Luchang Jin

The neutron is one of the most neutral particles in nature. But if you add a special parameter to the laws of physics, called the theta term Standard Model, a tiny charge imbalance appears inside the neutron. One edge becomes slightly positive, the other slightly negative.

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.