A pool of the purest water, into which tiny, almost invisible droplets occasionally fall. Each leaves a barely perceptible ripple, and capturing it requires years of patience. That is what the hunt for solar neutrinos looks like in the DEAP-3600 detector — a huge vat of liquid argon hidden deep underground.

Neutrinos are the most elusive of known particles. They are born in the Sun's core and stream through us by the billions every second, barely interacting with anything. But occasionally one does collide with an atomic nucleus. In this experiment, physicists looked for events where a neutrino from the decay of boron-8 (produced in the chain of thermonuclear reactions inside the Sun) is absorbed by an argon-40 nucleus. The nucleus then becomes potassium-40 and emits an electron, which produces a flash of light in the liquid argon. Over more than seven tonne-years of accumulated data (as if one tonne of argon had been 'listening' to the sky for seven years), six such flashes were isolated.

The result was surprising: the cross-section (a measure of interaction probability) turned out to be more than twice as high as predicted by 2009 calculations. This is not just a statistical fluctuation — the signal's significance reached 4 sigma, meaning the probability of a chance coincidence is less than one in ten thousand. Now theorists must revisit the models describing how neutrinos interact with nuclei. And the detector itself, originally built to search for dark matter, has proven it can also serve as a sensitive ear for solar particles.

That very pool of argon, which waited for years for rare cosmic visitors, has finally heard their voices, and those voices sound louder than expected. We now know that solar neutrinos are slightly more eager to engage in conversation with matter, opening the way to a more precise understanding of processes inside stars.