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

International audienceThe 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

Anisotropic flow and flow fluctuations of identified hadrons in Pb–Pb collisions at sNN \sqrt{s_{\textrm{NN}}} = 5.02 TeV

International audienceThe first measurements of elliptic flow of π$^{±}$, K$^{±}$, $\textrm{p}+\overline{\textrm{p}}$, ${\textrm{K}}_{\textrm{S}}^0$, $\Lambda +\overline{\Lambda}$, ϕ, ${\Xi}^{-}+{\overline{\Xi}}^{+}$, and ${\varOmega}^{-}+{\overline{\varOmega}}^{+}$ using multiparticle cumulants in Pb–Pb collisions at $\sqrt{s_{\textrm{NN}}}$ = 5.02 TeV are resented. Results obtained with two- (v$_{2}${2}) and four-particle cumulants (v$_{2}${4}) are shown as a function of transverse momentum, p$_{T}$, for various collision centrality intervals. Combining the data for both v$_{2}${2} and v$_{2}${4} also allows us to report the first measurements of the mean elliptic flow, elliptic flow fluctuations, and relative elliptic flow fluctuations for various hadron species. These observables probe the event-by-event eccentricity fluctuations in the initial state and the contributions from the dynamic evolution of the expanding quark–gluon plasma. The characteristic features observed in previous p$_{T}$-differential anisotropic flow measurements for identified hadrons with two-particle correlations, namely the mass ordering at low p$_{T}$ and the approximate scaling with the number of constituent quarks at intermediate p$_{T}$, are similarly present in the four-particle correlations and the combinations of v$_{2}${2} and v$_{2}${4}. In addition, a particle species dependence of flow fluctuations is observed that could indicate a significant contribution from final state hadronic interactions. The comparison between experimental measurements and CoLBT model calculations, which combine the various physics processes of hydrodynamics, quark coalescence, and jet fragmentation, illustrates their importance over a wide p$_{T}$ range.[graphic not available: see fulltext

Measurement of the non-prompt D-meson fraction as a function of multiplicity in proton-proton collisions at s \sqrt{s} = 13 TeV

International audienceThe fractions of non-prompt (i.e. originating from beauty-hadron decays) D$^{0}$ and D$^{+}$ mesons with respect to the inclusive yield are measured as a function of the charged-particle multiplicity in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV with the ALICE detector at the LHC. The results are reported in intervals of transverse momentum (p$_{T}$) and integrated in the range 1 < p$_{T}$< 24 GeV/c. The fraction of non-prompt D$^{0}$ and D$^{+}$ mesons is found to increase slightly as a function of p$_{T}$ in all the measured multiplicity intervals, while no significant dependence on the charged-particle multiplicity is observed. In order to investigate the production and hadronisation mechanisms of charm and beauty quarks, the results are compared to PYTHIA 8 as well as EPOS 3 and EPOS 4 Monte Carlo simulations, and to calculations based on the colour glass condensate including three-pomeron fusion.[graphic not available: see fulltext

Azimuthal correlations of heavy-flavor hadron decay electrons with charged particles in pp and p–Pb collisions at sNN\pmb {\sqrt{s_{\mathrm{{NN}}}}} = 5.02 TeV

International audienceThe azimuthal ($\Delta \varphi$) correlation distributions between heavy-flavor decay electrons and associated charged particles are measured in pp and p–Pb collisions at $\sqrt{s_{\mathrm{{NN}}}} = 5.02$ TeV. Results are reported for electrons with transverse momentum $4<p_{\textrm{T}}<16\textrm{GeV}/c$ and pseudorapidity $|\eta |<0.6$. The associated charged particles are selected with transverse momentum $1<p_{\textrm{T}}<7\textrm{GeV}/c$, and relative pseudorapidity separation with the leading electron $|\Delta \eta | < 1$. The correlation measurements are performed to study and characterize the fragmentation and hadronization of heavy quarks. The correlation structures are fitted with a constant and two von Mises functions to obtain the baseline and the near- and away-side peaks, respectively. The results from p–Pb collisions are compared with those from pp collisions to study the effects of cold nuclear matter. In the measured trigger electron and associated particle kinematic regions, the two collision systems give consistent results. The $\Delta \varphi$ distribution and the peak observables in pp and p–Pb collisions are compared with calculations from various Monte Carlo event generators

