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A three-dimensional fine-segmented sampling calorimeter enables us to measure the profiles of generated shower particles along the photon’s direction. For a feasibility study, a toy detector was designed via Geant4 simulation. It consists of alternating layers of a 1-mm-thick lead absorber and a 5-mm-thick plastic scintillator. The plastic scintillator is segmented into 15-mm-wide strips, alternately oriented in the vertical and horizontal directions. The energy deposits of each strip are used to train the machine learning algorithm (XGboost) to deduce the given angle, and the resolution of its angle reconstruction is expected to be 1.3 degrees for 1 GeV photon. We fabricate a small sampling calorimeter to validate the simulation results. We use 0.15-mm-thick tungsten strips instead of lead plates and 1mm-square scintillating fibers instead of plastic scintillators for better energy resolution. This updated configuration indicates no significant difference in the angular resolution, while the energy resolution significantly improves. Its performance shows a reasonable agreement between the Monte Carlo expectation and the obtained data with a positron beam. A detailed study is underway to understand the measured data thoroughly.

The J-PARC Hadron Experimental Facility was constructed with an aim to explore the origin and evolution of matter in the universe through experiments with intense particle beams. In the past decade, many results from particle and nuclear physics experiments have been obtained at the present facility. To expand the physics programs to as yet unexplored regions, the extension project of the Hadron Experimental Facility has been extensively discussed. This contribution presents the physics of the extension of the Hadron Experimental Facility to resolve issues related to strangeness nuclear physics, hadron physics, and flavor physics.

We report on the simulation results for the angular resolution of an electromagnetic (EM) sampling calorimeter with photons in the range of 100~MeV to 2~GeV. The simulation model of the EM calorimeter consists of alternating layers of a 1-mm-thick lead plate and a 5-mm-thick plastic scintillator plate. The scintillator plates are alternately segmented into horizontal and vertical strips. In this study, we obtain energy deposits in individual strips using Geant4 simulations and reconstruct the incident photon angles using XGBoost with gradient-boosted decision trees. The performance of the angle reconstruction depends on the detector configuration and the accuracy of machine learning. The angular resolution is well described by the expression \\(0.24^ 1.25^/E_\\), where \\(E_\\) is the incident photon energy in GeV, for strips of 15 mm and 32 layers. This energy dependence is consistent for different incident angles in the range of 10\\(^\\) to 40\\(^\\).

The KOTO II experiment is proposed to measure the branching ratio of the decay \\(K_L^0\\) at J-PARC. With a beamline to extract long-lived neutral kaons at 5 degrees from a production target, the single event sensitivity of the decay is \\(8.5 10^-13\\), which is much smaller than the Standard Model prediction \\(3 10^-11\\). This allows searches for new physics beyond the Standard Model and the first discovery of the decay with a significance exceeding \\(5\\). As the only experiment proposed in the world dedicated to rare kaon decays, KOTO II will be indispensable in the quest for a complete understanding of flavor dynamics in the quark sector. Moreover, by combining efforts from the kaon community worldwide, we plan to develop the KOTO II detector further and expand the physics reach of the experiment to include measurements of the branching ratio of the \\(K_L^0^+^-\\) decays, studies of other \\(K_L\\) decays, and searches for dark photons, axions, and axion-like particles. KOTO II will therefore obtain a comprehensive understanding of \\(K_L\\) decays, providing further constraints on new physics scenarios with existing \\(K^+\\) results.

The J-PARC Hadron Experimental Facility was constructed with an aim to explore the origin and evolution of matter in the universe through the experiments with intense particle beams. In the past decade, many results on particle and nuclear physics have been obtained at the present facility. To expand the physics programs to unexplored regions never achieved, the extension project of the Hadron Experimental Facility has been extensively discussed. This white paper presents the physics of the extension of the Hadron Experimental Facility for resolving the issues in the fields of the strangeness nuclear physics, hadron physics, and flavor physics.