KEK Theory Meeting on Particle Physics Phenomenology (KEK-PH2026winter)

16 Feb 2026, 01:00 → 19 Feb 2026, 17:00 Asia/Tokyo

Seminar hall (San-go-kan (Building #3))

The annual Theory Meeting on Particle Physics Phenomenology (KEK-­PH2026winter) in joint with IPNS workshop will be held at KEK between Feb 16th -19th. This meeting covers many topics in Particle Physics and Cosmology, from the Standard Model and models beyond the Standard Model to particle cosmology and astrophysics. We plan to discuss recent developments in modeling Physics Beyond the Standard Model with their phenomenological aspects and also learn about the latest results and status of various ongoing and planned experiments. 

The meeting includes talks by invited speakers and short talks by participants. All the talks will be given in English. We aim to provide an opportunity, not only for experts but also for master/Ph.D course students, to learn recent developments in particle physics phenomenology and to activate intensive discussions.

Invited speakers

Yohei Ema (U. of Florida)
Yannis Georis (IPMU)
Julia Harz (Mainz U.)
Shunsaku Horiuchi (ISCT, Tokyo)
Ryusuke Jinno (Kobe U.)
Shinya Kanemura (U. Osaka)
Kentaro Kawade (Shinshu U.)
Zoltan Ligeti (LBNL)
Yu Nakahama (KEK)
Yu Nakazawa (KEK)
Mihoko Nojiri (KEK)
Toshiya Namikawa (Cambridge U. & IPMU)
Christiane Scherb (LBNL)
Michael Spannowsky (Durham U.)
Kohsaku Tobioka (Florida State U.)
Daichi Yoshikawa (Kyushu U.)
Jure Zupan (Cincinnati U.)

Important dates

Deadline for workshop registration/KEK User registration/registration fee payment: February 1st (Sunday)  closed
For short talks and travel support applications: January 5th (Monday)
For those who need visa documents from KEK: December 19th (Friday)

A limited amount of support for local expenses (travel within Japan and KEK dormitory expenses) is available primarily for young participants. We cannot cover International travel. Selections will be made if there are too many applicants.

Registration fee: 2,500JPY

Local organizers

Motoi Endo, Kaori Fuyuto, Toru Goto, Ahmed Hammad, Akimasa Ishikawa, Takashi Kaneko, Kohtaroh Miura, Kyohei Mukaida, Yushi Mura, Mihoko Nojiri, Ippei Obata, Yasuhiro Okada, Yutaka Sakamura, Tenta Tsuji, Hidenaga Watanabe

Secretary

Tomoko Numata

Monday, 16 February

10:00 → 10:10 opening

opening.pdf

10:10 → 11:30 plenary: Michael Spannowsky, Jure Zupan

Convener: Motoi Endo

10:10 Quantum Machine Learning for High-Energy Physics

Speaker: Michael Spannowsky

10:50 Light new physics and rare tau decays

Speaker: Jure Zupan

KEK_PH_Zupan.pdf

11:30 → 13:00 lunch

13:00 → 14:20 plenary: Yu Nakazawa, Zoltan Ligeti

Convener: Takashi Kaneko

13:00 Current Status and Future Prospects of Belle II

Speaker: Yu Nakazawa

20260216.yunakazawa.kekph.v2.pdf

13:40 B physics: known and unknown

Speaker: Zoltan Ligeti

Ligeti.pdf

14:20 → 14:40 coffee

14:40 → 15:20 plenary: Kentaro Kawade

Convener: Kaori Fuyuto

14:40 The EIC and ePIC Experiments: Current Status and Synergies with High-Energy Physics

Speaker: Kentaro Kawade

15:20 → 16:30 coffee

16:30 → 17:30 parallel session A: Cosmo

Convener: Fuminobu Takahashi

16:30 Walking-Dilaton Hybrid Inflation in a Model with Dynamical Scalegenesis

We propose a hybrid inflationary scenario based on eight-flavor hidden QCD with the hidden colored fermions being in part gauged under B−L gauge symmetry. This hidden QCD is almost scale-invariant, so-called walking, and predicts the light scalar meson (the walking dilaton) associated with the spontaneous scale breaking, which develops the Coleman-Weinberg (CW) type potential as the consequence of the nonperturbative scale anomaly, hence plays the role of an inflaton of the small-field inflation. The B−L Higgs is coupled to the walking dilaton inflaton, which is dynamically induced from the so-called bosonic seesaw mechanism. We explore the hybrid inflation system involving the walking dilaton inflaton and the B−L Higgs as a waterfall field. We find that observed inflation parameters tightly constrain the B−L breaking scale as well as the walking dynamical scale to be around 10^9GeV and 10^{14}GeV, respectively, so as to make the waterfall mechanism worked. The lightest walking pion mass is then predicted to be around 500GeV.

Speaker: HIROYUKI ISHIDA

KEK-PH2026_HiroyukiISHIDA.pdf

16:50 Emission of Nambu-Goldstone bosons from semilocal string networks

Cosmic strings arise from the spontaneous breaking of a $U(1)$ symmetry, which can be either global or local, leading to distinct classes of string solutions. Their energy-loss mechanisms are known to differ significantly, which results in qualitatively different spectra of particles emitted from the string networks. Semilocal strings arise when global and local symmetries are simultaneously broken and combine characteristic features of both local and global strings. As a result, their particle-emission properties cannot be straightforwardly characterized as those of global or local string networks. In this talk, we present results from numerical lattice simulations of semilocal string networks, focusing on their particle-emission properties for the first time. We find that semilocal string networks can efficiently emit Nambu-Goldstone bosons, with particle-emission properties similar to those of axion emission from global string networks. Motivated by this result, we discuss a scenario in which the Nambu-Goldstone bosons acquire a soft-breaking mass and account for the observed dark matter abundance via non-thermal production from the string networks. This work is based on arXiv: 2510.07894.

