Mahboubeh Shahrbaf Motlagh, University of Wroclaw / Davood Rafiei, University of Wroclaw
[KEK-JAEA Joint Seminar] Strangeness in Neutron Stars: From Realistic Interactions to Astrophysical Signatures / Neutron Stars: Promising Natural Laboratories for Exotic and Dark Matter
[Abstract of Mahboubeh Shahrbaf Motlagh]
Strange particles are abundantly produced and studied in dedicated hadron and hypernuclear physics experiments at J-PARC, as well as in high-energy heavy-ion collision experiments such as HADES at GSI, ALICE at CERN, STAR at RHIC, and NA61/SHINE at CERN. These experiments provide valuable constraints on the properties of dense baryonic matter and hyperon interactions. Under the extreme densities reached in neutron star (NS) interiors, strange degrees of freedom are also expected to appear; however, their actual presence remains one of the most fundamental open questions in dense matter physics. Understanding their role is essential for constraining the equation of state (EoS) of strongly interacting matter and for connecting terrestrial measurements with astrophysical observations.
In the first part of this talk, I will briefly review my microscopic study of hyperonic matter within the lowest-order constrained variational (LOCV) framework. Using realistic spin- and parity-dependent ΛN and ΛΛ interactions constrained by hypernuclear data, I will discuss the resulting EoS and the macroscopic properties of NSs. The inclusion of Λ hyperons leads to a moderate softening of the EoS and reduces the maximum mass of NS, giving rise to the so-called hyperon puzzle. Possible mechanisms to resolve this puzzle will be discussed in light of current observational constraints from NICER and gravitational-wave measurements.
In the second part, I will consider a relativistic mean-field framework, specifically the DD2Y-T model, and explore how the inclusion of a deeply bound H-dibaryon-like particle, the so-called sexaquark with quark content uuddss, can modify the hyperonic EoS. This combined treatment of hyperons and sexaquarks highlights how these two strangeness-bearing components may jointly influence the internal composition and observable properties of NSs. Our results suggest that, for a favorable sexaquark mass range, the onset of sexaquark degrees of freedom in NS matter may facilitate an early transition to deconfined quark matter within a smooth crossover construction.
[Abstract of Dr. Davood Rafiei]
Dark matter makes up more than 85% of the matter in the Universe, yet its microscopic nature remains unknown. Neutron stars, because of their extreme density and strong gravity, provide unique natural laboratories to probe dark matter and other exotic degrees of freedom through multimessenger observations.
In this talk, I will discuss two approaches to probing dark-matter-admixed neutron stars. First, I will present a two-fluid framework in which normal neutron-star matter and sub-GeV self-interacting bosonic dark matter are treated as two separate fluids that interact only
through gravity. Depending on the dark matter mass, coupling strength, and fraction, the
dark component may form either a compact core or an extended halo, affecting the mass-radius relation, tidal deformability, and pulse-profile modeling. I will show how current multimessenger constraints can restrict the corresponding dark matter parameter space.
Second, I will discuss a single-fluid framework in which exotic or dark components are produced inside dense neutron-star matter and directly modify the effective equation of state. I will focus on f-mode oscillation frequencies, damping times, and quasi-universal relations, with applications to gravitational-wave asteroseismology. In this context, I
will consider hyperons, sexaquarks or deeply bound H-dibaryons, and deconfined quark matter. These studies show that exotic and dark components can leave observable signatures in future gravitational-wave detections.
Tatsuya Seko, Shizuoka U
Description of curved spacetimes by finite-size matrices in the type IIB matrix model
The type IIB matrix model is expected to give a nonperturbative formulation of superstring theory. Its covariant derivative interpretation provides a method to describe curved spacetimes in the model. There, matrices are identified with certain covariant derivatives which can be viewed as infinite-size matrices. Here, by using the Berezin Toeplitz quantization, we develop a method to regularize these matrices as finite-size ones, which is needed to calculate quantum effects in the interpretation or in particular to apply the interpretation to the results of numerical simulations. As an example, we examine the case of S^2 in detail.
Shunzo Kumano, IMP&KEK
Prospects of generalized parton distributions and related projects at KEK
Masses and spins of hadrons are fundamental quantities in physics; however, their origins are not well understood, and their investigation is one of the major purposes of building electron-ion colliders (EICs) in the 2030’s. Both quantities can be investigated by generalized parton distributions (GPDs). In this seminar, I explain various experimental processes for measuring the GPDs and experimental possibilities at KEK. So far, the GPDs have been investigated mainly by the deeply virtual Compton scattering and deeply virtual meson production processes at charged-lepton accelerator facilities. They could be investigated by high-energy neutrino facilities in the future.
