CMS-B2G-20-003

CMS-B2G-20-003 ; CERN-EP-2022-020
Search for new particles in an extended Higgs sector with four b quarks in the final state at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
1 March 2022
Phys. Lett. B 835 (2022) 137566
Abstract: A search for a massive resonance X decaying to a pair of spin-0 bosons $ \phi $ that themselves decay to pairs of bottom quarks, is presented. The analysis is restricted to the mass ranges $ m_\phi $ from 25 to 100 GeV and $ m_\mathrm{X} $ from 1 to 3 TeV. For these mass ranges, the decay products of each $ \phi $ boson are expected to merge into a single large-radius jet. Jet substructure and flavor identification techniques are used to identify these jets. The search is based on CERN LHC proton-proton collision data at $ \sqrt{s}= $ 13 TeV, collected with the CMS detector in 2016--2018, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Model-specific limits, where the two new particles arise from an extended Higgs sector, are set on the product of the production cross section and branching fraction for $ \mathrm{X} \to \phi\phi \to (\mathrm{b}\overline{\mathrm{b}})(\mathrm{b}\overline{\mathrm{b}}) $ as a function of the resonances' masses, where both the $ \mathrm{X} \to \phi\phi $ and $ \phi \to \mathrm{b}\overline{\mathrm{b}} $ branching fractions are assumed to be 100%. These limits are the first of their kind on this process, ranging between 30 and 1 fb at 95% confidence level for the considered mass ranges.
Links: e-print arXiv:2203.00480 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of the production and decay of $ \mathrm{X} \to \phi\phi \to (\mathrm{b}\overline{\mathrm{b}})(\mathrm{b}\overline{\mathrm{b}}) $. The dominant production mechanism occurs via a fermion loop as shown in the diagram. Additional partons may be present, produced by initial-state or final-state radiation.
png pdf	Figure 2: Distributions of the ratio of event passing and failing the $ D^{\mathrm{b}\mathrm{b}}_{\mathrm{j}_2} > $ 0.6 requirement, as a function of the subleading jet groomed mass $ m_{\mathrm{j}_2} $, in 2018 data, for the TSR (filled markers) and the tight $ \|\Delta\eta\| $ sidebands (open markers). The arctangent $ R_{\text{p/f}} $ fit from which the background is estimated is shown by the solid line in the case of the TSR, and by the thick dashed line for the sideband, with the resulting $ \chi^2 $ and degrees of freedom indicated in the legend. The 1$ \sigma $ uncertainty band of the fit in the TSR, from which systematic uncertainties on the QCD background estimate are derived, is shown by the thin dashed lines. Similar results were obtained for 2016 and 2017 data and in the three LSRs. The lower panel shows the difference between the observed data and the fits, divided by the statistical uncertainty of the ratio of passing and failing events in the data for each bin.
png pdf	Figure 3: Distributions of average jet mass (left) and dijet mass (right), and background estimate of the combined search regions after the final fit is performed. The blue (solid) line represents the sum of the estimated QCD and $ \mathrm{t} \overline{\mathrm{t}} $ backgrounds, and the red filled histogram shows the $ \mathrm{t} \overline{\mathrm{t}} $ contribution alone. The shaded areas around the background estimate in the upper panels represent the total uncertainty in the total background estimate in that bin. The shapes of two representative signals, each normalized to cross sections of 50 fb, are indicated by solid colored lines. The lower panel shows the difference between the observed data and the background prediction, divided by $ \sigma_{\text{data}} $, the statistical uncertainty of the data in each bin.
png pdf	Figure 3-a: Distribution of average jet mass, and background estimate of the combined search regions after the final fit is performed. The blue (solid) line represents the sum of the estimated QCD and $ \mathrm{t} \overline{\mathrm{t}} $ backgrounds, and the red filled histogram shows the $ \mathrm{t} \overline{\mathrm{t}} $ contribution alone. The shaded area around the background estimate in the upper panel represents the total uncertainty in the total background estimate in that bin. The shapes of two representative signals, each normalized to cross sections of 50 fb, are indicated by solid colored lines. The lower panel shows the difference between the observed data and the background prediction, divided by $ \sigma_{\text{data}} $, the statistical uncertainty of the data in each bin.
png pdf	Figure 3-b: Distribution of average dijet mass, and background estimate of the combined search regions after the final fit is performed. The blue (solid) line represents the sum of the estimated QCD and $ \mathrm{t} \overline{\mathrm{t}} $ backgrounds, and the red filled histogram shows the $ \mathrm{t} \overline{\mathrm{t}} $ contribution alone. The shaded area around the background estimate in the upper panel represents the total uncertainty in the total background estimate in that bin. The shapes of two representative signals, each normalized to cross sections of 50 fb, are indicated by solid colored lines. The lower panel shows the difference between the observed data and the background prediction, divided by $ \sigma_{\text{data}} $, the statistical uncertainty of the data in each bin.
png pdf	Figure 4: The average jet mass distributions in consecutive dijet mass intervals. The vertical dashed lines separate the average jet mass distributions in each bin of $ M_{\mathrm{jj}} $. The individual bins within such subdivisions correspond to the $ \hat{m} $ spectrum (from 15 to 200 GeV), as seen in Fig. 3 (left). Representative signal shapes are also shown; we note that they peak in the $ \hat{m} $ spectrum within subdivisions, and may appear in multiple $ M_{\mathrm{jj}} $ bins. The blue (solid) line represents the sum of the estimated QCD and $ \mathrm{t} \overline{\mathrm{t}} $ backgrounds, and the red filled histogram shows the $ \mathrm{t} \overline{\mathrm{t}} $ contribution alone. The shaded areas around the background estimate in the upper panels represent the total uncertainty in the total background estimate in that bin. The shapes of three representative signals, each normalized to cross sections of 50 fb are indicated by solid colored lines. The lower panel shows the difference between the observed data and the background prediction, divided by $ \sigma_{\text{data}} $, the statistical uncertainty of the data in each bin.
png pdf	Figure 5: Upper limits at 95% CL on the cross section of the process $ \mathrm{p}\mathrm{p} \to \mathrm{X} \to \phi\phi \to (\mathrm{b}\overline{\mathrm{b}})(\mathrm{b}\overline{\mathrm{b}}) $, as a function of the mass of $ m_\mathrm{X} $, for different values of $ m_\phi $. Both the $ \mathrm{X} \to \phi\phi $ and $ \phi \to \mathrm{b} \overline{\mathrm{b}} $ branching fractions are assumed to be 100%. Each subpanel shows the limits for a fixed value of $ m_\phi $. The observed limits are shown as solid black lines with markers, while the expected limits are dotted. The yellow (outer) and green (inner) bands represent one and two standard deviation intervals. The theoretical cross section for different values of the parameter $ m_{\mathrm{X}} N/f $ are shown with dotted and dashed curves.

