• Energy Research

The transverse-momentum ($p_{\rm T}$) spectra and coalescence parameters $B_2$ of (anti)deuterons are measured in pp collisions at $\sqrt{s} = 13$ TeV for the first time in and out of jets. In this measurement, the direction of the leading particle with the highest $p_{\rm T}$ in the event ($p_{\rm T}^{\rm{ lead}} > 5$ GeV/$c$) is used as an approximation for the jet axis. The event is consequently divided into three azimuthal regions and the jet signal is obtained as the difference between the Toward region, that contains jet fragmentation products in addition to the underlying event (UE), and the Transverse region, which is dominated by the UE. The coalescence parameter in the jet is found to be approximately a factor of 10 larger than that in the underlying event. This experimental observation is consistent with the coalescence picture and can be attributed to the smaller average phase-space distance between nucleons inside the jet cone as compared to the underlying event. The results presented in this Letter are compared to predictions from a simple nucleon coalescence model, where the phase space distributions of nucleons are generated using PYTHIA 8 with the Monash 2013 tuning, and to predictions from a deuteron production model based on ordinary nuclear reactions with parametrized energy-dependent cross sections tuned on data. The latter model is implemented in PYTHIA 8.3. Both models reproduce the observed large difference between in-jet and out-of-jet coalescence parameters, although the almost flat trend of the $B^{\rm Jet}_2$ is not reproduced by the models, which instead give a decreasing trend. 18 pages, 2 captioned figures, authors from page 13, published version + fix from erratum, figures at http://alice-publications.web.cern.ch/node/8658

The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of $J/��$ vector mesons off proton targets in ultra-peripheral p-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV. The e$^+$e$^-$ and $��^+��^-$ decay channels are used to measure the cross section as a function of the rapidity of the $J/��$ in the range $-2.5 < y < 2.7$, corresponding to an energy in the $��$p centre-of-mass in the interval $40 < W_{��\mathrm{p}}<550$ GeV. The measurements, which are consistent with a power law dependence of the exclusive $J/��$ photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements. 25 pages, 3 captioned figures, 3 tables, authors from page 19, published version, figures at http://alice-publications.web.cern.ch/node/4554

Abstract Measurements of anisotropic flow coefficients with two- and multi-particle cumulants for inclusive charged particles in Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ s N N = 5.02 and 2.76 TeV are reported in the pseudorapidity range |η| < 0.8 and transverse momentum 0.2 < p T < 50 GeV/c. The full data sample collected by the ALICE detector in 2015 (2010), corresponding to an integrated luminosity of 12.7 (2.0) μb−1 in the centrality range 0-80%, is analysed. Flow coefficients up to the sixth flow harmonic (v 6) are reported and a detailed comparison among results at the two energies is carried out. The p T dependence of anisotropic flow coefficients and its evolution with respect to centrality and harmonic number n are investigated. An approximate power-law scaling of the form v n (p T) ∼ p T n/3 is observed for all flow harmonics at low p T (0.2 < p T < 3 GeV/c). At the same time, the ratios v n /v m n/ m are observed to be essentially independent of p T for most centralities up to about p T = 10 GeV/c. Analysing the differences among higher-order cumulants of elliptic flow (v 2), which have different sensitivities to flow fluctuations, a measurement of the standardised skewness of the event-by-event v 2 distribution P(v 2) is reported and constraints on its higher moments are provided. The Elliptic Power distribution is used to parametrise P(v 2), extracting its parameters from fits to cumulants. The measurements are compared to different model predictions in order to discriminate among initial-state models and to constrain the temperature dependence of the shear viscosity to entropy-density ratio.

Abstract Inclusive J/ψ production is studied in minimum-bias proton-proton collisions at a centre-of-mass energy of $$ \sqrt{s} $$ s = 5.02 TeV by ALICE at the CERN LHC. The measurement is performed at mid-rapidity (|y| < 0.9) in the dielectron decay channel down to zero transverse momentum p T, using a data sample corresponding to an integrated luminosity of L int = 19.4 ± 0.4 nb −1. The measured p T-integrated inclusive J/ψ production cross sec- tion is dσ/dy = 5.64 ± 0.22(stat.) ± 0.33(syst.) ± 0.12(lumi.) μb. The p T-differential cross section d2 σ/dp Tdy is measured in the p T range 0–10 GeV/c and compared with state-of- the-art QCD calculations. The J/ψ 〈p T〉 and $$ \left\langle {p}_{\mathrm{T}}^2\right\rangle $$ p T 2 are extracted and compared with results obtained at other collision energies.