Laboratory-frame tests of quantum entanglement in $H\to WW$

Quantum entanglement between the two $W$ bosons resulting from the decay of a Higgs boson may be investigated in the dilepton channel $H\to WW\to \mathrm{\ell }\nu \mathrm{\ell }\nu$ using laboratory-frame observables that only involve the charged leptons $\mathrm{\ell }=e,\mu$. The dilepton invariant mass distribution, already measured by the ATLAS and CMS Collaborations at the LHC, can be used to observe the quantum entanglement of the $WW$ pair with a statistical sensitivity of $7\sigma$ with Run 2 data, and of $6\sigma$ when including theoretical systematics. As a by-product, the relation between $W$ rest frame (four-dimensional) angular distributions, $H\to WW$ decay amplitudes, and spin correlation coefficients, is written down.