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| CMS-PAS-EXO-23-012 | ||
| Search for long-lived particles decaying to photons in decays of the Higgs boson at $ \sqrt{s} = 13 \mathrm{TeV} $ | ||
| CMS Collaboration | ||
| 2026-08-06 | ||
| Abstract: A search for long-lived particles decaying into photons in rare decays of the Higgs boson is presented. The search uses proton-proton collision data from the CMS detector at the CERN LHC collected between 2016 and 2018, corresponding to an integrated luminosity of 138 fb$ ^{-1} $ at a center-of-mass energy of 13 TeV. A novel technique is introduced that features a kinematic fit to estimate the position of the decay of the hypothetical long-lived particle using the CMS electromagnetic calorimeter, which provides excellent position and energy resolution but lacks longitudinal segmentation. No significant excess above the background prediction is observed. The results are interpreted as upper limits on the branching fraction of the Higgs boson to long-lived particles as a function of the hypothetical lifetime of the long-lived candidate and its branching fraction to photons. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Reconstructed diphoton invariant mass for signal with $ m_\Phi = 30\ \text{GeV} $ for different values of the $ \Phi $ lifetime. |
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Figure 2:
Left: Geometric configuration used to reconstruct the displaced vertex of the diphoton decay. Solid blue lines represent particle trajectories, while the dashed green lines represent the locus of possible decay vertices. Right: Resolution of the reconstructed transverse displacement $ L_{xy} $ in signal events for different values of the $ \Phi $ candidate mass. |
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Figure 2-a:
Left: Geometric configuration used to reconstruct the displaced vertex of the diphoton decay. Solid blue lines represent particle trajectories, while the dashed green lines represent the locus of possible decay vertices. Right: Resolution of the reconstructed transverse displacement $ L_{xy} $ in signal events for different values of the $ \Phi $ candidate mass. |
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Figure 2-b:
Left: Geometric configuration used to reconstruct the displaced vertex of the diphoton decay. Solid blue lines represent particle trajectories, while the dashed green lines represent the locus of possible decay vertices. Right: Resolution of the reconstructed transverse displacement $ L_{xy} $ in signal events for different values of the $ \Phi $ candidate mass. |
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Figure 3:
Comparison between data and predicted background events for events with $ L_{xy} < - $ 10 cm and $ m_{\gamma\gamma} > 62.5\ \text{GeV} $ for W categories (left) and Z categories (right), demonstrating good agreement between the background prediction and data. The impact parameter was calculated with an assumption of $ m_\Phi = 30\ \text{GeV} $. |
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Figure 3-a:
Comparison between data and predicted background events for events with $ L_{xy} < - $ 10 cm and $ m_{\gamma\gamma} > 62.5\ \text{GeV} $ for W categories (left) and Z categories (right), demonstrating good agreement between the background prediction and data. The impact parameter was calculated with an assumption of $ m_\Phi = 30\ \text{GeV} $. |
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Figure 3-b:
Comparison between data and predicted background events for events with $ L_{xy} < - $ 10 cm and $ m_{\gamma\gamma} > 62.5\ \text{GeV} $ for W categories (left) and Z categories (right), demonstrating good agreement between the background prediction and data. The impact parameter was calculated with an assumption of $ m_\Phi = 30\ \text{GeV} $. |
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Figure 4:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 15, 20, and 30 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
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Figure 4-a:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 15, 20, and 30 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
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Figure 4-b:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 15, 20, and 30 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
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Figure 4-c:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 15, 20, and 30 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
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Figure 4-d:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 15, 20, and 30 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
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Figure 4-e:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 15, 20, and 30 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
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Figure 4-f:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 15, 20, and 30 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
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Figure 5:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 40, 50, and 55 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
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Figure 5-a:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 40, 50, and 55 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
png pdf |
Figure 5-b:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 40, 50, and 55 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
png pdf |
Figure 5-c:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 40, 50, and 55 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
png pdf |
Figure 5-d:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 40, 50, and 55 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
png pdf |
Figure 5-e:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 40, 50, and 55 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
png pdf |
Figure 5-f:
Comparison of data yields with the background prediction for $ m_\Phi $ hypotheses of 40, 50, and 55 GeV for the W categories (left) and the Z categories (right). The distributions from all periods of data taking have been summed together. Expected signal contributions are overlaid for $ \mathrm{c}\tau = $ 0 mm and $ \mathrm{c}\tau = $ 100 mm under the assumption of $ \mathcal{B} \left (\mathrm{H} \rightarrow \Phi\Phi \right ) =1% $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right )=50% $. |
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Figure 6:
Summarized 95% CL upper limits on the branching fraction of the Higgs boson to the new scalar particle $ \Phi $. Top: Upper limits presented as a function of $ m_\Phi $ and lifetime for a value of the branching fraction of $ \Phi $ to photons of 0.05). Bottom: Upper limits presented as a function of $ m_\Phi $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right ) $ for a lifetime of 100 mm. |
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Figure 6-a:
Summarized 95% CL upper limits on the branching fraction of the Higgs boson to the new scalar particle $ \Phi $. Top: Upper limits presented as a function of $ m_\Phi $ and lifetime for a value of the branching fraction of $ \Phi $ to photons of 0.05). Bottom: Upper limits presented as a function of $ m_\Phi $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right ) $ for a lifetime of 100 mm. |
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Figure 6-b:
Summarized 95% CL upper limits on the branching fraction of the Higgs boson to the new scalar particle $ \Phi $. Top: Upper limits presented as a function of $ m_\Phi $ and lifetime for a value of the branching fraction of $ \Phi $ to photons of 0.05). Bottom: Upper limits presented as a function of $ m_\Phi $ and $ \mathcal{B} \left (\Phi \rightarrow \gamma\gamma \right ) $ for a lifetime of 100 mm. |
| Summary |
| A search for rare decays of the SM Higgs boson to LLPs decaying to a pair of photons has been presented, using pp collisions at 13 TeV collected with the CMS detector from 2016--2018, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. A novel technique using kinematic constraints to geometrically reconstruct the decay vertex of the two photons has been employed to set upper limits on the branching fraction of the Higgs boson. Expected 95% CL upper limits probe branching fractions of the Higgs boson to $ \Phi $ candidates below 10% for modest branching fraction of the $ \Phi $ candidate to photons. This analysis represents the first search by the CMS experiment for long-lived particle decays to photons in a mass range of 15--55 GeV and proper lifetimes up to 1000 mm. |
| Additional Figures | |
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Additional Figure 1:
Summarized 95% CL upper limits on the branching fraction of the Higgs boson to the new scalar particle $\phi$ as a function of $m_\phi$ and lifetime for a value of the branching fraction of $\phi$ to photons of 0.01. |
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Additional Figure 2:
Summarized 95% CL upper limits on the branching fraction of the Higgs boson to the new scalar particle $\phi$ as a function of $m_\phi$ and lifetime for a value of the branching fraction of $\phi$ to photons of 0.02. |
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Additional Figure 3:
Summarized 95% CL upper limits on the branching fraction of the Higgs boson to the new scalar particle $\phi$ as a function of $m_\phi$ and lifetime for a value of the branching fraction of $\phi$ to photons of 0.1. |
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Additional Figure 4:
Summarized 95% CL upper limits on the branching fraction of the Higgs boson to the new scalar particle $\phi$ as a function of $m_\phi$ and lifetime for a value of the branching fraction of $\phi$ to photons of 0.5. |
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Additional Figure 5:
Summarized 95% CL upper limits on the branching fraction of the Higgs boson to the new scalar particle $\phi$ as a function of $m_\phi$ and lifetime for a value of the branching fraction of $\phi$ to photons of 1. |
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Additional Figure 6:
Comparison between data and predicted background events for events with $L_{xy} < -10\unit{cm}$ and $m_{\gamma\gamma} > 62.5$ GeV for the W categories. The impact parameter was calculated with an assumption of $m_\phi = 15$ GeV. |
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Additional Figure 7:
Comparison between data and predicted background events for events with $L_{xy} < -10\unit{cm}$ and $m_{\gamma\gamma} > 62.5$ GeV for the W categories. The impact parameter was calculated with an assumption of $m_\phi = 55$ GeV. |
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Additional Figure 8:
Prefit distribution of the invariant mass of the two photons under the assumption that \phi decayed at the primary vertex for the W categories and a mass hypothesis of $m_\phi = 30$ GeV. No requirements on the $L_{xy}$ are applied. |
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Additional Figure 9:
Prefit distribution of the invariant mass of the two photons under the assumption that \phi decayed at the primary vertex for the Z categories and a mass hypothesis of $m_\phi = 30$ GeV. No requirements on the $L_{xy}$ are applied. |
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Additional Figure 10:
Comparison of the event yield as a function of $L_{xy}$ between the number of expected events in simulation and the number of predicted events by the background estimation method for a mass hypothesis of $m_\phi = 15$ GeV. |
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Additional Figure 11:
Comparison of the event yield as a function of $L_{xy}$ between the number of expected events in simulation and the number of predicted events by the background estimation method for a mass hypothesis of $m_\phi = 30$ GeV. |
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Additional Figure 12:
Comparison of the event yield as a function of $L_{xy}$ between the number of expected events in simulation and the number of predicted events by the background estimation method for a mass hypothesis of $m_\phi = 55$ GeV. |
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Additional Figure 13:
Background extrapolation factor measured in data events with a single muon from W decay and a single photon. |
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Additional Figure 14:
Background extrapolation factor measured in data events with a single electron from W decay and a single photon. |
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Compact Muon Solenoid LHC, CERN |
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