Recent results and open questions on collective type phenomena from A-A to pp collisions

TL;DR

This paper reviews collective phenomena in heavy ion and proton-proton collisions, highlighting experimental evidence of hydrodynamic-like behavior in high-multiplicity pp collisions at 7 TeV and discussing related open questions.

Contribution

It provides a comprehensive overview of collective effects across different collision systems and discusses new experimental insights into hydrodynamic signatures in pp collisions.

Findings

Evidence of collective flow in high-multiplicity pp collisions at 7 TeV.

Correlations between freeze-out temperature and transverse expansion velocity.

Estimates of energy densities and event selection perspectives.

Abstract

A review of the main results on the collective type expansion of the compressed and hot fireball formed in heavy ion collisions and some remarks to be considered when comparing multiplicity wise phenomena taking place in A-A, p-A and pp collisions, are followed by a discussion of the experimental results which seem to evidence collective type phenomena in pp collisions at $\sqrt{s}$ = 7 TeV at high charged particle multiplicity. Correlations among the kinetic freeze-out temperature, the average transverse expansion velocity and its profile, as a function of centrality and multiplicity, extracted from the fits of experimental transverse momentum spectra with an expression inspired by hydrodynamical models, estimates on Bjorken energy densities and perspectives in selecting soft and close to azimuthal isotropic events in pp collisions are presented.