A scale capable of weighing the invisible: that's exactly how the ratio of decays of light mesons (pion and kaon) into electrons and muons works. If nature follows the same rules for all generations of particles, the pans of this scale show a strictly defined number. But if an unknown force lurks somewhere, the needle will waver.
The authors of the work have for the first time calculated the most subtle correction to this ratio using lattice methods. They accounted not only for the strong interaction but also for electromagnetic effects — how particles exchange photons (quanta of light) during the decay. Such a calculation demands enormous computational resources: spacetime is divided into a grid, and at each node, quantum fluctuations (random field fluctuations) are computed. Previously, approximations sufficed; now they achieved the physical pion mass and took the continuum limit — eliminating the error associated with the cell size.
The key tool was the technique of "infinite volume reconstruction" with photons in Coulomb gauge. It sharply reduced statistical noise and eliminated artifacts — false signals caused by the finite size of the computational lattice. As a result, the most accurate predictions of the Standard Model to date were obtained: for the pion, the decay ratio is 1.23501(10)×10⁻⁴, and for the kaon, 2.47653(34)×10⁻⁵.
These numbers will serve as a target for next-generation experiments such as PIONEER and NA62. If the measured value deviates from the calculated one, it will indicate the existence of particles or interactions beyond current theories. For now, all known forces of nature continue to operate with Swiss-watch precision.
A pion lives on average 26 nanoseconds — during that time, light travels less than eight meters. But even in such a fleeting moment, it manages to "choose" which particle to decay into, and this ratio holds the key to the mysteries of the universe.