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M. Fabbrichesi

Publications and source records attributed to M. Fabbrichesi.

At least 19 recordsLinked to original sources
arXiv2025

Measuring CP violation using quantum state tomography

We investigate direct CP violation in neutral meson decays by reconstructing the associated density matrices and measuring their difference using the trace distance. Our results cover neutral kaon decays into two scalar triplets of isospin space, specifically the pions, and decays of $B$- and $D$- mesons into two scalar octets of SU(3) flavor space. We briefly discuss the quantum properties of these states, including entanglement, contextuality, and nonlocality. Additionally, we demonstrate a comparable approach for spin-1 final states by employing a density matrix describing states in the space of helicities. The significance of CP violation obtained through this method is consistently comparable, and often surpasses that obtained using only single, or combinations of, asymmetries.

hep-ph↗
arXiv2025

Local vs. nonlocal entanglement in top-quark pairs at the LHC

We show that the entanglement observed in top-antitop quark spin states at the LHC is local in the energy region close to the production threshold. In contrast, nonlocal entanglement is observed in the central boosted region defined by a top-quark pair invariant mass $m_{t\bar t} > 800$ GeV and scattering angles $Θ$ satisfying $|\cos Θ|<0.2$. This makes top-quark pairs a unique laboratory for studying the interplay between entanglement and Bell locality. The locality of entanglement near the production threshold is further supported by a recent CMS analysis, which reports a significance of more than $5σ$. We also demonstrate that there exists a kinematic region where the spin states of the top-antitop quark are separable, yet they exhibit non-zero discord and magic.

hep-ph↗
arXiv2025

About testing Bell locality at colliders

High-energy colliders enable the testing of quantum mechanics at its most fundamental level, in the presence of strong and electroweak interactions, with systems that consist of qubits (fermions) and qutrits (massive spin-1 bosons). Quantum state tomography at colliders enables the witnessing of entanglement and Bell non-locality, two defining characteristics of quantum mechanics. We offer a comprehensive explanation of the underlying principles and the methods employed to achieve this remarkable feat.

quant-ph↗
arXiv2025

Quantum pions

We show that two- and three-pion states produced in the decay of neutral kaons are contextual, entangled, and Bell nonlocal in isospin space. By reinterpreting the experimental values of the different isospin amplitudes, we can determine the amount of entanglement enjoyed by these states and the extent to which they violate the non-contextuality and Bell locality inequalities. Notably, the three-pion state offers a genuine multipartite test of Bell nonlocality with qutrits.

hep-ph↗
arXiv2025

Tests of quantum contextuality in particle physics

Quantum contextuality refers to the impossibility of assigning a predefined, intrinsic value to a physical property of a system independently of the context in which the property is measured. It is, perhaps, the most fundamental feature of quantum mechanics. The many states with different spin that particle physics provides are the ideal setting for testing contextuality. We verify that the polarization states of single spin-1 massive particles produced at colliders are contextual. We test $W^{+}$ gauge bosons produced in top-quark decays, $J/ψ$ and $K^{*}(892)^0$ mesons in $B$-meson decays and $ϕ$ mesons in $χ^0_c$ and $χ^1_c$ charmonium decays by reinterpreting the data and the analyses of the ATLAS, LHCb, Belle II and BESIII experimental collaborations, respectively. The polarization states of these four particles show contextuality with a significance larger than $5σ$. We also discuss the presence of quantum contextuality in spin states of bipartite systems formed by spin-1/2 particles. We test $Λ$ and $Σ$ baryons reinterpreting two BESIII data analyses, and pairs of top quarks utilizing a recent analysis of the CMS collaboration. Quantum contextuality is present with a significance exceeding $5σ$ also in these cases. In addition, we study the feasibility of testing quantum contextuality by means of $Z$ boson production in association with the Higgs boson, $Z$ and $W$ bosons pairs created in Higgs boson decays and with pairs of $τ$ leptons. For the latter, we use Monte Carlo simulations that mimic the settings of SuperKEKB and of future lepton colliders. Experiments at high energies, though not designed for the purpose, perform surprisingly well in testing for quantum contextuality.

hep-ph↗
arXiv2025

Quantum contextuality of spin-1 massive particles

Contextuality is a fundamental property of quantum mechanics. Contrary to entanglement, which can only exist in composite systems, contextuality is also present for single entities. The case of a three-level system is of particular interest because--in agreement with the Bell-Kochen-Specker theorem--it is the simplest in which quantum contextuality is necessarily present. We verify that the polarizations of spin-1 massive particles produced at collider experiments indeed exhibit contextuality. To this purpose we consider $W$ gauge bosons produced in top-quark decays, $J/ψ$ and $K^{*}(892)^0$ mesons created in $B$-meson decays and $ϕ$ mesons resulting from $χ^0_c$ charmonium decays, making use of the data collected and analyzed by the ATLAS, LHCb and BESIII collaborations, respectively. The polarizations of all these four particles show contextuality with a significance of more than $5σ$.

