CMS-PAS-B2G-16-008

CMS-PAS-B2G-16-008
Search for heavy resonances decaying to a pair of Higgs bosons in the four b quark final state in proton-proton collisions at $\sqrt{s}=$ 13 TeV
CMS Collaboration
July 2016

Abstract: A search for heavy resonances decaying to a pair of standard model Higgs bosons (H) is performed using data from proton-proton collisions at a centre-of-mass energy of 13 TeV, collected by the CMS experiment in 2015, corresponding to an integrated luminosity of 2.7 fb$^{-1}$. The final state under consideration consists of both Higgs bosons decaying to b quark-antiquark pairs. For resonance masses above 1 TeV the Higgs bosons are highly Lorentz-boosted and thus each ${\rm H}\rightarrow{\rm b\overline{b}}$ is usually reconstructed as one hadronic jet. The signal is characterized as a peak in the distribution of the invariant mass of such dijet candidates. The background consists mostly of standard model multijet processes. The signal strength for different assumed resonance masses is estimated by a combined likelihood fit of background and signal shapes to the data. The data are found to be consistent with the standard model, and upper limits on the $s$-channel production cross sections of narrow bulk gravitons and scalar radions in warped extradimensional models are reported.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: The reconstructed signal mass distribution, ${M_{\rm jj}^{\rm red}} $, for different generated masses of the radion, modelled using the Crystal Ball function for the 4 b tagged event categories of the subjet b tagger analysis. The distributions are normalized to a signal cross section of 10 fb.
png pdf	Figure 2-a: The distribution of ${M_{\rm jj}^{\rm red}} $ for the data (black markers) with a background only fit using the main background model, the levelled-exponential (red line), and an alternative background model, the power law (blue line). The 68% and 95% confidence interval bands on the main background model is given. The expected contribution of the signal for a spin 2 bulk graviton hypothesis is also shown (magenta dotted line). The a (b) figures are for the 3 b (4 b) tagged categories.
png pdf	Figure 2-b: The distribution of ${M_{\rm jj}^{\rm red}} $ for the data (black markers) with a background only fit using the main background model, the levelled-exponential (red line), and an alternative background model, the power law (blue line). The 68% and 95% confidence interval bands on the main background model is given. The expected contribution of the signal for a spin 2 bulk graviton hypothesis is also shown (magenta dotted line). The a (b) figures are for the 3 b (4 b) tagged categories.
png pdf	Figure 3-a: Fit to the $m_{\rm J}$ sideband regions for the pass-fail ratio $R_{p/f}$ along with the fit uncertainties (a). The (b) figure shows the predicted ${M_{\rm jj}^{\rm red}} $ background distribution in the signal region, after applying the ``Alphabet'' method. The statistical and total uncertainties on the predicted background are also shown. The black markers with the error bars are the data. The lower panel shows the difference between the observed and predicted events for each histogram bin, divided by the statistical uncertainty on the observed events.
png pdf	Figure 3-b: Fit to the $m_{\rm J}$ sideband regions for the pass-fail ratio $R_{p/f}$ along with the fit uncertainties (a). The (b) figure shows the predicted ${M_{\rm jj}^{\rm red}} $ background distribution in the signal region, after applying the ``Alphabet'' method. The statistical and total uncertainties on the predicted background are also shown. The black markers with the error bars are the data. The lower panel shows the difference between the observed and predicted events for each histogram bin, divided by the statistical uncertainty on the observed events.
png pdf	Figure 4: The reconstructed signal mass distribution, ${M_{\rm jj}^{\rm red}} $, for different generated masses of the radion for the double-b tagger analysis. The distributions are normalized to a signal cross section of 10fb.
png pdf	Figure 5-a: The combined limits for the radion (a) and the bulk graviton (b) models. The double-b tagger analysis is used to set limits for the resonance masses 900-1200 GeV and 2000-3000 GeV, while the subjet b tagger analysis sets limits between 1200-2000 GeV.
png pdf	Figure 5-b: The combined limits for the radion (a) and the bulk graviton (b) models. The double-b tagger analysis is used to set limits for the resonance masses 900-1200 GeV and 2000-3000 GeV, while the subjet b tagger analysis sets limits between 1200-2000 GeV.

Tables

png pdf

Table 1:
Comparison of expected limits on the production cross section of a resonance decaying to HH for the subjet b tagger and double-b tagger analyses for the bulk graviton and the radion signal hypotheses, for different values of the resonance mass. The results of the double-b tagger analysis are shown over the range 900-3000 GeV, while the results of the subjet b tagger analysis are shown within the mass range 1200-2000 GeV, for reasons explained in Section 1.2. In the mass range where both analyses can be used, the expected limits are compatible. In this range, the final result is quoted for the subjet b tagger analysis, as it has a more precise estimate of the expected limits due to the fact that the uncertainty on the double-b tagging scale factor is larger than that on the subjet b tagging scale factor (Section 6).

Summary

A search for a new massive resonance decaying to two standard model Higgs bosons is conducted using 2.7 fb$^{-1}$ of CMS pp collision data, collected in 2015 at $ \sqrt{s} = $ 13 TeV. The ${\mathrm{ H }\rightarrow\mathrm{b}\mathrm{ \bar{b} }}$ decay, having the largest branching fraction, is used, resulting in a final state with four b quarks. For the resonance masses considered in this search, the Higgs bosons are highly Lorentz-boosted and the decay products of each Higgs boson are merged into one large-radius jet. Thus the signal events result in a dijet topology, with each Higgs identified as a massive jet with two subjets arising from the hadronization of two b quarks. Two complementary analyses are used to reconstruct such a boosted ${\mathrm{ H }\rightarrow\mathrm{b}\mathrm{ \bar{b} }} $ decay: one using b tagged subjets, and the other using a dedicated double-b tagging algorithm without explicit recourse to subjets. The main background is multijet production through QCD interactions, which is estimated entirely from the data using two different methods: the smoothness test and the ``Alphabet'' methods. The complementarity of the approaches helps to extend the search sensitivity over a wide range of resonance masses. The results are interpreted in terms of narrow radions and bulk gravitons in warped extra-dimensions models. In the absence of any excess in the data above the standard model background, we place upper limits on the production cross section times the branching fraction ${\rm X} \to \mathrm{ H }\mathrm{ H } \to \mathrm{ b \bar{b} }$ for assumed resonance masses between 900 and 3000 GeV. The search excludes a narrow radion (with $\Lambda_{R} =$ 1 TeV) for masses below 1720 GeV at 95% CL. More data is required to exclude a bulk graviton in this mass range for the assumed WED model parameters.

Additional Figures

png pdf

Additional Figure 1:
Final observed (black line), expected (dashed line) 95% CL upper limits on the product of cross section of a narrow resonance decaying to two Higgs bosons using the 13+8 TeV combined data. From 1.2 TeV the limit is evaluated using the Alphabet Method for the background estimation. Note that the limit is presented for the 8 TeV analysis-only before 1.2 TeV. In the range 1.2 TeV to 2 TeV the limit is extracted in the "Smoothness Test" version of the analysis. For resonance masses greater than 2 TeV the limit is taken from the Alphabet Method again, up to 3 TeV. There is no overlap in the transition regions, (Alphabet and Smoothnes Test methods). Theory corresponding to $\Lambda =$ 1 Radion spin-0 resonance hypothesis in the WED scenario is also shown (red line).

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