|In some Civil War battles, the shooting was dense enough and prolonged enough for bullets to collide. The atoms of the metal bullets redistributed themselves as a liquid, much like the quarks and gluons of heavy ion collisions. Image source: brotherswar.com|
In most particle physics experiments, physicists attempt to concentrate as much energy as possible into a point of space. This allows the formation of new, exotic particles like Higgs bosons that reveal the basic workings of the universe. Other collider experiments have a different goal: to spread the energy among enough particles to make a continuous medium, a droplet of fluid millions of times hotter than the center of the sun.
The latter studies, often referred to as heavy-ion physics, require collisions of large nuclei, such as gold or lead, to produce amorphous splashes instead of point-like collisions. Lead ions, for instance, contain 208 protons and neutrons. When two lead ions hit each other squarely head-on in the LHC, many of the 416 protons and neutrons are involved in the collision, unlike the single-proton-on-single-proton collisions used to search for the Higgs boson. With so many collisions in such close proximity, the debris of the nuclei mingles and re-collides with itself like atoms in a liquid. Instead of just splitting in half, the nuclei literally melt.
This is a bit like what happens when two bullets collide in mid-air. Immediately after impact, the atoms in the bullets have enough energy to temporarily melt. Similarly, the quarks and gluons in the colliding lead nuclei spread and mingle as a droplet of fluid before evaporating into thousands of semi-stable particles.
This short-lived state of quark matter is unlike any other known to science. All other liquids, gases, gels and plasmas are governed by forces that weaken with distance. Water, for instance, is made of molecules that electromagnetically attract each other and repel oil. Clouds of interstellar dust are gathered by gravity and congealed by electromagnetism. In contrast, the quarks and gluons loosed by a heavy-ion collision are attracted to one another by the nuclear strong force, which does not weaken with distance. As two quarks start to separate from each other, new pairs of quarks and antiquarks join the mix with an attraction of their own.
This difference in the strong force law leads to surprising effects in the droplet as a whole. Experiments indicate that it is dense and strongly interacting, but with zero or almost no viscosity. As a result, it splashes through itself without friction. This differs from colliding bullets, which behave like clay because of the viscosity of liquid metal.
Quark matter is the stuff the big bang was made of. In the first microseconds of the universe, all matter was a freely flowing quark-gluon soup, which later evaporated into the protons and neutrons that we know today. Yet it is far from understood. It can only be produced in collisions and it is so short-lived that its properties have to be inferred from patterns in the particles that spatter away. Heavy-ion collisions in the LHC and RHIC at Brookhaven will tell us more about the origin of our universe.
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