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CMS-HIG-16-015 ; CERN-EP-2016-292
Search for light bosons in decays of the 125 GeV Higgs boson in proton-proton collisions at $ \sqrt{s} = $ 8 TeV
JHEP 10 (2017) 076
Abstract: A search is presented for decays beyond the standard model of the 125 GeV Higgs bosons to a pair of light bosons, based on models with extended scalar sectors. Light boson masses between 5 and 62.5 GeV are probed in final states containing four $\tau$ leptons, two muons and two b quarks, or two muons and two $\tau$ leptons. The results are from data in proton-proton collisions corresponding to an integrated luminosity of 19.7 fb$^{-1}$, accumulated by the CMS experiment at the LHC at a center-of-mass energy of 8 TeV. No evidence for such exotic decays is found in the data. Upper limits are set on the product of the cross section and branching fraction for several signal processes. The results are also compared to predictions of two-Higgs-doublet models, including those with an additional scalar singlet.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Comparison, for the $ \mathrm {h} \to \mathrm {a} \mathrm {a} \to 4\tau $ search, of $m_{\mu +\text {X}}$ distributions for data (black markers) and the misidentified jet background estimate (solid histogram) in the low-$ {m_\mathrm {T}} $ (left) and high-$ {m_\mathrm {T}} $ (right) bins. Predicted signal distributions (dotted lines) for each of the four Higgs boson production mechanisms are also shown; the distributions are normalized to an integrated luminosity of the data sample of 19.7 fb$^{-1}$, assuming SM Higgs boson production cross sections and $\mathcal {B}( \mathrm {h}\to \mathrm {a} \mathrm {a} ) \mathcal {B}^{2}( \mathrm {a} \to \tau ^+\tau ^-) = $ 0.1. The last bin on the right contains all the events with $m_{\mu +\text {X}}\geq $ 4 GeV, which correspond to the numbers reported in Table 3.

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Figure 1-a:
Comparison, for the $ \mathrm {h} \to \mathrm {a} \mathrm {a} \to 4\tau $ search, of $m_{\mu +\text {X}}$ distributions for data (black markers) and the misidentified jet background estimate (solid histogram) in the low-$ {m_\mathrm {T}} $ bin. Predicted signal distributions (dotted lines) for each of the four Higgs boson production mechanisms are also shown; the distributions are normalized to an integrated luminosity of the data sample of 19.7 fb$^{-1}$, assuming SM Higgs boson production cross sections and $\mathcal {B}( \mathrm {h}\to \mathrm {a} \mathrm {a} ) \mathcal {B}^{2}( \mathrm {a} \to \tau ^+\tau ^-) = $ 0.1. The last bin on the right contains all the events with $m_{\mu +\text {X}}\geq $ 4 GeV, which correspond to the numbers reported in Table 3.

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Figure 1-b:
Comparison, for the $ \mathrm {h} \to \mathrm {a} \mathrm {a} \to 4\tau $ search, of $m_{\mu +\text {X}}$ distributions for data (black markers) and the misidentified jet background estimate (solid histogram) in the high-$ {m_\mathrm {T}} $ bin. Predicted signal distributions (dotted lines) for each of the four Higgs boson production mechanisms are also shown; the distributions are normalized to an integrated luminosity of the data sample of 19.7 fb$^{-1}$, assuming SM Higgs boson production cross sections and $\mathcal {B}( \mathrm {h}\to \mathrm {a} \mathrm {a} ) \mathcal {B}^{2}( \mathrm {a} \to \tau ^+\tau ^-) = $ 0.1. The last bin on the right contains all the events with $m_{\mu +\text {X}}\geq $ 4 GeV, which correspond to the numbers reported in Table 3.

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Figure 2:
The best fit to the data for a signal-plus-background model with $ m_{ \mathrm {a} } =$ 35 GeV, including profiling of the uncertainties, in the search for $ mathrm {h} \to \mathrm {a} \mathrm {a} \to 2\mu 2\mathrm{ b } $ events.

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Figure 3:
Background and signal ($ m_{ \mathrm {a} } = $ 35 GeV) models, scaled to their expected yields, for the combination of all final states in the search for $ \mathrm {h} \to \mathrm {a} \mathrm {a} \to 2\mu 2\tau $ decays. The two components of the background model, ZZ and reducible processes, are drawn. The signal sample is scaled with $\sigma _{ \mathrm {h} }$ as predicted in the SM, assuming $\mathcal {B}( \mathrm {h} \to \mathrm {a} \mathrm {a} ) = $ 10%, and considering decays of the pseudoscalar $ \mathrm {a} $ boson to leptons only ($\mathcal {B}( \mathrm {a} \rightarrow {\tau ^{+}}{\tau ^{-}})+\mathcal {B}( \mathrm {a} \rightarrow \mu ^+\mu ^-)+\mathcal {B}( \mathrm {a} \rightarrow \mathrm {e} ^+\mathrm {e} ^-) = 1$) using Eq.(\ref {eq:2hdm}). The results are shown after a simultaneous maximum likelihood fit in all five channels that takes into account the systematic uncertainties described in Section 6.

