Can a minimal radiative seesaw explain the LZ 248 keV event?

Deep in the quiet, shielded depths of a mine, a massive tank of liquid xenon waits to catch a ghost. This is the LUX-ZEPLIN experiment, a detector designed to find dark matter, the invisible substance that makes up most of the matter in our universe. For decades, physicists have searched for dark matter by looking for tiny flashes of light created when a dark matter particle bumps into an atom in the tank. The challenge is that these particles are notoriously shy; they rarely interact with normal matter, and when they do, the energy they transfer is often too small to see. Recently, however, the LZ detector recorded a single, unusual event: a heavy atomic nucleus recoiling with an energy of 248 kiloelectronvolts. This is a surprisingly high energy for such a rare collision, and it has sparked a new wave of curiosity. Is this a genuine signal from dark matter, or just a random fluctuation? To answer this, a team of researchers has proposed a specific, intricate story about what dark matter might be and how it behaves, using a framework that also explains why neutrinos, the universe's most elusive particles, have mass.

The story begins with a simple idea: what if dark matter is not a single, static particle, but comes in two slightly different versions? In the model proposed by Hiroshi Okada, Yoshihiro Shigekami, and Jia-Jun Wu, the dark matter candidate is a neutral particle that exists in two states, like a coin that can be heads or tails. These two states are separated by a tiny energy difference, less than the mass of a single electron. This small gap is the key to the entire puzzle. Under normal circumstances, dark matter particles moving at typical speeds would not have enough energy to flip from the lighter state to the heavier one. This means that for most of the dark matter in our galaxy, the detector would see nothing, because the collision simply cannot happen. This mechanism elegantly explains why the detector has not been flooded with signals from the slow-moving bulk of the dark matter halo, which would have been detected by previous experiments.

However, the universe is not perfectly uniform. The dark matter halo surrounding our galaxy has a "tail" of particles moving much faster than the average. The researchers suggest that the 248 keV event was caused by one of these rare, high-speed particles. When such a fast-moving particle strikes a xenon atom, it has just enough kinetic energy to overcome the tiny energy gap and flip into the heavier state. This process, known as inelastic scattering, releases the specific amount of energy observed in the LZ event. Because this interaction requires such high speeds, it is kinematically forbidden for the vast majority of dark matter, allowing the model to avoid the strict limits set by other experiments that look for slower, more common collisions. This specific energy gap, calculated to be around 360 to 380 kiloelectronvolts, acts as a filter, letting only the fastest particles through to create the signal.

The researchers did not stop at explaining the single event; they had to ensure their model could also account for the total amount of dark matter in the universe. In simpler models, heavy dark matter particles would need to interact very strongly with each other to disappear at the right rate after the Big Bang, but such strong interactions would also make them easy to detect in other ways, which they are not. The authors solved this by introducing a partner for their dark matter particle: a heavy, invisible fermion. These partners are nearly identical in mass to the dark matter particle. In the early universe, the dark matter particles and these partners would have swapped places and annihilated each other in a process called co-annihilation. This interaction allowed the dark matter to reach the correct abundance observed today without requiring the dangerous, strong interactions that would have been spotted by other detectors. This mechanism allows the dark matter to be as heavy as approximately 1 TeV, a mass range that was previously difficult to explain without conflicting with other data.

There was, however, a significant problem with this elegant setup. If dark matter particles can bounce off protons in the Sun, they would lose energy, get trapped by the Sun's gravity, and eventually sink to the core. There, they would annihilate and produce high-energy neutrinos that should be detectable by the IceCube observatory in Antarctica. The minimal version of the model predicted that the Sun should be a bright source of these neutrinos, yet IceCube sees nothing. To fix this, the researchers added a new layer to their theory: a hidden force carried by a new particle. This new force interacts differently with protons and neutrons. By carefully tuning the properties of this hidden force, the researchers showed that the dark matter's ability to bounce off protons could be completely canceled out, while its ability to bounce off neutrons remained strong. Since the Sun is mostly hydrogen (protons), the dark matter passes right through it without getting trapped. But the xenon in the LZ detector is rich in neutrons, so the dark matter can still collide with it and create the 248 keV signal. This "isospin-violating" scenario resolves the conflict with the neutrino limits while preserving the explanation for the LZ event.

The team ran extensive computer simulations to test if this complex picture could hold together. They checked the model against a wide range of constraints, from the precise measurements of neutrino masses to the limits on how often certain rare particle decays occur. They found dozens of specific combinations of particle masses and interaction strengths that satisfy every condition. In these scenarios, the dark matter mass ranges from about 691 to 1,479 gigaelectronvolts, and the energy gap remains in the narrow window required to explain the event. The simulations predicted that the LZ detector should see between 0.2 and 4.1 events of this type, which aligns perfectly with the single event they observed. Furthermore, they verified that the long-lived excited state of the dark matter particle would not disrupt the formation of elements in the early universe or the cosmic microwave background, ensuring the model is safe from cosmological objections.

This work does not claim to have definitively proven the nature of dark matter, but it offers a robust and testable explanation for a puzzling anomaly. It suggests that the 248 keV event is not a fluke, but a signature of a dark sector that is more complex than previously thought, involving a hidden force and a subtle mass difference between two states of the same particle. The model makes clear predictions for future experiments. It suggests that the hidden force carrier, a new type of particle, should be detectable at high-energy colliders like the Large Hadron Collider or in precision experiments at facilities like Belle II. It also predicts that the dark matter particle has a specific mass and interaction strength that could be confirmed or ruled out by the next generation of dark matter detectors. By weaving together the mystery of the LZ event, the origin of neutrino masses, and the abundance of dark matter, this research provides a coherent narrative that turns a single, strange flash of light into a potential window into a hidden world of physics.