arXiv subjects

Kohsaku Tobioka

Publications and source records attributed to Kohsaku Tobioka.

At least 19 recordsLinked to original sources

arXiv2026

New Bounds on Heavy QCD Axions from Big Bang Nucleosynthesis

We study Big Bang Nucleosynthesis (BBN) constraints on heavy QCD axions. BBN offers a powerful probe of new physics that modifies the neutron-to-proton ratio during the process, thanks to the precisely measured primordial Helium-4 abundance. A heavy QCD axion provides an attractive target for this probe, because not only is it a well-motivated hypothetical particle by the strong CP problem, but also it dominantly decays to hadrons if kinematically allowed. A range of its lifetime is thus excluded where the hadronic decays would significantly alter the neutron-to-proton ratio. We compute axion-induced modification of the neutron-to-proton ratio, and obtain robust upper bounds on the axion lifetimes, as low as 0.017 s for the axion mass higher than 300 MeV. Remarkably, this is stronger than projected future CMB bounds via $N_{\rm eff}$. Our bounds are largely insensitive to uncertainties in hadronic cross sections and the axion's branching fractions into various hadrons, as well as to the precise value of the initial axion abundance. We also incorporate, for the first time, several key improvements, such as scattering processes by energetic $K_L$ and secondary hadrons, that can also be important for studying general hadronic injections during BBN, not limited to those from axion decays.

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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.

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arXiv2025

Irreducible Bhabha background in the detection of muonium-antimuonium conversion

Experiments such as MACS and the proposed MACE study muonium-antimuonium conversion by the energies of the final-state $e^\pm$. The $e^+$ and $e^-$ from an antimuonium decay tend to be non-relativistic and relativistic, respectively, and vice versa for muonium. However, these $e^\pm$ can exchange their energies by hard Bhabha scattering, causing muonium to fake an antimuonium decay signal. We compute the rate for this background and find that, while negligible for MACE, it will become larger than the signal for conversion probabilities less than $10^{-18}$. Measuring the helicity of the $e^-$ will reduce this to $10^{-22}$.

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arXiv2025

Neutrino properties from muonium-antimuonium mixing

The nature of neutrino mass -- whether neutrinos are Dirac or Majorana particles -- remains one of the central open questions in particle physics. While observation of neutrinoless double beta decay would confirm the Majorana case, the absence of a signal offers no definitive insight. In light of this, we investigate muonium-antimuonium mixing -- proposed for further study at the MACE experiment -- as an alternative probe of neutrino properties. We compute the mixing amplitude in the Standard Model minimally extended to include massive Dirac or Majorana neutrinos, and correct previous calculations by properly treating the relevant infrared scales. As the GIM mechanism strongly suppresses the Dirac contribution, we explore whether relaxing unitarity of the PMNS matrix can enhance the mixing without obscuring neutrino properties. Surprisingly, the answer to this question is negative. We also examine the pseudo-Dirac case -- predominantly Dirac neutrinos with small Majorana masses -- and find that this scenario can significantly enhance the mixing compared to the pure Dirac case, especially for normal mass ordering.

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arXiv2025

High Energy Vector Boson Scattering in Four-Body Final States to Probe Higgs Cubic, Quartic, and HEFT interactions

We compute the energy scales of perturbative unitarity violation in various $2 \to 4$ vector boson scattering (VBS) processes and compare them to lower multiplicity processes. The final states include $h^4$, $V_L V_L h h$, and $V_L^4$, where $V_L$ represents the longitudinal mode of $Z$ or $W$ boson, and $h$ the Higgs boson. We consider scenarios with modified cubic and quartic Higgs self-couplings, including those derived from the Standard Model Effective Field Theory (SMEFT), as well as scenarios involving derivative operators from Higgs Effective Field Theory (HEFT). Modified Higgs self-couplings typically lead to perturbative unitarity violation in at least the $2\to 3$ VBS; however, the corresponding energy scales of unitarity violation are often very high. Our analysis reveals that, in the case of modified Higgs potentials, $2\to4$ processes exhibit significantly lower energy scales of unitarity violation compared to $2 \to 3$ processes. This, combined with the fact that the cross sections of $2 \to 4$ processes increase with energy, suggests they generate more signal events at high energies, $\sqrt{\hat{s}} \gtrsim 2$ TeV, which could be achieved in future colliders. In contrast, for HEFT derivative interactions, higher multiplicity offers diminished benefits, as $2 \to 4$ cross sections are often smaller than those of related $2 \to 3$ processes within the valid energy range. This study shows that $2 \to 4$ VBS processes are particularly compelling for probing Higgs potential modification, including the cubic and quartic couplings, but are less advantageous when derivative interactions dominate.

