How to measure the size of the quark-gluon plasma? And why is it important?

Heavy-ions are brought to ultrarelativistic energies in dedicated accelerators. When the ions collide, a system is produced with high temperature and energy density. Its size is initially comparable to the size of a nucleus. Theory predicts that a deconfined state of matter – a Quark-Gluon Plasma is produced in these conditions. However such description implicitly uses thermodynamic properties, such as temperature, chemical potentials etc. In physics such quantities are usually defined only for “large” systems. A question of the actual size of the system in the heavy-ion collision then becomes crucial. A methodology based on two-particle correlations, called “femtoscopy” is used to measure this size. In this work we briefly describe the technique, which is based on the quantum mechanical properties of particle pairs. We discuss examples of correlation functions and describe a procedure of extracting the “femtoscopic radii”. We show selected recent experimental results for these radii. We also give the theoretical interpretation and consequences for the model description of the heavy-ion collision evolution.