Analysis of CONUS+ run 2: Detector performance, background model and CE$\nu$NS measurement

Neutrinos are the most abundant particles in the universe, yet they are also the most elusive. They zip through planets, stars, and human bodies without leaving a trace, interacting with matter so rarely that detecting them usually requires massive tanks of liquid or huge detectors buried deep underground. For decades, physicists have relied on these giant instruments to catch a glimpse of these ghostly particles. However, a specific type of interaction called coherent elastic neutrino-nucleus scattering offers a different path. In this rare event, a neutrino hits an entire atomic nucleus at once rather than a single particle inside it, causing the whole nucleus to recoil. Because the neutrino pushes the entire nucleus, the force is much stronger than usual, but the resulting movement is incredibly tiny. To see this, scientists must build detectors sensitive enough to feel a vibration smaller than the width of a single atom, all while keeping the instrument perfectly quiet so that background noise does not drown out the signal.

A team of researchers at the CONUS+ experiment, located inside a nuclear power plant in Switzerland, has successfully captured this faint signal using a new generation of detectors. By upgrading their equipment with larger, more sensitive crystals, they were able to observe the recoil of germanium atoms caused by antineutrinos streaming from the reactor core. The team analyzed data collected over a period that amounted to 1,296 days of exposure with the reactor running. Their analysis revealed a clear excess of events that matched the predicted behavior of neutrinos, confirming the presence of 1,555 signal events with a statistical certainty that leaves almost no room for chance. This result aligns closely with the standard predictions of physics, demonstrating that these tiny detectors can now measure neutrino properties with a precision that rivals much larger experiments.

The experiment is situated just 20 meters from the core of the Leibstadt nuclear power plant, placing it in an environment flooded with antineutrinos. To catch the faint signal of a nucleus recoiling, the researchers needed to lower the energy threshold of their detectors to a level where even the smallest vibrations could be recorded. In their first phase of operation, they used detectors weighing one kilogram each. For this second run, they replaced three of those with new diodes weighing 2.42 kilograms each. These larger crystals offered a significant advantage: because they have more mass relative to their surface area, they are less affected by background noise coming from the surface of the material. This design change allowed the new detectors to achieve a background level per unit of mass that was roughly half that of the original detectors, while still maintaining the ability to detect energy deposits as small as 140 to 170 electron-volts.

The team faced the challenge of distinguishing the genuine neutrino signal from a variety of background sources, such as cosmic rays hitting the Earth's atmosphere or natural radioactivity in the surrounding rock and equipment. To solve this, they built a multi-layered shield around their detectors using lead, polyethylene, and borated polyethylene to block external radiation. They also surrounded the setup with plastic scintillator plates that act as a watchdog, instantly identifying and rejecting any events caused by cosmic muons passing through the facility. The detectors themselves were kept in a radon-free environment and cooled to extremely low temperatures to minimize electronic noise. Throughout the data collection period, which began in early 2025, the researchers monitored the stability of their equipment, ensuring that temperature fluctuations or power issues did not distort the results. They even paused data collection after a power outage to recalibrate the system, ensuring that only the most reliable data was used for the final analysis.

Once the data was collected, the researchers applied a series of rigorous filters to remove any events that did not look like a true neutrino interaction. They looked at the shape of the electrical signals produced by each event, checking for characteristics that would indicate a single, clean hit on a nucleus rather than a messy interaction involving multiple particles or surface noise. They also compared the data collected while the reactor was running against data collected while the reactor was shut down. Since the reactor is the source of the antineutrinos, any signal that appears only when the reactor is active and disappears when it is off must be related to the neutrinos. This comparison allowed them to isolate the signal from the background with high confidence.

The final analysis yielded a definitive result. The team counted 1,555 events that they identified as coherent elastic neutrino-nucleus scattering, with an uncertainty of 152 events. This number corresponds to a statistical significance of 10.2 sigma, a measure of certainty that indicates the probability of this result being a random fluke is virtually zero. The observed number of events was very close to the 1,701 events predicted by the Standard Model of particle physics, with the ratio between the observed and predicted values being 0.91. This agreement suggests that the current understanding of how neutrinos interact with matter is correct. The researchers also noted that their measurement was limited more by the statistics of the data than by systematic errors, meaning that collecting more data in the future would directly improve the precision of the measurement.

This success marks a transition for the CONUS+ experiment from a proof-of-concept study to a tool for precision measurement. The upgrade to larger detectors not only improved the signal-to-noise ratio but also provided a more robust dataset for testing the fundamental properties of neutrinos. The team plans to continue taking data, aiming to accumulate even more exposure to further reduce statistical uncertainties. By refining their background models and potentially applying additional techniques to filter out noise, they hope to push the precision of these measurements even further. This work opens the door for future studies that could probe for new physics beyond the current Standard Model, using compact, kilogram-scale detectors to explore questions that previously required massive, multi-ton facilities. The ability to measure these interactions with such clarity in a relatively small setup represents a significant step forward in the field of neutrino physics.