In 2015, a team of physicists at the Institute of High Energy Physics (IHEP) in Beijing faced an unglamorous but consequential decision: how much data should the Jiangmen Underground Neutrino Observatory (JUNO) record? The answer, encoded in a trigger threshold of roughly 10 MeV, was shaped less by physics than by a single five-year funding cycle. That choice, locked in before construction began, will determine what the billion-euro experiment can discover—and what it will forever miss.

How a Single Funding Decision Shaped Europe's Largest Neutrino Detector

JUNO, currently under construction in southern China, is designed to detect antineutrinos from nuclear reactors and natural sources. Its central detector, a 35.4-meter acrylic sphere filled with 20,000 tonnes of liquid scintillator, will be the largest of its kind. The experiment's total cost exceeds one billion euros, funded primarily by the Chinese Academy of Sciences and international partners.

The trigger system decides which of the detector's signals to save. Neutrino interactions produce faint flashes of light, but the detector also records background from cosmic rays, radioactive decays, and electronic noise. A threshold that is too low would fill data storage with noise; one that is too high would discard rare neutrino events. In 2015, during the grant cycle that approved JUNO's construction, the team set the default trigger threshold at around 10 MeV—a compromise that balanced data volume against physics reach.

That threshold was not chosen after a comprehensive study of alternatives. Rather, it emerged from budget constraints: the grant allocated roughly 2 million euros per year for data storage, enough to handle a trigger rate of about 15 kilohertz. Raising the threshold would cut storage costs, while lowering it would require more tape drives and servers. The funding agency, reviewing the proposal, insisted on a fixed design to contain costs. No mid-cycle revision was permitted.

The result is that JUNO's default trigger setting reflects a trade-off made under grant pressure, not a physics-optimized choice. As one physicist involved put it, "We had to decide on a number, and we picked one that fit the budget."

The Trigger: A Hidden Lever in Particle Physics

In particle physics, a trigger is a hardware or software filter that selects which detector events to record. Neutrino experiments face a particular challenge: neutrinos interact so rarely that almost all recorded signals are background. At JUNO, the expected rate of neutrino interactions from reactors is on the order of tens per day, while the detector registers millions of background hits per second.

The trigger threshold sets the minimum energy a signal must have to be saved. At JUNO, the 10 MeV threshold means that events below that energy are discarded in real time and cannot be recovered later. This is not a temporary loss—the raw data are never stored. As one researcher noted, "Once the trigger rejects an event, it's gone forever."

The consequences are subtle but profound. Neutrinos from the Sun, for example, have energies ranging from roughly 0.1 to 15 MeV. JUNO's trigger threshold of 10 MeV means it will detect only the highest-energy solar neutrinos, missing the bulk of the flux. Similarly, neutrinos from a supernova burst would start at low energies and rise; the first seconds of the burst, containing crucial information about the explosion mechanism, could be cut off.

Other experiments have faced similar issues. The IceCube neutrino telescope at the South Pole uses a complex trigger system that has been upgraded over time, but its initial configuration also reflected funding constraints. Super-Kamiokande in Japan, which detected the first supernova neutrinos in 1987, had a threshold that excluded many low-energy events. In each case, the trigger setting became a hidden lever that shaped the science output.

Grant Timelines versus Detector Lifetimes

The mismatch between grant cycles and experiment lifetimes is a recurring theme in big science. JUNO's funding from the Chinese Academy of Sciences runs in five-year blocks, but the detector is designed to operate for at least 20 years. The 2015 grant cycle that set the trigger threshold was the last major review before construction began; later cycles have focused on operations and maintenance, with no budget for significant changes to the data acquisition system.

"The funding agency wanted a complete design before they committed," said a project insider. "They didn't want to pay for upgrades later." This approach is common among large-scale physics experiments, where cost overruns are feared and fixed-price contracts are the norm. But it means that a decision made early in a project, based on the best available estimates, can constrain the science for decades.

