At the Large Hadron Collider, protons collide billions of times per second, and only once in a trillion collisions does a pair of Standard Model Higgs bosons (the particles responsible for the mass of everything) emerge. In the CMS experiment, physicists hunted for exactly such events. Both Higgses almost instantly decay into four 'heavy electrons'—bottom quarks — which the detector sees as four narrow jets of particles.

The main challenge is that the same four jets can also be produced by ordinary processes without any Higgses, like a whisper in the roar of a waterfall. To isolate the signal, the scientists improved the trigger (the system that rapidly selects interesting events), learned to separate merging jets, and applied machine learning. The sensitivity more than doubled compared to previous searches on the same data.

Combining data from two run periods (at 13 and 13.6 TeV), the physicists determined that if Higgs pair production occurs, it happens no more than 4.7 times more intensely than the theoretical prediction. This is the most stringent limit for this decay channel. They also constrained the strength of the 'triple contact' of Higgses — a key parameter that determines the shape of the Higgs potential and governs the mass of all particles.

Now this potential has been probed a little deeper. Like geologists who infer the structure of the Earth's interior from seismic waves, physicists reconstruct the invisible landscape where quarks and electrons gain weight from rare pair events.