Scientists at CERN's Large Hadron Collider (LHC) have reported new evidence that collisions between relatively small atomic nuclei can recreate quark-gluon plasma, the state of matter thought to fill the universe in the first millionths of a second after the Big Bang. In this plasma, temperatures reach more than 100,000 times those at the center of the Sun, and protons and neutrons melt into their building blocks: quarks, which are elementary particles, and gluons, the particles that normally bind quarks together.
Until recently, physicists assumed that such extreme conditions required smashing together very heavy nuclei. The new findings challenge that view. One year after the first oxygen collisions at the LHC, the ALICE, ATLAS, CMS and LHCb experiments have each reported signs of quark-gluon plasma in collisions of oxygen and neon nuclei.
Several independent signs point to the same conclusion
ATLAS examined pairs of particle jets, narrow sprays of particles produced by energetic quarks and gluons, and found that one jet often carried less energy than its partner. This "parton energy loss" - energy drained as a particle pushes through hot, dense matter - was strongest in head-on collisions, where the most plasma is expected. CMS and LHCb found that certain energetic particles were produced less often in oxygen and neon collisions than in proton collisions, and the reduction was larger for the heavier neon nuclei. ALICE then compared its oxygen data in a way that ruled out alternative explanations, providing clear evidence of energy loss. The teams also studied bound pairs of a heavy quark and its antiquark, which break apart inside plasma to different degrees, and ALICE observed a directional, flowing pattern in emerging particles that is considered a plasma signature. A related CMS result, the first direct observation of a wake left behind by a parton moving through the plasma, adds to this picture.
These results matter because they show that the matter of the newborn universe can be studied in much smaller and simpler collisions than once thought. That gives scientists a new way to investigate how quark-gluon plasma forms and behaves. As CERN upgrades the LHC into the more powerful High-Luminosity LHC, researchers expect to collect far more collision data and explore the universe's first moments in greater detail.