Search for pair production of massive particles decaying into three quarks with the ATLAS detector in sqrt(s) = 7 TeV pp collisions at the LHC

TL;DR

This paper reports a search for pair-produced massive colored particles decaying to six quarks, using fully hadronic final states in $\,sqrt{\text{ s}}=7$ TeV $pp$ collisions with ATLAS and $4.6\,\mathrm{fb}^{-1}$ of data. It employs two complementary channels: a resolved channel with six well-separated jets ($p_T$-thresholds optimized by gluino mass) and a boosted channel using two large-$R$ jets with jet-substructure tagging ($\tau_{32}$) to capture highly boosted decays. No excess over the Standard Model is observed; the analysis sets the most stringent 95% CL limits to date on RPVSUSY gluinos, excluding $m_{\tilde{g}}$ up to 666 GeV in the resolved channel and 255 GeV in the boosted channel, with the two approaches offering largely orthogonal, cross-validated sensitivity. The combination demonstrates the effectiveness of complementary hadronic strategies for accessing both high- and low-mass regimes in RPV SUSY scenarios.

Abstract

A search is conducted for hadronic three-body decays of a new massive coloured particle in sqrt(s) = 7 TeV pp collisions at the LHC using an integrated luminosity of 4.6/fb collected by the ATLAS detector. Supersymmetric gluino pair production in the context of a model with R-parity violation is used as a benchmark scenario. The analysis is divided into two search channels, each optimised separately for their sensitivity to high-mass and low-mass gluino production. The first search channel uses a stringent selection on the transverse momentum of the six leading jets and is performed as a counting experiment. The second search channel focuses on low-mass gluinos produced with a large boost. Large-radius jets are selected and the invariant mass of each of the two leading jets is used as a discriminant between the signal and the background. The results are found to be consistent with Standard Model expectations and limits are set on the allowed gluino mass.

Figures (6)

• Figure 1: Predicted event yield in the 5-jet bin is compared with expectations that are determined by projecting from lower jet multiplicity. The horizontal axis represents the $p_{\mathrm{T}}$ selection that is applied when counting jets, and the vertical axis represents the number of events that have exactly five jets with a $p_{\mathrm{T}}$ above this threshold. Such comparisons are used to assign a systematic uncertainty to the background normalisation, which is shown as the shaded green band of the ratio plot. The same relative normalisation systematic uncertainty is applied on the background in the signal region.
• Figure 2: Data and the baseline background prediction along with three example signal distributions in the signal region (n $\ge 6$). Background uncertainties include statistical and systematic effects.
• Figure 3: In the lower mass signal region (SR1), the distributions of \ref{['fig:results:SR:tau']} jet $\tau_{32}$ for the two leading jets in each event with $m^{\rm jet}\xspace>60$ GeV and \ref{['fig:results:SR:mass']} jet mass ($m^{J_{1}\xspace}$ and $m^{J_{2}\xspace}$) for jets with $\tau_{32}\xspace<0.7$ are shown for the data, the signal $m_{\tilde{g}}\xspace=100$ GeV, and the background MCs for comparison. In the higher mass signal region (SR2), the same distributions of \ref{['fig:results:SR2:tau']}$\tau_{32}$ and \ref{['fig:results:SR2:mass']} jet mass are shown, but in this case for $m_{\tilde{g}}\xspace=300$ GeV. In each case, the data are compared to the two MC models used to estimate the correlation correction factor, $\alpha$, for the background extrapolation.
• Figure 4: Distributions of the first leading and subleading (in $p_{\rm T}\xspace^\mathrm{jet}$) jet masses from which the correlation coefficients ($\rho$) are determined in \ref{['fig:signal-control:masscorr:data']} data ($\rho=1.05\%$), \ref{['fig:signal-control:masscorr:powheg']}POWHEG+PYTHIA MC samples ($\rho=0.2\%$), \ref{['fig:signal-control:masscorr100:data']} data with $m^{\rm jet}\xspace>100$ GeV ($\rho=10.1\%$), and \ref{['fig:signal-control:masscorr100:powheg']}POWHEG+PYTHIA MC samples with $m^{\rm jet}\xspace>100$ GeV ($\rho=10.9\%$).
• Figure 5: The expected and observed 95% confidence limits are shown for the resolved analyses channel. The published CMS results using 35 ${\rm pb}^{-1}$ of 2010 data and using 5 ${\rm fb}^{-1}$ of 2011 data are shown for comparison.