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CMS-BPH-23-008 ; CERN-EP-2025-108

Search for the rare decay $ \mathrm{D^0}\to\mu^{+}\mu^{-} $ in proton-proton collisions at $ \sqrt{s} = $ 13.6 TeV

CMS Collaboration

6 June 2025

Phys. Rev. Lett. 135 (2025) 151803

Abstract: A search for the rare decay $ \mathrm{D^0}\to\mu^{+}\mu^{-} $ is reported using proton-proton collision events at $ \sqrt{s} = $ 13.6 TeV collected by the CMS detector in 2022-2023, corresponding to an integrated luminosity of 64.5 fb$ ^{-1} $. This is the first analysis to use a newly developed inclusive dimuon trigger, expanding the scope of the CMS flavor physics program. The search uses $ \mathrm{D^0} $ mesons obtained from $ \mathrm{D}^{*+}\to \mathrm{D^0} \pi^{+} $ decays. No significant excess is observed. A limit on the branching fraction of $ \mathcal{B}(\mathrm{D^0}\to\mu^{+}\mu^{-}) < $ 2.4 $\times$ 10$^{-9} $ at 95% confidence level is set. This is the most stringent upper limit set on any flavor changing neutral current decay in the charm sector.

Links: e-print arXiv:2506.06152 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures
png pdf	Figure 1: The distributions of the dipion invariant mass $ m_{\pi\pi} $ (left) and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (right) along with the associated projections of the full fit (solid curve), the signal contribution (hatched area), and the background contributions (other curves).
png pdf	Figure 1-a: The distributions of the dipion invariant mass $ m_{\pi\pi} $ (left) and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (right) along with the associated projections of the full fit (solid curve), the signal contribution (hatched area), and the background contributions (other curves).
png pdf	Figure 1-b: The distributions of the dipion invariant mass $ m_{\pi\pi} $ (left) and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (right) along with the associated projections of the full fit (solid curve), the signal contribution (hatched area), and the background contributions (other curves).
png pdf	Figure 2: The distributions of $ m_{\mu\mu} $ (left) and $ \Delta m $ (right) of the fit with the requirements 0.145 $ < \Delta m < $ 0.146 GeV and 1.84 $ < m_{\mu\mu} < $ 1.89 GeV, respectively, along with the associated projections of the full fit (solid curve), signal contribution (hatched area), and background contributions (other curves). The $ \mathrm{D^0} $ meson components are scaled up by 20 in the upper panel. The lower panel shows the data and the fit result after subtracting the total background component of the fit. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Figure 2-a: The distributions of $ m_{\mu\mu} $ (left) and $ \Delta m $ (right) of the fit with the requirements 0.145 $ < \Delta m < $ 0.146 GeV and 1.84 $ < m_{\mu\mu} < $ 1.89 GeV, respectively, along with the associated projections of the full fit (solid curve), signal contribution (hatched area), and background contributions (other curves). The $ \mathrm{D^0} $ meson components are scaled up by 20 in the upper panel. The lower panel shows the data and the fit result after subtracting the total background component of the fit. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Figure 2-b: The distributions of $ m_{\mu\mu} $ (left) and $ \Delta m $ (right) of the fit with the requirements 0.145 $ < \Delta m < $ 0.146 GeV and 1.84 $ < m_{\mu\mu} < $ 1.89 GeV, respectively, along with the associated projections of the full fit (solid curve), signal contribution (hatched area), and background contributions (other curves). The $ \mathrm{D^0} $ meson components are scaled up by 20 in the upper panel. The lower panel shows the data and the fit result after subtracting the total background component of the fit. The gray error band represents the statistical and systematic uncertainties in the total background component.

Tables
png pdf	Table 1: Summary of systematic uncertainties for the $ \mathrm{D^0}\to\mu^{+}\mu^{-} $ branching fraction measurement with their corresponding contributions in the signal channel.
png pdf	Table 2: The post-fit event yields for the signal, the combinatorial background, the $ \mathrm{D^0}\to\pi^{+}\pi^{-} $ background, and the $ \mathrm{D^0}\to \pi^{-}\mu^{+}\nu $ background. The observed numbers of events are given in the Data column. The subrange is in one dimension with a full range in the other dimension.

