arXiv subjects

J. A. Aguilar-Saavedra

Publications and source records attributed to J. A. Aguilar-Saavedra.

At least 19 recordsLinked to original sources

arXiv2026

Toponium effects on quantum steering and Bell nonlocality of top quarks

We investigate quantum steering and Bell nonlocality in top-quark pair production at the LHC near threshold. The toponium contribution strengthens the spin-singlet component and substantially enhances the entanglement between the two quark spins. With current LHC data, quantum steering appears observable with a statistical significance around $10σ$. For Bell nonlocality, the statistical significance can also be high close to threshold, reaching about $9σ$, although the feasibility of such a measurement will depend crucially on the control of systematic uncertainties.

hep-ph↗

arXiv2026

Understanding Bell locality tests at colliders

For decades, it has been known that local hidden variable theories cannot be disproved by collider experiments involving decaying particles. However, if these theories satisfy a small set of mild assumptions, they become testable. In particular, they can be disproved using Bell-like inequalities for $μ^+ μ^-$ and $τ^+ τ^-$ pairs.

hep-ph↗

arXiv2026

Quantum tomography of $H \to ZZ, WW$ beyond leading order

We revisit quantum tomography of $H \to ZZ$ and $H \to WW$ in the presence of higher-order corrections. We verify that neither the use of an effective spin analysing power (only for $ZZ$) or a photon veto are sufficient to render the naively-constructed spin density operators physical. A subtraction of higher-order corrections is thus necessary to perform consistent quantum tomography. Such corrections are small when compared to expected experimental uncertainties with current data. As a by-product, we point out the striking possibility to observe parity-violating effects in $H \to WW$.

hep-ph↗

arXiv2026

Mass-unspecific classifiers for mass-dependent searches

Searches for new particles often span a wide range of mass scales, where the shape of potential signals and the SM background varies significantly. We make use of a multivariate method that fully exploits the correlation between signal and background features and the explored mass scale, and is trained on a sample that is balanced across the entire mass range. The classifiers, either a neural network or a boosted decision tree, produce a continuous output across the full mass range and, at a given mass, achieve nearly the same performance as a classifier specifically trained for that mass. The performance of the classifiers is better than the one obtained with parameterised neural networks and similar methods.

hep-ph↗

arXiv2025

Momentum entanglement at colliders: the $H \to WW,ZZ$ case

We address momentum entanglement in Higgs decays to weak boson pairs, $H \to WW,ZZ$, by discretising momentum space. The momenta of the two weak bosons are entangled, as well as the degrees of freedom in momentum and spin spaces. For $H \to ZZ$ in the four-lepton final state, we also estimate the statistical sensitivity of entanglement measures at the Large Hadron Collider and future upgrades. The discretisation method introduced here is broadly applicable, offering a framework for studies of momentum entanglement at the energy frontier.

hep-ph↗

arXiv2025

ECFA Higgs, electroweak, and top Factory Study

The ECFA Higgs, electroweak, and top Factory Study ran between 2021 and 2025 as a broad effort across the experimental and theoretical particle physics communities, bringing together participants from many different proposed future collider projects. Activities across three main working groups advanced the joint development of tools and analysis techniques, fostered new considerations of detector design and optimisation, and led to a new set of studies resulting in improved projected sensitivities across a wide physics programme. This report demonstrates the significant expansion in the state-of-the-art understanding of the physics potential of future e+e- Higgs, electroweak, and top factories, and has been submitted as input to the 2025 European Strategy for Particle Physics Update.

hep-ex↗

arXiv2025

The top quark in 2025 -- International year of Quantum Science and Technology

Since its discovery at the Tevatron in 1995, the top quark has been extensively studied due to its unique properties. We discuss how the remarkable progress in top-quark physics has opened the possibility of using the top quark as a tool to test quantum mechanics at the energy frontier. After reviewing the motivations for exploring beyond quantum mechanics, we present an example: probing post-quantum correlations at the energy frontier with boosted top quarks.

hep-ph↗

arXiv2025

Quantum tomography beyond the leading order

Quantum tomography, as a tool to probe foundational aspects of quantum mechanics, relies on extracting spin information from angular distributions. This is inherently a leading-order technique, ill-defined when higher-order corrections are significant. For those cases, we propose to treat higher-order corrections as a background, to be modeled and subtracted from data in the same way as other backgrounds are. We illustrate this procedure for Higgs decays $H \to ZZ \to e^+ e^- μ^+ μ^-$, which is of high interest for upcoming qutrit entanglement tests at the Large Hadron Collider.

hep-ph↗

arXiv2025

Time, quantum entanglement, and particle decay

We investigate the role of time ordering in entanglement experiments involving unstable particles, focusing on $μ^+ μ^-$ pairs produced in a maximally-entangled spin state. We analyse the correlations between measurements performed by two experimenters, Alice (who measures $μ^-$ spin) and Bob (who measures $μ^+$ decay products). Remarkably, the observed correlations persist irrespective of whether Bob's muon decays before or after Alice's spin measurement. We further discuss different interpretations of the same empirical results depending on the observer's reference frame. The findings reinforce the viewpoint that the Copenhagen interpretation of measurement is a mathematical tool rather than a literal account of physical reality.

hep-ph↗

arXiv2025

$H \to ZZ$ as a double-slit experiment

The decay $H \to ZZ \to 4\ell$, with $\ell = e,μ$, can be used to test quantum interference in analogy to the famous double-slit experiment. The observations corresponding to `covering a slit' can be extracted from data in the $eeμμ$ channel, while the observations when both `slits' are open, which include the interference, are measured in the $4e/4μ$ channels. In this way, this process offers a unique opportunity to investigate identical-particle effects at high energies, and provide the first evidence of identical-particle behaviour for muons. Sensitivity at the $4σ$ level could be achieved at the high-luminosity upgrade of the Large Hadron Collider.

hep-ph↗

arXiv2024

Gradient Boosting MUST taggers for highly-boosted jets

The MUST (Mass Unspecific Supervised Tagging) method has proven to be successful in implementing generic jet taggers capable of discriminating various signals over a wide range of jet masses. We implement the MUST concept by using eXtreme Gradient Boosting (XGBoost) classifiers instead of neural networks (NNs) as previously done. We build both fully-generic and specific multi-pronged taggers, to identify 2, 3, and/or 4-pronged signals from SM QCD background. We show that XGBoost-based taggers are not only easier to optimize and much faster than those based in NNs, but also show quite similar performance, even when testing with signals not used in training. Therefore, they provide a quite efficient alternative machine-learning implementation for generic jet taggers.

hep-ph↗

arXiv2024

Toponium Hunter's Guide

We address the discovery and characterisation of toponium production at the Large Hadron Collider. In the dilepton decay mode, multivariate analyses of spin and colour observables could provide evidence that an excess of events present near the $t \bar t$ threshold corresponds to a spin-zero colour singlet. The semileptonic decay mode may also exhibit an excess near threshold, but is not expected to play any role in the toponium characterisation.

hep-ph↗

arXiv2024

Entanglement autodistillation from particle decays

Particle decays do not constitute a spin ``measurement'' in the quantum-mechanical sense, but still modify the spin state, in particular for an entangled system. We show that for a spin-entangled pair of particles the entanglement of the system can increase after the decay of one particle. This unique phenomenon has no equivalent for stable particles and could be observable in top pair production at a high-energy polarized $e^+ e^-$ collider.

hep-ph↗

arXiv2024

Full quantum tomography of top quark decays

Quantum tomography in high-energy physics processes has usually been restricted to the spin degrees of freedom. We address the case of top quark decays $t \to W b$, in which the orbital angular momentum ($L$) and the spins of $W$ and $b$ are intertwined into a 54-dimensional $LWb$ density operator. The entanglement between $L$ and the $W$ or $b$ spin is large and could be determined for decays of single top quarks produced at the Large Hadron Collider with Run 2 data. With the foreseen statistical and systematic uncertainties, the significance is well above $5σ$ from the separability hypothesis for $L$-$W$ entanglement, and $3.2σ$ for $L$-$b$. These would be the first entanglement measurements between orbital and spin angular momenta in high-energy physics. Likewise, the genuine tripartite entanglement between $L$ and the two spins could be established with more than $5σ$. The method presented paves the way for similar measurements in other processes.

hep-ph↗

arXiv2024

Tripartite entanglement in $H \to ZZ,WW$ decays

The decays of the Higgs boson produce a state in which the spins of the decay products and the orbital angular momentum ($L$) are highly entangled. We obtain the tripartite density operator, as well as reduced operators, for the decay into two weak bosons. For $H \to ZZ$ in the four-lepton final state we also estimate the statistical sensitivity at the Large Hadron Collider and future upgrades, using a binned method to reconstruct the density operators from distributions. With the expected Run 3 data, establishing genuine tripartite entanglement would be possible beyond the $5σ$ level. The violation of Bell inequalities involving the spins of the two weak bosons could also be established beyond the $5σ$ level.

hep-ph↗

arXiv2024

A closer look at post-decay $t \bar t$ entanglement

Top pair production is ideally suited to observe post-decay entanglement, thus providing a novel test of quantum mechanics. We provide top polarised decay amplitudes that can be used to compute semi-analytical predictions and, in particular, to better understand {\em why} the post-decay entanglement arises. We obtain predictions for the LHC, identifying general phase space regions where experimental measurements of $tW$ entanglement are feasible. We also give predictions for polarised $e^+ e^-$ collisions, focusing on the possibility that the post-decay $tW$ entanglement is larger than the $t \bar t$ one.

hep-ph↗

arXiv2023

Decay of entangled fermion pairs with post-selection

We consider a pair of unstable fermions in a spin-entangled state. After the decay of one fermion, a spin measurement is performed on the surviving partner, with a Stern-Gerlach experiment or similar. The measurement not only projects the spin of the surviving fermion, but is also physically equivalent to a spin projection for the decayed one -- even when it no longer exists. This post-selection effect would be experimentally accessible using muon pairs in a maximally-entangled state, produced either in the decay of a scalar particle, or in $e^+ e^-$ collisions at wide angles.

hep-ph↗

arXiv2023

Post-decay quantum entanglement in top pair production

Top pairs produced at the Large Hadron Collider exhibit quantum entanglement of their spins near threshold and for boosted, central $t \bar t$ pairs. The entanglement is maintained between the decay products, in particular between the top quark and the $W^-$ boson from the anti-quark (or vice-versa, between $\bar t$ and $W^+$) in certain kinematical regions. Therefore, $t \bar t$ production provides a rare opportunity to verify the spin entanglement between a fermion and a boson. The $tW$ entanglement can be probed at the $7σ$ level near threshold with Run 2 data, and at the $5σ$ level in the boosted region with the foreseen Run 3 luminosity. In addition, the entanglement between the two $W$ bosons can be probed at the $4σ$ level at the LHC Run 3.

hep-ph↗