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arXiv2024

Energy Growth in $V_LV_L\to V_LV_L,\ V_LV_Lh$ Scattering to Probe Higgs Cubic and HEFT Interactions

We compute the energy scales of perturbative unitarity violation in $V_L V_L \to V_L V_L h$ processes and compare them to $V_L V_L \to hhh$ process, where $V_L$ refers to a longitudinal mode of $Z$ or $W$ boson, and $h$ the Higgs boson. Using these energy scales, we determine which process is more sensitive to potential modifications in the Higgs sector at high-energy colliders. Within the Higgs Effective Field Theory (HEFT), we consider the Higgs cubic coupling and other interactions with and without derivatives. Any HEFT interactions predict the perturbative unitarity violation at a finite scale, and in a generic case, the minimalistic process is $2\to 3$ scattering. Our analysis reveals that the energy scales for unitarity violation in $V_L V_L \to V_L V_L h$ and $V_L V_L \to hhh$ processes are similar across all scenarios considered. If the backgrounds are similar, $V_L V_L h$ final states are more feasible because $V_L V_L h$ has higher branching ratios in cleaner decay modes than $hhh$. We also investigate HEFT derivative interactions derived from various UV models. In these cases, both $V_L V_L \to V_L V_L$ and $V_L V_L \to hh$ processes exhibit unitarity violating behavior. We demonstrate that the energy scales for unitarity violation in $V_L V_L$ final states are comparable to or even lower than those in the $hh$ final state.

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arXiv2024

Decoding the $B \to K νν$ excess at Belle II: kinematics, operators, and masses

An excess in the branching fraction for $B^+ \to K^+ νν$ recently measured at Belle II may be a hint of new physics. We perform thorough likelihood analyses for different new physics scenarios such as $B \to KX$ with a new invisible particle $X$, or $B\to Kχχ$ through a scalar, vector, or tensor current with $χ$ being a new invisible particle or a neutrino. We find that vector-current 3-body decay with $m_X \simeq 0.6$ GeV - which may be dark matter - is most favored, while 2-body decay with $m_X \simeq 2$ GeV is also competitive. The best-fit branching fractions for the scalar and tensor cases are a few times larger than for the 2-body and vector cases. Past BaBar measurements provide further discrimination, although the best-fit parameters stay similar.

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arXiv2023

MeV Sterile Neutrino in light of the Cabibbo-Angle Anomaly

A modified neutrino sector could imprint a signature on precision measurements of the quark sector because many such measurements rely on the semi-leptonic decays of the charged currents. Currently, global fits of the determinations of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements point to a $3σ$-level deficit in the first-row CKM unitarity test, commonly referred to as the Cabibbo-angle anomaly. We find that a MeV sterile neutrino that mixes with the electron-type neutrino increases the extracted $|V_{ud}|$, accommodating the Cabibbo-angle anomaly. This MeV sterile neutrino affects the superallowed nuclear $β$ decays and neutron decay, but it barely modifies the other measurements of the CKM elements. While various constraints may apply to such a sterile neutrino, we present viable scenarios within an extension of the inverse seesaw model.

