Destello inexplicable en detector de materia oscura abre nueva línea de investigación

A mile beneath the surface of South Dakota, inside a chamber filled with ten tons of ultra-pure liquid xenon, something happened on the evening of June 16, 2023, that physicists still cannot explain. At 21:22:39 UTC, a detector called LUX-ZEPLIN registered a burst of energy—248 kiloelectronvolts—in precisely the region where scientists have long hoped to find the first direct evidence of dark matter. The signal survived months of scrutiny. It passed every test designed to rule it out. And yet it remains, stubbornly real, pointing toward a mystery that has haunted physics for decades.

Dark matter makes up roughly 85 percent of all the mass in the universe. We know it exists because we can see its gravitational fingerprints everywhere—holding galaxies together, shaping the cosmic web. But no one has ever caught it directly. It emits no light, reflects nothing, passes through ordinary matter as if the two barely acknowledge each other. For physicists, this absence of direct evidence is maddening. The leading candidates are particles called WIMPs—weakly interacting massive particles—theoretical objects that might occasionally collide with an atomic nucleus and leave behind a faint, detectable flash. The LZ experiment, a collaboration of 250 scientists and engineers from 39 institutions, was built to find exactly that flash.

The detector sits 1.6 kilometers down in the Sanford Underground Research Facility, shielded from cosmic rays by solid rock. When a particle strikes a xenon nucleus, the atom recoils and produces two signals: a burst of light and a cloud of electrons. Specialized sensors called photomultiplier tubes, positioned above and below the chamber, catch both signals. The ratio between them tells physicists whether the collision was with an electron—common, caused by background radiation—or with an atomic nucleus, which is what a dark matter particle would produce. Between March 2023 and April 2024, the team collected 220 days of data. They found one event that fit the profile.

What made this event unusual was its energy level. Previous searches had focused on collisions below 50 kiloelectronvolts, as if researchers had been listening to a radio at low volume and suddenly turned it up. This time, the team extended their search to 270 kiloelectronvolts, a less-explored territory where certain theoretical models predict dark matter would leave more obvious traces. The event's position within the detector was central, far from the edges where false signals tend to cluster. All the instrument's parameters were normal. The team had even injected artificial signals during analysis—a technique called salting—to guard against unconscious bias. When they removed those fake events, one real signal remained unexplained.

Rick Gaitskell, a physicist at Brown University and spokesperson for the LZ experiment, described the moment of discovery with careful restraint: the team was intrigued to see an event in the exact region where dark matter should appear, where competing background sources are extremely weak. But he was clear about what they were not claiming. "With a single event, we don't want to get ahead of ourselves," he said. "We're not saying we've seen dark matter. But we saw something interesting that we wanted to share with the scientific community." The finding was presented on September 1 at the TeV Particle Astrophysics conference in Tendo, Japan, and submitted to Physical Review Letters.

The team had spent months ruling out ordinary explanations. They considered radioactive decay in the xenon itself, neutron rebounds from the detector's own materials, atmospheric neutrino scattering, and a phenomenon called MSSI, where a particle leaves traces in two zones but only one produces a complete electrical signal. None of these accounted for what they observed. The statistical significance of the result is 2.6 sigma—roughly a 0.5 percent probability that the event is pure chance or miscalculated background noise. In particle physics, the threshold for claiming discovery is 5 sigma, a standard so stringent that the scientific community treats it as conclusive. This result falls far short. Yet it remains the most compelling evidence the LZ experiment has produced to date.

If the event was caused by a dark matter particle, that particle would weigh at least 200 GeV/c²—probably around 1,000 GeV/c². A proton, by comparison, weighs about 1 GeV/c². This hypothetical particle would be more than a thousand times heavier. The collision would also suggest a more complex interaction between dark matter and ordinary matter than simpler models assume—one compatible with advanced theories in which the dark matter particle changes state during impact, as if it gained mass in the collision itself. Aaron Manalaysay, a physicist at Lawrence Berkeley National Laboratory and chair of LZ's institutional board, called it "the first example, in any experiment I've worked on, of an atypical event that appears valid in every respect." Other experiments around the world are watching closely. PandaX in China and XENONnT in Italy are running similar searches with liquid xenon detectors. Elena Aprile, spokesperson for XENONnT, noted that her experiment has comparable sensitivity in that energy range. The next few years will tell whether this single flash in the dark was a glimpse of something real, or simply noise waiting to be explained.