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 exotic state of strongly interacting quantum chromodynamics matter is produced in the laboratory in heavy nuclei high-energy collisions, where an enhanced production of strange hadrons is observed^2,3,4,5,6. Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions⁷, 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, but the enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity proton–proton 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 the 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.

See paper for full list of authors - 23 pages, 7 captioned figures, 5 tables, author from page 19 to page 23In this Letter, comprehensive results on {\pi}+-, K+-, K^0_S, p(antiproton) and {\Lambda} anti-{\Lambda} production at mid-rapidity (0 < y_cms < 0.5) in p-Pb collisions at sqrt(s_NN) = 5.02 TeV, measured by the ALICE detector at the LHC, are reported. The transverse momentum distributions exhibit a hardening as a function of event multiplicity, which is stronger for heavier particles. This behavior is similar to what has been observed in pp and Pb--Pb collisions at the LHC. The measured pT distributions are compared to results at lower energy and with predictions based on QCD-inspired and hydrodynamic models

See paper for full list of authors - 26 pages, 4 captioned figures, 2 tables, Author from page 20 to page 23Angular correlations between unidentified charged trigger particles and various species of charged associated particles (unidentified particles, pions, kaons, protons and antiprotons) are measured by the ALICE detector in p-Pb collisions at a nucleon--nucleon centre-of-mass energy of 5.02 TeV in the transverse-momentum range 0.3 < p_T < 4 GeV/c. The correlations expressed as associated yield per trigger particle are obtained in the pseudorapidity range |eta_lab|<0.8. Fourier coefficients are extracted from the long-range correlations projected onto the azimuthal angle difference and studied as a function of $\pt$ and in intervals of event multiplicity. In high-multiplicity events, the second-order coefficient for protons, v_2^p, is observed to be smaller than that for pions, v_2^pi, up to about p_T = 2 GeV/c. To reduce correlations due to jets, the per-trigger yield measured in low-multiplicity events is subtracted from that in high-multiplicity events. A two-ridge structure is obtained for all particle species. The Fourier decomposition of this structure shows that the second-order coefficients for pions and kaons are similar. The v_2^p is found to be smaller at low p_T and larger at higher p_T than v_2^pi, with a crossing occurring at about 2 GeV. This is qualitatively similar to the elliptic-flow pattern observed in heavy-ion collisions. A mass ordering effect at low transverse momenta is consistent with expectations from hydrodynamic model calculations assuming a collectively expanding system

The ALICE measurement of $K_S^0$ and $\Lambda$ production at midrapidity in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ is presented. The transverse momentum ($p_T$) spectra are shown for several collision centrality intervals and in the $p_T$ range from $0.4\mathrm{GeV}/c$ ($0.6\mathrm{GeV}/c$ for $\Lambda$) to $12\mathrm{GeV}/c$. The $p_T$ dependence of the $\Lambda /K_S^0$ ratios exhibits maxima in the vicinity of $3\mathrm{GeV}/c$, and the positions of the maxima shift towards higher $p_T$ with increasing collision centrality. The magnitude of these maxima increases by almost a factor of three between most peripheral and most central Pb-Pb collisions. This baryon excess at intermediate $p_T$ is not observed in $pp$ interactions at $\sqrt{s}=0.9\mathrm{TeV}$ and at $\sqrt{s}=7\mathrm{TeV}$. Qualitatively, the baryon enhancement in heavy-ion collisions is expected from radial flow. However, the measured $p_T$ spectra above $2\mathrm{GeV}/c$ progressively decouple from hydrodynamical-model calculations. For higher values of $p_T$, models that incorporate the influence of the medium on the fragmentation and hadronization processes describe qualitatively the $p_T$ dependence of the $\Lambda /K_S^0$ ratio.

We report measurements of the primary charged-particle pseudorapidity density and transverse momentum distributions in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02\mathrm{TeV}$ and investigate their correlation with experimental observables sensitive to the centrality of the collision. Centrality classes are defined by using different event-activity estimators, i.e., charged-particle multiplicities measured in three different pseudorapidity regions as well as the energy measured at beam rapidity (zero degree). The procedures to determine the centrality, quantified by the number of participants $\left(N_{\mathrm{part}}\right)$ or the number of nucleon-nucleon binary collisions $\left(N_{\mathrm{coll}}\right)$ are described. We show that, in contrast to Pb-Pb collisions, in $p-\mathrm{Pb}$ collisions large multiplicity fluctuations together with the small range of participants available generate a dynamical bias in centrality classes based on particle multiplicity. We propose to use the zero-degree energy, which we expect not to introduce a dynamical bias, as an alternative event-centrality estimator. Based on zero-degree energy-centrality classes, the $N_{\mathrm{part}}$ dependence of particle production is studied. Under the assumption that the multiplicity measured in the Pb-going rapidity region scales with the number of Pb participants, an approximate independence of the multiplicity per participating nucleon measured at mid-rapidity of the number of participating nucleons is observed. Furthermore, at high-$p_{\mathrm{T}}$ the $p-\mathrm{Pb}$ spectra are found to be consistent with the $pp$ spectra scaled by $N_{\mathrm{coll}}$ for all centrality classes. Our results represent valuable input for the study of the event-activity dependence of hard probes in $p-\mathrm{Pb}$ collisions and, hence, help to establish baselines for the interpretation of the Pb-Pb data.

14 pages, 4 captioned figures, 1 table, authors from page 9 to page 11 ; see paper for full list of authorsTransverse momentum spectra of Pi+/-, K+/- and p(anti-p) up to pT = 20 GeV/c at mid-rapidity, |y| ~< 0.8, in pp and Pb-Pb collisions at sqrt(sNN) = 2.76 TeV have been measured using the ALICE detector at the LHC. At intermediate pT (2-8 GeV/c) an enhancement of the proton-to-proton ratio, (p + anti-p)/(Pi+ + Pi-), with respect to pp collisions is observed and the ratio reaches ~0.80 in central Pb-Pb collisions. The measurement of the nuclear modification factors for Pi+/-, K+/- and p(anti-p) indicates that within the systematic and statistical uncertainties they are the same at high pT (> 10 GeV/c), suggesting that the chemical composition of leading particles from jets in the medium is similar to that of vacuum jets