Shadows in the Quark-Gluon Plasma express
Deep inside the Large Hadron Collider, lead nuclei collide with such force that, for an instant, matter hotter than the Sun is born. This is the quark-gluon plasma — a soup of the tiniest building blocks of matter that existed only in the first microseconds after the Big Bang. Physicists from the ALICE experiment made this plasma reveal itself: they tracked messenger particles flying out of the fireball.
The main messenger was the neutral pion — a light particle that immediately decays into two photons. These photons are caught by a special calorimeter, like a net for light. Alongside the pion, physicists looked for other charged particles that flew out in the same direction or in the opposite one. It turned out that if the pion flies along the collision plane, its companions are clearly visible. But if it flies across, there are noticeably fewer of them near the pion. The plasma, like thick jelly, slows down particles flying through its bulk more strongly.
But the most interesting part came next. When the pion's companions had high energy, above 3 gigaelectronvolts, the difference between directions disappeared. This means that the fastest particles pierce the plasma almost without loss, like a bullet through fog. Theorists compared the data with the JEWEL model, which describes energy loss in an expanding medium. The model predicted that there should be no difference between directions at all. But the experiment saw one. So, there must be some additional quenching mechanism in the plasma that we don't yet know about.
Thus, the plasma born for an instant in the accelerator tells us how matter cooled after the Big Bang. Each caught pion is a step toward understanding why our world consists of ordinary matter, not a quark soup.
Original abstract
as written by the authors arXiv:2602.22866
Per-trigger yields of $$\pi^0$$-hadron correlations were measured in semicentral Pb-Pb collisions at $$\sqrt{s_{\rm NN}} = 5.02$$ TeV in ALICE at the LHC. The reconstructed $$\pi^0 \rightarrow \gamma \gamma$$, with a transverse momentum of $$11 < p_{\rm T}(\pi^0) < 14$$ GeV/$$c$$, is used as the trigger particle to calculate yields of associated charged particles on the near- and away-side. The photons are reconstructed using the ALICE Electromagnetic Calorimeter, and the charged particles are measured in the ALICE central barrel within a pseudorapidity range of $$|\eta|<0.8$$. The yields are reported relative to the orientation of the $$\pi^0$$ with the $$2^{\rm nd}$$-order event plane, and are background subtracted using the reaction plane fit method. The data give an indication of a suppression of the associated charged-particle yields near $$p_{\rm T} \approx 2$$ GeV/$$c$$ when comparing out-of-plane to in-plane trigger particles. At associated charged-particle $$p_{\rm T} > 3$$ GeV/$$c$$, no significant event-plane dependence is observed within uncertainties. The results are compared with predictions from the JEWEL model, which implements jet energy loss in an expanding medium. JEWEL predicts no significant modification of either the near- or away-side associated yields, independent of whether medium recoils are included. The observed behavior may indicate the presence of additional energy-loss mechanisms beyond those governed by path-length dependence.
A neutral pion decays into two photons so quickly that it travels less than a millimeter before vanishing. The detector catches not the particle itself, but its glowing trail.
In Greg Egan's novel Schild's Ladder, physicists create exotic matter to study the birth of the universe — somewhat like attempts to recreate the quark-gluon plasma in a collider.