At the Large Hadron Collider, protons collide at nearly the speed of light. Sometimes they don't smash apart but exchange photons — particles of light. These photons can produce a pair: one particle the detector sees, and another that flies off unnoticed. This process is like a game of billiards where after the strike, one ball stays on the table while the other disappears into a pocket without leaving a trace.

Scientists from the ATLAS collaboration used special detectors that catch protons deflected after the collision. From the energy of these protons, they reconstructed the total energy of the pair produced by the photons. Then they subtracted the energy of the visible particle to get the 'missing mass'—the mass of the invisible particle. It's like weighing a gift box without opening it: first weigh the whole box, then subtract the weight of the box itself.

In the mass range from 100 to 900 gigaelectronvolts (about 100 to 900 proton masses), physicists saw no significant excess over Standard Model expectations. This means that new particles, if they exist, are produced less frequently than would be noticeable in this data. Upper limits were set on the rate of such processes — from 128 to 2.5 femtobarns depending on the model.

This method is like listening for silence in a room where noise was expected. If the silence is too pure, you can say how loud a sound might have been that we didn't hear. Now physicists know that in this mass range, invisible particles are not produced too often, narrowing the field for future searches.