Search for New Physics in the Dijet Mass Distribution using 1 fb^-1 of pp Collision Data at sqrt(s) = 7 TeV collected by the ATLAS Detector

TL;DR

This ATLAS study searches for narrow resonances in the dijet mass spectrum using 1.0 fb^-1 of 7 TeV pp collision data, employing a data-driven background and a Bayesian framework to set limits on hypothetical resonances. The analysis uses a robust jet reconstruction and event selection, models the smoothly falling background, and scans for localized excesses with the BumpHunter algorithm, finding no significant signals. Model-dependent 95% CL limits exclude excited quarks up to 2.99 TeV, axigluons up to 3.32 TeV, and color-octet scalars up to 1.92 TeV, while model-independent Gaussian limits provide versatile guidance for broader BSM scenarios. The results extend the mass reach beyond 2010 data and present the first ATLAS limits on color-octet scalar resonances, representing the most stringent constraints to date in this channel.

Abstract

Invariant mass distributions of jet pairs (dijets) produced in LHC proton-proton collisions at a centre-of-mass energy sqrt(s)=7 TeV have been studied using a data set corresponding to an integrated luminosity of 1.0 fb^-1 recorded in 2011 by ATLAS. Dijet masses up to ~4 TeV are observed in the data, and no evidence of resonance production over background is found. Limits are set at 95% CL for several new physics hypotheses: excited quarks are excluded for masses below 2.99 TeV, axigluons are excluded for masses below 3.32 TeV, and colour octet scalar resonances are excluded for masses below 1.92 TeV.

Figures (3)

• Figure 1: The reconstructed dijet mass distribution (filled points) fitted with a smooth functional form describing the QCD background. The bin-by-bin significance of the data-background difference is shown in the lower panel. Vertical lines show the most significant excess found by the BumpHunter algorithm (see text).
• Figure 2: The 95% CL upper limits on $\sigma\times {\cal A}$ as a function of particle mass (black filled circles). The black dotted curve shows the 95% CL upper limit expected from Monte Carlo and the light and dark yellow shaded bands represent the 68% and 95% contours of the expected limit, respectively. Theoretical predictions for $\sigma\times {\cal A}$ are shown in (a) for excited quarks (blue dashed) and axigluons (green dot-dashed), and in (b) for colour octet scalar resonances (blue dashed). For a given new physics model, the observed (expected) limit occurs at the crossing of its $\sigma\times {\cal A}$ curve with the observed (expected) 95% CL upper limit curve.
• Figure 3: The 95% CL upper limits on $\sigma\times {\cal A}$ for a simple Gaussian resonance decaying to dijets as a function of the mean mass, $m_{\text{G}}$, for four values of $\sigma_{\text{G}}/m_{\text{G}}$, taking into account both statistical and systematic uncertainties.