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In the first moments after two lead nuclei collide at the Large Hadron Collider, a tiny fireball is born — a quark soup where particles move freely. If a very strong magnetic field arises at that moment, it can separate positive and negative charges, much like a comb parts hair into strands. This effect is called the chiral magnetic effect, and it was searched for in data from the ALICE detector.

To check whether this effect exists, physicists measured how charged particles fly apart relative to the reaction plane — an imaginary line along which the nuclei collided. They used two approaches. In the first, they varied the shape of the fireball itself to separate the signal from the background. In the second, they compared the particle flight relative to two different planes: one tied to the participants in the collision, where the background is maximal, and the other tied to the spectators (nuclei that passed by), where the signal should be stronger.

Both approaches gave the same result: no sign of the chiral magnetic effect. This doesn't mean the effect doesn't exist in nature: it might be too weak or fade too quickly. But now scientists know how small it must be, which helps rule out incorrect theories. The new limit, obtained from comparing the planes, is stricter than previous estimates.

This search is like trying to hear a very quiet sound in a noisy room. So far, only noise is audible, but now we know how loud the sound would need to be to notice it. And that brings us closer to understanding what happened to matter in the first moments after the Big Bang.