617 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

    Exploring the strong interaction of three-body systems at the LHC

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    International audienceDeuterons are atomic nuclei composed of a neutron and a proton held together by the strong interaction. Unbound ensembles composed of a deuteron and a third nucleon have been investigated in the past using scattering experiments and they constitute a fundamental reference in nuclear physics to constrain nuclear interactions and the properties of nuclei. In this work K+−^{+}-d and p−-d femtoscopic correlations measured by the ALICE Collaboration in proton−-proton (pp) collisions at s=13\sqrt{s}=13 TeV at the Large Hadron Collider (LHC) are presented. It is demonstrated that correlations in momentum space between deuterons and kaons or protons allow us to study three-hadron systems at distances comparable with the proton radius. The analysis of the K+−^{+}-d correlation shows that the relative distances at which deuterons and proton/kaons are produced are around 2 fm. The analysis of the p−-d correlation shows that only a full three-body calculation that accounts for the internal structure of the deuteron can explain the data. In particular, the sensitivity of the observable to the short-range part of the interaction is demonstrated. These results indicate that correlations involving light nuclei in pp collisions at the LHC will also provide access to any three-body systems in the strange and charm sectors

    Measurement of inclusive charged-particle jet production in pp and p-Pb collisions at sNN=5.02\sqrt{s_{\rm NN}}=5.02 TeV