Abstract
Jet substructure quantities are measured using jets groomed with the soft-drop grooming procedure in dijet events from 32.9 fb$^{-1}$ of $pp$ collisions collected with the ATLAS detector at $\sqrt{s} = 13$ TeV. These observables are sensitive to a wide range of QCD phenomena. Some observables, such as the jet mass and opening angle between the two subjets which pass the soft-drop condition, can be described by a high-order (resummed) series in the strong coupling constant $\alpha_S$. Other observables, such as the momentum sharing between the two subjets, are nearly independent of $\alpha_S$. These observables can be constructed using all interacting particles or using only charged particles reconstructed in the inner tracking detectors. Track-based versions of these observables are not collinear safe, but are measured more precisely, and universal non-perturbative functions can absorb the collinear singularities. The unfolded data are directly compared with QCD calculations and hadron-level Monte Carlo simulations. The measurements are performed in different pseudorapidity regions, which are then used to extract quark and gluon jet shapes using the predicted quark and gluon fractions in each region. All of the parton shower and analytical calculations provide an excellent description of the data in most regions of phase space.
Highlights
Jets are collimated sprays of particles that are initiated by high-energy quarks and gluons
The effects of pileup were simulated with unbiased pp collisions using the PYTHIA8.186 generator with the A2 [51] set of tuned parameters and the MSTW2008LO [52] parton distribution function (PDF) set; these events were overlaid on the nominal dijet events
Unfolded measurements of three substructure observables were shown for both the calorimeter-based observables unfolded to the all-particle level and track-based observables unfolded to the charged-particles level
Summary
Jets are collimated sprays of particles that are initiated by high-energy quarks and gluons. Grooming techniques systematically remove soft and wide-angle radiation, making the structure of the jet robust against contamination from multiple simultaneous proton-proton interactions (pileup) as well as against final-state radiation and the underlying event This internal structure of a jet has been successfully used to tag the origin of jets in precision measurements and searches at the Large Hadron Collider (LHC) [1,2]. In the region where the calculations are expected to be accurate, the agreement with the data is excellent, and nonperturbative effects [15] have become the most important theoretical source of uncertainty instead of higherorder effects This analysis goes beyond the jet mass by adding other soft-drop jet observables that are connected with the grooming procedure. Previous measurements of groomed jet observables have been conducted at the LHC by CMS [14,20], ATLAS [13,21], and ALICE [22], and at RHIC by the STAR Collaboration [23] and additional studies at the detector level have been performed using CMS data [24,25,26,27]
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