Speaker: Yukihiro Kanda

KEKPH2026_Kanda.pdf

17:10 Relaxing Cosmological Constraints on the Minimal $U(1)_{L_\mu - L_\tau}$ Model with One-Loop Corrections

Recently, Ref.~\cite{...} showed that the minimal gauged $U(1)_{L_\mu - L_\tau}$ model is constrained by cosmological bounds on the sum of neutrino masses.
In this work, we compute the one-loop corrections to the neutrino mass matrix in this model and examine whether these cosmological constraints can be relaxed.

Speaker: Jun Miyamoto

Jun_Miyamoto_2026_KEKPH.pdf

16:30 → 17:30 parallel session B: Higgs

Convener: Kei Yagyu

16:30 Higgs production in association with a Z boson at TeV-scale lepton colliders

We study the neutrino-associated Zh production process at lepton colliders, whose cross section exceeds that of Zh production in the multi-TeV energy regime.
The scattering amplitudes are classified into three main groups according to the topology of the corresponding Feynman diagrams, and their interference patterns are systematically analyzed.
We demonstrate that the delicate gauge cancellations appearing at high energies in the unitary gauge are absent in the recently proposed Feynman-diagram gauge, which enables a transparent interpretation of physical distributions in terms of contributions from each subgroup.
In particular, we show that the interference among amplitudes in the Feynman-diagram gauge provides a clear physical understanding of the observed kinematic features.

Speaker: Hiroyuki Furusato

KEKPH2026_Furusato.pdf

16:50 Chasing Long-lived Scalars at the Future Lepton Colliders

In this talk, I will discuss novel strategies to search for long-lived scalars at future lepton colliders in the context of the Type-II seesaw model. In the low-mass range, and for an appropriate choice of the triplet vev, these scalars can become long-lived, leading to displaced-vertex signatures. Depending on the proposed center-of-mass energy, we consider the pair production of these scalars at the ILC and at future muon colliders. Taking into account the relevant theoretical and experimental constraints, we explore this possibility in multi-lepton plus missing-energy final states. While displaced-vertex signatures highlight the long-lived nature of the scalars, the invariant-mass distribution of same-sign dileptons can be used to discover the non-standard charged Higgs at the ILC and muon colliders.

Speaker: Nivedita Ghosh

NiveditaGhosh.pdf

17:10 Impact of quark flavor violating SUSY on h(125) decays at future lepton colliders

We study the CP-even neutral Higgs boson decays $h^0 \to c \bar{c}, b\bar{b}, b \bar{s}, \gamma \gamma, g g$ in the Minimal Supersymmetric Standard Model (MSSM) with general quark flavor violation (QFV) due to squark generation mixings, identifying the $h^0$ as the Higgs boson with a mass of 125 GeV. We compute the widths of the $h^0$ decays to $c \bar c, b \bar b, b \bar s$ at full one-loop level. For the $h^0$ decays to $\gamma \gamma$ and $g g$ we compute the widths at NLO QCD level. For the first time, we perform a systematic MSSM parameter scan for these widths respecting all the relevant theoretical and experimental constraints, such as those from B-meson data, and the 125 GeV Higgs boson data from recent LHC experiments, as well as the limits on Supersymmetric (SUSY) particle (sparticle) masses from the LHC experiments. We also take into account the expected sparticle mass limits from the future HL-LHC experiment in our analysis. In strong contrast to the usual studies in the MSSM with Minimal Flavor Violation (MFV), we find that the deviations of these MSSM decay widths from the Standard Model (SM) values can be quite sizable and that there are significant correlations among these deviations. All of these sizable deviations in the $h^0$ decays are mainly due to large scharm-stop mixing and large sstrange-sbottom mixing. Such sizable deviations from the SM can be observed at high signal significance in future lepton colliders such as ILC, CLIC, CEPC, FCC-ee and muon collider EVEN AFTER the failure of SUSY particle discovery at the HL-LHC. In case the deviation pattern shown here is really observed at the lepton colliders, then it would strongly suggest the discovery of QFV SUSY (the MSSM with general QFV).
Authors: H. Eberl, K. Hidaka, E. Ginina;
Reference: arXiv:2511.20523 (to be published in Phys. Rev. D);
Link: https://arxiv.org/pdf/2511.20523

Speaker: Keisho Hidaka

Hidaka_h125@KEKPH2026_Fina.pdf

Hidaka_h125@KEKPH2026_Fina.pptx

17:30 → 19:30 reception

Tuesday, 17 February

09:00 → 10:20 plenary: Shunsaku Horiuchi, Toshiya Namikawa

Convener: Ippei Obata

09:00 Searching for dark matter in the gamma-ray sky

Speaker: Shunsaku Horiuchi

horiuchi_KEK2026.pdf

09:40 A brief overview of ACT DR6 results

Speaker: Toshiya Namikawa

10:20 → 10:40 coffee

10:40 → 11:40 parallel session A: Theory

Convener: Kohtaroh Miura

10:40 Analysis of Monopoles Associated with Multi-Step Spontaneous Breaking of Gauge Symmetries

The ’t Hooft–Polyakov monopole is absent in the Standard Model, yet Cho and Maison revealed that the electroweak sector admits a spherically symmetric magnetic monopole solution. The resulting Cho–Maison monopole exhibits unresolved theoretical issues, most notably an energy divergence at the spatial origin. In this work, we analyze a scenario in which a ’t Hooft–Polyakov monopole arising in a high-energy completion is mapped onto the Cho–Maison monopole in the low-energy effective theory, thereby providing a unified understanding of monopole configurations across energy scales.

