Prompt D-0, D+, and D*(+) production in Pb-Pb collisions at root S-NN=5.02 TeV

The production of prompt D-0, D+, and D*(+) mesons was measured at midrapidity (vertical bar y vertical bar < 0.5) in Pb-Pb collisions at the centre-of-mass energy per nucleon-nucleon pair root S-NN = 5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decay channels and their production yields were measured in central (0-10%) and semicentral (30-50%) collisions. The measurement was performed up to a transverse momentum (p(T)) of 36 or 50 GeV/c depending on the D meson species and the centrality interval. For the first time in Pb-Pb collisions at the LHC, the yield of D-0 mesons was measured down to p(T) = 0, which allowed a model-independent determination of the p(T)-integrated yield per unit of rapidity (dN/dy). A maximum suppression by a factor 5 and 2.5 was observed with the nuclear modification factor (R-AA) of prompt D mesons at p(T) = 6-8 GeV/c for the 0-10% and 30-50% centrality classes, respectively. The D-meson R-AA is compared with that of charged pions, charged hadrons, and J/psi mesons as well as with theoretical predictions. The analysis of the agreement between the measured R-AA, elliptic (v(2)) and triangular (v(3)) flow, and the model predictions allowed us to constrain the charm spatial diffusion coefficient D-s. Furthermore the comparison of R-AA and v(2) with different implementations of the same models provides an important insight into the role of radiative energy loss as well as charm quark recombination in the hadronisation mechanisms

Charm-quark fragmentation fractions and production cross section at mid rapidity in pp collisions at the LHC

Recent p(T)-integrated cross-section measurements of the ground-state charm mesons and baryons, D-0, D+, D-s(+), Lambda(+)(c), and Xi(0)(c) are used to evaluate the charm fragmentation fractions and production cross section per unit of rapidity at midrapidity (vertical bar y vertical bar < 0.5), in pp collisions at root s = 5.02 TeV at the LHC. The latter is d sigma(c<(c)over) (bar>)/dy vertical bar(vertical bar y vertical bar<0.5) = 1165 +/- 44(stat)(-101)(+131) (syst) mu b. These measurements were obtained for the first time in hadronic collisions at the LHC, including the charm baryon states, recently measured by ALICE at midrapidity. The charm fragmentation fractions differ significantly from the values measured in e(+)e(-) and ep collisions, providing evidence of the dependence of the parton-to-hadron fragmentation fractions on the collision system, indicating that the assumption of their universality is not supported by the measured cross sections. An increase of a factor of about 3.3 for the fragmentation fraction for the Lambda(+)(c) with a significance of 5 sigma between the values obtained in pp collisions and those obtained in e(+)e(-) (ep) collisions is reported. The fragmentation fraction for the Xi(0)(c) was obtained for the first time in any collision system. The measured fragmentation fractions were used to update the c (c) over bar cross sections per unit of rapidity at vertical bar y vertical bar < 0.5 at root s = 2.76 and 7 TeV, which are about 40% higher than the previously published results. The data were compared with perturbative-QCD calculations and lie at the upper edge of the theoretical bands

Anisotropic flow of identified hadrons in Xe-Xe collisions at root s(NN)=5.44 TeV

Measurements of elliptic (v(2)) and triangular (v(3)) flow coefficients of pi(+/-), K-+/-, p+(p) over bar, K-S(0), and Lambda+(Lambda) over bar obtained with the scalar product method in Xe-Xe collisions at root sNN = 5.44TeV are presented. The results are obtained in the rapidity range |y| < 0.5 and reported as a function of transverse momentum, p(T), for several collision centrality classes. The flow coefficients exhibit a particle mass dependence for pT < 3 GeV/c, while a grouping according to particle type (i.e., meson and baryon) is found at intermediate transverse momenta (3 < p(T) < 8 GeV/c). The magnitude of the baryon v(2) is larger than that of mesons up to pT = 6 GeV/c. The centrality dependence of the shape evolution of the p(T)-differential v(2) is studied for the various hadron species. The v(2) coefficients of pi(+/-), K-+/-, and p+(p) over bar are reproduced by MUSIC hydrodynamic calculations coupled to a hadronic cascade model (UrQMD) for p(T) < 1 GeV/c. A comparison with vn measurements in the corresponding centrality intervals in Pb-Pb collisions at root sNN = 5.02TeV yields an enhanced v(2) in central collisions and diminished value in semicentral collisions

( )Measurement of Prompt D-0, Lambda(+)(c), and Sigma(0,++)(c) (2455) Production in Proton-Proton Collisions at root s=13 TeV

