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At the Large Hadron Collider nuclei are accelerated to speeds comparable to the speed of light. They are then collided and the resulting fireball quickly expands and is converted into thousands of particles. The LHC has the possibility to collide Pb ions and protons. A separate running mode enables asymmetric collisions of protons with Pb ions. A Pb ion, consisting of 208 nucleons can be considered “large” and a collision of two such ions produces a system which is at least an order of magnitude larger than its constituents. It is predicted that a new state of matter – Quark Gluon Plasma, consisting of deconfined quarks and gluons is created in such conditions. In contrast the collision of two protons is expected to be “small”. It is often treated as a “reference system”, where the creation of the QGP is not expected. However, this reasoning is based mostly on the expected size of the system created in pp collisions. It is therefore crucial to measure the size of both systems and confront it with the expectations. An intriguing question arises for p-Pb collisions, whether they should be treated as “small” or “large”. We shortly introduce the technique of femtoscopy, used to measure the size of the particle emitting system. We discuss its connection to the dynamics of the colllision evolution. We present recent femtoscopic results for pp, p-Pb and Pb-Pb collisions and discuss the similarities and differences observed.
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