Imagine a football stadium where a hundred billion spectators sneeze at once, and only seven or ten do so in complete silence. That's the kind of rarity the NA62 experiment at CERN hunts for, studying the decay of a charged kaon (K+) into a pion (π+) and a pair of neutrinos (νν̄). This event is a quiet whisper in a roaring crowd: neutrinos leave no trace in the detector, so physicists have to infer their presence from missing energy.
The star of the show here is the Standard Model itself, the rulebook governing elementary particles. It predicts how many such 'silent' decays should occur. But if reality deviates even slightly from the calculations, that points to new particles or interactions. The new result, based on 2023–2024 data, is (7.2 ± 2.2) × 10⁻¹¹. When you combine all data from 2016 to 2024, you get (9.6 ± 1.9) × 10⁻¹¹, with 20% precision. So far, the numbers align with theory.
Why does this matter? The decay of a kaon into a pion and neutrinos is an ultra-sensitive probe. Even a tiny admixture of 'new physics'—hypothetical particles like dark photons or supersymmetric partners — would alter its rate. For now, the detector sees what's expected, but the uncertainty is still large. With each new run, precision will improve, and then it will become clear: will the Standard Model remain unshaken, or will it crack like ice under a warm breeze?
as written by the authors arXiv:2607.16413
The NA62 experiment has measured the branching ratio of the rare decay $$K^+\to\pi^+ u\bar{ u}$$ using data collected in 2023 and 2024 at the CERN SPS, obtaining $${\rm B}_{\rm 2023-2024}(K^+\to\pi^+ u\bar{ u}) = \left(7.2^{+2.3}_{-2.1}\right)\times10^{-11}$$. With results from data collected between 2016 and 2022, the analysis gives $${\rm B}_{\rm 2016-2024}(K^+\to\pi^+ u\bar{ u}) = \left(9.6^{+1.9}_{-1.8}\right)\times10^{-11}$$, corresponding to a relative precision of $$20\%$$. This value is consistent with Standard Model predictions.
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