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Abstract 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 bstract The first measurement of the e + e − pair production at low lepton pair transverse momentum ( p T , ee ) and low invariant mass ( m ee ) in non-central Pb–Pb collisions at $$ {\sqrt{s}}_{\textrm{NN}} $$ s NN = 5 . 02 TeV at the LHC is presented. The dielectron production is studied with the ALICE detector at midrapidity ( |η e | < 0 . 8) as a function of invariant mass (0.4 ≤ m ee < 2 . 7 GeV/ c 2 ) in the 50–70% and 70–90% centrality classes for p T , ee < 0.1 GeV/ c , and as a function of p T , ee in three m ee intervals in the most peripheral Pb–Pb collisions. Below a p T , ee of 0.1 GeV/ c , a clear excess of e + e − pairs is found compared to the expectations from known hadronic sources and predictions of thermal radiation from the medium. The m ee excess spectra are reproduced, within uncertainties, by different predictions of the photon–photon production of dielectrons, where the photons originate from the extremely strong electromagnetic fields generated by the highly Lorentz-contracted Pb nuclei. Lowest-order quantum electrodynamic (QED) calculations, as well as a model that takes into account the impact-parameter dependence of the average transverse momentum of the photons, also provide a good description of the p T , ee spectra. The measured $$ \sqrt{\left\langle {p}_{\textrm{T},\textrm{ee}}^2\right\rangle } $$ p T , ee 2 of the excess p T , ee spectrum in peripheral Pb–Pb collisions is found to be comparable to the values observed previously at RHIC in a similar phase-space region. 

A bstract We report about the properties of the underlying event measured with ALICE at the LHC in pp and p–Pb collisions at $$ \sqrt{s_{\textrm{NN}}} $$ s NN = 5 . 02 TeV. The event activity, quantified by charged-particle number and summed- p T densities, is measured as a function of the leading-particle transverse momentum $$ \left({p}_{\textrm{T}}^{\textrm{trig}}\right) $$ p T trig . These quantities are studied in three azimuthal-angle regions relative to the leading particle in the event: toward, away, and transverse. Results are presented for three different p T thresholds (0.15, 0.5 and 1 GeV/ c ) at mid-pseudorapidity (| η | < 0 . 8). The event activity in the transverse region, which is the most sensitive to the underlying event, exhibits similar behaviour in both pp and p–Pb collisions, namely, a steep increase with $$ {p}_{\textrm{T}}^{\textrm{trig}} $$ p T trig for low $$ {p}_{\textrm{T}}^{\textrm{trig}} $$ p T trig , followed by a saturation at $$ {p}_{\textrm{T}}^{\textrm{trig}}\approx 5 $$ p T trig ≈ 5 GeV/ c . The results from pp collisions are compared with existing measurements at other centre-of-mass energies. The quantities in the toward and away regions are also analyzed after the subtraction of the contribution measured in the transverse region. The remaining jet-like particle densities are consistent in pp and p–Pb collisions for $$ {p}_{\textrm{T}}^{\textrm{trig}}>10 $$ p T trig > 10 GeV/ c , whereas for lower $$ {p}_{\textrm{T}}^{\textrm{trig}} $$ p T trig values the event activity is slightly higher in p–Pb than in pp collisions. The measurements are compared with predictions from the PYTHIA 8 and EPOS LHC Monte Carlo event generators. 

