arXiv subjects

Fuminobu Takahashi

Publications and source records attributed to Fuminobu Takahashi.

At least 19 recordsLinked to original sources

arXiv2026

Nuclear Recoils from Invisible Neutron-Pair Annihilation and the LZ event

Two bound neutrons can annihilate into a single invisible scalar carrying baryon number two, producing a monochromatic nuclear recoil. Pair removal connects the $0^+$ ground states of even-even nuclei and, for several detector isotopes, leaves a stable daughter where single-neutron removal would leave a radioactive one. Below the first daughter excitation, the leading transition produces neither nuclear de-excitation nor subsequent daughter decay. We identify the corresponding mass windows in argon and xenon, including argon recoils up to $108\,\mathrm{keV}$, and obtain an approximate partial-lifetime bound of $8.5 \times 10^{26}\,\mathrm{yr}$ from DEAP-3600 data. Recoil lines in different isotopes reconstruct a common invisible-particle mass. The emitted scalar can itself be dark matter, making these searches a probe of ordinary matter converting into a dark sector.

hep-ph↗

arXiv2026

Gravitational Decays of Secluded Scalars and Graviton Dark Radiation

We discuss graviton dark radiation produced by the decay of a secluded scalar field that couples to the Standard Model (SM) only through gravity. Such scalar fields are long-lived, and their decay channels generically include gravitons. If such particles existed and dominated the early universe, a sizable branching ratio into gravitons would yield non-negligible dark radiation that significantly alters the subsequent thermal history of the universe. In this work, we focus on the dark glueball as a representative secluded hidden scalar and compare the decay rates into SM particles via a non-minimal coupling to gravity with those into gravitons, paying attention to how the breaking of conformal invariance affects the amount of graviton dark radiation. We find that decays into the SM are dominated by two-body decay channels into Higgs bosons and gluons. In particular, when the Higgs field has a large non-minimal coupling to gravity, the production of graviton dark radiation is naturally suppressed in the metric formalism, and the SM sector is preferentially reheated and energy transfer to other hidden sectors is suppressed. Finally, we present the expected gravitational-wave spectrum resulting from dark glueball domination.

hep-ph↗

arXiv2026

Multi-field oscillons/I-balls in the Friedberg-Lee-Sirlin model

We study oscillon/I-ball solutions in a real scalar version of the Friedberg-Lee-Sirlin (FLS) model. Using the multiple-scale analysis, we derive the conditions for oscillon solutions and explore multi-field oscillon configurations. In these configurations, the two fields form co-located oscillons that oscillate with frequencies set by their respective masses. These multi-field oscillons can be viewed as a bound state of two oscillons due to attractive interactions between the fields. We confirm these analytical predictions through numerical lattice calculations. This work extends the standard picture of single-field oscillons and may be relevant for cosmological scenarios involving multiple interacting real scalar fields.

hep-ph↗

arXiv2026

Axion Isocurvature Perturbations Survive the Scaling Evolution of Axion Domain Walls

We revisit the evolution of axion domain walls seeded by inflationary fluctuations. In our previous work, we showed that such domain-wall networks retain superhorizon correlations even after entering the scaling regime. We extend our previous analysis to the case with large initial fluctuations, where many minima of the axion potential are already populated when the axion starts to oscillate. Although the conventional misalignment contribution can have suppressed long-wavelength isocurvature perturbations when many vacua are averaged over, axions produced by domain-wall collapse provide an additional contribution that can dominate when the walls enter the scaling regime before annihilation. In particular, the biased vacuum energy released during wall annihilation inherits the superhorizon correlations of the inflationary fluctuations and transfers them to the axion energy density. We find that sizable isocurvature perturbations can therefore survive even after the walls annihilate. We also discuss generic isocurvature constraints on dark matter produced by domain-wall collapse.

hep-ph↗

arXiv2026

Nelson-Barr Inflation

The Nelson-Barr mechanism solves the strong CP problem through spontaneous CP violation, but the resulting CP-conjugate vacua generically lead to stable domain walls. We propose that the Nelson-Barr scalar itself drives hilltop inflation. A CP-symmetric double-well potential for its imaginary component forms two CP-breaking valleys, with inflation proceeding along one of them. Since the lowest CP-invariant ridge is generically much higher than the energy available after inflation, transitions between the two branches are energetically inaccessible, and the domain walls cannot be regenerated after inflation. A small CP-preserving linear deformation raises the scalar spectral index of quartic hilltop inflation and yields CMB-compatible parameter regions. The Nelson-Barr sector necessarily provides a portal to visible-sector reheating. When coupled to right-handed neutrinos, it can also transmit the spontaneous CP phase to the lepton sector and, in an extension that allows a higher CP-breaking scale, even realize nonthermal leptogenesis. This establishes a unified cosmological realization of the Nelson--Barr mechanism in which the same CP-breaking dynamics solves the strong CP problem, can generate quark and lepton CP violation, drives inflation and reheating, and enables baryogenesis, while simultaneously eliminating the associated domain-wall problem.

hep-ph↗

arXiv2026

Arithmetic Symmetry in Ideal Prouhet-Tarry-Escott Solutions

Motivated in part by anomaly cancellation for integral charge spectra in chiral gauge theory, we study the symmetric locus in the ideal degree-three Prouhet-Tarry-Escott problem. A symmetric integer solution is one whose entries are paired about a common center $c\in \frac12\mathbb Z$. This symmetry reduces the problem to a sum-of-two-squares equation, $x^2+y^2=u^2+v^2$, in integer variables, subject to the appropriate parity conditions. Thus the problem is governed by representations as sums of two squares. For the full symmetric locus, let $N_{\mathrm{sym}}(H)$ denote the number of nontrivial symmetric integer solutions of height at most $H$, counted with unordered multiset conventions and summed over the admissible centers. Then \begin{align*} N_{\mathrm{sym}}(H) = \frac{4\log 2}{3π^2}H^3\log H+O(H^3). \end{align*} The logarithmic enhancement comes from the second moment of the sum-of-two-squares representation function. In particular, the symmetric locus is larger than one would expect from the naive $H^3$ degree-weighted box-counting scale alone. This asymptotic identifies a large arithmetically structured subfamily of the ideal degree-three solution space, and suggests that paired anomaly-free integral charge spectra reflect a fundamental number-theoretic structure.

math.NT↗

arXiv2026

Domain-wall Quintessence

We investigate a dark energy model driven by a planar domain-wall-like structure with a thickness comparable to, or larger than, the current Hubble radius, focusing on its intrinsic anisotropy and observational viability. Near the centre of the domain wall (DW), the spacetime is anisotropic, with distinct expansion rates parallel and perpendicular to the wall. This anisotropic structure induces direction-dependent cosmic expansion and modifies photon geodesics from cosmological sources, leaving characteristic signatures in cosmological observables. We confront the model with recent observational data. We first compute the anisotropic Cosmic Microwave Background (CMB) temperature multipoles generated by the DW and impose constraints from the Planck 2018 measurements. These constraints severely limit the allowed DW abundance, requiring the DW energy density to be less than $\mathcal{O}(10^{-5})$ of the current critical density in order to suppress the quadrupole contributions. We then perform a Markov Chain Monte Carlo (MCMC) analysis using Type Ia supernova (SNe Ia) data, including the Pantheon+ SH0ES and DESY5 samples, to compare the DW scenario with the standard $Λ$CDM model. We find that although the DW naturally realises anisotropic accelerated expansion, the combined constraints from the CMB and SNe Ia favour the $Λ$CDM limit, in which the DW contribution is negligible, and the universe is effectively isotropic. Our results demonstrate that a Hubble-scale domain wall is tightly constrained by current observations and can only play a subdominant role in the late-time cosmic acceleration.

astro-ph.CO↗

arXiv2026

Number Theory in Quantum Physics: Minicharged Particles and the Prouhet-Tarry-Escott Problem

In quantum gauge theories, anomaly cancellation severely restricts the allowed patterns of chiral charges. Here we show that, in a phenomenologically motivated framework for light minicharged particles, the anomaly cancellation conditions are equivalent to the degree $k=3$ Prouhet-Tarry-Escott problem in number theory. This correspondence immediately implies that the hidden sector must contain at least four minicharged states. For constructions based on minimal ideal solutions, the mass spectrum generically exhibits a near-degenerate doublet structure, so that the discovery of one minicharged particle would point to a partner state with the same minicharge and a nearby mass. Our results uncover an unexpected link between quantum consistency and number theory, with direct implications for model building and future searches.

hep-ph↗

arXiv2026

Resolving the QCD Axion Domain Wall Problem with a Light Axion

We propose two novel solutions to the domain wall problem of the QCD axion by introducing a massless or light axion that also couples to gluons. The first solution applies when the new axion forms strings after inflation. Due to its mixing with the QCD axion, domain walls of the QCD axion are bounded by these strings and confined into cosmologically safe string bundles. This scenario predicts the existence of such string bundles, which may survive until today and leave observable signatures, such as gravitational waves, cosmic birefringence, and CMB anisotropies. The simultaneous detection of the QCD axion and any of these cosmological signatures would serve as a smoking-gun signal. The second solution assumes a homogeneous initial condition for the new axion. If it is sufficiently light, its potential temporarily induces a bias in the QCD axion potential before the onset of oscillations, rendering the domain walls unstable. In both scenarios, the Peccei-Quinn mechanism remains effective, and the strong CP problem is not reintroduced. We identify the viable parameter regions and discuss the resulting dark matter abundance.

hep-ph↗

arXiv2026

Dynamical Prevention of Topological Defect Formation

Topological defects can have significant cosmological consequences, so their production must be examined carefully. It is usually assumed that topological defects are produced if the temperature becomes sufficiently high, but in reality their formation depends on the post-inflationary dynamics of a symmetry-breaking scalar. We analyze the dynamics of a symmetry-breaking scalar field in the early universe within models that provide an effective negative mass term at the origin, and show that the symmetry can remain broken so that topological defects are never formed. In particular, we demonstrate that nonthermally produced particles (such as the Standard Model Higgs) during preheating can generate such an effective negative mass term, allowing the scalar field to follow a time-dependent minimum even in renormalizable models with a quartic coupling. We also discuss the implications of this result for the Peccei-Quinn scalar in axion models.

hep-ph↗

arXiv2025

Why $w \ne -1$? Anthropic Selection in a $Λ$ + Axion Dark Energy Model

We study a dark energy model composed of a bare negative cosmological constant and a single ultra-light axion, motivated by the string axiverse. Assuming that intelligent observers arise and observe, as in our universe, the onset of dark-energy-driven acceleration following matter domination, and that this acceleration persists to the present, we derive nontrivial constraints on both the axion mass and the bare cosmological constant. The axion mass is bounded from above to avoid fine-tuning of the initial misalignment angle near the hilltop, and from below because too light axions cannot achieve accelerated expansion due to their limited energy budget. As a result, the anthropically allowed axion mass range typically lies around $m = \mathcal{O}(10)\, H_0$ for a decay constant close to the Planck scale, where $H_0$ is the observed value of the Hubble constant. In this framework, the dark energy equation-of-state parameter $w_0$ generically deviates from $-1$ by $\mathcal{O}(0.1)$, providing a natural explanation for why $w \ne -1$ may be expected. We also find that, for a decay constant slightly smaller than the Planck scale, the peak value of dark energy density is significantly smaller than the anthropic bound on the cosmological constant and can be close to the observed value. These outcomes are intriguingly consistent with recent DESI hints of time-varying dark energy, and offer a compelling anthropic explanation within the $Λ$ + axion framework.

hep-ph↗

arXiv2025

Sign-Flipping Axion Potentials via Kapitza-Type Modulation by Heavy Axions

We show that the potential of a light axion can flip sign, or even nearly vanish, as a result of coherent oscillations of a heavier axion with which it mixes. This phenomenon is analogous to the Kapitza pendulum, where a high-frequency external force stabilizes an otherwise unstable configuration, but here it arises naturally from the inherent mass hierarchy and mixing among axions in the axiverse, without the need for any externally imposed modulation. We further show that a late-time sign flip of the potential can significantly enhance the abundance of the light axion, which has important cosmological and observational consequences.

hep-ph↗

arXiv2025

Isotropic cosmic birefringence from string axion domain walls without cosmic strings, and DESI results

Recently, results from the Atacama Cosmology Telescope (ACT) DR6 have shown a preference for isotropic cosmic birefringence, consistent with previous analyses based on Planck and WMAP data. Separately, the Dark Energy Spectroscopic Instrument (DESI) DR2 results suggest that dark energy evolves over cosmic history, pointing to new physics in the late-time universe. In this paper, we propose that domain walls associated with the string axion can naturally explain the isotropic cosmic birefringence, focusing on the case in which the axion starts near a hilltop. Interestingly, to avoid the domain wall problem, these walls must form well after recombination. The predicted rotation angle, $β\approx 0.21\,c_γ$ degrees (with anomaly coefficient $c_γ\approx 1$), is in excellent agreement with observations. This scenario can be further tested by probing anisotropic birefringence of photons emitted long after recombination, as well as gravitational waves. Moreover, starting the axion oscillation from a hilltop naturally enhances its abundance via anharmonic effects, thus contributing to the dark energy component. We discuss how this hilltop axion scenario may connect with the DESI results.

hep-ph↗

arXiv2025

Clustering of Primordial Black Holes from QCD Axion Bubbles

We study the clustering of primordial black holes (PBHs) and axion miniclusters produced in the model proposed to explain the LIGO/Virgo events or the seeds of the supermassive black holes (SMBHs) in arXiv:2006.13137. It is found that this model predicts large isocurvature perturbations due to the clustering of PBHs and axion miniclusters, from which we obtain stringent constraints on the model parameters. Specifically, for the axion decay constant $f_a=10^{16}~\mathrm{GeV}$, which potentially accounts for the seeds of the SMBHs, the PBH fraction in dark matter should be $f_\mathrm{PBH}\lesssim7\times 10^{-10}$. Assuming that the mass of PBHs increases by more than a factor of $\mathcal{O}(10)$ due to accretion, this is consistent with the observed abundance of SMBHs. On the other hand, for $f_a=10^{17}~\mathrm{GeV}$ required to produce PBHs of masses detected in the LIGO/Virgo, the PBH fraction should be $f_\mathrm{PBH}\lesssim6\times 10^{-8}$, which may be too small to explain the LIGO/Virgo events, although there is a significant uncertainty in calculating the merger rate in the presence of clustering.

astro-ph.CO↗

arXiv2025

Primordial Black Hole Formation via Inverted Bubble Collapse

We propose a novel mechanism of primordial black hole (PBH) formation through inverted bubble collapse. In this scenario, bubbles nucleate sparsely in an incomplete first-order phase transition, such that they remain isolated and do not percolate or collide with each other due to the extremely low nucleation rate. This is followed by a bulk phase transition in the rest of the universe that inverts these pre-existing bubbles into false vacuum regions. These spherically symmetric false-vacuum bubbles subsequently collapse to form PBHs. Unlike conventional PBH formation mechanisms associated with domain wall collapse or bubble coalescence, our inverted bubble collapse mechanism naturally ensures spherical collapse. We demonstrate that, when applied to the singlet extension of the Standard Model, this mechanism can produce highly monochromatic PBHs with masses up to ${\cal O}(10^{-7}\,\text{-}\,10^{-5}) M_\odot$, which potentially explain the microlensing events observed in the OGLE and Subaru HSC data.

astro-ph.CO↗

arXiv2025

Q-balls Under Spontaneously Broken U(1) Symmetry

We study the evolution of Q-balls under a spontaneously broken global $U(1)$ symmetry. Q-balls are stabilized by the conservation of $U(1)$ charge, but when the symmetry is spontaneously broken, the resulting Nambu-Goldstone (NG) boson can carry charge away from the Q-ball, potentially leading to charge leakage. To study this process in a controlled setting, we consider a scenario where Q-balls first form under an unbroken $U(1)$ symmetry, which is then spontaneously broken. We introduce two complex scalar fields: one responsible for forming the Q-ball, and the other for spontaneously breaking the $U(1)$ symmetry, allowing us to clearly separate the formation and symmetry-breaking phases. Using numerical simulations in a spherically symmetric system, we find that the evolution of Q-balls depends sensitively on the structure of the interaction between the two fields and the magnitude of symmetry breaking. Depending on parameters, Q-balls can completely decay, evaporate into smaller, stable Q-balls, or transition into oscillons/I-balls. In particular, we find that stable, localized remnants often survive the evolution over long timescales, especially when the symmetry-breaking scale is small. These results demonstrate that, even though spontaneous $U(1)$ breaking can lead to significant energy and charge loss from Q-balls, stable localized objects with reduced or no charge can frequently survive and potentially contribute to cosmological relics.

hep-ph↗

arXiv2025

Stability of domain walls with inflationary fluctuations under potential bias, and gravitational wave signatures

A recent study has shown that domain walls with inflationary initial fluctuations exhibit remarkable stability against population bias due to long-range correlations, challenging the claims of prior research. In this paper, we study the dynamics of these domain walls in the presence of potential bias and show that they collapse with a lifetime several times longer than that due to thermal fluctuations. This is interpreted as a difference in the average distance between domain walls, leading us to derive a new formula for the domain wall lifetime which depends on the area parameter in a qualitatively different way from previous studies. In addition, we compute the spectrum of gravitational waves generated by such domain walls and find that both the peak frequency and the peak abundance are lowered in a manner that depends on the area parameter. Based on these findings, we also determine the necessary degree of vacuum degeneracy for axion domain walls to explain the isotropic cosmic birefringence.

hep-ph↗

arXiv2025

More is Different: Multi-Axion Dynamics Changes Topological Defect Evolution

We study topological defects in multi-axion models arising from multiple Peccei-Quinn (PQ) scalars. Using a simplified two-axion system, we reveal fundamental differences in the evolution of these defects compared to single-axion scenarios. This finding is particularly significant because, despite the fact that integrating out heavier axions reduces these models to an effective single PQ scalar theory at low energies, the actual physical behavior of topological defects differs markedly from single-axion predictions. Unlike single-axion models where conventional cosmic strings form, multi-axion scenarios with post-inflationary or mixed initial conditions generically produce networks of strings interconnected by high-tension domain walls. This results in a severe cosmological domain wall problem. We determine string-wall network instability conditions and discuss cosmological implications including the application to the QCD axion and gravitational wave generation. Our findings highlight that multi-axion dynamics can lead to qualitatively different outcomes for topological defects, challenging the conventional picture of cosmic evolution of topological defects based on single-axion models.

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