Abstract The jet angularities are a class of jet substructure observables which characterize the angular and momentum distribution of particles within jets. These observables are sensitive to ...momentum scales ranging from perturbative hard scatterings to nonperturbative fragmentation into final-state hadrons. We report measurements of several groomed and ungroomed jet angularities in pp collisions at s $$ \sqrt{s} $$ = 5.02 TeV with the ALICE detector. Jets are reconstructed using charged particle tracks at midrapidity (|η| < 0.9). The anti-k T algorithm is used with jet resolution parameters R = 0.2 and R = 0.4 for several transverse momentum p T ch $$ {p}_{\mathrm{T}}^{\mathrm{ch}} $$ jet intervals in the 20–100 GeV/c range. Using the jet grooming algorithm Soft Drop, the sensitivity to softer, wide-angle processes, as well as the underlying event, can be reduced in a way which is well-controlled in theoretical calculations. We report the ungroomed jet angularities, λ α , and groomed jet angularities, λ α,g, to investigate the interplay between perturbative and nonperturbative effects at low jet momenta. Various angular exponent parameters α = 1, 1.5, 2, and 3 are used to systematically vary the sensitivity of the observable to collinear and soft radiation. Results are compared to analytical predictions at next-to-leading-logarithmic accuracy, which provide a generally good description of the data in the perturbative regime but exhibit discrepancies in the nonperturbative regime. Moreover, these measurements serve as a baseline for future ones in heavy-ion collisions by providing new insight into the interplay between perturbative and nonperturbative effects in the angular and momentum substructure of jets. They supply crucial guidance on the selection of jet resolution parameter, jet transverse momentum, and angular scaling variable for jet quenching studies.
Abstract The study of the production of nuclei and antinuclei in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic ...collisions. In this paper, the production of protons, deuterons and $$^{3}\mathrm {He}$$ 3 He and their charge conjugates at midrapidity is studied as a function of the charged-particle multiplicity in inelastic pp collisions at $$\sqrt{s}=5.02$$ s = 5.02 TeV using the ALICE detector. Within the uncertainties, the yields of nuclei in pp collisions at $$\sqrt{s}=5.02$$ s = 5.02 TeV are compatible with those in pp collisions at different energies and to those in p–Pb collisions when compared at similar multiplicities. The measurements are compared with the expectations of coalescence and Statistical Hadronisation Models. The results suggest a common formation mechanism behind the production of light nuclei in hadronic interactions and confirm that they do not depend on the collision energy but on the number of produced particles.
Abstract Angular correlations of heavy-flavour and charged particles in high-energy proton–proton collisions are sensitive to the production mechanisms of heavy quarks and to their fragmentation as ...well as hadronisation processes. The measurement of the azimuthal-correlation function of prompt D mesons with charged particles in proton–proton collisions at a centre-of-mass energy of $$\sqrt{s} = 13$$ s = 13 TeV with the ALICE detector is reported, considering $$\mathrm D^{0} $$ D 0 , $$\mathrm D^{+} $$ D + , and $$\mathrm D^{*+} $$ D ∗ + mesons in the transverse-momentum interval $$3< p_{\mathrm{T}} < 36$$ 3 < p T < 36 GeV/ $$c$$ c at midrapidity ( $$|y| < 0.5$$ | y | < 0.5 ), and charged particles with $$p_{\mathrm{T}} > 0.3$$ p T > 0.3 GeV/ $$c$$ c and pseudorapidity $$|\eta | < 0.8$$ | η | < 0.8 . This measurement has an improved precision and provides an extended transverse-momentum coverage compared to previous ALICE measurements at lower energies. The study is also performed as a function of the charged-particle multiplicity, showing no modifications of the correlation function with multiplicity within uncertainties. The properties and the transverse-momentum evolution of the near- and away-side correlation peaks are studied and compared with predictions from various Monte Carlo event generators. Among those considered, PYTHIA8 and POWHEG+PYTHIA8 provide the best description of the measured observables. The obtained results can provide guidance on tuning the generators.
Abstract The production of J/ψ is measured at midrapidity (|y| < 0.9) in proton-proton collisions at s $$ \sqrt{s} $$ = 5.02 and 13 TeV, through the dielectron decay channel, using the ALICE detector ...at the Large Hadron Collider. The data sets used for the analyses correspond to integrated luminosities of L $$ \mathcal{L} $$ int = 19.4 ± 0.4 nb −1 and L $$ \mathcal{L} $$ int = 32.2 ± 0.5 nb −1 at s $$ \sqrt{s} $$ = 5.02 and 13 TeV, respectively. The fraction of non-prompt J/ψ mesons, i.e. those originating from the decay of beauty hadrons, is measured down to a transverse momentum p T = 2 GeV/c (1 GeV/c) at s $$ \sqrt{s} $$ = 5.02 TeV (13 TeV). The p T and rapidity (y) differential cross sections, as well as the corresponding values integrated over p T and y, are carried out separately for prompt and non-prompt J/ψ mesons. The results are compared with measurements from other experiments and theoretical calculations based on quantum chromodynamics (QCD). The shapes of the p T and y distributions of beauty quarks predicted by state-of-the-art perturbative QCD models are used to extrapolate an estimate of the b b ¯ $$ \mathrm{b}\overline{\mathrm{b}} $$ pair cross section at midrapidity and in the total phase space. The total b b ¯ $$ \mathrm{b}\overline{\mathrm{b}} $$ cross sections are found to be σ b b ¯ $$ {\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$ = 541 ± 45 (stat.) ± 69 syst . − 12 + 10 $$ {\left(\mathrm{syst}.\right)}_{-12}^{+10} $$ (extr.) μb and σ b b ¯ $$ {\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$ = 218±37 (stat.)±31 syst . − 9.1 + 8.2 $$ {\left(\mathrm{syst}.\right)}_{-9.1}^{+8.2} $$ (extr.) μb at s $$ \sqrt{s} $$ = 13 and 5.02 TeV, respectively. The value obtained from the combination of ALICE and LHCb measurements in pp collisions at s $$ \sqrt{s} $$ = 13 TeV is also provided.
