An electron beam strikes a proton like a billiard ball hitting another ball. But between them is no empty space: the particles exchange virtual photons and Z bosons, like players passing a ball behind their backs. This exchange slightly alters the scattering trajectory, and hidden within it is a hint of new physics.
Lattice calculations break spacetime into tiny cells and compute interactions within each. The authors applied this method for the first time to the gamma-Z exchange contribution in electron–proton scattering. For a beam energy of 155 MeV, they obtained a value twice as precise as previous estimates.
The new calculation gives a weak charge of the proton of 0.06987 with an uncertainty of just 0.00050. It's like weighing a grain of sand on scales that used to be off by two grains, and now by one. The upcoming P2 experiment in Mainz will run at the same energy of 155 MeV, and its results will be compared with this number.
An energy of 155 MeV is millions of times lower than the energy of the Large Hadron Collider, but it is precisely at this energy that the weak interaction effect is most pronounced.