Flavor Catalog v0.4
102 entries
Catalog · Top / Higgs / EW · T016

T016 $\mathcal{B}(Z\to e^\pm\tau^\mp)$

Lepton-flavor-violating Z decay Z $ \to $ e $\tau$
Status REVIEWED VERIFIED High Code: NO Priority Medium

PDG / equivalent values

Observable Value Year Experiment / source Provenance
B(Z $ \to $ e+- $\tau^-+$), charge-summed PDG listing < 5.0e-6 branching fraction 2025 PDG2025:T016:zetau_limit source ↑
B(Z $ \to $ e $\tau$), ATLAS combined leptonic plus hadronic $\tau$ analyses < 5.0e-6 branching fraction 2021 ATLAS2021:T016:zetau_combined_limit source ↑
B(Z $ \to $ e $\tau$), ATLAS leptonically decaying $\tau$ analysis only < 7.0e-6 branching fraction 2021 ATLAS2021:T016:zetau_leptonic_limit source ↑
ATLAS Run-2 dataset for leptonically decaying $\tau$ Z LFV search 139 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy 2021 ATLAS2021:T016:dataset source ↑
B(Z $ \to $ e $\tau$), ATLAS hadronic $\tau$ analysis < 8.1e-6 branching fraction 2021 ATLAS2020:T016:zetau_hadronic_limit source ↑
ATLAS hadronic $\tau$ Z LFV combined dataset 139 fb^-1 at sqrt(s) = 13 TeV and 20.3 fb^-1 at sqrt(s) = 8 TeV integrated luminosity and collision energy 2021 ATLAS2020:T016:dataset source ↑
B(Z $ \to $ e $\tau$), CMS direct search < 13.8e-6 branching fraction 2025 CMS2025:T016:zetau_limit source ↑
CMS dataset for Z $ \to $ e $\tau$ direct search 138 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy 2025 CMS2025:T016:dataset source ↑
Future Z-pole factory scale discussed for LFV Z searches O(1e12) Z decays produced Z bosons 2021 CalibbiMarcanoRoy2021:T016:teraZ source ↑

Why this constrains the RS scan

Warped or partially composite lepton sectors can generate non-universal neutral-current couplings before rotating to charged-lepton mass eigenstates. After that rotation, a non-diagonal \(Z e\tau\) vertex can appear from gauge KK mixing, zero-mode coupling distortions, or lepton-sector flavor breaking. This process is therefore a direct collider probe of electroweak lepton-flavor violation in a lepton-sector extension, not a constraint implemented in the current quark \(\Delta F=2\) scan.

What's changed since the original paper

The arXiv:0804.1954 warped-flavor baseline predates the LHC \(Z\)-LFV program. Since then ATLAS superseded the old LEP-era \(Z\to e\tau\) sensitivity with Run-2 searches using hadronically and leptonically decaying tau channels; the combined ATLAS result is now the PDG-listed \(5.0\times10^{-6}\) bound (PDG2025:T016:zetau\_limit). CMS added a full-Run-2 search covering \(Z\to e\mu\), \(Z\to e\tau\), and \(Z\to\mu\tau\); its \(e\tau\) result is weaker than the ATLAS/PDG bound but is an independent experimental cross-check. Phenomenology work on future \(e^+e^-\) colliders has also sharpened the interpretation: a Tera-\(Z\) program with \(\mathcal{O}(10^{12})\) \(Z\) decays is expected to be more promising for \(Z\to\tau\ell\) than for \(Z\to\mu e\), because low-energy \(\mu\)-flavor constraints are much more restrictive (CalibbiMarcanoRoy2021:T016:teraZ).

Validity and model dependence

Experimentally this is a clean direct upper limit on a forbidden Standard Model decay, normalized to the inclusive \(Z\) branching fraction. As a model constraint it is not universal: the same effective \(Z e\tau\) structures may also induce tau LFV decays, dipoles, or four-lepton operators depending on the UV completion. In the catalog it should be classified as a direct collider constraint on an off-diagonal \(Z e\tau\) coupling, with correlations to low-energy tau observables left for a future lepton-sector likelihood.

