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The current studies of microlensing planets are limited by small numberstatistics. Follow-up observations of high-magnification microlensing eventscan efficiently form a statistical planetary sample. Since 2020, the KoreaMicrolensing Telescope Network (KMTNet) and the Las Cumbres Observatory (LCO)global network have been conducting a follow-up program for high-magnificationKMTNet events. Here, we report the detection and analysis of a microlensingplanetary event, KMT-2023-BLG-1431, for which the subtle (0.05 magnitude) andshort-lived (5 hours) planetary signature was characterized by the follow-upfrom KMTNet and LCO. A binary-lens single-source (2L1S) analysis reveals aplanet/host mass ratio of $q = (0.72 \pm 0.07) \times 10^{-4}$, and thesingle-lens binary-source (1L2S) model is excluded by $\Delta\chi^2 = 80$. ABayesian analysis using a Galactic model yields estimates of the host star massof $M_{\rm host} = 0.57^{+0.33}_{-0.29}~M_\odot$, the planetary mass of $M_{\rmplanet} = 13.5_{-6.8}^{+8.1}~M_{\oplus}$, and the lens distance of $D_{\rm L} =6.9_{-1.7}^{+0.8}$ kpc. The projected planet-host separation of $a_\perp =2.3_{-0.5}^{+0.5}$ au or $a_\perp = 3.2_{-0.8}^{+0.7}$, subject to theclose/wide degeneracy. We also find that without the follow-up data, thesurvey-only data cannot break the degeneracy of central/resonant caustics andthe degeneracy of 2L1S/1L2S models, showing the importance of follow-upobservations for current microlensing surveys.

KMT-2023-BLG-1431Lb: A New $q < 10^{-4}$ Microlensing Planet from a Subtle Signature