  The current studies of microlensing planets are limited by small number
statistics. Follow-up observations of high-magnification microlensing events
can efficiently form a statistical planetary sample. Since 2020, the Korea
Microlensing Telescope Network (KMTNet) and the Las Cumbres Observatory (LCO)
global network have been conducting a follow-up program for high-magnification
KMTNet events. Here, we report the detection and analysis of a microlensing
planetary event, KMT-2023-BLG-1431, for which the subtle (0.05 magnitude) and
short-lived (5 hours) planetary signature was characterized by the follow-up
from KMTNet and LCO. A binary-lens single-source (2L1S) analysis reveals a
planet/host mass ratio of $q = (0.72 \pm 0.07) \times 10^{-4}$, and the
single-lens binary-source (1L2S) model is excluded by $\Delta\chi^2 = 80$. A
Bayesian analysis using a Galactic model yields estimates of the host star mass
of $M_{\rm host} = 0.57^{+0.33}_{-0.29}~M_\odot$, the planetary mass of $M_{\rm
planet} = 13.5_{-6.8}^{+8.1}~M_{\oplus}$, 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 the
close/wide degeneracy. We also find that without the follow-up data, the
survey-only data cannot break the degeneracy of central/resonant caustics and
the degeneracy of 2L1S/1L2S models, showing the importance of follow-up
observations for current microlensing surveys.
