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| CMS-SUS-23-005 ; CERN-EP-2026-200 | ||
| Search for a light pseudoscalar Higgs boson in final states with boosted muon pairs and tau lepton pairs in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 11 September 2026 | ||
| Submitted to the Journal of High Energy Physics | ||
| Abstract: A search is presented for a light pseudoscalar Higgs boson, $ a $, using LHC proton-proton data at $ \sqrt{s} = $ 13 TeV collected by the CMS experiment from 2016 to 2018, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The analysis targets the process $ \mathrm{H} \to a a \to \mu\mu\tau\tau $, where the scalar H can be either the 125 GeV Higgs boson or a heavier state. Because of the large mass difference between H and $ a $, the two tau leptons are highly Lorentz boosted and collimated, often failing standard CMS tau lepton reconstruction. To address this, dedicated ditau reconstruction techniques, including a deep neural network, are developed to improve the efficiency for events in which at least one tau lepton decays to hadrons. No significant excess over the standard model (SM) background is observed. Model-independent upper limits at the 95% confidence level are set on the product of the Higgs boson production cross section and branching fraction, divided by the SM cross section. For a Higgs boson mass ($ m_{\mathrm{H}} $) equal to 125 GeV, these limits range from 5 $ \times 10^{-5} $ to 2.7 $ \times 10^{-4} $ for pseudoscalar masses between 3.6 and 21 GeV. For the first time using LHC data, upper limits are also obtained in final states with tau leptons for a hypothetical scalar boson with a mass up to 1 TeV. For $ m_{\mathrm{H}} = $ 1 TeV, the limits range from 3.5 $ \times 10^{-4} $ to 1.5 $ \times 10^{-3} $ for pseudoscalar masses between 3.6 and 50 GeV. In addition, limits are set for SM extensions involving two-Higgs-doublet plus singlet models for $ m_{\mathrm{H}} = $ 125 GeV, extending previous LHC results. | ||
| Links: e-print arXiv:2609.12683 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Feynman diagram of a Higgs boson produced via gluon fusion and decaying into two pseudoscalar bosons, one of which subsequently decays into two muons and the other decays into two tau leptons. |
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Figure 2:
The $ \tau\tau $ reconstruction efficiency of the standard $ \tau $ HPS algorithm (open markers) and the modified $ \tau_{\mu}\tau_\mathrm{h} $ (left) or $ \tau_{\mathrm{e}}\tau_\mathrm{h} $ (right) HPS algorithm (solid markers) as a function of $ m_{a } $ for $ m_{\mathrm{H}} = $ 125, 250, 500, 750, and 1000 GeV. Because of the requirement $ \Delta R(\tau_{\ell}, \tau_\mathrm{h}) < $ 0.8, the efficiency for $ m_{\mathrm{H}} = $ 125 GeV decreases above $ m_{a } = $ 12 GeV in the $ \tau_{\mu}\tau_\mathrm{h} $ channel and above $ m_{a } = $ 14 GeV in the $ \tau_{\mathrm{e}}\tau_\mathrm{h} $ channel, corresponding to the region in which the $ \tau\tau $ topology becomes increasingly resolved. In the right plot, the dip in the standard HPS efficiency for the low $ m_{a } $ region is due to two competing effects: the requirement of $ p_{\mathrm{T}}^{\mathrm{e}} > $ 10 GeV, which loses efficiency with increasing $ m_{a } $, and the requirement of passing an anti-electron discriminant, which increases in efficiency as $ m_{a } $ increases. This analysis deploys the modified HPS reconstruction which shows a significantly higher efficiency than the standard $ \tau $ HPS scenario. |
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Figure 2-a:
The $ \tau\tau $ reconstruction efficiency of the standard $ \tau $ HPS algorithm (open markers) and the modified $ \tau_{\mu}\tau_\mathrm{h} $ (left) or $ \tau_{\mathrm{e}}\tau_\mathrm{h} $ (right) HPS algorithm (solid markers) as a function of $ m_{a } $ for $ m_{\mathrm{H}} = $ 125, 250, 500, 750, and 1000 GeV. Because of the requirement $ \Delta R(\tau_{\ell}, \tau_\mathrm{h}) < $ 0.8, the efficiency for $ m_{\mathrm{H}} = $ 125 GeV decreases above $ m_{a } = $ 12 GeV in the $ \tau_{\mu}\tau_\mathrm{h} $ channel and above $ m_{a } = $ 14 GeV in the $ \tau_{\mathrm{e}}\tau_\mathrm{h} $ channel, corresponding to the region in which the $ \tau\tau $ topology becomes increasingly resolved. In the right plot, the dip in the standard HPS efficiency for the low $ m_{a } $ region is due to two competing effects: the requirement of $ p_{\mathrm{T}}^{\mathrm{e}} > $ 10 GeV, which loses efficiency with increasing $ m_{a } $, and the requirement of passing an anti-electron discriminant, which increases in efficiency as $ m_{a } $ increases. This analysis deploys the modified HPS reconstruction which shows a significantly higher efficiency than the standard $ \tau $ HPS scenario. |
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Figure 2-b:
The $ \tau\tau $ reconstruction efficiency of the standard $ \tau $ HPS algorithm (open markers) and the modified $ \tau_{\mu}\tau_\mathrm{h} $ (left) or $ \tau_{\mathrm{e}}\tau_\mathrm{h} $ (right) HPS algorithm (solid markers) as a function of $ m_{a } $ for $ m_{\mathrm{H}} = $ 125, 250, 500, 750, and 1000 GeV. Because of the requirement $ \Delta R(\tau_{\ell}, \tau_\mathrm{h}) < $ 0.8, the efficiency for $ m_{\mathrm{H}} = $ 125 GeV decreases above $ m_{a } = $ 12 GeV in the $ \tau_{\mu}\tau_\mathrm{h} $ channel and above $ m_{a } = $ 14 GeV in the $ \tau_{\mathrm{e}}\tau_\mathrm{h} $ channel, corresponding to the region in which the $ \tau\tau $ topology becomes increasingly resolved. In the right plot, the dip in the standard HPS efficiency for the low $ m_{a } $ region is due to two competing effects: the requirement of $ p_{\mathrm{T}}^{\mathrm{e}} > $ 10 GeV, which loses efficiency with increasing $ m_{a } $, and the requirement of passing an anti-electron discriminant, which increases in efficiency as $ m_{a } $ increases. This analysis deploys the modified HPS reconstruction which shows a significantly higher efficiency than the standard $ \tau $ HPS scenario. |
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Figure 3:
Receiver operating characteristic curve for the DEEPDITAU classifier. The tagging efficiency corresponds to the fraction of $ \tau_\mathrm{h}\tau_\mathrm{h} $ jets correctly identified as $ \tau_\mathrm{h}\tau_\mathrm{h} $ jets, while the mistagging rate corresponds to the fraction of light jets and b jets misidentified as $ \tau_\mathrm{h}\tau_\mathrm{h} $ jets. The dashed lines show that the medium working point used in this analysis corresponds to a tagging efficiency of 70% and a mistagging rate of approximately 1%. |
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Figure 4:
Schematic of the analysis strategy. In the Z boson mass range, the tight-to-loose ratio is derived from the $ \mathrm{Z}\to\mu\mu $+jet control region and sideband. In the analysis mass range, events with two isolated muons and no loosely selected $ \tau\tau $ candidates enter the control region. Events that additionally contain a loosely selected $ \tau\tau $ candidate are further categorized into the signal region or sideband, as described in the text. The sideband data, weighted by the tight-to-loose ratio, are used to estimate the background in the signal region. In addition, by inverting the isolation requirement on the muon that did not trigger the event, two analogous regions are defined for validating the tight-to-loose method. |
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Figure 5:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_{\mu}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 5-a:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_{\mu}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 5-b:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_{\mu}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 5-c:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_{\mu}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 5-d:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_{\mu}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 5-e:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_{\mu}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 5-f:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_{\mu}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
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Figure 6:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 6-a:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 6-b:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 6-c:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 6-d:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 6-e:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 6-f:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 125 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 7:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 250 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 7-a:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 250 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 7-b:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 250 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 7-c:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 250 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 7-d:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 250 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 7-e:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 250 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 7-f:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 250 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 8:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 500 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 8-a:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 500 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 8-b:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 500 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 8-c:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 500 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 8-d:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 500 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 