CMS-PAS-BPH-23-008

CMS-PAS-BPH-23-008
Search for rare charm decays into two muons
CMS Collaboration
28 July 2024

Abstract: A search for the rare decay ${\mathrm D}^0\to\mu^{+}\mu^{-}$ was conducted using $\sqrt{s} =$ 13.6 TeV proton-proton collision data collected by the CMS experiment during the 2022-2023 operation of the CERN LHC, with an integrated luminosity of 64.5 fb $^{-1}$ . The search exploits a new high-rate trigger that records events with two muons with transverse momenta as low as 4 and 3 GeV. Rare decays of the charm quark, which are less explored than rare bottom quark decays, offer a unique way to probe for new physics effects beyond the reach of modern colliders. No significant excess above the expected background was observed, leading to an upper limit of $\mathcal{B}({\mathrm D}^0\to\mu^{+}\mu^{-}) <$ 2.6 $\times$ 10 $^{-9}$ at the 95% confidence level. This is the most sensitive measurement to date, imposing additional constraints on new physics models.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: 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 after MVA $_{\mathrm{D}}$ selection.
png pdf	Figure 1-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 after MVA $_{\mathrm{D}}$ selection.
png pdf	Figure 1-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 after MVA $_{\mathrm{D}}$ selection.
png pdf	Figure 2: 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 bottom panel shows the data and the fit result after subtraction of the total background component. The grey error band represents the statistical and systematic uncertainties of the total background component.
png pdf	Figure 2-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 bottom panel shows the data and the fit result after subtraction of the total background component. The grey error band represents the statistical and systematic uncertainties of the total background component.
png pdf	Figure 2-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 bottom panel shows the data and the fit result after subtraction of the total background component. The grey error band represents the statistical and systematic uncertainties of the total background component.
png pdf	Figure 3: The projection of $m_{\mu\mu}$ (left) and $\Delta m$ (right) of the fit with requirements 0.145 $< \Delta m <$ 0.146 GeV and 1.84 $< m_{\mu\mu} <$ 1.89 GeV, respectively. The bottom panel shows the data and the fit result after subtraction of the total background component. The grey error band represents the statistical and systematic uncertainties of the total background component.
png pdf	Figure 3-a: The projection of $m_{\mu\mu}$ (left) and $\Delta m$ (right) of the fit with requirements 0.145 $< \Delta m <$ 0.146 GeV and 1.84 $< m_{\mu\mu} <$ 1.89 GeV, respectively. The bottom panel shows the data and the fit result after subtraction of the total background component. The grey error band represents the statistical and systematic uncertainties of the total background component.
png pdf	Figure 3-b: The projection of $m_{\mu\mu}$ (left) and $\Delta m$ (right) of the fit with requirements 0.145 $< \Delta m <$ 0.146 GeV and 1.84 $< m_{\mu\mu} <$ 1.89 GeV, respectively. The bottom panel shows the data and the fit result after subtraction of the total background component. The grey error band represents the statistical and systematic uncertainties of the total background component.
png pdf	Figure 4: The profile likelihood scan as a function of $\mathrm{D^0}\to\mu^{+}\mu^{-}$ decay branching fraction.

Tables
png pdf	Table 1: Fitted yields with zero bias events after MVA $_{\mathrm{D}}$ selection.
png pdf	Table 2: Summary of systematic uncertainties for the $\mathrm{D^0}\to\mu^{+}\mu^{-}$ branching ratio measurement.
png pdf	Table 3: The expected event yields for signal, the combinatorial background, the peaking background, and the semileptonic background are summarized (post-fit). The total expected and observed event yields are given in Total and Data column, respectively. The subrange is in one dimension with a full range in the other dimension.

Summary

The measurement of the branching fraction (

$\mathcal{B}$ ) of the

$\mathrm{D^0}\to\mu^{+}\mu^{-}$ decay, based on a data set of proton-proton collisions at

$\sqrt{s} =$ 13.6 TeV collected by the CMS experiment corresponding to an integrated luminosity of 64.5 fb

$^{-1}$ , has been presented. The branching fraction is measured using the cascade decay

$\mathrm{D}^{*+}\to\mathrm{D^0}\pi^{+}$ ,

$\mathrm{D^0}\to\mu^{+}\mu^{-}$ with the

$\mathrm{D^0}\to\pi^{+}\pi^{-}$ decay mode as the normalization channel, considering both prompt and non-prompt

$\mathrm{D^0}$ meson production. No significant excess above the background expectation was observed and an upper limit on the branching fraction has been set to be

$\mathcal{B}$ (

$\mathrm{D^0}\to\mu^{+}\mu^{-}$ ) <

$2.6$ \times

$10$ ^-9

$at 95% CL. The obtained value of$ \mathrm{D^0}\to\mu^{+}\mu^{-}

$branching fraction is found to be$ \mathcal{B}(\mathrm{D^0}\to\mu^{+}\mu^{-}) =

$(1.0$ \pm

$0.9)$ \times

$10$ ^{-9}$. The measurement is the most sensitive to date, representing a 35% improvement over the current best measurement [23]. This measurement provides the most stringent limit on flavor-changing neutral currents in the charm sector, setting additional constraints on new physics models that modify the decay branching fraction of

$\mathrm{D^0}\to\mu^{+}\mu^{-}$ . The analysis is the first measurement at CMS using the newly developed low-mass double muon parking trigger [24]. It also marks the first publication on the

$\mathrm{D^0}\to\mu^{+}\mu^{-}$ branching fraction measurement at CMS, made possible by the enriched dimuon events collected by this innovative trigger.

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