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Figure 4:
Observed 95% CL limits on the branching fraction $\mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} ) \mathcal {B}^{2}( \mathrm {a} \rightarrow \tau ^{+}\tau ^{-})$ assuming SM h production rates, compared to expected limits for pseudoscalar mass points between 5 and 15 GeV.

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Figure 5:
Observed and expected upper limits at 95% CL on the h boson production normalized to the SM prediction times $\mathcal {B}( { \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\mathrm{ b } )$.

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Figure 6:
Expected and observed upper limits at 95% CL on the h boson production normalized to the SM prediction times $\mathcal {B}( {{ \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\tau } )$ in the $\mu ^+\mu ^-\tau_\mathrm {e}^+\tau_\mathrm {e}^-$ (top left), $\mu ^+\mu ^-\tau _{e}^\pm \tau _{\mu }^\mp $ (top right), $\mu ^+\mu ^-\tau_\mathrm {e}^\pm \tau _{\rm {h}}^\mp $ (center left), $\mu ^+\mu ^-\tau _{\mu }^\pm {\tau _{\rm h}}^\mp $ (center right), and $\mu ^+\mu ^-{\tau _{\rm h}}^+{\tau _{\rm h}}^-$ (bottom left) final states, and for the combination of these five final states (bottom right). None of the event excesses exceed two standard deviations in global significance.

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Figure 6-a:
Expected and observed upper limits at 95% CL on the h boson production normalized to the SM prediction times $\mathcal {B}( {{ \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\tau } )$ in the $\mu ^+\mu ^-\tau_\mathrm {e}^+\tau_\mathrm {e}^-$ final state. None of the event excesses exceed two standard deviations in global significance.

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Figure 6-b:
Expected and observed upper limits at 95% CL on the h boson production normalized to the SM prediction times $\mathcal {B}( {{ \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\tau } )$ in the $\mu ^+\mu ^-\tau _{e}^\pm \tau _{\mu }^\mp $ final state. None of the event excesses exceed two standard deviations in global significance.

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Figure 6-c:
Expected and observed upper limits at 95% CL on the h boson production normalized to the SM prediction times $\mathcal {B}( {{ \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\tau } )$ in the $\mu ^+\mu ^-\tau_\mathrm {e}^\pm \tau _{\rm {h}}^\mp $ final state. None of the event excesses exceed two standard deviations in global significance.

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Figure 6-d:
Expected and observed upper limits at 95% CL on the h boson production normalized to the SM prediction times $\mathcal {B}( {{ \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\tau } )$ in the $\mu ^+\mu ^-\tau _{\mu }^\pm {\tau _{\rm h}}^\mp $ final state. None of the event excesses exceed two standard deviations in global significance.

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Figure 6-e:
Expected and observed upper limits at 95% CL on the h boson production normalized to the SM prediction times $\mathcal {B}( {{ \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\tau } )$ in the $\mu ^+\mu ^-{\tau _{\rm h}}^+{\tau _{\rm h}}^-$ final state. None of the event excesses exceed two standard deviations in global significance.

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Figure 6-f:
Expected and observed upper limits at 95% CL on the h boson production normalized to the SM prediction times $\mathcal {B}( {{ \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\tau } )$ for the combination of the five final states. None of the event excesses exceed two standard deviations in global significance.

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Figure 7:
Expected and observed 95% CL exclusion limits on $({\sigma _{ \mathrm {h} }}/{\sigma _{\textrm {SM}}}) \mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} ) \mathcal {B}^2( \mathrm {a} \rightarrow \mu ^+\mu ^-)$ for various exotic h boson decay searches performed with data collected at 8 TeV with the CMS detector, assuming that the branching fractions of the pseudoscalar boson to muons, $\tau $ leptons and b quarks follow Eqs.(1)-(2). This assumption implies that the limit shown for $ { \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\mathrm{ b } $ is valid only in type-1 and -2 2HDM+S.

