Multiplicity dependence of light (anti-)nuclei production in p–Pb collisions at sNN=5.02 TeV

The measurement of the deuteron and anti-deuteron production in the rapidity range −1 < y < 0 as a function of transverse momentum and event multiplicity in p–Pb collisions at √sNN = 5.02 TeV is presented. (Anti-)deuterons are identified via their specific energy loss dE/dx and via their time-of- flight. Their production in p–Pb collisions is compared to pp and Pb–Pb collisions and is discussed within the context of thermal and coalescence models. The ratio of integrated yields of deuterons to protons (d/p) shows a significant increase as a function of the charged-particle multiplicity of the event starting from values similar to those observed in pp collisions at low multiplicities and approaching those observed in Pb–Pb collisions at high multiplicities. The mean transverse particle momenta are extracted from the deuteron spectra and the values are similar to those obtained for p and particles. Thus, deuteron spectra do not follow mass ordering. This behaviour is in contrast to the trend observed for non-composite particles in p–Pb collisions. In addition, the production of the rare 3He and 3He nuclei has been studied. The spectrum corresponding to all non-single diffractive p-Pb collisions is obtained in the rapidity window −1 < y < 0 and the pT-integrated yield dN/dy is extracted. It is found that the yields of protons, deuterons, and 3He, normalised by the spin degeneracy factor, follow an exponential decrease with mass number

Dielectron production in proton-proton collisions at s=7 \sqrt{s}=7 TeV

The first measurement of e$^{+}$e$^{−}$ pair production at mid-rapidity (|η$_{e}$| < 0.8) in pp collisions at $\sqrt{s}=7$ TeV with ALICE at the LHC is presented. The dielectron production is studied as a function of the invariant mass (m$_{ee}$ < 3.3 GeV/c$^{2}$), the pair transverse momentum (p$_{T,ee}$ < 8 GeV/c), and the pair transverse impact parameter (DCA$_{ee}$), i.e., the average distance of closest approach of the reconstructed electron and positron tracks to the collision vertex, normalised to its resolution. The results are compared with the expectations from a cocktail of known hadronic sources and are well described when PYTHIA is used to generate the heavy-flavour contributions. In the low-mass region (0.14 < m$_{ee}$ < 1.1 GeV/c$^{2}$), prompt and non-prompt e$^{+}$e$^{−}$ sources can be separated via the DCA$_{ee}$. In the intermediate-mass region (1.1 < m$_{ee}$ < 2.7 GeV/c$^{2}$), a double-differential fit to the data in m$_{ee}$ and p$_{T,ee}$ and a fit of the DCA$_{ee}$ distribution allow the total $\mathrm{c}\overline{\mathrm{c}}$ and $\mathrm{b}\overline{\mathrm{b}}$ cross sections to be extracted. Two different event generators, PYTHIA and POWHEG, can reproduce the shape of the two-dimensional m$_{ee}$ and p$_{T,ee}$ spectra, as well as the shape of the DCA$_{ee}$ distribution, reasonably well. However, differences in the $\mathrm{c}\overline{\mathrm{c}}$ and $\mathrm{b}\overline{\mathrm{b}}$ cross sections are observed when using the generators to extrapolate to full phase space. Finally, the ratio of inclusive to decay photons is studied via the measurement of virtual direct photons in the transverse-momentum range 1 < p$_{T}$ < 8 GeV/c. This is found to be unity within the statistical and systematic uncertainties and consistent with expectations from next-to-leading order perturbative quantum chromodynamic calculations

Inclusive J/ψ\psi production in Xe–Xe collisions at sNN\sqrt{s_{\rm NN}} = 5.44 TeV

Inclusive J/$\psi$ production is studied in Xe-Xe interactions at a centre-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}}= 5.44$ TeV, using the ALICE detector at the CERN LHC. The J/$\psi$ meson is reconstructed via its decay into a muon pair, in the centre-of-mass rapidity interval $2.5<y<4$ and down to zero transverse momentum. In this Letter, the nuclear modification factors $R_{\rm AA}$ for inclusive J/$\psi$, measured in the centrality range 0-90% as well as in the centrality intervals 0-20% and 20-90% are presented. The $R_{\rm AA}$ values are compared to previously published results for Pb-Pb collisions at $\sqrt{s_{\rm NN}}= 5.02$ TeV and to the calculation of a transport model. A good agreement is found between Xe-Xe and Pb-Pb results as well as between data and the model

