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| CMS-HIG-18-024 ; CERN-EP-2020-061 | ||
| Search for a light pseudoscalar Higgs boson in the boosted $\mu\mu\tau\tau$ final state in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 18 May 2020 | ||
| JHEP 08 (2020) 139 | ||
| Abstract: A search for a light pseudoscalar Higgs boson (a) decaying from the 125 GeV (or a heavier) scalar Higgs boson (H) is performed using the 2016 LHC proton-proton collision data at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$, collected by the CMS experiment. The analysis considers gluon fusion and vector boson fusion production of the H, followed by the decay $\mathrm{H}\to\mathrm{a}\mathrm{a}\to\mu\mu\tau\tau$, and considers pseudoscalar masses in the range 3.6 $ < {m_{\mathrm{a}}} < $ 21 GeV. Because of the large mass difference between the H and the a bosons and the small masses of the a boson decay products, both the $\mu\mu$ and the $\tau\tau$ pairs have high Lorentz boost and are collimated. The $\tau\tau$ reconstruction efficiency is increased by modifying the standard technique for hadronic $\tau$ lepton decay reconstruction to account for a nearby muon. No significant signal is observed. Model-independent limits are set at 95% confidence level, as a function of ${m_{\mathrm{a}}}$, on the branching fraction (${\mathcal{B}}$) for $\mathrm{H}\to\mathrm{a}\mathrm{a}\to\mu\mu\tau\tau$, down to 1.5 (2.0) $\times$ 10$^{-4}$ for ${m_{\mathrm{H}}} =$ 125 (300) GeV. Model-dependent limits on ${\mathcal{B}}(\mathrm{H}\to\mathrm{a}\mathrm{a})$ are set within the context of two Higgs doublets plus singlet models, with the most stringent results obtained for Type-III models. These results extend current LHC searches for heavier a bosons that decay to resolved lepton pairs and provide the first such bounds for an H boson with a mass above 125 GeV. | ||
| Links: e-print arXiv:2005.08694 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The efficiency of the standard HPS (dashed lines) and $ {\tau _{\mu}} {\tau _{\mathrm{h}}} $ HPS reconstruction used in this search (solid lines) as a function of pseudoscalar boson mass for $ {m_{\mathrm{H}}} =$ 125 (red) and 300 GeV (green). The events are required to have two reconstructed muons passing identification and isolation criteria. The efficiency is measured by additionally requiring a third muon passing identification requirements and a ${\tau _{\mathrm{h}}}$ candidate reconstructed using either the standard HPS algorithm or the $ {\tau _{\mu}} {\tau _{\mathrm{h}}} $ HPS algorithm and passing isolation requirements. |
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Figure 2:
Schematic of the fit regions in the analysis. Events with two isolated muons and no $ {\tau _{\mu}} {\tau _{\mathrm{h}}} $ candidates constitute the control region (blue). Events that have a $ {\tau _{\mu}} {\tau _{\mathrm{h}}} $ candidate are further divided based on the isolation of the ${\tau _{\mathrm{h}}}$ candidate with isolated $ {\tau _{\mu}} {\tau _{\mathrm{h}}} $ candidates forming the signal region (green) and the remaining $ {\tau _{\mu}} {\tau _{\mathrm{h}}} $ candidates forming the sideband (red). Additionally, the $\mu \mu $ candidates that fail the muon isolation selection form two analogous regions for the validation of the background fit model (gray). |
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Figure 3:
Background model fits and observed data in the control region ${m(\mu \mu)}$ distribution. The figures are divided into three fit ranges: 2.5 $ < {m(\mu \mu)} < $ 8.5 GeV (left), 6 $ < {m(\mu \mu)} < $ 14 GeV (middle), and 11 $ < {m(\mu \mu)} < $ 25 GeV (right). |
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Figure 3-a:
Background model fit and observed data in the control region ${m(\mu \mu)}$ distribution, in the fit range 2.5 $ < {m(\mu \mu)} < $ 8.5 GeV. |
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Figure 3-b:
Background model fit and observed data in the control region ${m(\mu \mu)}$ distribution, in the fit range 6 $ < {m(\mu \mu)} < $ 14 GeV. |
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Figure 3-c:
