602 research outputs found

    Data-driven precision determination of the material budget in ALICE

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    International audienceThe knowledge of the material budget with a high precision is fundamental for measurements of direct photon production using the photon conversion method due to its direct impact on the total systematic uncertainty. Moreover, it influences many aspects of the charged-particle reconstruction performance. In this article, two procedures to determine data-driven corrections to the material-budget description in ALICE simulation software are developed. One is based on the precise knowledge of the gas composition in the Time Projection Chamber. The other is based on the robustness of the ratio between the produced number of photons and charged particles, to a large extent due to the approximate isospin symmetry in the number of produced neutral and charged pions. Both methods are applied to ALICE data allowing for a reduction of the overall material budget systematic uncertainty from 4.5% down to 2.5%. Using these methods, a locally correct material budget is also achieved. The two proposed methods are generic and can be applied to any experiment in a similar fashion

    Measurement of the angle between jet axes in Pb-Pb collisions at sNN=5.02\sqrt{s_{\rm NN}} = 5.02 TeV

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    International audienceThis letter presents the first measurement of the angle between different jet axes (denoted as ΔR{\Delta}R) in Pb-Pb collisions. The measurement is carried out in the 0-10% most-central events at sNN=5.02\sqrt{s_{\rm NN}} = 5.02 TeV. Jets are assembled by clustering charged particles at midrapidity using the anti-kTk_{\rm T} algorithm with resolution parameters R=0.2R=0.2 and 0.40.4 and transverse momenta in the intervals 40<pTchjet<14040 < p_{\rm T}^{\rm ch jet} < 140 GeV/cc and 80<pTchjet<14080 < p_{\rm T}^{\rm ch jet} < 140 GeV/cc, respectively. Measurements at these low transverse momenta enhance the sensitivity to quark-gluon plasma (QGP) effects. A comparison to models implementing various mechanisms of jet energy loss in the QGP shows that the observed narrowing of the Pb-Pb distribution relative to pp can be explained if quark-initiated jets are more likely to emerge from the medium than gluon-initiated jets. These new measurements discard intra-jet pTp_{\rm T} broadening as described in a model calculation with the BDMPS formalism as the main mechanism of energy loss in the QGP. The data are sensitive to the angular scale at which the QGP can resolve two independent splittings, favoring mechanisms that incorporate incoherent energy loss

    Measurement of inclusive J/ψ\psi pair production cross section in pp collisions at s=13\sqrt{s} = 13 TeV

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    International audienceThe production cross section of inclusive J/ψ\psi pairs in pp collisions at a centre-of-mass energy s=13\sqrt{s} = 13 TeV is measured with ALICE. The measurement is performed for J/ψ\psi in the rapidity interval 2.502.5 0. The production cross section of inclusive J/ψ\psi pairs is reported to be 10.3±2.3(stat.)±1.3(syst.)10.3 \pm 2.3 {\rm (stat.)} \pm 1.3 {\rm (syst.)} nb in this kinematic interval. The contribution from non-prompt J/ψ\psi (i.e. originated from beauty-hadron decays) to the inclusive sample is evaluated. The results are discussed and compared with data

    Measurement of the angle between jet axes in Pb-Pb collisions at sNN=5.02\sqrt{s_{\rm NN}} = 5.02 TeV