Decadal upgrade strategy for KAGRA toward post-O5 gravitational-wave astronomy

TL;DR

This paper presents a comprehensive ten-year upgrade plan for the KAGRA gravitational wave detector, focusing on optimizing scientific outcomes like binary neutron star parameter estimation while balancing technical feasibility and costs.

Contribution

It introduces a prioritized upgrade strategy emphasizing high-frequency sensitivity improvements, supported by detailed feasibility and impact assessments.

Findings

High-frequency upgrades improve sky localization to <0.5 deg$^2$ at 100 Mpc.

Upgrade enhances tidal deformability measurement precision by ~10%.

Broadband sensitivity increase above 200 Hz offers optimal scientific and technical balance.

Abstract

The KAGRA Collaboration has investigated a ten-year upgrade strategy for the KAGRA gravitational wave detector, considering a total of 14 upgrade options that vary in mirror mass, quantum noise reduction techniques, and the quality of cryogenic suspensions. We evaluated the scientific potential of these configurations with a focus on key targets such as parameter estimation of compact binary coalescences, binary neutron star post-merger signals, and continuous gravitational waves. Rather than aiming to improve all science cases uniformly, we prioritized those most sensitive to the detector configuration. Technical feasibility was assessed based on required hardware developments, associated R\&D efforts, cost, and risk. Our study finds that a high-frequency upgrade plan that enhances sensitivity over a broad frequency range above ~200 Hz offers the best balance between scientific return and technical feasibility. Such an upgrade would enable sky localization of binary neutron star mergers at 100 Mpc to better than 0.5 deg$^2$ in a LIGO-Virgo-KAGRA network, and improve the measurement precision of tidal deformability parameter by approximately 10% at median, compared to a network without KAGRA.