CMS-PAS-B2G-20-003

CMS-PAS-B2G-20-003
Search for new particles in an extended Higgs sector in the four b quark final state at $\sqrt{s}= $ 13 TeV
CMS Collaboration
July 2021

Abstract: A search for new particles in an extended Higgs sector, characterized by a massive resonance $X$ decaying to a pair of scalar bosons $a$, which themselves decay to pairs of b quarks, is presented. The analysis is restricted to the mass ranges $m_a$ from 25 to 100 GeV and $m_X$ from 1 to 3 TeV. For these mass ranges, the decay products of each $a$ are expected to merge into a single large radius jet. Jet substructure and flavor identification techniques are used to identify these jets from background jets. The search is based on 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 at 95% confidence level on the production cross section times branching fraction as a function of mass for $X\rightarrow aa\rightarrow (\mathrm{b}\bar{\mathrm{b}})(\mathrm{b}\bar{\mathrm{b}})$ are set, where both the $X \rightarrow a a $ and $ a \rightarrow \mathrm{b}\bar{\mathrm{b}}$ branching fractions are assumed to be 100%. These are the first such limits on this process, and range between 1 and 30 fb for the considered mass ranges.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 835 (2022) 137566. The superseded preliminary plots can be found here.

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of the benchmark signal production and decay of $X \rightarrow aa \rightarrow (\mathrm{b} \mathrm{\bar{b}})(\mathrm{b} \mathrm{\bar{b}})$. The dominant production mechanism occurs via a fermion loop as shown in the diagram. Additional partons are possible from either initial state or final state radiation.
png pdf	Figure 2: The average jet mass (left) and dijet mass (right) distributions and background estimate of the combined search regions after the final fit is performed. The shaded areas around the background estimate in the top panels represent the total uncertainty on the background estimate in that bin. The bottom panel shows the difference between the observed data and the background prediction, divided by the statistical uncertainty of the data in each bin.
png pdf	Figure 2-a: The average jet mass distribution and background estimate of the combined search regions after the final fit is performed. The shaded areas around the background estimate in the top panel represents the total uncertainty on the background estimate in that bin. The bottom panel shows the difference between the observed data and the background prediction, divided by the statistical uncertainty of the data in each bin.
png pdf	Figure 2-b: The average dijet mass distribution and background estimate of the combined search regions after the final fit is performed. The shaded areas around the background estimate in the top panel represents the total uncertainty on the background estimate in that bin. The bottom panel shows the difference between the observed data and the background prediction, divided by the statistical uncertainty of the data in each bin.
png pdf	Figure 3: The unrolled average jet mass distributions in consecutive dijet mass intervals. The vertical dashed grey lines separate the average jet mass distributions in each bin of dijet mass. The individual bins within such subdivisions correspond to the average jet mass spectrum (from 15 to 200 GeV), as seen in Fig. xxxxx (left). Representative signal shapes are also shown; note that they peak in the average jet mass spectrum within subdivisions, and may appear in multiple dijet mass bins. The shaded areas around the background estimate in the top panel represents the total uncertainty on the background estimate in that bin. The bottom panel shows the difference between the observed data and the background prediction, divided by the statistical uncertainty of the data in each bin.
png pdf	Figure 4: Upper limits at 95% CL on the cross section of the process ${\mathrm{p}} {\mathrm{p}} \rightarrow X \rightarrow aa \rightarrow (\mathrm{b} \mathrm{\bar{b}})(\mathrm{b} \mathrm{\bar{b}})$, as a function of the mass of the $X$ resonance, for different values of the $a$ boson mass. Both the $X \rightarrow a a$ and $a \rightarrow b \mathrm{\bar{b}} $ branching fractions are assumed to be 100%. Each sub-panel shows the limits for a fixed choice of $a$ mass. The observed limits are shown as solid black lines with markers, while the expected limits are dotted. The green (dark) and yellow (light) bands represent 1 and 2 standard deviation intervals. The theoretical cross section for different values of the parameter $(m_{X} N)/\textrm {f}$ 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^{bb}_{j2} > $ 0.6 further separates each region into the passing and failing categories.
png pdf	Table 2: Sources of systematic uncertainties considered in the analysis. Parameters denoted by the $\star $ symbol affect only the normalization; otherwise both the shape and normalization of the process are affected. The parameters affecting the normalization have log-normal priors, and those affecting the shape have Gaussian priors, unless marked with the $\dagger $ symbol, which denotes that this parameter was sampled from a $\Gamma $-distribution. Uncertainties marked with $^{\textrm {C}}$ are correlated between the LR and SR for a given year of data-taking, and those marked with $^{\textrm {Y}}$ 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 on the parameter. When a range is given, it indicates the variation of the size of the uncertainty over the average jet mass and dijet mass distribution. Note that all ${\mathrm{t} {}\mathrm{\bar{t}}}$ uncertainties are propagated to the QCD estimate.

