Exploration of jet substructure using iterative declustering in pp and Pb-Pb collisions at LHC energies

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The ALICE collaboration at the CERN LHC reports novel measurements of jet substructure in pp collisions at $\sqrt{s}$= 7 TeV and central Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. Jet substructure of track-based jets is explored via iterative declustering and grooming techniques. We present the measurement of the momentum sharing of two-prong substructure exposed via grooming, the $z_{\rm{g}}$, and its dependence on the opening angle, in both pp and Pb-Pb collisions. We also present the first measurement of the distribution of the number of branches obtained in the iterative declustering of the jet, which is interpreted as the number of its hard splittings. In Pb-Pb collisions, we observe a suppression of symmetric splittings at large opening angles and an enhancement of splittings at small opening angles relative to pp collisions, with no significant modification of the number of splittings. The results are compared to predictions from various Monte Carlo event generators to test the role of important concepts in the evolution of the jet in the medium such as color coherence.

 

PLB 802 (2020) 135227
e-Print: arXiv:1905.02512 | PDF | inSPIRE
CERN-EP-2019-087

Figure 1

Fully corrected \zg~ distribution in pp collisions for 40 $\leq \pTjetch < $ 60 GeV/{\it c} compared with predictions from PYTHIA simulations. The statistical uncertainties are shown as vertical bars and the systematic uncertainties are represented by a shaded area.

Figure 2

Fully corrected \nsd~distribution in pp collisions for 40 $\leq \pTjetch < $ 60 GeV/{\it c}, compared with predictions from PYTHIA simulations. The statistical uncertainties are shown as vertical bars and the systematic uncertainties are represented by a shaded area.

Figure 3

Detector-level Pb--Pb distributions of \zg~for $R=0.4$ jets with varying minimum/maximum angular separation of subjets ($\DeltaR$) for jets in the range 80 $\leq \pTjetch < $ 120\, GeV/$c$. The systematic uncertainties are represented by the shaded area. The corresponding values for the embedded PYTHIA reference (open symbols), Hybrid model (dashed line) and JEWEL (solid line) are also shown in the plot. The lower plots show the ratios of data, Hybrid and JEWEL model to the embedded PYTHIA reference.

Figure 4

The number of SD branches for jets reconstructed in Pb--Pb data are shown. The systematic uncertainties are represented by the shaded area. The datapoints are compared to jets found in PYTHIA events embedded into Pb--Pb events (open markers). The Hybrid model and JEWEL predictions correspond to the red (dashed) and blue (solid) lines. The lower panel shows the ratio of the $n_{\rm{SD}}$ distribution in data and the embedded PYTHIA reference (grey). The ratios of the Hybrid and JEWEL models to the embedded PYTHIA reference are also shown and their uncertainties are purely statistical