a digital camera.

“Essentially, we’re reconstructing something akin to an image. If you have a larger number of smaller pixels, you can improve the resolution in the image,” he said.

The new lead tungstate crystals are also radiation-hard, which means they can better withstand the onslaught of particles generated in experiments than the original lead glass modules. Centering the new crystal modules closest to the target allows them to take the brunt of the onslaught, protecting the lead glass modules from the highest-energy particles and resulting in a longer overall detector lifetime.

Funded by the lab, the Department of Energy and the National Science Foundation, the upgrade of FCAL to ECAL cost about $5 million. Several work groups at the lab also contributed to the upgrade, including the Radiation Detector & Imaging group, the Detector Support group, the Radiation Control Department, and the Fast Electronics group. 

After four years of R&D, the full-scale upgrade took about three years to complete, beginning in 2022 and wrapping up in early 2025. According to Alexander Somov, a staff scientist at Jefferson Lab and a spokesperson for the JEF experiment who led the ECAL construction, the detector has already proven its versatility.

“It is the largest lead-tungstate crystal calorimeter in the U.S., and it’s been successfully commissioned and integrated,” said Somov. “It took a few weeks to commission, and all modules are operational. It was working fine for the first run of approximately five months.”

ECAL was successfully commissioned in April 2025, with data acquired for JEF and GlueX-II in late spring and summer 2025. So far, the detector has collected about 75 days of data as counted by the Program Advisory Committee for the two experiments.

“And the first results we’re getting show that the performance of ECAL is in good agreement with our basic expectations. But there’s more analysis to come,” said Somov.

University of North Carolina Wilmington (UNCW) Physics Professor Liping Gan, George Washington University undergraduate student Olivia Nippe-Jeakins, UNCW undergraduate students Shane Whaley and Ben Simpson, Jefferson Lab Hall D Staff Scientist Alexander “Sasha” Somov, and UNCW postdoc Laveen Puthiya Veetil pose for a photo inside Jefferson Lab’s Experimental Hall D. (Aileen Devlin | Jefferson Lab)

The upgraded detector has also served as a training ground for undergraduate physics majors interested in pursuing a career in nuclear physics. In all, 28 undergraduate students from the JEF experiment’s 11 collaborating universities participated in ECAL fabrication, installation and commissioning. The project also involved three graduate students, six postdoctoral nuclear physicists and one visiting scientist. 

“These are valuable experiences to undergraduates and then later on, they go to graduate school and some of them already were talking about going to a university that has GlueX collaborators. So, the hardware work is an absolutely amazing experience for students,” said Papandreou.

“You built a time machine? Out of a DeLorean?”

The second and final run for these experiments is currently scheduled to resume this summer. In it, the collaborators also aim to collect data that may reveal new clues to our understanding of other phenomena, such as why matter beat out antimatter in the first second of the early universe.

George Washington University undergraduate Quinn Stefan, left, and graduate student Phoebe Sharp, right, monitor experiment progress inside the Experimental Hall D counting house at Jefferson Lab in Newport News, Va., on June 17, 2025. (Aileen Devlin | Jefferson Lab)

“For example, right now we see large asymmetry between matter and antimatter in the universe, and one of the goals is to search for maybe there’s some new symmetry violation beyond the Standard Model. The third goal is to understand why quantum chromodynamics (QCD) prevents quarks from being isolated via tests of fundamental symmetries at low energy,” Somov said.

Alongside these searches, the GlueX-II experiment collaborators are looking to expand their data on particles that include charm quarks, such as the J/ψ (J/psi) meson. While the more familiar protons and neutrons are made of up and down quarks, other subatomic particles include other quark flavors, such as the charm quark. These data offer a rare opportunity to look at in depth at how such particles are made.

“So that means we can do things like study J/ψ production with a lot higher statistics and look for the charm production mechanism with a lot more precision than we had before,” said Stevens.

As the collaborators prepare for their final data run, they have also already begun preparing for the next opportunity. The proposed GlueX-III run aims to increase the intensity of the photon beam. For GlueX, it achieved about 10 million photons per second, and GlueX-II has an intensity of about 50 million photons per second. The goal for GlueX-III is to slowly crank that up to around one hundred million photons per second.

“Once you understand the detector, you just drive up the rate,” Stevens said. “So, we’re going to keep pushing up the intensity as high as we can, because we just get more data. You can look at more rare decays, more rare processes.”

In the meantime, the collaborators have their hands full preparing for their next run this summer and are looking forward to what their data reveal about the light and dark sides of our universe.

Further Reading
GWU Research Magazine: The Particle Whisperers
UNCW: Undergraduates Step Into Physics Research
Technical Paper: Eta Decays with Emphasis on Rare Neutral Modes: The JLab Eta Factory (JEF) Experiment
Technical Paper: Update to the JEF proposal (PR12-14-004)
Technical Paper: Light monitoring system for the lead tungstate calorimeter in Hall D at Jefferson Lab