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.
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
One of the fundamental issues in quantum gravity is how to describe the initial state of the Universe. In quantum cosmology, the quantum state of the entire Universe is formulated as the “wave function of the Universe,” and its origin has been investigated using the path integral approach. However, conventional formulations based on the Euclidean path integral suffer from intrinsic problems.
In this talk, I will review recent developments in quantum cosmology based on the Lorentzian path integral and introduce our work applying Picard-Lefschetz theory and resurgence theory to this framework. First, I will explain how introducing an infinitesimal deformation in the parameter space fixes the Lefschetz thimble structure, naturally selecting the tunneling wave function. Second, I will demonstrate that the Borel resummation ambiguity of the perturbative series around the saddle is exactly canceled by the non-perturbative contribution from the other saddle. These results illustrate how non-perturbative aspects of quantum gravity offer new perspectives on the origin of the Universe. If time permits, I will also discuss the perturbation issues in quantum cosmology.
Jianing Wang, Kavli IPMU
PBH Formation and Its Origin in Inflation
Primordial black holes (PBHs) are believed to form through the gravitational collapse of overdense regions in the early Universe. They may serve as seeds for galaxy formation and are also considered one of the important candidates for cold dark matter (DM).
In particular, I will focus on several representative toy models of single-field inflation. The enhanced primordial perturbations in these models can not only produce PBHs, but also generate gravitational waves through higher-order effects. I will further extend the discussion to the possibility of a PBH-dominated era, which could leave observable signatures if PBH evaporation produces stable relics.
These studies demonstrate the significant potential of PBHs as probes of the early Universe, naturally leading to the important question of how to accurately estimate the PBH abundance. In the latter part of the talk, I will introduce a method based on peaks theory for estimating the abundance of primordial black holes. Our approach works well for arbitrary forms of the power spectrum, and by incorporating more systematic statistical methods, we expect it to provide useful cross-checks in combination with future gravitational-wave observations and related cosmological probes.
Fukutaro Miya, Osaka University
Generalizations and UV completions of Cho–Maison monopole
Magnetic monopoles are important topological solitons predicted in gauge theories with spontaneous symmetry breaking. In the electroweak theory, Cho and Maison constructed a monopole configuration by allowing a singular behavior at the origin. Since its essential structure is tied to electroweak-type symmetry breaking, analogous monopoles are expected to arise in a wider class of gauge theories.
In this talk, I will show that Cho-Maison-like monopole configurations can indeed be constructed in broad classes of models. I will also discuss how the electroweak Cho-Maison monopole can be embedded into a regular ‘t Hooft-Polyakov monopole as its low-energy effective description. In particular, I will show that a monopole in the Pati-Salam model reduces to the electroweak Cho-Maison monopole after heavy degrees of freedom are integrated out. This provides a possible ultraviolet origin of the Cho-Maison monopole and clarifies its generality beyond the Standard Model.
Sunao Sugiyama, Kavli IPMU
[IPNS Joint Experimental-Theoretical Cosmology Seminar] Probing Primordial Black Hole Dark Matter with Subaru HSC Microlensing Observations of M31
