arXiv subjects

Yuki Fujii

Publications and source records attributed to Yuki Fujii.

10 recordsLinked to original sources

arXiv2026

Designing Recommendation Exposure and Favorite Lists: A Field Experiment in a Spot-Work Platform

How should recommender systems be designed when recommendations shape access to scarce, short-lived opportunities? We study this question in a production setting: Timee, Japan's largest platform for spot work, where workers favorite job templates and receive notifications when firms post shifts from those templates. Maximizing predicted favoriting can generate misdirected concentration: recommendations accumulate on popular templates that create few viable job openings, while templates with unmet labor demand receive too little exposure. We design exposure-control mechanisms for favorite-list management, reallocating template exposure based on posting activity and unfilled capacity. The proposed recommender, thresholded eligibility control (TEC), is fully parallelizable and suitable for large-scale digital platforms. In simulations calibrated to Timee data, TEC raises the per-round job-finding rate from 57.6% to 70.0%. A prefecture-level randomized field experiment increases realized matches and exposure per active template, reduces the share of low-exposure templates, and improves impression-level favoriting and downstream matching.

econ.GN↗

arXiv2025

Estimating the spectral radius of Bell-type operator via finite dimensional approximation of orthogonal projections

We establish a new decomposition formula for two orthogonal projections P and Q on a separable Hilbert space V. This formula yields an orthogonal direct sum decomposition of V into invariant subspaces under P and Q, each of which is either at most two dimensional or infinite dimensional. On every infinite dimensional component, the pair (P,Q) admits a matrix representation that we call the "one-shifted form". This representation diagonalizes both P and Q into blocks of size at most two, and moreover, both projections can be explicitly approximated by orthogonal projections on finite dimensional subspaces. This approximation scheme offers a way to derive infinite dimensional results from their finite dimensional counterparts and is also useful in numerical computations. This decomposition provides a useful framework for analyzing a wide range of problems involving two orthogonal projections in infinite dimensions. In particular, several spectral problems for operators generated by P and Q (including polynomials in P and Q) can be reduced to the case where the pair admits a one-shifted form. More concretely, we can estimate the spectral radius of [P,Q], which is equivalent to estimating the spectral radius of the Bell-CHSH operator, a quantity of fundamental importance in quantum mechanics. We provide an upper bound and a lower bound for the spectral radius of [P,Q], which become exact when the matrix representations of P and Q are in "constant-angle one-shifted form".

math.RT↗

arXiv2025

Development of the Range Counter for the COMET Phase-$α$ Experiment

The COMET Phase-$α$ experiment aims to evaluate the novel muon transport beamline for the muon-to-electron conversion search at J-PARC, Japan. A dedicated Range Counter (RC) was developed to measure the momentum spectrum of transported negative muons with momenta of 30--100 MeV/$c$. The RC consists of graphite momentum degraders, a muon absorber, and plastic scintillation counters ($\rm T_0$, $\rm T_1$, and $\rm T_2$) to detect decay-in-orbit (DIO) electrons from stopped muons. The number of muons stopped in the absorber is reconstructed from the decay time distribution. A copper absorber was selected due to the short lifetime of muonic atoms in copper, which enhances signal separation. The counters' performance was evaluated experimentally. The $\rm T_0$ Counter, made of a $200\times 200\times 0.5~{\rm mm^3}$ scintillator plate, achieved a muon-trigger efficiency exceeding 99.9%. The $\rm T_1$ and $\rm T_2$ Counters also demonstrated high electron-detection efficiencies of $>99$%. Based on these results, simulation studies estimate the acceptance for reconstructing the number of DIO electrons from the absorber to be approximately 47% with a corresponding signal purity of 60% against muon capture-induced backgrounds.

physics.ins-det↗

arXiv2023

A search for a muon to electron conversion in COMET

The COMET experiment at J-PARC, Japan, aims to search for muon to electron conversion with aluminium nuclei, achieving a sensitivity four orders of magnitude higher than the current upper limit at a 90% confidence level. The proton beam line has recently been completed, and muons have been successfully transported through the curved solenoid in the Phase-alpha of the experiment. In this paper, we will present preliminary results from the Phase-alpha beam measurement, the status of the intermediate sensitivity experiment (COMET Phase-I), and the ultimate goal of COMET Phase-II.

hep-ex↗

arXiv2021

An FPGA-based Trigger System with Online Track Recognition in COMET Phase-I

An FPGA-based online trigger system has been developed for the COMET Phase-I experiment. This experiment searches for muon-to-electron conversion, which has never been observed yet. A drift chamber and trigger counters detect a mono-energetic electron from the conversion process in a 1-T solenoidal magnetic field. A highly intense muon source is applied to reach unprecedented experimental sensitivity. It also generates undesirable background particles, and a trigger rate due to these particles is expected to be much higher than an acceptable trigger rate in the data acquisition system. By using hit information from the drift chamber too, the online trigger system efficiently suppresses a background trigger rate while keeping signal-event acceptance large. A characteristic of this system is the utilization of the machine learning technique in the form of look-up tables on hardware. An initial simulation study indicates that the signal-event acceptance of the online trigger is 96% while the background trigger rate is reduced from over $90\,\mathrm{kHz}$ to $13\,\mathrm{kHz}$. For this scenario, we have produced trigger-related electronics that construct a distributed trigger architecture. The total latency of the trigger system was estimated to be $3.2\,\mathrm{μs}$, and the first operation test was carried out by using a part of the drift-chamber readout region.

physics.ins-det↗

arXiv2021

Radiation tolerance of online trigger system for COMET Phase-I

The COMET experiment aims to search for the neutrinoless muon to electron transition process with new sensitivity levels. The online trigger system is an integral part of achieving the sensitivity levels required and will be subject to an expected neutron fluence of up to $10^{12}$ $n \cdot \mathrm{cm}^{-2}\;$ within regions inside the detector solenoid. Consequently a significant number of soft errors in the logic of the onboard field programmable gate arrays (FPGA) can occur, requiring error correction for single event upsets and firmware reprogramming schemes for unrecoverable soft errors. We studied the radiation tolerance of the COMET Phase-I front-end trigger system, called COTTRI, subject to neutron fluence on order $10^{12}$ $n \cdot \mathrm{cm}^{-2}\;$ with multiple error correcting codes and automatic firmware reconfiguration. The regions measured were the configuration RAM, block RAM and also in a multi-gigabit transfer link using copper cables that will be used for communication between different trigger boards during Phase-I. The resulting cross sections observed suggest the most significant impact to the experiment will come from unrecoverable soft errors in configuration RAM, with dead time expected to be $(4.2 \pm 1.3)\%$. The effect of multi-bit errors in block RAM was found to be almost negligible in COMET Phase-I. In addition, multiple solutions have already been proposed in order to suppress these errors further. Soft errors observed in the multi-gigabit transfer links were measured to be of two orders of magnitude less impact compared to the unrecoverable errors in configuration RAM. We concluded that the COTTRI system meets the trigger requirement in COMET Phase-I.

physics.ins-det↗

arXiv2021

Gigabit Ethernet Daisy-Chain on FPGA for COMET Read-out Electronics

The COMET experiment at J-PARC aims to search for the neutrinoless transition of a muon to an electron. We have developed the readout electronics board called ROESTI for the COMET straw tube tracker. We plan to install the ROESTI in the gas manifold of the detector. The number of vacuum feedthroughs needs to be reduced due to space constraints and cost limitations. In order to decrease the number of vacuum feedthroughs drastically, we developed a network processor with a daisy-chain function of Gigabit Ethernet for the FPGA on the ROESTI. We implemented two SiTCPs, which are hardware-based TCP processors for Gigabit Ethernet, in the network processor. We also added the data path controllers which handle the Ethernet frames and the event data. The network processor enables ROESTI to process the slow control over UDP/IP and to transfer event data over TCP/IP. By using the network processor, we measured the throughput, the stability, and the data loss rate for two to six ROESTIs. In any number of boards, the throughput of the event data transfer achieved the theoretical limit of TCP over the Gigabit Ethernet stably and ROESTI stably sent 100% of the data.

physics.ins-det↗

arXiv2019

Radiation hardness study for the COMET Phase-I electronics

Radiation damage on front-end readout and trigger electronics is an important issue in the COMET Phase-I experiment at J-PARC, which plans to search for the neutrinoless transition of a muon to an electron. To produce an intense muon beam, a high-power proton beam impinges on a graphite target, resulting in a high-radiation environment. We require radiation tolerance to a total dose of $1.0\,\mathrm{kGy}$ and $1\,\mathrm{MeV}$ equivalent neutron fluence of $1.0\times10^{12}\,\mathrm{n_{eq}\,cm^{-2}}$ including a safety factor of 5 over the duration of the physics measurement. The use of commercially-available electronics components which have high radiation tolerance, if such components can be secured, is desirable in such an environment. The radiation hardness of commercial electronic components has been evaluated in gamma-ray and neutron irradiation tests. As results of these tests, voltage regulators, ADCs, DACs, and several other components were found to have enough tolerance to both gamma-ray and neutron irradiation at the level we require.

physics.ins-det↗

arXiv2013

An Analysis on Minimum s-t Cut Capacity of Random Graphs with Specified Degree Distribution

The capacity (or maximum flow) of an unicast network is known to be equal to the minimum s-t cut capacity due to the max-flow min-cut theorem. If the topology of a network (or link capacities) is dynamically changing or unknown, it is not so trivial to predict statistical properties on the maximum flow of the network. In this paper, we present a probabilistic analysis for evaluating the accumulate distribution of the minimum s-t cut capacity on random graphs. The graph ensemble treated in this paper consists of weighted graphs with arbitrary specified degree distribution. The main contribution of our work is a lower bound for the accumulate distribution of the minimum s-t cut capacity. From some computer experiments, it is observed that the lower bound derived here reflects the actual statistical behavior of the minimum s-t cut capacity of random graphs with specified degrees.

cs.IT↗

arXiv2012

A Coding Theoretic Approach for Evaluating Accumulate Distribution on Minimum Cut Capacity of Weighted Random Graphs

The multicast capacity of a directed network is closely related to the $s$-$t$ maximum flow, which is equal to the $s$-$t$ minimum cut capacity due to the max-flow min-cut theorem. If the topology of a network (or link capacities) is dynamically changing or have stochastic nature, it is not so trivial to predict statistical properties on the maximum flow. In this paper, we present a coding theoretic approach for evaluating the accumulate distribution of the minimum cut capacity of weighted random graphs. The main feature of our approach is to utilize the correspondence between the cut space of a graph and a binary LDGM (low-density generator-matrix) code with column weight 2. The graph ensemble treated in the paper is a weighted version of Erdős-Rényi random graph ensemble. The main contribution of our work is a combinatorial lower bound for the accumulate distribution of the minimum cut capacity. From some computer experiments, it is observed that the lower bound derived here reflects the actual statistical behavior of the minimum cut capacity.

cs.IT↗