Speaker: Fukutaro MIYA

MIYA_KEK_Ph2026.pdf

11:00 Supersymmetric Gauge Theories with Confining Phases

Confinement is one of the unresolved problems in modern physics. rgies. The analysis of confinement requires non-perturbative approaches due to asymptotic freedom. Supersymmetric gauge theories provide a powerful framework in which such non-perturbative analyses can be carried out. They are more tractable than their non-supersymmetric counterparts.In particular, the holomorphy of the superpotential enables various exact calculations, including non-perturbative effects. Motivated by the goal of deepening our understanding of the confinement mechanism, we classify supersymmetric gauge theories that possess confining phases in the strict sense. Then, we identify condensation of some operators in each case, which must be related to confinement. If possible, we also discuss the consequences of supersymmetry breaking in these theories.

Speaker: Riku Ishikawa

ishikawa_kekph.pdf

10:40 → 11:40 parallel session B: Cosmo

Convener: Yohei Ema

10:40 Thermal precondensation in gauge-fermion theories and its cosmological implications

Speaker: Alvaro Pastor Gutierrez

AlvaroPastorGutierrez_precondensation_2_26.pdf

11:00 Screening Effects of Finite-Mass Fermions in Monopole Baryogenesis Scenario

To investigate the finite mass effects in recently proposed monopole baryogenesis scenario, this study analyzes screening effect for the electric monopole charge, induced by CP-violating theta term, within a 't Hooft-Polyakov monopole background. Utilizing partial wave expansion and bosonized effective field theory for the massive charged fermions, we demonstrate that fermion finite masses lead to distinct screening behaviors compared to the original scenario, which provides crucial insights into baryogenesis mechanisms with realistic fermion dynamics.

Speaker: Yukito Kato

yukito_kato_KEK-PH2026winter.pdf

11:40 → 13:00 lunch

13:00 → 14:20 plenary: Shinya Kanemura, Yannis Georis

Convener: Yushi Mura

13:00 Extended Higgs sectors and a scenario of electroweak baryogenesis

Speaker: Shinya Kanemura

KEK-PH2026_Kanemura.pdf

13:40 Testable leptogenesis with three right-handed neutrinos

Speaker: Yannis Georis

KEKPH_YannisGeoris.pdf

14:20 → 14:50 photo, coffee

14:50 → 16:10 parallel session A: DM

Convener: Shunsaku Horiuchi

14:50 Phenomenology of electroweakly interacting spin-1 dark matter with Sommerfeld enhancement

Weakly Interacting Massive Particles (WIMPs) are leading candidates for dark matter and have been widely studied. However, direct detection experiments have placed stringent constraints on many WIMP models. Some scenarios, such as Higgsino, Wino, and minimal dark matter, can evade these constraints. In these models, the dark sector contains an SU(2)_L multiplet, and the dark matter candidate is one of its components. Such candidates are typically assumed to be either spin-0 or spin-1/2. In this talk, I focus on a spin-1 electroweakly interacting dark matter model proposed in Ref. 2004.00884 and discuss its phenomenology, including the effects of Sommerfeld enhancement. We determine the mass spectrum of the new particles required to reproduce the measured value of the dark matter energy density and also discuss prospects for indirect detection.

Speaker: Tomohiro Abe

kek-ph2026_0217-abe-tomohiro.pdf

15:10 Minimal Dark Matter in the sky: updated Indirect Detection probes

Minimal Dark Matter is among the simplest and most predictive Dark Matter frameworks, with the Majorana SU(2) 5-plet as its smallest accidentally stable real representation. We present a comprehensive reassessment of its indirect-detection signals. The γ-ray flux from both Sommerfeld-enhanced annihilations and bound-state formation is calculated, incorporating next-to-leading-order corrections and next-to-leading-log resummation of the relevant electroweak effects. In the Milky Way halo, bound-state formation dominates the flux near 100 GeV. The corresponding low-energy spectrum is used to place constraints based on Fermi-LAT observations of Galactic diffuse emission, while the high-energy part of the spectrum is employed to forecast the required observation time for several of the Milky Way's dwarf spheroidal galaxies using the Cherenkov Telescope Array Observatory (CTAO). Fermi-LAT data strongly disfavor the lower edge of the thermal mass window, even under conservative assumptions about the inner Galaxy density profile. Furthermore, several hundred hours of forthcoming CTAO observations of northern dwarfs should be sufficient to probe the central mass value.

