The Quark-Gluon Plasma is an extremely hot and dense medium, made of deconfined quarks and gluons, defined by strong collective interactions. Its production and characterisation in laboratory primarily relies on ultra-relativistic heavy-ion (A-A) collisions, such as gold-gold, copper-copper, ... available at RHIC or lead-lead (Pb-Pb) at LHC.
With the advent of the latter accelerator, performing hadron collisions at the TeV scale, there has been an increased interest by the QCD community for the studies dedicated to the so-called "small systems", i.e. proton-lead (p-Pb) and proton-proton (pp). Indeed, coming with the LHC run I (2009-2013) and II (2015-2018), various observables that are traditionally associated with high-energy A-A have shown similar behaviours in high-multiplicity p-Pb or even pp events; such an experimental input, normally bound to the formation of a QGP, raises a compelling question : "do we produce a certain QGP not only in A-A but now also in pp ?". The question is straightforward, the answer, expectedly complex... the investigation remains however first and foremost exhilarating.
The talk will deal with a possible masterpiece of that puzzle, it concerns Strangeness Enhancement. The production enhancement of the strange quarks and thus strange hadrons is among the most known and advertised signatures of the QGP-physics in A-A. With a recent publication in Nature Physics (http://arxiv.org/abs/1606.07424 = 10.1038/nphys4111), the ALICE collaboration has reported the observation of a similar pattern in high-multiplicity pp collisions at √s = 7 TeV : the integrated yields of strange and multi-strange hadrons relative to pions increase significantly with the event charged-particle multiplicity, following quantitatively the trend set in p-Pb and reaching the values measured in Pb-Pb, where a QGP is created. After a reminder of the strangeness enhancement conceived in A-A as a direct consequence of the deconfined medium, the ALICE experimental results in pp will be presented. The talk will then try to frame the discussion about possible interpretations.
