arXiv subjects

Kazuki Sakurai

Publications and source records attributed to Kazuki Sakurai.

At least 19 recordsLinked to original sources

arXiv2026

Extremal Magic States from Symmetric Lattices

Magic, a key quantum resource beyond entanglement, remains poorly understood in terms of its structure and classification. In this paper, we demonstrate a striking connection between high-dimensional symmetric lattices and quantum magic states. By mapping vectors from the $E_8, B W_{16}$, and $E_6$ lattices into Hilbert space, we construct and classify stabiliser and maximal magic states for two-qubit, three-qubit and one-qutrit systems. In particular, this geometric approach gives closed-form lattice representatives of maximal magic states and motivates a conjecture for the number of three-qubit maximal states. In the three-qubit case, we further classify the extremal magic states according to their entanglement structure. We also show that the 45 one-qutrit maximal magic states selected by the second shell of $E_6$ split into two Clifford orbits. Our findings suggest that deep algebraic and geometric symmetries underlie the structure of extremal magic states.

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arXiv2026

Qubit-qubit-qutrit quantum correlations in $H \to f \bar f V$

We perform an extensive analysis of the quantum correlations carried by the qubit-qubit-qutrit pure state arising in the decay of a massive scalar into a fermion-antifermion pair and a massive gauge boson, $H \to f \bar f V$, specialising to the Higgs boson decay $h \to τ^- τ^+ Z$. Working with the exact tree-level spin state and its systematic expansion around the massless-fermion limit, we obtain analytic control over the entire phase space: the bipartite entanglement measures, the genuine $2 \otimes 2 \otimes 3$ entanglement structure (the Miyake classification), as well as the Bell-inequality violations and the non-stabiliserness (magic) are all mapped and reproduced by compact formulas. The bipartite measures exhibit a monogamy-like trade-off between the fermion pair and the fermion-boson pairs. The state is genuinely $2 \otimes 2 \otimes 3$ entangled over almost the entire phase space, most strongly in the collinear regions. We derive, for the first time, semi-analytical expressions for the tight $4 \times 4 \times 2$ Bell inequalities of the $2 \otimes 2 \otimes 3$ system, generalising the optimisation previously available only for three qubits, and find that the local-hidden-variable bound is violated over the entire phase space, reaching within a few per cent of the quantum bound at the upper endpoint of the di-tau mass spectrum. We further extend the stabiliser Rényi entropy and the non-local magic to systems with unequal local dimensions, and show that the near-endpoint state carries almost exactly one bit of non-local magic, which peaks at $\log_2 \frac{27}{7} \simeq 1.95$ in the collinear regions. The differential decay rate concentrates precisely in the most nonclassical region of the phase space.

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arXiv2026

Experimental prospects for quantum decoherence measurements at colliders

We study the impact of radiation on quantum systems defined by the spins of elementary fermion-antifermion pairs produced at colliders. We present predictions for several processes, showing that energetic final-state radiation can induce decoherence and significantly reduce the entanglement of quantum systems formed by elementary fermion pairs. We investigate the feasibility of observing this effect experimentally in exclusive samples with energetic radiation. A statistically significant signal can be obtained with current data in associated $pp \rightarrow t\bar{t}(g)$ production at the LHC and in $e^+e^- \rightarrow τ^{+}τ^{-}(γ)$ production at Belle 2. Future electron-positron colliders operated at the $Z$ pole or well above the $t\bar{t}$ production threshold will extend these prospects further.

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arXiv2026

Machine learning fully hadronic events with spectral functions

Characterising fully hadronic events is a difficult task at hadron colliders. Signal jets from the hard process are mingled with an arbitrary number of ISR and FSR jets, leading to a large combinatorial background. This also poses a challenge for machine-learning analyses, where the number of input features is fixed while the jet multiplicity fluctuates from event to event due to QCD radiation. In this work, we explore the use of the two-point correlation spectral function as an input feature for machine-learning analyses of such events. The spectral function maps the transverse-momentum data of an event into a one-dimensional function of the angular distance, encoding the event information modulo collider isometries and jet permutations, and is defined independently of the jet multiplicity. As a concrete benchmark we apply the method to discriminate gluino-pair production followed by $\tilde{g} \to t \bar{t} \tildeχ_1^0$ against the fully hadronic $t \bar{t}$ background. With $139~{\rm fb}^{-1}$ of $\sqrt{s} = 13$ TeV $pp$ collision data, a dense neural network supplied with spectral-function features improves the expected reach in gluino-mass by roughly 150 GeV relative to a recent ATLAS analysis, and by roughly 250 GeV relative to the same network trained on jet kinematics alone.