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arXiv2023

Light Dilaton in Rare Meson Decays and Extraction of its CP Property

The dilaton $ϕ$ is a pseudo-Nambu-Goldstone boson associated with the spontaneous breaking of scale invariance in a nearly conformal theory, and couples to the trace of the stress-energy tensor. We analyze experimental constraints on a light dilaton with mass in the MeV-GeV range from rare meson decays. New model-independent inclusive bounds for the $b \to s ϕ$ transition largely exclude the parameter space of a light dilaton that could explain the muon $g-2$ anomaly. Despite similarities between a dilaton and a Higgs-portal scalar, the dilaton-photon coupling is enhanced compared to the Higgs-portal scalar due to contributions from loops of the conformal sector. Consequently, the shortened lifetime of the dilaton relaxes bounds from $K \to π$ + invisible searches at the NA62 experiment and constraints from the Big Bang Nucleosynthesis. We utilize this fact to search for the dilaton signature at a lepton collider such as the ongoing Belle II experiment. Further, we demonstrate how to extract the CP property of the dilaton using the variation of the differential cross-section of $e^+ e^- \to e^+ e^- ϕ$ with the azimuthal angle between the outgoing leptons.

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arXiv2023

Probing long-lived axions at the KOTO experiment

While the main goal of the J-PARC KOTO experiment is to measure the rare decay $K_L \to π^0 ν\bar ν$, the unique setup of KOTO raises the possibility to search for physics beyond the Standard Model, in an attempt to probe parts of the parameter space which are not covered by other experiments. In this paper, we test the possibility of using KOTO to search for heavy QCD axions, or axion-like particles, a well-motivated extension of the Standard Model emerging in a variety of models. In particular, we estimate the sensitivity of the current KOTO setup as well as the KOTO Step-2 for various benchmark scenarios of axion coupling to the Standard Model. We find that KOTO Step-2 can probe new regions in the parameter space, while KOTO with its current form can only reaffirm the existing bounds. The obtained axion datasets are available as an update of the public code of the ALPINIST framework, including implementation of KOTO setups in the simulation, allowing for interpretation of various analyses as searches for axions in custom models.

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arXiv2023

New Physics Searches at Kaon and Hyperon Factories

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

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arXiv2023

Heavy QCD Axion in $b\to s$ transition: Enhanced Limits and Projections

We study a "heavy" QCD axion whose coupling to the standard model is dominated by $a G \widetilde{G}$ but with $m_a \gg m_πf_π/ f_a$. This is well motivated as it can solve the strong CP problem while evading the axion quality problem. It also poses interesting challenges for its experimental search due to its suppressed couplings to photons and leptons. Such axion with mass around a GeV is kinematically inaccessible or poorly constrained by most experimental probes except B-factories. We study $B \to K a$ transitions as a powerful probe of the heavy QCD axion by performing necessary 2-loop calculations for the first time, together with some improvement on the existing analysis strategy. We find some of the existing limits are enhanced by at least an order of magnitude. We also demonstrate that the bounds are robust against unknown UV physics. For forthcoming data sets of the Belle II experiment, we provide a projection that $f_a$ of a few TeV is within its future reach, which is relevant to the quality problem.

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arXiv2023

First Evaluation of Meson and $τ$ 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 $τ$ 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

Heavy QCD Axion at Belle II: Displaced and Prompt Signals

The QCD axion is a well-motivated addition to the standard model to solve the strong $C\!P$ problem. If the axion acquires mass dominantly from a hidden sector, it can be as heavy as $O(1)$ GeV, and the decay constant can be as low as $O(100)$ GeV without running into the axion quality problem. We propose new search strategies for such heavy QCD axions at the Belle II experiment, where the axions are expected to be produced via $B\to K a$. We find that a subsequent decay $a\to 3π$ with a displaced vertex leads to a unique signal with essentially no background, and that a dedicated search can explore the range $O(1$-$10)$ TeV of decay-constant values. We also show that $a\to γγ$ can cover a significant portion of currently unexplored region of $150 \lesssim m_a \lesssim 500$ MeV.

