GPPU Seminar

Date

15:00—17:00, February 18, 2026

Place

(hybrid) Room 745, Science Complex B (H-03), Zoom registration map

Abstract

1) Axion-like particles (ALPs) are among the leading dark-matter candidates. In the minimal cosine potential, cosmology imposes a robust requirement: by the epoch when ALPs behave as matter, the potential height needs to exceed the ALP energy density. This translates into a lower bound on the decay constant. A plausible loophole is to store most of the energy in nonzero-momentum modes while keeping the momenta non-relativistic. Using lattice simulations, we show that this does not evade the bound. When the condition is violated, the system behaves effectively as relativistic radiation rather than matter. For sufficiently small typical momentum, it also undergoes a nonlinear transition accompanied by the formation of “Baumkuchen-like” domain walls.

2) The Earth is a thermodynamic system that has evolved over 4.5 billion years, with its internal heat driving mantle convection and plate tectonics. Although the total surface heat flux is estimated to be approximately 46 ± 3TW, the relative contributions of primordial heat and radiogenic heat remain uncertain. This uncertainty is not merely quantitative; it directly impacts models of Earth’s thermal evolution and cooling history. Geo-neutrinos, electron antineutrinos produced in the decay chains of uranium and thorium inside the Earth, provide a unique and direct probe of radiogenic heat. Because neutrinos propagate through the Earth with negligible interaction, their flux carries information about the abundance of radioactive elements in the interior, particularly in the mantle, which is otherwise inaccessible to direct sampling. In this presentation, I report on geo neutrino observations with the KamLAND liquid scintillator detector. Using the inverse beta decay technique with delayed coincidence tagging, KamLAND has accumulated nearly two decades of data. The observed energy spectrum allows us to extract uranium and thorium contributions for the heat and to constrain radiogenic heat models. I also show current challenges in improving measurement precision, focusing on background reduction, to further suppress these backgrounds and enhance sensitivity.

3) KamLAND is a neutrino detector with a 1-kiloton liquid scintillator located 1000 meters underground in the Kamioka Mine. It successfully completed its 22-year KamLAND1 phase in August 2024. The KamLAND2 experiment is scheduled to begin in FY2027, aiming to enhance the light collection efficiency. To prepare for the start of the KamLAND2 experiment, the 13-meter-diameter balloon film used to contain the liquid scintillator will be remanufactured. While the new balloon will follow the same material and design as the proven KamLAND1 balloon, several new R&D efforts have been implemented to achieve even lower radioactivity. These include the introduction of a protective cover film to prevent contamination during production, research on film humidity management aiming for lower radioactivity than KamLAND1, and the adoption of low-radioactive materials for the suspension cords. In this presentation, I will report on the current status of these balloon production efforts and discuss their expected impact on the sensitivity of 7Be solar neutrino observations.

4) The ΛN interaction is a fundamental piece for understanding the baryon-baryon interactions and the properties of dense nuclear matter such as neutron stars. Since April 2025, we have been conducting a Λp scattering experiment at SPring-8 to measure the differential cross sections. By providing high-statistics data, we will impose some constraints on the current theoretical models that still involve uncertainties. Currently, the data analysis is in progress using the magnetic spectrometer for Λ identification and the recoil proton detector system (CATCH). In this talk, I will report the present status of the analysis.

Point

GSP 1