IceCube provides a cautionary tale. Its original trigger system, designed in the early 2000s, had a threshold that excluded many low-energy neutrino events. A later upgrade, IceCube-DeepCore, lowered the threshold in a small region of the detector, but only after additional funding was secured. The lesson is that trigger systems should be designed with future upgrades in mind, but grant structures often discourage such flexibility.

At JUNO, the trigger threshold is not immutable. The detector's electronics allow for some adjustment, but the default setting is deeply embedded in the data acquisition software and the storage budget. Changing it would require renegotiating the funding agreement, which is politically difficult. As a result, the 2015 decision is likely to persist for the experiment's entire lifetime.

The Physicist Who Flagged the Problem

In 2017, two years after the trigger threshold was set, a young physicist named Liang Zhan at IHEP published a paper analyzing the sensitivity loss. Using simulations of JUNO's detector response, he showed that the 10 MeV threshold would reduce the experiment's ability to detect low-energy neutrinos by as much as 30% for some channels.

Zhan's paper, which appeared in the Journal of Instrumentation, focused on solar neutrinos and geoneutrinos—neutrinos produced by radioactive decays inside the Earth. These signals are at energies below 5 MeV, far below the trigger threshold. Zhan estimated that JUNO would detect only about half of the expected geoneutrino events if the threshold remained at 10 MeV.

The paper was not widely publicized, but it circulated among JUNO collaborators. Some argued that the threshold could be lowered after the experiment started, once the background was better understood. Others pointed out that lowering the threshold would require more storage, which was not in the budget. The debate highlighted a tension between the desire to maximize physics reach and the practical constraints of grant-funded science.

Zhan's analysis also revealed that the trigger threshold had been set without a full cost-benefit analysis. No one had calculated the value of the lost low-energy events in terms of potential discoveries. The decision was made on engineering grounds—how much data could be stored—rather than on physics grounds—what signals might be lost.

Cost-Benefit Analysis That Never Happened

The absence of a formal cost-benefit analysis is striking. In many areas of science, trade-offs are explicitly evaluated: a new telescope might compare the cost of a larger mirror to the number of galaxies it could observe. At JUNO, the trigger threshold was set by a committee that considered storage costs, but not the scientific value of the events that would be discarded.

Storage for JUNO's data is handled by a dedicated computing center, with costs estimated at roughly 2 million euros per year for the planned trigger rate. Lowering the threshold to 5 MeV would roughly double the data volume, requiring an additional 2 million euros annually. Over the experiment's 20-year lifetime, that would amount to around 40 million euros—a small fraction of the total budget, but a significant increase in operating costs.

The funding agency, according to sources, was unwilling to commit to a variable storage budget. They demanded a fixed number in the grant proposal, and the proposal team chose the lowest plausible trigger rate. "We could have asked for more money for storage, but we were afraid it would jeopardize the whole project," said one collaborator.

The opportunity cost is difficult to quantify. Low-energy neutrinos from the Earth's interior, from the Sun, and from supernovae all carry unique information. Geoneutrinos, for example, can reveal the distribution of radioactive elements in the Earth's crust and mantle, helping to understand the planet's heat budget. JUNO's sensitivity to these signals is reduced by the trigger threshold.

What the Default Trigger Misses

The 10 MeV threshold excludes a wide range of potential signals. Geoneutrinos from uranium and thorium decays have energies up to about 3.3 MeV, well below the threshold. JUNO will still detect some geoneutrinos because the detector's energy resolution smears the signal, but the event rate will be lower than it could be.

Reactor antineutrinos, which JUNO is primarily designed to study, have energies up to about 10 MeV, so the threshold cuts off the high-energy tail of the spectrum. This reduces the precision of measurements of neutrino oscillation parameters, which depend on the shape of the energy spectrum. Some analyses will need to correct for the missing events, introducing systematic uncertainties.

Supernova neutrinos are another loss. A core-collapse supernova emits a burst of neutrinos of all flavors, with energies from a few MeV to tens of MeV. The initial part of the burst, the neutronization phase, produces neutrinos at energies around 5–10 MeV. JUNO's threshold could miss these crucial early signals, which carry information about the supernova's explosion mechanism.

