Transverse-momentum and event-shape dependence of D-meson flow harmonics in Pb–Pb collisions at √sNN=5.02 TeV

The elliptic and triangular flow coefficients v2and v3of prompt D0, D+, and D∗+mesons were measured at midrapidity (|y| <0.8) in Pb–Pb collisions at the centre-of-mass energy per nucleon pair of √sNN=5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays in the transverse momentum interval 1 <pT<36 GeV/cin central (0–10%) and semi-central (30–50%) collisions. Compared to pions, protons, and J/ψmesons, the average D-meson vnharmonics are compatible within uncertainties with a mass hierarchy for pT 3GeV/c, and are similar to those of charged pions for higher pT. The coupling of the charm quark to the light quarks in the underlying medium is further investigated with the application of the event-shape engineering (ESE) technique to the D-meson v2and pT-differential yields. The D-meson v2is correlated with average bulk elliptic flow in both central and semi-central collisions. Within the current precision, the ratios of per-event D-meson yields in the ESE-selected and unbiased samples are found to be compatible with unity. All the measurements are found to be reasonably well described by theoretical calculations including the effects of charm-quark transport and the recombination of charm quarks with light quarks in a hydrodynamically expanding medium

Pseudorapidity distributions of charged particles as a function of mid- and forward rapidity multiplicities in pp collisions at √s=5.02, 7 and 13 TeV

The multiplicity dependence of the pseudorapidity density of charged particles in proton–proton (pp) collisions at centre-of-mass energies s√ = 5.02, 7 and 13 TeV measured by ALICE is reported. The analysis relies on track segments measured in the midrapidity range (|η|<1.5). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval |η|<1. The multiplicity dependence of the pseudorapidity density of charged particles is measured with mid- and forward rapidity multiplicity estimators, the latter being less affected by autocorrelations. A detailed comparison with predictions from the PYTHIA 8 and EPOS LHC event generators is also presented. The results can be used to constrain models for particle production as a function of multiplicity in pp collisions

J/ψ production as a function of charged-particle multiplicity in p-Pb collisions at √sNN= 8.16 TeV

Inclusive J/ψ yields and average transverse momenta in p-Pb collisions at a center-of-mass energy per nucleon pair √sNN=8.16 TeV are measured as a function of the charged-particle pseudorapidity density with ALICE. The J/ψ mesons are reconstructed at forward (2.03 < ycms < 3.53) and backward (−4.46 < ycms < −2.96) center-of-mass rapidity in their dimuon decay channel while the charged-particle pseudorapidity density is measured around midrapidity. The J/ψ yields at forward and backward rapidity normalized to their respective average values increase with the normalized charged-particle pseudorapidity density, the former showing a weaker increase than the latter. The normalized average transverse momenta at forward and backward rapidity manifest a steady increase from low to high charged-particle pseudorapidity density with a saturation beyond the average value

Measurement of the low-energy antideuteron inelastic cross section

In this Letter, we report the first measurement of the inelastic cross section for antideuteron-nucleus interactions at low particle momenta, covering a range of 0.3≤p<4  GeV/c. The measurement is carried out using p-Pb collisions at a center-of-mass energy per nucleon–nucleon pair of √sNN=5.02  TeV, recorded with the ALICE detector at the CERN LHC and utilizing the detector material as an absorber for antideuterons and antiprotons. The extracted raw primary antiparticle-to-particle ratios are compared to the results from detailed ALICE simulations based on the geant4 toolkit for the propagation of (anti)particles through the detector material. The analysis of the raw primary (anti)proton spectra serves as a benchmark for this study, since their hadronic interaction cross sections are well constrained experimentally. The first measurement of the inelastic cross section for antideuteron-nucleus interactions averaged over the ALICE detector material with atomic mass numbers (A)=17.4 and 31.8 is obtained. The measured inelastic cross section points to a possible excess with respect to the Glauber model parametrization used in geant4 in the lowest momentum interval of 0.3≤p<0.47  GeV/c up to a factor 2.1. This result is relevant for the understanding of antimatter propagation and the contributions to antinuclei production from cosmic ray interactions within the interstellar medium. In addition, the momentum range covered by this measurement is of particular importance to evaluate signal predictions for indirect dark-matter searches

Observation of a multiplicity dependence in the pT-differential charm baryon-to-meson ratios in proton–proton collisions at √s = 13 TeV

