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, 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 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 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.
Highlights
The ALICE Collaboration reports the first fully-corrected measurements of the N -subjettiness observable for track-based jets in heavy-ion collisions
In order to correct for the combinatorial yield without introducing a bias on the jet fragmentation, a data driven semi-inclusive approach using hadron-jet coincidences is extended for the first time to a substructure measurement
The impact of employing the semi-inclusive hadron-jet coincidence technique to suppress the combinatorial yield is shown for the measured data in figure 4, where the τ2/τ1 distributions measured in the recoil region of each trigger hadron class, along with the difference of these two distributions, are presented in three intervals of pcTh,jet
Summary
The calculation of N -subjettiness requires N subjet axes, which are themselves obtained by unwinding the reclustering history N −1 steps. The subjet axes are dependent on the choice of the reclustering algorithm used, with different algorithms returning axes that are sensitive to different regions of the splitting phase space. The reclustering algorithms employed in this analysis, which belong to the sequential recombination class of algorithms, are detailed as follows: 2.1.1 kT clustering. This metric clusters particles based on their pT and angular separation from one another [32]. Unwinding the last clustering step gives operational access to two hard subjets
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