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

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

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

Medium modification of the shape of small-radius jets in central Pb-Pb collisions at sNN=2.76 TeV\sqrt{s_{\mathrm {NN}}} = 2.76\,\rm{TeV}

We present the measurement of a new set of jet shape observables for track-based jets in central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV. The set of jet shapes includes the first radial moment or angularity, g, the momentum dispersion, p$_{T}$D, and the difference between the leading and sub-leading constituent track transverse momentum, LeSub. These observables provide complementary information on the jet fragmentation and can constrain different aspects of the theoretical description of jet-medium interactions. The jet shapes were measured for a small resolution parameter R = 0.2 and were fully corrected to particle level. The observed jet shape modifications indicate that in-medium fragmentation is harder and more collimated than vacuum fragmentation as obtained by PYTHIA calculations, which were validated with the measurements of the jet shapes in proton-proton collisions at $\sqrt{s}=7$ TeV. The comparison of the measured distributions to templates for quark and gluon-initiated jets indicates that in-medium fragmentation resembles that of quark jets in vacuum. We further argue that the observed modifications are not consistent with a totally coherent energy loss picture where the jet loses energy as a single colour charge, suggesting that the medium resolves the jet structure at the angular scales probed by our measurements (R = 0.2). Furthermore, we observe that small-R jets can help to isolate purely energy loss effects from other effects that contribute to the modifications of the jet shower in medium such as the correlated background or medium response

Azimuthal Anisotropy of Heavy-Flavor Decay Electrons in pp-Pb Collisions at sNN \sqrt{s_{\rm NN}} = 5.02 TeV

International audienceAngular correlations between heavy-flavor decay electrons and charged particles at midrapidity (|η|<0.8) are measured in p-Pb collisions at sNN=5.02  TeV. The analysis is carried out for the 0%–20% (high) and 60%–100% (low) multiplicity ranges. The jet contribution in the correlation distribution from high-multiplicity events is removed by subtracting the distribution from low-multiplicity events. An azimuthal modulation remains after removing the jet contribution, similar to previous observations in two-particle angular correlation measurements for light-flavor hadrons. A Fourier decomposition of the modulation results in a positive second-order coefficient (v2) for heavy-flavor decay electrons in the transverse momentum interval 1.5<pT<4  GeV/c in high-multiplicity events, with a significance larger than 5σ. The results are compared with those of charged particles at midrapidity and those of inclusive muons at forward rapidity. The v2 measurement of open heavy-flavor particles at midrapidity in small collision systems could provide crucial information to help interpret the anisotropies observed in such systems

Measurement of prompt D0^{0}, D+^{+}, D∗+^{*+}, and DS+ {\mathrm{D}}_{\mathrm{S}}^{+} production in p–Pb collisions at sNN \sqrt{{\mathrm{s}}_{\mathrm{NN}}} = 5.02 TeV

International audienceThe measurement of the production of prompt D$^{0}$, D$^{+}$, D$^{*+}$, and ${\mathrm{D}}_{\mathrm{S}}^{+}$ mesons in proton–lead (p–Pb) collisions at the centre-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV, with an integrated luminosity of 292 ± 11 μb$^{−1}$, are reported. Differential production cross sections are measured at mid-rapidity (−0.96 < y$_{cms}$< 0.04) as a function of transverse momentum (p$_{T}$) in the intervals 0 < p$_{T}$< 36 GeV/c for D$^{0}$, 1 < p$_{T}$< 36 GeV/c for D$^{+}$ and D$^{*+}$, and 2 < p$_{T}$< 24 GeV/c for D$^{+}$ mesons. For each species, the nuclear modification factor R$_{pPb}$ is calculated as a function of p$_{T}$ using a proton-proton (pp) ref- erence measured at the same collision energy. The results are compatible with unity in the whole p$_{T}$ range. The average of the non-strange D mesons R$_{pPb}$ is compared with theoretical model predictions that include initial-state effects and parton transport model predictions. The p$_{T}$ dependence of the D$^{0}$, D$^{+}$, and D$^{*+}$ nuclear modification factors is also reported in the interval 1 < p$_{T}$< 36 GeV/c as a function of the collision centrality, and the central-to-peripheral ratios are computed from the D-meson yields measured in different centrality classes. The results are further compared with charged-particle measurements and a similar trend is observed in all the centrality classes. The ratios of the p$_{T}$-differential cross sections of D$^{0}$, D$^{+}$, D$^{*+}$, and ${\mathrm{D}}_{\mathrm{S}}^{+}$ mesons are also reported. The ${\mathrm{D}}_{\mathrm{S}}^{+}$ and D$^{+}$ yields are compared as a function of the charged-particle multiplicity for several p$_{T}$ intervals. No modification in the relative abundances of the four species is observed with respect to pp collisions within the statistical and systematic uncertainties

Multiplicity dependence of (multi-)strange hadron production in proton-proton collisions at s\sqrt{s} = 13 TeV

The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity ($\left| y\right| < 0.5$) are measured in proton-proton collisions at $\sqrt{s}$ = 13 TeV as a function of the charged particle multiplicity, using the ALICE detector at the LHC. The production rates of $\mathrm{K}^{0}_{S}$, $\Lambda$, $\Xi$, and $\Omega$ increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV

Production of charged pions, kaons, and (anti-)protons in Pb-Pb and inelastic pppp collisions at sNN\sqrt {s_{NN}} = 5.02 TeV

International audienceMid-rapidity production of $\pi^{\pm}$, $\rm{K}^{\pm}$ and ($\bar{\rm{p}}$)p measured by the ALICE experiment at the LHC, in Pb-Pb and inelastic pp collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV, is presented. The invariant yields are measured over a wide transverse momentum ($p_{\rm{T}}$) range from hundreds of MeV/$c$ up to 20 GeV/$c$. The results in Pb-Pb collisions are presented as a function of the collision centrality, in the range 0$-$90%. The comparison of the $p_{\rm{T}}$-integrated particle ratios, i.e. proton-to-pion (p/$\pi$) and kaon-to-pion (K/$\pi$) ratios, with similar measurements in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV show no significant energy dependence. Blast-wave fits of the $p_{\rm{T}}$ spectra indicate that in the most central collisions radial flow is slightly larger at 5.02 TeV with respect to 2.76 TeV. Particle ratios (p/$\pi$, K/$\pi$) as a function of $p_{\rm{T}}$ show pronounced maxima at $p_{\rm{T}}$ $\approx$ 3 GeV/$c$ in central Pb-Pb collisions. At high $p_{\rm{T}}$, particle ratios at 5.02 TeV are similar to those measured in pp collisions at the same energy and in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. Using the pp reference spectra measured at the same collision energy of 5.02 TeV, the nuclear modification factors for the different particle species are derived. Within uncertainties, the nuclear modification factor is particle species independent for high $p_{\rm{T}}$ and compatible with measurements at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily