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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.

Original abstract

as written by the authors arXiv:2602.22900

The latest experimental studies related to the search for the Chiral Magnetic Effect (CME) in Pb-Pb collisions at $$\sqrt{s_{\rm NN}} = 5.02$$ TeV recorded with the ALICE detector at the Large Hadron Collider (LHC) are presented. Charge-dependent two-particle correlations relative to the reaction plane are measured for charged particles in the pseudorapidity range $$|\eta| < 0.8$$ and the transverse-momentum range $$0.2 < p_{\rm T} < 5$$ GeV/$$c$$. Two approaches have been employed: in the first method, the contribution of the background to the measurement is varied using the event shape engineering (ESE), while the second relies on changing the contribution of the potential CME signal by measuring azimuthal correlations relative to the participant plane, where the background contributions are maximized, and spectator plane, where the CME signal contribution is maximized. Both methods yield results consistent with the absence of a CME signal within the measurement uncertainties. The result obtained from correlations relative to different symmetry planes, a technique applied for the first time at LHC energies, gives the possibility to test independently and confirm the upper limits from previous measurements, while the new limit from the ESE analysis offers improved constraint relative to previous attempts.