Code coverage in this repo

Coverage is NO. The required broad greps and a targeted search for $Z \to e \tau$, $Z \to \tau$ e, Zetau, and related LFV-\(Z\) strings across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ found no \(Z\to e\tau\) observable implementation. The only charged-lepton LFV backend seen is \(\mu\to e\gamma\) (flavorConstraints/muToEGamma.py:75), called by 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 branching-ratio limit is simple, but a live RS constraint needs a new electroweak LFV operator convention, matching from lepton localization and \(Z\)-coupling distortions to left- and right-handed \(Z e\tau\) amplitudes, and a policy for combining the direct \(Z\)-pole bound with low-energy tau LFV limits.

Reason: MEDIUM to store the 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 tau LFV constraints.

Key references

PDG2025ZLFVListing; ATLAS2021ZLFVTau; ATLAS2020ZLFVTauHad; CMS2025ZLFV; CalibbiMarcanoRoy2021ZLFV; CsakiFalkowskiWeiler2008WarpedFlavor; PerezRandall2008WarpedLeptonFlavor.
Source SHAs
7 snapshot(s) tracked in flavor_catalog/processes/top_higgs_ew/T016.yaml
Access dates
2026-05-16
Worklog
flavor_catalog/worklogs/T016/
LaTeX source
flavor_catalog/processes/top_higgs_ew/T016.tex
Anchors generated
2026-05-18T01:54:07Z

PDG2025:T016:zetau_limit

https://pdgweb.lbl.gov/2025/listings/rpp2025-list-z-boson.pdf
accessed 2026-05-16 sha d6c641885851... snapshot T016/pdg2025_z_lfv_listing.txt RESOLVED
value < 5.0e-6
RESOLVED
Match snapshot line 16 
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: 
L19: Footnote [c]:
Snapshots live under flavor_catalog/references/<process_id>/. Source-line anchoring is automated; the status pill reflects match confidence, not editorial review.

ATLAS2021:T016:zetau_combined_limit

https://arxiv.org/abs/2105.12491
accessed 2026-05-16 sha 0869c301a050... snapshot T016/atlas2021_prl_z_ltau_arxiv_abs.txt RESOLVED
value < 5.0e-6
RESOLVED
Match snapshot line 28 
L25: B(Z -> mu tau) < 7.2 x 10^-6 at 95% confidence level.
L26: 
L27: The combination with the previously published analyses sets the strongest
L28: constraints to date: B(Z -> e tau) < 5.0 x 10^-6 and
L29: B(Z -> mu tau) < 6.5 x 10^-6 at 95% confidence level.
Snapshots live under flavor_catalog/references/<process_id>/. Source-line anchoring is automated; the status pill reflects match confidence, not editorial review.

ATLAS2021:T016:zetau_leptonic_limit

https://arxiv.org/abs/2105.12491
accessed 2026-05-16 sha 0869c301a050... snapshot T016/atlas2021_prl_z_ltau_arxiv_abs.txt AMBIGUOUS
value < 7.0e-6
AMBIGUOUS
Match 1 of 2 snapshot line 24 
L21: 
L22: The addition of leptonically decaying tau-leptons significantly improves the
L23: sensitivity reach for Z -> ell tau decays. The Z -> ell tau branching fractions
L24: are constrained in this analysis to B(Z -> e tau) < 7.0 x 10^-6 and
L25: B(Z -> mu tau) < 7.2 x 10^-6 at 95% confidence level.
L26: 
L27: The combination with the previously published analyses sets the strongest
Match 2 of 2 snapshot line 25 
L22: The addition of leptonically decaying tau-leptons significantly improves the
L23: sensitivity reach for Z -> ell tau decays. The Z -> ell tau branching fractions
L24: are constrained in this analysis to B(Z -> e tau) < 7.0 x 10^-6 and
L25: B(Z -> mu tau) < 7.2 x 10^-6 at 95% confidence level.
L26: 
L27: The combination with the previously published analyses sets the strongest
L28: constraints to date: B(Z -> e tau) < 5.0 x 10^-6 and
Snapshots live under flavor_catalog/references/<process_id>/. Source-line anchoring is automated; the status pill reflects match confidence, not editorial review.

ATLAS2021:T016:dataset

https://arxiv.org/abs/2105.12491
accessed 2026-05-16 sha 0869c301a050... snapshot T016/atlas2021_prl_z_ltau_arxiv_abs.txt RESOLVED
value 139 fb^-1 at sqrt(s) = 13 TeV
RESOLVED
Match snapshot line 17 
L14: 
L15: A search for lepton-flavor-violating Z -> e tau and Z -> mu tau decays with pp
L16: collision data recorded by the ATLAS detector at the LHC is presented. This
L17: analysis uses 139 fb^-1 of Run 2 pp collisions at sqrt(s) = 13 TeV and is
L18: combined with the results of a similar ATLAS search in the final state in which
L19: the tau-lepton decays hadronically, using the same data set as well as Run 1
L20: data.
Snapshots live under flavor_catalog/references/<process_id>/. Source-line anchoring is automated; the status pill reflects match confidence, not editorial review.