Tables
png pdf	Table 1: Search and control regions used in the analysis. A selection on the subleading jet double-b-tagger discriminant $ D^{\mathrm{b}\mathrm{b}}_{\mathrm{j}_2} > $ 0.6 further separates each region into ``passing'' and ``failing'' categories.
png pdf	Table 2: Sources of systematic uncertainties considered in the analysis. The uncertainty in the integrated luminosity only affects the normalization; for the rest, both the shape and normalization are affected. The parameters affecting only the normalization have log-normal priors, and those affecting the shape (or both the shape and normalization) have Gaussian priors, except for the statistical uncertainty in the failing region, whose parameters were sampled from a $ \Gamma $ distribution. Uncertainties marked with R are correlated between the TSR and LSR for a given year of data-taking, and those marked with Y are correlated between both search regions in all three years. All other uncertainties are uncorrelated between search regions. The values indicated in the table represent the pre-fit values of the uncertainty in the parameter. When a range is given, it indicates the typical variation of the size of the uncertainty over the average jet mass and dijet mass distribution. We note that all $ \mathrm{t} \overline{\mathrm{t}} $ uncertainties are propagated into the QCD backgroound estimate.

Summary

A search for massive resonances (X) decaying to pairs of spin-0 bosons ($ \phi $) that themselves decay into b quark-antiquark pairs has been presented. The analysis is restricted to the case where the mass ratio of the resonance and the scalar bosons is such that each pair of b quarks is reconstructed as a single large-radius jet. Data from proton-proton collisions at the LHC at $ \sqrt{s}= $ 13 TeV collected in 2016--2018 with the CMS detector, corresponding to an integrated luminosity of 138 fb$ ^{-1} $, have been used. Upper limits are set at 95% confidence level on the product of production cross section and branching fraction as a function of mass for $ \mathrm{X} \to \phi\phi\to (\mathrm{b}\overline{\mathrm{b}})(\mathrm{b}\overline{\mathrm{b}}) $, where both the $ \mathrm{X} \to \phi\phi $ and $ \phi \to \mathrm{b} \overline{\mathrm{b}} $ branching fractions are assumed to be 100%. These are the first limits on this process, and range between 30 and 1 fb for a $ \phi $ mass in the range 25--100 GeV and an X mass in the range 1--3 TeV.

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