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arXiv:2504.03841 · Journal of High Energy Physics

Quantum properties of H → VV*: precise predictions in the SM and sensitivity to new physics

Morgan Del Gratta, F. Fabbri, Priyanka Lamba, Fabio Maltoni B, Davide Pagani

Published Sep 1, 2025• 18 citations
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Abstract

A bstract We study the quantum properties of the Higgs-boson decays into four fermions via two vector bosons ( H → VV * → 4 f ). In particular, we focus on the case of two different-flavour lepton pairs ( H → ZZ * → μ + μ − e + e − ). We compute the quantum-information observables for the corresponding two-qutrit system ( ZZ ) at next-to-leading order electroweak (NLO EW) accuracy in the SM. We find that NLO EW corrections lead to giant (order 1) effects in some specific cases, significantly altering the extraction of observables quantifying the quantum correlations. We identify observables that are robust and can be used to extract reliable information. Finally, we discuss possible new physics (NP) effects, parametrised via an effective-field-theory approach. We show how quantum observables can increase the sensitivity to NP also for the process considered in this study.

DOI
10.1007/jhep09(2025)013

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A bstract We study the quantum properties of the Higgs-boson decays into four fermions via two vector bosons ( H → VV * → 4 f ). In particular, we focus on the case of two different-flavour lepton pairs ( H → ZZ * → μ + μ − e + e − ). We compute the quantum-information observables for the corresponding two-qutrit system ( ZZ ) at next-to-leading order electroweak (NLO EW) accuracy in the SM. We find that NLO EW corrections lead to giant (order 1) effects in some specific cases, significantly altering the extraction of observables quantifying the quantum correlations. We identify observables that are robust and can be used to extract reliable information. Finally, we discuss possible new physics (NP) effects, parametrised via an effective-field-theory approach. We show how quantum observables can increase the sensitivity to NP also for the process considered in this study.
Quantum Tomography and Entanglement in Semi-Leptonic h→VV∗h\to VV^*h→VV∗ Decays at Higher Orders
Angular correlations in Higgs decays to electroweak gauge bosons, h→ZZ∗,WW∗h \to ZZ^*, WW^*h→ZZ∗,WW∗, provide a powerful probe of both new physics effects and quantum information observables. We present a systematic study of semi-leptonic Higgs decays h→VV∗→ℓ+ℓ−qqˉh \to V V^* \to \ell^+\ell^- q\bar{q}h→VV∗→ℓ+ℓ−qqˉ​ and ℓ±νℓqqˉ′\ell^\pm ν_\ell q\bar{q}'ℓ±νℓ​qqˉ​′, including finite final state fermion masses, NLO QCD, and NLO electroweak corrections. We show that finite final state quark masses can induce effects that go beyond the two-qutrit description in more inclusive regimes, while remaining controllable with suitable kinematic selections. QCD corrections lead to modest percent-level shifts, whereas electroweak corrections can significantly modify the angular structure, particularly in the h→ZZ∗h\to ZZ^*h→ZZ∗ channels. We assess the impact of these effects on the reconstructed density matrix and entanglement measures, finding that, although they modify the angular observables, the semi-leptonic channels are better approximated by a two-qutrit description than the fully leptonic decay h→e+e−μ+μ−h\to e^+e^-μ^+μ^-h→e+e−μ+μ−.
A bstract We investigate the quantum properties of heavy-fermion pairs, such as tt‾ t\overline{t} tt t t ¯ or τ + τ − , produced in lepton-lepton collisions with polarised beams. Focusing on spin correlations, entanglement, Bell-inequality violation, and quantum-information-theoretic measures such as purity and magic, we analyse how beam polarisation shapes the structure of the spin-density matrix. We derive analytic expressions for a wide range of helicity configurations, including both Standard Model contributions and generic new physics effects parametrised by scalar, vector, and tensor four-fermion operators within an effective field theory framework. We show that beam polarisation unlocks a substantially richer set of spin configurations and significantly enhances sensitivity to non-standard interactions. As a phenomenological application, we study tt‾ t\overline{t} tt t t ¯ production at a future linear collider and demonstrate that quantum observables provide a comprehensive and complementary probe of top-quark interactions and stronger constraints on the scale of new physics.
We study how virtual next-to-leading-order electroweak corrections affect the quantum observables of entanglement and magic in lepton--lepton collisions. Treating the outgoing particles as multi-qudit systems, i.e.\textit{i.e.}i.e., qubits for spin-12\tfrac{1}{2}21​ fermions and qutrits for massive vector bosons, we compute entanglement entropy and the stabilizer second Rényi entropy for the processes ℓ+ℓ−→τ−τ+, ZZ\ell^+\ell^- \to τ^-τ^+,\, ZZℓ+ℓ−→τ−τ+,ZZ at one-loop accuracy. We show that radiative corrections coherently modify the quantum structure of the final state, shifting regions of maximal entanglement and altering the generated magic relative to leading order. Our results demonstrate that these quantum observables provide novel, precision-sensitive probes of electroweak dynamics at future lepton colliders and open a path toward their use in searches for new physics.
Entanglement is a key property of quantum systems. In this Letter the first measurements of quantum entanglement between spins in pairs of Z bosons are reported, using proton-proton collision data from the Large Hadron Collider (LHC) at center-of-mass energies of 13 and 13.6 TeV, recorded with the ATLAS detector. Measurements of angular observables sensitive to ZZ^{*} spin-density-matrix elements in the H→ZZ^{*}→ℓ^{+}ℓ^{-}ℓ^{+}ℓ^{-} process yield coefficients C_{2,1,2,-1}=-0.71±0.45 and C_{2,2,2,-2}=0.08±0.44, consistent with their Standard Model predictions. A complementary hypothesis test using the full angular distribution, and relying on several Standard Model assumptions in the decays, provides substantially higher sensitivity to quantum correlations and disfavors the separable-state hypothesis at a significance of 4.7 standard deviations (expected 4.9σ) relative to the entangled Standard Model hypothesis. These results provide strong evidence of quantum entanglement between massive bosons (spin qutrits) at the electroweak scale.
We investigate the quantum properties of heavy-fermion pairs, such as ttˉt\bar tttˉ or τ+τ−τ^+τ^-τ+τ−, produced in lepton-lepton collisions with polarised beams. Focusing on spin correlations, entanglement, Bell-inequality violation, and quantum-information-theoretic measures such as purity and magic, we analyse how beam polarisation shapes the structure of the spin-density matrix. We derive analytic expressions for a wide range of helicity configurations, including both Standard Model contributions and generic new-physics effects parametrised by scalar, vector, and tensor four-fermion operators within an effective field theory framework. We show that beam polarisation unlocks a substantially richer set of spin configurations and significantly enhances sensitivity to non-standard interactions. As a phenomenological application, we study ttˉt\bar tttˉ production at a future linear collider and demonstrate that quantum observables provide a comprehensive and complementary probe of top-quark interactions and stronger constraints on the scale of new physics.
We investigate the limitations of the two-qutrit quantum tomography in Higgs-boson decays to four charged leptons. At next-to-leading order, the standard reconstruction can yield an operator that is not positive semidefinite, even when radiatively improved spin-analysing powers are used. We trace this failure to the coherent combination of photon- and Z-mediated contributions to the off-shell dilepton system, whose distinct production and decay structures invalidate the assumed universal decay analysers. Nevertheless, the rank-two angular sector remains accessible and perturbatively stable, enabling a robust lower bound on the squared concurrence, and hence a reliable assessment of entanglement beyond leading order.