8-e:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 500 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 8-f:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 500 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 9:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 750 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 9-a:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 750 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 9-b:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 750 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 9-c:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 750 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 9-d:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 750 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 9-e:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 750 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 9-f:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 750 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5, 11, and 20 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 10:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two lower $ m_{\mu\mu} $ ranges (3 $ < m_{\mu\mu} < $ 8 GeV and 8 $ < m_{\mu\mu} < $ 12 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5 and 11 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 10-a:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two lower $ m_{\mu\mu} $ ranges (3 $ < m_{\mu\mu} < $ 8 GeV and 8 $ < m_{\mu\mu} < $ 12 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5 and 11 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 10-b:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two lower $ m_{\mu\mu} $ ranges (3 $ < m_{\mu\mu} < $ 8 GeV and 8 $ < m_{\mu\mu} < $ 12 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5 and 11 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 10-c:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two lower $ m_{\mu\mu} $ ranges (3 $ < m_{\mu\mu} < $ 8 GeV and 8 $ < m_{\mu\mu} < $ 12 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5 and 11 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 10-d:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two lower $ m_{\mu\mu} $ ranges (3 $ < m_{\mu\mu} < $ 8 GeV and 8 $ < m_{\mu\mu} < $ 12 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 5 and 11 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 11:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two higher $ m_{\mu\mu} $ ranges (12 $ < m_{\mu\mu} < $ 30 GeV and 30 $ < m_{\mu\mu} < $ 55 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 20 and 50 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 11-a:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two higher $ m_{\mu\mu} $ ranges (12 $ < m_{\mu\mu} < $ 30 GeV and 30 $ < m_{\mu\mu} < $ 55 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 20 and 50 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 11-b:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two higher $ m_{\mu\mu} $ ranges (12 $ < m_{\mu\mu} < $ 30 GeV and 30 $ < m_{\mu\mu} < $ 55 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 20 and 50 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 11-c:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two higher $ m_{\mu\mu} $ ranges (12 $ < m_{\mu\mu} < $ 30 GeV and 30 $ < m_{\mu\mu} < $ 55 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 20 and 50 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 11-d:
Projections of the post-fit two-dimensional background pdfs (blue) and the observed data onto the $ m_{\mu\mu} $ (left) and four-body visible mass (right) axes for the two higher $ m_{\mu\mu} $ ranges (12 $ < m_{\mu\mu} < $ 30 GeV and 30 $ < m_{\mu\mu} < $ 55 GeV) in the $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel for $ m_{\mathrm{H}} = $ 1000 GeV using the 2018 data set. The rows correspond to pseudoscalar masses of $ m_{a } = $ 20 and 50 GeV, from top to bottom. Example signal distributions (red) are overlaid assuming $ \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau) = 5 \times 10^{-4} $. |
png pdf |
Figure 12:
Expected and observed limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, combining the $ \tau_{\mu}\tau_{\mathrm{e}} $, $ \tau_{\mu}\tau_\mathrm{h} $, $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h} $ final states, for $ m_{\mathrm{H}} = $ 125, 250, 500, 750, and 1000 GeV. The vertical dotted lines indicate the boundaries of the $ m_{\mu\mu} $ regions used in the background fit, as described in Section 7. |
png pdf |
Figure 12-a:
Expected and observed limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, combining the $ \tau_{\mu}\tau_{\mathrm{e}} $, $ \tau_{\mu}\tau_\mathrm{h} $, $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h} $ final states, for $ m_{\mathrm{H}} = $ 125, 250, 500, 750, and 1000 GeV. The vertical dotted lines indicate the boundaries of the $ m_{\mu\mu} $ regions used in the background fit, as described in Section 7. |
png pdf |
Figure 12-b:
