Water freezes at zero and boils at one hundred degrees. Matter made of quarks and gluons also has boundaries between states, but instead of a thermometer, the temperature here is measured in millions of electron-volts. One such boundary is the line where quark soup stops being soup and turns into ordinary protons and neutrons. Until now, this line could only be drawn at high temperatures; at low temperatures, it got lost in the computational fog.
A new study has for the first time extended the line down to a temperature of about 108 MeV — roughly a hundred thousand times hotter than the surface of the Sun. The physicists used a trick with an imaginary chemical potential: instead of actually compressing the matter, they worked along an imaginary axis where calculations are stable. They then found a special point — the Lee-Yang edge — where the system's properties change sharply, like a crack in glass marking its breaking point. From this point, they derived a universal scaling relation, common to all phase transitions of this type, and used it to reconstruct how the boundary shifts under real compression.
The result matches previous estimates at small compression and what is observed in heavy-ion collisions, where a tiny fireball of quark matter is born. This means the method works and can be trusted. Now the road is open: step by step, adding new temperatures and more precise calculations, one can map the entire region of low temperatures and high densities — where the critical point, the final chord of the phase transition, may be hiding.
A temperature of 108 MeV corresponds to about 1.3 trillion degrees Celsius — 100,000 times hotter than the center of the Sun.