Catalog · Charm · C004

C004 $\mathcal{B}(D^0\to\mu^+\mu^-)$

Rare leptonic decay D0 $ \to \mu^+ \mu^-$

Status SUBTLETY-ADDED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
BR(D0 $ \to \mu^+ \mu^-$)	2.1e-9 branching fraction	2026	PDG Live/API S032.28	source ↑
BR(D0 $ \to \mu^+ \mu^-$)	2.4e-9 branching fraction	2025	CMS public result page / PRL 135 (2025) 151803	source ↑
BR(D0 $ \to \mu^+ \mu^-$)	3.1e-9 branching fraction	2023	LHCb PRL 131 (2023) 041804 / arXiv:2212.11203	source ↑
Long-distance two-photon contribution to D0 $ \to \mu^+ \mu^-$	? branching fraction	2002	Burdman, Golowich, Hewett, Pakvasa 2002	source ↑
Rare $\|\Delta$ c\| = $\|\Delta$ u\| = 1 global EFT context	?	2024	Gisbert, Hiller, Suelmann 2024	source ↑

Why this constrains the RS scan

Anarchic warped models can generate flavor-changing $c\to u$ neutral-current couplings through KK gauge exchange, electroweak $Z$-like effects, Higgs or radion scalar exchange, or lepton-sector extensions. This channel is therefore a useful up-sector $\Delta C=1$ complement to the implemented $\Delta F=2$ D-mixing constraint. For the present quark-only scan it is best treated as a catalog and roadmap entry; a live constraint would require lepton-current normalization in addition to the quark flavor spurions.

What's changed since the original paper

Since the arXiv:0804.1954 warped-flavor baseline, the experimental bound has moved by orders of magnitude. LHCb's full Run 1+2 search improved the rare charm dimuon limit to $3.1\times10^{-9}$ at 90\% C.L. CMS then used the Run 3 inclusive dimuon-trigger program, $64.5\,{\rm fb}^{-1}$ from 2022--2023, to set the stronger 95\% C.L. limit of $2.4\times10^{-9}$. On the theory side, the 2024 rare-charm EFT analysis incorporated recent $\Delta C=1$ data, including $D^0\to\mu^+\mu^-$, into constraints on $C_{7,9,10}^{(\prime)}$ while emphasizing that null-test observables still leave room for new physics.

Validity and model dependence

The experimental limit is robust, but the Standard Model prediction is not a clean short-distance number. The classic rare-charm calculation estimates the long-distance two-photon contribution as $\mathcal{B}(D^0\to\mu^+\mu^-)_{\gamma\gamma} \simeq 2.7\times10^{-5}\mathcal{B}(D^0\to\gamma\gamma)$, implying a long-distance floor of at least $3\times10^{-13}$ for the assumptions quoted there. The safe catalog use is therefore an upper bound on extra short-distance contributions, not a precision SM subtraction. In down-aligned or kaon-protected RS variants, up-sector observables become leading rather than secondary diagnostics.

Code coverage in this repo

NO. The required catalog greps and a focused search for D0.*mu, c.?->.?u, rare.?charm, C10, C\_S, C\_P, and scalar/pseudoscalar rare-decay terms found no live $D^0\to\mu^+\mu^-$ implementation in quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, or tests/. Nearby hits are unrelated: $D^0$ mixing appears at quarkConstraints/deltaf2.py:256 and quarkConstraints/deltaf2.py:941, while $\mu\to e\gamma$ is handled separately at flavorConstraints/muToEGamma.py:75.

Linked evidence (opens GitHub blob at flavor-catalog-website/2026q2):

Implementation difficulty

HIGH. $D^0\to\mu^+\mu^-$ is a new $c\to u$ dilepton mode requiring a new $\Delta C=1$ Wilson-coefficient normalization and a documented long-distance subtraction; it is not covered by the existing $\Delta F=2$ backend.

Reason: $D0 \to \mu^+$ mu- is a new $c \to u$ dilepton mode requiring a new $\Delta C = 1$ Wilson coefficient normalization plus long-distance subtraction; it is not covered by the existing $\Delta F = 2$ backend.

Key references

Process-local keys before bibliography consolidation: PDG2026:D0MuMu, CMS2025:D0MuMu, LHCb2023:D0MuMu, BurdmanGolowichHewettPakvasa2002:RareCharm, GisbertHillerSuelmann2024:RareCharmEFT, and CsakiFalkowskiWeiler2008:CompositeFlavor.

value 2.1e-09

RESOLVED

Match snapshot line 21

L18:   technique: CMS
L19:   comment: p p at 13.6 TeV
L20:   confidence_level: 90.0
L21:   value_text: <2.1 E-9
L22:   limit_type: U
L23:   used_in_average: true
L24:   used_in_fit: true

value 2.4e-09

RESOLVED

Match snapshot line 20

L17:   Integrated luminosity: 64.5 fb^-1.
L18:   Production/selection: D0 mesons from D*+ -> D0 pi+ decays.
L19:   Result: no significant excess is observed.
L20:   Limit: B(D0 -> mu+ mu-) < 2.4 x 10^-9 at 95% confidence level.
L21:   CMS summary states this was the most sensitive search to date and improved
L22:   on the previous best result.

value 3.1e-09

RESOLVED

Match snapshot line 21

L18:   Search strategy: optimized for D0 mesons from D*+ -> D0 pi+ decays, also
L19:   sensitive to D0 mesons from other sources.
L20:   Result: no evidence for an excess above expected background.
L21:   Limit: B(D0 -> mu+ mu-) < 3.1 x 10^-9 at 90% CL.
L22:   The arXiv abstract describes this as the world's most stringent limit at
L23:   the time of the LHCb publication.