ATLAS search for new phenomena in dijet mass and angular distributions using pp collisions at sqrt(s)=7 TeV

TL;DR

The ATLAS dijet analysis at 7 TeV tests for new phenomena by examining both the invariant mass m_jj and dijet angular distributions, employing data-driven backgrounds and QCD-based predictions for angular observables. No resonance or QCD deviation is observed; robust Bayesian 95% CL limits are set on a range of NP models (q*, s8, W', SR, QBH, CI) and on model-independent Gaussian signals, with complementary CI/QBH constraints from χ and F_χ channels. The study demonstrates a comprehensive methodology combining resonance searches, angular analyses, and rigorous treatment of systematics, establishing a benchmark for future high-energy dijet searches as the LHC moves to higher energies. These results extend previous limits and provide a framework for interpreting upcoming 8 TeV data in 2012.

Abstract

Mass and angular distributions of dijets produced in LHC proton-proton collisions at a centre-of-mass energy sqrt(s)=7 TeV have been studied with the ATLAS detector using the full 2011 data set with an integrated luminosity of 4.8/fb. Dijet masses up to 4.0 TeV have been probed. No resonance-like features have been observed in the dijet mass spectrum, and all angular distributions are consistent with the predictions of QCD. Exclusion limits on six hypotheses of new phenomena have been set at 95% CL in terms of mass or energy scale, as appropriate. These hypotheses include excited quarks below 2.83 TeV, colour octet scalars below 1.86 TeV, heavy W bosons below 1.68 TeV, string resonances below 3.61 TeV, quantum black holes with six extra space-time dimensions for quantum gravity scales below 4.11 TeV, and quark contact interactions below a compositeness scale of 7.6 TeV in a destructive interference scenario.

Figures (8)

• Figure 1: The reconstructed dijet mass distribution (filled points) fitted with a smooth functional form (solid line). Mass distribution predictions for three ${q^\ast}$ masses are shown above the background. The middle part of figure shows the data minus the background fit, divided by the fit. The bin-by-bin significance of the data-background difference is shown in the lower panel.
• Figure 2: The $\chi$ distributions for all dijet mass bins. The QCD predictions are shown with theoretical and total systematic uncertainties (bands), as well as the data with statistical uncertainties. The dashed line is the prediction for a QBH signal for $M_D =4.0$ TeV and $n = 6$ in the highest mass bin. The distributions have been offset by the amount shown in the legend to aid in visually comparing the shapes in each mass bin.
• Figure 3: The $F_{\chi}(m_{jj})$ distribution in $m_{jj}$. The QCD prediction is shown with theoretical and total systematic uncertainties (bands), and data (black points) with statistical uncertainties. The blue vertical line indicates the lower boundary of the search region for new phenomena. Various expected new physics signals are shown: a contact interaction with $\Lambda$ = 7.5 TeV, an excited quark with mass 2.5 TeV and a QBH signal with $M_D =4.0$ TeV.
• Figure 4: The 95% CL upper limits on $\sigma\times {\cal A}$ as a function of particle mass (black filled circles) using $m_{jj}$. The black dotted curve shows the 95% CL upper limit expected in the absence of any resonance signal, and the green and yellow bands represent the 68% and 95% contours of the expected limit, respectively. Theoretical predictions of $\sigma\times {\cal A}$ are shown (dashed) in (a) for excited quarks, and in (b) for colour octet scalars. For a given NP model, the observed (expected) limit occurs at the crossing of the dashed $\sigma\times {\cal A}$ curve with the observed (expected) 95% CL upper limit curve.
• Figure 5: In (a), 95% CL upper limits on $\sigma\times {\cal A}$$\times$ BR as a function of particle mass (black filled circles) from $m_{jj}$ analysis are shown for heavy gauge bosons, $W'$. The black dotted curve shows the 95% CL upper limit expected in the absence of any resonance signal, and the green and yellow bands represent the 68% and 95% contours of the expected limit, respectively. The observed (expected) limit occurs at the crossing of the dashed theoretical $\sigma\times {\cal A}$$\times$ BR curve with the observed (expected) 95% CL upper limit curve. In (b), 95% CL upper limits on $\sigma\times {\cal A}$ are shown for string resonances, SR, with the equivalent set of contours for this model, and the same method of limit determination.