hep-ph↗
arXiv2024

Quantum tomography with $τ$ leptons at the FCC-ee

The Future Circular Collider (FCC) -- in its first incarnation as a lepton collider -- will produce, according to the proposed design, more than 100 billion pairs of $τ$ leptons after working for four years at the energy of the $Z$-boson resonance. The $τ$ lepton is special because its relatively long lifetime makes it possible to reconstruct the momenta of neutrinos emitted in the single pion decay mode. The resulting large number of events is an ideal source for a full quantum tomography of the process that will test quantum entanglement and the violation of Bell inequality with unprecedented precision. In addition, the study of polarizations and spin correlations can provide a competitive determination of the Weinberg angle $ θ_W$ and constrain possible anomalous couplings in the neutral electroweak current. We utilize analytic results and Monte Carlo simulations to explore to what extent these goals might be accomplished.

hep-ph↗
arXiv2024

Bell inequality is violated in charmonium decays

The experimental data on the helicity amplitudes of charmonium decays allow us to measure entanglement in final state spin correlations and test possible violations of the Bell inequality. We find that the Bell inequality is violated with a significance of 5$σ$ or more in the decays $η_{c}, \, χ_{c}^{0}, \, J/ψ\to Λ+\bar Λ$ $J/ψ\to Ξ^- +\bar Ξ^+,\; Ξ^0 +\bar Ξ^0, \;Σ^{-} +\bar Σ^{+},\; Σ^{0} +\bar Σ^{0}$, $ψ(3686) \to Ξ^- +\bar Ξ^+,\; Σ^- +\bar Σ^+,\; Σ^{0} +\bar Σ^{0}$, $χ^{0}_{c}, \, χ_{c}^{1} \to ϕ+ ϕ$ . The decays $ψ(3686) \to Λ+\bar Λ$ and $Ξ^0 +\bar Ξ^0$ show the same violation but with less significance. The decay $ψ(3686) \to Ω^- + \bar Ω^+$ displays entanglement. These results firmly establish the presence of entanglement and quantum non-separability at high energies, in a setting with particles of different spins and interacting through electroweak and strong interactions. In addition, the relatively long lifetime of some of the strange baryons produced in the decays provides a natural probe to test whether quantum spin correlations remain after the particles have interacted with the beam pipe and the first few layers of the detector.

hep-ph↗
arXiv2024

Bell inequality is violated in $B^0\to J/ψ\, K^{\star}(892)^0$ decays

The violation of the Bell inequality is one of the hallmarks of quantum mechanics and can be used to rule out local deterministic alternative descriptions. We utilize the data analysis published by the LHCb collaboration on the helicity amplitudes for the decay $B^0\to J/ψ\,K^*(892)^0$ to compute the entanglement among the polarizations of the final vector mesons and the violation of the Bell inequality that it entails. We find that quantum entanglement can be detected with a significance well above 5$σ$ (nominally 84$σ$) and Bell inequality is violated with a significance well above 5$σ$ (nominally 36$σ$) -- thereby firmly establishing these distinguishing feature of quantum mechanics at high energies in a collider setting and in the presence of strong and weak interactions. Entanglement is also present and the Bell inequality is violated in other decays of the $B$ mesons into vector mesons, but with lesser significance.

hep-ph↗
arXiv2024

Dipole momenta and compositeness of the $τ$ lepton at Belle II

The large number of $τ$ leptons available at the Belle II experiment makes it possible to study their properties and the extent of their compositeness. Our strategy relies on three observables defined in terms of elements of the polarization density matrix of the produced $τ$ pairs, which we obtain via quantum tomography. The observables are able to explore values of the magnetic dipole moment down to $6.3 \times 10^{-4}$, which is two order of magnitude better than the current experimental limit, constrain the electromagnetic radius below $4.3 \times 10^{-3}$ fm and exclude an electric dipole moment larger than $1.7 \times 10^{-17}$ e cm. The quoted limits are at the $95\%$ confidence level and are obtained for a benchmark integrated luminosity of 1 ab$^{-1}$.

hep-ph↗
arXiv2023

Stringent bounds on $HWW$ and $HZZ$ anomalous couplings with quantum tomography at the LHC

Quantum tomography provides the full reconstruction of the density matrix of a state. We use it to study the Higgs boson decay into weak gauge bosons. Anomalous couplings beyond the Standard Model can be constrained by means of observables easily defined in terms of the polarization density matrix. We describe a strategy based on three observables that together provide the most stringent limits. Two of these observables are linked to the entanglement between the polarizations of the two gauge bosons, the other is based on CP-odd combinations of one momentum and two polarizations. We find for the $Z$ channel that this strategy offers, already with the available LHC data, limits competitive with the best available bounds. We argue that the inclusion of these observables in routine experimental analyses can lead to more stringent global fit limits.