Expected and observed limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, combining the $ \tau_{\mu}\tau_{\mathrm{e}} $, $ \tau_{\mu}\tau_\mathrm{h} $, $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h} $ final states, for $ m_{\mathrm{H}} = $ 125, 250, 500, 750, and 1000 GeV. The vertical dotted lines indicate the boundaries of the $ m_{\mu\mu} $ regions used in the background fit, as described in Section 7. |
png pdf |
Figure 12-c:
Expected and observed limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, combining the $ \tau_{\mu}\tau_{\mathrm{e}} $, $ \tau_{\mu}\tau_\mathrm{h} $, $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h} $ final states, for $ m_{\mathrm{H}} = $ 125, 250, 500, 750, and 1000 GeV. The vertical dotted lines indicate the boundaries of the $ m_{\mu\mu} $ regions used in the background fit, as described in Section 7. |
png pdf |
Figure 12-d:
Expected and observed limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, combining the $ \tau_{\mu}\tau_{\mathrm{e}} $, $ \tau_{\mu}\tau_\mathrm{h} $, $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h} $ final states, for $ m_{\mathrm{H}} = $ 125, 250, 500, 750, and 1000 GeV. The vertical dotted lines indicate the boundaries of the $ m_{\mu\mu} $ regions used in the background fit, as described in Section 7. |
png pdf |
Figure 12-e:
Expected and observed limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, combining the $ \tau_{\mu}\tau_{\mathrm{e}} $, $ \tau_{\mu}\tau_\mathrm{h} $, $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h} $ final states, for $ m_{\mathrm{H}} = $ 125, 250, 500, 750, and 1000 GeV. The vertical dotted lines indicate the boundaries of the $ m_{\mu\mu} $ regions used in the background fit, as described in Section 7. |
png pdf |
Figure 13:
Expected and observed limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, combining the final states for different Higgs boson mass hypotheses, for the pseudoscalar masses of 10 GeV (left) and 15 GeV (right). The straight lines between points are there to guide the eye. |
png pdf |
Figure 13-a:
Expected and observed limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, combining the final states for different Higgs boson mass hypotheses, for the pseudoscalar masses of 10 GeV (left) and 15 GeV (right). The straight lines between points are there to guide the eye. |
png pdf |
Figure 13-b:
Expected and observed limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, combining the final states for different Higgs boson mass hypotheses, for the pseudoscalar masses of 10 GeV (left) and 15 GeV (right). The straight lines between points are there to guide the eye. |
png pdf |
Figure 14:
Observed and expected limits in 2HDM+S models of Types I through IV. The contours at a branching fraction of 0.16 are motivated by a CMS combination that sets limits on Higgs boson decays to undetected modes, including modes not covered by the standard channels, such as the boosted topologies considered in this analysis [18]. In the Type-I model, $ \mathcal{B}(\mathrm{H}\rightarrow a a ) $ is independent of $ \tan\beta $. |
png pdf |
Figure 14-a:
Observed and expected limits in 2HDM+S models of Types I through IV. The contours at a branching fraction of 0.16 are motivated by a CMS combination that sets limits on Higgs boson decays to undetected modes, including modes not covered by the standard channels, such as the boosted topologies considered in this analysis [18]. In the Type-I model, $ \mathcal{B}(\mathrm{H}\rightarrow a a ) $ is independent of $ \tan\beta $. |
png pdf |
Figure 14-b:
Observed and expected limits in 2HDM+S models of Types I through IV. The contours at a branching fraction of 0.16 are motivated by a CMS combination that sets limits on Higgs boson decays to undetected modes, including modes not covered by the standard channels, such as the boosted topologies considered in this analysis [18]. In the Type-I model, $ \mathcal{B}(\mathrm{H}\rightarrow a a ) $ is independent of $ \tan\beta $. |
png pdf |
Figure 14-c:
Observed and expected limits in 2HDM+S models of Types I through IV. The contours at a branching fraction of 0.16 are motivated by a CMS combination that sets limits on Higgs boson decays to undetected modes, including modes not covered by the standard channels, such as the boosted topologies considered in this analysis [18]. In the Type-I model, $ \mathcal{B}(\mathrm{H}\rightarrow a a ) $ is independent of $ \tan\beta $. |
png pdf |
Figure 14-d:
Observed and expected limits in 2HDM+S models of Types I through IV. The contours at a branching fraction of 0.16 are motivated by a CMS combination that sets limits on Higgs boson decays to undetected modes, including modes not covered by the standard channels, such as the boosted topologies considered in this analysis [18]. In the Type-I model, $ \mathcal{B}(\mathrm{H}\rightarrow a a ) $ is independent of $ \tan\beta $. |
| Tables | |
png pdf |
Table 1:
Values of the scalar mass $ m_{\mathrm{H}} $ and pseudoscalar mass $ m_{a } $ considered in the search for $ \mathrm{H}\to a a \to\mu\mu\tau\tau $. The third column lists the generated pseudoscalar mass points or ranges; ranges are given together with the step size in braces. For $ m_{\mathrm{H}} = $ 125 GeV, the lightest neutral scalar corresponds to the SM Higgs boson. |
