Catalog · Top / Higgs / EW · T015

T015 $\mathcal{B}(Z\to e^\pm\mu^\mp)$

Lepton-flavor-violating Z decay Z $ \to $ e $\mu$

Status REVIEWED VERIFIED High Code: NO Priority Medium

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
B(Z $ \to $ e+- $\mu^-+$), charge-summed direct search	< 1.9e-7 branching fraction	2025	CMS2025:T015:zemu_limit	source ↑
CMS dataset for Z $ \to $ e $\mu$ direct search	138 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2025	CMS2025:T015:dataset	source ↑
B(Z $ \to $ e+- $\mu^-+$), charge-summed PDG listing	< 2.62e-7 branching fraction	2025	PDG2025:T015:zemu_limit	source ↑
B(Z $ \to $ e+- $\mu^-+$), ATLAS Run-2 direct search	< 2.62e-7 branching fraction	2023	ATLAS2023:T015:zemu_limit	source ↑
ATLAS Run-2 dataset for Z $ \to $ e $\mu$ direct search	139 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2023	ATLAS2023:T015:dataset	source ↑
B(Z $ \to $ e+- $\mu^-+$), ATLAS Run-1 direct search	< 7.5e-7 branching fraction	2014	ATLAS2014:T015:zemu_limit	source ↑
ATLAS Run-1 dataset for Z $ \to $ e $\mu$ direct search	20.3 fb^-1 at sqrt(s) = 8 TeV integrated luminosity and collision energy	2014	ATLAS2014:T015:dataset	source ↑
Future Z-pole factory scale discussed for LFV Z searches	O(1e12) Z decays produced Z bosons	2021	CalibbiMarcanoRoy2021:T015:teraZ	source ↑

Why this constrains the RS scan

Anarchic or partially composite lepton sectors can induce non-diagonal neutral currents after rotating to charged-lepton mass eigenstates. A direct $Z\to e\mu$ limit tests any effective off-diagonal $Z e\mu$ coupling from KK gauge mixing, zero-mode coupling non-universality, or lepton-sector flavor symmetry breaking. It is therefore relevant to a lepton/electroweak extension of the present pipeline, not to the existing quark $\Delta F=2$ scan lane.

What's changed since the original paper

The arXiv:0804.1954 warped-flavor baseline predates the LHC direct $Z$-LFV program (CsakiFalkowskiWeiler2008WarpedFlavor; PerezRandall2008WarpedLeptonFlavor). Since then ATLAS established the $8~\mathrm{TeV}$ $e\mu$ search at the $10^{-7}$ level (ATLAS2014ZLFVEmu), then improved it with the full Run-2 data set (ATLAS2023ZLFVEmu); CMS subsequently pushed the direct $e\mu$ limit below $2\times10^{-7}$ (CMS2025ZLFV). On the theory side, post-LEP EFT studies of future Tera-$Z$ facilities emphasize that indirect low-energy $\mu$-to-$e$ constraints can make $Z\to \mu e$ harder to observe than $Z\to\tau\ell$ even with $\mathcal{O}(10^{12})$ produced $Z$ bosons (CalibbiMarcanoRoy2021:T015:teraZ).

Validity and model dependence

Experimentally this is a clean direct upper limit on a forbidden Standard Model decay. Its use as a scan cut assumes SM-like inclusive $Z$ production and total $Z$ width. Model interpretations are less universal: in many lepton-flavor models the same operator structure is correlated with $\mu\to e\gamma$, $\mu\to3e$, or coherent $\mu$-to-$e$ conversion, so this should be treated as a direct collider constraint on $Z e\mu$, not as a complete charged-lepton flavor-violation likelihood.

Code coverage in this repo

Coverage is NO. The required broad greps and a targeted search for $Z \to e \mu$ , $Z \to \mu$ e, Zemu, and related LFV-$Z$ strings across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ found no $Z\to e\mu$ observable implementation. The only charged-lepton LFV backend seen is $\mu\to e\gamma$ (flavorConstraints/muToEGamma.py:75), wired into the scan at scanParams/scan.py:524; the modern phenomenology policy surface is limited to $\epsilon_K$, $K$, $B_d$, $B_s$, and $D^0$ (quarkConstraints/modern/phenomenology.py:23).

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

Implementation difficulty

Production integration is HIGH. Storing the external limit is straightforward, but a faithful RS constraint needs a new electroweak LFV operator convention, matching from lepton localization and gauge/Higgs-sector mixing to $Z e\mu$, and a decision on how to combine direct $Z$-pole limits with low-energy $\mu$-flavor observables.

Reason: MEDIUM to store the external branching-ratio upper limit, but HIGH for production-grade RS integration because the repo lacks LFV Z-coupling operators, electroweak/lepton-sector matching, and a policy for combining direct Z-pole and low-energy mu-to-e constraints.

Key references

PDG2025ZLFVListing; CMS2025ZLFV; ATLAS2023ZLFVEmu; ATLAS2014ZLFVEmu; CalibbiMarcanoRoy2021ZLFV; CsakiFalkowskiWeiler2008WarpedFlavor; PerezRandall2008WarpedLeptonFlavor.

value 138 fb^-1 at sqrt(s) = 13 TeV

RESOLVED

Match snapshot line 18

L15: A search for flavor violating decays of the Z boson to charged leptons is
L16: performed using data from proton-proton collisions at sqrt(s) = 13 TeV
L17: collected with the CMS detector at the LHC, corresponding to an integrated
L18: luminosity of 138 fb^-1. Each of the decays Z -> e mu, Z -> e tau, and
L19: Z -> mu tau is considered. The data are consistent with the backgrounds
L20: expected from standard model processes. For the Z -> e mu channel the observed
L21: (expected) 95% confidence level upper limit on the branching fraction is

value < 2.62e-7

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Match snapshot line 15

L12: 
L13: Relevant extracted listing text:
L14: 
L15: Gamma66  e+- mu-+    LF    [c] < 2.62 x 10^-7    CL=95%
L16: Gamma67  e+- tau-+   LF    [c] < 5.0  x 10^-6    CL=95%
L17: Gamma68  mu+- tau-+  LF    [c] < 6.5  x 10^-6    CL=95%
L18:

value < 2.62e-7

RESOLVED

Match snapshot line 19

L16: presented, using 139 fb^-1 of sqrt(s) = 13 TeV pp collision data collected by
L17: the ATLAS experiment at the LHC. An excess in the e mu invariant mass spectrum
L18: near the Z boson mass would be a striking signature of new physics. No excess
L19: is observed, and an upper limit B(Z -> e mu) < 2.62 x 10^-7 is placed on the
L20: branching fraction at 95% confidence level, which is the most stringent limit
L21: to date.

value 139 fb^-1 at sqrt(s) = 13 TeV

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Match snapshot line 16

L13: 
L14: Relevant abstract text:
L15: A search for the charged-lepton-flavor-violating process Z -> e mu is
L16: presented, using 139 fb^-1 of sqrt(s) = 13 TeV pp collision data collected by
L17: the ATLAS experiment at the LHC. An excess in the e mu invariant mass spectrum
L18: near the Z boson mass would be a striking signature of new physics. No excess
L19: is observed, and an upper limit B(Z -> e mu) < 2.62 x 10^-7 is placed on the

value < 7.5e-7

RESOLVED

Match snapshot line 21

L18: in the data sample is estimated using events of similar topology, Z -> ee and
L19: mu mu, significantly reducing the systematic uncertainty in the measurement.
L20: There is no evidence of an enhancement at the Z boson mass, resulting in an
L21: upper limit on the branching fraction, B(Z -> e mu) < 7.5 x 10^-7 at the 95%
L22: confidence level.

value 20.3 fb^-1 at sqrt(s) = 8 TeV

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Match snapshot line 15

L12: 
L13: Relevant abstract text:
L14: The ATLAS detector at the Large Hadron Collider is used to search for the
L15: lepton flavor violating process Z -> e mu in pp collisions using 20.3 fb^-1 of
L16: data collected at sqrt(s) = 8 TeV. An enhancement in the e mu invariant mass
L17: spectrum is searched for at the Z boson mass. The number of Z bosons produced
L18: in the data sample is estimated using events of similar topology, Z -> ee and

value O(1e12) Z decays

RESOLVED

Match snapshot line 17

L14: In this work we assess the potential of discovering new physics by searching
L15: for lepton-flavour-violating decays of the Z boson, Z -> l_i l_j, at the
L16: proposed circular e+e- colliders CEPC and FCC-ee. Both projects plan to run at
L17: the Z-pole as a Tera Z factory, i.e., collecting O(10^12) Z decays. In order
L18: to discuss the discovery potential in a model-independent way, we revisit the
L19: LFV Z decays in the context of the Standard Model effective field theory and
L20: study the indirect constraints from LFV mu and tau decays on the operators that