Summary

In summary, a search for $ \mathrm{D^0}\to\mu^{+}\mu^{-} $ decays by the CMS experiment, using proton-proton collision data at $ \sqrt{s} = $ 13.6 TeV corresponding to an integrated luminosity of 64.5 fb$ ^{-1} $, is presented. No significant excess above the fitted background is observed. An upper limit of $ \mathcal{B}(\mathrm{D^0}\to\mu^{+}\mu^{-}) < $ 2.4 $\times$ 10$^{-9} $ is set at 95% confidence level. This search is the most sensitive to date and provides a significant improvement over the previous best result [22], setting the most stringent limit on flavor changing neutral currents in the charm sector. It can be used to set constraints on scenarios that modify $ \mathcal{B}(\mathrm{D^0}\to\mu^{+}\mu^{-}) $. The search is made possible by a newly developed inclusive dimuon trigger and represents its first application. The result demonstrates the benefits of this trigger for flavor physics measurements and its potential to enable opportunities for a wide range of studies involving low-mass muon pairs.

Additional Figures
png pdf	Additional Figure 1: Distributions of the boosted decision tree discriminator $ d_{\text{MVA}} $ for data and $ \mathrm{D^0}\to\pi^{+}\pi^{-} $ Monte Carlo (MC) samples. The $ \mathrm{D^0}\to\pi^{+}\pi^{-} $ data distribution is extracted using the sPlot technique. The reweighted MC distribution, obtained by applying scale factors derived from a comparison between data and simulation in $ \mathrm{D^0}\to \mathrm{K^-}\pi^{+} $ control samples, is shown for illustration only and is not used in the analysis. The background events are extracted from the data sideband defined by 0.150 $ < \Delta m < $ 0.155 GeV and 1.81 $ < m_{\mu\mu} < $ 2.45 GeV.
png pdf	Additional Figure 2: The projection of $ m_{\pi\pi} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.144 GeV, 0.144 $ < \Delta m < $ 0.147 GeV, and 0.144 $ < \Delta m < $ 0.147 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\pi\pi} < $ 1.84 GeV, 1.84 $ < m_{\pi\pi} < $ 1.89 GeV, and 1.89 $ < m_{\pi\pi} < $ 1.94 GeV.
png pdf	Additional Figure 2-a: The projection of $ m_{\pi\pi} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.144 GeV, 0.144 $ < \Delta m < $ 0.147 GeV, and 0.144 $ < \Delta m < $ 0.147 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\pi\pi} < $ 1.84 GeV, 1.84 $ < m_{\pi\pi} < $ 1.89 GeV, and 1.89 $ < m_{\pi\pi} < $ 1.94 GeV.
png pdf	Additional Figure 2-b: The projection of $ m_{\pi\pi} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.144 GeV, 0.144 $ < \Delta m < $ 0.147 GeV, and 0.144 $ < \Delta m < $ 0.147 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\pi\pi} < $ 1.84 GeV, 1.84 $ < m_{\pi\pi} < $ 1.89 GeV, and 1.89 $ < m_{\pi\pi} < $ 1.94 GeV.
png pdf	Additional Figure 2-c: The projection of $ m_{\pi\pi} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.144 GeV, 0.144 $ < \Delta m < $ 0.147 GeV, and 0.144 $ < \Delta m < $ 0.147 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\pi\pi} < $ 1.84 GeV, 1.84 $ < m_{\pi\pi} < $ 1.89 GeV, and 1.89 $ < m_{\pi\pi} < $ 1.94 GeV.
png pdf	Additional Figure 2-d: The projection of $ m_{\pi\pi} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.144 GeV, 0.144 $ < \Delta m < $ 0.147 GeV, and 0.144 $ < \Delta m < $ 0.147 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\pi\pi} < $ 1.84 GeV, 1.84 $ < m_{\pi\pi} < $ 1.89 GeV, and 1.89 $ < m_{\pi\pi} < $ 1.94 GeV.
png pdf	Additional Figure 2-e: The projection of $ m_{\pi\pi} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.144 GeV, 0.144 $ < \Delta m < $ 0.147 GeV, and 0.144 $ < \Delta m < $ 0.147 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\pi\pi} < $ 1.84 GeV, 1.84 $ < m_{\pi\pi} < $ 1.89 GeV, and 1.89 $ < m_{\pi\pi} < $ 1.94 GeV.
png pdf	Additional Figure 2-f: The projection of $ m_{\pi\pi} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.144 GeV, 0.144 $ < \Delta m < $ 0.147 GeV, and 0.144 $ < \Delta m < $ 0.147 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\pi\pi} < $ 1.84 GeV, 1.84 $ < m_{\pi\pi} < $ 1.89 GeV, and 1.89 $ < m_{\pi\pi} < $ 1.94 GeV.
