A firework bursts, and one point explodes into four sparks at once. That's how the Z boson, the particle that carries the weak force, behaves when it decays into four charged leptons (electrons or muons). The process is extremely rare: for every million Z bosons produced, only about five decay this way.

At the Large Hadron Collider, in the CMS detector, protons collided at tremendous speeds, and physicists sifted through the debris looking for these 'four-lepton firework displays.' Over two run periods at energies of 8 and 13 TeV, they accumulated data corresponding to 19.7 and 138 inverse femtobarns (a measure of the total number of collisions). As a result, the overall probability of the decay Z → 4ℓ was measured with 3% precision: [4.67 ± 0.11 (stat) ± 0.10 (syst)] × 10⁻⁶. They also computed it separately for the combinations Z → 4μ, Z → 4e, and Z → 2μ2e.

The main intrigue wasn't just the probability itself. Physicists studied how the decay products fly apart in space: angles, energies, and mutual orientations. Special attention was paid to triple asymmetries — quantities that change sign under mirror reflection and swapping particles for antiparticles. Significant deviations from zero would point to CP violation, one of the fundamental principles of the Standard Model. The results were compared with theoretical predictions, and no discrepancies were found. Instead, they set constraints on the possible existence of new heavy particles — for example, additional gauge bosons that could interfere with this decay.

Such a firework display is not just a pretty sight. It's an ultra-sensitive test for new physics: even a tiny admixture of unknown forces would alter the pattern of particle flight. So far, everything is clean, but the measurement precision is now close to the level where long-awaited surprises might show up. And then, four sparks will tell us something about the world we don't yet know.