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Abstract Event-by-event fluctuations of the event-wise mean transverse momentum,$$\langle p_{\textrm{T}}\rangle $$ $⟨p_{\text{T}}⟩$, of charged particles produced in proton–proton (pp) collisions at$$\sqrt{s}$$ $\sqrt{s}$= 5.02 TeV, Xe–Xe collisions at$$\sqrt{s_{\textrm{NN}}}$$ $\sqrt{s_{\text{NN}}}$= 5.44 TeV, and Pb–Pb collisions at$$\sqrt{s_{\textrm{NN}}}$$ $\sqrt{s_{\text{NN}}}$= 5.02 TeV are studied using the ALICE detector based on the integral correlator$$\langle \!\langle \Delta p_\textrm{T}\Delta p_\textrm{T}\rangle \!\rangle $$ $⟨⟨\Delta p_{\text{T}}\Delta p_{\text{T}}⟩⟩$. The correlator strength is found to decrease monotonically with increasing produced charged-particle multiplicity measured at midrapidity in all three systems. In Xe–Xe and Pb–Pb collisions, the multiplicity dependence of the correlator deviates significantly from a simple power-law scaling as well as from the predictions of the HIJING and AMPT models. The observed deviation from power-law scaling is expected from transverse radial flow in semicentral to central Xe–Xe and Pb–Pb collisions. In pp collisions, the correlation strength is also studied by classifying the events based on the transverse spherocity,$$S_0$$ $S_0$, of the particle production at midrapidity, used as a proxy for the presence of a pronounced back-to-back jet topology. Low-spherocity (jetty) events feature a larger correlation strength than those with high spherocity (isotropic). The strength and multiplicity dependence of jetty and isotropic events are well reproduced by calculations with the PYTHIA 8 and EPOS LHC models. 

Abstract The ALICE Collaboration at the CERN LHC has measured the inclusive production cross section of isolated photons at midrapidity as a function of the photon transverse momentum ($$p_{\textrm{T}}^{\gamma }$$ $p_{\text{T}}^{\gamma }$), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of$$\sqrt{s_{\textrm{NN}}}~=~5.02$$ $\sqrt{s_{\text{NN}}}=5.02$ TeV. The photon transverse momentum range is between 10–14 and 40–140 GeV/$$c$$ $c$, depending on the collision system and on the Pb–Pb centrality class. The result extends to lower$$p_{\textrm{T}}^{\gamma }$$ $p_{\text{T}}^{\gamma }$than previously published results by the ATLAS and CMS experiments at the same collision energy. The covered pseudorapidity range is$$|\eta ^{\gamma } | <0.67$$ $|{\eta }^{\gamma }|<0.67$. The isolation selection is based on a charged particle isolation momentum threshold$$p_{\textrm{T}}^\mathrm{iso,~ch} = 1.5$$ $p_{\text{T}}^{\mathrm{iso},\mathrm{ch}}=1.5$ GeV/$$c$$ $c$within a cone of radii$$R=0.2$$ $R=0.2$and 0.4. The nuclear modification factor is calculated and found to be consistent with unity in all centrality classes, and also consistent with the HG-PYTHIA model, which describes the event selection and geometry biases that affect the centrality determination in peripheral Pb–Pb collisions. The measurement is compared to next-to-leading order perturbative QCD calculations and to the measurements of isolated photons and Z$$^{0}$$ $_{}^0$bosons from the CMS experiment, which are all found to be in agreement. 

Abstract The femtoscopic study of pairs of identical pions is particularly suited to investigate the effective source function of particle emission, due to the resulting Bose–Einstein correlation signal. In small collision systems at the LHC, pp in particular, the majority of the pions are produced in resonance decays, which significantly affect the profile and size of the source. In this work, we explicitly model this effect in order to extract the primordial source in pp collisions at$$\sqrt{s}~=~13$$ $\sqrt{s}=13$ TeV from charged$$\uppi $$ $\pi$–$$\uppi $$ $\pi$correlations measured by ALICE. We demonstrate that the assumption of a Gaussian primordial source is compatible with the data and that the effective source, resulting from modifications due to resonances, is approximately exponential, as found in previous measurements at the LHC. The universality of hadron emission in pp collisions is further investigated by applying the same methodology to characterize the primordial source of$$\textrm{K}$$ $\text{K}$–$$\textrm{p}$$ $\text{p}$ pairs. The size of the primordial source is evaluated as a function of the transverse mass ($$m_{\textrm{T}}$$ $m_{\text{T}}$) of the pairs, leading to the observation of a common scaling for both$$\uppi $$ $\pi$–$$\uppi $$ $\pi$and$$\textrm{K}$$ $\text{K}$–$$\textrm{p}$$ $\text{p}$ , suggesting a collective effect. Further, the present results are compatible with the$$m_{\textrm{T}}$$ $m_{\text{T}}$scaling of the$$\textrm{p}$$ $\text{p}$–$$\textrm{p}$$ $\text{p}$ and p$$-\Lambda $$ $-\Lambda$primordial source measured by ALICE in high multiplicity pp collisions, providing additional evidence for the presence of a common emission source for all hadrons in small collision systems at the LHC. This will allow the determination of the source function for any hadron–hadron pairs with high precision, granting access to the properties of the possible final-state interaction among pairs of less abundantly produced hadrons, such as strange or charmed particles. 

