Search for stable hadronising squarks and gluinos with the ATLAS experiment at the LHC

TL;DR

This study searches for slow-moving, long-lived hadronising particles (R-hadrons) predicted in supersymmetric and extra-dimensional theories. By exploiting complementary observables from the ATLAS detector—pixel detector $dE/dx$ and tile calorimeter time-of-flight—the analysis maintains robustness against uncertain R-hadron charge states after matter interactions. Using 34 pb^-1 of LHC data at 7 TeV and data-driven background estimation, the paper sets 95% CL mass limits: $m_{ ilde{b}} > 294$ GeV, $m_{ ilde{t}} > 309$ GeV, and $m_{ ilde{g}} > 586$ GeV, with model-dependent variations for gluinos. These results represent the most stringent direct limits to date on stable R-hadrons and significantly extend the exploration of coloured SMPs at the LHC. The work also highlights the importance of modelling R-hadron interactions in matter through multiple hadronisation and scattering models, informing future searches with larger datasets.

Abstract

Hitherto unobserved long-lived massive particles with electric and/or colour charge are predicted by a range of theories which extend the Standard Model. In this paper a search is performed at the ATLAS experiment for slow-moving charged particles produced in proton-proton collisions at 7 TeV centre-of-mass energy at the LHC, using a data-set corresponding to an integrated luminosity of 34 pb-1. No deviations from Standard Model expectations are found. This result is interpreted in a framework of supersymmetry models in which coloured sparticles can hadronise into long-lived bound hadronic states, termed R-hadrons, and 95% CL limits are set on the production cross-sections of squarks and gluinos. The influence of R-hadron interactions in matter was studied using a number of different models, and lower mass limits for stable sbottoms and stops are found to be 294 and 309 GeV respectively. The lower mass limit for a stable gluino lies in the range from 562 to 586 GeV depending on the model assumed. Each of these constraints is the most stringent to date.

Figures (4)

• Figure 1: Distributions of $\mathrm{d}E\!/\!\mathrm{d}x_{\rm Pixel}$ (left) and ${\beta}_{\mathrm{Tile}}$ (right) in data after the transverse momentum selection $p_{\mathrm{T}}>50$ GeV. Spectra for simulated background processes are plotted for comparison. The uncertainty shown on the background is the Monte Carlo statistical uncertainty.
• Figure 2: Mass estimated by the pixel detector (left) and the tile calorimeter (right). To obtain a mass estimate, a cut of $\mathrm{d}E\!/\!\mathrm{d}x_{\rm Pixel}>1.1\,MeV\rm{g}^{-1}\rm{cm}^{2}$ is imposed for the pixel detector distribution. This is a looser cut than used in the analysis itself. For the tile calorimeter, the requirement is that ${\beta}_{\mathrm{Tile}}<1$.
• Figure 3: Background estimates for the pixel detector (left) and the tile calorimeter (right). Signal samples are superimposed on the background estimate. The total systematic uncertainty of the background estimate is indicated by the error band.
• Figure 4: Cross-section limits at 95% CL as a function of sparticle mass. Since five candidate events are observed for the mass windows used for the 100 GeV mass hypotheses, the mass points between 100 and 200 GeV are connected with a dotted line. This indicates that fluctuations in the excluded cross-section will occur. The mass limits quoted in the text are inferred by comparing the cross-section limits with the model predictions. Systematic uncertainties from the choice of PDF and the choice of renormalisation and factorisation scales are represented as a band in the cross-section curves. Previous mass limits are indicated by shaded vertical lines for sbottom (ALEPH), stop (CDF) and gluino (CMS).