Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Measurement of event shapes at large momentum transfer with the ATLAS detector in pp collisions at sqrt(s) = 7 TeV

ATLAS Collaboration

TL;DR

This ATLAS study measures six jet-based event shapes in 7 TeV pp collisions using 35 pb^-1 to probe multi-jet topologies and test LO Monte Carlo generators with leading-log parton showers. The observables are unfolded to particle level and analyzed as functions of the kinematic scale defined by $ rac{1}{2}H_{{ m T},2}$, revealing general agreement with Pythia and Alpgen predictions and highlighting areas where Herwig++ underperforms, especially for aplanarity. The results support the importance of leading-log resummation in MC models and provide data-driven input for tuning generators, with implications for precise QCD studies and background modeling in multi-jet final states. Overall, the HT2-dependence of the mean event-shape values mirrors the running of $oldsymbol{\\alpha_s}$ and demonstrates the capability of ATLAS jet-based event shapes to constrain QCD dynamics at high momentum transfers.

Abstract

A measurement of event shape variables is presented for large momentum transfer proton-proton collisions using the ATLAS detector at the Large Hadron Collider. Six event shape variables calculated using hadronic jets are studied in inclusive multi-jet events in 35 pb^-1 of integrated luminosity at a center-of-mass energy of sqrt(s) = 7 TeV. These measurements are compared to predictions by three Monte Carlo event generators containing leading-logarithmic parton showers matched to leading order matrix elements for 2 to 2 and 2 to n (n=2,...6) scattering. Measurements of the third-jet resolution parameter, aplanarity, thrust, sphericity, and transverse sphericity are generally well described. The mean value of each event shape variable is evaluated as a function of the average momentum of the two leading jets pT1 and pT2, with a mean pT approaching 1 TeV.

Measurement of event shapes at large momentum transfer with the ATLAS detector in pp collisions at sqrt(s) = 7 TeV

TL;DR

This ATLAS study measures six jet-based event shapes in 7 TeV pp collisions using 35 pb^-1 to probe multi-jet topologies and test LO Monte Carlo generators with leading-log parton showers. The observables are unfolded to particle level and analyzed as functions of the kinematic scale defined by , revealing general agreement with Pythia and Alpgen predictions and highlighting areas where Herwig++ underperforms, especially for aplanarity. The results support the importance of leading-log resummation in MC models and provide data-driven input for tuning generators, with implications for precise QCD studies and background modeling in multi-jet final states. Overall, the HT2-dependence of the mean event-shape values mirrors the running of and demonstrates the capability of ATLAS jet-based event shapes to constrain QCD dynamics at high momentum transfers.

Abstract

A measurement of event shape variables is presented for large momentum transfer proton-proton collisions using the ATLAS detector at the Large Hadron Collider. Six event shape variables calculated using hadronic jets are studied in inclusive multi-jet events in 35 pb^-1 of integrated luminosity at a center-of-mass energy of sqrt(s) = 7 TeV. These measurements are compared to predictions by three Monte Carlo event generators containing leading-logarithmic parton showers matched to leading order matrix elements for 2 to 2 and 2 to n (n=2,...6) scattering. Measurements of the third-jet resolution parameter, aplanarity, thrust, sphericity, and transverse sphericity are generally well described. The mean value of each event shape variable is evaluated as a function of the average momentum of the two leading jets pT1 and pT2, with a mean pT approaching 1 TeV.

Paper Structure

This paper contains 14 sections, 4 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Event Shape Definitions
  3. The ATLAS Detector
  4. Data Samples and Event Selection
  5. Data Sample and Event Selection
  6. Jet Reconstruction and Calibration
  7. Monte Carlo Simulation
  8. Results and Systematic Uncertainties
  9. Accounting for Detector Effects
  10. Systematic Uncertainties
  11. Event Shape Distributions
  12. Dependence on $\frac{1}{2}H_{{\rm T},2}\xspace$
  13. Summary
  14. Acknowledgments

Figures (3)

  • Figure 1: Inclusive jet trigger efficiency as a function of $\frac{1}{2}H_{{\rm T},2}\xspace$ evaluated offline. The efficiency to select events with $\frac{1}{2}H_{{\rm T},2}\xspace\xspace > 250$ GeV using this trigger is greater than 99.8%.
  • Figure 2: Unfolded hadron-level distributions of the \ref{['fig:eventshapes:unfolded:lny23']} third-jet resolution parameter, $\ln y_{23}\xspace$, \ref{['fig:eventshapes:unfolded:apl']} aplanarity, $A$, \ref{['fig:eventshapes:unfolded:thrustMajor']} transverse thrust, $\tau_{\perp}$, \ref{['fig:eventshapes:unfolded:thrustMinor']} minor component of the transverse thrust, $T_{m,\perp}$, \ref{['fig:eventshapes:unfolded:sph']} sphericity, $S$, and \ref{['fig:eventshapes:unfolded:sphtranv']} transverse sphericity, $S_{\perp}$. The uncertainty shown for the data includes statistical and systematic uncertainties.
  • Figure 3: Mean value of each event shape variable as a function of $\frac{1}{2}H_{{\rm T},2}\xspace$. Comparisons are made between the MC generators Herwig++, Alpgen and Pythia.