png pdf	Additional Figure 3: Dipion invariant mass $ m_{\pi\pi} $ (left) and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (right) from the reconstructed candidates from zero bias samples before $ d_{\text{MVA}} $ selection.
png pdf	Additional Figure 3-a: Dipion invariant mass $ m_{\pi\pi} $ (left) and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (right) from the reconstructed candidates from zero bias samples before $ d_{\text{MVA}} $ selection.
png pdf	Additional Figure 3-b: Dipion invariant mass $ m_{\pi\pi} $ (left) and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (right) from the reconstructed candidates from zero bias samples before $ d_{\text{MVA}} $ selection.
png pdf	Additional Figure 4: Invariant mass of $ \mathrm{K}\pi $ system $ m_{\mathrm{K}\pi} $ (left) and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (right) from the reconstructed candidates from zero bias samples after full selection in the $ \mathrm{D^0}\to \mathrm{K^-}\pi^{+} $ channel.
png pdf	Additional Figure 4-a: Invariant mass of $ \mathrm{K}\pi $ system $ m_{\mathrm{K}\pi} $ (left) and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (right) from the reconstructed candidates from zero bias samples after full selection in the $ \mathrm{D^0}\to \mathrm{K^-}\pi^{+} $ channel.
png pdf	Additional Figure 4-b: Invariant mass of $ \mathrm{K}\pi $ system $ m_{\mathrm{K}\pi} $ (left) and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (right) from the reconstructed candidates from zero bias samples after full selection in the $ \mathrm{D^0}\to \mathrm{K^-}\pi^{+} $ channel.
png pdf	Additional Figure 5: The one-dimensional fit to the distribution of $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ in the dimuon channel, where the $ \mathrm{K}\pi $ mass hypothesis is used to reconstruct $ \mathrm{D^0} $ candidate. The signal component represents the contribution from the $ \mathrm{D^0}\to \mathrm{K^-}\pi^{+} $ in which both of the hadrons are misidentified as muons. The semileptonic background comes from the $ \mathrm{D^0}\to \mathrm{K^-}\mu^+\nu $ and $ \mathrm{D^0}\to \pi^{-}\mu^+\nu $ processes. Both the signal and semileptonic background components are modeled using simulations.
png pdf	Additional Figure 6: The distribution of $ m_{\mu\mu} $ in 2022-2023 data used to extract the $ \mathrm{K^0_S}\to\pi^{+}\pi^{-} $ contribution, where both pions are misidentified as muons (left), and comparison of the $ m_{\mu\mu} $ modeling for this process extracted from data and simulation (right).
png pdf	Additional Figure 6-a: The distribution of $ m_{\mu\mu} $ in 2022-2023 data used to extract the $ \mathrm{K^0_S}\to\pi^{+}\pi^{-} $ contribution, where both pions are misidentified as muons (left), and comparison of the $ m_{\mu\mu} $ modeling for this process extracted from data and simulation (right).
png pdf	Additional Figure 6-b: The distribution of $ m_{\mu\mu} $ in 2022-2023 data used to extract the $ \mathrm{K^0_S}\to\pi^{+}\pi^{-} $ contribution, where both pions are misidentified as muons (left), and comparison of the $ m_{\mu\mu} $ modeling for this process extracted from data and simulation (right).
