This paper presents the measurement of charged-hadron and identified-hadron ($K^\mathrm{0}_\mathrm{S}$, $Λ$, $Ξ^\mathrm{-}$) yields in photo-nuclear collisions using 1.7 $\mathrm{nb^{-1}}$ of $\sqrt{s_\mathrm{NN}} = 5.02$ TeV Pb+Pb data collected in 2018 with the ATLAS detector at the Large Hadron Collider. Candidate photo-nuclear events are selected using a combination of tracking and calorimeter information, including the zero-degree calorimeter. The yields as a function of transverse momentum and rapidity are measured in these photo-nuclear collisions as a function of charged-particle multiplicity. These photo-nuclear results are compared with 0.1 $\mathrm{nb^{-1}}$ of $\sqrt{s_\mathrm{NN}} = 5.02$ TeV $p$+Pb data collected in 2016 by ATLAS using similar charged-particle multiplicity selections. These photo-nuclear measurements shed light on potential quark-gluon plasma formation in photo-nuclear collisions via observables sensitive to radial flow, enhanced baryon-to-meson ratios, and strangeness enhancement. The results are also compared with the Monte Carlo DPMJET-III generator and hydrodynamic calculations to test whether such photo-nuclear collisions may produce small droplets of quark-gluon plasma that flow collectively.

Energy-energy correlators are constructed by averaging the number of charged particle pairs within jets, weighted by the product of their transverse momenta, as a function of the angular separation of the particles within a pair. They are sensitive to a multitude of perturbative and nonperturbative quantum chromodynamics phenomena in high-energy particle collisions. Using lead-lead data recorded with the CMS detector, energy-energy correlators inside high transverse momentum jets are measured in heavy ion collisions for the first time. The data are obtained at a nucleon-nucleon center-of-mass energy of 5.02 TeV and correspond to an integrated luminosity of 1.70 nb$^{-1}$. A similar analysis is done for proton-proton collisions at the same center-of-mass energy to establish a reference. The ratio of lead-lead to proton-proton energy-energy correlators reveals significant jet substructure modifications in the quark-gluon plasma. The results are compared to different models that incorporate either color coherence or medium response effects, where the two effects predict similar substructure modifications.

The correlations between event-by-event fluctuations of symmetry planes are measured in Pb$-$Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV recorded by the ALICE detector at the Large Hadron Collider. This analysis is conducted using the Gaussian Estimator technique, which is insensitive to biases from correlations between different flow amplitudes. The study presents, for the first time, the centrality dependence of correlations involving up to five different symmetry planes. The correlation strength varies depending on the harmonic order of the symmetry plane and the collision centrality. Comparisons with measurements from lower energies indicate no significant differences within uncertainties. Additionally, the results are compared with hydrodynamic model calculations. Although the model predictions provide a qualitative explanation of the experimental results, they overestimate the data for some observables. This is particularly true for correlators that are sensitive to the non-linear response of the medium to initial-state anisotropies in the collision system. As these new correlators provide unique information - independent of flow amplitudes - their usage in future model developments can further constrain the properties of the strongly-interacting matter created in ultrarelativistic heavy-ion collisions.

The ALICE Collaboration reports measurements of the large relative transverse momentum ($k_{\text{T}}$) component of jet substructure in pp and in central and semicentral Pb$-$Pb collisions at center-of-mass energy per nucleon pair $\sqrt{s_{\text{NN}}}=5.02$ TeV. Enhancement in the yield of such large-$k_{\text{T}}$ emissions in central Pb$-$Pb collisions is predicted to arise from partonic scattering with quasi-particles of the quark-gluon plasma. The analysis utilizes charged-particle jets reconstructed by the anti-$k_{\text{T}}$ algorithm with resolution parameter $R=0.2$ in the transverse-momentum interval $60 < p_{\text{T,ch jet}} < 80$$\text{GeV}/c$. The soft drop and dynamical grooming algorithms are used to identify high transverse momentum splittings in the jet shower. Comparison of measurements in Pb$-$Pb and pp collisions shows medium-induced narrowing, corresponding to yield suppression of high-$k_{\text{T}}$ splittings, in contrast to the expectation of yield enhancement due to quasi-particle scattering. The measurements are compared to theoretical model calculations incorporating jet quenching, both with and without quasi-particle scattering effects. These measurements provide new insight into the underlying mechanisms and theoretical modeling of jet quenching.

