Search for high-mass dilepton resonances in pp collisions at sqrt(s) = 8 TeV with the ATLAS detector

TL;DR

ATLAS conducts a shape-based search for high-mass dilepton resonances in $pp$ collisions at $\sqrt{s}=8$ TeV using the full 2012 dielectron and dimuon datasets, testing a broad suite of beyond-Standard-Model scenarios. By modeling SM backgrounds with data-driven and MC techniques and exploiting the invariant-mass spectrum, the analysis sets 95% CL limits on cross-sections or couplings for Z′ bosons, RS gravitons, QBHs, and MWT states, with the combined dielectron+dimuon channel yielding the strongest exclusions. The results exclude Z′-like resonances up to about 2.9 TeV, RS gravitons up to ~2.7 TeV for certain couplings, and impose stringent QBH and MWT constraints, representing the most stringent limits to date in many benchmarks. These findings tighten the available parameter space for several hierarchy-problem and extra-dimensions scenarios and guide future LHC searches in the dilepton channel.

Abstract

The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb-1 in the dielectron channel and 20.5 fb-1 in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including a grand-unification model based on the E6 gauge group, Z* bosons, Minimal Z' Models, a spin-2 graviton excitation from Randall-Sundrum models, quantum black holes and a Minimal Walking Technicolor model with a composite Higgs boson.

Figures (10)

• Figure 1: Product of acceptance and efficiency for the dielectron (upper distribution) and dimuon (lower distribution) selections as a function of the pole mass.
• Figure 2: Dielectron (top) and dimuon (bottom) invariant mass ( ) distributions after event selection, with two selected signals overlaid, compared to the stacked sum of all expected backgrounds, and the ratios of data to background expectation. The bin width is constant in $\log \mll$. The green band in the ratio plot shows the systematic uncertainties described in Sec. \ref{['sec:sys']}.
• Figure 3: Median expected (dashed line) and observed (solid red line) 95% CL upper limits on cross-section times branching ratio () in the combined dilepton channel, along with predicted for production. The inner and outer bands show the range in which the limit is expected to lie in 68% and 95% of pseudo-experiments, respectively. The thickness of the theory curve represents the theoretical uncertainty from the PDF error set and $\alpha_S$, as well as the choice of PDF.
• Figure 4: Median expected (dashed line) and observed (solid line) 95% CL upper limits on cross-section times branching ratio () for production for the exclusive dimuon and dielectron channels, and for both channels combined. The width of the theory band represents the theoretical uncertainty from the PDF error set, the choice of PDF as well as $\alpha_S$.
• Figure 5: Observed upper cross-section times branching ratio () limits at 95% CL for , -motivated and bosons using the combined dilepton channel. In addition, theoretical cross-sections on are shown for the same models. The stars indicate the lower mass limits for each considered model. The width of the band represents the theoretical uncertainty from the PDF error set, the choice of PDF as well as $\alpha_S$. The width of the band applies to the -motivated curves as well.