The production of prompt D0, D+ s , and + c hadrons, and their ratios, D+ s /D0 and + c /D0, are measured in proton–proton collisions at √s = 13 TeV at midrapidity (|y| < 0.5) with the ALICE detector at the LHC. The measurements are performed as a function of the charm-hadron transverse momentum (pT) in intervals of charged-particle multiplicity, measured with two multiplicity estimators covering different pseudorapidity regions. While the strange to non-strange D+ s /D0 ratio indicates no significant multiplicity dependence, the baryon-to-meson pT-differential + c /D0 ratio shows a multiplicity-dependent enhancement, with a significance of 5.3σ for 1 < pT < 12 GeV/c, comparing the highest multiplicity interval with respect to the lowest one. The measurements are compared with a theoretical model that explains the multiplicity dependence by a canonical treatment of quantum charges in the statistical hadronisation approach, and with predictions from event generators that implement colour reconnection mechanisms beyond the leading colour approximation to model the hadronisation process. The + c /D0 ratios as a function of pT present a similar shape and magnitude as the /K0 S ratios in comparable multiplicity intervals, suggesting a potential common mechanism for light- and charmhadron formation, with analogous multiplicity dependence. The pT-integrated ratios, extrapolated down to pT = 0, do not show a significant dependence on multiplicity within the uncertainties

Production of muons from heavy-flavour hadron decays at high transverse momentum in Pb–Pb collisions at √sNN=5.02 and 2.76 TeV

Measurements of the production of muons from heavy-flavour hadron decays in Pb–Pb collisions at √sNN = 5.02 and 2.76 TeV using the ALICE detector at the LHC are reported. The nuclear modification factor RAA at √sNN = 5.02 TeV is measured at forward rapidity (2.5 7 GeV/c). The RAA shows an increase of the suppression of the yields of muons from heavy-flavour hadron decays with increasing centrality. A suppression by a factor of about three is observed in the 10% most central collisions. The RAA at √sNN = 5.02 TeV is similar to that at √sNN = 2.76 TeV. The precise RAA measurements have the potential to distinguish between model predictions implementing different mechanisms of parton energy loss in the high-density medium formed in heavy-ion collisions. They place important constraints for the understanding of the heavy-quark interaction with the hot and dense QCD medium

Nuclear modification factor of light neutral-meson spectra up to high transverse momentum in p–Pb collisions at √sNN = 8.16 TeV

Neutral pion (π0) and η meson production cross sections were measured up to unprecedentedly high transverse momenta (pT) in p–Pb collisions at √sNN = 8.16 TeV. The mesons were reconstructed via their two-photon decay channel in the rapidity interval −1.3 10 GeV/c, leaving essentially no room for final state energy loss. The new data provide strong constraints for nuclear parton distribution and fragmentation functions over a broad kinematic range and are compared to model predictions as well as previous results at √sNN = 5.02 TeV

Anisotropic flow of identified particles in Pb-Pb collisions at √sNN = 5.02 TeV

The elliptic (v2), triangular (v3), and quadrangular (v4) ow coe cients of , K , p + p, + , K0 S, and the -meson are measured in Pb-Pb collisions at p sNN = 5:02TeV. Results obtained with the scalar product method are reported for the rapidity range jyj < 0.5 as a function of transverse momentum, pT, at di erent collision centrality intervals between 0{70%, including ultra-central (0{1%) collisions for , K , and p + p. For pT < 3 GeV=c, the ow coe cients exhibit a particle mass dependence. At intermediate transverse momenta (3 < pT < 8{10 GeV=c), particles show an approximate grouping according to their type (i.e., mesons and baryons). The -meson v2, which tests both particle mass dependence and type scaling, follows p + p v2 at low pT and v2 at intermediate pT. The evolution of the shape of vn(pT) as a function of centrality and harmonic number n is studied for the various particle species. Flow coe cients of , K , and p + p for pT < 3 GeV=c are compared to iEBE-VISHNU and MUSIC hydrodynamical calculations coupled to a hadronic cascade model (UrQMD). The iEBE-VISHNU calculations describe the results fairly well for pT < 2:5 GeV=c, while MUSIC calculations reproduce the measurements for pT < 1 GeV=c. A comparison to vn coe cients measured in Pb-Pb collisions at p sNN = 2.76TeV is also provided

Neutral pion and η meson production at midrapidity in Pb-Pb collisions at √sNN = 2.76 TeV

Neutral pion and η meson production in the transverse momentum range 1 < pT < 20 GeV/c have been measured at midrapidity by the ALICE experiment at the Large Hadron Collider (LHC) in central and semicentral Pb-Pb collisions at √ sNN = 2.76 TeV. These results were obtained using the photon conversion method as well as the Photon Spectrometer (PHOS) and Electromagnetic Calorimeter detectors. The results extend the upper pT reach of the previous ALICE π0 measurements from 12 to 20 GeV/c and present the first measurement of η meson production in heavy-ion collisions at the LHC. The η/π0 ratio is similar for the two centralities and reaches at high pT a plateau value of 0.457 ± 0.013stat ± 0.018syst . A suppression of similar magnitude for π0 and η meson production is observed in Pb-Pb collisions with respect to their production in pp collisions scaled by the number of binary nucleon-nucleon collisions. We discuss the results in terms of Next to Leading Order (NLO) pQCD predictions and hydrodynamic models. The measurements show a stronger suppression than observed at lower center-of-mass energies in the pT range 6< pT < 10 GeV/c. For pT < 3 GeV/c, hadronization models describe the π0 results while for the η some tension is observed