Anisotropic flow in Xe-Xe collisions at sNN=5.44\mathbf{\sqrt{s_{\rm{NN}}} = 5.44} TeV

The first measurements of anisotropic flow coefficients vn for mid-rapidity charged particles in Xe–Xe collisions at sNN=5.44 TeV are presented. Comparing these measurements to those from Pb–Pb collisions at sNN=5.02 TeV, v2 is found to be suppressed for mid-central collisions at the same centrality, and enhanced for central collisions. The values of v3 are generally larger in Xe–Xe than in Pb–Pb at a given centrality. These observations are consistent with expectations from hydrodynamic predictions. When both v2 and v3 are divided by their corresponding eccentricities for a variety of initial state models, they generally scale with transverse density when comparing Xe–Xe and Pb–Pb, with some deviations observed in central Xe–Xe and Pb–Pb collisions. These results assist in placing strong constraints on both the initial state geometry and medium response for relativistic heavy-ion collisions

Measurement of the inclusive J/ ψ\psi polarization at forward rapidity in pp collisions at s=8\mathbf {\sqrt{s} = 8} TeV

We report on the measurement of the inclusive J/ $\psi$ polarization parameters in pp collisions at a center of mass energy $\sqrt{s} = 8$  TeV with the ALICE detector at the LHC. The analysis is based on a data sample corresponding to an integrated luminosity of 1.23 pb$^{-1}$ . J/ $\psi$ resonances are reconstructed in their di-muon decay channel in the rapidity interval $2.5< y < 4.0$ and over the transverse-momentum interval $2< p_\mathrm{T} < 15$   $\mathrm {GeV}/c$ . The three polarization parameters ( $\lambda _\theta$ , $\lambda _\varphi$ , $\lambda _{\theta \varphi }$ ) are measured as a function of $p_\mathrm{T}$ both in the helicity and Collins-Soper reference frames. The measured J/ $\psi$ polarization parameters are found to be compatible with zero within uncertainties, contrary to expectations from all available predictions. The results are compared with the measurement in pp collisions at $\sqrt{s} = 7$  TeV

Inclusive J/ψ\psi production in Xe–Xe collisions at sNN\sqrt{s_{\rm NN}} = 5.44 TeV

Inclusive J/$\psi$ production is studied in Xe-Xe interactions at a centre-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}}= 5.44$ TeV, using the ALICE detector at the CERN LHC. The J/$\psi$ meson is reconstructed via its decay into a muon pair, in the centre-of-mass rapidity interval $2.5<y<4$ and down to zero transverse momentum. In this Letter, the nuclear modification factors $R_{\rm AA}$ for inclusive J/$\psi$, measured in the centrality range 0-90% as well as in the centrality intervals 0-20% and 20-90% are presented. The $R_{\rm AA}$ values are compared to previously published results for Pb-Pb collisions at $\sqrt{s_{\rm NN}}= 5.02$ TeV and to the calculation of a transport model. A good agreement is found between Xe-Xe and Pb-Pb results as well as between data and the model

Inclusive J/ψ\psi production at forward and backward rapidity in p-Pb collisions at sNN\sqrt{s_{\rm NN}} = 8.16 TeV

