CMS-PAS-EXO-19-019

CMS-PAS-EXO-19-019
Search for a narrow resonance in high-mass dilepton final states in proton-proton collisions using 140 fb$^{-1}$ of data at $\sqrt{s}= $ 13 TeV
CMS Collaboration
August 2019

Abstract: A search for physics beyond the standard model is presented using electron or muon pairs with high invariant mass. A data set of proton-proton collisions collected by the CMS experiment at the LHC at $\sqrt{s}= $ 13 TeV recorded in years 2016 to 2018 and corresponding to a total integrated luminosity of up to 140 fb$^{-1}$ is analyzed. No significant deviation is observed with respect to the expectation from the standard model backgrounds. Upper limits are set on the ratio of the production cross section times branching ratio of a new narrow dilepton resonance to that of the Z boson and converted into lower limits on the masses of various hypothetical particles. A $\mathrm{Z}'_{SSM}$ ($\mathrm{Z}'_{\psi}$) particle, arising in the sequential standard model (superstring-inspired model) is excluded below a mass of 5.15 (4.56) TeV at 95% confidence level.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Accepted by JHEP. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The invariant mass distribution of pairs of (left) electrons and (right) muons observed in data (black dots with statistical error bars) and expected from the SM processes (stacked histograms). For the dimuon channel, a prescaled trigger with a ${p_{\mathrm {T}}}$ threshold of 27 GeV was used to collect events in the normalization region (NR) with $m_{\mu \mu} < $ 120 GeV. The corresponding offline threshold is 30 GeV. Events in the signal region (SR) corresponding to masses above 120 GeV are collected using an unprescaled single muon trigger. The bin width gradually increases with mass. The ratio of the data yields after background subtraction to the background yields is shown on the bottom plots. The blue band represents the various statistical and systematic uncertainties on the background.
png pdf	Figure 1-a: The invariant mass distribution of pairs of (left) electrons and (right) muons observed in data (black dots with statistical error bars) and expected from the SM processes (stacked histograms). For the dimuon channel, a prescaled trigger with a ${p_{\mathrm {T}}}$ threshold of 27 GeV was used to collect events in the normalization region (NR) with $m_{\mu \mu} < $ 120 GeV. The corresponding offline threshold is 30 GeV. Events in the signal region (SR) corresponding to masses above 120 GeV are collected using an unprescaled single muon trigger. The bin width gradually increases with mass. The ratio of the data yields after background subtraction to the background yields is shown on the bottom plots. The blue band represents the various statistical and systematic uncertainties on the background.
png pdf	Figure 1-b: The invariant mass distribution of pairs of (left) electrons and (right) muons observed in data (black dots with statistical error bars) and expected from the SM processes (stacked histograms). For the dimuon channel, a prescaled trigger with a ${p_{\mathrm {T}}}$ threshold of 27 GeV was used to collect events in the normalization region (NR) with $m_{\mu \mu} < $ 120 GeV. The corresponding offline threshold is 30 GeV. Events in the signal region (SR) corresponding to masses above 120 GeV are collected using an unprescaled single muon trigger. The bin width gradually increases with mass. The ratio of the data yields after background subtraction to the background yields is shown on the bottom plots. The blue band represents the various statistical and systematic uncertainties on the background.
png pdf	Figure 2: The upper limits at 95% CL 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 of a Z boson, for the (top left) dielectron channel, (top right) dimuon channel, and (bottom) their combination. The shaded bands correspond to the 68 and 95% quantiles for the expected limits. Theoretical predictions for the spin-1 $ {\mathrm{Z'} _\mathrm {SSM}} $ and $ {\mathrm{Z'} _\psi} $ resonances are shown for comparison.
png pdf	Figure 2-a: The upper limits at 95% CL 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 of a Z boson, for the (top left) dielectron channel, (top right) dimuon channel, and (bottom) their combination. The shaded bands correspond to the 68 and 95% quantiles for the expected limits. Theoretical predictions for the spin-1 $ {\mathrm{Z'} _\mathrm {SSM}} $ and $ {\mathrm{Z'} _\psi} $ resonances are shown for comparison.
png pdf	Figure 2-b: The upper limits at 95% CL 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 of a Z boson, for the (top left) dielectron channel, (top right) dimuon channel, and (bottom) their combination. The shaded bands correspond to the 68 and 95% quantiles for the expected limits. Theoretical predictions for the spin-1 $ {\mathrm{Z'} _\mathrm {SSM}} $ and $ {\mathrm{Z'} _\psi} $ resonances are shown for comparison.
png pdf	Figure 2-c: The upper limits at 95% CL 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 of a Z boson, for the (top left) dielectron channel, (top right) dimuon channel, and (bottom) their combination. The shaded bands correspond to the 68 and 95% quantiles for the expected limits. Theoretical predictions for the spin-1 $ {\mathrm{Z'} _\mathrm {SSM}} $ and $ {\mathrm{Z'} _\psi} $ resonances are shown for comparison.

Tables
png pdf	Table 1: Benchmark models considered in this analysis with their corresponding mixing angles; their branching fraction ($\mathcal {B}$) to dileptons; the $c_{\mathrm{u}}$ and $c_{\mathrm{d}}$ parameter values and their ratio; and the ratio of width to mass of the associated Z' boson.
png pdf	Table 2: Systematic uncertainties considered in this analysis and their magnitude.
png pdf	Table 3: Observed and expected yields for different mass ranges in the (top) dielectron channel and (bottom) dimuon channel. The sum of all background contributions is shown as well as a breakdown into the three main categories. The quoted uncertainty includes both the statistical and the systematic components.
png pdf	Table 4: The observed and expected 95% CL lower limits on the masses of spin-1 $ {\mathrm{Z'} _\mathrm {SSM}} $ and $ {\mathrm{Z'} _\psi} $ bosons, assuming a signal width of 0.6% (3.0%) of the resonance mass for $ {\mathrm{Z'} _\psi} $ ($ {\mathrm{Z'} _\mathrm {SSM}} $).

Summary

A search for new heavy resonances in the dilepton invariant mass spectrum in proton-proton collisions at $\sqrt{s} = $ 13 TeV corresponding to an integrated luminosity of 140 fb$^{-1}$ has been presented. High-mass dielectron and dimuon events are reconstructed and selected with algorithms optimized for high-${p_{\mathrm{T}}}$ electrons and muons. Standard Model backgrounds are primarily estimated from simulation, with the dominant Drell-Yan background corrected to the highest order calculations available, including the contribution from photon induced processes. The background estimate is normalized to the data in a control region around the Z boson peak. No significant deviation from Standard Model expectation is observed. Limits are set on the ratio of the cross section of the new resonance to that of the Standard Model Z boson at 95% CL using Bayesian techniques known to have good frequentist coverage properties. The limits are interpreted in the context of a sequential standard model, and a superstring-inspired one, predicting spin-1 resonances. Lower mass limits of 5.15 TeV (4.56 TeV) are set in the $\mathrm{Z}'_{SSM}$ ($\mathrm{Z}'_{\psi}$) models.

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