Background model fit and observed data in the control region ${m(\mu \mu)}$ distribution, in the fit range 11 $ < {m(\mu \mu)} < $ 25 GeV. |
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Figure 4:
Projections of 2D background model fits and observed data in the sideband on the ${m(\mu \mu)}$ (left), and ${m(\mu \mu {\tau _{\mu}} {\tau _{\mathrm{h}}})}$ (right) axes with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV. The figures are divided into three fit ranges: 2.5 $ < {m(\mu \mu)} < $ 8.5 GeV (upper), 6 $ < {m(\mu \mu)} < $ 14 GeV (middle), and 11 $ < {m(\mu \mu)} < $ 25 GeV (lower). |
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Figure 4-a:
Projection of the 2D background model fit and observed data in the sideband on the ${m(\mu \mu)}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV, in the fit range 2.5 $ < {m(\mu \mu)} < $ 8.5 GeV. |
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Figure 4-b:
Projection of the 2D background model fit and observed data in the sideband on the ${m(\mu \mu {\tau _{\mu}} {\tau _{\mathrm{h}}})}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV, in the fit range 2.5 $ < {m(\mu \mu)} < $ 8.5 GeV. |
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Figure 4-c:
Projection of the 2D background model fit and observed data in the sideband on the ${m(\mu \mu)}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV, in the fit range 6 $ < {m(\mu \mu)} < $ 14 GeV. |
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Figure 4-d:
Projection of the 2D background model fit and observed data in the sideband on the ${m(\mu \mu {\tau _{\mu}} {\tau _{\mathrm{h}}})}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV, in the fit range 6 $ < {m(\mu \mu)} < $ 14 GeV. |
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Figure 4-e:
Projection of the 2D background model fit and observed data in the sideband on the ${m(\mu \mu)}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV, in the fit range 11 $ < {m(\mu \mu)} < $ 25 GeV. |
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Figure 4-f:
Projection of the 2D background model fit and observed data in the sideband on the ${m(\mu \mu {\tau _{\mu}} {\tau _{\mathrm{h}}})}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV, in the fit range 11 $ < {m(\mu \mu)} < $ 25 GeV. |
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Figure 5:
Projections of 2D background model fits and observed data in the signal region on the ${m(\mu \mu)}$ (left), and ${m(\mu \mu {\tau _{\mu}} {\tau _{\mathrm{h}}})}$ (right) axes with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV. The figures are divided into three fit ranges: 2.5 $ < {m(\mu \mu)} < $ 8.5 GeV (upper), 6 $ < {m(\mu \mu)} < $ 14 GeV (middle), and 11 $ < {m(\mu \mu)} < $ 25 GeV (lower). |
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Figure 5-a:
Projection of the 2D background model fit and observed data in the signal region on the ${m(\mu \mu)}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV in the fit range 2.5 $ < {m(\mu \mu)} < $ 8.5 GeV. |
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Figure 5-b:
Projection of the 2D background model fit and observed data in the signal region on the ${m(\mu \mu {\tau _{\mu}} {\tau _{\mathrm{h}}})}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV in the fit range 2.5 $ < {m(\mu \mu)} < $ 8.5 GeV. |
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Figure 5-c:
Projection of the 2D background model fit and observed data in the signal region on the ${m(\mu \mu)}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV in the fit range 6 $ < {m(\mu \mu)} < $ 14 GeV. |
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Figure 5-d:
Projection of the 2D background model fit and observed data in the signal region on the ${m(\mu \mu {\tau _{\mu}} {\tau _{\mathrm{h}}})}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV in the fit range 6 $ < {m(\mu \mu)} < $ 14 GeV. |
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Figure 5-e:
Projection of the 2D background model fit and observed data in the signal region on the ${m(\mu \mu)}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV in the fit range 11 $ < {m(\mu \mu)} < $ 25 GeV. |