Speaker: Mohammad Aghaie

MDM and its Indirect detection.pdf

15:30 Dark matter explanations for the neutrino emission from the Seyfert galaxy NGC 1068

We investigate the possibility that the high-energy neutrino flux observed from the Seyfert galaxy NGC 1068 originates from dark matter annihilations within the density spike surrounding the supermassive black hole at its center. The comparatively lower gamma-ray flux is attributed to a dark sector that couples predominantly to Standard Model neutrinos. To explain the absence of a corresponding neutrino signal from the center of the Milky Way, we propose two scenarios: (i) the disruption of the dark matter spike at the Milky Way center due to stellar heating, or (ii) the annihilation into a dark scalar that decays exclusively into neutrinos, with a decay length longer than the size of the Milky Way but shorter than the distance from Earth to NGC 1068.

Speaker: Kensuke Akita

KEK-PH2026_Akita_v2.pdf

15:50 Light neutrinophilic thermal dark matter

No signal of the electroweak (EW) scale WIMP has been observed so far, and the lighter WIMP, whose mass is much lighter than the EW scale, attracts attention. However, light WIMP receives severe constraints from cosmological (CMB and BBN) observation when the WIMP couples mainly to electromagnetically interacting particles. We constructed a model where the light WIMP interacts mainly with harmless particles (i.e., neutrinos) in a simple B-L extension of the standard model. It turns out that the model evades not only the cosmological constraint but also those from dark matter searches, such as the direct, indirect, and collider dark matter detections while explaining the observed dark matter abundance vis the well-known freeze-out mechanism and solving the small-scale crisis vis its self-interaction.

Speaker: Tatsuya Aonashi

tatsuya_aonashi.pdf

14:50 → 16:10 parallel session B: Higgs

Convener: Shinya Kanemura

14:50 Two Higgs doublet models with a new U(1) gauge symmetry

In this talk, we discuss two Higgs doublet models in which a new U(1) gauge symmetry is introduced. We investigate if these models are allowed by current phenomenological data without introducing a scalar field except for two Higgs doublet ones. We find they are excluded by constraints from scalar boson decays associated with new gauge boson Z'. Then a dark vector-like fermion is introduced to modify branching ratio of Z' and we searched for allowed parameter region taking all the phenomenological constraints into account. Finally we show allowed region that can avoid all the constraints.

Speaker: Takaaki Nomura

Takaaki-Nomura0217.pdf

15:10 Testing the Higgs triplet model via 125GeV Higgs boson decays with radiative corrections

The complex Higgs triplet model (CHTM) can explain the neutrino masses via the Type II seesaw mechanism. A feature of CHTM is that the electroweak rho parameter (\rho) deviates from unity at the tree-level. Thereby, how one renormalize electroweak sector is different from models with \rho=1. In this talk, we discuss imact of the radiative corrections to the 125 GeV Higgs boson decays in CHTM. We demonstrate that the CHTM is distinguishable from other simple extended Higgs models by investigating the pattern of deviations from the standard model in the Higgs decays.

Speaker: Kodai Sakurai

KS_KEK-PH.pdf

15:30 Probing spontaneous CP-violation through precision Higgs observables

Spontaneous CP-violation (SCPV) is one of the interesting possibilities for explaining observed CP-violation. As a simple realization of SCPV, we consider 2 Higgs doublet models (2HDMs), where complex phases of the Higgs doublets can be a source of SCPV. We discuss two important phenomenological consequences of the SCPV 2HDMs,
i.e., non-decoupling nature of extra Higgs bosons and constrained Yukawa structure. We also discuss how we can explore such a scenario at High-Luminosity LHC.

Speaker: Kei Yagyu

KEKPH_2026_Yagyu.pdf

16:10 → 16:40 coffee

16:40 → 17:40 parallel session A: DM

Convener: Toshiya Namikawa

16:40 Forbidden Dark Matter with Sommerfeld Enhancement

We discuss the Sommefeld enhancement of the annihilation cross section of dark matter into heavier unstable particles which have long-range interaction with each other. Since annihilation products become non-relativistic near kinematical threshold, the wave function is modified from the plain wave, and significant enhancement of the annihilation cross section can be induced. In this talk, we discuss the formulation of the Sommerfeld enhancement from the annihilation products, and show that the finite decay width of the annihilation products induces resonances of bound-states near threshold in analogy with top-antitop pair production.

Speaker: Takumu Yamanaka

KEKph2026_yamanaka (2).pdf

17:00 Asymmetric Dark Matter from Low-Scale Spontaneous Leptogenesis

"We investigate a novel type of asymmetric dark matter (ADM) model in which the dark matter asymmetry and the baryon asymmetry in our universe (BAU) are produced simultaneously via low-scale spontaneous leptogenesis, where the mass scale of right-handed neutrino is much lower than the Davidson-Ibarra bound $M_1 \ll 10^{9}~\mr{GeV}$.
In our scenario, both asymmetries are predominantly sourced by a dynamical $CP$ phase, namely the majoron.
Its kinetic misalignment provides a sufficiently large, time-dependent effective $CP$ phase, allowing efficient asymmetry production even for low-scale right-handed neutrinos.
In our framework, the sources of $CP$ violation responsible for the BAU and ADM are correlated with each other, leading to a predictive relation for the dark matter mass. In particular, when the dark matter asymmetry reaches its equilibrium value before freeze-out, the dark matter mass is typically predicted to lie in the range
$\mathcal{O}(0.1)~\mathrm{GeV} \lesssim m_{\chi} \lesssim \mathcal{O}(100)~\mathrm{GeV}$,
which lies within the sensitivity of direct detection experiments.
On the other hand, if the dark matter asymmetry does not reach its equilibrium value due to weak coupling, the allowed mass range extends over a broader interval,
$\mathcal{O}(0.1)~\mathrm{GeV} \lesssim m_{\chi} \lesssim \mathcal{O}(10)~\mr{TeV}$."

Speaker: Hiroki Takahashi

Hiroki_Takahashi_KEKPH2026.pdf

16:40 → 17:40 parallel session B: Flavor

Convener: Kohsaku Tobioka

16:40 Possible scenarios for baryogenesis in the presence of a lepton number violating operator in the rare meson decays

We discuss lepton number violating operators motivated by the flavor-changing rare meson decays, B -> Kvv and K ->πvv.
They can erase a pre-existing baryon asymmetry via electroweak sphaleron, and we analyze their impacts on baryon number washout in the early Universe,
We also discuss other topics related to the lepton number violation, the Majorana masses of the active neutrinos and the neutrinoless double beta decay, which are induced by such operators.
Combining constraints from those experimental data, we identify regions of parameter space where sizable effects in the rare meson decays are compatible with successful baryogenesis.

Speaker: Yushi Mura

KEKph2026winter_mura.pdf

17:00 Rare meson decay constraints on dark photon and the like

We evaluate constraints from flavor changing rare meson decays to a light vector boson X, followed by the decay of the on-shell X into the SM fermions. Our theoretical evaluation of the branching ratio of charged B meson decay and charged kaon decay is compared to experimental results, and we derive new constraints for dark photon, U(1)B−L​ and U(1)R​ models.

Speaker: Osamu Seto

osamu_seto.pdf

17:20 Radiative Corrections to $D_s \to \ell \nu$ and CKM Unitarity Test

Recently, a violation of the CKM unitarity condition has been reported in the latest charm-meson data and the latest lattice results, once the universal electroweak correction is taken into account. In this talk, we analytically derive for the first time the complete one-loop electroweak (EW) and QED corrections to the $D_{s}^+ \to \ell^+ \nu_\ell$ decays for $\ell = \mu, \tau$. Our analysis incorporates both short-distance EW-QED corrections, which are beyond the leading-logarithmic approximation (the so-called Sirlin factor), and long distance soft photon corrections depending on the maximum total energy of undetected photons with their resummation. Although the inclusive photon QED corrections to the meson leptonic decays are well known, they do not match the actual measurement circumstances in $D_s^+ \to \mu^+ \nu_\mu$. We find $|V_{cs}|_{D_s} = 0.991 \pm 0.007 $ from the latest data on $D_s^+$ leptonic decays. We show that properly including these radiative corrections is essential to bring the second-column CKM unitarity tests into agreement with the Standard Model expectation. The study emphasizes that the current limiting factor in confirming CKM unitarity is the precision of QED corrections, and it points out that improving lattice simulations, taking the QED corrections into account, would be desirable for a more robust confirmation.

Speaker: Kota Sasaki

KEKPH2026_Kota_Sasaki.pdf

Wednesday, 18 February

09:00 → 10:20 plenary: Julia Harz, Yohei Ema

Convener: Kyohei Mukaida

09:00 Advancing Standards in Early Universe Physics: The Impact of Bound States & Thermal Effects

Speaker: Julia Harz

AnnualTheoryMeetingKEK_2026.pdf

09:40 Cutting rule for cosmological correlators and its applications

Speaker: Yohei Ema

Yohei_Ema_KEKPH2026.pdf

10:20 → 10:40 coffee

10:40 → 11:40 parallel session A: DM

Convener: Julia Harz

10:40 Sommerfeld Enhancement in the light of Halo gamma-ray excess

We examine Sommerfeld enhancement in dark matter annihilation as a potential origin of the halo-like gamma-ray excess near E ~ 20 GeV reported by Totani. A minimal model with a light CP-even scalar mediator naturally produces a velocity-dependent annihilation cross section consistent with thermal freeze-out, the Milky Way excess, and limits from dwarf spheroidal galaxies. We discuss future prospects on this issue with other Galactic gamma-ray and cosmic ray excess.

Speaker: Yongsoo Jho

kek-ph2026_talk_yongsoo_jho.pdf

11:00 Composite Asymmetric Dark Matter from Primordial Black Holes

We investigate a cogenesis scenario for composite asymmetric dark matter framework: a dark sector has a similar strong dynamics to quantum chromodynamics in the standard model, and the dark-sector counterpart of baryons is the dark matter candidate. The Hawking evaporation of primordial black holes plays the role of a source of heavy scalar particles whose CP-violating decay into quarks and dark quarks provides particle-antiparticle asymmetries in baryons and dark matter, respectively. Primordial black holes should evaporate after the electroweak phase transition and before the big-bang nucleosynthesis for explaining the baryon asymmetry of the Universe and for consistent cosmology. We find that this scenario explains the observed values for both baryon and dark matter energy densities when the heavy scalar particles have a mass of 10^6 - 10^9 GeV and the primordial black holes have masses of 10^7 - 10^9 g.

