Reactor Antineutrino Detection with JUNO-TAO: Commissioning Performance and First Results

Deep beneath the earth, in a quiet corner of a nuclear power plant in China, a new kind of eye has opened to watch the invisible. For decades, physicists have been trying to solve a puzzle about the universe's most abundant ghostly particles: the neutrino. These particles, born in the hearts of stars and nuclear reactors, pass through almost everything without leaving a trace. Yet, they hold the key to understanding why the universe is made of matter rather than antimatter. To catch them, scientists need detectors of immense sensitivity, capable of spotting the faint flash of light that occurs when a neutrino finally interacts with an atom. The Jiangmen Underground Neutrino Observatory, or JUNO, is a massive experiment designed to weigh these particles and determine their order of mass. But to do this with the precision required, JUNO needs a perfect reference point—a way to know exactly what the stream of neutrinos looks like before it is altered by the journey through the earth. This is where a smaller, highly specialized companion experiment, the Taishan Antineutrino Observatory, or TAO, comes in.

The TAO detector was built to stand just forty-three meters away from a reactor core, a distance close enough to catch the raw, unfiltered stream of antineutrinos before they have a chance to change. Its job is to measure the energy spectrum of these particles with extreme precision, acting as a calibration standard for the much larger JUNO detector nearby. The paper describes the successful completion of this detector, its first months of operation, and the initial results that prove it is working exactly as designed. The team has shown that the machine can operate stably in extreme cold, that it can distinguish real particle interactions from background noise with high accuracy, and that it has already begun to record the first clear signals of reactor antineutrinos.

The heart of the TAO detector is a sphere filled with a special liquid that glows when a particle passes through it. This liquid is kept at a temperature of minus fifty degrees Celsius, a necessary condition to quiet the electronic noise of the sensors that watch for the light. Surrounding this sphere are thousands of tiny light sensors, each made of silicon, which are so sensitive they can detect a single particle of light. These sensors are packed tightly together, covering more than eighty percent of the sphere's inner surface, ensuring that almost no flash of light goes unnoticed. To protect this delicate system from the constant rain of cosmic rays hitting the earth from space, the detector is surrounded by a tank of pure water and a layer of plastic scintillator panels, which act as a shield and a veto system to flag any unwanted particles.

Building a machine of this complexity required overcoming significant engineering hurdles, particularly regarding the extreme cold and the sheer number of electronic channels. The researchers had to ensure that the liquid inside remained perfectly clear and that the sensors did not freeze or malfunction. They also had to manage the data from over eight thousand individual channels, filtering out the ones that were noisy or broken. After a period of testing and calibration, the team found that about ninety percent of the channels were working perfectly. They established a stable operating temperature and tuned the voltage of the sensors to get the best possible performance. The result is a detector that produces a very bright signal for every unit of energy deposited, a crucial feature that allows it to measure energy with a precision that is among the best ever achieved for this type of experiment.

With the detector running smoothly, the team turned their attention to the first physics data. They collected information over thirty-one days while the reactor was running at full power and compared it with data from twenty-nine days when the reactor was turned off. This comparison is the gold standard for identifying the signal, as any difference between the two periods must be caused by the reactor itself. The analysis revealed a clear and significant excess of events when the reactor was on. Specifically, the detector saw about two thousand nine hundred events per day when the reactor was active, compared to only about four hundred events per day when it was off. These events matched the expected signature of an inverse beta decay interaction, where an antineutrino hits a proton and produces a positron and a neutron. The timing and energy of these events fit the theoretical predictions, confirming that the detector is successfully capturing the antineutrino stream.

The paper also details how the team reconstructed the path and energy of these particles. By analyzing the pattern of light hitting the sensors, they could determine where the interaction happened inside the sphere with an accuracy of a few centimeters. They also measured the energy of the particles with a resolution of about two to three percent, a level of sharpness that allows them to see fine details in the energy spectrum. This precision is vital because it enables TAO to serve as a nearly perfect reference for JUNO. By knowing the exact shape of the antineutrino spectrum coming from the reactor, JUNO can more accurately measure how these particles oscillate, or change, as they travel to the larger detector.

The success of the TAO detector marks a major milestone in the broader effort to understand neutrinos. The team has demonstrated that the complex system of cryogenics, sensors, and electronics can work together reliably in a real-world environment. They have validated their methods for calibrating the detector and filtering out background noise. While the initial data serves primarily as a proof of concept and a performance check, it confirms that the machine is ready for the long-term physics program. The researchers plan to continue refining the detector, replacing faulty components, and reducing electronic noise to push the performance even higher. As more data is collected, TAO will provide the high-precision measurements needed to help JUNO solve the mystery of the neutrino mass ordering, bringing scientists one step closer to understanding the fundamental nature of the universe.