Abstract The transverse momentum ($$p_{\textrm{T}}$$ $p_{\text{T}}$) differential production cross section of the promptly produced charm-strange baryon$$\mathrm {\Xi _{c}^{0}}$$ ${\Xi }_c^0$(and its charge conjugate$$\overline{\mathrm {\Xi _{c}^{0}}}$$ $\overline{{\Xi }_c^0}$) is measured at midrapidity via its hadronic decay into$$\mathrm{\pi ^{+}}\Xi ^{-}$$ ${\pi }^{+}{\Xi }^{-}$in p–Pb collisions at a centre-of-mass energy per nucleon–nucleon collision$$\sqrt{s_{\textrm{NN}}}~=~5.02$$ $\sqrt{s_{\text{NN}}}=5.02$ TeV with the ALICE detector at the LHC. The$$\mathrm {\Xi _{c}^{0}}$$ ${\Xi }_c^0$nuclear modification factor ($$R_{\textrm{pPb}}$$ $R_{\text{pPb}}$), calculated from the cross sections in pp and p–Pb collisions, is presented and compared with the$$R_{\textrm{pPb}}$$ $R_{\text{pPb}}$of$$\mathrm {\Lambda _{c}^{+}}$$ ${\Lambda }_c^{+}$baryons. The ratios between the$$p_{\textrm{T}}$$ $p_{\text{T}}$-differential production cross section of$$\mathrm {\Xi _{c}^{0}}$$ ${\Xi }_c^0$baryons and those of$$\mathrm {D^0}$$ $D^0$mesons and$$\mathrm {\Lambda _{c}^{+}}$$ ${\Lambda }_c^{+}$baryons are also reported and compared with results at forward and backward rapidity from the LHCb Collaboration. The measurements of the production cross section of prompt$$\Xi ^0_\textrm{c}$$ ${\Xi }_{\text{c}}^0$baryons are compared with a model based on perturbative QCD calculations of charm-quark production cross sections, which includes only cold nuclear matter effects in p–Pb collisions, and underestimates the measurement by a factor of about 50. This discrepancy is reduced when the data is compared with a model that includes string formation beyond leading-colour approximation or in which hadronisation is implemented via quark coalescence. The$$p_{\textrm{T}}$$ $p_{\text{T}}$-integrated cross section of prompt$$\Xi ^0_\textrm{c}$$ ${\Xi }_{\text{c}}^0$-baryon production at midrapidity extrapolated down to$$p_{\textrm{T}}$$ $p_{\text{T}}$= 0 is also reported. These measurements offer insights and constraints for theoretical calculations of the hadronisation process. Additionally, they provide inputs for the calculation of the charm production cross section in p–Pb collisions at midrapidity. 

Abstract The production cross section of inclusive isolated photons has been measured by the ALICE experiment at the CERN LHC in pp collisions at centre-of-momentum energy of$$\sqrt{s} =13$$ $\sqrt{s}=13$ TeV collected during the LHC Run 2 data-taking period. The measurement is performed by combining the measurements of the electromagnetic calorimeter EMCal and the central tracking detectors ITS and TPC, covering a pseudorapidity range of$$|\eta ^{\gamma }|<0.67$$ $|{\eta }^{\gamma }|<0.67$and a transverse momentum range of$$7 $7<p_{\text{T}}^{\gamma }<200$GeV/$$c$$ $c$. The result extends to lower$$p_\textrm{T}^{\gamma }$$ $p_{\text{T}}^{\gamma }$and$$x_\textrm{T}^{\gamma } = 2p_\textrm{T}^{\gamma }/\sqrt{s} $$ $x_{\text{T}}^{\gamma }=2p_{\text{T}}^{\gamma }/\sqrt{s}$ranges, the lowest$$x_\textrm{T}^{\gamma }$$ $x_{\text{T}}^{\gamma }$of any isolated photon measurements to date, extending significantly those measured by the ATLAS and CMS experiments towards lower$$p_\textrm{T}^{\gamma }$$ $p_{\text{T}}^{\gamma }$at the same collision energy with a small overlap between the measurements. The measurement is compared with next-to-leading order perturbative QCD calculations and the results from the ATLAS and CMS experiments as well as with measurements at other collision energies. The measurement and theory prediction are in agreement with each other within the experimental and theoretical uncertainties. 

