Ice, water, steam — phase transitions we all know. But what happens if you squeeze and heat an atomic nucleus itself? Theorists think protons and neutrons inside it 'melt,' releasing smaller building blocks — quarks and gluons. The result is a hot soup called quark-gluon plasma, which filled the Universe in the first microseconds after the Big Bang.
At CERN, on the SPS accelerator, the NA61/SHINE setup works like a giant microscope peering into such micro-explosions. Heavy ions (like lead nuclei) are accelerated to near the speed of light and smashed into a stationary target. The impact energy briefly creates temperatures of thousands of billions of degrees — enough to 'melt' nuclear matter. The experiment scans beam energies (from 13 to 158 gigaelectronvolts per nucleon) and colliding nuclei, from single protons to heavy lead, as if adjusting the flame under a pot and the pot's size.
The main clue to plasma formation is the behavior of 'strange' quarks. Ordinary matter has almost none, but they abound in plasma. Scientists track the ratio of strange particles to pions (the lightest particles produced in abundance). According to the Statistical Model of the early stage, a plot of this ratio versus collision energy should show a 'horn'—a sharp rise followed by a drop. This resembles the boiling point: water temperature stops rising even as heating continues, because energy goes into breaking bonds, not raising temperature.
Recent NA61/SHINE data indeed show such a horn around 30 gigaelectronvolts, with the peak shape depending on the size of the colliding nuclei. Results are compared with data from other experiments worldwide, and the picture converges: the transition from ordinary nuclear matter to quark-gluon plasma is not sharp but passes through a blurred region, like melting butter, not ice. This systematic 'two-dimensional' exploration, by energy and system size, helps map the full landscape of nuclear matter states, from cold nuclei to searing plasma.
Ice, water, steam… and quark-gluon plasma. Physicists add another state of matter to this list, one that can be created only by briefly heating a tiny droplet of the Universe to unimaginable temperatures.
The temperature inside the fireball from lead nucleus collisions is about 100,000 times higher than at the Sun's core.
In Charles Stross's novel 'Accelerando,' a 'quark soup' is described as the computational medium of an alien intelligence.