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

Measurement of electrons from beauty-hadron decays in p-Pb collisions at sNN=5.02 \sqrt{s_{\mathrm{NN}}}=5.02 TeV and Pb-Pb collisions at sNN=2.76 \sqrt{s_{\mathrm{NN}}}=2.76 TeV

The production of beauty hadrons was measured via semi-leptonic decays at mid-rapidity with the ALICE detector at the LHC in the transverse momentum interval 1<p$_{T}$ < 8 GeV/c in minimum-bias p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV and in 1.3 < p$_{T}$ < 8 GeV/c in the 20% most central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV. The pp reference spectra at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV and $\sqrt{s}=2.76$ TeV, needed for the calculation of the nuclear modification factors R$_{pPb}$ and R$_{PbPb}$, were obtained by a pQCD-driven scaling of the cross section of electrons from beauty-hadron decays measured at $\sqrt{s}=7$ TeV. In the p$_{T}$ interval 3 < p$_{T}$ < 8 GeV/c, a suppression of the yield of electrons from beauty-hadron decays is observed in Pb-Pb compared to pp collisions. Towards lower p$_{T}$, the R$_{PbPb}$ values increase with large systematic uncertainties. The R$_{pPb}$ is consistent with unity within systematic uncertainties and is well described by theoretical calculations that include cold nuclear matter effects in p-Pb collisions. The measured R$_{pPb}$ and these calculations indicate that cold nuclear matter effects are small at high transverse momentum also in Pb-Pb collisions. Therefore, the observed reduction of R$_{PbPb}$ below unity at high p$_{T}$ may be ascribed to an effect of the hot and dense medium formed in Pb-Pb collisions

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

π0\pi ^{0} and η\eta meson production in proton-proton collisions at s=8\sqrt{s}=8 TeV

An invariant differential cross section measurement of inclusive $\pi ^{0}$ and $\eta$ meson production at mid-rapidity in pp collisions at $\sqrt{s}=8$  TeV was carried out by the ALICE experiment at the LHC. The spectra of $\pi ^{0}$ and $\eta$ mesons were measured in transverse momentum ranges of $0.33.5$   $\text{ GeV/c }$ . However, a deviation from this empirical scaling rule is observed for transverse momenta below $p_{ \text{ T }} <3.5$   $\text{ GeV/c }$ in the $\eta /\pi ^0$ ratio with a significance of $6.2\sigma$

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

W and Z boson production in p-Pb collisions at sNN\sqrt{s_{\rm NN}} = 5.02 TeV

The W and Z boson production was measured via the muonic decay channel in proton-lead collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV at the Large Hadron Collider with the ALICE detector. The measurement covers backward (−4.46 10 GeV/c are determined. The results are compared to theoretical calculations both with and without including the nuclear modification of the parton distribution functions. The W-boson production is also studied as a function of the collision centrality: the cross section of muons from W-boson decays is found to scale with the average number of binary nucleon-nucleon collisions within uncertainties

Evolution of the longitudinal and azimuthal structure of the near-side jet peak in Pb-Pb collisions at sNN=2.76\sqrt{s_{\rm NN}} = 2.76 TeV

In two-particle angular correlation measurements, jets give rise to a near-side peak, formed by particles associated to a higher-pT trigger particle. Measurements of these correlations as a function of pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the centrality and pT dependence of the shape of the near-side peak in the pT range 1<pT<8 GeV/c in Pb-Pb and pp collisions at sNN = 2.76 TeV. A combined fit of the near-side peak and long-range correlations is applied to the data and the peak shape is quantified by the variance of the distributions. While the width of the peak in the Δφ direction is almost independent of centrality, a significant broadening in the Δη direction is found from peripheral to central collisions. This feature is prominent for the low-pT region and vanishes above 4 GeV/c. The widths measured in peripheral collisions are equal to those in pp collisions in the Δφ direction and above 3 GeV/c in the Δη direction. Furthermore, for the 10% most central collisions and 1<pT,assoc< 2 GeV/c, 1<pT,trig< 3 GeV/c, a departure from a Gaussian shape is found: a depletion develops around the center of the peak. The results are compared to A Multi-Phase Transport (AMPT) model simulation as well as other theoretical calculations indicating that the broadening and the development of the depletion are connected to the strength of radial and longitudinal flow

Anomalous evolution of the near-side jet peak shape in Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 2.76 TeV

The measurement of two-particle angular correlations is a powerful tool to study jet quenching in a pT region inaccessible by direct jet identification. In these measurements pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the shape of the near-side peak formed by particles associated with a higher pT trigger particle (1<pT,trig<8 GeV/c). A combined fit of the near-side peak and long-range correlations is applied to the data allowing the extraction of the centrality evolution of the peak shape in Pb-Pb collisions at sNN=2.76 TeV. A significant broadening of the peak in the Δη direction at low pT is found from peripheral to central collisions, which vanishes above 4 GeV/c, while in the Δφ direction the peak is almost independent of centrality. For the 10% most central collisions and 1<pT,assoc<2 GeV/c, 1<pT,trig<3 GeV/c a novel feature is observed: a depletion develops around the center of the peak. The results are compared to pp collisions at the same center of mass energy and ampt model simulations. The comparison to the investigated models suggests that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow