Catalog · Charm · C005

C005 $\mathcal{B}(D^0\to e^+e^-)$

Rare leptonic decay D0 $ \to $ e+ e-

Status REVIEWED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
BR(D0 $ \to $ e+ e-)	7.9e-8 branching fraction	2026	PDG Live/API S032.39	source ↑
BR(D0 $ \to $ e+ e-)	7.9e-8 branching fraction	2010	Belle Phys. Rev. D 81 (2010) 091102 / arXiv:1003.2345	source ↑
BR(D0 $ \to $ e+ e-)	1.7e-7 branching fraction	2012	BABAR Phys. Rev. D 86 (2012) 032001 / arXiv:1206.5419	source ↑

Why this constrains the RS scan

Anarchic warped models can induce up-sector flavor-changing neutral currents through nonuniversal KK gauge couplings, flavor-violating $Z$-like effects, Higgs/radion scalar exchange, or lepton-sector extensions (CsakiFalkowskiWeiler2008:CompositeFlavor). C005 therefore tests a pure $c\to u$ dilepton amplitude rather than the four-quark $\Delta F=2$ operator basis used for neutral $D$-mixing. For the current quark-only scan it is a catalog and roadmap process; a live veto would need both the $c\to u$ current and electron-current normalization.

What's changed since the original paper

The 2008 RS-flavor baseline predates the Belle and BaBar leptonic $D^0$ searches (CsakiFalkowskiWeiler2008:CompositeFlavor). The main post-2008 experimental development is Belle's 2010 improvement to the current PDG limit, followed by BaBar's 2012 cross-check in a separate $e^+e^-\to c\bar c$ sample (Belle2010:D0Ee, BaBar2012:D0EeMuMuEMu). Unlike C004, which has newer hadron-collider dimuon searches, the accessed C005 PDG record still uses the Belle result as the headline input. Rare-charm theory work emphasizes that purely leptonic $D^0$ limits are clean null tests experimentally but require care when separating short-distance new physics from long-distance charm dynamics (BurdmanGolowichHewettPakvasa2002:RareCharm).

Validity and model dependence

The upper limit itself is experimentally robust. Its interpretation as an RS constraint is model dependent because the electron mode is helicity sensitive, bremsstrahlung and electron identification enter the experimental acceptance, and the Standard Model rare-charm amplitude is not a precision short-distance prediction. A conservative use is an upper bound on additional short-distance $c\to u e^+e^-$ amplitudes in a specified operator basis, not a direct subtraction against a sharply predicted Standard Model rate (BurdmanGolowichHewettPakvasa2002:RareCharm).

Code coverage in this repo

NO. The author greps and a focused search for D0.*e+e-, D0.*ee, c.?->.?u.*e, and rare.?charm found no $D^0\to e^+e^-$ implementation in quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, or tests/. Nearby hits are not C005: $D^0$ mixing is configured at quarkConstraints/deltaf2.py:252 and evaluated at quarkConstraints/deltaf2.py:941; the modern policy likewise lists only $D^0$ mixing at quarkConstraints/modern/phenomenology.py:23 and quarkConstraints/modern/phenomenology.py:668. The $\mu\to e\gamma$ dipole check at flavorConstraints/muToEGamma.py:75 is a separate LFV observable.

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

Implementation difficulty

HIGH. Production use would require a new $\Delta C=1$ rare leptonic module with vector, axial-vector, scalar, and pseudoscalar normalizations, electron-mode acceptance conventions, and a documented long-distance treatment. The existing $\Delta F=2$ SLL/SLR/VLL/VRR/LR basis is not enough.

Reason: $D0 \to e^+$ e- needs a new $\Delta C = 1$ rare leptonic observable with lepton-current normalization, electron-mode acceptance conventions, and long-distance treatment; the existing $\Delta F = 2$ basis does not cover it.

Key references

Process-local keys before bibliography consolidation: PDG2026:D0Ee, Belle2010:D0Ee, BaBar2012:D0EeMuMuEMu, BurdmanGolowichHewettPakvasa2002:RareCharm, and CsakiFalkowskiWeiler2008:CompositeFlavor. Numeric claims above are tied to these sidecar source keys and local snapshots.

value 7.9e-08

RESOLVED

Match snapshot line 66

L63: 
L64: Extraction note:
L65:   The current PDG/API headline value for C005 is the Belle 2010 upper limit
L66:   B(D0 -> e+ e-) < 7.9e-8 at 90% CL. No LHCb D0 -> e+ e- measurement appears
L67:   in this PDG API measurement list at the access date above.

value 7.9e-08

RESOLVED

Match snapshot line 22

L19:   D0 -> e+- mu-+ using 660 fb^-1 of data collected with the Belle detector at
L20:   the KEKB asymmetric-energy e+e- collider. No evidence was found for any of
L21:   these decays. Belle obtained upper limits on the branching fractions:
L22:   B(D0 -> mu+mu-) < 1.4 x 10^-7, B(D0 -> e+e-) < 7.9 x 10^-8, and
L23:   B(D0 -> e+mu-) + B(D0 -> mu+e-) < 2.6 x 10^-7 at 90% confidence level.
L24: 
L25: Extraction note:

value 1.7e-07

RESOLVED

Match snapshot line 22

L19:   The observed event yields were consistent with expected backgrounds, except
L20:   for a D0 -> mu+mu- upward background fluctuation at the 5% level. Using the
L21:   Feldman-Cousins method, BABAR set 90% confidence-level intervals:
L22:   B(D0 -> e+e-) < 1.7 x 10^-7, B(D0 -> mu+mu-) in [0.6, 8.1] x 10^-7, and
L23:   B(D0 -> e mu) < 3.3 x 10^-7.
L24: 
L25: Extraction note: