The CMS Collaboration at the Large Hadron Collider searched for a new heavy particle that would decay to two Z bosons, with each Z boson then decaying to a pair of electrons or muons. The study used proton–proton collision data collected in 2016–2018 at a center-of-mass energy of 13 teraelectronvolts (TeV), corresponding to an integrated luminosity of 138 inverse femtobarns (fb⁻¹). No significant excess over the standard model background was found.

The search looked for a generic heavy scalar resonance, called X, with a mass anywhere between 130 gigaelectronvolts (GeV) and 3 teraelectronvolts (TeV). The experimental signature is four clean, well-measured leptons (electrons or muons) coming from two Z bosons. The team considered two main ways such a particle might be produced in the collisions: gluon fusion (where two gluons collide) and vector boson fusion (where two W or Z bosons from the protons interact).

The analysis tested both “narrow-width” and “broad-width” scenarios. Width is a measure of how quickly a particle decays; a broader width means a shorter lifetime and a wider feature in the measured mass distribution. For broad widths the study also accounted for interference effects. Interference means the new signal could add to or subtract from the known processes, including production of the 125 GeV Higgs boson and the continuum background of two Z bosons produced directly by the standard model.

To estimate what signal and background would look like, the collaboration used detailed simulation tools that include higher-order theoretical corrections. Dominant backgrounds come from standard model production of two Z bosons through quark-antiquark collisions and through gluon fusion. The analysis compared the observed four-lepton mass distribution to those expectations and set upper limits on the product of the production rate and the probability that X decays to two Z bosons.