Search for narrow resonances in dilepton mass spectra in proton-proton collisions at sqrt(s) = 13 TeV and combination with 8 TeV data

TL;DR

The CMS collaboration conducts a comprehensive search for narrow resonances decaying to dielectron or dimuon pairs using 13 TeV data, and enhances sensitivity by combining with 8 TeV results. Employing a Bayesian unbinned extended likelihood and a model-agnostic treatment of resonance shapes, the study sets upper limits on the production cross section times branching ratio and derives lower mass limits for Z' bosons (SSM and ψ) and RS gravitons. The analysis meticulously handles trigger, reconstruction, and background uncertainties, including data-driven validations and cross-checks with Z peaks. The resulting limits surpass previous 8 TeV constraints, tightening the viable parameter space for TeV-scale gauge bosons and extra-dimensional gravitons. Overall, no evidence for non-Standard-Model resonances is observed, but the work provides the most stringent dilepton resonance constraints to date in the explored mass ranges.

Abstract

A search for narrow resonances in dielectron and dimuon invariant mass spectra has been performed using data obtained from proton-proton collisions at sqrt(s) = 13 TeV collected with the CMS detector. The integrated luminosity for the dielectron sample is 2.7 inverse femtobarns and for the dimuon sample 2.9 inverse femtobarns. The sensitivity of the search is increased by combining these data with a previously analysed set of data obtained at sqrt(s) = 8 TeV and corresponding to a luminosity of 20 inverse femtobarns. No evidence for non-standard-model physics is found, either in the 13 TeV data set alone, or in the combined data set. Upper limits on the product of production cross section and branching fraction have also been calculated in a model-independent manner to enable interpretation in models predicting a narrow dielectron or dimuon resonance structure. Limits are set on the masses of hypothetical particles that could appear in new-physics scenarios. For the Z'[SSM] particle, which arises in the sequential standard model, and for the superstring inspired Z'[psi] particle, 95% confidence level lower mass limits for the combined data sets and combined channels are found to be 3.37 and 2.82 TeV, respectively. The corresponding limits for Kaluza-Klein gravitons arising in the Randall-Sundrum model of extra dimensions with coupling parameters 0.01 and 0.10 are 1.46 and 3.11 TeV, respectively. These results significantly exceed the limits based on the 8 TeV LHC data.

Figures (6)

• Figure 1: The invariant mass spectrum of (top) dielectron and (bottom) dimuon events at $\sqrt{s}=13\,\text{Te\spaceV}\xspace$. The points with error bars represent the data. The histograms represent the expectations from SM processes. The bins have equal width in logarithmic scale but the width in $\,\text{Ge\spaceV}$ becomes larger with increasing mass. Example signal shapes for a narrow resonance with a mass of 2$\,\text{Te\spaceV}$ are shown by the stacked open histograms.
• Figure 2: The 95% CL upper limits on the product of production cross section and branching fraction for a spin-1 resonance with a width equal to 0.6% of the resonance mass, relative to the product of production cross section and branching fraction for a Z boson, for the (top) dielectron and (bottom) dimuon channels in the 13$\,\text{Te\spaceV}$ data. The shaded bands correspond to the 68 and 95% quantiles for the expected limits. Theoretical predictions for the spin-1 ${Z}'_\mathrm{SSM}$ and ${Z}'_\psi$ resonances are shown for comparison.
• Figure 3: The 95% CL upper limits on the product of production cross section and branching fraction for a spin-1 resonance for widths equal to 0, 0.6, and 3.0% of the resonance mass, relative to the product of production cross section and branching fraction for a Z boson, for the (top) dielectron and (bottom) dimuon channels in the 13$\,\text{Te\spaceV}$ data. Theoretical predictions for the spin-1 ${Z}'_\mathrm{SSM}$ and ${Z}'_\psi$ resonances are also shown.
• Figure 4: The 95% CL upper limits on the product of production cross section and branching fraction for a spin-1 resonance, relative to the product of production cross section and branching fraction for a Z boson, for the combined dielectron and dimuon channels in the 13$\,\text{Te\spaceV}$ data, (top) for a resonance width equal to 0.6% of the resonance mass and (bottom) for resonance widths equal to 0, 0.6, and 3.0% of the resonance mass. The shaded bands correspond to the 68 and 95% quantiles for the expected limits. Theoretical predictions for the spin-1 ${Z}'_\mathrm{SSM}$ and ${Z}'_\psi$ resonances are also shown.
• Figure 5: The 95% CL upper limits on the product of production cross section and branching fraction for a spin-1 resonance with a width equal to 0.6% of the resonance mass, relative to the product of production cross section and branching fraction for a Z boson, for the combined 8 and 13$\,\text{Te\spaceV}$ data in the (top) dielectron and (bottom) dimuon channel. The shaded bands correspond to the 68 and 95% quantiles for the expected limits. Theoretical predictions for the spin-1 ${Z}'_\mathrm{SSM}$ and ${Z}'_\psi$ resonances are also shown.