Universality in hadronic and nuclear collisions at high energy

Recent experimental results in proton-proton and in proton-nucleus collisions at Large Hadron Collider energies show a strong similarity to those observed in nucleus-nucleus collisions, where the formation of a quark-gluon plasma is expected. We discuss the comparison between small colliding systems and nucleus-nucleus collisions, for (a) the strangeness suppression factor ${\ensuremath{\gamma}}_{s}$ and yields of multi-strange hadrons; (b) the average transverse momentum, ${p}_{t}$, with particular attention to the low ${p}_{t}$ region where soft, nonperturbative effects are important; and (c) the elliptic flow scaled by the participant eccentricity. The universal behavior in hadronic and nuclear high energy collisions emerges for all these observables in terms of a specific dynamical variable which corresponds to the entropy density of initial system in the collision and which takes into account the transverse size of the initial configuration and its fluctuations.

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