First measurements of N-subjettiness in central Pb–Pb collisions at √ sNN = 2.76 TeV

Acharya, S. and Adamova, D. and Adler, A. and Adolfsson, J. and Rinella, G. Aglieri and Agnello, M. and Agrawal, N. and Ahammed, Z. and Ahmad, S. and Ahn, S. U. and Ahuja, I and Akbar, Z. and Akindinov, A. and Al-Turany, M. and Alam, S. N. and Aleksandrov, D. and Alessandro, B. and Alfanda, H. M. and Alfaro, Molina R. and Ali, B. and Ali, Y. and Alici, A. and Alizadehvandchali, N. and Alkin, A. and Alme, J. and Alt, T. and Altenkamper, L. and Altsybeev, I and Anaam, M. N. and Andrei, C. and Barnaföldi, Gergely Gábor and Bencédi, Gyula and Berényi, Dániel and Bíró, Gábor and Hamar, Gergő and Lévai, Péter and Varga, Dezső and Varga, Zoltán (2021) First measurements of N-subjettiness in central Pb–Pb collisions at √ sNN = 2.76 TeV. JOURNAL OF HIGH ENERGY PHYSICS (10). ISSN 1126-6708

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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 √ s = 7 TeV and √ sNN = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, τ2/τ1, 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 τ2/τ1 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 τ2/τ1, together with the splitting aperture angle ∆R, are also performed in pp collisions at √ s = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at √ 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 τ2/τ1 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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