Speaker: Takumi Kuwahara

KEK-PH2026_Kuwahara.pdf

10:40 → 11:40 parallel session B: Cosmo

Convener: Yannis Georis

10:40 Quintessence with a sudden transitional feature and its late-time cosmological signals

Combinations of recent cosmological observations, including Dark Energy Spectroscopic Instrument, show hints of a dynamical nature for dark energy. While the data suggest the possibility of the phantom crossing, it is worth exploring thoroughly quintessence models. As phenomenological parametrizations of the equation-of-state parameter $w(a)$ with a sharp transitional feature fit the data well, we study the realization of such models by quintessence. In the late Universe, the quintessence field begins to oscillate abruptly, changing the behavior of $w$. Naturally, such a model entails tachyonic instability, and particle production modifies $w$. We perform numerical lattice simulations to study the time dependence of $w$. In addition, the violent particle production produces significant density perturbations and the stochastic gravitational-wave background, whose characteristic scale depends on the mass scale of the quintessence around the minimum of the potential.
We discuss the observability of these late-time cosmological signals through cosmic microwave background, quasar astrometry, pulsar timing arrays, and others.

Speaker: Takahiro Terada

KEK-PH2026winter_Feb18_Terada.pdf

11:00 Gravitational Wave signatures of Reheating after Warm Axion Inflation

Before the Big Bang, the Universe experienced an era of accelerated expansion known as inflation.
Reheating is the process that bridges this inflationary phase to the hot Big Bang, and it also provides one of the few opportunities for information from the inflaton sector to survive into the later Universe. Although many details of the inflaton’s interactions are erased as the Universe thermalizes, gravitational waves offer a uniquely robust channel capable of preserving these microscopic imprints.

In this talk, I will show that when the inflaton is an axion coupled to a dark gauge sector, the reheating dynamics become inherently non-trivial. The axion’s Chern–Simons coupling activates strong gauge-field production, glueball formation, and a delayed transfer of energy into light degrees of freedom. A key consequence is that integrating out the heavy glueballs induces effective higher-curvature operators, enabling the inflaton to decay directly into gravitons—an effect that cannot arise in standard Einstein gravity. This mechanism leads to a characteristic enhancement of the high-frequency gravitational-wave background.
I will present the resulting gravitational-wave spectrum and discuss possible cosmological implications.

Speaker: Tenta Tsuji

GW Signatures of Reheating After Warm Axion Inflation KEK-PH.pdf

11:20 Quantum sensing of high-frequency gravitational waves with ion traps

Speaker: Ryoto Takai

RyotoTakai.pdf

11:40 → 13:00 lunch

13:00 → 14:20 plenary: Yu Nakahama, Daichi Yoshikawa

Convener: Mihoko Nojiri

13:00 Selected highlights from the LHC-ATLAS experiment

Speaker: Yu Nakahama

13:40 The recent progress from FASER, focusing on the Axion-Like Particle search and Dark Photon search

FASER (the ForwArd Search ExpeRiment) is an experiment at the CERN Large Hadron Collider (LHC) designed to measure Standard Model neutrinos and to search for light, weakly interacting Beyond the Standard Model (BSM) particles. These particles are produced at the ATLAS interaction point in the forward direction and subsequently decay inside the FASER detector.
In this talk, we present the latest progress from FASER on searches for dark photons and for axion-like particles (ALPs).

Speaker: Daichi Yoshikawa

KEK-PH.pdf

14:20 → 14:50 coffee

14:50 → 15:50 parallel session A: EW

Convener: Yasuhiro Okada

14:50 Three-body entanglement and nonlocality in particle decays

The investigation and collider measurement of quantum information generated in elementary particle interactions have attracted growing interest in recent years. To date, most studies have focused on bipartite systems, for example, spin correlations in top-quark pair production, while the rich structure of multipartite quantum correlations remains largely unexplored in high-energy physics.
In this talk, based on 2310.01477 and 2502.19470, I will present a novel framework for analysing genuine three-body quantum correlations in 1 \to 3 particle decays. In particular, I will discuss the monogamy of entanglement and demonstrate the presence of two distinct classes of nonlocality in tripartite quantum systems.

Speaker: Kazuki Sakurai

2026_02_18_Sakurai_KEK-PH (3).pdf

15:10 An Analytic Prescription for t-channel Singularities

The t-channel singularity is a divergence in the scattering amplitude which occurs when a stable particle propagating in t-channel scattering process becomes an on-shell state. Such situations appear either in the system of collider experiments or in the context of the cosmological particle production. No scheme which is generally applicable is known. In this work, we propose a new formulation to identify and remove the source of the divergence. The scheme is fully analytical and various applications can be expected. This work provides a valuable tool in this research field. This talk Based on Phys.Rev.D 112 (2025) 7, 076020.

Speaker: Ryusei Sato

2026_KEK-PH_Ryusei_V2.pdf

15:30 Searching for Leptoquarks at Future Circular Colliders

We study the sensitivities of the Future Circular Collider (FCC) under electron-positron collision and hadron-hadron collision scenarios for the search of Leptoquarks. In particular, we focus on the Z-factory mode and the high-pT mode. For the Z-factory mode, we summarize past analytical works on Z-boson decays introducing Leptoquarks at loop-level. For the high-pT mode, we update the current search status of Leptoquarks at the Large Hadron Collider(LHC) and project the sensitivity for the Future Circular Collider based on it. We also comment on the potential for FCC to probe flavour anomalies via leptoquarks.

Speaker: Christian Heiderijk

Christian_Heiderijk_KEKPH2026.pdf

14:50 → 15:50 parallel session B: ML

Convener: Mihoko Nojiri

14:50 Exploring Lepton Flavor Structures with a Latent Diffusion Model

In this talk, we present a model-independent analysis based on a latent diffusion model to address the flavor structure of leptons. The latent diffusion model combines a diffusion model with a variational autoencoder as a generative AI framework. By generating a wide variety of parameter sets consistent with experimental observations, we find non-trivial features characterizing lepton flavor structures.

