CR005 $pp \to (\gamma^{(1)}, Z^{(1)})_{KK} \to l^+ l^-$
Dilepton high-mass resonance (KK electroweak) Status REVIEWED VERIFIED High Code: NO Priority Medium
PDG / equivalent values
| Observable | Value | Year | Experiment / source | Provenance |
|---|---|---|---|---|
| Observed lower mass limit for spin-1 $Z'_{SSM} \to $ e+e- and $\mu^+\mu^-$ | m(Z'_SSM) > 5.15 TeV 95% CL (lower_limit) | 2021 | CMS | source ↑ |
| Observed lower mass limit for spin-1 $Z'_\psi \to $ e+e- and $\mu^+\mu^-$ | m(Z'_psi) > 4.56 TeV 95% CL (lower_limit) | 2021 | CMS | source ↑ |
| Observed lower mass limit for spin-1 $Z'_{SSM} \to $ e+e- and $\mu^+\mu^-$ | m(Z'_SSM) > 5.1 TeV 95% CL (lower_limit) | 2019 | ATLAS | source ↑ |
| Observed fiducial cross-section times branching-ratio upper limit for a zero-width dilepton resonance at 6 TeV | sigma_fid x B < about 0.014 fb at m_X = 6 TeV 95% CL (upper_limit) | 2019 | ATLAS | source ↑ |
| PDG summary of ATLAS/CMS Z' $ \to $ e+e- and $\mu^+\mu^-$ cross-section upper limits | cross-section upper limits as low as 0.02 fb 95% CL (upper_limit) | 2024 | ATLAS/CMS summary | source ↑ |
Why this constrains the RS scan
For an RS model with bulk electroweak gauge fields, this search constrains
the first neutral electroweak KK tower, schematically
\((\gamma^{(1)},Z^{(1)},Z'_{\rm custodial})\), only after specifying the
light-quark production coupling, the lepton coupling, the total width, and
the branching fraction to \(e^+e^-\) and \(\mu^+\mu^-\). A sequential
\(Z'\) limit is therefore a useful collider benchmark, but it is not a
model-independent lower bound on the RS \(M_{\rm KK}\).
The distinction matters for the anarchic-flavor scan. The current quark
scan methodology note quotes the low-energy \(\Delta F=2\) headline crossing
\(M_{\rm KK}^{\min}=47.26~{\rm TeV}\) at \(g_*=3\) and 50\% acceptance.
The direct dilepton reach is an order of magnitude lower in the SSM-like
benchmark and can weaken further in custodial RS if the neutral KK bosons
couple weakly to light quarks or leptons, are broad, or preferentially decay
to \(t\bar t\), \(b\bar b\), \(W_LW_L\), \(Zh\), Higgs-sector states, or
fermion partners. CR005 is therefore a PRIMARY collider cross-check and a
handle on non-anarchic or nonstandard coupling patterns, not the leading
constraint for the anarchic-flavor RS baseline.
What's changed since the original paper
The post-2010 search history is a steady luminosity and energy progression.
ATLAS
arXiv:1209.2535 is the Run-1 reference most directly naming
Kaluza--Klein \(Z/\gamma\) bosons among the interpreted high-mass dilepton
signals. ATLAS arXiv:1707.02424 and CMS
arXiv:1803.06292 moved the same prompt \(ee/\mu\mu\) strategy to
the early 13 TeV Run-2 dataset and set the predecessor high-mass resonance
limits. ATLAS arXiv:1903.06248 then used the full Run-2 ATLAS
sample and provided generic width-dependent fiducial cross-section limits
plus \(Z'_{\rm SSM}\), \(Z'_\chi\), \(Z'_\psi\), and HVT interpretations.
CMS arXiv:2103.02708 used the full Run-2 CMS electron and muon
datasets, supplied the sharpest combined \(Z'_{\rm SSM}\) mass benchmark for
this entry, and also published recast-friendly coupling-plane information in
HEPData.Validity and model dependence
The measured collider objects are cross-section upper limits and benchmark
mass exclusions. Translating them into custodial-RS parameters requires
model input: production is usually \(q\bar q\)-initiated, the \(Z'\) or KK
state is treated as a narrow or moderately broad spin-1 resonance, and the
quoted SSM benchmark assumes SM-like fermion couplings. In realistic
custodial RS, the first neutral electroweak resonances can have suppressed
light-fermion couplings and enhanced couplings to IR-localized third
generation fermions and longitudinal electroweak bosons. The dilepton
branching fraction may therefore be much smaller than in \(Z'_{\rm SSM}\).
Conversely, a non-anarchic flavor structure or a model with deliberately
large lepton couplings can make this channel more competitive than the
minimal custodial-RS expectation.
Code coverage in this repo
NO. Required greps over
quarkConstraints/, qcd/,
flavorConstraints/, neutrinos/, yukawa/,
warpConfig/, solvers/, scanParams/, and
tests/ found no direct LHC dilepton-resonance likelihood, no
ATLAS/CMS/HEPData collider-limit ingestion, and no neutral electroweak KK
filter. The adjacent code is low-energy or spectrum bookkeeping:
quarkConstraints/deltaf2.py:1--10 describes a tree-level
KK-gluon-inspired \(\Delta F=2\) benchmark, and
quarkConstraints/deltaf2.py:320--324 matches a compact meson-mixing
operator basis at \(M_{\rm KK}\). quarkConstraints/couplings.py:96--124
builds quark mass-basis KK-gluon couplings, not electroweak dilepton
production. The only collider-looking grep hit was
tests/test\_alpha\_s.py:88--90, a CMS/RunDec \(\alpha_s\) example,
not an LHC search constraint.Implementation difficulty
HIGH. A faithful implementation is not a one-line mass cut. It
needs a signal model for the neutral electroweak KK spectrum and couplings,
parton luminosities or event generation, widths and branching fractions, and
a likelihood or external reinterpretation of the ATLAS/CMS dilepton spectra
and limits. A practical implementation would likely use exported HEPData
tables plus a recasting tool or a validated internal likelihood, and would
need separate branches for narrow \(Z'\)-like, broad, and third-generation
dominated custodial-RS scenarios.
Reason: A faithful live constraint needs a neutral electroweak KK signal model, parton luminosities or event generation, total-width and branching-ratio predictions, and a likelihood or external reinterpretation of ATLAS/CMS dilepton limits. A simple universal M_KK mass cut would overstate the SSM benchmark.
Key references
Process-local source keys before bibliography consolidation are
CMS2021\_HighMassDilepton,
ATLAS2019\_HighMassDilepton,
PDG2024\_ZPrimeSearches,
HEPDataCMS2021\_HighMassDilepton,
HEPDataATLAS2019\_HighMassDilepton,
DavoudiaslHewettRizzo1999\_BulkGauge,
AgasheDelgadoMaySundrum2003\_CustodialRS,
AgasheServant2004\_WarpedGUT,
BellaEtAl2010\_KKEWGauge,
ATLAS2012\_Run1Dilepton,
ATLAS2017\_Run2Dilepton36fb, and
CMS2018\_Run2Dilepton36fb.