CMS-PAS-SMP-25-011

CMS-PAS-SMP-25-011
Search for W boson decays to three charged hadrons
CMS Collaboration
2026-07-08

Abstract: A search is presented for the rare decay of the W boson into the hadronic final state of three charged hadrons. This search is based on proton-proton collision data collected by the CMS experiment at the CERN LHC at center-of-mass energies of 13 and 13.6 TeV, corresponding to integrated luminosities of 138 and 308 fb$ ^{-1} $, respectively. No evidence for the existence of a signal is observed. An upper limit of 2.4 $ \times 10^{-7} $ is set at 95% confidence level on the branching fraction of the W boson to three charged hadrons, improving previous results by about a factor of four.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Observed and expected triplet invariant mass distributions for the combination of all data-taking periods, for the HP (left) and LP (right) categories. The bin contents are normalized by the bin width. The signal is shown on top of the expected backgrounds with its fitted braching fraction $ \mathcal{B}(\mathrm{W}^{\pm}\to 3\mathrm{h}^{\pm})= $ 0.5 $ ^{+1.0}_{-0.5} $ \times 10^-7, as well as overlayed, displayed as a line, and enhanced by a factor of 10 for better visibility of the shape. The ``Other'' background includes processes with top quarks and processes with two or three vector bosons.
png pdf	Figure 1-a: Observed and expected triplet invariant mass distributions for the combination of all data-taking periods, for the HP (left) and LP (right) categories. The bin contents are normalized by the bin width. The signal is shown on top of the expected backgrounds with its fitted braching fraction $ \mathcal{B}(\mathrm{W}^{\pm}\to 3\mathrm{h}^{\pm})= $ 0.5 $ ^{+1.0}_{-0.5} $ \times 10^-7, as well as overlayed, displayed as a line, and enhanced by a factor of 10 for better visibility of the shape. The ``Other'' background includes processes with top quarks and processes with two or three vector bosons.
png pdf	Figure 1-b: Observed and expected triplet invariant mass distributions for the combination of all data-taking periods, for the HP (left) and LP (right) categories. The bin contents are normalized by the bin width. The signal is shown on top of the expected backgrounds with its fitted braching fraction $ \mathcal{B}(\mathrm{W}^{\pm}\to 3\mathrm{h}^{\pm})= $ 0.5 $ ^{+1.0}_{-0.5} $ \times 10^-7, as well as overlayed, displayed as a line, and enhanced by a factor of 10 for better visibility of the shape. The ``Other'' background includes processes with top quarks and processes with two or three vector bosons.
png pdf	Figure 2: Observed and expected 95% CL limit on $ \mathcal{B}(\mathrm{W}^{\pm}\to 3\mathrm{h}^{\pm}) $.

Summary

In summary, a search for the rare decay of a W boson into three charged hadrons has been presented using the data collected at $\sqrt{s}=13$ and 13.6 TeV by the CMS experiment. No evidence for the $\mathrm{W}^{\pm}\to 3\mathrm{h}^{\pm}$ signal is observed. The most stringent upper limit on the $\mathrm{W}^{\pm}\to 3\mathrm{h}^{\pm}$ branching fraction is set at $ 2.4 \times 10^{-7} $ at 95% confidence level.

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