Elliptic flow of charged particles at midrapidity relative to the spectator plane in Pb–Pb and Xe–Xe collisions

Measurements of the elliptic flow coefficient relative to the collision plane defined by the spectator neutrons v2{ SP} in collisions of Pb ions at center-of-mass energy per nucleon–nucleon pair √ 2.76 TeV and Xe ions at √ sNN = sNN =5.44 TeV are reported. The results are presented for charged particles produced at midrapidity as a function of centrality and transverse momentum for the 5–70% and 0.2–6 GeV/c ranges, respectively. The ratio between v2{ SP} and the elliptic flow coefficient relative to the participant plane v2{4}, estimated using four-particle correlations, deviates by up to 20% from unity depending on centrality. This observation differs strongly from the magnitude of the corresponding eccentricity ratios predicted by the TRENTo and the elliptic power models of initial state fluctuations that are tuned to describe the participant plane anisotropies. The differences can be interpreted as a decorrelation of the neutron spectator plane and the reaction plane because of fragmentation of the remnants from the colliding nuclei, which points to an incompleteness of current models describing the initial state fluctuations. A significant transverse momentum dependence of the ratio v2{ SP}/v2{4} is observed in all but the most central collisions, which may help to understand whether momentum anisotropies at low and intermediate transverse momentum have a common origin in initial state f luctuations. The ratios of v2{ SP} and v2{4} to the corresponding initial state eccentricities for Xe–Xe and Pb–Pb collisions at similar initial entropy density show a difference of (7.0 ±0.9)%with an additional variation of +1.8% when including RHIC data in the TRENTo parameter extraction. These observations provide new experimental constraints for viscous effects in the hydrodynamic modeling of the expanding quark–gluon plasma produced in heavy-ion collisions at the LHC

J/psi production as a function of charged-particle pseudorapidity density in p-Pb collisions at root s(NN)=5.02 TeV

We report measurements of the inclusive J/ψ yield and average transverse momentum as a function of charged-particle pseudorapidity density dNch/dη in p–Pb collisions at sNN=5.02TeV with ALICE at the LHC. The observables are normalised to their corresponding averages in non-single diffractive events. An increase of the normalised J/ψ yield with normalised dNch/dη, measured at mid-rapidity, is observed at mid-rapidity and backward rapidity. At forward rapidity, a saturation of the relative yield is observed for high charged-particle multiplicities. The normalised average transverse momentum at forward and backward rapidities increases with multiplicity at low multiplicities and saturates beyond moderate multiplicities. In addition, the forward-to-backward nuclear modification factor ratio is also reported, showing an increasing suppression of J/ψ production at forward rapidity with respect to backward rapidity for increasing charged-particle multiplicity

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

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

Measurement of ψ\psi(2S) production as a function of charged-particle pseudorapidity density in pp collisions at s\sqrt{s} = 13 TeV and p-Pb collisions at sNN\sqrt{s_{\rm NN}} = 8.16 TeV with ALICE at the LHC

International audienceProduction of inclusive charmonia in pp collisions at center-of-mass energy of $\sqrt{s}$ = 13 TeV and p–Pb collisions at center-of-mass energy per nucleon pair of $\sqrt{s_{\textrm{NN}}}$ = 8.16 TeV is studied as a function of charged-particle pseudorapidity density with ALICE. Ground and excited charmonium states (J/ψ, ψ(2S)) are measured from their dimuon decays in the interval of rapidity in the center-of-mass frame 2.5 < y$_{cms}$< 4.0 for pp collisions, and 2.03 < y$_{cms}$< 3.53 and −4.46 < y$_{cms}$< −2.96 for p–Pb collisions. The charged-particle pseudorapidity density is measured around midrapidity (|η| < 1.0). In pp collisions, the measured charged-particle multiplicity extends to about six times the average value, while in p-Pb collisions at forward (backward) rapidity a multiplicity corresponding to about three (four) times the average is reached. The ψ(2S) yield increases with the charged-particle pseudorapidity density. The ratio of ψ(2S) over J/ψ yield does not show a significant multiplicity dependence in either colliding system, suggesting a similar behavior of J/ψ and ψ(2S) yields with respect to charged-particle pseudorapidity density. Results for the ψ(2S) yield and its ratio with respect to J/ψ agree with available model calculations.[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

Measurements of the groomed jet radius and momentum splitting fraction with the soft drop and dynamical grooming algorithms in pp collisions at s \sqrt{s} = 5.02 TeV

International audienceThis article presents measurements of the groomed jet radius and momentum splitting fraction in pp collisions at $\sqrt{s}$ = 5.02 TeV with the ALICE detector at the Large Hadron Collider. Inclusive charged-particle jets are reconstructed at midrapidity using the anti-k$_{T}$ algorithm for transverse momentum $60<{p}_{\textrm{T}}^{\textrm{ch}\;\textrm{jet}}<80$ GeV/c. We report results using two different grooming algorithms: soft drop and, for the first time, dynamical grooming. For each grooming algorithm, a variety of grooming settings are used in order to explore the impact of collinear radiation on these jet substructure observables. These results are compared to perturbative calculations that include resummation of large logarithms at all orders in the strong coupling constant. We find good agreement of the theoretical predictions with the data for all grooming settings considered.[graphic not available: see fulltext