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Figure 8:
Expected and observed 95% CL limits on $( \sigma _{ \mathrm {h} }/\sigma _{\textrm {SM}}) \mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} )$ in 2HDM+S type-1 (top left), type-2 with $\tan\beta =$ 2 (top right), type-3 with $\tan\beta =$ 5 (bottom left), and type-4 with $\tan\beta =$ 0.5 (bottom right). Limits are shown as a function of the mass of the light boson, $ {m_{ \mathrm {a} }} $. The branching fractions of the pseudoscalar boson to SM particles are computed following a model described in Ref. [8]. Grey shaded regions correspond to regions where theoretical predictions for the branching fractions of the pseudoscalar boson to SM particles are not reliable.

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Figure 8-a:
Expected and observed 95% CL limits on $( \sigma _{ \mathrm {h} }/\sigma _{\textrm {SM}}) \mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} )$ in 2HDM+S type-1. Limits are shown as a function of the mass of the light boson, $ {m_{ \mathrm {a} }} $. The branching fractions of the pseudoscalar boson to SM particles are computed following a model described in Ref. [8]. Grey shaded regions correspond to regions where theoretical predictions for the branching fractions of the pseudoscalar boson to SM particles are not reliable.

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Figure 8-b:
Expected and observed 95% CL limits on $( \sigma _{ \mathrm {h} }/\sigma _{\textrm {SM}}) \mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} )$ in type-2 with $\tan\beta =$ 2. Limits are shown as a function of the mass of the light boson, $ {m_{ \mathrm {a} }} $. The branching fractions of the pseudoscalar boson to SM particles are computed following a model described in Ref. [8]. Grey shaded regions correspond to regions where theoretical predictions for the branching fractions of the pseudoscalar boson to SM particles are not reliable.

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Figure 8-c:
Expected and observed 95% CL limits on $( \sigma _{ \mathrm {h} }/\sigma _{\textrm {SM}}) \mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} )$ in type-3 with $\tan\beta =$ 5. Limits are shown as a function of the mass of the light boson, $ {m_{ \mathrm {a} }} $. The branching fractions of the pseudoscalar boson to SM particles are computed following a model described in Ref. [8]. Grey shaded regions correspond to regions where theoretical predictions for the branching fractions of the pseudoscalar boson to SM particles are not reliable.

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Figure 8-d:
Expected and observed 95% CL limits on $( \sigma _{ \mathrm {h} }/\sigma _{\textrm {SM}}) \mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} )$ in type-4 with $\tan\beta =$ 0.5. Limits are shown as a function of the mass of the light boson, $ {m_{ \mathrm {a} }} $. The branching fractions of the pseudoscalar boson to SM particles are computed following a model described in Ref. [8]. Grey shaded regions correspond to regions where theoretical predictions for the branching fractions of the pseudoscalar boson to SM particles are not reliable.

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Figure 9:
The 95% CL limit on $( \sigma _{ \mathrm {h} }/\sigma _{\textrm {SM}} ) \mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} )$ in 2HDM+S type-3 (left) and type-4 (right) for different $\tan\beta $ values, for the $ { \mathrm {h} }\to \mathrm {a} \mathrm {a} \to 2\mu 2\tau $ and $ { \mathrm {h} }\to \mathrm {a} \mathrm {a} \to 2\mu 2\mathrm{ b } $ analyses at $ {m_{ \mathrm {a} }} =$ 40 GeV. The branching fractions of the pseudoscalar boson to SM particles are computed following the prescriptions in Ref. [8].

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Figure 9-a:
The 95% CL limit on $( \sigma _{ \mathrm {h} }/\sigma _{\textrm {SM}} ) \mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} )$ in 2HDM+S type-3 for different $\tan\beta $ values, for the $ { \mathrm {h} }\to \mathrm {a} \mathrm {a} \to 2\mu 2\tau $ and $ { \mathrm {h} }\to \mathrm {a} \mathrm {a} \to 2\mu 2\mathrm{ b } $ analyses at $ {m_{ \mathrm {a} }} =$ 40 GeV. The branching fractions of the pseudoscalar boson to SM particles are computed following the prescriptions in Ref. [8].

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Figure 9-b:
The 95% CL limit on $( \sigma _{ \mathrm {h} }/\sigma _{\textrm {SM}} ) \mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} )$ in 2HDM+S and type-4 for different $\tan\beta $ values, for the $ { \mathrm {h} }\to \mathrm {a} \mathrm {a} \to 2\mu 2\tau $ and $ { \mathrm {h} }\to \mathrm {a} \mathrm {a} \to 2\mu 2\mathrm{ b } $ analyses at $ {m_{ \mathrm {a} }} =$ 40 GeV. The branching fractions of the pseudoscalar boson to SM particles are computed following the prescriptions in Ref. [8].
Tables

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Table 1:
Doublets to which the different types of fermions couple in the four types of 2HDM without FCNC at lowest order.

