ATLAS measurements of the properties of jets for boosted particle searches

TL;DR

This ATLAS study investigates jet substructure in 7 TeV $pp$ collisions to enhance boosted-particle searches. It measures jet mass, width, eccentricity, planar flow, and angularity for anti-$k_T$ jets with $R=0.6$ and $R=1.0$, applying pileup and detector corrections to yield particle-level distributions that are compared to multiple Monte Carlo predictions. Overall, MC predictions describe the data well for most observables, with notable exceptions such as a mass-shift in some Herwig++ versions and some planar-flow features; angularity agrees with small-angle QCD expectations. The work validates the use of jet-substructure observables in boosted-object discrimination and establishes a framework for correcting and interpreting such measurements in early LHC data.

Abstract

Measurements are presented of the properties of high transverse momentum jets, produced in proton-proton collisions at a center-of-mass energy of sqrt(s) = 7 TeV. The data correspond to an integrated luminosity of 35 pb^-1 and were collected with the ATLAS detector in 2010. Jet mass, width, eccentricity, planar flow and angularity are measured for jets reconstructed using the anti-kt algorithm with distance parameters R = 0.6 and 1.0, with transverse momentum pT > 300 GeV and pseudorapidity |eta| < 2. The measurements are compared to the expectations of Monte Carlo generators that match leading-logarithmic parton showers to leading-order, or next-to-leading-order, matrix elements. The generators describe the general features of the jets, although discrepancies are observed in some distributions.

Figures (10)

• Figure 1: The correlation coefficients between pairs of variables calculated in Pythia at particle level for $R =$ 1.0 jets with no mass constraint (top) and with a mass constraint of $M >$ 100 GeV (bottom).
• Figure 2: The size of the mass shift in anti-$k_{t}$$R =$ 0.6 jets with 300 $< p_{T}\xspace\ <$ 400 GeV in jets with pileup and UE ($N_{\rm{PV}}$$>$ 1, average $N_{\rm{PV}} \simeq$ 2.2) and with UE alone ($N_{\rm{PV}}$ = 1). The curves are fits of the form $\Delta M = p_{0_M} + \frac{p_{1_M}}{M}$. The difference between the curves gives the contribution to the jet mass from pileup only. The $1\sigma$ uncertainties on the fits are shown in the error bands.
• Figure 3: The mass, width and eccentricity distributions before and after the pileup corrections. The (red) squares indicate the uncorrected data in the full data set, the (black) circles indicate the subset of this data with $N_{\rm{PV}}$ = 1 and the (blue) triangles indicate the full data set after pileup corrections. The mean value of each distribution is indicated in the legend with the corresponding statistical uncertainty. The lower region of each figure shows the measured ratio of $N_{\rm{PV}}$$>$ 1 to $N_{\rm{PV}}$ = 1 events.
• Figure 4: The dominant sources of systematic uncertainty on the measurements are those resulting in large variations in the detector correction factors $C$. These correction factors are found bin-by-bin using $R =$ 0.6 jets in a PythiaAMBT1 sample with upward and downward variations of the cluster energy scale (first and second columns), and by using Herwig++ (third column) and PythiaPerugia$2010$ (fourth column) in place of PythiaAMBT1. The differences $\Delta C$ found when comparing the correction factors obtained with the baseline PythiaAMBT1 sample are shown here for each of the properties measured in $R =$ 0.6 jets. The shaded bands indicate the statistical uncertainties.
• Figure 5: