This paper presents a practical method to study a particular Higgs boson decay in which two neutrinos escape detection. The decay is h → ℓ ν̄ℓ ℓ̄′ νℓ′ with the two charged leptons ℓ and ℓ′ taken to be different flavors (for example muon and electron). Because the neutrinos are invisible to detectors, a full map of all angles in the four-body final state is not directly measurable. The authors reorganize the kinematics so the measured quantities depend on a charged-lepton pair and a neutrino pair. That change makes it possible to write the decay rate in terms of observables that experiments can access, including the invariant mass of the neutrino pair inferred from the missing momentum.

What the researchers did is derive an expression for the differential decay rate using an Effective Field Theory (EFT) framework. They start from the charged-current amplitudes mediated by the W boson and rewrite the particle-spin factors so the neutrino-pair invariant mass appears explicitly. The full kinematics is described by five variables: the two invariant masses (the charged-lepton pair and the neutrino pair) and three angles. Because the neutrino angles cannot be measured, the authors integrate analytically over the azimuthal angle that describes the neutrino plane and then integrate numerically over one remaining neutrino-associated angle. The result is a differential distribution in experimentally accessible variables, for example the neutrino-pair invariant mass and the angle of one charged lepton.

At a high level this works because the charged leptons are measured well in collider detectors and their momenta let physicists infer the total missing momentum carried by the two neutrinos. By treating the neutrino pair as a single system, the authors can keep an experimentally accessible degree of freedom — the neutrino-pair invariant mass squared — and remove the dependence on angles that are not observed. They also use standard properties of the W-boson propagator and complex analysis to carry out the azimuthal-angle integration cleanly.