Collective flow in high-multiplicity proton-proton collisions

We present evidence of strong radial flow in high-multiplicity pp collisions. We analyze the CMS Collaboration data on the inclusive spectra of the charged pions, kaons and protons in the Large Hadron Collider s=7 TeV collisions. For (Ntracks)75 we demonstrate the consistency of the hydrodynamic description with the (idealized) Gubser flow. Using a one-parameter fit of the model to experimental data, we obtain the initial fireball size to be of the order of 1 fm. At smaller multiplicities, the fit cannot be performed, which shows a limitation of the hydrodynamic approach and provides us with falsifiability of our theory.