Femtoscopic signature of strong radial flow in high-multiplicity pp collisions

Hydrodynamic simulations are used to calculate the identical pion Hanbury-Brown-Twiss (HBT) radii, as a function of the pair momentum kT. This dependence is sensitive to the magnitude of the collective radial flow in the transverse plane, and thus comparison to ALICE data enables us to derive its magnitude. By using hydro solutions with variable initial parameters we conclude that in this case fireball explosions start with a very small initial size, well below 1 fm.