Measurements of the $p_{\rm T}$-dependent flow vector fluctuations in Pb-Pb collisions at $\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV}$ using azimuthal correlations with the ALICE experiment at the Large Hadron Collider are presented. A four-particle correlation approach [1] is used to quantify the effects of flow angle and magnitude fluctuations separately. This paper extends previous studies to additional centrality intervals and provides measurements of the $p_{\rm T}$-dependent flow vector fluctuations at $\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV}$ with two-particle correlations. Significant $p_{\rm T}$-dependent fluctuations of the $\vec{V}_{2}$ flow vector in Pb-Pb collisions are found across different centrality ranges, with the largest fluctuations of up to $\sim$15% being present in the 5% most central collisions. In parallel, no evidence of significant $p_{\rm T}$-dependent fluctuations of $\vec{V}_{3}$ or $\vec{V}_{4}$ is found. Additionally, evidence of flow angle and magnitude fluctuations is observed with more than $5\sigma$ significance in central collisions. These observations in Pb-Pb collisions indicate where the classical picture of hydrodynamic modeling with a common symmetry plane breaks down. This has implications for hard probes at high $p_{\rm T}$, which might be biased by $p_{\rm T}$-dependent flow angle fluctuations of at least 23% in central collisions. Given the presented results, existing theoretical models should be re-examined to improve our understanding of initial conditions, quark--gluon plasma properties, and the dynamic evolution of the created system.

The first measurement of the polarisation of coherently photoproduced J$/\psi$ mesons in ultra-peripheral Pb-Pb collisions, using data at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, is presented. The J/$\psi$ meson is measured via its dimuon decay channel in the forward rapidity interval $-4.0 < y < -2.5$ using the ALICE detector at the CERN LHC. An event sample corresponding to an integrated luminosity of 750 $\mu\text{b}^{-1}$$\pm$ 5% (syst) is analysed. Hadronic activity is highly suppressed since the interaction is mediated by a photon. The polar and azimuthal angle distributions of the decay muons are measured, and the polarisation parameters $\mathbf{\lambda_{\theta}}$, $\mathbf{\lambda_{\varphi}}$, $\mathbf{\lambda_{\theta\varphi}}$ are extracted. The analysis is carried out in the helicity frame. The results are found to be consistent with a transversely polarised J/$\psi$. These values are compared with previous measurements by the H1 and ZEUS experiments. The polarisation parameters of coherent J/$\psi$ photoproduction in Pb-Pb collisions are found to be consistent with the $s$-channel helicity conservation hypothesis.

The ALICE Collaboration presents a new suite of jet substructure measurements in Pb-Pb and pp collisions at a center-of-mass energy per nucleon pair $\sqrt{s}_{\rm NN} = 5.02$ TeV. These measurements provide access to the internal structure of jets via the momentum and angle of their constituents, probing how the quark--gluon plasma modifies jets, an effect known as jet quenching. Jet grooming additionally removes soft wide-angle radiation to enhance perturbative accuracy and reduce experimental uncertainties. We report the groomed and ungroomed jet mass $m_{\rm jet}$ and jet angularities $\lambda_\alpha^\kappa$ using $\kappa=1$ and $\alpha>0$. Charged-particle jets are reconstructed at midrapidity using the anti-$k_\mathrm{T}$ algorithm with resolution parameter $R = 0.2$. A narrowing of the jet mass and angularity distributions in Pb-Pb collisions with respect to pp is observed and is enhanced for groomed results, confirming modification of the jet core. By using consistent jet definitions and kinematic cuts between the mass and angularities for the first time, previous inconsistencies in the interpretation of quenching measurements are resolved, rectifying a hurdle for understanding how jet quenching arises from first principles and highlighting the importance of a well-controlled baseline. These results are compared with a variety of theoretical models of jet quenching, providing constraints on jet energy-loss mechanisms in the quark--gluon plasma.

