CMS-SUS-23-007

CMS-SUS-23-007 ; CERN-EP-2025-028
Search for the nonresonant production of a pair of additional Higgs bosons in the Type-X two-Higgs-doublet model in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
15 May 2026
Submitted to Physical Review Letters
Abstract: A search is presented for the production of two additional Higgs bosons from an off-shell Z boson, where both additional particles decay to $ \tau $ lepton pairs. The search is performed with a data sample collected with the CMS detector from proton-proton collisions at the LHC at $ \sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. No deviation from the standard model background is observed. Exclusion limits are set on the Type-X two-Higgs-doublet model alignment scenario. These results rule out this model as an explanation to the potential tension between the experimental and theoretical values of the muon anomalous magnetic moment.
Links: e-print arXiv:2605.16655 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram for the production of two BSM neutral Higgs bosons from an off-shell Z boson and their decay to $ \tau $ leptons.
png pdf	Figure 2: Distributions of the discriminating variable $ m_{\mathrm{T}}^{\text{tot}} $ after a background-only fit to the data. The $ \tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $ channel (left) and the $ \mathrm{e}\tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $, $ \mu\tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $ channels, as well $ \mathrm{e}\mathrm{e}\tau_\mathrm{h}\tau_\mathrm{h} $ SS, $ \mu\mu\tau_\mathrm{h}\tau_\mathrm{h} $ SS, $ \mathrm{e}\mu\tau_\mathrm{h}\tau_\mathrm{h} $ SS, and $ \mathrm{e}\mu\tau_\mathrm{h}\tau_\mathrm{h} $ OS leptons categories are shown (right). The background uncertainty represents the combined statistical and systematic uncertainties. An example signal with mass hypotheses $ m_{\phi} = $ 200 GeV and $ m_{A } = $ 160 GeV, with the cross section scaled to 0.01 pb, is shown as a blue line.
png pdf	Figure 2-a: Distributions of the discriminating variable $ m_{\mathrm{T}}^{\text{tot}} $ after a background-only fit to the data. The $ \tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $ channel (left) and the $ \mathrm{e}\tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $, $ \mu\tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $ channels, as well $ \mathrm{e}\mathrm{e}\tau_\mathrm{h}\tau_\mathrm{h} $ SS, $ \mu\mu\tau_\mathrm{h}\tau_\mathrm{h} $ SS, $ \mathrm{e}\mu\tau_\mathrm{h}\tau_\mathrm{h} $ SS, and $ \mathrm{e}\mu\tau_\mathrm{h}\tau_\mathrm{h} $ OS leptons categories are shown (right). The background uncertainty represents the combined statistical and systematic uncertainties. An example signal with mass hypotheses $ m_{\phi} = $ 200 GeV and $ m_{A } = $ 160 GeV, with the cross section scaled to 0.01 pb, is shown as a blue line.
png pdf	Figure 2-b: Distributions of the discriminating variable $ m_{\mathrm{T}}^{\text{tot}} $ after a background-only fit to the data. The $ \tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $ channel (left) and the $ \mathrm{e}\tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $, $ \mu\tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $, and $ \tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h}\tau_\mathrm{h} $ channels, as well $ \mathrm{e}\mathrm{e}\tau_\mathrm{h}\tau_\mathrm{h} $ SS, $ \mu\mu\tau_\mathrm{h}\tau_\mathrm{h} $ SS, $ \mathrm{e}\mu\tau_\mathrm{h}\tau_\mathrm{h} $ SS, and $ \mathrm{e}\mu\tau_\mathrm{h}\tau_\mathrm{h} $ OS leptons categories are shown (right). The background uncertainty represents the combined statistical and systematic uncertainties. An example signal with mass hypotheses $ m_{\phi} = $ 200 GeV and $ m_{A } = $ 160 GeV, with the cross section scaled to 0.01 pb, is shown as a blue line.
png pdf	Figure 3: Observed 95% CL upper limits on the product of the cross section ($ \sigma $) for the production of two BSM Higgs bosons produced via an off-shell Z boson, and the branching fractions ($ B $) for their decay into $ \tau $ leptons. This is shown as a function of $ m_{A } $ and $ m_{\phi} $. No limits are set in the red hatched region.
png pdf	Figure 4: Observed and expected 95% CL exclusion contours on the Type-X 2HDM alignment scenario for $ m_{\phi}= $ 200 GeV. This is shown as a function of $ m_{A } $ and $ \tan\beta $. The $ y $-axis scales logarithmically below 100 and linearly above 100. The dashed line represents the expected median, the dark and bright grey bands correspond to the central 68 and 95% expected intervals, and the blue area highlights the parameter space that has been excluded. Constraints from previous searches, obtained by HIGGSTOOLS-1 [66], are shown in red. The allowed region in the normal alignment scenario accommodating the current measurement of the muon anomalous magnetic moment, as detailed in Ref. [9], is shown in green.
png pdf	Figure 5: Observed 95% CL exclusions of the Type-X 2HDM alignment scenario in the $ m_{A }-m_{\phi} $ plane are shown as black solid contours, with the $ \tan\beta $ exclusion of each bounded region written on the plot. The complete exclusion of this model, from the union of the observed limits and those obtained by HIGGSTOOLS-1 [66], is enclosed by the red dot-dashed contour. The allowed regions accommodating the current measurement of the muon anomalous magnetic moment, as detailed in Ref. [9], are shown as green dashed boxes, with the $ \tan\beta $ allowed range written on the plot. Hatched regions indicate parameter space for which no exclusion limit is set.

Summary

In summary, a search has been presented for beyond-the-standard-model Higgs bosons produced via off-shell Z bosons that decay into $\tau$ leptons in pp collisions at the LHC, using a data sample collected with the CMS detector at $\sqrt{s} = 13$ TeV.A good agreement between the background-only fit and the data is observed. Upper limits on the production cross sections times the branching fractions range from 0.4 fb at $m_A = 600$ GeV and $m_{\phi} = 800$ GeV to 190 fb at $m_A = 40$ GeV and $m_{\phi} = 60$ GeV. The data, combined with the constraints from previous searches, exclude a large portion of the Type-X two-Higgs-doublet model's phase space. This search rules out this model as an explanation for the muon anomalous magnetic moment, as described in Ref. [9].

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