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Figure 5-f:
Projection of the 2D background model fit and observed data in the signal region on the ${m(\mu \mu {\tau _{\mu}} {\tau _{\mathrm{h}}})}$ axis with sample signal distributions that assume H boson masses of $ {m_{\mathrm{H}}} =$ 125 and 300 GeV in the fit range 11 $ < {m(\mu \mu)} < $ 25 GeV. |
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Figure 6:
Observed data distribution, as a function of the 4-body visible mass and $\mu \mu $ invariant mass for the signal region; 614 events are observed. |
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Figure 7:
Model-independent 95% CL upper limits on $ {\sigma _{\mathrm{H}}} {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a} \to \mu \mu \tau \tau)/ {\sigma _{\text {SM}}} $ as a function of pseudoscalar boson mass for a Higgs boson with $ {m_{\mathrm{H}}} = $ 125 GeV (left), and 300 GeV (right). The vertical dashed lines indicate the transition between the $\mu \mu $ mass fit ranges for a given mass hypothesis, occurring at $ {m_{\mathrm{a}}} =$ 8 and 11.5 GeV. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. |
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Figure 7-a:
Model-independent 95% CL upper limits on $ {\sigma _{\mathrm{H}}} {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a} \to \mu \mu \tau \tau)/ {\sigma _{\text {SM}}} $ as a function of pseudoscalar boson mass for a Higgs boson with $ {m_{\mathrm{H}}} = $ 125 GeV. The vertical dashed lines indicate the transition between the $\mu \mu $ mass fit ranges for a given mass hypothesis, occurring at $ {m_{\mathrm{a}}} =$ 8 and 11.5 GeV. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. |
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Figure 7-b:
Model-independent 95% CL upper limits on $ {\sigma _{\mathrm{H}}} {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a} \to \mu \mu \tau \tau)/ {\sigma _{\text {SM}}} $ as a function of pseudoscalar boson mass for a Higgs boson with $ {m_{\mathrm{H}}} = $ 300 GeV. The vertical dashed lines indicate the transition between the $\mu \mu $ mass fit ranges for a given mass hypothesis, occurring at $ {m_{\mathrm{a}}} =$ 8 and 11.5 GeV. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. |
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Figure 8:
Observed (black) and expected (blue, median and 68%) model-specific 95% CL upper limits on $ {\sigma _{\mathrm{H}}} {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a})/ {\sigma _{\text {SM}}} $ as a function of ${m_{\mathrm{a}}}$ for the Type-I 2HDM+S at $ {\tan\beta} = $ 1.5 and $ {m_{\mathrm{H}}} = $ 125 GeV. The assumed model branching fractions for pseudoscalar Higgs boson decay to $\mu \mu $ and $\tau \tau $ are taken from Ref. [71] and are approximately independent of ${\tan\beta}$. |
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Figure 9:
Model-specific 95% CL upper limits on $ {\sigma _{\mathrm{H}}} {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a})/ {\sigma _{\text {SM}}} $ for three model types of the 2HDM+S as a function of ${\tan\beta}$ and ${m_{\mathrm{a}}}$, for $ {m_{\mathrm{H}}} = $ 125 GeV. Contours for two values of $ {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a})$ are shown for reference. The assumed model branching fractions for pseudoscalar Higgs boson decay to $\mu \mu $ and $\tau \tau $ are taken from Ref. [71]. |
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Figure 9-a:
Model-specific 95% CL upper limits on $ {\sigma _{\mathrm{H}}} {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a})/ {\sigma _{\text {SM}}} $ for model 2HDM+S Type II as a function of ${\tan\beta}$ and ${m_{\mathrm{a}}}$, for $ {m_{\mathrm{H}}} = $ 125 GeV. Contours for two values of $ {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a})$ are shown for reference. The assumed model branching fractions for pseudoscalar Higgs boson decay to $\mu \mu $ and $\tau \tau $ are taken from Ref. [71]. |
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Figure 9-b:
Model-specific 95% CL upper limits on $ {\sigma _{\mathrm{H}}} {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a})/ {\sigma _{\text {SM}}} $ for model 2HDM+S Type III as a function of ${\tan\beta}$ and ${m_{\mathrm{a}}}$, for $ {m_{\mathrm{H}}} = $ 125 GeV. Contours for two values of $ {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a})$ are shown for reference. The assumed model branching fractions for pseudoscalar Higgs boson decay to $\mu \mu $ and $\tau \tau $ are taken from Ref. [71]. |
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Figure 9-c:
Model-specific 95% CL upper limits on $ {\sigma _{\mathrm{H}}} {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a})/ {\sigma _{\text {SM}}} $ for model 2HDM+S Type IV as a function of ${\tan\beta}$ and ${m_{\mathrm{a}}}$, for $ {m_{\mathrm{H}}} = $ 125 GeV. Contours for two values of $ {\mathcal {B}}(\mathrm{H} \to \mathrm{a} \mathrm{a})$ are shown for reference. The assumed model branching fractions for pseudoscalar Higgs boson decay to $\mu \mu $ and $\tau \tau $ are taken from Ref. [71]. |
| Tables | |
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Table 1:
Background model parameters and their relations among the three fit regions in the analysis. The ${\mu \mu}$ background model includes the five meson resonances modeled using a Voigt function over an exponential continuum. The 4-body background model includes an error function multiplied with the sum of two exponential distributions. Three types of fit region relations are used: (a) constrained, in which the parameters are the same in the indicated regions, (b) free, in which the parameter is not related to those in any other region, and (c) related via the $ {\tau _{\mu}} {\tau _{\mathrm{h}}} $ tight-to-loose ratio, in which the indicated parameter in the signal region is constrained to the corresponding parameter in the sideband via a linear transformation. |
| Summary |
| A search for Higgs boson (H) decays to a pair of light pseudoscalar bosons (a) is presented, including the first such LHC results for an H with mass above 125 GeV. The light pseudoscalars decay to $\mu\mu$ and $\tau\tau$ with substantial overlap between the leptons because of the Lorentz boost. This difficult topology motivates the development of a dedicated $\tau_{\mu}\tau_{\mathrm{h}}$ reconstruction method to increase the acceptance. Data collected by the CMS Collaboration at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$, are examined and no significant excess over standard model (SM) processes is observed. This analysis obtains model-independent upper limits at 95% confidence level on the branching fraction (${\mathcal{B}}$) of a SM-like Higgs boson (H), decaying to a pair of pseudoscalar bosons (a) in the $\mu\mu\tau\tau$ final state, ${\sigma_{\mathrm{H}}} {\mathcal{B}}(\mathrm{H}\to\mathrm{a}\mathrm{a}\to\mu\mu\tau\tau)/{\sigma_{\text{SM}}} $, as well as model-specific upper limits on ${\sigma_{\mathrm{H}}} {\mathcal{B}}(\mathrm{H}\to\mathrm{a}\mathrm{a})/{\sigma_{\text{SM}}} $ for Type-I, -II, -III, and -IV two Higgs doublets plus singlet models. In the Type-I model, the upper limit on the allowed branching fraction is approximately independent of ${\tan\beta}$, with the most stringent limit of 5% set for ${m_{\mathrm{a}}} \approx $ 4.5 GeV. For the Type-II and -III models with ${m_{\mathrm{a}}} $ below the $\mathrm{b\bar{b}}$ threshold, upper limits on ${\mathcal{B}}(\mathrm{H}\to\mathrm{a}\mathrm{a})$ are stronger than the 0.47 inferred from combined measurements of SM Higgs couplings for ${\tan\beta}\gtrsim $ 0.8-0.9, becoming as strong as 10% for ${\tan\beta}\gtrsim $ 1.5. In the Type-III models, the predicted branching fraction to leptons increases with ${\tan\beta}$, leading to strong upper limits for all pseudoscalar boson masses tested when ${\tan\beta}\gtrsim $ 1.5. In contrast, the strongest upper limits for Type-IV models are set when ${\tan\beta} < $ 1. These results significantly extend upper limits obtained by earlier searches by the CMS and ATLAS Collaborations, such as those obtained by CMS with 8 TeV data [39], and are complementary to present searches (e.g. Ref. [40]) at higher ${m_{\mathrm{a}}} $ that lead to resolved $\mu\mu$ and $\tau\tau$ final states. |
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