Summary

A search for massive resonances decaying to pairs of scalar bosons which themselves decay to b quark-antiquark pairs is 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 analyzed. Upper limits are set at 95% confidence level on the production cross section times branching fraction as a function of mass for $X\rightarrow aa\rightarrow (\mathrm{b}\mathrm{\bar{b}})(\mathrm{b}\mathrm{\bar{b}})$, where both the $X \rightarrow a a$ and $a \rightarrow b \mathrm{\bar{b}}$ branching fractions are assumed to be 100%. These are the first such limits on this process, and range between 1 and 30 fb for $a$ mass between 25 and 100 GeV and $X$ mass between 1 and 3 TeV.

Additional Figures
png pdf	Additional Figure 1: Observed upper limits at 95% CL on the cross section of the process $ {\mathrm {p}} {\mathrm {p}}\rightarrow X \rightarrow aa \rightarrow ({\mathrm {b}} {\overline {\mathrm {b}}})({\mathrm {b}} {\overline {\mathrm {b}}})$, as a function of the mass of $X$, for different values of the $a$ mass. Both the $X \rightarrow a a$ and $a \rightarrow {\mathrm {b}} {\overline {\mathrm {b}}} $ branching fractions are assumed to be 100%. The theoretical cross section for different values of the parameter $m_{X} N/\textrm {f}$ are shown with dotted curves, where $m_{X}$ is the mass of the particle $X$, $N$ is the number of fermions in the production diagram, and $\textrm {f}$ is the expectation value of $X$.
png pdf	Additional Figure 2: The pass-to-fail ratio vs. the soft drop mass of the subleading jet. The black (blue) points show the tight search region (control region) and the red (pink) is a fit to that region, in the 2016 data. The fit uncertainty in the control region is also shown as the dashed pink lines.
png pdf	Additional Figure 3: The pass-to-fail ratio vs. the soft drop mass of the subleading jet. The black (blue) points show the loose search region (control region) and the red (pink) is a fit to that region, in the 2016 data. The fit uncertainty in the control region is also shown as the dashed pink lines.
png pdf	Additional Figure 4: The pass-to-fail ratio vs. the soft drop mass of the subleading jet. The black (blue) points show the tight search region (control region) and the red (pink) is a fit to that region, in the 2017 data. The fit uncertainty in the control region is also shown as the dashed pink lines.
png pdf	Additional Figure 5: The pass-to-fail ratio vs. the soft drop mass of the subleading jet. The black (blue) points show the loose search region (control region) and the red (pink) is a fit to that region, in the 2017 data. The fit uncertainty in the control region is also shown as the dashed pink lines.
png pdf	Additional Figure 6: The pass-to-fail ratio vs. the soft drop mass of the subleading jet. The black (blue) points show the tight search region (control region) and the red (pink) is a fit to that region, in the 2018 data. The fit uncertainty in the control region is also shown as the dashed pink lines.
png pdf	Additional Figure 7: The pass-to-fail ratio vs. the soft drop mass of the subleading jet. The black (blue) points show the loose search region (control region) and the red (pink) is a fit to that region, in the 2018 data. The fit uncertainty in the control region is also shown as the dashed pink lines.
png pdf	Additional Figure 8: Local p-values for each signal mass considered in the analysis. The minimum p-value, for the 1 TeV $X$, 70 GeV $a$ signal point is 0.0008, corresponding to a local significance of 3.12$\sigma $.
png pdf	Additional Figure 9: Candidate event display 1.
png pdf	Additional Figure 10: Candidate event display 2.

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