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First measurements of $N$-subjettiness in central Pb$-$Pb collisions at ${\sqrt{\it{s}_{\rm {NN}}}} = 2.76$ TeV

The ALICE collaboration
CERN-EP-2021-082, 2021.

Abstract (data 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 $\sqrt{s_{\rm NN}} = 2.76$ TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, $\tau_{2}/\tau_{1}$, 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 $40 \leq p_{\rm T,\rm jet} \leq 60$ 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 $\tau_{2}/\tau_{1}$ 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 $\tau_{2}/\tau_{1}$, together with the splitting aperture angle $\Delta R$, 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 $\tau_{2}/\tau_{1}$ 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.

  • $\Delta R$ with $k_{T}$ reclustering in pp

    Data from Figure 5 kT

    10.17182/hepdata.111055.v1/t1

    The $\Delta R$ (with $k_{T}$) fully corrected data distributions for jetsin the jet transverse momentum range 40-60 GeV/c in pp...

  • $\Delta R$ with CA reclustering in pp

    Data from Figure 5 C/A

    10.17182/hepdata.111055.v1/t2

    The $\Delta R$ (with C/A) fully corrected data distributions for jetsin the jet transverse momentum range 40-60 GeV/c in pp...

  • $\Delta R$ with Soft Drop in pp

    Data from Figure 5 Soft Drop

    10.17182/hepdata.111055.v1/t3

    The $\Delta R$ (with Soft Drop) fully corrected data distributions for jetsin the jet transverse momentum range 40-60 GeV/c in...

  • $\frac{\tau_{2}}{\tau_{1}}$ with $k_{T}$ reclustering in pp

    Data from Figure 6 kT

    10.17182/hepdata.111055.v1/t4

    The $\tau_{2}/\tau_{1}$ (with $k_{T}$) fully corrected data distributions for jetsin the jet transverse momentum range 40-60 GeV/c in pp collisions

  • $\frac{\tau_{2}}{\tau_{1}}$ with CA reclustering in pp

    Data from Figure 6 C/A

    10.17182/hepdata.111055.v1/t5

    The $\tau_{2}/\tau_{1}$ (with C/A) fully corrected data distributions for jetsin the jet transverse momentum range 40-60 GeV/c in pp collisions

  • $\frac{\tau_{2}}{\tau_{1}}$ with Soft Drop in pp

    Data from Figure 6 Soft Drop

    10.17182/hepdata.111055.v1/t6

    The $\tau_{2}/\tau_{1}$ (with Soft Drop) fully corrected data distributions for jetsin the jet transverse momentum range 40-60 GeV/c in pp...

  • $\frac{\tau_{2}}{\tau_{1}}$ with $k_{T}$ reclustering in Pb-Pb

    Data from Figure 7 kT

    10.17182/hepdata.111055.v1/t7

    The $\tau_{2}/\tau_{1}$ (with $k_{T}$) fully corrected data distributions for recoil jetsin the jet transverse momentum range 40-60 GeV/c in PbPb...

  • $\frac{\tau_{2}}{\tau_{1}}$ with CA reclustering in Pb-Pb

    Data from Figure 7 C/A

    10.17182/hepdata.111055.v1/t8

    The $\tau_{2}/\tau_{1}$ (with C/A) fully corrected data distributions for recoil jetsin the jet transverse momentum range 40-60 GeV/c in PbPb...

  • $\frac{\tau_{2}}{\tau_{1}}$ with Soft Drop in Pb-Pb

    Data from Figure 7 Soft Drop

    10.17182/hepdata.111055.v1/t9

    The $\tau_{2}/\tau_{1}$ (with Soft Drop) fully corrected data distributions for recoil jetsin the jet transverse momentum range 40-60 GeV/c in...

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