Symmetry plane correlations in Pb-Pb collisions at sNN=2.76\sqrt{s_{\rm NN}} = 2.76TeV

International audienceA newly developed observable for correlations between symmetry planes, which characterize the direction of the anisotropic emission of produced particles, is measured in Pb–Pb collisions at $\sqrt{s_\text {NN}}$ = 2.76 TeV with ALICE. This so-called Gaussian Estimator allows for the first time the study of these quantities without the influence of correlations between different flow amplitudes. The centrality dependence of various correlations between two, three and four symmetry planes is presented. The ordering of magnitude between these symmetry plane correlations is discussed and the results of the Gaussian Estimator are compared with measurements of previously used estimators. The results utilizing the new estimator lead to significantly smaller correlations than reported by studies using the Scalar Product method. Furthermore, the obtained symmetry plane correlations are compared to state-of-the-art hydrodynamic model calculations for the evolution of heavy-ion collisions. While the model predictions provide a qualitative description of the data, quantitative agreement is not always observed, particularly for correlators with significant non-linear response of the medium to initial state anisotropies of the collision system. As these results provide unique and independent information, their usage in future Bayesian analysis can further constrain our knowledge on the properties of the QCD matter produced in ultrarelativistic heavy-ion collisions

Jet-like correlations with respect to KS0^{0}_{\rm S} and Λ\Lambda (Λˉ\bar{\Lambda}) in pp and Pb-Pb collisions at sNN\mathbf{\it\sqrt{s_\mathrm{NN}}} = 5.02 TeV

International audienceTwo-particle correlations with $\textrm{K}^{0}_\mathrm{{S}}$, $\Lambda$/$\overline{\Lambda }$, and charged hadrons as trigger particles in the transverse momentum range 8{3 GeV/$c$ as expected from strong in-medium energy loss, while an enhancement develops at low $p_{{\textrm{T}},{\textrm{assoc}}}$ on both the near and away sides, reaching $I_{\textrm{AA}}\approx 1.8$ and 2.7 respectively. These findings are in good agreement with previous ALICE measurements from two-particle correlations triggered by neutral pions ($\pi ^{0}$–h) and charged hadrons (h–h) in Pb–Pb collisions at $\sqrt{s_{\textrm{NN}}}~=~2.76$ TeV. Moreover, the correlations with $\textrm{K}^{0}_\mathrm{{S}}$ mesons and $\Lambda$/$\overline{\Lambda }$ baryons as trigger particles are compared to those of inclusive charged hadrons. The results are compared with the predictions of Monte Carlo models

Σ(1385)±\Sigma (1385)^{\pm } resonance production in Pb–Pb collisions at sNN = 5.02\sqrt{s_{\textrm{NN}}}~=~5.02 TeV

International audienceHadronic resonances are used to probe the hadron gas produced in the late stage of heavy-ion collisions since they decay on the same timescale, of the order of 1–10 fm/c, as the decoupling time of the system. In the hadron gas, (pseudo)elastic scatterings among the products of resonances that decayed before the kinetic freeze-out and regeneration processes counteract each other, the net effect depending on the resonance lifetime, the duration of the hadronic phase, and the hadronic cross sections at play. In this context, the $\Sigma (1385)^{\pm }$ particle is of particular interest as models predict that regeneration dominates over rescattering despite its relatively short lifetime of about 5.5 fm/c. The first measurement of the $\Sigma (1385)^{\pm }$ resonance production at midrapidity in Pb–Pb collisions at $\sqrt{s_{\textrm{NN}}}= 5.02$ TeV with the ALICE detector is presented in this Letter. The resonances are reconstructed via their hadronic decay channel, $\Lambda \pi$, as a function of the transverse momentum ($p_\textrm{T}$) and the collision centrality. The results are discussed in comparison with the measured yield of pions and with expectations from the statistical hadronization model as well as commonly employed event generators, including PYTHIA8/Angantyr and EPOS3 coupled to the UrQMD hadronic cascade afterburner. None of the models can describe the data. For $\Sigma (1385)^{\pm }$, a similar behaviour as $\textrm{K}^{*} (892)^{0}$ is observed in data unlike the predictions of EPOS3 with afterburner

