CMS-EXO-24-006

CMS-EXO-24-006 ; CERN-EP-2026-022
Search for heavy resonances decaying into four-lepton final states via light bosons in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
14 April 2026
Submitted to Physical Review Letters
Abstract: A search for a resonance heavier than 250 GeV decaying into four leptons via two intermediate bosons is presented. The search uses proton-proton collision data at $ \sqrt{s} = $ 13 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Novel techniques are used to enhance the sensitivity to a collimated pair of dileptons reconstructed as a single merged object, resulting from the decay of an intermediate light boson. No significant excess of data over the background predictions is observed. Upper limits are set on the production cross section for a four-lepton resonance, including the previously unexplored phase space at the LHC with a dilepton mass of 0.4--15 GeV.
Links: e-print arXiv:2604.14236 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The observed and predicted $ m_{4\ell} $ (left and center) and $ m_{\mathrm{T}} $ (right) distributions after the likelihood fit with the background-only hypothesis. The lower panel shows the ratio of the number of observed events to the predicted background. The shaded band represents the post-fit background uncertainty, incorporating both systematic and statistical sources. The red histogram shows a simulated signal mass spectrum for a given mass and decay mode hypothesis, normalized to the cross section times branching fraction of 10fb.
png pdf	Figure 2: The observed limit on the product of the cross section and branching fraction for (left) $ \mathrm{X}\to{\mathrm{Y}} {\mathrm{Y}} \to4\ell $ and (right) $ \mathrm{X}\to\mathrm{Z}{\mathrm{Y}} \to4\ell $ signal hypotheses. The orange (red) solid line represents the 1 (2) s.d. observed local significance contour.
png pdf	Figure 3: The observed limit on the product of the cross section and branching fraction for (upper left) $ \mathrm{X}\to{\mathrm{Y}} {\mathrm{Y}} \to4\mathrm{e} $, (upper right) $ \mathrm{X}\to{\mathrm{Y}} {\mathrm{Y}} \to4\mu $, (lower left) $ \mathrm{X}\to\mathrm{Z}{\mathrm{Y}} \to2\ell2\mathrm{e} $, and (lower right) $ \mathrm{X}\to\mathrm{Z}{\mathrm{Y}} \to2\ell2\mu $ signal hypotheses, where the Z boson decays into either a dielectron or dimuon. The orange (red) solid line represents the 1 (2) s.d. observed local significance contour.

Tables
png pdf	Table 1: The SRs, where each region is defined by the listed reconstructed leptons in an event. The e and $ \mu $ represent resolved leptons, while $ \mathrm{e}_{\mathrm{ME}} $ and $ \mu_{\mathrm{MM}} $ represent the merged electron and missing muon, respectively.
png pdf	Table 2: A summary of the CRs used to derive the misidentification factors for different lepton classes. The Z represents an SM Z boson candidate, formed by two opposite-sign same-flavor resolved leptons passing the ID criteria with an invariant mass of 91 $ \pm $ 7 GeV, and $ \ell^{\prime} $ denotes a resolved lepton with a different flavor (e or $ \mu $) than \ell.

Summary

In summary, a model-independent search for a resonance heavier than 250 GeV decaying into four-lepton final states has been presented for the $X \to YY \to 4\ell$ and $X \to ZY \to 4\ell$ channels, where X and Y are hypothetical new bosons with masses $m_{X}$ and $m_{Y}$. The search uses proton-proton collision data at $\sqrt{s} = 13$ TeV collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb$ ^{-1}$. Decays of a light Y boson with a mass $\lesssim$10 GeV can result in final states with a large Lorentz boost, resulting in highly collimated dielectron or dimuon signatures. A dedicated merged-electron identification is developed to enhance signal sensitivity to collimated dielectrons from a low-mass $m_{Y}$. Events with collimated dimuons are analyzed using the missing transverse momentum vector as a proxy for the missing muon. The maximum local (global) observed significance is 2.4 (1.8) standard deviations at $m_{X} = 550$ GeV and $m_{Y} = 0.8$ GeV in the $X \to ZY \to 4\ell$ channel. The 95% confidence level upper limits on the product of cross section and branching fraction to 4$\ell$ final states are set in the range $m_{X} > 250$ GeV and $m_{Y} > 0.4$ GeV.

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