Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Relative particle yield fluctuations in Pb-Pb collisions at sNN=2.76\sqrt{s_{\rm NN}} = 2.76 TeV

First results on K/$\pi$, p/$\pi$ and K/p fluctuations are obtained with the ALICE detector at the CERN LHC as a function of centrality in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. The observable $\nu_{\rm dyn}$, which is defined in terms of the moments of particle multiplicity distributions, is used to quantify the magnitude of dynamical fluctuations of relative particle yields and also provides insight into the correlation between particle pairs. This study is based on a novel experimental technique, called the Identity Method, which allows one to measure the moments of multiplicity distributions in case of incomplete particle identification. The results for p/$\pi$ show a change of sign in $\nu_{\rm dyn}$ from positive to negative towards more peripheral collisions. For central collisions, the results follow the smooth trend of the data at lower energies and $\nu_{\rm dyn}$ exhibits a change in sign for p/$\pi$ and K/p.First results on $\hbox {K}/\pi$ , $\hbox {p}/\pi$ and K/p fluctuations are obtained with the ALICE detector at the CERN LHC as a function of centrality in $\text{ Pb--Pb }$ collisions at $\sqrt{s_\mathrm{{NN}}} =2.76\hbox { TeV}$ . The observable $\nu _{\mathrm{dyn}}$ , which is defined in terms of the moments of particle multiplicity distributions, is used to quantify the magnitude of dynamical fluctuations of relative particle yields and also provides insight into the correlation between particle pairs. This study is based on a novel experimental technique, called the Identity Method, which allows one to measure the moments of multiplicity distributions in case of incomplete particle identification. The results for $\hbox {p}/\pi$ show a change of sign in $\nu _{\mathrm{dyn}}$ from positive to negative towards more peripheral collisions. For central collisions, the results follow the smooth trend of the data at lower energies and $\nu _{\mathrm{dyn}}$ exhibits a change in sign for $\hbox {p}/\pi$ and K/p

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

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

Analysis of the apparent nuclear modification in peripheral Pb–Pb collisions at 5.02 TeV

International audienceCharged-particle spectra at midrapidity are measured in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair sNN=5.02 TeV and presented in centrality classes ranging from most central (0–5%) to most peripheral (95–100%) collisions. Possible medium effects are quantified using the nuclear modification factor ( RAA ) by comparing the measured spectra with those from proton–proton collisions, scaled by the number of independent nucleon–nucleon collisions obtained from a Glauber model. At large transverse momenta ( 8<pT<20GeV/c ), the average RAA is found to increase from about 0.15 in 0–5% central to a maximum value of about 0.8 in 75–85% peripheral collisions, beyond which it falls off strongly to below 0.2 for the most peripheral collisions. Furthermore, RAA initially exhibits a positive slope as a function of pT in the 8–20 GeV/c interval, while for collisions beyond the 80% class the slope is negative. To reduce uncertainties related to event selection and normalization, we also provide the ratio of RAA in adjacent centrality intervals. Our results in peripheral collisions are consistent with a PYTHIA-based model without nuclear modification, demonstrating that biases caused by the event selection and collision geometry can lead to the apparent suppression in peripheral collisions. This explains the unintuitive observation that RAA is below unity in peripheral Pb–Pb, but equal to unity in minimum-bias p–Pb collisions despite similar charged-particle multiplicities

Anisotropic flow of identified particles in Pb-Pb collisions at sNN=5.02 {\sqrt{s}}_{\mathrm{NN}}=5.02 TeV

The elliptic (v$_{2}$), triangular (v$_{3}$), and quadrangular (v$_{4}$) flow coefficients of π$^{±}$, K$^{±}$, $\mathrm{p}+\overline{\mathrm{p}},\kern0.5em \Lambda +\overline{\Lambda},\kern0.5em {\mathrm{K}}_{\mathrm{S}}^0$ , and the ϕ-meson are measured in Pb-Pb collisions at ${\sqrt{s}}_{\mathrm{NN}}=5.02$ TeV. Results obtained with the scalar product method are reported for the rapidity range |y| < 0.5 as a function of transverse momentum, p$_{T}$, at different collision centrality intervals between 0–70%, including ultra-central (0–1%) collisions for π$^{±}$, K$^{±}$, and $\mathrm{p}+\overline{\mathrm{p}}$ . For p$_{T}$ < 3 GeV/c, the flow coefficients exhibit a particle mass dependence. At intermediate transverse momenta (3 < p$_{T}$ < 8–10 GeV/c), particles show an approximate grouping according to their type (i.e., mesons and baryons). The ϕ-meson v$_{2}$, which tests both particle mass dependence and type scaling, follows $\mathrm{p}+\overline{\mathrm{p}}$ v$_{2}$ at low p$_{T}$ and π$^{±}$ v$_{2}$ at intermediate p$_{T}$. The evolution of the shape of v$_{n}$(p$_{T}$) as a function of centrality and harmonic number n is studied for the various particle species. Flow coefficients of π$^{±}$, K$^{±}$, and $\mathrm{p}+\overline{\mathrm{p}}$ for p$_{T}$ < 3 GeV/c are compared to iEBE-VISHNU and MUSIC hydrodynamical calculations coupled to a hadronic cascade model (UrQMD). The iEBE-VISHNU calculations describe the results fairly well for p$_{T}$ < 2.5 GeV/c, while MUSIC calculations reproduce the measurements for p$_{T}$ < 1 GeV/c. A comparison to v$_{n}$ coefficients measured in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV is also provided