Abstract The transverse-momentum $$(p_{\textrm{T}})$$ ( p T ) spectra of K $$^{*}(892)^{0}~$$ ∗ ( 892 ) 0 and $$\mathrm {\phi (1020)}~$$ ϕ ( 1020 ) measured with the ALICE detector up to $$p_{\textrm{T}} $$ p T  = 16 GeV/ c in the rapidity range $$-1.2< y < 0.3,$$ - 1.2 < y < 0.3 , in p–Pb collisions at the center-of-mass energy per nucleon–nucleon collision $$\sqrt{s_{\textrm{NN}}} = 5.02$$ s NN = 5.02  TeV are presented as a function of charged particle multiplicity and rapidity. The measured $$p_{\textrm{T}} $$ p T distributions show a dependence on both multiplicity and rapidity at low $$p_{\textrm{T}} $$ p T whereas no significant dependence is observed at high $$p_{\textrm{T}} $$ p T . A rapidity dependence is observed in the $$p_{\textrm{T}} $$ p T -integrated yield (d N /d y ), whereas the mean transverse momentum $$\left( \langle p_{\textrm{T}} \rangle \right) $$ ⟨ p T ⟩ shows a flat behavior as a function of rapidity. The rapidity asymmetry ( $$Y_{\textrm{asym}}$$ Y asym ) at low $$p_{\textrm{T}} $$ p T (< 5 GeV/ c ) is more significant for higher multiplicity classes. At high $$p_{\textrm{T}} $$ p T , no significant rapidity asymmetry is observed in any of the multiplicity classes. Both K $$^{*}(892)^{0}~$$ ∗ ( 892 ) 0 and $$\mathrm {\phi (1020)}~$$ ϕ ( 1020 ) show similar $$Y_{\textrm{asym}}$$ Y asym . The nuclear modification factor $$(Q_{\textrm{CP}})$$ ( Q CP ) as a function of $$p_{\textrm{T}} $$ p T shows a Cronin-like enhancement at intermediate $$p_{\textrm{T}} $$ p T , which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high $$p_{\textrm{T}}$$ p T (> 5 GeV/ $$c$$ c ), the $$Q_{\textrm{CP}}$$ Q CP values are greater than unity and no significant rapidity dependence is observed. 

Abstract This article presents measurements of the groomed jet radius and momentum splitting fraction in pp collisions at $$ \sqrt{s} $$ s = 5 . 02 TeV with the ALICE detector at the Large Hadron Collider. Inclusive charged-particle jets are reconstructed at midrapidity using the anti- k T algorithm for transverse momentum $$ 60<{p}_{\textrm{T}}^{\textrm{ch}\;\textrm{jet}}<80 $$ 60 < p T ch jet < 80 GeV/ c . We report results using two different grooming algorithms: soft drop and, for the first time, dynamical grooming. For each grooming algorithm, a variety of grooming settings are used in order to explore the impact of collinear radiation on these jet substructure observables. These results are compared to perturbative calculations that include resummation of large logarithms at all orders in the strong coupling constant. We find good agreement of the theoretical predictions with the data for all grooming settings considered. 

Abstract This article presents new measurements of the fragmentation properties of jets in both proton–proton (pp) and heavy-ion collisions with the ALICE experiment at the Large Hadron Collider (LHC). We report distributions of the fraction z r of transverse momentum carried by subjets of radius r within jets of radius R . Charged-particle jets are reconstructed at midrapidity using the anti- k T algorithm with jet radius R = 0 . 4, and subjets are reconstructed by reclustering the jet constituents using the anti- k T algorithm with radii r = 0 . 1 and r = 0 . 2. In proton–proton collisions, we measure both the inclusive and leading subjet distributions. We compare these measurements to perturbative calculations at next-to-leading logarithmic accuracy, which suggest a large impact of threshold resummation and hadronization effects on the z r distribution. In heavy-ion collisions, we measure the leading subjet distributions, which allow access to a region of harder jet frag- mentation than has been probed by previous measurements of jet quenching via hadron fragmentation distributions. The z r distributions enable extraction of the parton-to-subjet fragmentation function and allow for tests of the universality of jet fragmentation functions in the quark–gluon plasma (QGP). We find no significant modification of z r distributions in Pb–Pb compared to pp collisions. However, the distributions are also consistent with a hardening trend for z r < 0 . 95, as predicted by several jet quenching models. As z r → 1 our results indicate that any such hardening effects cease, exposing qualitatively new possibilities to disentangle competing jet quenching mechanisms. By comparing our results to theoretical calculations based on an independent extraction of the parton-to-jet fragmentation function, we find consistency with the universality of jet fragmentation and no indication of factorization breaking in the QGP.