The ALICE experiment was proposed in 1993, to study strongly-interacting matter at extreme energy densities and temperatures. This proposal entailed a comprehensive investigation of nuclear collisions at the LHC. Its physics programme initially focused on the determination of the properties of the quark–gluon plasma (QGP), a deconfined state of quarks and gluons, created in such collisions. The ALICE physics programme has been extended to cover a broader ensemble of observables related to Quantum Chromodynamics (QCD), the theory of strong interactions. The experiment has studied Pb–Pb, Xe–Xe, p–Pb and pp collisions in the multi-TeV centre of mass energy range, during the Run 1–2 data-taking periods at the LHC (2009–2018). The aim of this review is to summarise the key ALICE physics results in this endeavor, and to discuss their implications on the current understanding of the macroscopic and microscopic properties of strongly-interacting matter at the highest temperatures reached in the laboratory. It will review the latest findings on the properties of the QGP created by heavy-ion collisions at LHC energies, and describe the surprising QGP-like effects in pp and p–Pb collisions. Measurements of few-body QCD interactions, and their impact in unraveling the structure of hadrons and hadronic interactions, will be discussed. ALICE results relevant for physics topics outside the realm of QCD will also be touched upon. Finally, prospects for future measurements with the ALICE detector in the context of its planned upgrades will also be briefly described.

A Large Ion Collider Experiment (ALICE) has been conceived and constructed as a heavy-ion experiment at the LHC. During LHC Runs 1 and 2, it has produced a wide range of physics results using all collision systems available at the LHC. In order to best exploit new physics opportunities opening up with the upgraded LHC and new detector technologies, the experiment has undergone a major upgrade during the LHC Long Shutdown 2 (2019–2022). This comprises the move to continuous readout, the complete overhaul of core detectors, as well as a new online event processing farm with a redesigned online-offline software framework. These improvements will allow to record Pb-Pb collisions at rates up to 50 kHz, while ensuring sensitivity for signals without a triggerable signature.

The interaction of $$\textrm{K}^{-}$$K - with protons is characterised by the presence of several coupled channels, systems like $${\overline{\textrm{K}}}^0$$K ¯ 0 n and $$\uppi \Sigma $$π Σ with a similar mass and the same quantum numbers as the $$\textrm{K}^{-}$$K - p state. The strengths of these couplings to the $$\textrm{K}^{-}$$K - p system are of crucial importance for the understanding of the nature of the $$\Lambda (1405)$$Λ ( 1405 ) resonance and of the attractive $$\textrm{K}^{-}$$K - p strong interaction. In this article, we present measurements of the $$\textrm{K}^{-}$$K - p correlation functions in relative momentum space obtained in pp collisions at $$\sqrt{s}~=~13$$s = 13 Te, in p–Pb collisions at $$\sqrt{s_{\textrm{NN}}}~=~5.02$$s NN = 5.02 Te, and (semi)peripheral Pb–Pb collisions at $$\sqrt{s_{\textrm{NN}}}~=~5.02$$s NN = 5.02 Te. The emitting source size, composed of a core radius anchored to the $$\textrm{K}^{+}$$K + p correlation and of a resonance halo specific to each particle pair, varies between 1 and 2 fm in these collision systems. The strength and the effects of the $${\overline{\textrm{K}}}^0$$K ¯ 0 n and $$\uppi \Sigma $$π Σ inelastic channels on the measured $$\textrm{K}^{-}$$K - p correlation function are investigated in the different colliding systems by comparing the data with state-of-the-art models of chiral potentials. A novel approach to determine the conversion weights $$\omega $$ω , necessary to quantify the amount of produced inelastic channels in the correlation function, is presented. In this method, particle yields are estimated from thermal model predictions, and their kinematic distribution from blast-wave fits to measured data. The comparison of chiral potentials to the measured $$\textrm{K}^{-}$$K - p interaction indicates that, while the $$\uppi \Sigma $$π Σ – $$\textrm{K}^{-}$$K - p dynamics is well reproduced by the model, the coupling to the $${\overline{\textrm{K}}}^0$$K ¯ 0 n channel in the model is currently underestimated.

The most precise measurements to date of the ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ lifetime $\tau$ and $\mathrm{\Lambda }$ separation energy ${\mathrm{B}}_{\mathrm{\Lambda }}$ are obtained using the data sample of Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ collected by ALICE at the LHC. The ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ is reconstructed via its charged two-body mesonic decay channel (${}_{\mathrm{\Lambda }}{}^3\mathrm{H}\to {}^3\mathrm{He}+{\pi }^{-}$ and the charge-conjugate process). The measured values $\tau =[253\pm 11(\mathrm{stat})\pm 6(\mathrm{syst})]\mathrm{ps}$ and ${\mathrm{B}}_{\mathrm{\Lambda }}=[102\pm 63(\mathrm{stat})\pm 67(\mathrm{syst})]\mathrm{keV}$ are compatible with predictions from effective field theories and confirm that the ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ structure is consistent with a weakly bound system.

The two-particle momentum correlation functions between charm mesons (${\mathrm{D}}^{*\pm }$ and ${\mathrm{D}}^{\pm }$) and charged light-flavor mesons (${\pi }^{\pm }$ and ${\mathrm{K}}^{\pm }$) in all charge combinations are measured for the first time by the ALICE Collaboration in high-multiplicity proton–proton collisions at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$. For DK and ${\mathrm{D}}^{*}\mathrm{K}$ pairs, the experimental results are in agreement with theoretical predictions of the residual strong interaction based on quantum chromodynamics calculations on the lattice and chiral effective field theory. In the case of $\mathrm{D}\pi$ and ${\mathrm{D}}^{*}\pi$ pairs, tension between the calculations including strong interactions and the measurement is observed. For all particle pairs, the data can be adequately described by Coulomb interaction only, indicating a shallow interaction between charm and light-flavor mesons. Finally, the scattering lengths governing the residual strong interaction of the $\mathrm{D}\pi$ and ${\mathrm{D}}^{*}\pi$ systems are determined by fitting the experimental correlation functions with a model that employs a Gaussian potential. The extracted values are small and compatible with zero.