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arXiv2026

Tripartite Entanglement in $e^+ e^- \to t \bar{t} Z$

Multipartite entanglement is a uniquely quantum form of correlation that captures collective properties of a composite quantum state beyond those encoded in its bipartite subsystems. We investigate this phenomenon in the process $e^+e^-\to t\bar tZ$ at a future lepton collider, where the final state spins span the tripartite Hilbert space $\mathscr{H} = \mathbb{C}^2 \otimes \mathbb{C}^2 \otimes \mathbb{C}^3$. Starting from the Standard Model helicity amplitudes, we reconstruct the full $12\times 12$ spin density matrix and characterise its entanglement structure through one-to-one negativities, one-to-other negativities, and the genuine multipartite negativity, evaluated at three increasingly inclusive levels of phase space integration. Pairwise entanglement is generally suppressed relative to the collective (one-to-other) and the genuine multipartite entanglement, and all measures decrease as more kinematic information is integrated out. Assuming quantum tomography in the fully leptonic decay channel at $\sqrt{s}=1$ TeV, we find that collective entanglement should be accessible at a realistic high-luminosity polarised lepton collider. By contrast, certifying genuine multipartite entanglement is more challenging, with only limited sensitivity projected for a specific polarisation benchmark within the expected ILC luminosity. The study establishes $e^+e^-\to t \bar{t}Z$ as an attractive laboratory for probing multipartite entanglement in high-energy collisions and provides a general mixed state framework that applies to any tripartite spin system.

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arXiv2026

Quantum properties of heavy-fermion pairs at a lepton collider with polarised beams

We investigate the quantum properties of heavy-fermion pairs, such as $t\bar t$ 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 $t\bar t$ 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.

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arXiv2026

Prospects for detecting charged long-lived BSM particles at MoEDAL-MAPP experiment: A mini-review

The search for physics beyond the Standard Model at the Large Hadron Collider is expanding to include unconventional signatures such as long-lived particles. This mini-review assesses the prospects for detecting electrically charged long-lived particles using the MoEDAL-MAPP experiment. We synthesize findings from recent studies that evaluate sensitivity to supersymmetric models, radiative neutrino mass scenarios, and generic multiply charged objects. A key component of this review is the comparative analysis of MoEDAL's reach against the general-purpose ATLAS and CMS experiments. We conclude that while MoEDAL is constrained by lower integrated luminosity, its passive, background-free detection methodology offers a unique advantage. Specifically, the experiment provides complementarity to the major detectors, particularly for signals involving slow-moving particles and stable states with intermediate electric charges.

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arXiv2025

Three-Body Non-Locality in Particle Decays

The exploration of entanglement and Bell non-locality among multi-particle quantum systems offers a profound avenue for testing and understanding the limits of quantum mechanics and local real hidden variable theories. In this work, we examine non-local correlations among three massless spin-1/2 particles generated from the three-body decay of a massive particle, utilizing a framework based on general four-fermion interactions. By analyzing several inequalities, we address the detection of deviations from quantum mechanics as well as violations of two key hidden variable theories: fully local-real and bipartite local-real theories. Our approach encompasses the standard Mermin inequality and the tight $4 \times 4 \times 2$ inequality, providing a comprehensive framework for probing three-partite non-local correlations. Our findings provide deeper insights into the boundaries of classical and quantum theories in three-particle systems, advancing the understanding of non-locality in particle decays and its relevance to particle physics and quantum foundations.

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arXiv2025

Quantum Information meets High-Energy Physics: Input to the update of the European Strategy for Particle Physics

Some of the most astonishing and prominent properties of Quantum Mechanics, such as entanglement and Bell nonlocality, have only been studied extensively in dedicated low-energy laboratory setups. The feasibility of these studies in the high-energy regime explored by particle colliders was only recently shown and has gathered the attention of the scientific community. For the range of particles and fundamental interactions involved, particle colliders provide a novel environment where quantum information theory can be probed, with energies exceeding by about 12 orders of magnitude those employed in dedicated laboratory setups. Furthermore, collider detectors have inherent advantages in performing certain quantum information measurements, and allow for the reconstruction of the state of the system under consideration via quantum state tomography. Here, we elaborate on the potential, challenges, and goals of this innovative and rapidly evolving line of research and discuss its expected impact on both quantum information theory and high-energy physics.

