ALICE upgrades during the LHC Long Shutdown 2

A Large Ion Collider Experiment (ALICE) has been conceivedand constructed as a heavy-ion experiment at the LHC. During LHCRuns 1 and 2, it has produced a wide range of physics results usingall collision systems available at the LHC. In order to bestexploit new physics opportunities opening up with the upgraded LHCand new detector technologies, the experiment has undergone a majorupgrade during the LHC Long Shutdown 2 (2019–2022). This comprisesthe move to continuous readout, the complete overhaul of coredetectors, as well as a new online event processing farm with aredesigned online-offline software framework. These improvementswill allow to record Pb-Pb collisions at rates up to 50 kHz, whileensuring sensitivity for signals without a triggerable signature

System-size dependence of the hadronic rescattering effect at energies available at the CERN Large Hadron Collider

The first measurements of K*(892)0 resonance production as a function of charged-particle multiplicity in Xe-Xe collisions at sNN=5.44 TeV and pp collisions ats=5.02 TeV using the ALICE detector are presented. The resonance is reconstructed at midrapidity (|y| < 0.5) using the hadronic decay channel K*0 →K±π∓. Measurements of transverse-momentum integrated yield, mean transverse-momentum, nuclear modification factor of K*0, and yield ratios of resonance to stable hadron (K*0/K) are compared across different collision systems (pp, p-Pb, Xe-Xe, and Pb-Pb) at similar collision energies to investigate how the production of K*0 resonances depends on the size of the system formed in these collisions. The hadronic rescattering effect is found to be independent of the size of colliding systems and mainly driven by the produced charged-particle multiplicity, which is a proxy of the volume of produced matter at the chemical freeze-out. In addition, the production yields of K*0 in Xe-Xe collisions are utilized to constrain the dependence of the kinetic freeze-out temperature on the system size using the hadron resonance gas–partial chemical equilibrium model

Dielectron production in central Pb−-Pb collisions at sNN\sqrt{s_\mathrm{NN}} = 5.02 TeV

The first measurement of the e$^+$e$^-$ pair production at midrapidity and low invariant mass in central Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV at the LHC is presented. The yield of e$^+$e$^-$ pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass ($m_{\rm ee}$) and pair transverse momentum ($p_{\rm T,ee}$) ranges m_{\rm ee} < 3.5 GeV$/c^2$ and p_{\rm T,ee} < 8 GeV$/c$. For 0.18 < m_{\rm ee} < 0.5 GeV$/c^2$ the ratio of data to the cocktail of hadronic contributions without $\rho$ mesons amounts to $1.42 \pm 0.12 \ ({\rm stat.}) \pm 0.17 \ ({\rm syst.}) \pm 0.12 \ ({\rm cocktail})$ and $1.44 \pm 0.12 \ ({\rm stat.}) \pm 0.17 \ ({\rm syst.}) ^{+0.17}_{-0.21} \ ({\rm cocktail})$, including or not including medium effects in the estimation of the heavy-flavor background, respectively. It is consistent with predictions from two different models for an additional contribution of thermal e$^+$e$^-$ pairs from the hadronic and partonic phases. In the intermediate-mass range (1.2 < m_{\rm ee} < 2.6 GeV$/c^2$), the pair transverse impact parameter of the e$^+$e$^-$ pairs (DCA$_{\rm ee}$) is used for the first time in Pb$-$Pb collisions to separate displaced dielectrons from heavy-flavor hadron decays from a possible (thermal) contribution produced at the interaction point. The data are consistent with a suppression of e$^+$e$^-$ pairs from ${\rm c\overline{c}}$ and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is reported via the study of virtual direct photons in the transverse momentum range 1 < p_{\rm T} < 5 GeV$/c$. A model including prompt photons, as well as photons from the pre-equilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties

Measurement of the angle between jet axes in Pb−-Pb collisions at sNN=5.02\sqrt{s_{\rm NN}} = 5.02 TeV