png pdf |
Table 2:
Measured scale factors and their uncertainties for $ \tau_\mathrm{h} $ candidates. The 2016 data comprise two eras with different strip tracker running conditions [71]. |
png pdf |
Table 3:
Tau lepton selection criteria in the four different channels. In the columns labeled $ \Delta R_{\mu\tau_{\ell}} $ and $ \Delta R_{\mu\tau_\mathrm{h}} $, $ \mu $ refers to either of the two muons reconstructed as the $ a \to\mu\mu $ candidate. In the column labeled $ \Delta R_{\mu\tau_{\ell}} $, $ \tau_{\ell} $ refers to either the $ \tau_{\mu} $ or $ \tau_{\mathrm{e}} $ candidate. In the final column, $ \mu_{2} $ refers to the nontriggering muon in the $ a \to\mu\mu $ pair. A dash ($ \text{---} $) means that the selection in the column header does not apply to the channel in that row. |
png pdf |
Table 4:
Four-body mass ranges used in the two-dimensional fit. For the $ \tau_{\mathrm{e}}\tau_\mathrm{h} $ channel, an additional requirement is imposed on $ m_{\mu\mu} $, as shown in the last row. All entries are given in GeV. |
png pdf |
Table 5:
The background model parameters and their relationships between the signal region and control region: shared, in which the parameters are identical in the indicated regions, and independent, in which the parameters vary freely and are not constrained by any other region. |
| Summary |
| A search for decays of the Higgs boson, H, to a pair of light pseudoscalar bosons, $ a $, is presented for both the standard model (SM) Higgs boson at 125 GeV and an SM-like scalar with masses in the range 250--1000 GeV. The light pseudoscalars decay to $ \mu\mu $ and $ \tau\tau $ with substantial overlap within each pair because of the large Lorentz boosts. This nonstandard topology motivates the development of dedicated $ \tau_{\mu}\tau_\mathrm{h} $, $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h} $ reconstruction and tagging methods to increase the acceptance. A dedicated neural network enables, for the first time, the inclusion of the high-rate but high-background $ \tau_\mathrm{h}\tau_\mathrm{h} $ final state. Data collected by the CMS experiment at $ \sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $, are examined. No significant excess over SM processes is observed. Model-independent upper limits at the 95% confidence level are set on the product of the Higgs boson production cross section and branching fraction, divided by the standard model cross section, $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $, with the most stringent limits reaching approximately 5 $ \times 10^{-5} $ for $ m_{\mathrm{H}} = $ 125 GeV and 3.5 $ \times 10^{-4} $ for $ m_{\mathrm{H}} = $ 1000 GeV. Model-specific upper limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a )/\sigma_{\mathrm{SM}} $ are also extracted for $ m_{\mathrm{H}} = $ 125 GeV in Type-I, -II, -III, and -IV two-Higgs-doublet plus singlet models. These results significantly extend the upper limits obtained in earlier searches by the CMS and ATLAS Collaborations and are complementary to existing searches, $ \mbox{e.g.} $, Ref. [85], at higher $ m_{a } $, where the $ \mu\mu $ and $ \tau\tau $ final states are resolved. Considering only the previous CMS search using boosted $ \tau_{\mu}\tau_\mathrm{h} $ reconstruction and a partial 13 TeV data set [47], the median expected upper limits on $ \sigma_{\mathrm{H}} \times \mathcal{B}(\mathrm{H}\to a a \to\mu\mu\tau\tau)/\sigma_{\mathrm{SM}} $ for $ m_{\mathrm{H}} = $ 125 GeV are stronger than those obtained in Ref. [47] by factors of 2--5, depending on $ m_{a } $, owing to the larger dataset and inclusion of more tau lepton decay channels in the analysis. In the Type-I and Type-II models, this result places the most stringent limits to date on $ \mathcal{B}(\mathrm{H}\to a a ) $ in the range 3.6 $ < m_{a } < $ 4.2 GeV when $ m_{\mathrm{H}} = $ 125 GeV. In the range 4.0 $ < m_{a } < $ 8.0 GeV, the upper limits in this work are stronger than those of Ref. [87] based on the same final state and dataset. Above 4.2 GeV, they are a bit weaker than those of Ref. [88], which reconstructs the predicted decay of the pseudoscalar to a pair of charm quarks as well as its dimuon decay. In the Type-III model, in which decays to leptons are enhanced over quarks, the results of this work in the range 13 $ < m_{a } < $ 15 GeV improve upon those of Ref. [43], which reconstructs the predicted decay of the pseudoscalar to a pair of bottom quarks as well as its dimuon decay, and those of Ref. [87]. Finally, this analysis is the first to search for the decay of a heavy neutral scalar to two promptly decaying pseudoscalars in the full Run 2 dataset of the LHC. |
| References | ||||
| 1 | CMS Collaboration | Measurement of the Higgs boson mass and width using the four-lepton final state in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | PRD 111 (2025) 092014 | CMS-HIG-21-019 2409.13663 |
