arXiv subjects

Ryosuke Sato

Publications and source records attributed to Ryosuke Sato.

At least 19 recordsLinked to original sources

arXiv2026

Finding Competitive Equilibria in Online Marketplaces with Platform Fees, Taxes, and Subsidies

We study equilibrium computation in multi-unit trading networks with valuations satisfying generalized full substitutes and agent- and transaction-specific piecewise-linear transfer functions. These transfers model platform fees, taxes, subsidies, and membership discounts in online marketplaces. Our ascending procedure raises prices at different rates and reaches the minimum equilibrium price vector in finitely many iterations, each implementable in strongly polynomial time under our oracle and arithmetic models. An equilibrium allocation can then be recovered efficiently. The method extends primal-dual techniques developed for ascending auctions to handle complementarities between purchases and sales through a submodular flow formulation. In single-seller markets with substitutes and complements, our framework also extends the double-track auctions of Sun and Yang (2009) and Shioura and Yang (2015) to bidder- and good-specific transfers.

cs.GT↗

arXiv2026

A Weakly Group Strategy-Proof and Pareto-Stable Matching Mechanism with Two-Sided Indifferences under Gross Substitutes

We study finite many-to-one matching markets with indifferences on both sides and publicly known firm preferences over sets of workers. A standard approach breaks ties uniformly at random and runs deferred acceptance, which is strategy-proof for workers but may select a weakly stable matching that is Pareto dominated under the agents' original preferences. For the broad class of markets in which each firm's preferences admit a gross substitutes (GS) representation, we construct a deterministic generalized deferred acceptance mechanism that is weakly stable, Pareto efficient with respect to both sides, worker weakly group strategy-proof, and implementable in strongly polynomial time given value oracles for the GS representations. This domain includes nonresponsive firm preferences, such as additive scores under matroid constraints and assignment values. Our analysis bridges ordinal matching with indifferences and competitive equilibria under imperfectly transferable utility. Although the mechanism uses only ordinal comparisons, it returns a seller-optimal equilibrium matching of a tiered auxiliary market, and the equilibrium structure of this market yields the stability, efficiency, and incentive guarantees.

cs.GT↗

arXiv2026

Sommerfeld enhancement from unstable final-state particles in dark matter annihilation

We study the Sommerfeld enhancement of the annihilation cross section of dark matter into heavier unstable particles. In this process, the annihilation products become non-relativistic near the kinematical threshold. If they experience long-range interactions with each other, their wave function is distorted from a plane wave, and the annihilation cross section can be significantly enhanced. When evaluating the Sommerfeld enhancement from the long-range interactions between the annihilation products, the decay of the products needs to be taken into account. We treat this issue by including the decay width in the Schrödinger equations of the two-body wave function of the annihilation products. We find that bound states of the annihilation products with a narrow decay width enhance the annihilation cross section through a resonant effect. At the same time, this formulation automatically includes the annihilation process with off-shell final state particles, which is relevant for a wide decay width. We show that the resonant effect significantly affects the prediction of the dark matter relic abundance.

hep-ph↗

arXiv2026

Ascending Auctions for Combinatorial Markets with Frictions: A Unified Framework via Discrete Convex Analysis

We develop a unified ascending-auction framework for computing Walrasian equilibria in combinatorial markets with strong substitutes valuations and piecewise-linear payment functions. Our auction extends the celebrated ascending auctions of Gul and Stacchetti (2000) and Ausubel (2006) to accommodate payment frictions (e.g., transaction taxes or commission fees). This is achieved by incorporating directional price updates that reflect heterogeneous payment structures. Our framework also generalizes the unit-demand imperfectly transferable utility models of Alkan (1989, 1992) to a fully combinatorial setting, thereby unifying these paradigms. Furthermore, this is the first study to compute the minimum -- also known as the buyer-optimal -- equilibrium in combinatorial markets with such frictions. Our analysis builds upon discrete convex analysis. Our main technical contribution is a characterization of valid price-update directions, together with a strongly polynomial-time algorithm for computing them. Notably, the algorithm uses only demand- and exchange-oracle queries and never requires handling information of exponential size. To compute such a direction, we formulate a lexicographic extension of the polymatroid sum problem and characterize its dual solution via a reduction to a convex flow problem. Exploiting the $\text{L}^\natural$-convexity of the dual objective, we show that the desired direction can be constructed from the minimal dual solution. This convexity also yields transparent economic and potential-based interpretations of the auction dynamics, strengthening the connection between ascending auctions and discrete optimization.

cs.GT↗

arXiv2026

$q$-deformed polyanalytic Ginibre point processes:construction and central limit theorems for linear statistics

Based on the representation theory of the $q$-CCR algebra, we develop a representation-theoretic construction of the $q$-deformed polyanalytic Fock spaces, their reproducing kernels, and the associated $q$-deformed polyanalytic Bargmann transforms. The associated determinantal point processes provide $q$-deformations of the pure and full polyanalytic Ginibre ensembles. Moreover, we determine their limiting density and establish central limit theorems for the fluctuations of their linear statistics as the number of particles tends to infinity and the deformation parameter $q$ tends to 1 simultaneously. Remarkably, although the $q$-deformation changes the macroscopic density and the size of the droplet, the limiting covariance structures coincide, after spatial rescaling, with those of the classical polyanalytic Ginibre ensembles.

math.PR↗

arXiv2026

Anomaly-free axion-like particle in Nelson-Barr models

We study Nelson--Barr models with a discrete $Z_N$ symmetry that solve the strong CP problem through spontaneous CP violation, and show that they naturally predict a light axion-like particle (ALP) without introducing any additional ingredients. Unlike the QCD axion, this ALP is anomaly-free: its couplings to photons and gluons are highly suppressed, rendering it naturally long-lived. Instead, it couples to quarks through flavor-violating interactions whose structure is dictated by the CKM matrix. These interactions induce rare meson decays, providing a unique probe of the Nelson--Barr mechanism. We study the cosmological production of the ALP through both freeze-in and misalignment mechanisms. We show that the parameter space in which the observed relic abundance is explained by the freeze-in mechanism is subject to stringent constraints from precision flavor experiments and stellar cooling bounds from SN1987A, leaving only a small viable region that will be comprehensively tested by future structure-formation observations such as the Vera Rubin Observatory and next-generation X-ray missions like Athena, GECCO and THESEUS. In contrast, misalignment production remains a robust and viable mechanism for explaining the observed dark matter abundance over a broad region of parameter space. Our results demonstrate that precision flavor measurements, cosmological observations, and X-ray searches provide complementary probes of this anomaly-free ALP and, consequently, of the Nelson--Barr solution to the strong CP problem.

hep-ph↗

arXiv2026

Generalizations and UV completions of Cho-Maison monopole

A monopole configuration in the electroweak theory was constructed by Cho and Maison, allowing for a singular behavior at the origin. Since the essential structure of the Cho-Maison monopole is based on an electroweak-type symmetry breaking, similar monopole configurations are expected to arise more generally in gauge theories containing such a structure. In this paper, we explicitly show that Cho-Maison-like monopole configurations can indeed be constructed in a broad class of models. We also show that the Cho-Maison monopole can be embedded into an 't Hooft-Polyakov monopole as its low-energy effective description. In particular, we find that a monopole in the Pati-Salam model behaves as the electroweak Cho-Maison monopole once degrees of freedom which are heavier than the electroweak scale are integrated out.

hep-ph↗

arXiv2026

Solvable Tuple Patterns and Their Applications to Program Verification

Despite the recent progress of automated program verification techniques, fully automated verification of programs manipulating recursive data structures remains a challenge. We introduce solvable tuple patterns (STPs) and conjunctive STPs (CSTPs), novel formalisms for expressing and inferring invariants between list-like recursive data structures. A distinguishing feature of STPs is that they can be efficiently inferred from only a small number of positive samples; no negative samples are required. After presenting properties and inference algorithms of STPs and CSTPs, we show how to incorporate the CSTP inference into a CHC (Constrained Horn Clauses) solver supporting list-like data structures, which serves as a uniform backend for automated program verification tools. A CHC solver incorporating the (C)STP inference has won the ADT-LIN category of CHC-COMP 2025 by a significant margin.

cs.PL↗

arXiv2026

A particle on a ring or: how I learned to stop worrying and love $θ$-vacua

Recently, Ai, Cruz, Garbrecht, and Tamarit (arXiv:2001.07152, arXiv:2404.16026, arXiv:2511.04216) claimed that the strong CP problem can be avoided by adopting a particular order of limits in the Euclidean path integral, in which the spacetime volume is taken to infinity before summing over all topological sectors. We critically examine this proposal using exactly solvable examples of one-dimensional quantum mechanics on a ring, namely the quantum rotor and the quantum pendulum. These systems provide fully controlled settings with known $θ$-dependent spectra. We find that the ACGT procedure fails to reproduce the correct energy spectrum. Since the spectrum is a direct physical observable, this result demonstrates that the proposed order of limits cannot be justified and conclusions about CP conservation in QCD cannot be based on this prescription alone.

hep-ph↗

arXiv2026

QCD axion from chiral gauge theories

We present models of axion based on supersymmetric chiral gauge theories. In these models, the PQ symmetry is spontaneously broken by the non-perturbative dynamics of chiral gauge theory. Thanks to supersymmetry, IR dynamics of the models are calculable. We also present an example of a QCD axion model that is compatible with SU(5) grand unification. We find that in order to realize the gauge coupling unification with a certain precision, the GUT scale is the same with the PQ breaking scale, and the SUSY breaking scale is ${\cal O} (10^9)~{\rm GeV} $.

hep-ph↗

arXiv2026

Excluding MeV-scale QCD axions by $K_L \to π^0π^0 a$ at KTeV

An interesting proposal suggests that a QCD axion $(a)$ coupling to the up quark, down quark, and electron remains viable for an axion mass near 10~MeV. In this paper, this possibility is reexamined by deriving new bounds from kaon decays. In particular, we perform a detailed analysis of the $K_L \to π^0 π^0 e^+ e^-$ measurement reported by the KTeV experiment, and reinterpret $K^+$ decay measurements at the E949 and NA62 experiments to constrain both the diphoton decay and effectively invisible decay modes of the axion. We find that, combined with the previously known bounds, the viable window for the MeV-scale QCD axion is excluded, primarily due to the KTeV bound. Uncertainties associated with the chiral Lagrangian are further examined, and the scenario remains excluded even after accounting for these uncertainties, except for a tiny region of parameter space where higher-order corrections must finely cancel the leading-order contribution, suppressing the branching ratio of $K_L\to π^0π^0 a$ by three orders of magnitude.

hep-ph↗

arXiv2026

A universal bound on the duration of a kination era

We show that primordial adiabatic curvature fluctuations generate an instability of the scalar field sourcing a kination era. We demonstrate that the generated higher Fourier modes constitute a radiation-like component dominating over the kination background after about $11$ e-folds of cosmic expansion. Current constraints on the extra number of neutrino flavors $ΔN_{\rm eff}$ thus imply the observational bound of approximately 10 e-folds, representing the most stringent bound to date on the stiffness of the equation of state of the pre-Big-Bang-Nucleosynthesis universe.

hep-ph↗

arXiv2026

New Source for QCD Axion Dark Matter Production: Curvature Induced

We discuss a novel mechanism for generating dark matter from a fast-rolling scalar field, relevant for both inflation and rotating axion models, and apply it specifically to the (QCD) axion. Dark matter comes from scalar field fluctuations generated by the product of the curvature perturbation and the fast-rolling background field. These fluctuations can explain the totality of dark matter in a vast axion parameter space, particularly for the QCD axion, which will be targeted by upcoming experiments. We review the constraints on this mechanism and potential gravitational-wave signatures.

hep-ph↗

arXiv2025

Regulating Sommerfeld resonances for multi-state systems and higher partial waves

Long-range attractive interactions between dark matter particles can significantly enhance their annihilation, particularly at low velocities. This ``Sommerfeld enhancement'' is typically computed by evaluating the deformation of the two-particle wavefunction due to the long-range potential, while ignoring the physics associated with the annihilation, and then scaling the appropriate annihilation matrix elements by factors that depend on the wavefunction in the limit where the particles approach zero relative separation. It has long been recognized that this approach is a valid approximation only in the limit where the annihilation rate is small, and breaks down in the regime where the enhanced annihilation rate approaches the unitarity bound, in which case ignoring the impact of the annihilation physics on the two-particle wavefunction cannot be justified and leads to apparent violations of unitarity. In the case where the physics relevant to annihilation occurs at a parametrically shorter distance scale (higher energy scale) compared with the long-range potential, we provide a simple prescription for correcting the Sommerfeld enhancement for the effects of the short-range physics, valid for all partial waves and for systems where multiple states are coupled by the long-range potential.

hep-ph↗

arXiv2025

Conservation operator processes from asymptotic representation theory and their CLT

In this paper, we examine applications of the theory of operator-valued processes to algebraic methods in probability theory. We show a central limit theorem for general conservation operator processes. Utilizing this, we analyze the asymptotic behavior of processes derived from unitary groups and quantum unitary groups as their ranks tend to infinity, thereby providing applications of asymptotic representation theory.

math.PR↗

arXiv2025

SimpleBounce : a simple package for the false vacuum decay

We present SimpleBounce, a C++ package for finding the bounce solution for the false vacuum decay. This package is based on a flow equation which is proposed by the author and solves Coleman-Glaser-Martin's reduced problem: the minimization problem of the kinetic energy while fixing the potential energy. The bounce configuration is obtained by a scale transformation of the solution of this problem. For models with 1--8 scalar field(s), the bounce action can be calculated with O(0.1) % accuracy in O(0.1) s. This package is available at https://github.com/rsato64/SimpleBounce.

hep-ph↗

arXiv2025

Simple Gradient Flow Equation for the Bounce Solution

Motivated by the recent work of Chigusa, Moroi, and Shoji, we propose a new simple gradient flow equation to derive the bounce solution which contributes to the decay of the false vacuum. Our discussion utilizes the discussion of Coleman, Glaser, and Martin and we solve a minimization problem of the kinetic energy while fixing the potential energy. The bounce solution is derived as a scale-transformed of the solution of this problem. We also show that the convergence of our method is robust against a choice of the initial configuration.

hep-ph↗

arXiv2025

Variations on quantum de Finetti theorems and operator valued Martin boundaries: a Choquet-Deny approach

We revisit the quantum de Finetti theorem. We state and prove a couple of variants thereof. In parallel, we introduce an operator version of the Martin boundary on quantum groups and prove generalizations of Biane's theoresm. Our proof of the de Finetti theorem is new in the sense that it is based on an analogy with the theory of operator valued Martin boundary that we introduce.

math.OA↗