This letter presents the first measurement of the angle between different jet axes (denoted as ${\Delta}R$) in Pb$-$Pb collisions. The measurement is carried out in the 0$-$10% most-central events at $\sqrt{s_{\rm NN}} = 5.02$ TeV. Jets are assembled by clustering charged particles at midrapidity using the anti-$k_{\rm T}$ algorithm with resolution parameters $R=0.2$ and $0.4$ and transverse momenta in the intervals 40 < p_{\rm T}^{\rm ch jet} < 140 GeV/$c$ and 80 < p_{\rm T}^{\rm ch jet} < 140 GeV/$c$, respectively. Measurements at these low transverse momenta enhance the sensitivity to quark$-$gluon plasma (QGP) effects. A comparison to models implementing various mechanisms of jet energy loss in the QGP shows that the observed narrowing of the Pb$-$Pb distribution relative to pp can be explained if quark-initiated jets are more likely to emerge from the medium than gluon-initiated jets. These new measurements discard intra-jet $p_{\rm T}$ broadening as described in a model calculation with the BDMPS formalism as the main mechanism of energy loss in the QGP. The data are sensitive to the angular scale at which the QGP can resolve two independent splittings, favoring mechanisms that incorporate incoherent energy loss

Measurement of the fraction of jet longitudinal momentum carried by <math display="inline"><msubsup><mi mathvariant="normal">Λ</mi><mi>c</mi><mo>+</mo></msubsup></math> baryons in <math display="inline"><mi>p</mi><mi>p</mi></math> collisions

Recent measurements of charm-baryon production in hadronic collisions have questioned the universality of charm-quark fragmentation across different collision systems. In this work the fragmentation of charm quarks into charm baryons is probed, by presenting the first measurement of the longitudinal jet momentum fraction carried by Λc+ baryons, z∥ch, in hadronic collisions. The results are obtained in proton-proton (pp) collisions at s=13  TeV at the LHC, with Λc+ baryons and charged (track-based) jets reconstructed in the transverse momentum intervals of 3≤pTΛc+&lt;15  GeV/c and 7≤pTjet ch&lt;15  GeV/c, respectively. The z∥ch distribution is compared to a measurement of D0-tagged charged jets in pp collisions as well as to pythia 8 simulations. The data hints that the fragmentation of charm quarks into charm baryons is softer with respect to charm mesons, in the measured kinematic interval, as predicted by hadronization models which include color correlations beyond leading-color in the string formation

Production of pions, kaons, and protons as a function of the relative transverse activity classifier in pp collisions at s \sqrt{s} = 13 TeV

The production of π$^{±}$, K$^{±}$, and $\left(\overline{\textrm{p}}\right)\textrm{p}$ is measured in pp collisions at $\sqrt{s}$ = 13 TeV in different topological regions of the events. Particle transverse momentum (p$_{T}$) spectra are measured in the "toward", "transverse", and "away" angular regions defined with respect to the direction of the leading particle in the event. While the toward and away regions contain the fragmentation products of the near-side and away-side jets, respectively, the transverse region is dominated by particles from the Underlying Event (UE). The relative transverse activity classifier, R$_{T}$ = N$_{T}$/〈N$_{T}$〉, is used to group events according to their UE activity, where N$_{T}$ is the measured charged-particle multiplicity per event in the transverse region and 〈N$_{T}$〉 is the mean value over all the analysed events. The first measurements of identified particle p$_{T}$ spectra as a function of R$_{T}$ in the three topological regions are reported. It is found that the yield of high transverse momentum particles relative to the R$_{T}$-integrated measurement decreases with increasing R$_{T}$ in both the toward and the away regions, indicating that the softer UE dominates particle production as R$_{T}$ increases and validating that R$_{T}$ can be used to control the magnitude of the UE. Conversely, the spectral shapes in the transverse region harden significantly with increasing R$_{T}$. This hardening follows a mass ordering, being more significant for heavier particles. Finally, it is observed that the p$_{T}$-differential particle ratios \left(\textrm{p}+\overline{\textrm{p}}\right)/\left({\uppi}^{+}+{\uppi}^{-}\right) and (K$^{+}$ + K$^{−}$)/(π$^{+}$ + π$^{−}$) in the low UE limit (R$_{T}$ → 0) approach expectations from Monte Carlo generators such as PYTHIA 8 with Monash 2013 tune and EPOS LHC, where the jet-fragmentation models have been tuned to reproduce e$^{+}$e$^{−}$ results.[graphic not available: see fulltext

