In order to minimise the dependence of the measurement on the specific model assumed for Higgs boson production and properties, the fiducial phase space for the ${\ensuremath{\PH \to 4\ell}\xspace}$ cross section measurements is defined to match as closely as possible the experimental acceptance defined by the reconstruction-level selection. This includes the definition of selection observables and selection requirements, as well as the definition of the algorithm for the topological event selection.

The fiducial phase space is defined using the leptons produced in the hard scattering, before any FSR occurs. This choice is motivated by the fact that the recovery of the FSR photons is explicitly performed at the reconstruction level. In the case of differential measurements as a function of jet-related observables, jets are reconstructed from the individual stable particles, excluding neutrinos and muons, using the anti-$k_t$ clustering algorithm with a distance parameter of 0.5. Jets are considered if they satisfy $\pt \ge 30\GeV$ and $|\eta| \le 4.7$.

The fiducial phase space requires at least four leptons (electrons, muons), with at least one lepton having $\pt > 20\GeV$, another lepton having $\pt > 10\GeV$, and the remaining electrons and muons having $\pt > 7\GeV$ and $\pt > 5\GeV$ respectively. All 