HADES is an internationally collaborative piece of equipment located in Germany, used by scientists all over the world to study matter as it might exist in some of the most intense events in the cosmos, like the merging of neutron stars. But how? Well, many other colliders around the world smash atoms of different elements together at nearly the speed of light. These kinds of experiments can let us take a closer look at the component parts of atoms, including their quarks. And quarks are the elementary particles inside protons and neutrons.

Extremely dense states of matter made up of free quarks and gluons are called quantum chromodynamics matter, which is matter governed by a theory called, appropriately, quantum chromodynamics, or QCD. Scientists sometimes just call these states ‘quark matter’, though, kinda like dark matter, which I think is really hilarious, and physicists think QCD matter, specifically quark-gluon plasma, is what the universe was made of right after the big bang, so states of matter like this require some pretty extreme conditions to exist. See, we know that quarks are held together by the strong nuclear force, also known as just ‘the strong force’, and QCD is the theory that describes how the strong force works on these fundamental particles.

But the theory is so complicated that we can’t actually use the math to predict how this matter will behave at extreme temperatures and densities. As you can imagine, the behavior of quarks in the quark matter has been pretty hard to even create in a collider, much less observe in any great detail. But HADES just got us one step closer. So the HADES team decided to leave the equations on the page and pursue some answers with a physical experiment instead. The group crushed gold molecules into a gold objective at about the speed of light, making a fireball of quark matter. After its initial creation, the fireball starts to shed particles called rho mesons, which are made of a quark and an antiquark. These rho mesons decay into ‘virtual’ photons (which is the coolest name ever), which then further decay into electron-positron pairs.



By using HADES to measure the electron-positron pairs left at the end of the experiment, the researchers gained brand new understanding into the behavior of the quark matter fireball itself. Their measurements indicated that the quark matter fireball could reach temperatures of 800 billion degrees Celsius, so now. Pretty freakin’ hot. The material also reaches a density that’s pretty much what you would get if you crammed New York City into a sugar cube. At these extreme conditions, this kind of quark matter doesn’t break up into free-floating quarks, like in that other phase of exotic matter called quark-gluon plasma. Instead, the quarks bunch up into clusters, forming grape-like bunches of six to nine quarks.







This is the first experiment to measure what the behavior and state of quark matter would be in an interaction like a neutron star collision. And this is important because events like a neutron star collision are a place in the universe where there’s an imbalance of matter: it mostly matters and very little antimatter and our calculations start to fall apart in scenarios like this. We don’t have the math, or previously, the experimental conditions to study it. But many questions still remain, and the HADES team is thinking about what’s next.