Speaker: Haruto Kitagawa

KEKPH2026_HarutoKitagawa.pdf

15:10 Machine learning the likelihoods

Searches for BSM Physics have produced many theoretical ideas, but only a few can be directly tested at the LHC. Reinterpreting existing results is therefore essential for constraining a wider range of models. Independent of the specific reinterpretation method, the final step always involves statistical analysis and hypothesis testing. Accurate tests require detailed information on correlations between signal regions and detector-related nuisance parameters. Although ATLAS provides this information, using the full statistical model is computationally expensive. In this talk, I will present a project that builds NN-based surrogates of ATLAS statistical models. These surrogates reproduce log-likelihoods with high accuracy while reducing computation time by several orders of magnitude, making large parameter scans much more feasible.

Speaker: Rafal Maselek

RMaselek.pdf

15:30 Systematic mapping of $U(1)_{L_{e}-L_{mu}-L_{tau}}$ flavor model via reinforcement learning

Recent studies actively apply machine learning to the exploration of flavor physics. In analyzing the flavor structure of the lepton sector, we apply reinforcement learning to a $U(1)$ flavor model with $L_{e}-L_{mu}-L_{tau}$ charges. By testing multiple architectures to explore charge combinations, we develop a strategy to efficiently achieve high-precision solutions. It turns out that the proposed approach successfully finds parameter sets that reproduce the observed lepton masses and mixing angles.

Speaker: Satsuki Nishimura

KEK-PH2026_Satsuki_Nishimura.pdf

15:50 → 16:20 coffee

16:20 → 17:20 parallel session A: EW/Theory

Convener: Tomohiro Abe

16:20 The Electric Dipole Moment of electron induced by Electroweak Multiplets at full Three-loop

There has been remarkable progress in recent years in the electric dipole moment (EDM) measurements of electron using paramagnetic atom or molecule. In a previous study, we calculated the contribution to the electron EDM induced by the CP-violating Yukawa interaction of electroweak multiplets at the three-loop level in effective field theory. We found that this contribution might reach the sensitivity of future EDM experiments. However, this calculation involves an uncertainty relating to threshold corrections that cannot be evaluated in the effective field theory. In this study, we calculate the electron EDM induced by electroweak multiplets at full three-loop. As a result, we found that the threshold correction contributes at the same order as the result of the effective field theory and that the full result is larger than the previous study.

Speaker: Kiyoto Ogawa

KEKph_Ogawa.pdf

16:40 Gradient-flowed operator product expansion without IR renormalons

A long-standing problem concerns the question how to consistently combine perturbative expansions in QCD with power corrections in the context of the operator product expansion (OPE), since the former exhibit ambiguities due to infrared renormalons, which are of the same order as the power corrections. We propose to use the gradient flow time as a factorization scale and to express the OPE in terms of IR renormalon-free subtracted perturbative expansions and unambiguous matrix elements of gradient-flow regularized local operators. We show on the example of the Adler function and its leading power correction from the gluon condensate that this method dramatically improves the convergence of the perturbative expansion.

Speaker: Hiromasa Takaura

KEKPH2026takaura.pdf

17:00 Toponium at the LHC

We review the signature of toponium formation at the LHC. A color-singlet bound state effect at the threshold region is included in the MonteCarlo simulation. The top pair dileptonic decay final states distributions shows distinctive signature in the small dilepton correlation region. We also discuss the potential effects of P wave contribution.

Speaker: Ya-Juan Zheng

ZhengYajuan_toponium_KEKPH2026 (3).pdf

16:20 → 17:20 parallel session B: Flavor/Baryogenesis

Convener: Hiroyuki Ishida

16:20 A TeV-scale model for neutrino mass, dark matter and baryon asymmetry of the universe and its phenomenology

The Standard Model successfully describes particle physics but cannot explain neutrino oscillations, the baryon asymmetry of the universe (BAU), and dark matter. The Aoki-Kanemura-Seto (AKS) model is a new physics model that can explain these three phenomena simultaneously at TeV scale testable by future experiments. However, in the original model published in 2009, the baryon number was not evaluated. In the present work [Enomoto, Kanemura, Taniguchi, JHEP06(2025)036], we introduced CP violation to the original AKS model for evaluating the baryon number and found a benchmark scenario to avoid the current constraint on the electron electric dipole moment while keeping a large enough CP-violating phase for electroweak baryogenesis. In this talk, we evaluate the baryon number in this model, using the previously found viable parameter regions. Furthermore, we present benchmark points that can simultaneously explain neutrino masses, dark matter, and BAU, under various experimental and theoretical constraints. We also discuss prospects for testing the model at future experiments. The new particles predicted by the AKS model, including $S^\pm$ and the additional Higgs bosons $H_2$ and $H_3$, could be produced at future collider experiments such as the High-Luminosity LHC and electron-positron Higgs factories, and are expected to be tested there. In addition, since electroweak baryogenesis requires a strong first-order electroweak phase transition, which can generate characteristic gravitational wave signals, we also explore the potential for detection of such signals by future space-based observatories such as DECIGO.

Speaker: Sora Taniguchi

KEK-PH2026_SoraTaniguchi (2).pdf

16:40 Probing Scalar-Mediator Quark Couplings via CLFV Lepton-Nucleon Scattering

We investigate charged lepton flavor violating (CLFV) deep-inelastic scattering, focusing on the gluon-initiated subprocess $\ell_i g \to \ell_j g$ via the gluon effective operator $\phi\, G_{\mu \nu}^a G_a^{\mu \nu}$, and demonstrate how to probe the nature of the CLFV mediator $\phi$, specifically its mass and interaction with quarks.
We consider two benchmark scenarios for the mediator-quark coupling: (i) $h$-like scenario, in which the mediator couples to heavy quarks in proportion to their masses, and (ii) $b$-only scenario, where the coupling is restricted to bottom quark only. We demonstrate that these scenarios can be discriminated by examining the dependence of the differential cross section on the momentum transfer. Furthermore, we show that the peak position of the differential cross section exhibits a pronounced sensitivity to both the mass of the mediator and the coupling strengths with quarks.

Speaker: Masato Yamanaka

KEKPH2026-Yamanaka.pdf

Thursday, 19 February

09:00 → 10:20 plenary: Kohsaku Tobioka, Christiane Scherb

Convener: Jure Zupan

09:00 New Axion Bounds from Big Bang Nucleosynthesis

Speaker: Kohsaku Tobioka

Tobioka_talk2026_KEK-PH_v2.pdf

09:40 Sweeping the pion chimney for axion-like particles with KOTO

Speaker: Christiane Scherb

Kaon_alps_Scherb.pdf

10:20 → 10:40 coffee

10:40 → 11:40 parallel session A: DM

Convener: Ryusuke Jinno

10:40 Diffuse Multi-messenger Signals of Dark Matter Powered Stars

Dark matter (DM) annihilation can power the first generation of stars as long lived dark stars (DSs) that grow to supermassive scales $M_{\rm DS}\gtrsim 10^{5} M_{\odot}$ and eventually collapse into heavy black holes that could seed the supermassive black holes observed at high redshifts. We compute the electromagnetic and neutrino emission from these objects and determine the diffuse background flux from a population which would seed supermassive black holes. Using data from Fermi-LAT, Super-K, and IceCube, we draw constraints on this scenario of SMBH production in terms of the DM models which power the stars.

Speaker: Tom Schwemberger

KEK-PH2026_schwemberger_tom.pdf

11:00 MeV-Scale Sterile Neutrino Dark Matter and Future Detection

Sterile neutrinos are well-motivated candidates for dark matter. In particular, sterile neutrino dark matter in the MeV mass range has recently attracted increasing attention due to the prospects of future indirect detection experiments. In this work, we focus on the Compton Spectrometer and Imager (COSI) as a next-generation gamma-ray detector and investigate the detectability of MeV-scale sterile neutrino dark matter.
By exploiting the complementary sensitivity of the 511 keV line associated with positronium decay and a monochromatic gamma-ray line from radiative decays, COSI is expected to explore previously unexplored regions of parameter space for MeV-scale sterile neutrino dark matter. Furthermore, we show that within part of this parameter space, COSI has the capability to simultaneously probe sterile neutrino dark matter through both the 511 keV and monochromatic gamma-ray signals.

Speaker: Subaru Fujisawa

KEKPH2026_S.Fujisawa.pdf

10:40 → 11:40 parallel session B: Flavor/Cosmo

Convener: Christiane Scherb

10:40 Constraints on MeV Axion Models from Kaon Decays with KTeV Data

The QCD axion is a compelling mechanism for solving the strong CP problem. Most studies have focused on axion models with a large decay constant, $f_a \gtrsim 10^9,\mathrm{GeV}$. However, recent work has pointed out that viable axion models may also exist in the low-scale regime, $f_a \sim \mathcal{O}(1),\mathrm{GeV}$, corresponding to an axion mass in the MeV range.

In this mass range, the axion can be produced in kaon decays such as $K_L \to \pi^0 \pi^0 a$, followed by the prompt decay $a \to e^+ e^-$. As a result, the final-state signature coincides with that of the rare decay $K_L \to \pi^0 \pi^0 e^+ e^-$. The KTeV experiment has placed a stringent upper bound on this channel, $\mathrm{Br}(K_L \to \pi^0 \pi^0 e^+ e^-) < 6.6 \times 10^{-9}$, which can therefore be directly reinterpreted as a strong constraint on MeV-scale QCD axion models. In our work, we perform a dedicated analysis of this reinterpretation and derive stringent limits on the axion parameter space from kaon decay measurements.

Speaker: Takaya Iwai

KEK-ph2026_Iwai.pdf

11:00 Thermal Lepton Oscillations in Leptogenesis

Oscillation phenomena occur in both active and sterile neutrinos, where the oscillation phases are triggered by vacuum mass difference. Before the electroweak gauge symmetry breaking, leptons are massless, and cannot oscillate in the conventional way. However, they can still oscillate in the background plasma. Using the flavor-covariant nonequilibrium quantum field theory, I will discuss how oscillations appear from thermal leptons at finite temperatures, and present new ideas that may have significant impacts on leptogenesis.

Speaker: Shaoping Li

leptogenesis_Shaoping.pdf

11:40 → 13:00 lunch

13:00 → 14:20 plenary: Ryusuke Jinno, Mihoko Nojiri

Convener: Kaori Fuyuto

13:00 First-order phase transitions in the early Universe: Recent theoretical/experimental progress

Speaker: Ryusuke Jinno

Jinno.pdf

13:40 How LLM can help particle physicists

Speaker: Mihoko Nojiri

kekph2026.pdf

14:20 → 14:30 closing