Inclusive J/ψ production is studied in p-Pb interactions at a centre-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\mathrm{NN}}}=8.16$ TeV, using the ALICE detector at the CERN LHC. The J/ψ meson is reconstructed, via its decay to a muon pair, in the centre-of-mass rapidity intervals 2.03 < y$_{cms}$ < 3.53 and −4.46 < y$_{cms}$ < −2.96, where positive and negative y$_{cms}$ refer to the p-going and Pb-going direction, respectively. The transverse momentum coverage is p$_{T}$ < 20 GeV/c. In this paper, y$_{cms}$- and p$_{T}$-differential cross sections for inclusive J/ψ production are presented, and the corresponding nuclear modification factors R$_{pPb}$ are shown. Forward results show a suppression of the J/ψ yield with respect to pp collisions, concentrated in the region p$_{T}$ ≲ 5 GeV/c. At backward rapidity no significant suppression is observed. The results are compared to previous measurements by ALICE in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV and to theoretical calculations. Finally, the ratios R$_{FB}$ between forward- and backward-y$_{cms}$ R$_{pPb}$ values are shown and discussed

Anisotropic flow in Xe-Xe collisions at sNN=5.44\mathbf{\sqrt{s_{\rm{NN}}} = 5.44} TeV

The first measurements of anisotropic flow coefficients vn for mid-rapidity charged particles in Xe–Xe collisions at sNN=5.44 TeV are presented. Comparing these measurements to those from Pb–Pb collisions at sNN=5.02 TeV, v2 is found to be suppressed for mid-central collisions at the same centrality, and enhanced for central collisions. The values of v3 are generally larger in Xe–Xe than in Pb–Pb at a given centrality. These observations are consistent with expectations from hydrodynamic predictions. When both v2 and v3 are divided by their corresponding eccentricities for a variety of initial state models, they generally scale with transverse density when comparing Xe–Xe and Pb–Pb, with some deviations observed in central Xe–Xe and Pb–Pb collisions. These results assist in placing strong constraints on both the initial state geometry and medium response for relativistic heavy-ion collisions

Suppression of Λ(1520)\Lambda(1520) resonance production in central Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 2.76 TeV

International audienceThe production yield of the Λ(1520) baryon resonance is measured at midrapidity in Pb-Pb collisions at sNN = 2.76 TeV with the ALICE detector at the Large Hadron Collider (LHC). The measurement is performed in the Λ(1520)→pK− (and charge conjugate) hadronic decay channel as a function of the transverse momentum (pT) and collision centrality. The ratio of the pT-integrated production of Λ(1520) baryons relative to Λ baryons in central collisions is suppressed by about a factor of 2 with respect to peripheral collisions. This is the first observation of the suppression of a baryonic resonance at the LHC and the first 3σ evidence of Λ(1520) suppression within a single collision system. The measured Λ(1520)/Λ ratio in central collisions is smaller than the value predicted by the statistical hadronization model calculations. The shape of the measured pT distribution and the centrality dependence of the suppression are reproduced by the EPOS3 Monte Carlo event generator. The measurement adds further support to the formation of a dense hadronic phase in the final stages of the evolution of the fireball created in heavy-ion collisions, lasting long enough to cause a significant reduction in the observable yield of short-lived resonances

Measurement of dielectron production in central Pb-Pb collisions at sNN\sqrt{{\textit{s}}_{\mathrm{NN}}} = 2.76 TeV

The first measurement of dielectron (e+e−) production in central (0–10%) Pb–Pb collisions at sNN=2.76TeV at the LHC is presented. The dielectron invariant-mass spectrum is compared to the expected contributions from hadron decays in the invariant-mass range 0<mee<3.5GeV/c2. The ratio of data and the cocktail of hadronic contributions without vacuum ρ0 is measured in the invariant-mass range 0.15<mee<0.7GeV/c2, where an excess of dielectrons is observed in other experiments, and its value is 1.40±0.28(stat.)±0.08(syst.)±0.27(cocktail). The dielectron spectrum measured in the invariant mass range 0<mee<1GeV/c2 is consistent with the predictions from two theoretical model calculations that include thermal dielectron production from both partonic and hadronic phases with in-medium broadened ρ0 meson. The fraction of direct virtual photons over inclusive virtual photons is extracted for dielectron pairs with invariant mass 0.1<mee<0.3GeV/c2 and in the transverse-momentum intervals 1<pT,ee<2GeV/c and 2<pT,ee<4GeV/c. The measured fraction of virtual direct photons is consistent with the measurement of real direct photons by ALICE and with the expectations from previous dielectron measurements at RHIC within the experimental uncertainties