| 2 | CMS Collaboration | Constraints on anomalous Higgs boson couplings from its production and decay using the WW channel in proton-proton collisions at $ \sqrt{s} = 13 \text {TeV} $ | EPJC 84 (2024) 779 | CMS-HIG-22-008 2403.00657 |
| 3 | CMS Collaboration | Measurement of simplified template cross sections of the Higgs boson produced in association with W or Z bosons in the H $ \rightarrow $ bb decay channel in proton-proton collisions at $ \sqrt{s} = 13 \text {TeV} $ | PRD 109 (2024) 092011 | CMS-HIG-20-001 2312.07562 |
| 4 | ATLAS and CMS Collaborations | Evidence for the Higgs boson decay to a Z boson and a photon at the LHC | PRL 132 (2024) 021803 | 2309.03501 |
| 5 | CMS Collaboration | Measurements of inclusive and differential cross sections for the Higgs boson production and decay to four-leptons in proton-proton collisions at $ \sqrt{s} = 13 \text {TeV} $ | JHEP 08 (2023) 040 | CMS-HIG-21-009 2305.07532 |
| 6 | CMS Collaboration | Measurement of the Higgs boson inclusive and differential fiducial production cross sections in the diphoton decay channel with pp collisions at $ \sqrt{s} = 13 \text {TeV} $ | JHEP 07 (2023) 091 | CMS-HIG-19-016 2208.12279 |
| 7 | ATLAS Collaboration | Measurements of $ WH $ and $ ZH $ production with Higgs boson decays into bottom quarks and direct constraints on the charm yukawa coupling in 13 TeV $ pp $ collisions with the ATLAS detector | JHEP 04 (2025) 075 | 2410.19611 |
| 8 | ATLAS Collaboration | Differential cross-section measurements of Higgs boson production in the H $ \rightarrow $ \ensuremath\tau$ ^{+} $\ensuremath\tau$ ^{-} $ decay channel in pp collisions at $ \sqrt{s} = $ 13 TeV with the ATLAS detector | JHEP 03 (2025) 010 | 2407.16320 |
| 9 | ATLAS Collaboration | Measurement of the associated production of a top-antitop-quark pair and a Higgs boson decaying into a $ b\bar{b} $ pair in pp collisions at $ \sqrt{s}= $ 13 TeV using the ATLAS detector at the LHC | EPJC 85 (2025) 210 | 2407.10904 |
| 10 | ATLAS Collaboration | Determination of the relative sign of the Higgs boson couplings to W and Z bosons using WH production via vector-boson fusion with the ATLAS detector | PRL 133 (2024) 141801 | 2402.00426 |
| 11 | ATLAS Collaboration | Measurement of the VH, H $ \rightarrow $ \ensuremath\tau\ensuremath\tau process with the ATLAS detector at 13 TeV | PLB 855 (2024) 138817 | 2312.02394 |
| 12 | ATLAS Collaboration | Measurement of the Higgs boson mass with H $ \rightarrow $ \ensuremath\gamma\ensuremath\gamma decays in 140 fb$ ^{-1} $ of $ \sqrt{s} = 13 \text {TeV} $ pp collisions with the ATLAS detector | PLB 847 (2023) 138315 | 2308.07216 |
| 13 | ATLAS Collaboration | Combined measurement of the Higgs boson mass from the H $ \rightarrow $ \ensuremath\gamma\ensuremath\gamma and H $ \rightarrow $ ZZ* $ \rightarrow $ 4\ensuremath\ell decay channels with the ATLAS detector using $ \sqrt s = $ 7, 8, and 13 TeV pp collision data | PRL 131 (2023) 251802 | 2308.04775 |
| 14 | ATLAS Collaboration | Integrated and differential fiducial cross-section measurements for the vector boson fusion production of the Higgs boson in the $ H \rightarrow WW^{\ast}\rightarrow e\nu\mu\nu $ decay channel at 13 $ \text{TeV} $ with the ATLAS detector | PRD 108 (2023) 072003 | 2304.03053 |
| 15 | D. Curtin et al. | Exotic decays of the 125 GeV Higgs boson | PRD 90 (2014) 075004 | 1312.4992 |
| 16 | G. C. Branco et al. | Theory and phenomenology of two-Higgs-doublet models | Phys. Rept. 516 (2012) 1 | 1106.0034 |
| 17 | U. Ellwanger, C. Hugonie, and A. M. Teixeira | The next-to-minimal supersymmetric standard model | Phys. Rept. 496 (2010) 1 | 0910.1785 |
| 18 | CMS Collaboration | A portrait of the Higgs boson by the CMS experiment ten years after the discovery | Nature 607 (2022) 60 | CMS-HIG-22-001 2207.00043 |
| 19 | CMS Collaboration | Combined measurements and interpretations of Higgs boson production and decay in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | CMS-HIG-21-018 2602.18611 |
|
| 20 | R. Dermisek and J. F. Gunion | Escaping the large fine tuning and little hierarchy problems in the next to minimal supersymmetric model and $ h\rightarrow aa $ decays | PRL 95 (2005) 041801 | hep-ph/0502105 |
| 21 | R. Dermisek and J. F. Gunion | The NMSSM close to the R-symmetry limit and naturalness in $ h\rightarrow aa $ decays for $ m(a) < 2m(b) $ | PRD 75 (2007) 075019 | hep-ph/0611142 |
| 22 | S. Chang, R. Dermisek, J. F. Gunion, and N. Weiner | Nonstandard Higgs boson decays | Ann. Rev. Nucl. Part. Sci. 58 (2008) 75 | 0801.4554 |
| 23 | S. F. King, M. Muhlleitner, R. Nevzorov, and K. Walz | Natural NMSSM Higgs bosons | NPB 870 (2013) 323 | 1211.5074 |
| 24 | A. Celis, V. Ilisie, and A. Pich | LHC constraints on two-Higgs doublet models | JHEP 07 (2013) 053 | 1302.4022 |
| 25 | B. Grinstein and P. Uttayarat | Carving out parameter space in Type-II two Higgs doublets model | JHEP 06 (2013) 094 | 1304.0028 |
| 26 | B. Coleppa, F. Kling, and S. Su | Constraining Type II 2HDM in light of LHC Higgs searches | JHEP 01 (2014) 161 | 1305.0002 |
| 27 | C.-Y. Chen, S. Dawson, and M. Sher | Heavy Higgs searches and constraints on two Higgs doublet models | PRD 88 (2013) 015018 | 1305.1624 |