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Table 2:
Ratio of the Yukawa couplings of the pseudoscalar boson a of the 2HDM relative to those of the Higgs boson of the SM, in the four types of 2HDM without FCNC at lowest order.

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Table 3:
Expected signal yields for the $ \mathrm {h} \to \mathrm {a} \mathrm {a} \to 4\tau $ process for a representative pseudoscalar mass of 9 GeV, in both $ m_\mathrm {T} $ bins, assuming SM cross sections and $\mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} ) \mathcal {B}^{2}( \mathrm {a} \rightarrow \tau ^{+}\tau ^{-})=$ 0.1, in the context of the $ \mathrm {h} \to \mathrm {a} \mathrm {a} \to 4\tau $ search. Expected background yields as well as observed numbers of events are also quoted. Only the statistical uncertainty is given for signal yields.

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Table 4:
Expected signal and background yields, together with the number of observed events, for the $ \mathrm {h} \to \mathrm {a} \mathrm {a} \to 2\mu 2\mathrm{ b } $ search, in the range 20 $ \leq {m_{\mu \mu }} \leq $ 70 GeV. Signal yields are evaluated assuming $\mathcal {B}( \mathrm {h} \rightarrow \mathrm {a} \mathrm {a} )= $ 10% and $\mathcal {B}( \mathrm {a} \mathrm {a} \rightarrow \mu ^+\mu ^- \mathrm{ b } \overline{ \mathrm{ b } })= 1.7\times 10^{-3}$, with the latter obtained in the context of type-3 2HDM+S with $\tan\beta =$ 2.

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Table 5:
Expected and observed yields in the search for $ { \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\tau $ decays. The signal samples are scaled with the production cross section for the SM h boson, assuming $\mathcal {B}( \mathrm {h} \to \mathrm {a} \mathrm {a} ) = 10%$ and considering decays of the pseudoscalar $ \mathrm {a} $ boson to leptons only. Background yields are obtained after a maximum likelihood fit to observed data, taking into account the systematic uncertainties detailed in Section 6.

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Table 6:
Sources of systematic uncertainties, and their effects on process yields, for the three different searches. Ranges for the $ {{ \mathrm {h} }\to { \mathrm {a} \mathrm {a} }\to 2\mu 2\tau } $ search correspond to different final states.

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Table 7:
Branching fractions of the pseudoscalar boson a to muons, $\tau $ leptons, and b quarks, in the four 2HDM+S scenarios considered in Fig. 8, as a function of the light boson mass. The branching fraction $\mathcal {B}( \mathrm {a} \to \mathrm{b} \overline{ \mathrm{ b } } )$ is not indicated in the mass range $ {m_{ \mathrm {a} }} \in $ [5,15] GeV because it is not used to interpret the results.
Summary
Searches for the decay of the SM-like Higgs boson to pairs of light scalar particles have been performed using 19.7 fb$^{-1}$ of pp collisions at a center-of-mass energy of 8 TeV, collected by the CMS experiment at the LHC, in final states with $\tau$ leptons, muons, or b quark jets. Such signatures are motivated in light of the non-negligible branching fraction provided in recent experimental constraints for non-SM ${{\mathrm{h}}} $ decays. The data were found to be compatible with SM predictions. Whereas indirect measurements from the combination of data collected by the ATLAS and CMS collaborations at the LHC at 8 TeV center-of-mass energy set an upper limit of 34% on branching fraction of the Higgs boson to BSM, direct limits provide complementarity and improve the sensitivity to the 2HDM+S models for particular scenarios and pseudoscalar masses. Upper limits at 95% CL on $({\sigma_{{{\mathrm{h}}} }}/{\sigma_{\textrm{SM}}}) \, \mathcal{B}({{\mathrm{h}}} \rightarrow {\mathrm{a}} {\mathrm{a}} )$, assuming SM production of the 125 GeV Higgs boson, are as low as 17, 16, and 4%, and have been determined for the ${{{{\mathrm{h}}} }\to{{\mathrm{a}} {\mathrm{a}} }\to4\tau} $, ${{{{\mathrm{h}}} }\to{{\mathrm{a}} {\mathrm{a}} }\to2\mu2\mathrm{ b }} $, and ${{{{\mathrm{h}}} }\to{{\mathrm{a}} {\mathrm{a}} }\to2\mu2\tau} $ analyses, respectively.
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