Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions with the ATLAS Detector at the LHC

TL;DR

The paper reports a search for new heavy particles decaying to dijets in $pp$ collisions at $\,\sqrt{s}=7$ TeV, using a data sample corresponding to an integrated luminosity of $315~\text{nb}^{-1}$ collected by ATLAS. It adopts a model-independent approach by examining the dijet mass spectrum $m^{jj}$ for resonant structures on top of a smooth background described by a fitted parametric function; simulated excited-quark signals with various masses are used to set acceptance and width expectations, with the resonances expected to be narrow compared to the detector resolution. No evidence for a resonance is observed, and 95% credibility-level upper limits on $\sigma\cdot\mathcal{A}$ are derived as a function of the test-mass $m_{q^*}$; the observed exclusion region using MRST2007 PDFs is $0.30 < m_{q^*} < 1.26$ TeV, extending prior limits. These results demonstrate ATLAS’s capability to probe dijet resonances and set competitive constraints in the early LHC data era.

Abstract

A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the Large Hadron Collider (LHC) and correspond to an integrated luminosity of 315 nb^-1 collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (q*) production as a function of q* mass. These exclude at the 95% CL the q* mass interval 0.30 < mq* < 1.26 TeV, extending the reach of previous experiments.

Figures (2)

• Figure 1: The data (D) dijet mass distribution (filled points) fitted using a binned background (B) distribution described by Eqn. \ref{['eq:f']} (histogram). The predicted $q^*$ signals Baur:1987gaBaur:1989kv for excited-quark masses of 500, 800, and 1200 ${\rm GeV}$ are overlaid, and the bin-by-bin significance of the data-background difference is shown.
• Figure 2: The 95% CL upper limit on $\sigma\cdot {\cal A}$ as a function of dijet resonance mass (black filled circles). The black dotted curve shows the expected 95% CL upper limit and the light and dark yellow shaded bands represent the 68% and 95% credibility intervals of the expected limit, respectively. The dashed curves represent excited-quark $\sigma\cdot {\cal A}$ predictions for different MC tunes, each using a different PDF set.