| 28 | N. Craig, J. Galloway, and S. Thomas | Searching for signs of the second Higgs doublet | 1305.2424 | |
| 29 | L. Wang and X.-F. Han | Status of the aligned two-Higgs-doublet model confronted with the Higgs data | JHEP 04 (2014) 128 | 1312.4759 |
| 30 | J. Cao et al. | A light Higgs scalar in the NMSSM confronted with the latest LHC Higgs data | JHEP 11 (2013) 018 | 1309.4939 |
| 31 | N. D. Christensen, T. Han, Z. Liu, and S. Su | Low-mass Higgs bosons in the NMSSM and their LHC implications | JHEP 08 (2013) 019 | 1303.2113 |
| 32 | D. G. Cerdeno, P. Ghosh, and C. B. Park | Probing the two light Higgs scenario in the NMSSM with a low-mass pseudoscalar | JHEP 06 (2013) 031 | 1301.1325 |
| 33 | G. Chalons and F. Domingo | Analysis of the Higgs potentials for two doublets and a singlet | PRD 86 (2012) 115024 | 1209.6235 |
| 34 | A. Ahriche, A. Arhrib, and S. Nasri | Higgs phenomenology in the two-singlet model | JHEP 02 (2014) 042 | 1309.5615 |
| 35 | J. Baglio, O. Eberhardt, U. Nierste, and M. Wiebusch | Benchmarks for Higgs pair production and heavy Higgs boson searches in the two-Higgs-doublet model of Type II | PRD 90 (2014) 015008 | 1403.1264 |
| 36 | B. Dumont, J. F. Gunion, Y. Jiang, and S. Kraml | Constraints on and future prospects for two-Higgs-doublet models in light of the LHC Higgs signal | PRD 90 (2014) 035021 | 1405.3584 |
| 37 | H. Davoudiasl, R. Marcarelli, N. Miesch, and E. T. Neil | Searching for flavor-violating ALPs in Higgs boson decays | PRD 104 (2021) 055022 | 2105.05866 |
| 38 | M. Bauer, M. Neubert, and A. Thamm | Collider probes of axion-like particles | JHEP 12 (2017) 044 | 1708.00443 |
| 39 | ATLAS Collaboration | Search for Higgs boson decays into a pair of pseudoscalar particles in the $ bb\mu\mu $ final state with the ATLAS detector in $ pp $ collisions at $ \sqrt s = $ 13 TeV | PRD 105 (2022) 012006 | 2110.00313 |
| 40 | CMS Collaboration | Search for the decay of the Higgs boson to a pair of light pseudoscalar bosons in the final state with four bottom quarks in proton-proton collisions at $ \sqrt{\textrm{s}} = $ 13 TeV | JHEP 06 (2024) 097 | CMS-HIG-18-026 2403.10341 |
| 41 | CMS Collaboration | Search for exotic Higgs boson decays $ H \to \mathcal{A}\mathcal{A} \to 4\gamma $ with events containing two merged diphotons in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | PRL 131 (2023) 101801 | CMS-HIG-21-016 2209.06197 |
| 42 | CMS Collaboration | Search for the exotic decay of the Higgs boson into two light pseudoscalars with four photons in the final state in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | JHEP 07 (2023) 148 | CMS-HIG-21-003 2208.01469 |
| 43 | CMS Collaboration | Search for exotic decays of the Higgs boson to a pair of pseudoscalars in the $ \mu\mu $bb and $ \tau\tau $bb final states | EPJC 84 (2024) 493 | CMS-HIG-22-007 2402.13358 |
| 44 | CMS Collaboration | Model-independent search for pair production of new bosons decaying into muons in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | JHEP 12 (2024) 172 | CMS-HIG-21-004 2407.20425 |
| 45 | CMS Collaboration | Summary of 2HDM+S searches at 13 TeV (Run 2) | https://twiki.cern.ch/twiki/bin/view/CMSPublic/Summary2HDMSRun2 | |
| 46 | CMS Collaboration | 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 | CMS-HIG-16-015 1701.02032 |
| 47 | CMS Collaboration | 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 | JHEP 08 (2020) 139 | CMS-HIG-18-024 2005.08694 |
| 48 | LHC Higgs Cross Section Working Group | Handbook of LHC Higgs cross sections: 4. Deciphering the nature of the Higgs sector | CERN Yellow Reports: Monographs., 2017 link |
1610.07922 |
| 49 | J. Bernon et al. | Scrutinizing the alignment limit in two-Higgs-doublet models: m$ _h = $ 125 GeV | PRD 92 (2015) 075004 | 1507.00933 |
| 50 | J. Bernon, J. F. Gunion, Y. Jiang, and S. Kraml | Light Higgs bosons in two-Higgs-doublet models | PRD 91 (2015) 075019 | 1412.3385 |
| 51 | CMS Collaboration | HEPData record for this analysis | link | |
| 52 | CMS Collaboration | The CMS trigger system | JINST 12 (2017) P01020 | CMS-TRG-12-001 1609.02366 |
| 53 | CMS Collaboration | Performance of the CMS level-1 trigger in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | JINST 15 (2020) P10017 | CMS-TRG-17-001 2006.10165 |
| 54 | CMS Collaboration | Performance of the CMS high-level trigger during LHC Run 2 | JINST 19 (2024) P11021 | CMS-TRG-19-001 2410.17038 |
| 55 | CMS Collaboration | The CMS experiment at the CERN LHC | JINST 3 (2008) S08004 | 0805.1757 |
| 56 | CMS Collaboration | Development of the CMS detector for the CERN LHC Run 3 | JINST 19 (2024) P05064 | CMS-PRF-21-001 2309.05466 |
| 57 | J. Alwall et al. | The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations | JHEP 07 (2014) 079 | 1405.0301 |
| 58 | T. Sjöstrand et al. | An introduction to PYTHIA 8.2 | Comput. Phys. Commun. 191 (2015) 159 | 1410.3012 |
| 59 | CMS Collaboration | Extraction and validation of a new set of CMS PYTHIA8 tunes from underlying-event measurements | EPJC 80 (2020) 4 | CMS-GEN-17-001 1903.12179 |
