Muon Detection and Direction Reconstruction with the Upgraded 1-ton Water Cherenkov Prototype Detector at CJPL-I
The upgraded 1-ton water Cherenkov prototype detector at China Jinping Underground Laboratory (CJPL-I) demonstrated a 59% increase in muon detection efficiency and a 70% reduction in direction reconstruction uncertainty, successfully measuring the underground muon flux and identifying a rare up-going muon event to enable future neutrino-induced studies.
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Deep beneath the mountains of Sichuan Province in China lies the world's deepest underground laboratory, a place where the rock above is so thick that it blocks out almost all of the natural radiation and particles raining down from space. This immense depth creates a quiet, dark environment where scientists can listen for the faintest whispers of the universe, such as elusive particles called neutrinos that rarely interact with matter. To hear these whispers, researchers must first understand the background noise, which comes mostly from cosmic-ray muons. These muons are high-speed particles created when cosmic rays hit the atmosphere; they can mimic the signals scientists are looking for or create other radioactive particles that clutter the data. By measuring exactly how many muons reach the bottom of the mine and where they are coming from, scientists can design better shields and filters to protect their sensitive experiments, ensuring that what they detect is truly a new discovery and not just a cosmic impostor.
At the China Jinping Underground Laboratory, a team of researchers recently completed a significant upgrade to a small, one-ton prototype detector designed to study these muons. Originally built to test technologies for a much larger future experiment, this device was recently overhauled to improve its ability to catch and track these fast-moving particles. The researchers replaced the old light sensors inside the tank with sixty new, highly sensitive tubes and redesigned the internal structure to hold them. They then filled the detector with water instead of the liquid scintillator it had used before, turning the tank into a water Cherenkov detector. In this setup, when a muon zips through the water faster than light travels through that same water, it creates a faint cone of blue light, similar to the sonic boom created by a jet breaking the sound barrier. The new sensors are designed to catch this light, allowing the team to count the muons and determine their direction with much greater precision than before.
Over the course of several months, the detector operated in two distinct phases, first without a special light-blocking shield and later with one installed to block stray light from the water outside the main viewing area. By analyzing the data collected during these periods, the team found that the upgraded detector was remarkably efficient. In the second phase, with the new shielding in place, the detector successfully identified muons with an efficiency nearly sixty percent higher than the previous version of the experiment. The team calculated that the number of muons passing through the detector is approximately 3.55 per square centimeter every second, a result that matches perfectly with earlier measurements taken at the same location using different equipment. This agreement confirms that the new water-based technology is reliable and provides a valuable cross-check for the scientific community.
Perhaps the most exciting development was the detector's ability to pinpoint the direction of these particles. Because the new sensors are more numerous and cover more of the tank's surface, and because the researchers developed a smarter way to compare the timing and brightness of the light hits, they could reconstruct the path of a muon with an uncertainty of only about six degrees. This is a dramatic improvement, reducing the margin of error to just thirty percent of what it was in the past. This level of precision is crucial because it allows scientists to distinguish between muons coming from above, which are common, and those coming from below, which are extremely rare.
During the data collection, the team spotted a single, very unusual event: a muon that appeared to be traveling upward, emerging from the rock beneath the detector. While cosmic-ray muons are always pushed down by gravity and the atmosphere, a muon moving upward suggests it was created by a neutrino interacting with the rock or water below the detector and shooting a muon back up through the tank. The researchers carefully checked this event against all possible errors and background noise, finding that the chance of it being a mistake or a downward-moving particle misidentified was less than one in ten thousand. This single event marks the first time such a neutrino-induced particle has been clearly identified at this laboratory, opening a new window for studying these ghostly particles. The success of this upgraded water detector proves that a cost-effective, water-based system can effectively measure cosmic rays and detect neutrino events, paving the way for the larger, more powerful detectors planned for the future.
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