Measurements are presented of the W and Z boson production cross sections in proton-proton collisions at a center-of-mass energy of 13.6 TeV. Data collected in 2022 and corresponding to an integrated luminosity of 5.01 fb$^{-1}$ with one or two identified muons in the final state are analyzed. The results for the products of total inclusive cross sections and branching fractions for muonic decays of W and Z bosons are 11.93 $\pm$ 0.08 (syst) $\pm$ 0.17 (lumi) $^{+0.07}_{-0.07}$ (acc) nb for W$^+$ boson production, 8.86 $\pm$ 0.06 (syst) $\pm$ 0.12 (lumi) $^{+0.05}_{-0.06}$ (acc) nb for W$^-$ boson production, and 2.021 $\pm$ 0.009 (syst) $\pm$ 0.028 (lumi) $^{+0.011}_{-0.013}$ (acc) nb for the Z boson production in the dimuon mass range of 60-120 GeV, all with negligible statistical uncertainties. Furthermore, the corresponding fiducial cross sections, as well as cross section ratios for both fiducial and total phase space, are provided. The ratios include charge-separated results for W boson production (W$^+$ and W$^-$) and the sum of the two contributions (W$^\pm$), each relative to the measured Z boson production cross section. Additionally, the ratio of the measured cross sections for W$^+$ and W$^-$ boson production is reported. All measurements are in agreement with theoretical predictions, calculated at next-to-next-to-leading order accuracy in quantum chromodynamics.

Incoherent J/$\psi$ photoproduction in heavy ion ultraperipheral collisions (UPCs), in which the photon interacts with localized, fluctuating gluonic hotspots rather than the entire nucleus, provides a unique probe of those fluctuations. This study presents the first measurement of the dependence of this photoproduction yield on the photon-nucleon center-of-mass energy ($W_\gamma^\mathrm{N}$), using PbPb UPCs at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The data corresponds to an integrated luminosity of 1.52 nb$^{-1}$, recorded by the CMS experiment. The measurement covers a wide $W_\gamma^\mathrm{N}$ range of $\approx$40-400 GeV, probing gluons carrying a fraction of nucleon momentum $x$ in the range between 5.9 $\times$ 10$^{-3}$ and 6.5 $\times$ 10$^{-5}$. The measured incoherent J/$\psi$ photoproduction cross section is suppressed relative to theoretical predictions without nuclear effects. However, the ratio of incoherent to coherent photoproduction remains constant across the probed $W_\gamma^\mathrm{N}$ and $x$ range. Together, these results pose significant challenges to current theoretical models which include gluon saturation or nuclear shadowing effects.

High-energy nuclear collisions create a quark-gluon plasma, whose initial condition and subsequent expansion vary from event to event, impacting the distribution of the event-wise average transverse momentum ($P([p_{\mathrm{T}}])$). Distinguishing between contributions from fluctuations in the size of the nuclear overlap area (geometrical component) and other sources at fixed size (intrinsic component) presents a challenge. Here, these two components are distinguished by measuring the mean, variance, and skewness of $P([p_{\mathrm{T}}])$ in $^{208}$Pb+$^{208}$Pb and $^{129}$Xe+$^{129}$Xe collisions at $\sqrt{s_{{\mathrm{NN}}}} = 5.02$ and 5.44 TeV, respectively, using the ATLAS detector at the LHC. All observables show distinct changes in behavior in ultra-central collisions, where the geometrical variations are suppressed as the overlap area reaches its maximum. These results demonstrate a new technique to disentangle geometrical and intrinsic fluctuations, enabling constraints on initial condition and properties of the quark-gluon plasma, such as the speed of sound.