| 60 | NNPDF Collaboration | Parton distributions from high-precision collider data | EPJC 77 (2017) 663 | 1706.00428 |
| 61 | GEANT4 Collaboration | GEANT 4---a simulation toolkit | NIM A 506 (2003) 250 | |
| 62 | CMS Collaboration | Particle-flow reconstruction and global event description with the CMS detector | JINST 12 (2017) P10003 | CMS-PRF-14-001 1706.04965 |
| 63 | M. Cacciari, G. P. Salam, and G. Soyez | The anti-$ k_{\mathrm{T}} $ jet clustering algorithm | JHEP 04 (2008) 063 | 0802.1189 |
| 64 | M. Cacciari, G. P. Salam, and G. Soyez | FastJet user manual | EPJC 72 (2012) 1896 | 1111.6097 |
| 65 | CMS Collaboration | Technical proposal for the Phase-II upgrade of the Compact Muon Solenoid | CMS Technical Proposal CERN-LHCC-2015-010, CMS-TDR-15-02, 2015 CDS |
|
| 66 | CMS Collaboration | Performance of the CMS muon detector and muon reconstruction with proton-proton collisions at $ \sqrt{s}= $ 13 TeV | JINST 13 (2018) P06015 | CMS-MUO-16-001 1804.04528 |
| 67 | CMS Collaboration | Performance of electron reconstruction and selection with the CMS detector in proton-proton collisions at $ \sqrt{s} = $ 8 TeV | JINST 10 (2015) P06005 | CMS-EGM-13-001 1502.02701 |
| 68 | CMS Collaboration | Determination of jet energy calibration and transverse momentum resolution in CMS | JINST 6 (2011) P11002 | CMS-JME-10-011 1107.4277 |
| 69 | CMS Collaboration | Performance of the pile up jet identification in CMS for Run 2 | technical report, 2020 CDS |
|
| 70 | CMS Collaboration | Performance of reconstruction and identification of $ \tau $ leptons decaying to hadrons and $ \nu_\tau $ in pp collisions at $ \sqrt{s}= $ 13 TeV | JINST 13 (2018) P10005 | CMS-TAU-16-003 1809.02816 |
| 71 | CMS Collaboration | Operation and performance of the CMS silicon strip tracker with proton-proton collisions at the CERN LHC | JINST 20 (2025) P08027 | CMS-TRK-20-002 2506.17195 |
| 72 | E. Bols et al. | Jet flavour classification using DeepJet | JINST 15 (2020) P12012 | 2008.10519 |
| 73 | CMS Collaboration | Identification of hadronic tau lepton decays using a deep neural network | JINST 17 (2022) P07023 | CMS-TAU-20-001 2201.08458 |
| 74 | CMS Collaboration | Electron and photon reconstruction and identification with the CMS experiment at the CERN LHC | JINST 16 (2021) P05014 | CMS-EGM-17-001 2012.06888 |
| 75 | CMS Collaboration | The CMS statistical analysis and combination tool: COMBINE | Comput. Softw. Big Sci. 8 (2024) 19 | CMS-CAT-23-001 2404.06614 |
| 76 | CMS Collaboration | Precision luminosity measurement in proton-proton collisions at $ \sqrt{s} = $ 13 TeV in 2015 and 2016 at CMS | EPJC 81 (2021) 800 | CMS-LUM-17-003 2104.01927 |
| 77 | CMS Collaboration | CMS luminosity measurement for the 2017 data-taking period at $ \sqrt{s} = $ 13 TeV | CMS Physics Analysis Summary, 2018 CMS-PAS-LUM-17-004 |
CMS-PAS-LUM-17-004 |
| 78 | CMS Collaboration | CMS luminosity measurement for the 2018 data-taking period at $ \sqrt{s} = $ 13 TeV | CMS Physics Analysis Summary, 2019 CMS-PAS-LUM-18-002 |
CMS-PAS-LUM-18-002 |
| 79 | CMS Collaboration | Precision luminosity measurement in proton-proton collisions at a center-of-mass energy of 13 TeV with the CMS detector at the Large Hadron Collider | Technical Report CERN-EP-2026-134, -003, 2026 CMS-PAS-LUM-20-001 |
CMS-PAS-LUM-20-001 |
| 80 | CMS Collaboration | Measurement of the inelastic proton-proton cross section at $ \sqrt{s}= $ 13 TeV | JHEP 07 (2018) 161 | CMS-FSQ-15-005 1802.02613 |
| 81 | T. Junk | Confidence level computation for combining searches with small statistics | NIM A 434 (1999) 435 | hep-ex/9902006 |
| 82 | A. L. Read | Presentation of search results: The CL$ _{s} $ technique | JPG 28 (2002) 2693 | |
| 83 | G. Cowan, K. Cranmer, E. Gross, and O. Vitells | Asymptotic formulae for likelihood-based tests of new physics | [Erratum: Eur. Phys. J. C 73 2501 ()], 2011 EPJC 71 (2011) 1554 |
1007.1727 |
| 84 | ATLAS and CMS Collaborations, and The LHC Higgs Combination Group | Procedure for the LHC Higgs boson search combination in Summer 2011 | Technical Report CMS-NOTE-2011-005. ATL-PHYS-PUB-2011-11, 2011 | |
| 85 | CMS Collaboration | Search for an exotic decay of the Higgs boson to a pair of light pseudoscalars in the final state of two muons and two $ \tau $ leptons in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | JHEP 11 (2018) 018 | CMS-HIG-17-029 1805.04865 |
| 86 | U. Haisch, J. F. Kamenik, A. Malinauskas, and M. Spira | Collider constraints on light pseudoscalars | JHEP 03 (2018) 178 | 1802.02156 |
| 87 | CMS Collaboration | Search for light pseudoscalar boson pairs produced from Higgs boson decays using the 4 tau and 2mu2au final states in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | JHEP 04 (2026) 132 | CMS-SUS-24-002 2508.06947 |
| 88 | D. Perez Adan | Search for an exotic decay of the 125 GeV Higgs boson to a pair of light pseudoscalars in the final state of two muons and two c-quarks in proton-proton collisions at $ \sqrt{s}= $ 13 TeV with CMS open data | JHEP 09 (2025) 096 | 2504.12161 |
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