Abstract The total charm-quark production cross section per unit of rapidity$$\textrm{d}\sigma ({{\textrm{c}}\overline{\textrm{c}}})/\textrm{d}y$$ $\text{d}\sigma \left(\text{c}\overline{\text{c}}\right)/\text{d}y$, and the fragmentation fractions of charm quarks to different charm-hadron species$$f(\textrm{c}\rightarrow {\textrm{h}}_{\textrm{c}})$$ $f(\text{c}\to {\text{h}}_{\text{c}})$, are measured for the first time in p–Pb collisions at$$\sqrt{s_\textrm{NN}} = 5.02~\text {Te}\hspace{-1.00006pt}\textrm{V} $$ $\sqrt{s_{\text{NN}}}=5.02\text{Te}\text{V}$at midrapidity ($$-0.96<0.04$$ $-0.96<y<0.04$in the centre-of-mass frame) using data collected by ALICE at the CERN LHC. The results are obtained based on all the available measurements of prompt production of ground-state charm-hadron species:$$\textrm{D}^{0}$$ ${\text{D}}^0$,$$\textrm{D}^{+}$$ ${\text{D}}^{+}$,$$\textrm{D}_\textrm{s}^{+}$$ ${\text{D}}_{\text{s}}^{+}$, and$$\mathrm {J/\psi }$$ $J/\psi$mesons, and$$\Lambda _\textrm{c}^{+}$$ ${\Lambda }_{\text{c}}^{+}$and$$\Xi _\textrm{c}^{0}$$ ${\Xi }_{\text{c}}^0$baryons. The resulting cross section is$$ \textrm{d}\sigma ({{\textrm{c}}\overline{\textrm{c}}})/\textrm{d}y =219.6 \pm 6.3\;(\mathrm {stat.}) {\;}_{-11.8}^{+10.5}\;(\mathrm {syst.}) {\;}_{-2.9}^{+8.3}\;(\mathrm {extr.})\pm 5.4\;(\textrm{BR})\pm 4.6\;(\mathrm {lumi.}) \pm 19.5\;(\text {rapidity shape})+15.0\;(\Omega _\textrm{c}^{0})\;\textrm{mb} $$ $\text{d}\sigma \left(\text{c}\overline{\text{c}}\right)/\text{d}y=219.6\pm 6.3\left(\mathrm{stat}.\right)_{-11.8}^{+10.5}\left(\mathrm{syst}.\right)_{-2.9}^{+8.3}\left(\mathrm{extr}.\right)\pm 5.4\left(\text{BR}\right)\pm 4.6\left(\mathrm{lumi}.\right)\pm 19.5\left(\text{rapidity shape}\right)+15.0\left({\Omega }_{\text{c}}^0\right)\text{mb}$, which is consistent with a binary scaling of pQCD calculations from pp collisions. The measured fragmentation fractions are compatible with those measured in pp collisions at$$\sqrt{s} = 5.02$$ $\sqrt{s}=5.02$and 13 TeV, showing an increase in the relative production rates of charm baryons with respect to charm mesons in pp and p–Pb collisions compared with$$\mathrm {e^{+}e^{-}}$$ $e^{+}{e}^{-}$and$$\mathrm {e^{-}p}$$ $e^{-}p$collisions. The$$p_\textrm{T}$$ $p_{\text{T}}$-integrated nuclear modification factor of charm quarks,$$R_\textrm{pPb}({\textrm{c}}\overline{\textrm{c}})= 0.91 \pm 0.04\;\mathrm{(stat.)} ^{+0.08}_{-0.09}\;\mathrm{(syst.)} ^{+0.05}_{-0.03}\;\mathrm{(extr.)} \pm 0.03\;\mathrm{(lumi.)}$$ $R_{\text{pPb}}\left(\text{c}\overline{\text{c}}\right)=0.91\pm 0.04{(\mathrm{stat}.)}_{-0.09}^{+0.08}{(\mathrm{syst}.)}_{-0.03}^{+0.05}(\mathrm{extr}.)\pm 0.03(\mathrm{lumi}.)$, is found to be consistent with unity and with theoretical predictions including nuclear modifications of the parton distribution functions. 