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

First measurement of Ξc0\Xi_{\rm c}^0 production in pp collisions at s\mathbf{\sqrt{s}} = 7 TeV

The production of the charm-strange baryon Ξc0 is measured for the first time at the LHC via its semileptonic decay into e Ξ−+νe in pp collisions at s=7 TeV with the ALICE detector. The transverse momentum ( pT ) differential cross section multiplied by the branching ratio is presented in the interval 1<pT<8 GeV/c at mid-rapidity, |y|<0.5 . The transverse momentum dependence of the Ξc0 baryon production relative to the D0 meson production is compared to predictions of event generators with various tunes of the hadronisation mechanism, which are found to underestimate the measured cross-section ratio

Production of the ρ\rho(770)0{^{0}} meson in pp and Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 2.76 TeV

The production of the $\rho(770)^{0}$ meson has been measured at mid-rapidity (|y| < 0.5) in pp and centrality differential Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV with the ALICE detector at the Large Hadron Collider. The particles have been reconstructed in the $\rho(770)^{0}\rightarrow \pi^{+}\pi^{-}$ decay channel in the transverse momentum ($p_{T}$) range 0.5 − 11 GeV/$c$. A centrality dependent suppression of the ratio of the integrated yields $2\rho(770)^{0}/(\pi^{+}+\pi^{-})$ is observed. The ratio decreases by ~40% from pp to central Pb-Pb collisions. A study of the $p_{T}$-differential $2\rho(770)^{0}/(\pi^{+}+\pi^{-})$ ratio reveals that the suppression occurs at low transverse momenta, $p_{T}$ < 2 GeV/$c$. At higher momentum, particle ratios measured in heavy-ion and pp collisions are consistent. The observed suppression is very similar to that previously measured for the $K^{*}(892)^{0}/K$ ratio and is consistent with EPOS3 predictions that may imply that rescattering in the hadronic phase is a dominant mechanism for the observed suppression.The production of the $\rho$(770)${^{0}}$ meson has been measured at mid-rapidity $(|y|<0.5)$ in pp and centrality differential Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV with the ALICE detector at the Large Hadron Collider. The particles have been reconstructed in the $\rho$(770)$\rightarrow\pi^{+}\pi^{-}$ decay channel in the transverse momentum ($p_{T}$) range $0.5-11$ GeV/$c$. A centrality dependent suppression of the ratio of the integrated yields $2\rho$(770)$^{0}/(\pi^{+}+\pi^{-})$ is observed. The ratio decreases by $\sim40\%$ from pp to central Pb-Pb collisions. A study of the $p_{T}$-differential $2\rho$(770)$^{0}/(\pi^{+}+\pi^{-})$ ratio reveals that the suppression occurs at low transverse momenta, $p_{T}<2$ GeV/$c$. At higher momentum, particle ratios measured in heavy-ion and pp collisions are consistent. The observed suppression is very similar to that previously measured for the $K^{*}$(892)$^{0}/K$ ratio and is consistent with EPOS3 predictions that may imply that rescattering in the hadronic phase is a dominant mechanism for the observed suppression.The production of the ρ(770)0 meson has been measured at midrapidity (|y|<0.5) in pp and centrality differential Pb-Pb collisions at sNN= 2.76 TeV with the ALICE detector at the Large Hadron Collider. The particles have been reconstructed in the ρ(770)0→π+π− decay channel in the transverse-momentum (pT) range 0.5–11 GeV/c. A centrality-dependent suppression of the ratio of the integrated yields 2ρ(770)0/(π++π−) is observed. The ratio decreases by ∼40% from pp to central Pb-Pb collisions. A study of the pT-differential 2ρ(770)0/(π++π−) ratio reveals that the suppression occurs at low transverse momenta, pT<2 GeV/c. At higher momentum, particle ratios measured in heavy-ion and pp collisions are consistent. The observed suppression is very similar to that previously measured for the K*(892)0/K ratio and is consistent with EPOS3 predictions that may imply that rescattering in the hadronic phase is a dominant mechanism for the observed suppression

Multiplicity dependence of light-flavor hadron production in pp collisions at s\sqrt{s} = 7 TeV

International audienceComprehensive results on the production of unidentified charged particles, π±, K±, KS0, K*(892)0, p, p¯, ϕ(1020), Λ, Λ¯, Ξ−, Ξ¯+, Ω−, and Ω¯+ hadrons in proton-proton (pp) collisions at s=7 TeV at midrapidity (|y|<0.5) as a function of charged-particle multiplicity density are presented. In order to avoid autocorrelation biases, the actual transverse momentum (pT) spectra of the particles under study and the event activity are measured in different rapidity windows. In the highest multiplicity class, the charged-particle density reaches about 3.5 times the value measured in inelastic collisions. While the yield of protons normalized to pions remains approximately constant as a function of multiplicity, the corresponding ratios of strange hadrons to pions show a significant enhancement that increases with increasing strangeness content. Furthermore, all identified particle-to-pion ratios are shown to depend solely on charged-particle multiplicity density, regardless of system type and collision energy. The evolution of the spectral shapes with multiplicity and hadron mass shows patterns that are similar to those observed in p-Pb and Pb-Pb collisions at Large Hadron Collider energies. The obtained pT distributions and yields are compared to expectations from QCD-based pp event generators as well as to predictions from thermal and hydrodynamic models. These comparisons indicate that traces of a collective, equilibrated system are already present in high-multiplicity pp collisions