Medium-induced modification of azimuthal correlations of electrons from heavy-flavor hadron decays with charged particles in Pb--Pb collisions at sNN=5.02\mathbf{\sqrt{s_{\rm{NN}}} = 5.02} TeV

The azimuthal-correlation distributions between electrons from the decays of heavy-flavor hadrons and associated charged particles in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV are reported for the 0-10% and 30-50% centrality classes. This is the first measurement to provide access to the azimuthal-correlation observables in the heavy-flavor sector in Pb-Pb collisions. The analysis is performed for trigger electrons from heavy-flavor hadron decays with transverse momentum $4 4$ GeV/$c$ on the away side. The $I_\mathrm{AA}$ for electron triggers from heavy-flavor hadron decays is compared with that for light-flavor and strange-particle triggers to investigate the dependence on different fragmentation processes and parton-medium dynamics, and is found to be the same within uncertainties.The azimuthal-correlation distributions between electrons from the decays of heavy-flavor hadrons and associated charged particles in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV are reported for the 0-10% and 30-50% centrality classes. This is the first measurement to provide access to the azimuthal-correlation observables in the heavy-flavor sector in Pb-Pb collisions. The analysis is performed for trigger electrons from heavy-flavor hadron decays with transverse momentum $4 4$ GeV/$c$ on the away side. The $I_{\rm AA}$ for electron triggers from heavy-flavor hadron decays is compared with that for light-flavor and strange-particle triggers to investigate the dependence on different fragmentation processes and parton-medium dynamics, and is found to be the same within uncertainties

Multiplicity dependence of f0_0(980) production in pp collisions at s=13\sqrt{s} = 13 TeV

International audienceThe dependence of f$_0$(980) production on the final-state charged-particle multiplicity is reported for proton-proton (pp) collisions at the centre-of-mass energy, $\sqrt{s}= 13$ TeV. The production of f$_0$(980) is measured with the ALICE detector via the f$_0(980) \rightarrow π^{+}π^{-}$ decay channel in a midrapidity region of $|y| < 0.5$. The evolution of the integrated yields and mean transverse momentum of f$_{0}$(980) as a function of charged-particle multiplicity measured in pp at $\sqrt{s} = 13$ TeV follows the trends observed in pp at $\sqrt{s} = 5.02$ TeV and in proton-lead (p-Pb) collisions at $\sqrt{s_{\rm{NN}}} = 5.02$ TeV. Particle yield ratios of f$_{0}$(980) to $π^{\pm}$ and K$^{*}$(892)$^{0}$ are found to decrease with increasing charged-particle multiplicity. These particle ratios are compared with calculations from the canonical statistical thermal model as a function of charged-particle multiplicity. The thermal model calculations provide a better description of the decreasing trend of particle ratios when no strange or antistrange quark composition for f$_{0}$(980) is assumed, which suggests that the tetraquark interpretation of the f$_{0}$(980) is disfavored

Evidence for J/ψ\psi suppression in incoherent photonuclear production

International audienceAccording to quantum chromodynamics, at sufficiently high energy, the structure of hadrons reveals a dynamic equilibrium between gluon splitting and gluon recombination -- a phenomenon known as saturation. The process of diffractive photonuclear production of a J/$\psi$ vector meson provides a direct insight into the gluon composition of hadrons. The J/$\psi$ production as a function of momentum transferred in the interaction, quantified by the Mandelstam-$t$ variable, serves as an excellent probe for studying the structure of hadrons within the impact-parameter plane, because different ranges in $t$ are sensitive to the dynamics of the gluon field at varying spatial size scales. The ALICE collaboration has measured the energy dependence of incoherent photonuclear production of J/$\psi$ mesons off lead ions, at $\sqrt{s_{\rm NN}} = 5.02$ TeV, for three Mandelstam-$t$ intervals. The energy dependence of the photonuclear cross section at the highest $|t|$ range measured, $(0.81< |t| <1.44)$ GeV$^2$, is sensitive to subnucleonic structures of the Pb target. The increase of the cross section with energy at large $|t|$ shows evidence of suppression with respect to the increase seen at low $|t|$. The observed pattern of the energy evolution in data is similar to that of gluon saturation models