arXiv subjects

Daiki Ueda

Publications and source records attributed to Daiki Ueda.

At least 19 recordsLinked to original sources

arXiv2026

Bounds on nonlinear electrodynamics via resummed relative entropy

We investigate nonlinear electrodynamic effective field theories (EFTs) through the relative entropy evaluated in suitable background electromagnetic fields. In this setup, the relative entropy encodes information about the infinite tower of higher-dimensional operators and provides a systematic probe of nonlinear EFT effects. We study these features in fermionic QED, scalar QED, and Dirac-Born-Infeld theory using perturbative analyses, resummation techniques such as Borel--Laplace resummation, and non-perturbative approaches including the Schwinger proper-time method. In the weak-coupling regime, we show that the non-negativity of the perturbative relative entropy imposes sign constraints on finite truncations of higher-dimensional operators, generalizing familiar positivity bounds on leading EFT coefficients. We further show that violations of non-negativity in the strong-coupling regime admit qualitatively different interpretations depending on the framework: perturbatively analyzed violations diagnose the breakdown of the truncated EFT expansion, whereas violations in resummed or genuinely non-perturbative relative entropy signal physical instabilities of the system, such as the Schwinger effect. Extending the analysis to broader classes of UV completions, including theories with factorial or power-law growth of EFT coefficients, we derive general constraints on nonlinear electrodynamic EFT effects from the non-negativity of the resummed relative entropy. Our results suggest that relative entropy provides a unified diagnostic of perturbative consistency and non-perturbative stability in nonlinear EFTs.

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arXiv2026

Bounds on nonlinear effective field theories via resurgent relative entropy

We study nonlinear effective field theories (EFTs) with factorially growing perturbative expansions, focusing on a class in which the relative entropy encodes an infinite tower of higher-dimensional operators. Using the resummed relative entropy, we derive bounds on EFT coefficients: the non-negativity of the resummed relative entropy fixes the sign of their asymptotic growth, while its violation signals nonperturbative effects such as instabilities. In fermionic QED, analytic continuation from Euclidean to Minkowski spacetime yields a concrete example: the Schwinger effect, a nonperturbative instability captured by the resummed relative entropy.

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arXiv2026

The Spurion Massive EFT (SMEFT)

We use the amplitude formulation of the SMEFT to introduce a spurion analysis of the SMEFT low-energy amplitudes in terms of the Higgs VEV. Each SMEFT contact-term is given as a sum of a few spurion structures, whose number depends on the electroweak charges of the external legs. The coefficients of these structures involve singlet combinations of Higgses from higher-order SMEFT contributions. We use this to derive the spurion expansions of the W- and Z-boson masses and mixing, and their three-point couplings to fermions. The textures of these couplings are saturated by the dimension-eight SMEFT. Our analysis can be generalized to higher-point amplitudes and nonzero Yukawa couplings.

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arXiv2026

Muon collider experiments as electron/positron beam sources: case studies of new light-particle searches

At muon colliders, muon decays naturally produce intense electrons and positrons with unique features, namely high energies, high repetition rates, and small intrinsic uncertainties, that are unavailable at existing accelerator facilities. We quantitatively study the feasibility of extracting such particles in two representative future muon collider designs, IMCC and $\mu$TRISTAN. Using Monte Carlo simulations with the corresponding design parameters, we study the spatial, angular, and energy distributions of decay electrons and positrons in the curved sections of the collider ring. We find that typical deflections of $0.1-10~\mathrm{mrad}$ can be achieved even for high-energy electrons carrying large energy fractions ($\simeq 0.6 - 1.0$) of the muon beam energy, with the ring bending magnets (or magnets providing an equivalent field) effectively serving as a pre-septum magnet, that partially deflects the beam before the main septum magnet, suggesting that the extraction scheme could be practically feasible. Exploiting the distinct beam properties of IMCC and $\mu$TRISTAN, we propose complementary search strategies, missing energy and momentum searches for dark matter at $\mu$TRISTAN and visible-decay searches for axion-like particles and light scalars at IMCC, which probe parameter space beyond the reach of current and other proposed experiments.

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arXiv2025

Quantum Resonance Beyond Direct Measurement: Insights from Weak Measurement

Aharonov's weak value amplification realized via weak measurement provides a versatile means to measure physical parameters with high accuracy, similar to quantum resonance epitomized by the Rabi and Ramsey resonances. The similarity between the two is not accidental: in fact, these two methods of precision measurement have recently been shown to be interconnected for the particular case of direct weak measurement. Here we show that this connection also holds in the case of indirect weak measurement, which is the standard scheme used mostly for studies of weak value amplification. We present a unified framework of weak measurements in which direct measurement appears as a special case of indirect measurement. This allows us to compare the measurement precision of different methods on a common basis, as demonstrated explicitly for the Rabi and Ramsey resonances, showing how the precision of the latter surpasses that of the former in the context of weak value amplification.

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arXiv2025

RGE effects on new physics searches via gravitational waves

Gravitational wave (GW) observations offer a promising probe of new physics associated with a first-order electroweak phase transition. Precision studies of the Higgs potential, including Fisher matrix analyses, have been extensively conducted in this context. However, significant theoretical uncertainties in the GW spectrum, particularly those due to renormalization scale dependence in the conventional daisy-resummed approach, have cast doubt on the reliability of such precision measurements. These uncertainties have been highlighted using the Standard Model Effective Field Theory (SMEFT) as a benchmark. To address these issues, we revisit Fisher matrix analyses based on the daisy-resummed approach, explicitly incorporating renormalization scale uncertainties. We then reassess the prospects for precise new physics measurements using GW observations. Adopting the SMEFT as a benchmark, we study the effects of one-loop RGE running of dimension-six operators on the Higgs effective potential via the Higgs self-couplings, top Yukawa coupling, and gauge couplings, in addition to the SMEFT tree-level effects. We find that future GW observations can remain sensitive to various dimension-six SMEFT effects, even in the presence of renormalization scale uncertainties, provided that the SMEFT $(H^{\dagger}H)^3$ operator is precisely measured, e.g., by future collider experiments.

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arXiv2024

Searching for neutrino self-interactions at future muon colliders

Multi-TeV muon colliders offer a powerful means of accessing new physics coupled to muons while generating clean and intense high-energy neutrino beams via muon decays. We study a fixed-target experiment leveraging the neutrino beams and a forward detector pointing at the interaction point of the muon collider. The sensitivity to neutrino self-interactions is analyzed as a feasibility study, focusing on the leptonic scalar $\phi$ exclusively coupled to the Standard Model neutrinos. Our work shows that projections from both the main and forward detectors can enhance the existing limits by two orders of magnitude, surpassing other future experiments.

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arXiv2024

Consistency of EFT illuminated via relative entropy: A case study in scalar field theory

Relative entropy is a non-negative quantity and offers a powerful means of achieving a unified understanding of fundamental properties in physics, including the second law of thermodynamics and positivity bounds on effective field theories (EFTs). We analyze the relative entropy in scalar field theories and show that the non-negativity of relative entropy is potentially violated in perturbative calculations based on operator and loop expansions. Conversely, this suggests that the consistency of the EFT description in the scalar field theory can be identified by the sign of the relative entropy. In fact, we revisit an EFT of single-field inflation and present a relation between its non-linear parameter $f_{\rm NL}$ and the consistency condition of the EFT description derived from the relative entropy method. We find that interesting regions of $f_{\rm NL}$ that are observationally allowed can be constrained from the relative entropy by imposing the consistency of the EFT description when the EFT is generated via the interaction with heavy fields in UV theories.

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arXiv2023

Quantum Resonance viewed as Weak Measurement

Quantum resonance, i.e., amplification in transition probability available under certain conditions, offers a powerful means for determining fundamental quantities in physics, including the time duration of the second adopted in the SI units and neutron's electric dipole moment which is directly linked to CP violation. We revisit two of the typical examples, the Rabi resonance and the Ramsey resonance, and show that both of these represent the weak value amplification when involving significant enhancement of transition probabilities and that near the resonance points they share exactly the same behavior of transition probabilities except for the measurement strength whose difference leads to the known advantage of the Ramsey resonance in the sensitivity. Conversely, as a by-product of the relationship, we may measure the weak value through quantum resonance. In fact, we argue that previous measurements of neutron electric dipole moment based on the Ramsey resonance have potentially determined the weak value of neutron's spin with much higher precision than the conventional weak value measurement.

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arXiv2023

Sub-GeV dark matter search at ILC beam dumps

Light dark matter particles may be produced in electron and positron beam dumps of the International Linear Collider (ILC). We propose an experimental setup to search for such events, the Beam-Dump eXperiment at the ILC (ILC-BDX). The setup consists of a muon shield placed behind the beam dump, followed by a multi-layer tracker and an electromagnetic calorimeter. The calorimeter can detect electron recoils due to elastic scattering of dark matter particles produced in the dump, while the tracker is sensitive to decays of excited dark-sector states into the dark matter particle. We study the production, decay and scattering of sub-GeV dark matter particles in this setup in several models with a dark photon mediator. Taking into account beam-related backgrounds due to neutrinos produced in the beam dump as well as the cosmic-ray background, we evaluate the sensitivity reach of the ILC-BDX experiment. We find that the ILC-BDX will be able to probe interesting regions of the model parameter space and, in many cases, reach well below the relic target.

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arXiv2022

Effective field theory in light of relative entropy

We study constraints on the effective field theory (EFT) from the relative entropy between two theories: we refer to these as target and reference theories. The consequence of the non-negativity of the relative entropy is investigated by choosing some reference theories for a given target theory involving field theories, quantum mechanical models, etc. It is found that the constraints on EFTs, e.g., the single massless scalar field with the dimension-eight operator, and SMEFT dimension-eight $SU(N)$ gauge bosonic operators, are consistent with the positivity bounds from the unitarity and causality when the higher-derivative operators are generated by the interaction between heavy and light fields. The constraints on Einstein-Maxwell theory with higher-derivative operators from the non-negativity of relative entropy are also investigated. The constraints on such EFTs from the relative entropy hold under an assumption that perturbative corrections from the interaction involving higher-derivative operators of light fields are not dominant in the EFTs. The consequence of this study on the weak gravity conjecture and the second law of thermodynamics is also discussed.

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arXiv2022

SMEFT effects on gravitational wave spectrum from electroweak phase transition

Future gravitational wave observations are potentially sensitive to new physics corrections to the Higgs potential once the first-order electroweak phase transition arises. We study the SMEFT dimension-six operator effects on the Higgs potential, where three types of effects are taken into account: (i) SMEFT tree level effect on $\varphi^6$ operator, (ii) SMEFT tree level effect on the wave function renormalization of the Higgs field, and (iii) SMEFT top-quark one-loop level effect. The sensitivity of future gravitational wave observations to these effects is numerically calculated by performing a Fisher matrix analysis. We find that the future gravitational wave observations can be sensitive to (ii) and (iii) once the first-order electroweak phase transition arises from (i). The dimension-eight $\varphi^8$ operator effects on the first-order electroweak phase transition are also discussed. The sensitivities of the future gravitational wave observations are also compared with those of future collider experiments.

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arXiv2022

First Evaluation of Meson and $\tau$ lepton Spectra and Search for Heavy Neutral Leptons at ILC Beam Dump

A beam dump experiment can be seamlessly added to the {proposed} International Linear Collider (ILC) program because the high energy electron beam should be dumped after the collision point. The ILC beam dump experiment will provide an excellent opportunity to search for new long-lived particles. Since many of them can be produced by a rare decay of standard model particles, we evaluate spectra of the mesons and $\tau$ lepton at the decay based on the PHITS and PYTHIA8 simulations. As a motivated physics case, we study the projected sensitivity of heavy neutral leptons at the ILC beam dump experiment. The heavy neutral leptons can also be produced via deep inelastic scattering and $Z$ boson decay at the ILC main detector, which we include in the projection. With the multi-track signal, the reach would be greatly extended in mass and coupling, even compared with the other proposed searches.

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arXiv2022

Entropy constraints on effective field theory

In effective field theory, the positivity bounds of higher derivative operators are derived from analyticity, causality, and unitarity. We show that the positivity bounds on some operators of the effective field theory, e.g., dimension-eight term of a single massless scalar field, the Standard Model Effective Field Theory dimension-eight $SU(N)$ gauge bosonic operators, and higher-derivative operators in the Einstein-Maxwell theory, generated by interactions between heavy and light degrees of freedom can be derived by the non-negativity of relative entropy. For such effective field theories, we prove that the interactions increase thermodynamic entropy at a fixed charge and an extremal point of energy, which is intimately connected with the extremality relations of black holes exhibiting Weak-Gravity-Conjecture. These arguments are applicable when corrections from the interactions involving higher-derivative operators of light fields are not dominant in the effective field theories. The entropy constraint is a consequence of the Hermiticity of Hamiltonian, and any theory violating the non-negativity of entropy would not respect the second law of thermodynamics.

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arXiv2021

New physics searches at the ILC positron and electron beam dumps

We study capability of the ILC beam dump experiment to search for new physics, comparing the performance of the electron and positron beam dumps. The dark photon, axion-like particles, and light scalar bosons are considered as new physics scenarios, where all the important production mechanisms are included: electron-positron pair-annihilation, Primakoff process, and bremsstrahlung productions. We find that the ILC beam dump experiment has higher sensitivity than past beam dump experiments, with the positron beam dump having slightly better performance for new physics particles which are produced by the electron-positron pair-annihilation.

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arXiv2020

Revisiting electroweak radiative corrections to $b\to s\ell\ell$ in SMEFT

We revisit electroweak radiative corrections to Standard Model Effective Field Theory (SMEFT) operators which are relevant for the $B$-meson semileptonic decays. The one-loop matching formulae onto the low-energy effective field theory are provided without imposing any flavor symmetry. The on-shell conditions are applied especially in dealing with quark-flavor mixings. Also, the gauge independence is shown explicitly in the $R_\xi$ gauge.

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arXiv2020

Search for new light particles at ILC main beam dump

We perform a feasibility study of a beam dump experiment at the International Linear Collider (ILC). To investigate the sensitivity to new light particles at the experiment, we consider models for axion-like particles (ALPs) and a light scalar particle coupled to charged leptons. For both models, we show that the detection sensitivity is almost an order of magnitude higher than other beam dump experiments in the small coupling region. For ALPs, it is shown that the ILC beam dump experiment is highly complementary to bounds from astrophysics. In addition, for the model of the scalar particle, the region favored by the muon $g-2$ experiment can be explored.

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arXiv2020

Novel approach to neutron electric dipole moment search using weak measurement

We propose a novel approach in a search for the neutron electric dipole moment (EDM) by taking advantage of signal amplification in a weak measurement, known as weak value amplification. Considering an analogy to the weak measurement that can measure the spin magnetic moment interaction, we examine an experimental setup with a polarized neutron beam through an external electric field with spatial gradient, where the signal is sensitive to the EDM interaction. In particular, a dedicated analysis of effects from impurities in pre- and post-selections is performed. We show that the weak value amplification occurs where the signal is enhanced by up to two orders of magnitude, and demonstrate a potential sensitivity of the proposed setup to the neutron EDM.

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