Abstract A measurement of the inclusive b-jet production cross section is presented in pp and p-Pb collisions at s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV, using data collected with the ALICE ...detector at the LHC. The jets were reconstructed in the central rapidity region |η| < 0.5 from charged particles using the anti-k T 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 $$ {R}_{\mathrm{pPb}}^{\mathrm{b}-\mathrm{jet}} $$ . 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 s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 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.
Abstract The production of prompt D0, D+, and D*+ mesons was measured at midrapidity (|y| < 0.5) in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair s NN $$ ...\sqrt{s_{\mathrm{NN}}} $$ = 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 D0 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 RAA is compared with that of charged pions, charged hadrons, and J/ψ 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.
Abstract Understanding the production mechanism of light (anti)nuclei is one of the key challenges of nuclear physics and has important consequences for astrophysics, since it provides an input for ...indirect dark-matter searches in space. In this paper, the latest results about the production of light (anti)nuclei in pp collisions at s $$ \sqrt{s} $$ = 13 TeV are presented, focusing on the comparison with the predictions of coalescence and thermal models. For the first time, the coalescence parameters B 2 for deuterons and B 3 for helions are compared with parameter-free theoretical predictions that are directly constrained by the femtoscopic measurement of the source radius in the same event class. A fair description of the data with a Gaussian wave function is observed for both deuteron and helion, supporting the coalescence mechanism for the production of light (anti)nuclei in pp collisions. This method paves the way for future investigations of the internal structure of more complex nuclear clusters, including the hypertriton.
Abstract 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 s $$ \sqrt{s} $$ = 7 TeV and s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, τ 2 /τ 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 τ 2 /τ 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 τ 2 /τ 1, together with the splitting aperture angle ∆R, are also performed in pp collisions at s $$ \sqrt{s} $$ = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at s $$ \sqrt{s} $$ = 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for τ 2 /τ 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.
Abstract The transverse momentum (p T) differential cross section of the charm-strange baryon Ξ c 0 $$ {\Xi}_{\mathrm{c}}^0 $$ is measured at midrapidity (|y| < 0.5) via its semileptonic decay into ...e+Ξ − ν e in pp collisions at s $$ \sqrt{s} $$ = 5.02 TeV with the ALICE detector at the LHC. The ratio of the p T-differential Ξ c 0 $$ {\Xi}_{\mathrm{c}}^0 $$ -baryon and D0-meson production cross sections is also reported. The measurements are compared with simulations with different tunes of the PYTHIA 8 event generator, with predictions from a statistical hadronisation model (SHM) with a largely augmented set of charm-baryon states beyond the current lists of the Particle Data Group, and with models including hadronisation via quark coalescence. The p T-integrated cross section of prompt Ξ c 0 $$ {\Xi}_{\mathrm{c}}^0 $$ -baryon production at midrapidity is also reported, which is used to calculate the baryon-to-meson ratio Ξ c 0 $$ {\Xi}_{\mathrm{c}}^0 $$ /D0 = 0.20 ± 0.04 stat . − 0.07 + 0.08 $$ {\left(\mathrm{stat}.\right)}_{-0.07}^{+0.08} $$ (syst.). These results provide an additional indication of a modification of the charm fragmentation from e+e − and e − p collisions to pp collisions.
Abstract Measurements of elliptic (v 2) and triangular (v 3) flow coefficients of π ±, K±, p+ p ¯ $$ \overline{\mathrm{p}} $$ , K S 0 $$ {\mathrm{K}}_{\mathrm{S}}^0 $$ , and Λ+ Λ ¯ $$ ...\overline{\Lambda} $$ obtained with the scalar product method in Xe-Xe collisions at s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.44 TeV 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 p T < 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 p T = 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 π ±, K±, and p+ p ¯ $$ \overline{\mathrm{p}} $$ are reproduced by MUSIC hydrodynamic calculations coupled to a hadronic cascade model (UrQMD) for p T < 1 GeV/c. A comparison with v n measurements in the corresponding centrality intervals in Pb-Pb collisions at s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV yields an enhanced v 2 in central collisions and diminished value in semicentral collisions.
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