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First measurements of N-subjettiness in central Pb-Pb collisions at \( \sqrt{s_{\mathrm{NN}}} \) = 2.76 TeV

A preprint version of the article is available at arXiv.

Abstract

The ALICE Collaboration reports the first fully-corrected measurements of the N-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data recorded in pp and Pb-Pb collisions at centre-of-mass energies of \( \sqrt{s} \) = 7 TeV and \( \sqrt{s_{\mathrm{NN}}} \) = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, τ21, which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy-ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of R = 0.4 and charged jet transverse momentum of 40 ≤ pT,jet ≤ 60 GeV/c, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the τ21 observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of τ21, together with the splitting aperture angle ∆R, are also performed in pp collisions at \( \sqrt{s} \) = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at \( \sqrt{s} \) = 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for τ21 are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions.

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