The polarization of the Λ and Λ ¯ hyperons along the beam ( z ) direction, Pz , has been measured in Pb-Pb collisions at sNN=5.02 TeV recorded with ALICE at the Large Hadron Collider (LHC). The main ...contribution to Pz comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient Pz,s2= ⟨ Pzsin ( 2φ-2 Ψ 2 ) ⟩ , where φ is the hyperon azimuthal emission angle and Ψ 2 is the elliptic flow plane angle. We report the measurement of Pz,s2 for different collision centralities and in the 30%–50% centrality interval as a function of the hyperon transverse momentum and rapidity. The Pz,s2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at sNN=200 GeV , with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon Pz in Pb-Pb collisions at the LHC. The comparison of the measured Pz,s2 with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear-induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase.
The production of non-prompt ${\rm D}^{0}$ mesons from beauty-hadron decays was measured at midrapidity ($\left| y \right| < 0.5$) in Pb-Pb collisions at a nucleon-nucleon center-of-mass energy of ...$\sqrt{s_{\rm NN}}=5.02~\mathrm{TeV}$ with the ALICE experiment at the LHC. Their nuclear modification factor ($R_{\rm AA}$), measured for the first time down to $p_{\rm T}=1~\mathrm{GeV}/c$ in the $0-10$% and $30-50$% centrality classes, indicates a significant suppression, up to a factor of about three, for $p_{\rm T} > 5~\mathrm{GeV}/c$ in the $0-10$% central Pb-Pb collisions. The data are described by models that include both collisional and radiative processes in the calculation of beauty-quark energy loss in the quark-gluon plasma, and quark recombination in addition to fragmentation as a hadronization mechanism. The ratio of the non-prompt to prompt ${\rm D}^{0}$-meson $R_{\rm AA}$ is larger than unity for $p_{\rm T} > 4~\mathrm{GeV}/c$ in the $0-10$% central Pb-Pb collisions, as predicted by models in which beauty quarks lose less energy than charm quarks in the quark-gluon plasma because of their larger mass.
The aim of this work is to explore possible medium modifications to the substructure of inclusive charged jets in Pb-Pb relative to proton-proton collisions by measuring a set of jet shapes. The set ...of shapes includes the radial moment, g, and the momentum dispersion pTD. They provide complementary information on the fragmentation and can help to discriminate between two different scenarios: intra-jet broadening or collimation as a result of jet quenching. The shapes are measured in Pb–Pb collisions at sNN=2.76 TeV with a constituent cutoff of 0.15 GeV/c and jet resolution R=0.2. New techniques for background subtraction are applied and a two-dimensional unfolding is performed to correct the shapes to particle level. The corrected jet shapes for jet pT 40≤pT,jet≤60 GeV/c are presented and discussed. The observed jet shape modifications suggest that the in-medium fragmentation is harder and more collimated than vacuum fragmentation as obtained by a PYTHIA calculation. The PYTHIA calculation is validated with proton-proton data at 7 TeV.
The 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.
ALICE measured the nuclear modification factor (RAA) and elliptic flow (v2) of D mesons (D0, D+, D⁎+ and Ds+) in semi-central Pb–Pb collisions at sNN=5.02TeV. The increased amount of data in ...semi-central Pb–Pb events obtained with the LHC Run 2 provides access to more precise measurements for the RAA and the non-strange D-meson v2 with respect to Run 1 results, as well as to the first measurement of the Ds-meson v2 at LHC energies. The first application of the Event Shape Engineering technique in the analysis of the v2 coefficient of D0 and D+ mesons is also illustrated.
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
ALICE at the LHC provides unique capabilities to study charmonium production at low transverse momenta (pT). At central rapidity, (|y|<0.8), ALICE can reconstruct J/ψ via their decay into two ...electrons down to zero pT. Results at mid-rapidity on the inclusive J/ψ nuclear modification factor RAA as a function of centrality in Pb-Pb collisions at sNN=5.02 TeV and comparisons to lower energies and models are shown. In addition, the yield of the J/ψ produced with very low pT in the most peripheral collisions and the J/ψ elliptic flow (v2) are also presented.
The ALICE collaboration has measured the inclusive charmonium production at forward rapidity in Pb–Pb and p–Pb collisions at sNN=5.02TeV and sNN=8.16TeV, respectively. In Pb–Pb collisions, the J/ψ ...and ψ(2S) nuclear modification factors (RAA), as well as the J/ψ elliptic flow (v2) and averaged squared transverse momentum (〈pT2〉), are presented. In p–Pb collisions, the J/ψ nuclear modification factor (RpPb) as a function of pt and rapidity is presented. In both systems, our measurements are compared to theoretical calculations and to the results at lower energies.
We report measurements of two jet shapes, the ratio of 2-Subjettiness to 1-Subjettiness (τ2/τ1) and the opening angle between the two axes of the 2-Subjettiness jet shape, which is obtained by ...reclustering the jet with the exclusive-kT algorithm S.D.Ellis and D.E.Soper, Phys.Rev.B 48, 3160 and undoing the final clustering step. The aim of this measurement is to explore a possible change in the rate of 2-pronged objects in Pb-Pb compared to pp due to colour coherence. Coherence effects Y.Mehtar-Tani, C.A.Salgado and K.Tywoniuk Phys. Rev. Lett. 106:122002, 2011 relate to the ability of the medium to resolve a jet's substructure, which has an impact on the energy loss magnitude and mechanism of the traversing jet. In both collision systems charged jets are found with the anti-kT algorithm M.Cacciari, G.P.Salam and G.Soyez JHEP 0804:063, 2008, a resolution parameter of R=0.4 and a constituent cut off of 0.15 GeV. This analysis uses hadron-jet coincidence techniques in Pb-Pb collisions to reject the combinatorial background and corrects further for background effects by employing various jet shape subtraction techniques and two dimensional unfolding. Measurements of the Nsubjettiness for jet momenta of 40–60 GeV/c in Pb-Pb collisions at sNN=2.76 TeV and pp collisions at s=7 TeV will be presented and compared to PYTHIA simulations.
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