ATLAS2020:T016:zetau_hadronic_limit

https://arxiv.org/abs/2010.02566
accessed 2026-05-16 sha e96e6d395dcd... snapshot T016/atlas2020_natphys_z_ltau_arxiv_abs.txt UNRESOLVED
value < 8.1e-6
UNRESOLVED

Automated anchoring could not pinpoint this value inside the snapshot. Manual verification is needed: open the live source and confirm against the context surrounding the value claim.

Open source manually
Snapshots live under flavor_catalog/references/<process_id>/. Source-line anchoring is automated; the status pill reflects match confidence, not editorial review.

ATLAS2020:T016:dataset

https://arxiv.org/abs/2010.02566
accessed 2026-05-16 sha e96e6d395dcd... snapshot T016/atlas2020_natphys_z_ltau_arxiv_abs.txt RESOLVED
value 139 fb^-1 at sqrt(s) = 13 TeV and 20.3 fb^-1 at sqrt(s) = 8 TeV
RESOLVED
Match snapshot line 21 
L18: 
L19: The branching fractions for these decays are measured to be less than
L20: 8.1 x 10^-6 (e tau) and 9.5 x 10^-6 (mu tau) at 95% confidence level using
L21: 139 fb^-1 of proton-proton collision data at a centre-of-mass energy
L22: sqrt(s) = 13 TeV and 20.3 fb^-1 at sqrt(s) = 8 TeV. These results supersede the
L23: limits from the Large Electron-Positron Collider experiments conducted more
L24: than two decades ago.
Snapshots live under flavor_catalog/references/<process_id>/. Source-line anchoring is automated; the status pill reflects match confidence, not editorial review.

CMS2025:T016:zetau_limit

https://arxiv.org/abs/2508.07512
accessed 2026-05-16 sha 9370bd0fcb6e... snapshot T016/cms2025_smp23003_arxiv_abs.txt UNRESOLVED
value < 13.8e-6
UNRESOLVED

Automated anchoring could not pinpoint this value inside the snapshot. Manual verification is needed: open the live source and confirm against the context surrounding the value claim.

Open source manually
Snapshots live under flavor_catalog/references/<process_id>/. Source-line anchoring is automated; the status pill reflects match confidence, not editorial review.

CMS2025:T016:dataset

https://arxiv.org/abs/2508.07512
accessed 2026-05-16 sha 9370bd0fcb6e... snapshot T016/cms2025_smp23003_arxiv_abs.txt RESOLVED
value 138 fb^-1 at sqrt(s) = 13 TeV
RESOLVED
Match snapshot line 19 
L16: A search for flavor violating decays of the Z boson to charged leptons is
L17: performed using data from proton-proton collisions at sqrt(s) = 13 TeV
L18: collected with the CMS detector at the LHC, corresponding to an integrated
L19: luminosity of 138 fb^-1. Each of the decays Z -> e mu, Z -> e tau, and
L20: Z -> mu tau is considered. The data are consistent with the backgrounds
L21: expected from standard model processes.
L22:
Snapshots live under flavor_catalog/references/<process_id>/. Source-line anchoring is automated; the status pill reflects match confidence, not editorial review.

CalibbiMarcanoRoy2021:T016:teraZ

https://arxiv.org/abs/2107.10273
accessed 2026-05-16 sha b6bc6608c1cd... snapshot T016/calibbi_marcano_roy2021_arxiv_abs.txt RESOLVED
value O(1e12) Z decays
RESOLVED
Match snapshot line 17 
L14: The authors assess the potential of discovering new physics by searching for
L15: lepton-flavour-violating decays of the Z boson, Z -> l_i l_j, at proposed
L16: circular e+e- colliders CEPC and FCC-ee. Both projects plan to run at the
L17: Z-pole as a Tera Z factory, collecting O(10^12) Z decays.
L18: 
L19: They revisit LFV Z decays in Standard Model effective field theory and study
L20: indirect constraints from LFV mu and tau decays on operators that can induce
Snapshots live under flavor_catalog/references/<process_id>/. Source-line anchoring is automated; the status pill reflects match confidence, not editorial review.