A Particle on the Brink: What Holds the Mysterious X(3872) Together
In the city of elementary particles, there is a house right on the edge — X(3872). It sits exactly on the line separating it from decaying into two lighter structures: a D meson and an anti-D meson. Give it a nudge, and it would fall apart. But the house stands.
Physicists have long debated what holds it together.