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arXiv2022

Feebly-Interacting Particles:FIPs 2020 Workshop Report

With the establishment and maturation of the experimental programs searching for new physics with sizeable couplings at the LHC, there is an increasing interest in the broader particle and astrophysics community for exploring the physics of light and feebly-interacting particles as a paradigm complementary to a New Physics sector at the TeV scale and beyond. FIPs 2020 has been the first workshop fully dedicated to the physics of feebly-interacting particles and was held virtually from 31 August to 4 September 2020. The workshop has gathered together experts from collider, beam dump, fixed target experiments, as well as from astrophysics, axions/ALPs searches, current/future neutrino experiments, and dark matter direct detection communities to discuss progress in experimental searches and underlying theory models for FIPs physics, and to enhance the cross-fertilisation across different fields. FIPs 2020 has been complemented by the topical workshop "Physics Beyond Colliders meets theory", held at CERN from 7 June to 9 June 2020. This document presents the summary of the talks presented at the workshops and the outcome of the subsequent discussions held immediately after. It aims to provide a clear picture of this blooming field and proposes a few recommendations for the next round of experimental results.

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arXiv2020

Light quark Yukawas in triboson final states

Triple heavy vector boson production, $p p \to VVV$ $(V = W, Z)$, has recently been observed for the first time. We propose that precision measurements of this process provide an excellent probe of the first generation light quark Yukawa couplings. Modified quark interactions with the off-shell Higgs in this process lead to a rapid growth of the partonic cross sections with energy, which manifests in an enhanced $p_T$ distribution of the final state leptons and quarks. We quantify this effect and estimate the present and future 2$σ$ sensitivity to the up, down, and strange Yukawas. In particular, we find that HL-LHC can reach $\mathcal{O}(400)$ sensitivity to the down Yukawa relative to the Standard Model value, improving the current sensitivity in this process by a factor of $10$, and which can be further improved to $\mathcal{O}(30)$ at FCC-hh. This is competitive with and complementary to constraints from global fits and other on-shell probes of the first generation Yukawas. The triboson sensitivity at HL-LHC corresponds to probing dimension-6 SMEFT operators suppressed by an $\mathcal{O}(1)$ TeV scale, similarly to other LHC Higgs probes.

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arXiv2020

A solar origin of the XENON1T excess without stellar cooling problems

Solar interpretations of the recent XENON1T excess events, such as axion or dark photon emissions from the sun, are thought to be at odds with stellar cooling bounds from the horizontal branch stars and red giants. We propose a simple effective field theory of a dark photon in which a $Z_2$ symmetry forbids a single dark photon emission in the dense stars, thereby evading the cooling bounds, while the $Z_2$ is spontaneously broken in the vacuum and sun, thereby explaining the XENON1T excess. The scalar responsible for the $Z_2$ breaking has an extremely flat potential, but the flatness can be maintained under quantum corrections. The UV completion of the EFT generally requires the existence of new electrically charged particles with sub-TeV masses with $O(1)$ couplings to the dark photon, offering the opportunity to test the scenario further and opening a new window into the dark sector in laboratory experiments.

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arXiv2020

Probing Muonic Forces and Dark Matter at Kaon Factories

Rare kaon decays are excellent probes of light, new weakly-coupled particles. If such particles $X$ couple preferentially to muons, they can be produced in $K\to μνX$ decays. In this letter we evaluate the future sensitivity for this process at NA62 assuming $X$ decays either invisibly or to di-muons. Our main physics target is the parameter space that resolves the $(g-2)_μ$ anomaly, where $X$ is a gauged $L_μ-L_τ$ vector or a muon-philic scalar. The same parameter space can also accommodate dark matter freeze out or reduce the tension between cosmological and local measurements of $H_0$ if the new force decays to dark matter or neutrinos, respectively. We show that for invisible $X$ decays, a dedicated single muon trigger analysis at NA62 could probe much of the remaining $(g-2)_μ$ favored parameter space. Alternatively, if $X$ decays to muons, NA62 can perform a di-muon resonance search in $K\to 3 μν$ events and greatly improve existing coverage for this process. Independently of its sensitivity to new particles, we find that NA62 is also sensitive to the Standard Model predicted rate for $K \to 3μν$, which has never been measured.

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