The p(T)-differential production cross sections of prompt D-0, Lambda(c)+, and Sigma(0,++)(c) (2455) charmed hadrons are measured at midrapidity (vertical bar y vertical bar < 0.5) in pp collisions at root s. = 13 TeV. This is the first measurement of Sigma(0,++)(c) production in hadronic collisions. Assuming the same production yield for the three Sigma(0,++)(c) isospin states, the baryon-to-meson cross section ratios Sigma(0,+,++)(c)/D-0 and Lambda(+)(c)/D-0 are calculated in the transverse momentum (p(T)) intervals 2 < p(T) < 12 and 1 < p(T) < 24 GeV/c. Values significantly larger than in e(+)e(-) collisions are observed, indicating for the first time that baryon enhancement in hadronic collisions also extends to the Sigma(c). The feed-down contribution to Lambda(+)(c) production from Sigma(0,+,++)(c) is also reported and is found to be larger than in e(+)e(-) collisions. The data are compared with predictions from event generators and other phenomenological models, providing a sensitive test of the different charm-hadronization mechanisms implemented in the models

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

Themultiplicity dependence of the pseudorapidity density of charged particles in proton-proton (pp) collisions at centre-of-mass energies root s = 5.02, 7 and 13 TeV measured by ALICE is reported. The analysis relies on track segments measured in the midrapidity range (|eta| < 1.5). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval |eta| < 1. The multiplicity dependence of the pseudorapidity density of charged particles is measured with midand forward rapidity multiplicity estimators, the latter being less affected by autocorrelations. Adetailed 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

Measurement of inclusive charged-particle b-jet production in pp and p-Pb collisions at root S-NN =5.02 TeV

A measurement of the inclusive b-jet production cross section is presented in pp and p-Pb collisions at root S-NN = 5.02 TeV, using data collected with the ALICE detector at the LHC. The jets were reconstructed in the central rapidity region vertical bar eta vertical bar < 0.5 from charged particles using the anti-kT algorithm with resolution parameter R = 0.4. Identification of b jets exploits the long lifetime of b hadrons, using the properties of secondary vertices and impact parameter distributions. The p(T)-differential inclusive production cross section of b jets, as well as the corresponding inclusive b-jet fraction, are reported for pp and p-Pb collisions in the jet transverse momentum range 10 <= p(T)(,)(ch)( jet) <= 100 GeV/c, together with the nuclear modification factor, R-pPb(b-jet). The analysis thus extends the lower p(T) limit of b-jet measurements at the LHC. The nuclear modification factor is found to be consistent with unity, indicating that the production of b jets in p-Pb at root S-NN = 5.02 TeV is not affected by cold nuclear matter effects within the current precision. The measurements are well reproduced by POWHEG NLO pQCD calculations with PYTHIA fragmentation

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 root s(NN) = 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

Experimental Evidence for an Attractive p-phi Interaction

This Letter presents the first experimental evidence of the attractive strong interaction between a proton and a phi meson. The result is obtained from two-particle correlations of combined p-phi circle plus (p) over bar-phi pairs measured in high-multiplicity pp collisions at root s = 13 TeV by the ALICE Collaboration. The spin-averaged scattering length and effective range of the p-phi interaction are extracted from the fully corrected correlation function employing the Lednicky-Lyuboshits approach. In particular, the imaginary part of the scattering length vanishes within uncertainties, indicating that inelastic processes do not play a prominent role for the p-phi interaction. These data demonstrate that the interaction is dominated by elastic p-phi scattering. Furthermore, an analysis employing phenomenological Gaussian-and Yukawa-type potentials is conducted. Under the assumption of the latter, the N-phi coupling constant is found to be g(N-phi) = 0.14 +/- 0.03(stat) +/- 0.02(syst). This work provides valuable experimental input to accomplish a self-consistent description of the N-phi interaction, which is particularly relevant for the more fundamental studies on partial restoration of chiral symmetry in nuclear medium

First measurements of N-subjettiness in central Pb-Pb collisions at p root s(NN)=2.76 TeV

The ALICE Collaboration reports the first fully-corrected measurements of the N-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data recorded in pp and Pb-Pb collisions at centre-of-mass energies of root s = 7 TeV and root s(NN) = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, tau(2)/tau(1), which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy-ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of R = 0.4 and charged jet transverse momentum of 40 <= p(T,jet) <= 60 GeV/c, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the tau(2)/tau(1) observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of tau(2)/tau(1), together with the splitting aperture angle increment R, are also performed in pp collisions at root s = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at root s = 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for tau(2)/tau(1) are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions