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

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.

 

JHEP 10 (2021) 003
HEP Data
e-Print: arXiv:2105.04936 | PDF | inSPIRE
CERN-EP-2021-082

Figure 1

The $\tau_{2}/\tau_{1}$ (left) and $\Delta R$ (right) observables are shown for a variety of different axis selection algorithms. Both plots are obtained using a PYTHIA6 Perugia 2011 simulation of pp collisions at $\sqrt{s}=7$ TeV, in the particle level charged jet transverse momentum interval of $40 \leq p_{\rm T,\rm jet}^{\rm{part},\rm{ch}} \leq 60$ GeV/$c$.

Figure 2

The $p_{\rm T,\rm jet}^{\rm ch}$ dependence of the $\tau_{2}/\tau_{1}$ (left) and $\Delta R$ (right) observables are shown, for the axes obtained using the $k_{\rm{T}}$ reclustering algorithm. These calculations are made using PYTHIA6 Perugia 2011 simulations of pp collisions at $\sqrt{s}=7$ TeV.

Figure 3

Left: The performances of the constituent (squares) and area-derivatives (triangles) subtraction methods on the $\tau_{2}/\tau_{1}$ observable, measured with the $k_\mathrm{T}$ reclustering algorithm, are evaluated by embedding PYTHIA MC events into real Pb--Pb data. The filled circles represent the case with no heavy-ion background and the diamonds represent the case with background but no subtraction. Right: The $\tau_{2}/\tau_{1}$ residual distributions characterising the observable resolution due to detector effects and uncorrelated background. Both plots are shown for $40

Figure 4

The uncorrected $\tau_{2}/\tau_{1}$ distributions of the jets recoiling from a trigger hadron, measured in Pb--Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV, are presented. For each trigger hadron class, three consecutive uncorrected $p_{\rm{T},\rm{jet}}^{\rm{ch}}$ bins of $20 \leq p_{\rm{T},\rm{jet}}^{\rm{ch}}

Figure 5

Fully corrected $\Delta R$ distributions, measured with the $k_{\rm{T}}$, C/A and C/A with Soft Drop grooming algorithms, in pp collisions at $\sqrt{s} = 7$\,TeV for jets with $R = 0.4$ in the jet $p_{\rm T,\rm jet}^{\rm ch}$ interval of $40$--$60$\,GeV$/c$, are shown. The systematic uncertainties are given by the grey boxes. The results are compared with PYTHIA6 Perugia 2011 and PYTHIA8 Monash. The uncertainties presented for the PYTHIA distributions are purely statistical.

Figure 7

Fully corrected $\tau_{2}/\tau_{1}$ distributions, measured with the $k_{\rm{T}}$, C/A and C/A with Soft Drop grooming algorithms, in Pb--Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$\,TeV for jets with $R = 0.4$ in the jet $p_{\rm T,\rm jet}^{\rm ch}$ interval of $40$--$60$\,GeV$/c$, are shown. The systematic uncertainties are given by the grey boxes. The results are self normalised and compared with PYTHIA6 Perugia 2011 and PYTHIA8 Monash. The uncertainties presented for the PYTHIA distributions are purely statistical.