Testing Bell inequalities in Higgs boson decays

Abstract

Higgs boson decays produce pairs of $W$ bosons in a maximally entangled state, the spins of which can be expected to violate Bell inequalities. We show that the spin density matrix of the $W^{\pm }$ pair may be reconstructed experimentally from the directions of the charged lepton decay products, and from it the expectation values of various Bell operators determined. Numerical simulations of $H\to W{W}^{\ast }$ decays indicate that violation of a generalised CHSH inequality is unlikely to be measurable, however the CGLMP inequality is near-maximally violated. Experimental Bell tests could be performed at a variety of colliders and in different production channels. If reconstruction effects and backgrounds can be controlled then statistically significant violations could be observable even with datasets comparable to those already collected at the LHC.

Sign corrections to Eqns. (12) (13) (23). Results and conclusions unchanged

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• Quantum information and CP measurement in $H\to {\tau }^{+}{\tau }^{-}$ at future lepton colliders
• Quantum entanglement and top spin correlations in SMEFT at higher orders
• Measurement of Bell-type inequalities and quantum entanglement from $\Lambda$-hyperon spin correlations at high energy colliders
• Laboratory-frame tests of quantum entanglement in $H\to WW$
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• Comment on "Spin correlations in elastic $e^{+}{e}^{-}$ scattering in QED"
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