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.

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.

We report the first results of elliptic ($v_2$), triangular ($v_3$), and quadrangular ($v_4$) flow of charged particles in Pb-Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ with the ALICE detector at the CERN Large Hadron Collider. The measurements are performed in the central pseudorapidity region $|\eta |<0.8$ and for the transverse momentum range $0.2<p_T<5\mathrm{GeV}/c$. The anisotropic flow is measured using two-particle correlations with a pseudorapidity gap greater than one unit and with the multiparticle cumulant method. Compared to results from Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$, the anisotropic flow coefficients $v_2$, $v_3$, and $v_4$ are found to increase by $(3.0\pm 0.6)\%$, $(4.3\pm 1.4)\%$, and $(10.2\pm 3.8)\%$, respectively, in the centrality range 0%–50%. This increase can be attributed mostly to an increase of the average transverse momentum between the two energies. The measurements are found to be compatible with hydrodynamic model calculations. This comparison provides a unique opportunity to test the validity of the hydrodynamic picture and the power to further discriminate between various possibilities for the temperature dependence of shear viscosity to entropy density ratio of the produced matter in heavy-ion collisions at the highest energies.

Directed and elliptic flows of neutrons and light-charged particles were measured for the reaction ${}^{197}\mathrm{Au}+{}^{197}\mathrm{Au}$ at 400 MeV/nucleon incident energy within the ASY-EOS experimental campaign at the GSI laboratory. The detection system consisted of the Large Area Neutron Detector (LAND), combined with parts of the CHIMERA multidetector, the ALADIN Time-of-flight Wall, and the Washington University Microball detector. The latter three arrays were used for the event characterization and reaction-plane reconstruction. In addition, an array of triple telescopes, KRATTA was used for complementary measurements of the isotopic composition and flows of light-charged particles. From the comparison of the elliptic-flow ratio of neutrons with respect to charged particles with UrQMD predictions, a value $\gamma =0.72\pm 0.19$ is obtained for the power-law coefficient describing the density dependence of the potential part in the parametrization of the symmetry energy. It represents a new and more stringent constraint for the regime of suprasaturation density and confirms, with a considerably smaller uncertainty, the moderately soft to linear density dependence deduced from the earlier FOPI-LAND data. The densities probed are shown to reach beyond twice saturation.

We report the measured transverse momentum (p _T) spectra of primary charged particles from pp, p-Pb and Pb-Pb collisions at a center-of-mass energy $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ s N N = 5.02 TeV in the kinematic range of 0.15 < p _T < 50 GeV/c and |η| < 0.8. A significant improvement of systematic uncertainties motivated the reanalysis of data in pp and Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ s N N = 2.76 TeV, as well as in p-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ s N N = 5.02 TeV, which is also presented. Spectra from Pb-Pb collisions are presented in nine centrality intervals and are compared to a reference spectrum from pp collisions scaled by the number of binary nucleon-nucleon collisions. For central collisions, the p _T spectra are suppressed by more than a factor of 7 around 6–7 GeV/c with a significant reduction in suppression towards higher momenta up to 30 GeV/c. The nuclear modification factor R _pPb, constructed from the pp and p-Pb spectra measured at the same collision energy, is consistent with unity above 8 GeV/c. While the spectra in both pp and Pb-Pb collisions are substantially harder at $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ s N N = 5.02 TeV compared to 2.76 TeV, the nuclear modification factors show no significant collision energy dependence. The obtained results should provide further constraints on the parton energy loss calculations to determine the transport properties of the hot and dense QCD matter.

25 pages, 6 captioned figures, 2 tables, authors from page 20, submitted to PLB, figures at http://aliceinfo.cern.ch/ArtSubmission/node/1887 ; see paper for full list of authorsInternational audienceDirect photon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76$ TeV was studied in the transverse momentum range $0.9 < p_\mathrm{T} < 14$ GeV$/c$. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the $e^+e^-$ pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0-20%, 20-40%, and 40-80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for $p_\mathrm{T} \gtrsim 5$ GeV$/c$. Direct photon spectra down to $p_\mathrm{T} \approx 1$ GeV$/c$ could be extracted for the 20-40% and 0-20% centrality classes. The significance of the direct photon signal for $0.9 < p_\mathrm{T} < 2.1$ GeV$/c$ is $2.6\sigma$ for the 0-20% class. The spectrum in this $p_\mathrm{T}$ range and centrality class can be described by an exponential with an inverse slope parameter of $(297 \pm 12^\mathrm{stat}\pm 41^\mathrm{syst})$ MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties