Highlights of recent results from the ALICE collaboration are presented. The collision systems investigated are Pb–Pb, p–Pb, and pp, and results from studies of bulk particle production, azimuthal ...correlations, open and hidden heavy flavor, and hard probes are shown. The results provide key constraints on transport properties of the QCD matter, the initial conditions, the dynamics of heavy quarks in hot and dense systems, and the coupling of hard probes with the medium. A brief description of the ALICE upgrade program is also given.
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.
The second and the third order anisotropic flow, V2 and V3, are determined by the corresponding initial spatial anisotropy coefficients, ε2 and ε3, in the initial density distribution. On the ...contrary, the higher order anisotropic flow Vn(n>3), in addition to their dependence on the same order initial anisotropy coefficient εn, have a significant contribution from lower order initial anisotropy coefficients, which leads to mode-coupling effects. In this contribution, we present the investigations on linear and non-linear modes in higher order anisotropic flow (V4, V5 and V6) in Pb–Pb collisions at sNN=2.76 TeV using the ALICE detector at the Large Hadron Collider (LHC). A significant contribution from a non-linear mode is observed. A new observable, the non-linear response coefficient, is measured as well. The comparison to theoretical calculations provides crucial information on dynamic of the created system especially at the freeze-out conditions, which are poorly known from previous flow measurements.
In ultra relativistic heavy-ion collisions at the Large Hadron Collider (LHC), conditions are met to produce a hot, dense and strongly interacting medium known as the Quark Gluon Plasma (QGP). Quarks ...and gluons from incoming nuclei collide to produce partons at high momenta early in the collisions. By fragmenting into collimated sprays of hadrons, these partons form 'jets'. The outgoing partons scatter and interact with the medium, leading to a manifestation of medium modifications of jets in the final state, known as jet quenching. Within the framework of perturbative QCD, jet production is well understood in pp collisions. We use jets measured in pp interactions as a baseline reference for comparing to heavy-ion collision systems to detect and study jet quenching. The jet quenching mechanism can be studied through the angular correlations of jets with charged hadrons and is examined in transverse momentum (pT) bins of the jets, pT bins of the associated hadrons, and as a function of collision centrality. A robust and precise background subtraction method is used in this analysis to remove the complex, flow dominated, heavy-ion background. The analysis of angular correlations for different orientations of the jet relative to the event plane allows for the study of the path-length dependence of medium modifications to jets. The event plane dependence of azimuthal angular correlations of charged hadrons with respect to the axis of an R=0.2 reconstructed full (charged + neutral) jet in Pb–Pb collisions at sNN=2.76 TeV in ALICE is presented. Results are compared for three angular bins of the jet relative to the event plane in mid-peripheral events. The yields relative to the event plane are presented and then quantified through yield ratio calculations. The results show no significant path-length dependence on the medium modifications.
Direct photon production in heavy-ion collisions provides a valuable set of observables to study the hot QCD medium. The direct photons are produced at different stages of the collision and escape ...the medium unaffected. In heavy-ion collisions, the direct photon yield at high transverse momentum (pT>5GeV/c) is dominated by prompt photons produced in hard scattering of incoming partons and provides information on nuclear parton distribution functions and on the initial parton dynamics. The low momentum component (pT≲5GeV/c) of the direct photon production is dominated by thermal radiation by the hot and dense matter created, carrying information on its space-time evolution, collective flow and temperature.
We present recent ALICE results on direct photon production Pb-Pb collisions at 2.76 TeV and on direct photon production in pp at 7 TeV using isolation techniques. The results are compared to theoretical predictions and previous measurements.
The second and the third order anisotropic flow, V2and V3, are mostly determined by the corresponding initial spatial anisotropy coefficients, ε2and ε3, in the initial density distribution. In addition ...to their dependence on the same order initial anisotropy coefficient, higher order anisotropic flow, Vn(n > 3), can also have a significant contribution from lower order initial anisotropy coefficients, which leads to mode-coupling effects. In this Letter we investigate the linear and non-linear modes in higher order anisotropic flow Vnfor n = 4, 5, 6 with the ALICE detector at the Large Hadron Collider. The measurements are done for particles in the pseudorapidity range |η| < 0.8andthe transverse momentum range 0.2 < pT< 5.0GeV/c as a function of collision centrality. The results are compared with theoretical calculations and provide important constraints on the initial conditions, including initial spatial geometry and its fluctuations, as well as the ratio of the shear viscosity to entropy density of the produced system.
Elliptic (v2) and higher harmonic (v3,v4) flow coefficients of π±, K±, p(p‾), and the ϕ-meson, measured in Pb-Pb collisions at the highest-ever center-of-mass energy of sNN=5.02 TeV, are presented. ...The results were obtained with the scalar product method, correlating hadrons with reference particles from a different η region. The vn exhibit a clear mass ordering for pT ≲ 2 GeV/c and only approximate particle type scaling for pT ≳ 2 GeV/c. Reasonable agreement with hydrodynamic calculations (IP-Glasma+MUSIC+UrQMD) is seen at pT ≲ 1 GeV/c.
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