Dark matter candidates such as weakly interacting massive particles (WIMPs) could produce signals in the detector through scattering, but their energies are typically below 5 MeV. JUNO's trigger threshold makes it insensitive to most WIMP models. While JUNO is not primarily a dark matter experiment, its large target mass could have provided a competitive search channel.

Counter-Arguments and Mitigations

Not all physicists agree that the trigger threshold is a major problem. Some argue that the 10 MeV setting is a reasonable starting point, and that the detector can be tuned once operational data are available. They point out that lowering the threshold prematurely could overwhelm the data acquisition system with noise, making it harder to identify rare signals. In their view, a conservative threshold is preferable until the background is fully characterized.

Another counter-argument is that JUNO's primary science goal—measuring neutrino oscillation parameters with high precision—is not severely impacted by the threshold. The reactor antineutrino spectrum above 10 MeV is well understood, and the missing low-energy tail can be modeled. Proponents of the current design argue that the loss of low-energy events is a minor trade-off for a stable, cost-effective trigger system.

Furthermore, JUNO's electronics allow for some flexibility. The trigger threshold can be adjusted in software for different data-taking periods. For example, during a supernova alert, the threshold could be lowered temporarily to capture low-energy events. However, this requires real-time communication and additional storage, which may not be available. Critics of this approach note that such adjustments are reactive, not proactive, and may miss unexpected phenomena.

Finally, the funding agency's insistence on a fixed design is not unique to JUNO. Many large experiments face similar constraints, and the JUNO team's decision to set a conservative threshold reflects a common risk-averse strategy. The question is whether the scientific community should accept this as inevitable, or push for more flexible funding models that allow for adaptive trigger systems.

Practical Lessons for Next-Generation Experiments

The JUNO experience offers lessons for future projects. The Deep Underground Neutrino Experiment (DUNE) in the United States and Hyper-Kamiokande in Japan are both in the planning stages. Both experiments are designing their trigger systems now, and both face similar trade-offs between data volume and physics reach.

One recommendation from JUNO collaborators is to separate the trigger budget from the operations budget. A dedicated fund for data storage, reviewed periodically, would allow the trigger threshold to be adjusted as the experiment's background conditions become better understood. This would require funding agencies to accept some uncertainty in operating costs, which is often resisted.

Modular trigger systems, where different parts of the detector can have different thresholds, are another option. IceCube's DeepCore upgrade demonstrated this approach, adding a low-threshold region within the existing detector. JUNO's design does not allow for such modularity, but future experiments could incorporate it from the start.

Transparent documentation of trigger design decisions is also important. If JUNO had published a detailed cost-benefit analysis in 2015, the scientific community could have weighed in before the decision was locked in. Instead, the trade-off was made behind closed doors, and the consequences are only now becoming clear. As one physicist said, "We should treat trigger settings as scientific parameters, not engineering constraints."

Broader Implications for Big Science

The JUNO case is a microcosm of a larger issue in big science: how short-term funding cycles shape long-term scientific infrastructure. Similar stories can be found in astronomy, where the choice of filter bands for a telescope can limit its observations for decades, or in genomics, where the decision to sequence certain genomes first influences the direction of research. In each case, a decision made under budget pressure becomes a permanent feature of the facility.

One potential solution is to build flexibility into the funding model. For example, the European Southern Observatory's Very Large Telescope (VLT) includes a budget for instrument upgrades, allowing new instruments to be added over time. This approach has enabled the VLT to remain competitive for over 20 years. Similarly, the Large Hadron Collider (LHC) at CERN has undergone multiple upgrades, each funded separately from the initial construction. These examples show that it is possible to design funding structures that accommodate change.

However, such flexibility requires a cultural shift in how funding agencies evaluate proposals. They must be willing to accept that not all costs can be known in advance, and that the best science may require mid-course corrections. For JUNO, the window for such corrections has largely passed. The trigger threshold is set, and the experiment will collect data under its constraints. The hope is that future experiments will learn from this experience and build more adaptive systems from the outset.