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Measurement of the Ratio of the 3-jet to 2-jet Cross Sections in pp Collisions at sqrt(s) = 7 TeV

CMS Collaboration

TL;DR

This study measures the inclusive ratio $R_{32}$ of 3-jet to 2-jet cross sections as a function of $H_T$ in $pp$ collisions at $\sqrt{s}=7$ TeV using CMS data, extending the kinematic reach to $H_T$ up to $2.5$ TeV. Jets were reconstructed with the anti-$k_T$ algorithm ($R=0.5$) and a $p_T$ threshold of 50 GeV, enabling a robust cross-section ratio that benefits from cancellations of major systematic uncertainties. The data are compared with several LO QCD MC models, showing good agreement for $H_T>0.5$ TeV (with MadGraph matching the data across the full range) and highlighting differences among LO generators at lower $H_T$. The $R_{32}$ observable thus provides a precise test of multijet production in the TeV regime and guides MC tuning for accurate background predictions in high-energy LHC analyses.

Abstract

A measurement of the ratio of the inclusive 3-jet to 2-jet cross sections as a function of the total jet transverse momentum, HT, in the range 0.2 < HT < 2.5 TeV is presented. The data have been collected at a proton-proton centre-of-mass energy of 7 TeV with the CMS detector at the LHC, and correspond to an integrated luminosity of 36 inverse picobarns. Comparisons are made between the data and the predictions of different QCD-based Monte Carlo models for multijet production. All models considered in this study are consistent with the data for HT > 0.5 TeV. This measurement extends to an HT range that has not been explored before.

Measurement of the Ratio of the 3-jet to 2-jet Cross Sections in pp Collisions at sqrt(s) = 7 TeV

TL;DR

This study measures the inclusive ratio of 3-jet to 2-jet cross sections as a function of in collisions at TeV using CMS data, extending the kinematic reach to up to TeV. Jets were reconstructed with the anti- algorithm () and a threshold of 50 GeV, enabling a robust cross-section ratio that benefits from cancellations of major systematic uncertainties. The data are compared with several LO QCD MC models, showing good agreement for TeV (with MadGraph matching the data across the full range) and highlighting differences among LO generators at lower . The observable thus provides a precise test of multijet production in the TeV regime and guides MC tuning for accurate background predictions in high-energy LHC analyses.

Abstract

A measurement of the ratio of the inclusive 3-jet to 2-jet cross sections as a function of the total jet transverse momentum, HT, in the range 0.2 < HT < 2.5 TeV is presented. The data have been collected at a proton-proton centre-of-mass energy of 7 TeV with the CMS detector at the LHC, and correspond to an integrated luminosity of 36 inverse picobarns. Comparisons are made between the data and the predictions of different QCD-based Monte Carlo models for multijet production. All models considered in this study are consistent with the data for HT > 0.5 TeV. This measurement extends to an HT range that has not been explored before.

Paper Structure

This paper contains 7 sections, 3 equations, 3 figures.

Table of Contents

  1. Introduction
  2. The CMS Detector
  3. Data Selection and Reconstruction
  4. Monte Carlo Models and Comparisons with Data
  5. Extraction of $R_\mathrm{32}$ from the Data
  6. Summary
  7. The CMS Collaboration

Figures (3)

  • Figure 1: $H_\mathrm{T}$ distributions (top) for inclusive 2-jet (left) and 3-jet (right) events for data (solid circles) and Pythia6 tune Z2 MC generator (histogram). Ratios between the MC predictions and the data (bottom).
  • Figure 2: The measured $R_\mathrm{32}$ (solid circles) as a function of $H_\mathrm{T}$, and the predictions of Pythia6, Pythia8, MadGraph, Alpgen, and Herwig++ (curves). Error bars represent statistical uncertainties. The shaded area indicates the size of the combined systematic uncertainty.
  • Figure 3: The ratios of the predicted $R_\mathrm{32}$ values from the Pythia6, Pythia8, MadGraph, Alpgen, and Herwig++ Monte Carlo generators to the measured value, as a function of $H_\mathrm{T}$. The shaded area indicates the size of the combined statistical and systematic uncertainty.