png pdf	Additional Figure 7: The projection of $ m_{\mu\mu} $ (left) and $ \Delta m $ (right) of the fit in the full $ m_{\mu\mu} $ and $ \Delta m $ range. The $ \mathrm{D^0} $ meson components are scaled up by 20 in the upper panel. The lower panel shows the data and the fit result after subtracting the total background component of the fit. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Additional Figure 7-a: The projection of $ m_{\mu\mu} $ (left) and $ \Delta m $ (right) of the fit in the full $ m_{\mu\mu} $ and $ \Delta m $ range. The $ \mathrm{D^0} $ meson components are scaled up by 20 in the upper panel. The lower panel shows the data and the fit result after subtracting the total background component of the fit. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Additional Figure 7-b: The projection of $ m_{\mu\mu} $ (left) and $ \Delta m $ (right) of the fit in the full $ m_{\mu\mu} $ and $ \Delta m $ range. The $ \mathrm{D^0} $ meson components are scaled up by 20 in the upper panel. The lower panel shows the data and the fit result after subtracting the total background component of the fit. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Additional Figure 8: The projection of $ m_{\mu\mu} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.145 GeV and 0.145 $ < \Delta m < $ 0.150 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\mu\mu} < $ 1.84 GeV and 1.89 $ < m_{\mu\mu} < $ 1.94 GeV. The $ \mathrm{D^0} $ meson components are scaled by 20 in the upper panel. The lower panel shows the data and the fit result after subtraction of the total background component. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Additional Figure 8-a: The projection of $ m_{\mu\mu} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.145 GeV and 0.145 $ < \Delta m < $ 0.150 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\mu\mu} < $ 1.84 GeV and 1.89 $ < m_{\mu\mu} < $ 1.94 GeV. The $ \mathrm{D^0} $ meson components are scaled by 20 in the upper panel. The lower panel shows the data and the fit result after subtraction of the total background component. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Additional Figure 8-b: The projection of $ m_{\mu\mu} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.145 GeV and 0.145 $ < \Delta m < $ 0.150 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\mu\mu} < $ 1.84 GeV and 1.89 $ < m_{\mu\mu} < $ 1.94 GeV. The $ \mathrm{D^0} $ meson components are scaled by 20 in the upper panel. The lower panel shows the data and the fit result after subtraction of the total background component. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Additional Figure 8-c: The projection of $ m_{\mu\mu} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.145 GeV and 0.145 $ < \Delta m < $ 0.150 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\mu\mu} < $ 1.84 GeV and 1.89 $ < m_{\mu\mu} < $ 1.94 GeV. The $ \mathrm{D^0} $ meson components are scaled by 20 in the upper panel. The lower panel shows the data and the fit result after subtraction of the total background component. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Additional Figure 8-d: The projection of $ m_{\mu\mu} $ (top) of the fit with requirements 0.140 $ < \Delta m < $ 0.145 GeV and 0.145 $ < \Delta m < $ 0.150 GeV, and $ \mathrm{D}^{*} $-$ \mathrm{D^0} $ mass difference $ \Delta m $ (bottom) of the fit with requirements 1.81 $ < m_{\mu\mu} < $ 1.84 GeV and 1.89 $ < m_{\mu\mu} < $ 1.94 GeV. The $ \mathrm{D^0} $ meson components are scaled by 20 in the upper panel. The lower panel shows the data and the fit result after subtraction of the total background component. The gray error band represents the statistical and systematic uncertainties in the total background component.
png pdf	Additional Figure 9: The profile likelihood scan as a function of $ \mathrm{D^0}\to\mu^{+}\mu^{-} $ decay branching fraction.
png pdf	Additional Figure 10: The upper limits on the $ \mathrm{D^0}\to\mu^{+}\mu^{-} $ branching fraction using the CLs method.
png pdf	Additional Figure 11: Historical progression of the upper limit (UL) on the branching fraction of $ \mathrm{D^0}\to\mu^{+}\mu^{-} $, including the CMS result from this analysis.
png pdf	Additional Figure 12: Historical progression of the upper limit (UL) on the branching fraction of $ \mathrm{D^0}\to\mu^{+}\mu^{-} $, including the CMS result from this analysis and the standard model (SM) prediction.

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