Form-shifting collisions probe secrets and techniques of early Universe

This summer season, the Large Hadron Collider (LHC) took a breath of recent air. Normally full of beams of protons, the 27-km ring was reconfigured to allow its first oxygen–oxygen and neon–neon collisions. First outcomes from the brand new knowledge, recorded over a interval of six days by the ALICE, ATLAS, CMS and LHCb experiments, have been offered throughout the Initial Stages conference held in Taipei, Taiwan, on 7–12 September.

Smashing atomic nuclei into each other permits physicists to review the quark–gluon plasma (QGP), an excessive state of matter that mimics the situations of the Universe throughout its first microseconds, earlier than atoms shaped. Until now, exploration of this scorching and dense state of free particles on the LHC relied on collisions between heavy ions (like lead or xenon), which maximise the scale of the plasma droplet created.

Collisions between lighter ions, corresponding to oxygen, open a brand new window on the QGP to higher perceive its traits and evolution. Not solely are they smaller than lead or xenon, permitting a greater investigation of the minimal dimension of nuclei wanted to create the QGP, however they’re much less common in form. A neon nucleus, for instance, is predicted to be elongated like a bowling pin – an image that has now been introduced into sharper focus due to the brand new LHC outcomes.

The experiments centered on measurements of delicate patterns within the angles and instructions of the particles flying outward because the QGP droplet expands and cools, that are attributable to small distortions within the authentic collision zone. Remarkably, these “flow” patterns could be described utilizing the identical fluid-dynamics calculations which can be used to mannequin on a regular basis fluids, permitting researchers to probe each the properties of the QGP and the geometry of the colliding nuclei. Accurate mannequin predictions allow a extra exact exploration of movement in oxygen–oxygen and neon–neon collisions than in proton–proton and proton–lead collisions.

ALICE, which specialises within the examine of the QGP, in addition to the general-purpose experiments ATLAS and CMS, have measured sizeable elliptic and triangular movement in oxygen–oxygen and neon–neon collisions, and located that these rely strongly on whether or not the collisions are glancing or head-on. The degree of settlement between principle and knowledge is akin to that obtained for collisions of heavier xenon and lead ions, regardless of the a lot smaller system dimension. This supplies sturdy proof that movement in oxygen–oxygen and neon–neon collisions is pushed by nuclear geometry, supporting the bowling-pin construction of the neon nucleus and demonstrating that hydrodynamic movement emerges robustly throughout collision techniques on the LHC.

Complementary outcomes offered final week by the LHCb collaboration affirm the bowling-pin form of the neon nucleus. The outcomes are primarily based on lead–argon and lead–neon collisions in a fixed-target configuration, utilizing knowledge recorded in 2024 with its SMOG equipment. The LHCb collaboration has additionally began to analyse the oxygen–oxygen and neon–neon collision knowledge.

“Taken together, these results bring fresh perspectives on nuclear structure and how matter emerged after the Big Bang,” says CERN Director for Research and Computing Joachim Mnich.

Further materials

Animation showing side-by-side comparison of lead-lead and oxygen-oxygen collision

Animations showing the quark–gluon plasma formed in collisions between heavy ions

Animation showing the quark-gluon plasma produced in a lead-lead collision and formed by squeezing a group of protons together

Quark-gluon plasma explained – YouTube Video