Abstract The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton–proton (pp) collisions is reported. The mean charged-particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at center-of-mass energy$$\sqrt{s}$$ $\sqrt{s}$= 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region ($$|\eta | < 0.9$$ $\left|\eta \right|<0.9$) using the sequential recombination anti-$$k_{\textrm{T}}$$ $k_{\text{T}}$algorithm with jet resolution parametersR= 0.2, 0.3, and 0.4 for the transverse momentum ($$p_\textrm{T}$$ $p_{\text{T}}$) interval 5–110 GeV/c. The high-multiplicity events are selected by the forward V0 scintillator detectors. The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet$$p_\textrm{T}$$ $p_{\text{T}}$in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables$$z^{\textrm{ch}}$$ $z^{\text{ch}}$and$$\xi ^{\textrm{ch}}$$ ${\xi }^{\text{ch}}$are measured for different jet-$$p_\textrm{T}$$ $p_{\text{T}}$intervals. Jet-$$p_\textrm{T}$$ $p_{\text{T}}$independent fragmentation of leading jets is observed for wider jets except at high- and low-$$z^{\textrm{ch}}$$ $z^{\text{ch}}$values. The observed “hump-backed plateau” structure in the$$\xi ^{\textrm{ch}}$$ ${\xi }^{\text{ch}}$distribution indicates suppression of low-$$p_\textrm{T}$$ $p_{\text{T}}$particles. In high-multiplicity events, an enhancement of the fragmentation probability of low-$$z^{\textrm{ch}}$$ $z^{\text{ch}}$particles accompanied by a suppression of high-$$z^{\textrm{ch}}$$ $z^{\text{ch}}$particles is observed compared to minimum-bias events. This behavior becomes more prominent for low-$$p_\textrm{T}$$ $p_{\text{T}}$jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet$$p_\textrm{T}$$ $p_{\text{T}}$. These measurements provide important constraints to models of jet fragmentation. 

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. 

Abstract The interactions of kaons (K) and antikaons ($$\mathrm {\overline{K}}$$ $\overline{K}$) with few nucleons (N) were studied so far using kaonic atom data and measurements of kaon production and interaction yields in nuclei. Some details of the three-body KNN and$$\mathrm {\overline{K}}$$ $\overline{K}$NN dynamics are still not well understood, mainly due to the overlap with multi-nucleon interactions in nuclei. An alternative method to probe the dynamics of three-body systems with kaons is to study the final state interaction within triplet of particles emitted in pp collisions at the Large Hadron Collider, which are free from effects due to the presence of bound nucleons. This Letter reports the first femtoscopic study of p–p–K$$^+$$ ${}^{+}$and p–p–K$$^-$$ ${}^{-}$correlations measured in high-multiplicity pp collisions at$$\sqrt{s}$$ $\sqrt{s}$= 13 TeV by the ALICE Collaboration. The analysis shows that the measured p–p–K$$^+$$ ${}^{+}$and p–p–K$$^-$$ ${}^{-}$correlation functions can be interpreted in terms of pairwise interactions in the triplets, indicating that the dynamics of such systems is dominated by the two-body interactions without significant contributions from three-body effects or bound states. 

Abstract The azimuthal ($$\Delta \varphi $$ $\Delta \phi$) correlation distributions between heavy-flavor decay electrons and associated charged particles are measured in pp and p–Pb collisions at$$\sqrt{s_{\mathrm{{NN}}}} = 5.02$$ $\sqrt{s_{\mathrm{NN}}}=5.02$TeV. Results are reported for electrons with transverse momentum$$4<16$$ $4<p_{\text{T}}<16$$$\textrm{GeV}/c$$ $\text{GeV}/c$ and pseudorapidity$$|\eta |<0.6$$ $\left|\eta \right|<0.6$. The associated charged particles are selected with transverse momentum$$1<7$$ $1<p_{\text{T}}<7$$$\textrm{GeV}/c$$ $\text{GeV}/c$, and relative pseudorapidity separation with the leading electron$$|\Delta \eta | < 1$$ $\left|\Delta \eta \right|<1$. The correlation measurements are performed to study and characterize the fragmentation and hadronization of heavy quarks. The correlation structures are fitted with a constant and two von Mises functions to obtain the baseline and the near- and away-side peaks, respectively. The results from p–Pb collisions are compared with those from pp collisions to study the effects of cold nuclear matter. In the measured trigger electron and associated particle kinematic regions, the two collision systems give consistent results. The$$\Delta \varphi $$ $\Delta \phi$distribution and the peak observables in pp and p–Pb collisions are compared with calculations from various Monte Carlo event generators.