| Summary: | Using $pp$ collision data corresponding to an integrated luminosity of
$5.4\,{\rm fb}^{-1}$ collected with the LHCb detector at a center-of-mass
energy of $13\,{\rm TeV}$, the $B^0\to D^-D^+K^+\pi^-$ decay is studied. A new
excited $D_s^+$ meson is observed decaying into the $D^+K^+\pi^-$ final state
with large statistical significance. The pole mass and width, and the
spin-parity of the new state are measured with an amplitude analysis to be
$m_R=2591\pm6\pm7\,{\rm MeV}$, $\Gamma_R=89\pm16\pm12\,{\rm MeV}$ and
$J^P=0^-$, where the first uncertainty is statistical and the second
systematic. Fit fractions for all components in the amplitude analysis are also
reported. The new resonance, denoted as $D_{s0}(2590)^+$, is a strong candidate
to be the $D_s(2^1{S}_0)^+$ state, the radial excitation of the pseudoscalar
ground-state $D_s^+$ meson.
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