The Higgs boson is like a tuning fork that rings at a strictly defined note. But physicists have learned to hear it even when the note is off — in the so-called 'off-shell' region. It was there, in the data from the CMS detector, that answers were hiding about what the Higgs mechanism is really made of.
In proton collisions at the record energy of 13 TeV, pairs of Z bosons were produced, and through them, Higgs particles with masses far from the usual 125 GeV. This 'detuned' signal allowed the first direct test of the hypothesis: is the Higgs boson composite, like an atom or a proton? It turned out that if it does have an internal structure, it is hidden below 870 GeV. That's like a tuning fork showing no sign of composite parts even when struck with a hammer seven times harder than usual.
The second surprise came from the light quarks. Previously, their contribution to Higgs production was almost impossible to detect: it was drowned out by the 'noise' of heavier particles. But by combining data from the on-shell and off-shell regions, physicists managed to isolate this faint whisper and place the tightest constraints to date. Along the way, they measured the boson's lifetime: 5.1 MeV in energy units — about 10⁻²² seconds. That's comparable to the time it takes light to travel a distance a hundred times smaller than an atomic nucleus.
as written by the authors arXiv:2607.23352
Measurements of Higgs boson production in the off-shell region are presented, using the four-lepton decay channel. Data from proton-proton collisions at the CERN LHC, collected by the CMS experiment and corresponding to an integrated luminosity of 138 fb$$^{-1}$$ at a center-of-mass energy of 13 TeV, are utilized. The first direct test of composite Higgs boson models is performed, with a lower limit on the compositeness scale $$\Lambda_\mathrm{H}$$ set at 870 GeV at the 95% confidence level. Tests of gluon-fusion production within the standard model effective field theory framework are performed. The first constraint on the Higgs boson self-coupling in the off-shell region is obtained. By combining on- and off-shell measurements, the analysis sets the tightest constraints to date on light-quark Yukawa couplings, while relaxing assumptions such as the bound on the Higgs boson coupling to vector bosons, thereby providing more model-independent results. Constraints on the Higgs boson width are provided while accounting for a range of beyond-the-standard-model effects, including both light and heavy particles in the gluon-fusion production loop, as well as modified couplings to vector bosons. A combined analysis of the H$$\to$$ZZ and PH$$\to$$WW channels is performed to improve sensitivity to the Higgs boson width, yielding $$\Gamma_\mathrm{H}$$ = 5.1$$^{+2.0}_{-1.8}$$ MeV. The scenario of no off-shell Higgs boson production is excluded at a confidence level exceeding 5 standard deviations.
The width of the Higgs boson measured in this analysis is about 5 MeV. In more familiar units, that corresponds to a lifetime of roughly 10⁻²² seconds. In that time, light travels a distance a hundred times smaller than the diameter of a proton.
In Liu Cixin's novel 'The Three-Body Problem', an advanced civilization manipulates physical constants — searches like this for a composite nature of the Higgs field could be a first step toward detecting such interference.