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arXiv2025

Machine Learning Electroweakino Production

The system of light electroweakinos and heavy squarks gives rise to one of the most challenging signatures to detect at the LHC. It consists of missing transverse energy recoiled against a few hadronic jets originating either from QCD radiation or squark decays. The analysis generally suffers from the large irreducible Z + jets $(Z \to ν\bar ν)$ background. In this study, we explore Machine Learning (ML) methods for efficient signal/background discrimination. Our best attempt uses both reconstructed (jets, missing transverse energy, etc.) and low-level (particle-flow) objects. We find that the discrimination performance improves as the pT threshold for soft particles is lowered from 10 GeV to 1 GeV, at the expense of larger systematic uncertainty. In many cases, the ML method provides a factor two enhancement in $S/\sqrt{(S + B)}$ from a simple kinematical selection. The sensitivity on the squark-elecroweakino mass plane is derived with this method, assuming the Run-3 and HL-LHC luminosities. Moreover, we investigate the relations between input features and the network's classification performance to reveal the physical information used in the background/signal discrimination process.

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arXiv2025

Prospects for quantum process tomography at high energies

In quantum information theory, the evolution of an open quantum system -- a unitary evolution followed by a measurement -- is described by a quantum channel or, more generally, a quantum instrument. In this work, we formulate spin and flavour measurements in collider experiments as quantum instruments. We demonstrate that the Choi matrix, which completely determines input-output transitions, can be both theoretically computed from a given model and experimentally reconstructed from a set of final state measurements (quantum state tomography) using varied input states. The experimental reconstruction of the Choi matrix, known as quantum process tomography, offers a powerful new approach for probing potential extensions of the Standard Model within the quantum field theory framework and, at the same time, constitutes a new foundational test of quantum mechanics itself. As an example, we outline the quantum process tomography approach applied to the $e^+ e^- \to t \bar{t}$ process at a polarized lepton collider.

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arXiv2025

Enhancing anomaly detection with topology-aware autoencoders

Anomaly detection in high-energy physics is essential for identifying new physics beyond the Standard Model. Autoencoders provide a signal-agnostic approach but are limited by the topology of their latent space. This work explores topology-aware autoencoders, embedding phase-space distributions onto compact manifolds that reflect energy-momentum conservation. We construct autoencoders with spherical ($S^n$), product ($S^2 \otimes S^2$), and projective ($\mathbb{RP}^2$) latent spaces and compare their anomaly detection performance against conventional Euclidean embeddings. Our results show that autoencoders with topological priors significantly improve anomaly separation by preserving the global structure of the data manifold and reducing spurious reconstruction errors. Applying our approach to simulated hadronic top-quark decays, we show that latent spaces with appropriate topological constraints enhance sensitivity and robustness in detecting anomalous events. This study establishes topology-aware autoencoders as a powerful tool for unsupervised searches for new physics in particle-collision data.

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arXiv2024

Machine Learning Classification of Sphalerons and Black Holes at the LHC

In models with large extra dimensions, "miniature" black holes (BHs) might be produced in high-energy proton-proton collisions at the Large Hadron Collider (LHC). In the semi-classical regime, those BHs thermally decay, giving rise to large-multiplicity final states with jets and leptons. On the other hand, similar final states are also expected in the production of electroweak sphaleron/instanton-induced processes. We investigate whether one can discriminate these scenarios when BH or sphaleron-like events are observed in the LHC using machine learning (ML) methods. Classification among several BH scenarios with different numbers of extra dimensions and the minimal BH masses is also examined. In this study we consider three ML models: XGBoost algorithms with (1) high- and (2) low-level inputs, and (3) a Residual Convolutional Neural Network. In the latter case, the low-level detector information is converted into an input format of three-layer binned event images, where the value of each bin corresponds to the energy deposited in various detector subsystems. We demonstrate that only a small number of detected events are sufficient to effectively discriminate between the sphaleron and BH processes. Separation between BH scenarios with different minimal masses is possible with an order of 10 events passing the preselection. A sufficient number of events could be observed in combined Run-2 and -3 data, if the production cross section is not much smaller than the present limit ~ 0.1 fb. We find, however, that a large number of events is needed to discriminate between BH hypotheses with the same minimal BH mass, but different numbers of extra dimensions.

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arXiv2024

Three-body Entanglement in Particle Decays

Quantum entanglement has long served as a foundational pillar in understanding quantum mechanics, with a predominant focus on two-particle systems. We extend the study of entanglement into the realm of three-body decays, offering a more intricate understanding of quantum correlations. We introduce a novel approach for three-particle systems by utilising the principles of entanglement monotone concurrence and the monogamy property. Our findings highlight the potential of studying deviations from the Standard Model and emphasise its significance in particle phenomenology. This work paves the way for new insights into particle physics through multi-particle quantum entanglement, particularly in decays of heavy fermions and hadrons.

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arXiv2023

Phases of Pseudo-Nambu-Goldstone Bosons

We study the vacuum dynamics of pseudo-Nambu-Goldstone bosons (pNGBs) for $SO(N+1) \rightarrow SO(N)$ spontaneous and explicit symmetry breaking. We determine the magnitude of explicit symmetry breaking consistent with an EFT description of the effective potential at zero and finite temperatures. We expose and clarify novel additional vacuum transitions that can arise for generic pNGBs below the initial scale of $SO(N+1) \rightarrow SO(N)$ spontaneous symmetry breaking, which may have phenomenological relevance. In this respect, two phenomenological scenarios are analyzed: thermal and supercooled dark sector pNGBs. In the thermal scenario the vacuum transition is first-order but very weak. For a supercooled dark sector we find that, depending on the sign of the explicit symmetry breaking, one can have a symmetry-restoring vacuum transition $SO(N-1) \rightarrow SO(N)$ which can be strongly first-order, with a detectable stochastic gravitational wave background signal.

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arXiv2023

LHC constraints on electroweakino dark matter revisited

We revisit LHC searches for heavy invisible particles by exploiting QCD initial state radiation. We recast a dijet signal region in a general multijet plus MET search by ATLAS. We find that non-trivial mass limit can be obtained for various models of the electroweakino sector with the present data in hadronic channels. The winos are bound to be heavier than $m_{\tilde W} \gtrsim {160}$ GeV and higgsinos $m_{\tilde h} \gtrsim {100}$ GeV, depending on the chargino-neutralino mass splitting. The expected exclusion limits at the LHC Run 3 with $\mathcal{L} = 300$ fb$^{-1}$ increase to $m_{\tilde W} \gtrsim {200}$ GeV and $m_{\tilde h} \gtrsim {130}$ GeV for winos and higgsino, respectively. This is the first LHC limit for promptly decaying nearly mass-degenerate winos.

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arXiv2023

Quantum information and CP measurement in $H \to τ^+ τ^-$ at future lepton colliders

We introduce a methodology and investigate the feasibility of measuring quantum properties of tau lepton pairs in the $H \to τ^+ τ^-$ decay at future lepton colliders. In particular, observation of entanglement, steerability and violation of Bell inequalities are examined for the ILC and FCC-ee. We find that detecting quantum correlation crucially relies on precise reconstruction of the tau lepton rest frame and a simple kinematics reconstruction does not suffice due to the finite energy resolution of the colliding beams and detectors. To correct for energy mismeasurements, a log-likelihood method is developed that incorporates the information of impact parameters of tau lepton decays. We demonstrate that an accurate measurement of quantum properties is possible with this method. As a by-product, we show that a novel model-independent test of CP violation can be performed and the CP-phase of $H ττ$ interaction can be constrained with an accuracy comparable to dedicated analyses, i.e., up to $7.9^{\circ}$ and $5.4^{\circ}$ at ILC and FCC-ee, respectively.

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arXiv2022

Monojet signatures from gluino and squark decays

We study the monojet and dijet channels at the LHC as a tool for searching for squarks and gluinos. We consider two separate R-parity conserving supersymmetric scenarios. In the first scenario we postulate a large mass hierarchy between squarks ($\tilde q$) and winos ($\widetilde W$), and wino-like neutralino is assumed to be the lightest supersymmetric particle (LSP). The associated squark-wino production, $pp \to \tilde q \widetilde W$, then leads to a monojet-like signature, where the high $p_T$ jet is originated from the squark decay, $\tilde q \to q + \widetilde W$. We demonstrate that this associated production, as well as the $pp \to \widetilde W \widetilde W + {\rm jets}$ production, have a significant impact on the exclusion limit in the squark-neutralino mass plane. The second scenario postulates that the lighter of the squark and gluino is only a few GeV heavier than the LSP neutralino. The associated squark-gluino production, $pp \to \tilde q \tilde g$, then leads to a distinctive monojet signature, where the high $p_T$ jet is produced from the decay of the heavier coloured particle into the lighter one ($\tilde q \to q + \tilde g$ for $m_{\tilde q} > m_{\tilde g}$ and $\tilde g \to q + \tilde q$ for $m_{\tilde g} > m_{\tilde q}$). The lighter coloured particle is effectively regarded as an invisible particle since the decay products are soft due to the approximate mass degeneracy. We recast existing monojet and dijet analyses and find a non-trivial exclusion limit in the squark-gluino mass plane in this scenario.

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