Enhanced production of multi-strange hadrons in high-multiplicity proton-proton collisions

The ALICE collaboration Adam, Jaroslav ; Adamova, Dagmar ; Aggarwal, Madan Mohan ; et al.
Nature Phys. 13 (2017) 535-539, 2017.
Inspire Record 1471838 DOI 10.17182/hepdata.77284

At sufficiently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the Quark-Gluon Plasma (QGP) [1]. Such an extreme state of strongly-interacting QCD (Quantum Chromo-Dynamics) matter is produced in the laboratory with high-energy collisions of heavy nuclei, where an enhanced production of strange hadrons is observed [2-6]. Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions [7], is more pronounced for multi-strange baryons. Several effects typical of heavy-ion phenomenology have been observed in high-multiplicity proton-proton (pp) collisions [8,9]. Yet, enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity pp collisions. We find that the integrated yields of strange and multi-strange particles relative to pions increases significantly with the event charged-particle multiplicity. The measurements are in remarkable agreement with p-Pb collision results [10,11] indicating that the phenomenon is related to the final system created in the collision. In high-multiplicity events strangeness production reaches values similar to those observed in Pb-Pb collisions, where a QGP is formed.

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Particle-yield modification in jet-like azimuthal di-hadron correlations in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV

The ALICE collaboration Aamodt, K. ; Abelev, B. ; Abrahantes Quintana, A. ; et al.
Phys.Rev.Lett. 108 (2012) 092301, 2012.
Inspire Record 930312 DOI 10.17182/hepdata.58113

The yield of charged particles associated with high-$p_{\rm T}$ trigger particles ($8 < p_{\rm T} < 15$ GeV/$c$) is measured with the ALICE detector in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV relative to proton-proton collisions at the same energy. The conditional per-trigger yields are extracted from the narrow jet-like correlation peaks in azimuthal di-hadron correlations. In the 5% most central collisions, we observe that the yield of associated charged particles with transverse momenta $p_{\rm T}> 3$ GeV/$c$ on the away-side drops to about 60% of that observed in pp collisions, while on the near-side a moderate enhancement of 20-30% is found.

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