hep-ph↗
arXiv2023

Bell inequalities and quantum entanglement in weak gauge bosons production at the LHC and future colliders

Quantum entanglement of weak interaction gauge bosons produced at colliders can be explored by computing the corresponding polarization density matrix. To this end, we consider the Higgs boson decays $H\to W W^*$ and $H\to Z Z^*$, in which $W^*$ and $Z^*$ are off-shell states, and the $WW$, $WZ$ and $ZZ$ di-boson production in proton collisions. The polarization density matrix of the di-boson state is determined by the amplitude of the production process and can be experimentally reconstructed from the angular distribution of the momenta of the final states into which the gauge bosons decay. We show that a suitable instance of the Bell inequality is violated in $H\to Z Z^*$ to a degree that can be tested at the LHC with future data. The same Bell inequality is violated in the production of $WW$ and $ZZ$ boson pairs for invariant masses above 900 GeV and scattering angles close to $π/2$ in the center of mass frame. LHC data in this case are not sufficient to establish the violation of the Bell inequality. We also analyze the prospects for detecting Bell inequality violations in di-boson final states at future $e^+e^-$ and muon colliders. A further observable that provides a lower bound on the amount of polarization entanglement in the di-boson system is computed for each of the examined processes. The analytic expressions for the polarization density matrices are presented in full in an Appendix. We also provide the unitary matrices required in the optimization procedure necessary in testing the Bell inequalities.

hep-ph↗
arXiv2021

Testing Bell inequalities at the LHC with top-quark pairs

Entanglement between the spins of top-quark pairs produced at a collider can be used to test a (generalized) Bell inequality at energies never explored so far. We show how the measurement of a single observable can provide a test of the violation of the Bell inequality at the 98% CL with the data already collected at the Large Hadron Collider and at the 99.99% CL with the higher luminosity of the next run.

hep-ph↗
arXiv2020

An asymptotically safe SU(5) GUT

We minimally extend the Standard Model field content by adding new vector-like fermions at the TeV scale to allow gauge coupling unification at a realistic scale. We embed the model into a $SU(5)$ grand unified theory that is asymptotically safe and features an interacting fixed point for the gauge coupling. There are no Landau poles of the $U(1)$ gauge and Higgs couplings. Gauge, Yukawa and Higgs couplings are retraced from the fixed point and matched at the grand unification scale to those of the Standard Model rescaled up to the same energy. All couplings, their fixed point values and critical exponents always remain in the perturbative regime.

hep-ph↗
arXiv2020

$Z$-boson decays into an invisible dark photon at the LHC, HL-LHC and future lepton colliders

We study the decay of the $Z$ vector boson into a photon and a massless (invisible) dark photon in high-energy collisions. The photon can be used as trigger for the event, while the dark photon is detected indirectly as missing momentum in the event final state. We investigate the possibility of searching for such a dark photon at the LHC, HL-LHC and future lepton colliders, and compare the respective sensitivities. As expected, the best result is found for the lepton colliders running at the $Z$ mass, FCC-ee and CEPC, with a final sensitivity to branching ratios of order $O(10^{-11})$. We also discuss how to use the photon angular distribution of the events in lepton collisions to discriminate between the dark photon and a pseudo-scalar state like the axion.

hep-ph↗
arXiv2018

Neutral Hadrons Disappearing into the Darkness

We study the invisible decay of neutral hadrons in a representative model of the dark sector. The mesons $K_L$ and $B^0$ decay into the dark sector with branching rates that can be at the current experimental limits. The neutron decays with a rate that could either explain the neutron lifetime puzzle (although only for an extreme choice of the parameters and a fine tuned value of the masses) or be just above the current limit of its invisible decay ($τ_N^{\rm inv} \ge 10^{29}$ years) if kinematically allowed. These invisible decays of ordinary matter provide a novel and promising window into new physics that should be vigorously pursued.

hep-ph↗
arXiv2018

$Z$ Boson Decay into Light and Darkness

We study the $Z\rightarrow γ\bar γ$ process in which the $Z$ boson decays into a photon $γ$ and a massless dark photon $\bar γ$, when the latter couples to standard-model fermions via dipole moments. This is a simple yet nontrivial example of how the Landau-Yang theorem---ruling out the decay of a massive spin-1 particle into two photons---is evaded if the final particles can be distinguished. The striking signature of this process is a resonant monochromatic single photon in the $Z$-boson center of mass together with missing momentum. LEP experimental bounds allow a branching ratio up to about 10$^{-6}$ for such a decay. In a simplified model of the dark sector, the dark-photon dipole moments arise from one-loop exchange of heavy dark fermions and scalar messengers. The corresponding prediction for the rare $Z\to γ\bar γ$ decay width can be explored with the large samples of $Z$ bosons foreseen at future colliders.

hep-ph↗