Search for Resonances in the Dilepton Mass Distribution in pp Collisions at sqrt(s) = 7 TeV

TL;DR

This CMS study searches for narrow high-mass dilepton resonances in 7 TeV pp collisions using shape analyses of m_ll in the μμ and ee channels, benchmarking Z′ bosons (SSM and ψ) and RS KK gravitons. By normalizing to Z production and combining channels, it derives 95% CL limits on cross-section ratios and translates them into mass bounds, excluding Z′ masses up to ~1.14 TeV (SSM) and RS gravitons up to ~1.08 TeV depending on coupling. The analysis employs data-driven background estimates, robust lepton identification, and a careful treatment of PDFs and higher-order corrections, delivering competitive constraints with relatively low integrated luminosity. The results significantly extend previous limits and demonstrate the CMS detector’s capability for high-mass resonance searches in dilepton final states.

Abstract

A search for narrow resonances at high mass in the dimuon and dielectron channels has been performed by the CMS experiment at the CERN LHC, using pp collision data recorded at sqrt(s)=7 TeV. The event samples correspond to integrated luminosities of 40 inverse picobarns in the dimuon channel and 35 inverse picobarns in the dielectron channel. Heavy dilepton resonances are predicted in theoretical models with extra gauge bosons (Z') or as Kaluza-Klein graviton excitations(G_KK) in the Randall-Sundrum model. Upper limits on the inclusive cross section of Z'(G_KK) decaying to oppositely charged leptons relative to Z decaying to oppositely charged leptons are presented. These limits exclude at 95% confidence level a Z' with standard-model-like couplings below 1140 GeV, the superstring-inspired Z_psi below 887 GeV, and, for values of the coupling parameter k/M_Pl of 0.05 (0.1), Kaluza-Klein gravitons below 855 (1079) GeV.

Figures (5)

• Figure 1: The observed opposite-sign $\mathrm{e}^\pm{\mu}^\mp$ dilepton invariant mass spectrum (data points). The uncertainties on the data points (statistical only) represent 68% confidence intervals for the Poisson means. Filled histograms show contributions to the spectrum from ${t}\overline{{t}}\xspace$, other sources of prompt leptons ($\mathrm{t}\mathrm{W}$, diboson production, ${Z}\to\tau\tau$), and the multi-jet background (from Monte Carlo simulation).
• Figure 2: Invariant mass spectrum of ${\mu^+}{\mu^-}$ (left) and $\mathrm{e}\mathrm{e}$ (right) events. The points with error bars represent the data. The uncertainties on the data points (statistical only) represent 68% confidence intervals for the Poisson means. The filled histograms represent the expectations from SM processes: ${Z}{/}\gamma^*$, ${t}\overline{{t}}\xspace$, other sources of prompt leptons ($\mathrm{t}\mathrm{W}$, diboson production, ${Z}\to\tau\tau$), and the multi-jet backgrounds. The open histogram shows the signal expected for a ${Z}^\prime_\text{SSM}$ with a mass of 750$\,\text{Ge\spaceV}$.
• Figure 3: Cumulative distribution of invariant mass spectrum of ${\mu^+}{\mu^-}$ (left) and $\mathrm{e}\mathrm{e}$ (right) events. The points with error bars represent the data, and the filled histogram represents the expectations from SM processes.
• Figure 4: Upper limits as a function of resonance mass $M$, on the production ratio $R_{\sigma}$ of cross section times branching fraction into lepton pairs for ${Z}^\prime_\text{SSM}$ and $\mathrm{G}_\text{KK}$ production and ${Z}^\prime_\psi$ boson production. The limits are shown from (top) the ${\mu^+}{\mu^-}$ final state, (middle) the $\mathrm{e}\mathrm{e}$ final state and (bottom) the combined dilepton result. Shaded yellow and red bands correspond to the $68\%$ and $95\%$ quantiles for the expected limits. The predicted cross section ratios are shown as bands, with widths indicating the theoretical uncertainties.
• Figure 5: 95% C.L. lower limits on the ${Z}^\prime$ mass, represented by the thin continuous lines in the $(c_d,c_u)$ plane. Curves for three classes of model are shown. Colours on the curves correspond to different mixing angles of the generators defined in each model. For any point on a curve, the mass limit corresponding to that value of $(c_d,c_u)$ is given by the intersected contour.