At KEK, it is possible to investigate the GPDs using the e^+ e^- collider KEKB and the hadron-accelerator facility J-PARC. We can extract the s-channel GPDs and gravitational form factors of hadrons by the two-photon processes gamma^* + gamma -> h + h-bar, where h is a hadron. Actually, there was the first report on the determination of gravitational form factors and radii from KEKB measurements in 2018. At J-PARC, the GPDs will be investigated by the exclusive Drell-Yan process pi^- p -> mu^+ mu^- B, where the baryon B could be a nucleon or Delta. In the future, other processes could be studied for the GPDs. For example, the 2->3 reaction processes NN -> N pi B could be used for probing the GPDs in the ERBL (Efremov-Radyushkin-Brodsky-Lepage) region.
Hiroki Wada, Tohoku U
Anomalies in family unification models from bordism classification
Anomaly cancellation is one of the most fundamental consistency conditions in quantum field theory. In constructing models beyond the Standard Model, one must ensure the absence of anomalies that would render the theory inconsistent. Consequently, anomaly cancellation conditions impose strong constraints on the allowed matter content. Recent developments have made it possible to systematically study not only perturbative anomalies but also the more subtle global anomalies via bordism theory. This perspective has led to a deeper understanding of anomalies and their implications for model building. In this talk, I will discuss anomalies in family unification models, which provide a natural explanation for the three generations of quarks and leptons. We identify the relevant bordism groups that classify possible anomalies in these models and show that no global anomalies arise. We also revisit earlier studies of perturbative anomalies in these models dating back to the 1980s. This talk is based on joint work with Tsubasa Sugeno (Tohoku University), arXiv:2604.04393.
Hiroki Takahashi, Univ. of Tokyo
Asymmetric Dark Matter from 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 the right-handed neutrino is much lower than the Davidson–Ibarra bound. 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-mass 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 be of order 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, from GeV up to 10 TeV.
福山寛, 東大低温科学研究センター/KEK QUP
[金茶会] 超低エネルギー物理学の魅力 −量子凝縮系を中心に−
https://www-conf.kek.jp/kincha/
Sotaro Sugishita, Hokkaido University
Celestial holography meets dS/CFT
Celestial holography reformulates scattering amplitudes in (D+2)-dimensional flat spacetime as correlation functions on the D-dimensional celestial sphere, while in (D+1)-dimensional de Sitter spacetime cosmological correlators are also viewed as correlation functions on the D-dimensional sphere at late times. Thus, both frameworks naturally lead to correlators on spheres with the same dimension. We provide a concrete map between celestial amplitudes and cosmological correlators through the dS foliation of (D+2)-dimensional flat spacetime. This gives a cosmological-celestial dictionary relating celestial operators to operators at future and past infinity in dS. In the dictionary, massless fields in Minkowski spacetime are mapped to fields with continuous mass spectra in dS. In particular, the modes associated with asymptotic symmetries in flat spacetime have precise counterparts on the de Sitter side. For U(1) gauge theory, the counterpart is the massless sector within the continuous Proca spectrum. For linearized gravity, supertranslations and superrotations are encoded, respectively, in partially massless and strictly massless sectors of a spin-2 field with continuous spectrum. Using these identifications, we find that the Ward-Takahashi identities of cosmological correlators reproduce the conformally soft photon and graviton theorems through the cosmological-celestial dictionary.
Yukihiro Kanda, ICRR
Particle emission from semilocal string networks
Topological defects can arise as remnants of symmetry-breaking phase transitions in the early universe and are usually classified according to the homotopy groups of the vacuum manifold. However, field theories can admit stable localized configurations whose stability is ensured not only by topology but also by energetic considerations. Semilocal strings provide such an example: they are string-like defects that arise even when the fundamental group of the vacuum manifold is trivial. We present results from numerical lattice simulations of semilocal string networks, focusing for the first time on their particle-emission properties. We find that semilocal string networks efficiently emit Nambu-Goldstone bosons, with emission properties similar to axion radiation from global string networks. Motivated by this result, we discuss a scenario in which the Nambu-Goldstone bosons acquire a soft-breaking mass, and their non-thermal production from the string network accounts for the observed dark matter abundance.
Keisuke Harigaya, Univ. of Chicago
Kinetic isocurvature perturbations
We formulate a new class of primordial perturbations called kinetic isocurvature perturbations, where the mass density of dark matter is constant relative to the photon number density while the kinetic energy of dark matter fluctuates in space. Such perturbations naturally arise in scenarios where a nonrelativistic heavy field decays into relativistic dark matter particles with a spatially modulated rate. As dark matter cools and becomes nonrelativistic, these fluctuations in kinetic energy leave large-scale density perturbations essentially unaffected and therefore evade the Cosmic Microwave Background bounds on isocurvature perturbations, yet survive as spatial variations in the free-streaming scale, resulting in patch-by-patch variation of the matter power spectrum.
Hiroki Matsui, Osaka Metropolitan University
Perturbative and Non-Perturbative Aspects of Lorentzian Quantum Cosmology