Production of KS0{\rm K}^{0}_{\rm{S}}, Λ\Lambda (Λˉ\bar{\Lambda}), Ξ±\Xi^{\pm} and Ω±\Omega^{\pm} in jets and in the underlying event in pp and p−-Pb collisions

International audienceThe production of strange hadrons (${\textrm{K}}_{\textrm{S}}^0$, Λ, Ξ$^{±}$, and Ω$^{±}$), baryon-to-meson ratios (Λ/${\textrm{K}}_{\textrm{S}}^0$, Ξ/${\textrm{K}}_{\textrm{S}}^0$, and Ω/${\textrm{K}}_{\textrm{S}}^0$), and baryon-to-baryon ratios (Ξ/Λ, Ω/Λ, and Ω/Ξ) associated with jets and the underlying event were measured as a function of transverse momentum (p$_{T}$) in pp collisions at $\sqrt{s}$ = 13 TeV and p Pb collisions at $\sqrt{s_{\textrm{NN}}}$ = 5.02 TeV with the ALICE detector at the LHC. The inclusive production of the same particle species and the corresponding ratios are also reported. The production of multi-strange hadrons, Ξ$^{±}$ and Ω$^{±}$, and their associated particle ratios in jets and in the underlying event are measured for the first time. In both pp and p–Pb collisions, the baryon-to-meson and baryon-to-baryon yield ratios measured in jets differ from the inclusive particle production for low and intermediate hadron p$_{T}$ (0.6–6 GeV/c). Ratios measured in the underlying event are in turn similar to those measured for inclusive particle production. In pp collisions, the particle production in jets is compared with Pythia 8 predictions with three colour-reconnection implementation modes. None of them fully reproduces the data in the measured hadron p$_{T}$ region. The maximum deviation is observed for Ξ$^{±}$ and Ω$^{±}$ which reaches a factor of about six. The event multiplicity dependence is further investigated in p−Pb collisions. In contrast to what is observed in the underlying event, there is no significant event-multiplicity dependence for particle production in jets. The presented measurements provide novel constraints on hadronisation and its Monte Carlo description. In particular, they demonstrate that the fragmentation of jets alone is insufficient to describe the strange and multi-strange particle production in hadronic collisions at LHC energies.[graphic not available: see fulltext

Measurement of the angle between jet axes in pp collisions at s \sqrt{s} = 5.02 TeV

International audienceThis article reports measurements of the angle between differently defined jet axes in pp collisions at $\sqrt{s}$ = 5.02 TeV carried out by the ALICE Collaboration. Charged particles at midrapidity are clustered into jets with resolution parameters R = 0.2 and 0.4. The jet axis, before and after Soft Drop grooming, is compared to the jet axis from the Winner-Takes-All (WTA) recombination scheme. The angle between these axes, ∆R$_{axis}$, probes a wide phase space of the jet formation and evolution, ranging from the initial high-momentum-transfer scattering to the hadronization process. The ∆R$_{axis}$ observable is presented for 20 <${p}_{\textrm{T}}^{\textrm{ch}\ \textrm{jet}}$< 100 GeV/c, and compared to predictions from the PYTHIA 8 and Herwig 7 event generators. The distributions can also be calculated analytically with a leading hadronization correction related to the non-perturbative component of the Collins-Soper-Sterman (CSS) evolution kernel. Comparisons to analytical predictions at next-to-leading-logarithmic accuracy with leading hadronization correction implemented from experimental extractions of the CSS kernel in Drell-Yan measurements are presented. The analytical predictions describe the measured data within 20% in the perturbative regime, with surprising agreement in the non-perturbative regime as well. These results are compatible with the universality of the CSS kernel in the context of jet substructure.[graphic not available: see fulltext

Energy dependence of coherent photonuclear production of J/ψ mesons in ultra-peripheral Pb-Pb collisions at sNN \sqrt{{\textrm{s}}_{\textrm{NN}}} = 5.02 TeV

International audienceThe cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of $\sqrt{{\textrm{s}}_{\textrm{NN}}}$ = 5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y| < 4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17 < W$_{γ Pb,n}$ < 920 GeV, where W$_{γ Pb,n}$ is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1 × 10$^{−5}$ < x < 3.3 × 10$^{−2}$. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.[graphic not available: see fulltext