Recreating the intense fireball of quarks and gluons that existed immediately after the Big Bang, the quark–gluon plasma (QGP), traditionally requires high-energy collisions between heavy ions such as lead-on-lead. Recently, however, the ALICE experiment has seen tentative evidence that the extreme QGP state is created in much smaller systems generated by selected proton–proton collisions.
In a paper published in Nature Physics, the collaboration reports an enhanced production of strange and multi-strange hadrons in high-multiplicity proton–proton (pp) interactions at a centre-of-mass energy of 7 TeV. This phenomenon was one of the earliest proposed indicators for the formation of a QGP, and is very similar to that found in lead–lead (Pb–Pb) collisions and proton–lead (p–Pb) collisions. Measured at mid-rapidity, the production rate of strange particles increases with the event “activity” (quantified by the charged-particle multiplicity density) faster than that of non-strange ones, leading to an enhancement relative to pions.
The enhancement in strangeness is expected to be more pronounced for multi-strange baryons, and this was confirmed in collisions of heavy nuclei at the SPS, RHIC and the LHC. The remarkable similarity between strange particle production in pp, p–Pb and Pb–Pb collisions is complemented by other pp and p–Pb measurements. All exhibit characteristic features from high-energy heavy-ion collisions that are understood to be connected to the formation of a deconfined QCD phase at high temperature and energy density.
The observed multiplicity-dependent enhancement (see figure) follows a hierarchy connected to the strangeness in the hadron. No enhancement is observed for protons (which have no valence strange quarks), demonstrating that the observed increase is strangeness rather than mass related. The results have been compared with Monte Carlo models commonly used at the LHC, of which none can reproduce satisfactorily the observations.
It is not yet clear if the ALICE data truly signal the progressive onset of a QGP medium in small systems. On the other hand, these measurements unveil another remarkable similarity with phenomena known from high-energy nuclear reactions, opening up new possibilities to investigate the underlying dynamical mechanisms of the QGP. Either way, the ability to isolate QGP-like phenomena in a smaller and simpler system opens up an entirely new dimension for the study of the properties of the fundamental state that our universe emerged from.